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Unit-pockmarks and their potential significance for predicting fluid flow
Marine and Petroleum Geology 27 (2010) 1190e1199
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Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo
Unit-pockmarks and their potential significance for predicting fluid flow M. Hovland a, b, *, R. Heggland a, M.H. De Vries c, T.I. Tjelta a a
Statoil ASA, Forusbeen 50, N-4035 Stavanger, Norway Centre for Geobiology, University of Bergen, N-5020 Bergen, Norway c SINTEF, Building and Infrastructure, N-7465 Trondheim, Norway b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 August 2009 Received in revised form 5 February 2010 Accepted 6 February 2010 Available online 12 February 2010
Unit-pockmarks were recognized as more-or-less insignificant features on the seafloor in the early 1980s. However, this investigation, at four different regions in Norwegian waters, suggests they are more significant for seep detection than previously believed. They occur as circular depressions in the seafloor (diameter < 5 m) either as singular features, as strings, or as clusters. One of our main conclusions is that they are widespread and represent the most recent and most active local seep locations. This is based on their areal density distribution, the finding of relatively high hydrocarbon concentrations inside sampled unit-pockmarks and at locations where they are abundant, and on theoretical considerations. When unitpockmarks occur together with ‘normal-sized’ pockmarks, they often form to the side of the normalpockmark centre. Our study also suggests that (1) the driving force behind seafloor hydraulic activity, i.e., the formation of unit-pockmarks, normal-pockmarks, and many other fluid flow features, is pockets of buried free gas, and (2) whereas unit-pockmarks likely manifest cyclic pore-water seepage, their larger related, normal-pockmarks, likely manifest periodic or intermittent gas bursts (eruptions), with extended intervening periods of slow, diffusive, and cyclic pore-water seepage. Our findings suggest that seep detection is most efficiently performed by mapping the seafloor with high-resolution bathymetry (at least 1 m 1 m gridding), and acquiring geochemical samples where the density of unit-pockmarks is locally highest. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Unit-pockmarks Normal-pockmarks Hydraulic activity Fluid flow Capillary sealing Geochemical results Micro-seep Pore-water Nyegga
1. Introduction Seeps, including visible ones from mud volcanoes and invisible micro-seeps from pockmarks, may provide important information on sub-seafloor hydraulic and fluid conditions. Seeps have already provided valuable information for petroleum geology, exploration, tectonic studies, and geo-hazard studies, but will probably become increasingly important for marine environmental and ecological studies in the future (Paull et al., 2002; Niemann et al., 2005; Etiope et al., 2009; Webb et al., 2009; Foucher et al., 2009; Hovland, 2008; Judd and Hovland, 2007; Cathles et al., 2010). Since their discovery in the late 1960s, pockmarks have intrigued marine scientists (King and MacLean, 1970). Their association with seeping gases and pore-water was established by stable isotope results on carbonate-cemented sediments found inside North Sea pockmarks (Hovland et al., 1985). But, even so, a proper understanding of their formation and underground
* Corresponding author. Statoil ASA, Forusbeen 50, N-4035 Stavanger, Norway. Tel.: þ47 95802243; fax: þ47 51990050. E-mail address: [email protected] (M. Hovland). 0264-8172/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2010.02.005
plumbing system has yet to be established (Tjelta et al., 2007; Forsberg et al., 2007; De Vries et al., 2007a, b; Judd and Hovland, 2007; Webb et al., 2009; Cathles et al., 2010). It is inferred that pockmark craters form abruptly, when pockets of local over-pressured pore-water and gas erupt through the seafloor surface sediments, and that they are subsequently maintained by slow (tidally induced?) pore-water and gas seepage (Cathles et al., 2010). This inference is based on a combination of (i) laboratory studies and on modern detailed studies at pockmarks over the Troll field in the Norwegian Channel and elsewhere (Tjelta et al., 2007; De Vries et al., 2007a, b; Judd and Hovland, 2007), (ii) observations made from fossilized seepage locations (Aiello, 2005; Campbell et al., 2002; De Boever et al., 2009), (iii) high-resolution 3D-studies of seep locations (Heggland, 1998; Schroot et al., 2005; Cartwright et al., 2007), (iv) partly based on studies of mud volcanoes (Hovland et al., 1997; Dimitrov, 2002; Foucher et al., 2009; Etiope et al., 2009); and partly based on theoretical modelling (Bui et al., 2007; Cathles et al., 2010). However, it is expected that the exact mechanisms and processes involved in pockmark maintenance, i.e., the continuous local removal of suspension sediments, cannot be established before extended detailed monitoring of pockmarks has been performed. In this paper we examine
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the manifestation of seepage conveyed by the small ‘unit-pockmarks’, often seen, but often treated as mere curiosities and insignificant morphological seabed features. The term ‘Unit-Pockmark’ was coined by Hovland et al. (1984), during a comparison study of pockmark-associated features on both sides of the Atlantic Ocean. The term describes very distinct and more-or-less mono-sized, small pockmarks, at that time only seen on side scan sonar records where ‘normal’ pockmarks were often found to consist of the amalgamation of large numbers of ‘unit-pockmarks” (Hovland et al., 1984). Their definition is simply: “Unit-pockmarks are very small (<5 m) seabed depressions which are found in isolation, in groups, and in association with larger pockmarks” (Hovland and Judd, 1988). Since the advent of multi-beam swath echo-sounders, mounted on remotely operated or autonomous vehicles (ROV and AUVs), these distinctive, small features have been detected in numerous geological settings, most often where normal-pockmarks occur (Judd and Hovland, 2007; Webb et al., 2009). The objective of this paper is to describe the occurrence and configurations of unit-
Fig. 1. Map showing the selected locations for this study. The black asterisks show the locations of Troll (T), Nyegga (N), Morvin (M), and Haltenpipe Reef Cluster (H). The three latter locations are off mid-Norway (framed). Whereas T, M, and H occur on the continental shelf (water depths ranging from 300 to 400 m), area N is located on the continental slope (water depth 750 m). The two red dots on land are Trondheim (T) and Oslo (O).
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pockmarks in four different settings on the Norwegian continental shelf and slope, and to discuss their possible significance in improving the understanding of local seabed hydraulic activity and plumbing systems. The four selected study locations are the Norwegian Channel, near the Troll hydrocarbon field, the Haltenpipe Reef Cluster (HRC), the Morvin hydrocarbon field, and the Nyegga area (Fig. 1). 2. Methods, mapping and geochemistry Unit-pockmarks have been studied mainly by interpreting highresolution bathymetry, sub-bottom and side scan sonar data, gathered with ROVs. This type of seafloor mapping is conducted while the ROV, fitted with single- or dual swath echo-sounders, ‘flies’ between 10 and 15 m above the general seafloor. The digital terrain model (DTM) thus produced has been gridded to a resolution of up to 0.2 m 0.2 m (i.e., Figs. 2 and 3), however, gridding to a resolution of 1 m 1 m and 0.5 m 0.5 m has normally been done. ROV-mounted side scan sonar (100 kHz) has also been used often together with ROV-mounted high-resolution shallow seismic systems (e.g., Fig. 4). In addition to gathering high-resolution geophysical data, the main author has, whenever possible, acquired sediment cores for the geochemical analysis of light hydrocarbons remaining in the clay fraction of the sediments. The cores were either obtained by normal gravity corers, or 0.5 m long coring tubes operated by ROVs. Approximately 200 g clay samples were acquired from between 0.4 and 1 m below the seafloor (normally in the oxic zone). The samples were immediately packed in plastic bags and stored frozen (20 C) for onshore geochemical analysis (at GeolabNor, Trondheim). Both adsorbed and interstitial (occluded) gases were analysed (Hovland and Judd, 1988). Over the years, this method has provided interesting geochemical signals and indications of significant variations in the content of interstitial and adsorbed gases in the upper sediments. These variations are interpreted as proof of seepage, both the invisible micro-seepage and the visible macro-seepage of light hydrocarbons many places on the Norwegian and UK continental
Fig. 2. Perspective view of a shaded relief digital terrain model (DTM) from the Norwegian Trough, near the Troll field. The artificial illumination of the surface is from SE. There are solitary unit-pockmarks scattered over otherwise smooth portions of the seafloor. The density of the unit-pockmarks is clearly higher near some of the normalpockmarks. There is one cluster of five or six unit-pockmarks marked with a white arrow. In addition, there are numerous unit-pockmarks clearly associated with different generations of trawl-scars (some of these unit-pockmarks are marked with black arrows). Notice the cluster of unit-pockmarks inside the shallowest normalpockmark, located about 100 m SSE of the white arrow. According to Cathles et al. (2010), the next normal-pockmark may be formed at the light arrow. The two white patches are caused by missing depth data.
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shelves (Judd and Hovland, 2007). This type of geochemical data, are currently available at three of the studied locations: HRC, Morvin, and Nyegga. At Norwegian Channel (Troll) locations, however, sensitive geochemical data are currently unavailable (Tjelta et al., 2007; Forsberg et al., 2007; De Vries et al., 2007b). 3. Unit-pockmarks off Norway
Fig. 3. Perspective view of a shaded relief digital terrain model (DTM) from one of a series of “pockmark-families” in the Norwegian Trough, near the Troll field. Illumination is from the south. This family of about eight pockmarks and hundreds of unitpockmarks consists of one large parent-pockmark, located in the centre, with seven satellite-pockmarks surrounding it. Further morphological study, however, reveals that the parent-pockmark probably consists of three or four amalgamated satellite-pockmarks. Unit-pockmarks occur in varying density, which clearly increases towards the rims of the parent-pockmark. The black arrow points at a trawl-scar associated unitpockmark. The white arrow points at a possible ‘incipient’ pockmark, where there is a cluster of unit-pockmarks associated with a slight depression. Note that the pinnacle in the centre of the parent-pockmark consists of methane-derived authigenic carbonate (MDAC) which has been colonized by deep-water corals and giant bivalves, indicating on-going pore-water seepage (Hovland, 2008). The two white orthogonal lines indicate sub-bottom profiler lines, one of which is shown in Fig. 4.
Fig. 4. A sub-bottom profiler (SBP) record crossing the parent-pockmark, shown in Fig. 3. An interpreted version of this image is found on Fig. 12. Note the disturbance in the acoustic layering directly below the carbonate pinnacle in the centre of the pockmark. This pockmark is suspected to be sealed by MDAC, suspected to act as an efficient seal for free gas (Hovland, 2002; Cathles et al., 2010).
Over the last two decades, Statoil has conducted detailed seabed mapping in many offshore areas where unit-pockmarks have been detected. Four of these are discussed here (1) on the mid-continental shelf off Western-Norway, e.g., near the giant Troll hydrocarbon field in the Norwegian Channel (Tjelta et al., 2007; Forsberg et al., 2007; Webb et al., 2009); (2) on the mid-continental shelf off MidNorway, e.g., at the Haltenpipe Reef Cluster, HRC (Hovland and Risk, 2003; Hovland, 2008); (3) on the outer-continental shelf off MidNorway, e.g., at the Morvin hydrocarbon field (Hovland, 2005; Hovland, 2008); and (4) on the continental slope off Mid-Norway, e.g., at the Nyegga complex pockmark and hydrate pingo location (Hovland et al., 2005; Hovland and Svensen, 2006; Ivanov et al., 2007; Westbrook et al., 2008; Plaza-Faverola et al., 2010). The following is a description of the features documented at each of these sites and the manner in which the unit-pockmarks occur. Unit-pockmarks may occur as: (1) solitary features (in isolation), (2) curvilinear strings of varying length and density, and (3) clusters of varying density. Also the geochemical data obtained from the same areas are presented here. 3.1. Unit-pockmarks in the Norwegian Channel (Troll) The water depth at the Troll area of the Norwegian Channel (also called the Norwegian Trench), is about 330 m. The upper sediment layer, consists of a 20e30 m thick, acoustically finely layered glacimarine Late Quaternary and Holocene deposit, of soft, silty clay (Forsberg et al., 2007). Strings of small pockmarks were already identified on side scan sonar records in the middle of the Norwegian Channel during the first major seafloor mapping conducted there, in the late 1970s (Van Weering et al., 1973; Hovland, 1981, 1982; Hovland and Judd, 1988; Judd and Hovland, 2007). However, from the high-resolution mapping with ROV-mounted multi-beam echo-sounders and side scan sonars conducted over the last 5 years, the ‘habitat’ of unitpockmarks in the near-Troll area has been documented. The most remarkable occurrences are those associated with families or clusters of normal-pockmarks, where one large pockmark occurs in association with several ‘parasite’ or satellite-pockmarks (Forsberg et al., 2007; Webb et al., 2009). These systems also have an abundance of unit-pockmarks that are associated, not only as clusters of dense unit-pockmarks, but also as strings. In addition, there are a few solitary unit-pockmarks scattered about the seafloor. In 2005 and early 2006, four normal-pockmark clusters were targeted for visual observation and detailed bathymetric mapping. The four clusters each consisted of one large, central parent-pockmark surrounded by up to eight smaller and shallower ‘satellite’ normalpockmarks. On the smooth, even seafloor surface, between normal-pockmarks and clusters of normal-pockmarks, there is often a background scatter of solitary unit-pockmarks (Fig. 2). Crossing the smooth, flat seafloor, there are linear trawl-scars, imprinted in the seafloor by the trawl-doors of deep-sea trawling vessels. In Fig. 2, several generations of trawl-scars criss-cross the seafloor. Numerous unit-pockmarks have formed along these trawl-scars, clearly indicating that trawling somehow affects the hydraulic properties of the seafloor. At one particular location in Fig. 2 (white arrow), there is a cluster of five or six unit-pockmarks. This raises
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the question as to whether or not this cluster represents the incipient development of a new normal-pockmark. One of the four clustered normal-pockmarks mapped in 2005 is shown in Fig. 3. The parent-pockmark is about 50 m wide, 70 m long and 8 m deep. It has a central pinnacle, known to be a methane-derived authigenic carbonate (MDAC) mound (Forsberg et al., 2007). There are about eight satellite-pockmarks surrounding it and hundreds of unit-pockmarks (Fig. 3). Some of these unitpockmarks form distinct strings, seemingly radiating out from the centre of the large, parent-pockmark. It is inferred that the eight satellite normal-pockmarks surrounding the parent-pockmark occur as a consequence of selfsealing of the parent, by the formation of MDAC across its bottom. ROV-mounted sub-bottom profiling (SBP) was also performed across this parent-pockmark (Fig. 4). A vertical zone of disturbance and anomalous reflections beneath the centre of the pockmark can be seen (Fig. 4). This zone is probably caused partly by the occurrence of MDAC and also a presence of small amounts of free gas, suspected to represent a cylindrical ‘chimney’ below the pockmark. A further discussion and interpretation of this pockmark follows later. 3.2. Unit-pockmarks at the Haltenpipe Reef Cluster (HRC) The Haltenpipe Reef Cluster (HRC) contains nine large, up to 25 m high, deep-water coral reefs (Hovland et al., 1998; Hovland, 2008). A couple of kilometres to the east of these structures, Statoil laid a pipeline, the Haltenpipe, between shore and the Heidrun hydrocarbon field, located about 200 km offshore Mid-Norway (Hovland, 2008). The water depth at HRC ranges from 280 (at the reef tops) to 310 m. The surface sediments consist of relatively stiff boulder clay, with a top layer (about 0.5 m thick) of soft, silty, Holocene, clay. Underlying the boulder clay, which is no more than 10 m thick, there are sedimentary rocks of Paleocene age (Hovland, 2008). Immediately to the SE of HRC, there is a wide depression, without underlying boulder clay (Fig. 5). Here, low competent (mechanically weak) Cretaceous sedimentary rocks sub-crop, beneath a surface layer of about 10 m thickness, consisting of Quaternary glacimarine, acoustically well-layered sediments with numerous normal and unit-pockmarks (Fig. 5). Both Paleocene and Cretaceous formations
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Table 1 Adsorbed light hydrocarbon concentrations in clay samples related to a normalpockmark, near the Haltenpipe Reef Cluster. Values are given in ml gas per l sediment. The highest values, found adjacent to the centre of the pockmark are highlighted. ID
Methane
Ethane
Propane
n-Butane
Sum
19 20 21 22
417.74 692.16 329.67 70.29
66.76 108.67 53.27 11.22
36.36 58.55 29.04 6.43
16.44 27.64 13.97 3.16
537.30 887.02 425.95 91.10
have beds (strata) dipping westward at an angle of about 10 . These strata clearly bear reflection seismic evidence of gas-charge (Hovland et al., 1998). This is further documented by the numerous normal-pockmarks forming in the soft clays overlying the Cretaceous sub-crop basin. These pockmarks occur to the southeeast, within a distance of less than 500 m of the HRC (Fig. 5). The presence of upward migrating thermogenic light hydrocarbons, methaneepentane, in the Quaternary sediments has been documented previously (Hovland et al., 1998; Hovland, 2008), and, these geochemical data are also included herein (Tables 1 and 2). Unit-pockmarks form spectacular strings along curvilinear slight depressions in the seafloor across the pockmarked basin east of HRC (Fig. 5). Out of a total of 14 unit-pockmark strings in Fig. 5, there are four or five which are associated with two normalpockmarks. Side scan sonar, sub-bottom profiler, and multi-beam echo-sounder data acquired in 2000 show that there are numerous normal-pockmarks and unit-pockmarks, not only inside the Cretaceous general depression, but also on portions of the seabed where the Paleocene dipping layers sub-crop and where the reefs grow (Fig. 6). Unit-pockmarks occur in highest density close to the base of the three eastern-most coral reefs in the HRC (Reefs A, B, and C, in Fig. 6), strongly suggesting a link between these reefs and seabed fluid flow (Hovland and Risk, 2003). 3.2.1. Geochemical indications A sediment geochemical investigation over the Paleocene and Cretaceous sub-cropping rocks was conducted in 1991 and 1993. Some of the samples targeted the HRC and also one of the normalpockmarks located about 500 m east of the HRC (Hovland, 2008). These results prove that the upper sediments contain light thermogenic hydrocarbons. This geochemical investigation, therefore, confirms that both the large pockmark and at least one of the reefs are associated with seepage of light hydrocarbons (Hovland and Risk, 2003; Hovland, 2008). The geochemical results at HRC were first reported in Hovland et al. (1998), and are re-iterated here. Two locations were selected for sampling with gravity corer, (i) a normal-pockmark location, situated about 1 km east of the HRC (where four samples were acquired, Table 1), and (ii) the cluster of large coral reefs, the HRC (where nine samples were acquired, Table 2). Table 2 Adsorbed light hydrocarbon concentrations in clay samples related to the Haltenpipe Reef Cluster. Values are given in ml gas per l sediment. The highest concentrations are highlighted (see Fig. 6 for sample locations).
Fig. 5. A high-resolution DTM-strip showing the “Haltenpipe 20-inch pipeline” (HP) on the seafloor to the right and some intriguing unit-pockmark patterns to the left. Two normal-pockmarks of about 30 m diameter and 4 m depth, are also seen (vertical exaggeration is 5). Illumination is from the NW. The strings of unit-pockmarks are thought to have formed along vertical weakness zones in the layered, soft, glacimarine sediments, which here have a thickness of about 10 m above the boulder clay (Hovland, 2008).
ID
Methane
Ethane
Propane
n-Butane
Sum
G1 G2 G3 G4 G5 G6 G7 G8 G9
233.32 333.05 64.87 88.79 167.25 147.00 172.93 76.76 77.32
43.93 63.67 12.49 16.24 32.10 27.92 33.47 12.26 14.45
29.69 38.04 4.83 8.98 17.48 12.78 21.29 7.70 7.95
15.92 18.53 2.00 4.74 8.22 5.54 11.87 4.68 4.34
322.86 453.29 84.19 118.75 225.05 193.24 239.56 101.40 104.06
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Fig. 6. Solitary and strings of unit-pockmarks occur at the base of the HRC deep-water coral reefs. The eight coral reefs in the cluster are named Aeh (the three major reefs, have been given capital lettering). They range in height from A, 24 m high, to g, 1 m high (Hovland and Risk, 2003; Hovland, 2008). The black arrows point at unit-pockmarks. The black dots G1eG5, G7, and G9 are locations of sediment samples from which there are geochemical results (Table 2). The highest light hydrocarbon values were found where unit-pockmarks have a high density (i.e., at G1 and G2). Also locations at the base of coral reef C had relatively high values (see text). Illumination from lower left (S) (modified from Hovland and Risk, 2003).
3.2.1.1. The pockmark location. The pockmark results are especially interesting, as they show a clear trend and document that the highest concentrations are not necessarily found at the centre of the normalpockmark, but rather, to the side of the centre, a result supporting the notion of self-sealing seeps. The results of adsorbed light hydrocarbon gases (ml gas per l sediment) from the pockmark samples are shown in Table 1. Whereas the sum of concentrations (methane, ethane, propane, and n-butane) of the background sediments, as found outside the normal-pockmark is 91.10 ml/l, the highest concentration (887.02 ml/l) occurs adjacent to the centre of the pockmark, and is over eight times the background value. At the time of investigation, we did not target unit-pockmarks during such sampling.
Fig. 7. A shaded relief DTM image from the Morvin area. The coral reef ‘MRR’ (large red area) is seen to be located inside a 10 m deep and 130 m wide pockmark crater. This crater has no distinct unit-pockmarks, probably because the seafloor has been colonized by invertebrates (sponges, corals, etc.). In contrast, unit-pockmarks (arrowed) can be seen inside the depression to the north. These unit-pockmarks are located up-stream of the prevailing current and the adjacent live corals. “8” Marks a sampled unit-pockmark (see Table 3 and Fig. 9). Illumination is from NNW. Note that the northesouth and eastewest linear features are artefacts from the multi-beam echo-sounder mapping.
unit-pockmarks, and coral reefs were found. The seafloor at Morvin consists of boulder clay with a layer of silty, soft clay in the upper 0.5 m. One of the larger coral reefs, the Morvin Reference Reef MRR (Fig. 7), is about 80 m long, 25 m wide, and spans the elevation interval: 360e370 m (Hovland, 2008). It is located inside a normalpockmark (130 m 80 m 10 m). The reef occupies about one third of the pockmark, and is growing from the maximum depth (at 370 m), up along the northern side, to the rim of the depression (at 360 m water depth). The prevailing current in the Morvin area is from the SSW. There exists an abundance of solitary and clustered unit-pockmarks, sometimes apparently associated with the coral reefs and with normal-pockmarks at Morvin (Figs. 7 and 8). Unit-pockmarks at Morvin have an apparent highest abundance inside depressions, some of which are normal-pockmarks and some are partly infilled relict iceberg plough-marks.
3.2.1.2. The HRC-location. A total of nine gravity cores were acquired from the base of the large coral reefs at HRC (Fig. 6 and Table 2). Two of the samples (G1 and G2) were acquired at the base of Reef A (Hovland et al., 1998), where unit-pockmarks actually occur in clusters. Four of the samples (G5eG8) were acquired across the large coral reef Reef C (Fig. 6; Hovland et al., 1998). Whereas the regional background sum of concentrations of adsorbed gases (methane, ethane, propane, n-butane) is taken to be 100 ml/l (a conservative number, as the lowest acquired values are generally less than this), the unit-pockmark samples (G1 and G2) had two to three times this background value. Also the samples acquired across Reef C provided a sum of concentrations that is well above the background value, with up to 1.7 times. This fact alone, supports the ‘hydraulic theory’ for deepwater coral reefs, suggesting that seepage of hydrocarbons benefits and ‘fertilizes’ the reefs (Hovland and Risk, 2003; Hovland, 2008). 3.3. Unit-pockmarks and coral reefs at the Morvin hydrocarbon field During mapping and inspection with ROV for potential pipeline routes at Morvin hydrocarbon field, numerous normal-pockmarks,
Fig. 8. A high-resolution shaded relief DTM from the Morvin field (Hovland, 2008). It shows up to nine unit-pockmarks (three of which are marked with white arrows) and associated coral reefs. Live coral reefs appear as rough topography, whereas dead corals appear as smoother seafloor (generally located to the NNW of the live corals). The unitpockmarks are generally located up-stream relative to the prevailing current and the adjacent live corals. Illumination is from NNW. Eastewest linear features are artefacts from the multi-beam echo-sounder mapping.
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Table 4 Adsorbed light hydrocarbon concentrations in clay samples related to complex pockmarks at Nyegga. The highlighted sediment sample, G11-2 was acquired inside a local depression (unit-pockmark?) inside complex pockmark G11, where there is a bacterial mat on the seafloor (see Hovland, 2008).
Fig. 9. Geochemical sediment sampling inside a unit-pockmark (station 8) at Morvin reference reef (MRR), see “8” in Fig. 7 and “S8” in Table 3. Notice the slight depression it represents e although distinct on the shaded DTMs, it is visually non-distinct upon close approach. Sampling cylinder is 80 mm wide.
3.3.1. Geochemical indications During investigations of coral reef MRR, in 2008, a total of nine sediment core samples were acquired for geochemical analysis, including one core (S8) from the centre of a unit-pockmark (Figs. 7,8, Table 3). The total hydrocarbon concentrations in core S6 acquired on the seafloor outside the pockmark, is lowest and represents the expected regional background concentration (of about 100 ml/l, as for HRC), and is used as the relative reference value. The other cores acquired inside normal- and unit-pockmarks at and near MRR have between two and five times this concentration and are interpreted to represent micro-seep locations (Judd and Hovland, 2007; Hovland, 2008, Etiope et al., 2009). The highest interstitial hydrocarbon concentrations (not provided here) were found inside the unit-pockmark (S8, see Hovland, 2008).
ID
Methane
Ethane
Propane
n-Butane
Sum
C-3 G8-5 G11-1 G11-2
262.70 153.53 202.74 275.54
50.61 14.37 37.68 55.13
28.50 6.44 24.60 36.17
12.75 2.69 12.54 18.20
354.56 177.03 277.56 385.04
associated with a bottom simulating reflector (BSR), and several manifestations of gas migration at depth, including vertical conduits (pipes) seen on 2D- and 3D-reflection seismic data and also imaged in 3D by ocean bottom seismometers (Hustoft et al., 2007; Ivanov et al., 2007; Westbrook et al., 2008; Plaza-Faverola et al., 2010). There are also some distinct organic-rich sediment mounds, ‘hydrate pingoes’, up to 1 m high and 4 m wide (Hovland and Svensen, 2006; Hovland, 2008). Work by Ivanov et al. (2007), proved that the pockmarks contain nodules or layers of gas hydrate occurring 1e1.5 m below surface. This discovery has provided confirmation of their status as active methane seeps or vents. Unitpockmarks were mainly found as solitary features on the seafloor outside the complex pockmarks, as can be seen in Fig. 11. There is also an occurrence of clustered unit-pockmarks about 150 m to the west of complex pockmark G12 (Fig. 11). 3.4.1. Geochemical indications In 2004, when geochemical cores were acquired at Nyegga, they were taken independently of known unit-pockmarks, as they were not then regarded as being important. The four locations reported in Table 4 have all been acquired from within complex pockmarks, of which G11 is the most studied (Hovland and Svensen, 2006; Ivanov et al., 2007; Westbrook et al., 2008).
3.4. Unit-pockmarks at Nyegga On the continental slope at Nyegga (the north-eastern boundary of the Storegga slide scarp), off Mid-Norway, there are some large, complex pockmarks with carbonate ridges inside them, Fig. 10 (Hovland et al., 2005). The water depth ranges between 600 and 800 m and the upper sediments consist of soft, sandy and silty clay. The complex pockmarks and other fluid flow features are Table 3 Geochemical results from sediment sampling at Morvin Reference Reef (MRR). The three highlighted samples are the ones with the highest concentrations (ml/l). S1 is located near the centre of the pockmark within which MRR is located. S4 is along the side of the same pockmark. S8 was acquired within a unit-pockmark, adjacent to the coral reef MRR and at the base of a neighbouring coral reef (see Fig. 11 for locations and Fig. 12 for sampling of unit-pockmark at S8). ID
Methane
Ethane
Propane
n-Butane
Sum
S1 S2 S4 S5 S6 S7 S8 S9
472.73 148.67 371.07 151.45 103.11 169.50 249.94 173.13
79.84 23.86 61.80 24.33 16.88 29.42 36.22 28.54
43.81 13.36 36.08 13.63 9.54 16.59 19.80 15.85
18.73 6.12 16.55 6.18 4.43 7.37 8.81 6.85
615.11 182.01 485.50 195.59 133.96 222.88 314.77 224.37
Fig. 10. The Nyegga detailed survey area, which contains a series of large, complex pockmarks, of which G11 and G12 are shown (Hovland et al., 2005). In addition, there are numerous scattered and partly clustered unit-pockmarks, not seen at this scale. The upper rectangle delineates Fig. 11. Illumination from NW (modified from Hovland, 2008).
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2pR2 V [ FpR2 vchimney ; so V [ Fvchimney =2; where R is the radius of the cylindrical gas chimney, F is the porosity of the sediments, vchimney is the upward velocity of the chimney, and V is the water flux on the hemispherical surface in m3 of water per m2 area per second (Cathles et al., 2010). In Fig. 12, we have attempted to utilize the model of Cathles et al. (2010) to interpret the sub-bottom profiler (SBP) records at a parent-pockmark in the Norwegian Channel. Here, we infer that remaining gas trapped below the pockmark by sealing MDAC pumps up the pore-water pressure and forces pore-water out along the rims and sides of the pockmarks. We suggest this may explain the observed distribution of unit-pockmarks. We will now address each of the three questions listed above. 4.1. Timescale of pockmark development
Fig. 11. At complex pockmark G12, there are at least 10 unit-pockmarks, five of which are indicated here by white arrows. Illumination from NW (see Fig. 10 for location).
At Nyegga, the regional background sum of adsorbed gases is expected to be approximately 100 ml/l. All values reported here are up to 3.8 times greater than this value, indicating that complex pockmarks at Nyegga represent locations of active seepage. The highest value (sample G11-2, Table 4) is from a core taken at a bacterial mat, situated inside a slight seafloor depression, near the centre of complex pockmark G11 (Hovland, 2008).
In general, pockmarks appear to form very rapidly (see examples in Hovland and Judd, 1988; Judd and Hovland, 2007, and Cathles et al., 2010). The quantitative model proposed by Cathles et al. (2010), whereby rising gas saturates a chimney which rises through a capillary seal by driving water through the seafloor, is a model supported by the observations of unit-pockmarks provided herein and by the long pipe-structures below complex pockmarks and other surface fluid flow features at Nyegga (Hovland et al., 2005; Hovland and Svensen, 2006; Westbrook et al., 2008; PlazaFaverola et al., 2010). According to this model, the vertical rate of growth of the chimney accelerates as it rises closer toward the
4. Discussion From our compilation of unit-pockmark occurrences off Norway, it is evident that these features may provide important information on the most recent and on-going seepage activity within a region of the seafloor. The most interesting and perhaps important questions regarding pockmarks, in general, are (1) the timescale of development, (2) their likely fluid pathways, and (3) how parent-pockmarks are kept clear of new sedimentation. A couple of recent publications addressed aspects of these questions, including the formation of unit-pockmarks and improve our understanding of pockmarks. One of these (Bui et al., 2007) used the distinct element method (DEM) to calculate the distribution of near-surface stress fields induced in sediments during fluid flux. That study showed that the flux would move in highly focused conduits and that the overlying sediments would be strongly deformed. The other study (Cathles et al., 2010), addressed the formation of pockmarks directly, and amongst other interesting results claimed that a sub-surface gas chimney can start to deform the sediments overlying it when its height equals half the depth of its base below the seafloor. This means that the buoyancy force of the rising gas column more-or-less pushes water out of the sediments above it, and causes local doming of the seafloor, fluidization (‘quickening’) of surface sediments, and unit-pockmark formation by fluid (mainly water) escape (Cathles et al., 2010). They calculated that the gas column or pipe behaves like a piston moving vertically and is resisted by the pore-water being displaced radially from the top of the advancing piston. The flux of water from a hemisphere of the same radius as the chimney is calculated from the equation of mass balance:
Fig. 12. Interpretation of the most probable fluid flow conduits (black arrows) beneath the parent-pockmark shown in Fig. 3. The densest occurrence of unit-pockmarks is indicated with ‘Upm’ above the seafloor. Based on the model by Cathles et al. (2010), it is inferred that slightly over-pressured pore-waters (blue) causes the formation of unit-pockmarks and trapped free gas (red) provides the over-pressure (by tidal pumping), trapped below MDAC (for location see Fig. 3).
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seafloor (even assuming the sediment permeability is vertically constant). The observation of unit-pockmarks forming along modern day trawl-scars (Fig. 2) clearly documents their recent formation history. Therefore, we conclude that unit-pockmarks may form in a matter of days, weeks or possibly months. In the early stages, pore-waters will utilize all permeable pathways and minor fractures and eventually break surface and form unit-pockmarks. This is in full agreement with observations of unitpockmarks forming even along trawl-gear marks which must have disturbed the permeability integrity of the surface seafloor layer. When the rising gas chimney moves closer to the surface, there will be a vigorous outflow, according to Cathles et al. (2010). The intense clustering and amalgamation of unit-pockmarks is expected to occur immediately before the formation of a normal-pockmark with the same diameter as the chimney (Cathles et al., 2010). Thus, in conclusion, we may say that a rising gas chimney will, first, slightly deform the seafloor (doming). Unit-pockmarks will form, as the weakest zones in the seafloor transmit rising and focused overpressured pore-water. Finally, as the gas chimney comes close to the seafloor, the gas, sediment, and remaining pore-water catastrophically erupts (drains) into the water column. However, although this model probably works well for the first generation of pockmarks, after a high-sediment influx episode, for example, after a glaciation period (e.g., The Norwegian Channel, cf. Tjelta et al., 2007), it may not work for all unit-pockmark occurrences, as in well-established and ‘old’ normal-pockmark regions. Thus, it may no longer be expected that all unit-pockmarks will eventually lead to the formation of larger structures (normal-pockmarks).
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Fig. 13. A small MDAC-nodule retrieved from a Tommeliten ‘eyed-pockmark’ (Hovland and Judd, 1988). It clearly demonstrates the turtuousity in the sub-bottom plumbing system, as each conduit clogs up, a new one is developed beside it, thus forming a complex underground ‘root’ system. Note the small circular hole (bubble-hole) in the surface of the nodule, immediately above one of the cemented gas feeder conduits. The hole is suspected kept open by active ebullition, which has prevented cementation (modified from Hovland, 2002).
4.2. Fluid pathways below pockmarks According to both the capillary seal model by Cathles et al. (2010), and the DEM of Bui et al. (2007), there is very little reason to expect an extensive lateral root system below ‘first-generation’ normal-pockmarks. Both models predict that the roots are vertical rather than spread out laterally. This is especially true for uniform vertical permeabilites. However, if there are general dips, domes, or other lateral inhomogeneities in permeability distribution below the pockmark, the fluid pathways will be modified accordingly. It will always be the path of lowest resistance that is used by migrating pore-waters and gas. This means that diagenetic processes, and especially the subsurface formation of MDAC will cause an immediate impact upon the distribution of fluid pathways. If fluid flow conduits are blocked and sealed by mineral precipitation, then according to the principle of least resistance, these pathways will close down and new ones will form laterally. Methane-derived carbonate-cemented sediments were first found in North Sea pockmarks in 1983. This discovery ended the on-going discussion about pockmark formation (biota, dead-ice vs gas and fluids), and more-or-less proved that long-time seepage is responsible for their formation (Hovland et al., 1985). It also became clear that the very same process of cementation causes the sealing-up or clogging of the plumbing system e i.e., seeps tend to seal themselves (Hovland, 2002). An example of a sealed-up and contorted fluid flow pathway, is clearly demonstrated in a sub-surface sample of MDAC, acquired from a cemented (‘eyed’) pockmark at Tommeliten (Fig. 13, Hovland, 2002, 2008). From studies of fossilized fluid flow systems there is also ample evidence of branching fluid pathways below long-lived fluid flow features. Fossilized seep studies have been conducted at several places (Clari et al., 1988; Campbell et al., 2002; Aiello, 2005). It is, perhaps, the observations by De Boever et al. (2009) that best indicate the clustering of seepage conduits in long-lived structures. They studied over 800 large and small tubular cemented vertical structures that formed in sandy marine sediments. Thus, their
results conform to the predictions of the new model of Cathles et al. (2010) that the cylindrical shape was the most common tube type, as it primarily reflected the buoyancy-driven, vertical path of an ascending gas-bearing fluid through permeable, mainly unconsolidated sandy host sediments. They also documented that tube morphology was influenced by local stratigraphic anisotropies such as locally coarsening sediments (De Boever et al., 2009). With respect to the fluid flow pathways immediately below unit-pockmarks, it is suspected that these are mainly vertical and un-contorted, as they are inferred to be the result of pore-water seepage, rather than gas flow eruptions. According to our analysis of SBP data from the Troll area, they occur along the side-walls of old pockmarks and on the seafloor immediately beside these (Figs. 3 and 12). Whereas the gas bubbles are suspected to be trapped below the centre of the normal-pockmark, where its bottom sediments and sub-seafloor sediments are partly cemented, most of the water in the system is pressured by this gas reservoir and pushed out at the sides of the system. Thus, the unit-pockmarks are suspected to manifest the pore-water seepage and the flat centre of the parent-pockmark to manifest the cap of the gas reservoir. The fact that the highest densities of unit-pockmarks in the Norwegian Channel occurs along the flanks and rims of the pockmarks also agrees well with findings off the Eastern seaboard of USA, where some giant-pockmarks were mapped and studied in detail by Newman et al. (2007). The researchers used ‘sniffers’, methane detectors which mapped the concentrations of methane in the water column. They found that high methane concentrations come out on the flanks and rims of the craters rather than from the bottom (centre) of the giant-pockmarks. 4.3. How parent-pockmarks are kept clear of new sedimentation The inherent small size of unit-pockmarks suggests that they may easily be obliterated, by sediment infilling, unless they are,
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or recently have been active. It is well known from visual inspections that small-sized (shallow) depressions on the seafloor, such as trenches and drag-marks after anchors and trawl-boards, are rapidly covered by mobile (drifting) sediments. Even though the net sedimentation rate in Norwegian and UKwaters, is very low, we know that sediment transport along the seafloor may be significant and at times high. During construction work (mechanical disturbance of the seafloor) we have observed visually (with ROV) turbid clouds drifting along the seafloor. Thus we infer that unit-pockmarks probably only occur for periods of up to some decades, whereas normal-pockmarks are inferred to have life-spans of perhaps thousands of years before they are obliterated by recent sediments (Forsberg et al., 2007; Judd and Hovland, 2007). There is, however, a problem in understanding how normalpockmarks and parent-pockmarks, which are often flat-bottomed (and slightly larger than normal-pockmarks), prevent re-sedimentation. From the theory of Cathles et al. (2010), it is likely that free gas exists directly beneath such pockmarks, in the latter case by a sealing layer of MDAC. This is also suggested in our own interpretation of the parent-pockmark near the Troll field, shown in Figs. 3, 4, and 12. The hydraulic properties of the seafloor are complex and intriguing. Because gas bubbles trapped below the seafloor are constantly affected by cyclic loading, such as tidal waves and passing cyclonic/anticyclonic (low/high pressure) weather systems, a buried or trapped shallow gas pocket will act as an effective pump on the nearby pore-waters (Hovland and Judd, 1988). Whereas the gas will remain for long periods below ground, hindered by capillary force threshold-values, pore-water is free to move in the surrounding capillary pore-structure (Cathles et al., 2010). The pore-water will likely utilize all possible cracks and holes available in the MDAC seal, so that it passes out of the seafloor every time the pressure is reduced. On images showing the seafloor micro-structure, from close-up photographs taken at the base of corals and MDAC rocks within the parent-pockmark shown in Fig. 3 (see also Fig. 4.52 in Hovland, 2008), sediment deposition can be seen. However, on the broad scale, the sediment has been re-suspended and transported away from the bottom of the parent-pockmark. We do not believe the background current velocity at the bottom of the Norwegian Trough to be strong enough for sediment scouring (current-induced erosion). Therefore, we suggest that the remaining fine-grained sediments seen on close-up photographs mentioned above, represent the remains of fine-grained sediments that have not been removed by seepage processes because they have somehow settled in locations without upward pore-water flux. Based on the surface and near-surface observations provided herein, we, thus, infer the following: - Wherever unit-pockmarks are found on the surface, it is likely there is a sub-surface (trapped) reservoir of free gas, nearby. - This reservoired gas acts as a dynamic pump, which causes periodic (rhythmic) movement of pore-water through the sediments. Hovland and Curzi (1989) and Hovland et al. (1999) have examined the effect of such a ‘pump’ on the local porewater system (over-pressuring) and found acoustic evidence of over-pressured pore-waters. - It is likely that wherever in-situ pore-water channels or conduits branch vertically up to the seafloor, a unit-pockmark forms (provided there are fine-grained sediments amenable to pockmark formation). - Furthermore, wherever a crust of cement (MDAC) has formed at long-duration seepage locations such as inside normalpockmarks, a thicker layer of gas-charged sediments may build up beneath the cemented ‘seal’ (Hovland, 2002).
Therefore, we conclude that the hydraulic action of water moving in and out of such crusts (driven by trapped free gas) can cause fine-grained sediment particles to re-suspend, and thus prevent re-burial. Even though there may be very low water current velocities inside pockmarks, the sediment grains (silt and clay) are so small that they will easily re-suspend in the water column if there is a slight upward directed pore-water movement. Such re-suspension means that the grains will be carried away, out of the pockmark. The net sedimentation inside ‘actively pumping’ pockmarks is therefore likely to be relatively low. Using this hydraulic model, it is also possible to understand how the carbonate rubble and large carbonate ridges were formed inside the Nyegga complex pockmarks (Hovland et al., 2005; Hovland and Svensen, 2006). From the recent work by Westbrook et al. (2008) and Plaza-Faverola et al. (2010), we know that these pockmarks represent surface manifestations of deep-rooted chimneys (pipes) which tap into gas hydrate and free gas reservoirs. According to the concept of Cathles et al. (2010), it is likely that the pipes erupt catastrophically from time to time. Consequently, the broader picture of the seafloor acting on hydraulic principles of physics, may therefore be summed up as follows: - Generally, we can divide the seafloor into two main types e ‘hydraulically active’ (i.e., with pockmarks and fluid flow features) and ‘hydraulically passive’ (i.e., without any surface manifestations). - Wherever normal-pockmarks and complex pockmarks occur, there is likely to be an excess of migrating gas, which will, periodically, or intermittently vent through the seafloor (causing the normal-pockmark development, according to Cathles et al., 2010). - Wherever only unit-pockmarks occur on the seafloor, the gas supply from below, is likely low, and remains sub-surface. Thus, it will act as a buried hydraulic pump, causing only cyclic porewater movement without gas eruptions. An important corollary to this hydraulic theory is that it is also valid for all volumes of soil, which have a porosity system partly filled by liquid and partly filled by gas. Thus, it is valid for all ocean depths, lakes and swamps, as the driving gas-type (methane, carbon-dioxide, hydrogen-sulphide, or hydrogen) is immaterial. 5. Conclusions Since their first discovery and description, pockmarks have been found in all of the world's oceans and some of the lakes, mapped with sonars and swath echo-sounders. During the 1980s it was noticed that small, mono-sized pockmarks were also common on the seafloor, they were termed unit-pockmarks. Based on this analysis of the occurrence of unit-pockmarks and normal-pockmarks at four selected locations on the Norwegian continental shelf and slope, combined with new theoretical models, we find that unit-pockmarks represent well defined manifestations of active fluid flow. Our main conclusions are as follows: - The local seafloor is either characterized as ‘hydraulically active’ or ‘hydraulically passive’, dependent on the occurrence of pockmarks and other fluid flow features (active) or the absence of such features (passive). - The type of surface fluid flow manifestations determines the type and vigour of activity, i.e., cyclic/periodic, high, or low hydraulic activity. - Whereas unit-pockmarks most likely represent cyclic porewater seepage, normal-pockmarks represent periodic or
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intermittent gas bursts (eruptions) with intervening periods of slow, diffusive, and cyclic pore-water seepage. - The driving force behind seafloor hydraulic activity is reservoired, buried gas pockets. In practical terms, this means that when sampling for seeping fluids, we recommend ‘seep-hunters’ to target the unit-pockmarks. The higher investment needed to ensure detection and mapping of the small unit-pockmarks, may be balanced by a higher success rate in sampling dissolved gases in the seeping pore-waters resulting from the active pumping by the trapped underground gas. Acknowledgements The marine crew and the ROV-operators on board the multipurpose vessels ‘Acergy Viking’, ‘Edda Fonn’, and ‘Edda Freya’ are thanked for their professional work during the mapping, sampling, and inspections at HRC, Morvin, Nyegga and Troll. Statoil are thanked for releasing the information contained herein. The reviewers Graham Westbrook and Peter Croker are thanked for their fruitful comments. References Aiello, I.W., 2005. Fossil seep structures of Monterey Bay region and tectonic/ structural controls on fluid flow in an active transform margin. Palaeogeography, Palaeoclimatology, Palaeoecology 227, 124e142. Bui, H., Dvorkin, J., Nur, A., 2007. Subsurface fluid flow and its implications for seabed pockmarks and mud volcanoes: an approach of distinct element method (DEM). In: Book of Extended Abstract, Societey of Exploration Geophysiscists, Annual Meeting. San Antonio, USA, pp. 2055e2059. Campbell, K.A., Farmer, J.D., Des Marais, D., 2002. Ancient hydrocarbon seeps from the Mesozoic convergent margin of California: carbonate geochemistry, fluids and palaeoenvironments. Geofluids 2 (2), 63e94. Cartwright, J., Huuse, M., Aplin, A., 2007. Seal bypass system. AAPG Bulletin 91 (8), 1141e1166. Cathles, L.M., Su, Z., Chen, D., 2010. The physics of gas chimney and pockmark formation, with implications for assessment of seafloor hazards and gas sequestration. Marine and Petroleum Geology 27, 82e91. Clari, P.A., Gagliardi, C., Governa, M.E., Ricci, B., Zuppi, G.M.,1988. I calcari di Marmorito: una testimonianza di processi diagenetici in presenza di metano. Bollettino del Museo Regionale di Scienze Naturali Torino 5, 197e216 (in Italian). De Boever, E., Huysmans, M., Muchez, P., Dimitrov, L., Swennen, R., 2009. Controlling factors on the morphology and spatial distribution of methane-related tubular concretions e case study of an Early Eocene seep system. Marine and Petroleum Geology 26, 1580e1591. De Vries, M.H., Svanø, G., Tjelta, T.I., 2007a. Small scale model testing of gas migration in a soft seabed as a basis for developing a mechanical model for gas migration. In: Proceedings of the Sixth International Offshore Site Investigation and Geotechnics Conference. SUT-OSIG, London, UK, pp. 231e236. De Vries, M.H., Svanø, G., Tjelta, T.I., Emdal, A.J., 2007b. Pockmarks: created by reduced sedimentation or a sudden blow-out?. In: Proceedings of the 17th International Offshore and Polar Engineering Conference. ISOPE, Lisbon, Portugal, pp. 1361e1365. Dimitrov, L., 2002. Mud volcanoes e the most important pathway for degassing deeply buried sediments. Earth-Science Reviews 59, 49e76. Etiope, G., Feyzullayev, A., Baciu, C.L., 2009. Terrestrial methane seeps and mud volcanoes: a global perspective of gas origin. Marine and Petroleum Geology 26, 333e344. Forsberg, C.F., Planke, S., Tjelta, T.I., Svanø, G., Strout, J.M., Svensen, H., 2007. Formation of pockmarks in the Norwegian Channel. In: Proceedings of the Sixth International Offshore Site Investigation and Geotechnics Conference. SUTOSIG, London, UK, pp. 221e230. Foucher, J.-P., Westbrook, G.K., Boetius, A., Ceramicola, S., Dupré, S., Mascle, J., Mienert, J., Pfannkuche, O., Pierre, C., Praeg, D., 2009. Structures and drivers of cold seep ecosystems. Oceanography 22 (1), 92e109. Heggland, R., 1998. Gas seepage as an indicator of deeper prospective reservoirs. A study on exploration 3D seismic data. Marine and Petroleum Geology 15, 1e9.
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Do Norwegian deep-water coral reefs rely on seeping fluids? Marine Geology 198, 83e96. Hovland, M., Svensen, H., 2006. Submarine pingoes: indicators of shallow gas hydrates in a pockmark at Nyegga, Norwegian Sea. Marine Geology 228, 15e23. Hovland, M., Judd, A.G., King, L.H., 1984. Characteristic features of pockmarks on the North Sea Floor and Scotian Shelf. Sedimentology 31, 471e480. Hovland, M., Talbot, M., Olaussen, S., Aasberg, L., 1985. Recently formed methanederived carbonates from the North Sea floor. In: Thomas, B.M. (Ed.), Petroleum Geochemistry in Exploration of the Norwegian Shelf. Norwegian Petroleum Society, Graham & Trotman, pp. 263e266. Hovland, M., Hill, A., Stokes, D., 1997. The structure and geomorphology of the Dashgil mud volcano, Azerbaijan. Geomorphology 21, 1e15. Hovland, M., Mortensen, P.B., Brattegard, T., Strass, P., Rokoengen, K., 1998. Ahermatypic coral banks off mid-Norway: evidence for a link with seepage of light hydrocarbons. Palaios 13, 189e200. 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Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo
Unit-pockmarks and their potential significance for predicting fluid flow M. Hovland a, b, *, R. Heggland a, M.H. De Vries c, T.I. Tjelta a a
Statoil ASA, Forusbeen 50, N-4035 Stavanger, Norway Centre for Geobiology, University of Bergen, N-5020 Bergen, Norway c SINTEF, Building and Infrastructure, N-7465 Trondheim, Norway b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 August 2009 Received in revised form 5 February 2010 Accepted 6 February 2010 Available online 12 February 2010
Unit-pockmarks were recognized as more-or-less insignificant features on the seafloor in the early 1980s. However, this investigation, at four different regions in Norwegian waters, suggests they are more significant for seep detection than previously believed. They occur as circular depressions in the seafloor (diameter < 5 m) either as singular features, as strings, or as clusters. One of our main conclusions is that they are widespread and represent the most recent and most active local seep locations. This is based on their areal density distribution, the finding of relatively high hydrocarbon concentrations inside sampled unit-pockmarks and at locations where they are abundant, and on theoretical considerations. When unitpockmarks occur together with ‘normal-sized’ pockmarks, they often form to the side of the normalpockmark centre. Our study also suggests that (1) the driving force behind seafloor hydraulic activity, i.e., the formation of unit-pockmarks, normal-pockmarks, and many other fluid flow features, is pockets of buried free gas, and (2) whereas unit-pockmarks likely manifest cyclic pore-water seepage, their larger related, normal-pockmarks, likely manifest periodic or intermittent gas bursts (eruptions), with extended intervening periods of slow, diffusive, and cyclic pore-water seepage. Our findings suggest that seep detection is most efficiently performed by mapping the seafloor with high-resolution bathymetry (at least 1 m 1 m gridding), and acquiring geochemical samples where the density of unit-pockmarks is locally highest. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Unit-pockmarks Normal-pockmarks Hydraulic activity Fluid flow Capillary sealing Geochemical results Micro-seep Pore-water Nyegga
1. Introduction Seeps, including visible ones from mud volcanoes and invisible micro-seeps from pockmarks, may provide important information on sub-seafloor hydraulic and fluid conditions. Seeps have already provided valuable information for petroleum geology, exploration, tectonic studies, and geo-hazard studies, but will probably become increasingly important for marine environmental and ecological studies in the future (Paull et al., 2002; Niemann et al., 2005; Etiope et al., 2009; Webb et al., 2009; Foucher et al., 2009; Hovland, 2008; Judd and Hovland, 2007; Cathles et al., 2010). Since their discovery in the late 1960s, pockmarks have intrigued marine scientists (King and MacLean, 1970). Their association with seeping gases and pore-water was established by stable isotope results on carbonate-cemented sediments found inside North Sea pockmarks (Hovland et al., 1985). But, even so, a proper understanding of their formation and underground
* Corresponding author. Statoil ASA, Forusbeen 50, N-4035 Stavanger, Norway. Tel.: þ47 95802243; fax: þ47 51990050. E-mail address: [email protected] (M. Hovland). 0264-8172/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2010.02.005
plumbing system has yet to be established (Tjelta et al., 2007; Forsberg et al., 2007; De Vries et al., 2007a, b; Judd and Hovland, 2007; Webb et al., 2009; Cathles et al., 2010). It is inferred that pockmark craters form abruptly, when pockets of local over-pressured pore-water and gas erupt through the seafloor surface sediments, and that they are subsequently maintained by slow (tidally induced?) pore-water and gas seepage (Cathles et al., 2010). This inference is based on a combination of (i) laboratory studies and on modern detailed studies at pockmarks over the Troll field in the Norwegian Channel and elsewhere (Tjelta et al., 2007; De Vries et al., 2007a, b; Judd and Hovland, 2007), (ii) observations made from fossilized seepage locations (Aiello, 2005; Campbell et al., 2002; De Boever et al., 2009), (iii) high-resolution 3D-studies of seep locations (Heggland, 1998; Schroot et al., 2005; Cartwright et al., 2007), (iv) partly based on studies of mud volcanoes (Hovland et al., 1997; Dimitrov, 2002; Foucher et al., 2009; Etiope et al., 2009); and partly based on theoretical modelling (Bui et al., 2007; Cathles et al., 2010). However, it is expected that the exact mechanisms and processes involved in pockmark maintenance, i.e., the continuous local removal of suspension sediments, cannot be established before extended detailed monitoring of pockmarks has been performed. In this paper we examine
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the manifestation of seepage conveyed by the small ‘unit-pockmarks’, often seen, but often treated as mere curiosities and insignificant morphological seabed features. The term ‘Unit-Pockmark’ was coined by Hovland et al. (1984), during a comparison study of pockmark-associated features on both sides of the Atlantic Ocean. The term describes very distinct and more-or-less mono-sized, small pockmarks, at that time only seen on side scan sonar records where ‘normal’ pockmarks were often found to consist of the amalgamation of large numbers of ‘unit-pockmarks” (Hovland et al., 1984). Their definition is simply: “Unit-pockmarks are very small (<5 m) seabed depressions which are found in isolation, in groups, and in association with larger pockmarks” (Hovland and Judd, 1988). Since the advent of multi-beam swath echo-sounders, mounted on remotely operated or autonomous vehicles (ROV and AUVs), these distinctive, small features have been detected in numerous geological settings, most often where normal-pockmarks occur (Judd and Hovland, 2007; Webb et al., 2009). The objective of this paper is to describe the occurrence and configurations of unit-
Fig. 1. Map showing the selected locations for this study. The black asterisks show the locations of Troll (T), Nyegga (N), Morvin (M), and Haltenpipe Reef Cluster (H). The three latter locations are off mid-Norway (framed). Whereas T, M, and H occur on the continental shelf (water depths ranging from 300 to 400 m), area N is located on the continental slope (water depth 750 m). The two red dots on land are Trondheim (T) and Oslo (O).
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pockmarks in four different settings on the Norwegian continental shelf and slope, and to discuss their possible significance in improving the understanding of local seabed hydraulic activity and plumbing systems. The four selected study locations are the Norwegian Channel, near the Troll hydrocarbon field, the Haltenpipe Reef Cluster (HRC), the Morvin hydrocarbon field, and the Nyegga area (Fig. 1). 2. Methods, mapping and geochemistry Unit-pockmarks have been studied mainly by interpreting highresolution bathymetry, sub-bottom and side scan sonar data, gathered with ROVs. This type of seafloor mapping is conducted while the ROV, fitted with single- or dual swath echo-sounders, ‘flies’ between 10 and 15 m above the general seafloor. The digital terrain model (DTM) thus produced has been gridded to a resolution of up to 0.2 m 0.2 m (i.e., Figs. 2 and 3), however, gridding to a resolution of 1 m 1 m and 0.5 m 0.5 m has normally been done. ROV-mounted side scan sonar (100 kHz) has also been used often together with ROV-mounted high-resolution shallow seismic systems (e.g., Fig. 4). In addition to gathering high-resolution geophysical data, the main author has, whenever possible, acquired sediment cores for the geochemical analysis of light hydrocarbons remaining in the clay fraction of the sediments. The cores were either obtained by normal gravity corers, or 0.5 m long coring tubes operated by ROVs. Approximately 200 g clay samples were acquired from between 0.4 and 1 m below the seafloor (normally in the oxic zone). The samples were immediately packed in plastic bags and stored frozen (20 C) for onshore geochemical analysis (at GeolabNor, Trondheim). Both adsorbed and interstitial (occluded) gases were analysed (Hovland and Judd, 1988). Over the years, this method has provided interesting geochemical signals and indications of significant variations in the content of interstitial and adsorbed gases in the upper sediments. These variations are interpreted as proof of seepage, both the invisible micro-seepage and the visible macro-seepage of light hydrocarbons many places on the Norwegian and UK continental
Fig. 2. Perspective view of a shaded relief digital terrain model (DTM) from the Norwegian Trough, near the Troll field. The artificial illumination of the surface is from SE. There are solitary unit-pockmarks scattered over otherwise smooth portions of the seafloor. The density of the unit-pockmarks is clearly higher near some of the normalpockmarks. There is one cluster of five or six unit-pockmarks marked with a white arrow. In addition, there are numerous unit-pockmarks clearly associated with different generations of trawl-scars (some of these unit-pockmarks are marked with black arrows). Notice the cluster of unit-pockmarks inside the shallowest normalpockmark, located about 100 m SSE of the white arrow. According to Cathles et al. (2010), the next normal-pockmark may be formed at the light arrow. The two white patches are caused by missing depth data.
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shelves (Judd and Hovland, 2007). This type of geochemical data, are currently available at three of the studied locations: HRC, Morvin, and Nyegga. At Norwegian Channel (Troll) locations, however, sensitive geochemical data are currently unavailable (Tjelta et al., 2007; Forsberg et al., 2007; De Vries et al., 2007b). 3. Unit-pockmarks off Norway
Fig. 3. Perspective view of a shaded relief digital terrain model (DTM) from one of a series of “pockmark-families” in the Norwegian Trough, near the Troll field. Illumination is from the south. This family of about eight pockmarks and hundreds of unitpockmarks consists of one large parent-pockmark, located in the centre, with seven satellite-pockmarks surrounding it. Further morphological study, however, reveals that the parent-pockmark probably consists of three or four amalgamated satellite-pockmarks. Unit-pockmarks occur in varying density, which clearly increases towards the rims of the parent-pockmark. The black arrow points at a trawl-scar associated unitpockmark. The white arrow points at a possible ‘incipient’ pockmark, where there is a cluster of unit-pockmarks associated with a slight depression. Note that the pinnacle in the centre of the parent-pockmark consists of methane-derived authigenic carbonate (MDAC) which has been colonized by deep-water corals and giant bivalves, indicating on-going pore-water seepage (Hovland, 2008). The two white orthogonal lines indicate sub-bottom profiler lines, one of which is shown in Fig. 4.
Fig. 4. A sub-bottom profiler (SBP) record crossing the parent-pockmark, shown in Fig. 3. An interpreted version of this image is found on Fig. 12. Note the disturbance in the acoustic layering directly below the carbonate pinnacle in the centre of the pockmark. This pockmark is suspected to be sealed by MDAC, suspected to act as an efficient seal for free gas (Hovland, 2002; Cathles et al., 2010).
Over the last two decades, Statoil has conducted detailed seabed mapping in many offshore areas where unit-pockmarks have been detected. Four of these are discussed here (1) on the mid-continental shelf off Western-Norway, e.g., near the giant Troll hydrocarbon field in the Norwegian Channel (Tjelta et al., 2007; Forsberg et al., 2007; Webb et al., 2009); (2) on the mid-continental shelf off MidNorway, e.g., at the Haltenpipe Reef Cluster, HRC (Hovland and Risk, 2003; Hovland, 2008); (3) on the outer-continental shelf off MidNorway, e.g., at the Morvin hydrocarbon field (Hovland, 2005; Hovland, 2008); and (4) on the continental slope off Mid-Norway, e.g., at the Nyegga complex pockmark and hydrate pingo location (Hovland et al., 2005; Hovland and Svensen, 2006; Ivanov et al., 2007; Westbrook et al., 2008; Plaza-Faverola et al., 2010). The following is a description of the features documented at each of these sites and the manner in which the unit-pockmarks occur. Unit-pockmarks may occur as: (1) solitary features (in isolation), (2) curvilinear strings of varying length and density, and (3) clusters of varying density. Also the geochemical data obtained from the same areas are presented here. 3.1. Unit-pockmarks in the Norwegian Channel (Troll) The water depth at the Troll area of the Norwegian Channel (also called the Norwegian Trench), is about 330 m. The upper sediment layer, consists of a 20e30 m thick, acoustically finely layered glacimarine Late Quaternary and Holocene deposit, of soft, silty clay (Forsberg et al., 2007). Strings of small pockmarks were already identified on side scan sonar records in the middle of the Norwegian Channel during the first major seafloor mapping conducted there, in the late 1970s (Van Weering et al., 1973; Hovland, 1981, 1982; Hovland and Judd, 1988; Judd and Hovland, 2007). However, from the high-resolution mapping with ROV-mounted multi-beam echo-sounders and side scan sonars conducted over the last 5 years, the ‘habitat’ of unitpockmarks in the near-Troll area has been documented. The most remarkable occurrences are those associated with families or clusters of normal-pockmarks, where one large pockmark occurs in association with several ‘parasite’ or satellite-pockmarks (Forsberg et al., 2007; Webb et al., 2009). These systems also have an abundance of unit-pockmarks that are associated, not only as clusters of dense unit-pockmarks, but also as strings. In addition, there are a few solitary unit-pockmarks scattered about the seafloor. In 2005 and early 2006, four normal-pockmark clusters were targeted for visual observation and detailed bathymetric mapping. The four clusters each consisted of one large, central parent-pockmark surrounded by up to eight smaller and shallower ‘satellite’ normalpockmarks. On the smooth, even seafloor surface, between normal-pockmarks and clusters of normal-pockmarks, there is often a background scatter of solitary unit-pockmarks (Fig. 2). Crossing the smooth, flat seafloor, there are linear trawl-scars, imprinted in the seafloor by the trawl-doors of deep-sea trawling vessels. In Fig. 2, several generations of trawl-scars criss-cross the seafloor. Numerous unit-pockmarks have formed along these trawl-scars, clearly indicating that trawling somehow affects the hydraulic properties of the seafloor. At one particular location in Fig. 2 (white arrow), there is a cluster of five or six unit-pockmarks. This raises
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the question as to whether or not this cluster represents the incipient development of a new normal-pockmark. One of the four clustered normal-pockmarks mapped in 2005 is shown in Fig. 3. The parent-pockmark is about 50 m wide, 70 m long and 8 m deep. It has a central pinnacle, known to be a methane-derived authigenic carbonate (MDAC) mound (Forsberg et al., 2007). There are about eight satellite-pockmarks surrounding it and hundreds of unit-pockmarks (Fig. 3). Some of these unitpockmarks form distinct strings, seemingly radiating out from the centre of the large, parent-pockmark. It is inferred that the eight satellite normal-pockmarks surrounding the parent-pockmark occur as a consequence of selfsealing of the parent, by the formation of MDAC across its bottom. ROV-mounted sub-bottom profiling (SBP) was also performed across this parent-pockmark (Fig. 4). A vertical zone of disturbance and anomalous reflections beneath the centre of the pockmark can be seen (Fig. 4). This zone is probably caused partly by the occurrence of MDAC and also a presence of small amounts of free gas, suspected to represent a cylindrical ‘chimney’ below the pockmark. A further discussion and interpretation of this pockmark follows later. 3.2. Unit-pockmarks at the Haltenpipe Reef Cluster (HRC) The Haltenpipe Reef Cluster (HRC) contains nine large, up to 25 m high, deep-water coral reefs (Hovland et al., 1998; Hovland, 2008). A couple of kilometres to the east of these structures, Statoil laid a pipeline, the Haltenpipe, between shore and the Heidrun hydrocarbon field, located about 200 km offshore Mid-Norway (Hovland, 2008). The water depth at HRC ranges from 280 (at the reef tops) to 310 m. The surface sediments consist of relatively stiff boulder clay, with a top layer (about 0.5 m thick) of soft, silty, Holocene, clay. Underlying the boulder clay, which is no more than 10 m thick, there are sedimentary rocks of Paleocene age (Hovland, 2008). Immediately to the SE of HRC, there is a wide depression, without underlying boulder clay (Fig. 5). Here, low competent (mechanically weak) Cretaceous sedimentary rocks sub-crop, beneath a surface layer of about 10 m thickness, consisting of Quaternary glacimarine, acoustically well-layered sediments with numerous normal and unit-pockmarks (Fig. 5). Both Paleocene and Cretaceous formations
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Table 1 Adsorbed light hydrocarbon concentrations in clay samples related to a normalpockmark, near the Haltenpipe Reef Cluster. Values are given in ml gas per l sediment. The highest values, found adjacent to the centre of the pockmark are highlighted. ID
Methane
Ethane
Propane
n-Butane
Sum
19 20 21 22
417.74 692.16 329.67 70.29
66.76 108.67 53.27 11.22
36.36 58.55 29.04 6.43
16.44 27.64 13.97 3.16
537.30 887.02 425.95 91.10
have beds (strata) dipping westward at an angle of about 10 . These strata clearly bear reflection seismic evidence of gas-charge (Hovland et al., 1998). This is further documented by the numerous normal-pockmarks forming in the soft clays overlying the Cretaceous sub-crop basin. These pockmarks occur to the southeeast, within a distance of less than 500 m of the HRC (Fig. 5). The presence of upward migrating thermogenic light hydrocarbons, methaneepentane, in the Quaternary sediments has been documented previously (Hovland et al., 1998; Hovland, 2008), and, these geochemical data are also included herein (Tables 1 and 2). Unit-pockmarks form spectacular strings along curvilinear slight depressions in the seafloor across the pockmarked basin east of HRC (Fig. 5). Out of a total of 14 unit-pockmark strings in Fig. 5, there are four or five which are associated with two normalpockmarks. Side scan sonar, sub-bottom profiler, and multi-beam echo-sounder data acquired in 2000 show that there are numerous normal-pockmarks and unit-pockmarks, not only inside the Cretaceous general depression, but also on portions of the seabed where the Paleocene dipping layers sub-crop and where the reefs grow (Fig. 6). Unit-pockmarks occur in highest density close to the base of the three eastern-most coral reefs in the HRC (Reefs A, B, and C, in Fig. 6), strongly suggesting a link between these reefs and seabed fluid flow (Hovland and Risk, 2003). 3.2.1. Geochemical indications A sediment geochemical investigation over the Paleocene and Cretaceous sub-cropping rocks was conducted in 1991 and 1993. Some of the samples targeted the HRC and also one of the normalpockmarks located about 500 m east of the HRC (Hovland, 2008). These results prove that the upper sediments contain light thermogenic hydrocarbons. This geochemical investigation, therefore, confirms that both the large pockmark and at least one of the reefs are associated with seepage of light hydrocarbons (Hovland and Risk, 2003; Hovland, 2008). The geochemical results at HRC were first reported in Hovland et al. (1998), and are re-iterated here. Two locations were selected for sampling with gravity corer, (i) a normal-pockmark location, situated about 1 km east of the HRC (where four samples were acquired, Table 1), and (ii) the cluster of large coral reefs, the HRC (where nine samples were acquired, Table 2). Table 2 Adsorbed light hydrocarbon concentrations in clay samples related to the Haltenpipe Reef Cluster. Values are given in ml gas per l sediment. The highest concentrations are highlighted (see Fig. 6 for sample locations).
Fig. 5. A high-resolution DTM-strip showing the “Haltenpipe 20-inch pipeline” (HP) on the seafloor to the right and some intriguing unit-pockmark patterns to the left. Two normal-pockmarks of about 30 m diameter and 4 m depth, are also seen (vertical exaggeration is 5). Illumination is from the NW. The strings of unit-pockmarks are thought to have formed along vertical weakness zones in the layered, soft, glacimarine sediments, which here have a thickness of about 10 m above the boulder clay (Hovland, 2008).
ID
Methane
Ethane
Propane
n-Butane
Sum
G1 G2 G3 G4 G5 G6 G7 G8 G9
233.32 333.05 64.87 88.79 167.25 147.00 172.93 76.76 77.32
43.93 63.67 12.49 16.24 32.10 27.92 33.47 12.26 14.45
29.69 38.04 4.83 8.98 17.48 12.78 21.29 7.70 7.95
15.92 18.53 2.00 4.74 8.22 5.54 11.87 4.68 4.34
322.86 453.29 84.19 118.75 225.05 193.24 239.56 101.40 104.06
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Fig. 6. Solitary and strings of unit-pockmarks occur at the base of the HRC deep-water coral reefs. The eight coral reefs in the cluster are named Aeh (the three major reefs, have been given capital lettering). They range in height from A, 24 m high, to g, 1 m high (Hovland and Risk, 2003; Hovland, 2008). The black arrows point at unit-pockmarks. The black dots G1eG5, G7, and G9 are locations of sediment samples from which there are geochemical results (Table 2). The highest light hydrocarbon values were found where unit-pockmarks have a high density (i.e., at G1 and G2). Also locations at the base of coral reef C had relatively high values (see text). Illumination from lower left (S) (modified from Hovland and Risk, 2003).
3.2.1.1. The pockmark location. The pockmark results are especially interesting, as they show a clear trend and document that the highest concentrations are not necessarily found at the centre of the normalpockmark, but rather, to the side of the centre, a result supporting the notion of self-sealing seeps. The results of adsorbed light hydrocarbon gases (ml gas per l sediment) from the pockmark samples are shown in Table 1. Whereas the sum of concentrations (methane, ethane, propane, and n-butane) of the background sediments, as found outside the normal-pockmark is 91.10 ml/l, the highest concentration (887.02 ml/l) occurs adjacent to the centre of the pockmark, and is over eight times the background value. At the time of investigation, we did not target unit-pockmarks during such sampling.
Fig. 7. A shaded relief DTM image from the Morvin area. The coral reef ‘MRR’ (large red area) is seen to be located inside a 10 m deep and 130 m wide pockmark crater. This crater has no distinct unit-pockmarks, probably because the seafloor has been colonized by invertebrates (sponges, corals, etc.). In contrast, unit-pockmarks (arrowed) can be seen inside the depression to the north. These unit-pockmarks are located up-stream of the prevailing current and the adjacent live corals. “8” Marks a sampled unit-pockmark (see Table 3 and Fig. 9). Illumination is from NNW. Note that the northesouth and eastewest linear features are artefacts from the multi-beam echo-sounder mapping.
unit-pockmarks, and coral reefs were found. The seafloor at Morvin consists of boulder clay with a layer of silty, soft clay in the upper 0.5 m. One of the larger coral reefs, the Morvin Reference Reef MRR (Fig. 7), is about 80 m long, 25 m wide, and spans the elevation interval: 360e370 m (Hovland, 2008). It is located inside a normalpockmark (130 m 80 m 10 m). The reef occupies about one third of the pockmark, and is growing from the maximum depth (at 370 m), up along the northern side, to the rim of the depression (at 360 m water depth). The prevailing current in the Morvin area is from the SSW. There exists an abundance of solitary and clustered unit-pockmarks, sometimes apparently associated with the coral reefs and with normal-pockmarks at Morvin (Figs. 7 and 8). Unit-pockmarks at Morvin have an apparent highest abundance inside depressions, some of which are normal-pockmarks and some are partly infilled relict iceberg plough-marks.
3.2.1.2. The HRC-location. A total of nine gravity cores were acquired from the base of the large coral reefs at HRC (Fig. 6 and Table 2). Two of the samples (G1 and G2) were acquired at the base of Reef A (Hovland et al., 1998), where unit-pockmarks actually occur in clusters. Four of the samples (G5eG8) were acquired across the large coral reef Reef C (Fig. 6; Hovland et al., 1998). Whereas the regional background sum of concentrations of adsorbed gases (methane, ethane, propane, n-butane) is taken to be 100 ml/l (a conservative number, as the lowest acquired values are generally less than this), the unit-pockmark samples (G1 and G2) had two to three times this background value. Also the samples acquired across Reef C provided a sum of concentrations that is well above the background value, with up to 1.7 times. This fact alone, supports the ‘hydraulic theory’ for deepwater coral reefs, suggesting that seepage of hydrocarbons benefits and ‘fertilizes’ the reefs (Hovland and Risk, 2003; Hovland, 2008). 3.3. Unit-pockmarks and coral reefs at the Morvin hydrocarbon field During mapping and inspection with ROV for potential pipeline routes at Morvin hydrocarbon field, numerous normal-pockmarks,
Fig. 8. A high-resolution shaded relief DTM from the Morvin field (Hovland, 2008). It shows up to nine unit-pockmarks (three of which are marked with white arrows) and associated coral reefs. Live coral reefs appear as rough topography, whereas dead corals appear as smoother seafloor (generally located to the NNW of the live corals). The unitpockmarks are generally located up-stream relative to the prevailing current and the adjacent live corals. Illumination is from NNW. Eastewest linear features are artefacts from the multi-beam echo-sounder mapping.
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Table 4 Adsorbed light hydrocarbon concentrations in clay samples related to complex pockmarks at Nyegga. The highlighted sediment sample, G11-2 was acquired inside a local depression (unit-pockmark?) inside complex pockmark G11, where there is a bacterial mat on the seafloor (see Hovland, 2008).
Fig. 9. Geochemical sediment sampling inside a unit-pockmark (station 8) at Morvin reference reef (MRR), see “8” in Fig. 7 and “S8” in Table 3. Notice the slight depression it represents e although distinct on the shaded DTMs, it is visually non-distinct upon close approach. Sampling cylinder is 80 mm wide.
3.3.1. Geochemical indications During investigations of coral reef MRR, in 2008, a total of nine sediment core samples were acquired for geochemical analysis, including one core (S8) from the centre of a unit-pockmark (Figs. 7,8, Table 3). The total hydrocarbon concentrations in core S6 acquired on the seafloor outside the pockmark, is lowest and represents the expected regional background concentration (of about 100 ml/l, as for HRC), and is used as the relative reference value. The other cores acquired inside normal- and unit-pockmarks at and near MRR have between two and five times this concentration and are interpreted to represent micro-seep locations (Judd and Hovland, 2007; Hovland, 2008, Etiope et al., 2009). The highest interstitial hydrocarbon concentrations (not provided here) were found inside the unit-pockmark (S8, see Hovland, 2008).
ID
Methane
Ethane
Propane
n-Butane
Sum
C-3 G8-5 G11-1 G11-2
262.70 153.53 202.74 275.54
50.61 14.37 37.68 55.13
28.50 6.44 24.60 36.17
12.75 2.69 12.54 18.20
354.56 177.03 277.56 385.04
associated with a bottom simulating reflector (BSR), and several manifestations of gas migration at depth, including vertical conduits (pipes) seen on 2D- and 3D-reflection seismic data and also imaged in 3D by ocean bottom seismometers (Hustoft et al., 2007; Ivanov et al., 2007; Westbrook et al., 2008; Plaza-Faverola et al., 2010). There are also some distinct organic-rich sediment mounds, ‘hydrate pingoes’, up to 1 m high and 4 m wide (Hovland and Svensen, 2006; Hovland, 2008). Work by Ivanov et al. (2007), proved that the pockmarks contain nodules or layers of gas hydrate occurring 1e1.5 m below surface. This discovery has provided confirmation of their status as active methane seeps or vents. Unitpockmarks were mainly found as solitary features on the seafloor outside the complex pockmarks, as can be seen in Fig. 11. There is also an occurrence of clustered unit-pockmarks about 150 m to the west of complex pockmark G12 (Fig. 11). 3.4.1. Geochemical indications In 2004, when geochemical cores were acquired at Nyegga, they were taken independently of known unit-pockmarks, as they were not then regarded as being important. The four locations reported in Table 4 have all been acquired from within complex pockmarks, of which G11 is the most studied (Hovland and Svensen, 2006; Ivanov et al., 2007; Westbrook et al., 2008).
3.4. Unit-pockmarks at Nyegga On the continental slope at Nyegga (the north-eastern boundary of the Storegga slide scarp), off Mid-Norway, there are some large, complex pockmarks with carbonate ridges inside them, Fig. 10 (Hovland et al., 2005). The water depth ranges between 600 and 800 m and the upper sediments consist of soft, sandy and silty clay. The complex pockmarks and other fluid flow features are Table 3 Geochemical results from sediment sampling at Morvin Reference Reef (MRR). The three highlighted samples are the ones with the highest concentrations (ml/l). S1 is located near the centre of the pockmark within which MRR is located. S4 is along the side of the same pockmark. S8 was acquired within a unit-pockmark, adjacent to the coral reef MRR and at the base of a neighbouring coral reef (see Fig. 11 for locations and Fig. 12 for sampling of unit-pockmark at S8). ID
Methane
Ethane
Propane
n-Butane
Sum
S1 S2 S4 S5 S6 S7 S8 S9
472.73 148.67 371.07 151.45 103.11 169.50 249.94 173.13
79.84 23.86 61.80 24.33 16.88 29.42 36.22 28.54
43.81 13.36 36.08 13.63 9.54 16.59 19.80 15.85
18.73 6.12 16.55 6.18 4.43 7.37 8.81 6.85
615.11 182.01 485.50 195.59 133.96 222.88 314.77 224.37
Fig. 10. The Nyegga detailed survey area, which contains a series of large, complex pockmarks, of which G11 and G12 are shown (Hovland et al., 2005). In addition, there are numerous scattered and partly clustered unit-pockmarks, not seen at this scale. The upper rectangle delineates Fig. 11. Illumination from NW (modified from Hovland, 2008).
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2pR2 V [ FpR2 vchimney ; so V [ Fvchimney =2; where R is the radius of the cylindrical gas chimney, F is the porosity of the sediments, vchimney is the upward velocity of the chimney, and V is the water flux on the hemispherical surface in m3 of water per m2 area per second (Cathles et al., 2010). In Fig. 12, we have attempted to utilize the model of Cathles et al. (2010) to interpret the sub-bottom profiler (SBP) records at a parent-pockmark in the Norwegian Channel. Here, we infer that remaining gas trapped below the pockmark by sealing MDAC pumps up the pore-water pressure and forces pore-water out along the rims and sides of the pockmarks. We suggest this may explain the observed distribution of unit-pockmarks. We will now address each of the three questions listed above. 4.1. Timescale of pockmark development
Fig. 11. At complex pockmark G12, there are at least 10 unit-pockmarks, five of which are indicated here by white arrows. Illumination from NW (see Fig. 10 for location).
At Nyegga, the regional background sum of adsorbed gases is expected to be approximately 100 ml/l. All values reported here are up to 3.8 times greater than this value, indicating that complex pockmarks at Nyegga represent locations of active seepage. The highest value (sample G11-2, Table 4) is from a core taken at a bacterial mat, situated inside a slight seafloor depression, near the centre of complex pockmark G11 (Hovland, 2008).
In general, pockmarks appear to form very rapidly (see examples in Hovland and Judd, 1988; Judd and Hovland, 2007, and Cathles et al., 2010). The quantitative model proposed by Cathles et al. (2010), whereby rising gas saturates a chimney which rises through a capillary seal by driving water through the seafloor, is a model supported by the observations of unit-pockmarks provided herein and by the long pipe-structures below complex pockmarks and other surface fluid flow features at Nyegga (Hovland et al., 2005; Hovland and Svensen, 2006; Westbrook et al., 2008; PlazaFaverola et al., 2010). According to this model, the vertical rate of growth of the chimney accelerates as it rises closer toward the
4. Discussion From our compilation of unit-pockmark occurrences off Norway, it is evident that these features may provide important information on the most recent and on-going seepage activity within a region of the seafloor. The most interesting and perhaps important questions regarding pockmarks, in general, are (1) the timescale of development, (2) their likely fluid pathways, and (3) how parent-pockmarks are kept clear of new sedimentation. A couple of recent publications addressed aspects of these questions, including the formation of unit-pockmarks and improve our understanding of pockmarks. One of these (Bui et al., 2007) used the distinct element method (DEM) to calculate the distribution of near-surface stress fields induced in sediments during fluid flux. That study showed that the flux would move in highly focused conduits and that the overlying sediments would be strongly deformed. The other study (Cathles et al., 2010), addressed the formation of pockmarks directly, and amongst other interesting results claimed that a sub-surface gas chimney can start to deform the sediments overlying it when its height equals half the depth of its base below the seafloor. This means that the buoyancy force of the rising gas column more-or-less pushes water out of the sediments above it, and causes local doming of the seafloor, fluidization (‘quickening’) of surface sediments, and unit-pockmark formation by fluid (mainly water) escape (Cathles et al., 2010). They calculated that the gas column or pipe behaves like a piston moving vertically and is resisted by the pore-water being displaced radially from the top of the advancing piston. The flux of water from a hemisphere of the same radius as the chimney is calculated from the equation of mass balance:
Fig. 12. Interpretation of the most probable fluid flow conduits (black arrows) beneath the parent-pockmark shown in Fig. 3. The densest occurrence of unit-pockmarks is indicated with ‘Upm’ above the seafloor. Based on the model by Cathles et al. (2010), it is inferred that slightly over-pressured pore-waters (blue) causes the formation of unit-pockmarks and trapped free gas (red) provides the over-pressure (by tidal pumping), trapped below MDAC (for location see Fig. 3).
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seafloor (even assuming the sediment permeability is vertically constant). The observation of unit-pockmarks forming along modern day trawl-scars (Fig. 2) clearly documents their recent formation history. Therefore, we conclude that unit-pockmarks may form in a matter of days, weeks or possibly months. In the early stages, pore-waters will utilize all permeable pathways and minor fractures and eventually break surface and form unit-pockmarks. This is in full agreement with observations of unitpockmarks forming even along trawl-gear marks which must have disturbed the permeability integrity of the surface seafloor layer. When the rising gas chimney moves closer to the surface, there will be a vigorous outflow, according to Cathles et al. (2010). The intense clustering and amalgamation of unit-pockmarks is expected to occur immediately before the formation of a normal-pockmark with the same diameter as the chimney (Cathles et al., 2010). Thus, in conclusion, we may say that a rising gas chimney will, first, slightly deform the seafloor (doming). Unit-pockmarks will form, as the weakest zones in the seafloor transmit rising and focused overpressured pore-water. Finally, as the gas chimney comes close to the seafloor, the gas, sediment, and remaining pore-water catastrophically erupts (drains) into the water column. However, although this model probably works well for the first generation of pockmarks, after a high-sediment influx episode, for example, after a glaciation period (e.g., The Norwegian Channel, cf. Tjelta et al., 2007), it may not work for all unit-pockmark occurrences, as in well-established and ‘old’ normal-pockmark regions. Thus, it may no longer be expected that all unit-pockmarks will eventually lead to the formation of larger structures (normal-pockmarks).
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Fig. 13. A small MDAC-nodule retrieved from a Tommeliten ‘eyed-pockmark’ (Hovland and Judd, 1988). It clearly demonstrates the turtuousity in the sub-bottom plumbing system, as each conduit clogs up, a new one is developed beside it, thus forming a complex underground ‘root’ system. Note the small circular hole (bubble-hole) in the surface of the nodule, immediately above one of the cemented gas feeder conduits. The hole is suspected kept open by active ebullition, which has prevented cementation (modified from Hovland, 2002).
4.2. Fluid pathways below pockmarks According to both the capillary seal model by Cathles et al. (2010), and the DEM of Bui et al. (2007), there is very little reason to expect an extensive lateral root system below ‘first-generation’ normal-pockmarks. Both models predict that the roots are vertical rather than spread out laterally. This is especially true for uniform vertical permeabilites. However, if there are general dips, domes, or other lateral inhomogeneities in permeability distribution below the pockmark, the fluid pathways will be modified accordingly. It will always be the path of lowest resistance that is used by migrating pore-waters and gas. This means that diagenetic processes, and especially the subsurface formation of MDAC will cause an immediate impact upon the distribution of fluid pathways. If fluid flow conduits are blocked and sealed by mineral precipitation, then according to the principle of least resistance, these pathways will close down and new ones will form laterally. Methane-derived carbonate-cemented sediments were first found in North Sea pockmarks in 1983. This discovery ended the on-going discussion about pockmark formation (biota, dead-ice vs gas and fluids), and more-or-less proved that long-time seepage is responsible for their formation (Hovland et al., 1985). It also became clear that the very same process of cementation causes the sealing-up or clogging of the plumbing system e i.e., seeps tend to seal themselves (Hovland, 2002). An example of a sealed-up and contorted fluid flow pathway, is clearly demonstrated in a sub-surface sample of MDAC, acquired from a cemented (‘eyed’) pockmark at Tommeliten (Fig. 13, Hovland, 2002, 2008). From studies of fossilized fluid flow systems there is also ample evidence of branching fluid pathways below long-lived fluid flow features. Fossilized seep studies have been conducted at several places (Clari et al., 1988; Campbell et al., 2002; Aiello, 2005). It is, perhaps, the observations by De Boever et al. (2009) that best indicate the clustering of seepage conduits in long-lived structures. They studied over 800 large and small tubular cemented vertical structures that formed in sandy marine sediments. Thus, their
results conform to the predictions of the new model of Cathles et al. (2010) that the cylindrical shape was the most common tube type, as it primarily reflected the buoyancy-driven, vertical path of an ascending gas-bearing fluid through permeable, mainly unconsolidated sandy host sediments. They also documented that tube morphology was influenced by local stratigraphic anisotropies such as locally coarsening sediments (De Boever et al., 2009). With respect to the fluid flow pathways immediately below unit-pockmarks, it is suspected that these are mainly vertical and un-contorted, as they are inferred to be the result of pore-water seepage, rather than gas flow eruptions. According to our analysis of SBP data from the Troll area, they occur along the side-walls of old pockmarks and on the seafloor immediately beside these (Figs. 3 and 12). Whereas the gas bubbles are suspected to be trapped below the centre of the normal-pockmark, where its bottom sediments and sub-seafloor sediments are partly cemented, most of the water in the system is pressured by this gas reservoir and pushed out at the sides of the system. Thus, the unit-pockmarks are suspected to manifest the pore-water seepage and the flat centre of the parent-pockmark to manifest the cap of the gas reservoir. The fact that the highest densities of unit-pockmarks in the Norwegian Channel occurs along the flanks and rims of the pockmarks also agrees well with findings off the Eastern seaboard of USA, where some giant-pockmarks were mapped and studied in detail by Newman et al. (2007). The researchers used ‘sniffers’, methane detectors which mapped the concentrations of methane in the water column. They found that high methane concentrations come out on the flanks and rims of the craters rather than from the bottom (centre) of the giant-pockmarks. 4.3. How parent-pockmarks are kept clear of new sedimentation The inherent small size of unit-pockmarks suggests that they may easily be obliterated, by sediment infilling, unless they are,
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or recently have been active. It is well known from visual inspections that small-sized (shallow) depressions on the seafloor, such as trenches and drag-marks after anchors and trawl-boards, are rapidly covered by mobile (drifting) sediments. Even though the net sedimentation rate in Norwegian and UKwaters, is very low, we know that sediment transport along the seafloor may be significant and at times high. During construction work (mechanical disturbance of the seafloor) we have observed visually (with ROV) turbid clouds drifting along the seafloor. Thus we infer that unit-pockmarks probably only occur for periods of up to some decades, whereas normal-pockmarks are inferred to have life-spans of perhaps thousands of years before they are obliterated by recent sediments (Forsberg et al., 2007; Judd and Hovland, 2007). There is, however, a problem in understanding how normalpockmarks and parent-pockmarks, which are often flat-bottomed (and slightly larger than normal-pockmarks), prevent re-sedimentation. From the theory of Cathles et al. (2010), it is likely that free gas exists directly beneath such pockmarks, in the latter case by a sealing layer of MDAC. This is also suggested in our own interpretation of the parent-pockmark near the Troll field, shown in Figs. 3, 4, and 12. The hydraulic properties of the seafloor are complex and intriguing. Because gas bubbles trapped below the seafloor are constantly affected by cyclic loading, such as tidal waves and passing cyclonic/anticyclonic (low/high pressure) weather systems, a buried or trapped shallow gas pocket will act as an effective pump on the nearby pore-waters (Hovland and Judd, 1988). Whereas the gas will remain for long periods below ground, hindered by capillary force threshold-values, pore-water is free to move in the surrounding capillary pore-structure (Cathles et al., 2010). The pore-water will likely utilize all possible cracks and holes available in the MDAC seal, so that it passes out of the seafloor every time the pressure is reduced. On images showing the seafloor micro-structure, from close-up photographs taken at the base of corals and MDAC rocks within the parent-pockmark shown in Fig. 3 (see also Fig. 4.52 in Hovland, 2008), sediment deposition can be seen. However, on the broad scale, the sediment has been re-suspended and transported away from the bottom of the parent-pockmark. We do not believe the background current velocity at the bottom of the Norwegian Trough to be strong enough for sediment scouring (current-induced erosion). Therefore, we suggest that the remaining fine-grained sediments seen on close-up photographs mentioned above, represent the remains of fine-grained sediments that have not been removed by seepage processes because they have somehow settled in locations without upward pore-water flux. Based on the surface and near-surface observations provided herein, we, thus, infer the following: - Wherever unit-pockmarks are found on the surface, it is likely there is a sub-surface (trapped) reservoir of free gas, nearby. - This reservoired gas acts as a dynamic pump, which causes periodic (rhythmic) movement of pore-water through the sediments. Hovland and Curzi (1989) and Hovland et al. (1999) have examined the effect of such a ‘pump’ on the local porewater system (over-pressuring) and found acoustic evidence of over-pressured pore-waters. - It is likely that wherever in-situ pore-water channels or conduits branch vertically up to the seafloor, a unit-pockmark forms (provided there are fine-grained sediments amenable to pockmark formation). - Furthermore, wherever a crust of cement (MDAC) has formed at long-duration seepage locations such as inside normalpockmarks, a thicker layer of gas-charged sediments may build up beneath the cemented ‘seal’ (Hovland, 2002).
Therefore, we conclude that the hydraulic action of water moving in and out of such crusts (driven by trapped free gas) can cause fine-grained sediment particles to re-suspend, and thus prevent re-burial. Even though there may be very low water current velocities inside pockmarks, the sediment grains (silt and clay) are so small that they will easily re-suspend in the water column if there is a slight upward directed pore-water movement. Such re-suspension means that the grains will be carried away, out of the pockmark. The net sedimentation inside ‘actively pumping’ pockmarks is therefore likely to be relatively low. Using this hydraulic model, it is also possible to understand how the carbonate rubble and large carbonate ridges were formed inside the Nyegga complex pockmarks (Hovland et al., 2005; Hovland and Svensen, 2006). From the recent work by Westbrook et al. (2008) and Plaza-Faverola et al. (2010), we know that these pockmarks represent surface manifestations of deep-rooted chimneys (pipes) which tap into gas hydrate and free gas reservoirs. According to the concept of Cathles et al. (2010), it is likely that the pipes erupt catastrophically from time to time. Consequently, the broader picture of the seafloor acting on hydraulic principles of physics, may therefore be summed up as follows: - Generally, we can divide the seafloor into two main types e ‘hydraulically active’ (i.e., with pockmarks and fluid flow features) and ‘hydraulically passive’ (i.e., without any surface manifestations). - Wherever normal-pockmarks and complex pockmarks occur, there is likely to be an excess of migrating gas, which will, periodically, or intermittently vent through the seafloor (causing the normal-pockmark development, according to Cathles et al., 2010). - Wherever only unit-pockmarks occur on the seafloor, the gas supply from below, is likely low, and remains sub-surface. Thus, it will act as a buried hydraulic pump, causing only cyclic porewater movement without gas eruptions. An important corollary to this hydraulic theory is that it is also valid for all volumes of soil, which have a porosity system partly filled by liquid and partly filled by gas. Thus, it is valid for all ocean depths, lakes and swamps, as the driving gas-type (methane, carbon-dioxide, hydrogen-sulphide, or hydrogen) is immaterial. 5. Conclusions Since their first discovery and description, pockmarks have been found in all of the world's oceans and some of the lakes, mapped with sonars and swath echo-sounders. During the 1980s it was noticed that small, mono-sized pockmarks were also common on the seafloor, they were termed unit-pockmarks. Based on this analysis of the occurrence of unit-pockmarks and normal-pockmarks at four selected locations on the Norwegian continental shelf and slope, combined with new theoretical models, we find that unit-pockmarks represent well defined manifestations of active fluid flow. Our main conclusions are as follows: - The local seafloor is either characterized as ‘hydraulically active’ or ‘hydraulically passive’, dependent on the occurrence of pockmarks and other fluid flow features (active) or the absence of such features (passive). - The type of surface fluid flow manifestations determines the type and vigour of activity, i.e., cyclic/periodic, high, or low hydraulic activity. - Whereas unit-pockmarks most likely represent cyclic porewater seepage, normal-pockmarks represent periodic or
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intermittent gas bursts (eruptions) with intervening periods of slow, diffusive, and cyclic pore-water seepage. - The driving force behind seafloor hydraulic activity is reservoired, buried gas pockets. In practical terms, this means that when sampling for seeping fluids, we recommend ‘seep-hunters’ to target the unit-pockmarks. The higher investment needed to ensure detection and mapping of the small unit-pockmarks, may be balanced by a higher success rate in sampling dissolved gases in the seeping pore-waters resulting from the active pumping by the trapped underground gas. Acknowledgements The marine crew and the ROV-operators on board the multipurpose vessels ‘Acergy Viking’, ‘Edda Fonn’, and ‘Edda Freya’ are thanked for their professional work during the mapping, sampling, and inspections at HRC, Morvin, Nyegga and Troll. Statoil are thanked for releasing the information contained herein. The reviewers Graham Westbrook and Peter Croker are thanked for their fruitful comments. References Aiello, I.W., 2005. Fossil seep structures of Monterey Bay region and tectonic/ structural controls on fluid flow in an active transform margin. Palaeogeography, Palaeoclimatology, Palaeoecology 227, 124e142. Bui, H., Dvorkin, J., Nur, A., 2007. Subsurface fluid flow and its implications for seabed pockmarks and mud volcanoes: an approach of distinct element method (DEM). In: Book of Extended Abstract, Societey of Exploration Geophysiscists, Annual Meeting. San Antonio, USA, pp. 2055e2059. Campbell, K.A., Farmer, J.D., Des Marais, D., 2002. Ancient hydrocarbon seeps from the Mesozoic convergent margin of California: carbonate geochemistry, fluids and palaeoenvironments. Geofluids 2 (2), 63e94. Cartwright, J., Huuse, M., Aplin, A., 2007. Seal bypass system. AAPG Bulletin 91 (8), 1141e1166. Cathles, L.M., Su, Z., Chen, D., 2010. The physics of gas chimney and pockmark formation, with implications for assessment of seafloor hazards and gas sequestration. Marine and Petroleum Geology 27, 82e91. Clari, P.A., Gagliardi, C., Governa, M.E., Ricci, B., Zuppi, G.M.,1988. I calcari di Marmorito: una testimonianza di processi diagenetici in presenza di metano. Bollettino del Museo Regionale di Scienze Naturali Torino 5, 197e216 (in Italian). De Boever, E., Huysmans, M., Muchez, P., Dimitrov, L., Swennen, R., 2009. Controlling factors on the morphology and spatial distribution of methane-related tubular concretions e case study of an Early Eocene seep system. Marine and Petroleum Geology 26, 1580e1591. De Vries, M.H., Svanø, G., Tjelta, T.I., 2007a. Small scale model testing of gas migration in a soft seabed as a basis for developing a mechanical model for gas migration. In: Proceedings of the Sixth International Offshore Site Investigation and Geotechnics Conference. SUT-OSIG, London, UK, pp. 231e236. De Vries, M.H., Svanø, G., Tjelta, T.I., Emdal, A.J., 2007b. Pockmarks: created by reduced sedimentation or a sudden blow-out?. In: Proceedings of the 17th International Offshore and Polar Engineering Conference. ISOPE, Lisbon, Portugal, pp. 1361e1365. Dimitrov, L., 2002. Mud volcanoes e the most important pathway for degassing deeply buried sediments. Earth-Science Reviews 59, 49e76. Etiope, G., Feyzullayev, A., Baciu, C.L., 2009. Terrestrial methane seeps and mud volcanoes: a global perspective of gas origin. Marine and Petroleum Geology 26, 333e344. Forsberg, C.F., Planke, S., Tjelta, T.I., Svanø, G., Strout, J.M., Svensen, H., 2007. Formation of pockmarks in the Norwegian Channel. In: Proceedings of the Sixth International Offshore Site Investigation and Geotechnics Conference. SUTOSIG, London, UK, pp. 221e230. Foucher, J.-P., Westbrook, G.K., Boetius, A., Ceramicola, S., Dupré, S., Mascle, J., Mienert, J., Pfannkuche, O., Pierre, C., Praeg, D., 2009. Structures and drivers of cold seep ecosystems. Oceanography 22 (1), 92e109. Heggland, R., 1998. Gas seepage as an indicator of deeper prospective reservoirs. A study on exploration 3D seismic data. Marine and Petroleum Geology 15, 1e9.
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