Mud volcanoes in deepwater Nigeria

Marine and Petroleum Geology 17 (2000) 959–974 www.elsevier.com/locate/marpetgeo

Mud volcanoes in deepwater Nigeria K. Graue Statoil, INT-WA, 4035 Stavanger, Norway Received 2 August 1999; received in revised form 6 March 2000; accepted 10 March 2000

Abstract Detailed study of 3D seismic data from deepwater Nigeria has revealed the presence of features interpreted to be mud volcanoes. They occur in an upper slope environment seen as 1–2 km circular features at the seabed. Seabed cores from the mud volcanoes contain oil, gas and sand/shale–clast content richer than the seabed background. Pliocene fossils have been identified in the cores, demonstrating transport of material from depth. The features show a high seabed seismic amplitude above a chimney of chaotic seismic reflections and data wipe-out. The mud volcanoes in the study area shows two distinct clusters located over deeper structural culminations. Four active mud volcanoes (Area 1) are located above a rollover anticline in the central part of the area. Cuspate listric faults reach the seabed on the up-slope side of these mud volcanoes. Four abandoned mud volcanoes have also been identified, with progressively older ages towards the crest of the underlying structure. These abandoned features are associated with an extremely chaotic seismic signature. A combination of over-pressured carrier beds and low integrity top seals are believed to be responsible for the formation of these mud volcanoes. It is further believed that gas expansion, subsequent to seal failure, was the main driving force for what must have been violent eruptions. The long lived mud volcano activity over the deep structural closure suggests a plumbing system that focuses on escaping compaction water and hydrocarbons through time. In the south west of the study area, another cluster of five mud volcanoes is located above a shale diapir. The seabed expression of the members of this cluster is more varied. Some show a positive relief at the seabed while others show circular depressions. The largest feature represents an assemblage of many smaller mud volcanoes with a common root system. Their seismic expression is also different from that of Area 1, with a well-defined sediment wedge at surface and a shallower root system. These features are believed to represent a less violent type of mud volcano characterised by ductile flow. Tectonic stress, due to growth of the underlying diapir, is thought to have played an important role during eruption, in addition to focused methane escape and the low mechanical strength of the overburden. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: Mud volcanoes; Deepwater Nigeria; Seismic data

1. Introduction This paper presents a detailed study of mud volcanoes seen on a 3D seismic data set situated on the western slope of the Niger Delta, approximately 50 km from the present coastline (Fig. 1). The survey is located just seaward of the present shelf edge in an upper slope setting with water depths ranging from 250 m in the NE to 850 m in the SW (Fig. 2). The deepwater area (⬎200 m WD) of Nigeria was opened for oil exploration activity in the early 1990s. Numerous seabed anomalies were observed during early regional 2D seismic evaluations. They were generally characterised by positive topographic relief and very high seabed seismic amplitude above a vertical chimney with almost complete data wipe-out. Whenever seen on seismic data, these E-mail address: [email protected] (K. Graue).

anomalies always occur above deeper structural culminations. The poor seismic imaging associated with these features allows for some ambiguity as to their interpretation and consequently a number of models can be suggested for their formation: 1. 2. 3. 4. 5.

shallow gas pockets (gas blisters); chemosynthetic carbonate biotas on top of gas seeps; escape routes for decompaction water and gas; gas hydrates; mud volcanoes.

The objective of this paper is to present evidence supporting the interpretation that these features are mud volcanoes. Little has been published on mud volcanoes in Nigeria. Heggland, Nygaard and Gallagher (1996) showed a picture of mud volcanoes from the same 3D dataset used in this study.

0264-8172/00/$ - see front matter 䉷 2000 Elsevier Science Ltd. All rights reserved. PII: S0264-817 2(00)00016-7

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Fig. 1. Seabed dip map over the 3D area. Mud volcanoes are seen as 1–2 km circular features. Note the cuspate faults and numerous pockmarks. Circled numbers show location of seabed cores, black lines show location of seismic lines and the frames show location of side-scan sonar images. See Fig. 2 for location of the study area.

2. Nature of mud volcanoes Mud volcanoes were defined by Guliev (1992) as “periodic expulsion from a deep part of the sedimentary cover of mixtures of water, various gases and solid material”. Earlier works on mud volcanoes concentrated on onshore areas. Much of the published material is therefore based on surface

description of “features” that were generally considered geological anomalies at the time. Recent developments in technology have opened vast deepwater areas for research and it is now clear that mud volcanism is also widespread there. Mud volcanoes have been described from Albania (Ali-Zade, Shnyukov, Grigoryants, Aliyev, & Rakhmanov, 1984), Azerbaijan (Jakubov, Ali-Zade & Zeinalov, 1971),

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Fig. 2. Regional structural elements and depobelts of the Niger Delta (Modified after Doust & Omatsola, 1990). Depobelts from old to young are labelled I– VII. Seabed contours are seen as dotted lines.

Barbados (Langseth, Westbrook & Hobart, 1988), Burma (Ali-Zade et al., 1984), Colombia (Vernadette, 1989), Ecuador (Higgins & Saunders, 1974), Gulf of Mexico (Prior, Doyle & Kaluza, 1989), Iran (Snead, 1972), Sumatra (Banerjee, 1975), India (Snead, 1972), Italy (Higgins & Saunders, 1974), Japan (Ali-Zade et al., 1984), Malaysia (McManus & Tate, 1986), Mexico (Higgins & Saunders, 1974), New Zealand (Ridd, 1970), Pakistan (Ali-Zade et al., 1984), Romania (Higgins & Saunders, 1974), Taiwan (Shih, 1967), Timor (Barber, Tjokrosapoetro & Charlton, 1986) and Trinidad (Yassir, 1989). Most classification schemes of mud volcanoes, as found in published literature, are based on surface morphologies. A kinetic classification was presented by Guliev (1992): Class (1) expresses powerful phreatic explosions of gas and large volumes of argillaceous material. Eruptions are short but intense/fierce and usually associated with spontaneous combustion of gas. Class (2) represents eruption of mud breccia with associated gas that does not ignite. Class (3) is characterised by emission of a low-viscosity mud and breccia without any intense gas jets of flames. Class (4) breccia is squeezed out from the mouth of the mud volcano together with small amounts of gas. Such expulsions may be prolonged, sometimes years. The size range of mud volcanoes varies between large cones up to 3–4 km in diameter and 400 m high, and

meter-sized outlets for liquefied mud and gas (Ali-Zade et al., 1984; Hain, Apressov & Mitchink, 1937; Jakubov et al., 1971; Tamrazyan, 1972). A total of 5 mill m 3 of ejected material has been reported for one single eruption in Azerbaijan and totals ejected during the lifetime of a mud volcano range up to 11.4 km 3. Clasts up to 5 m have been found among the erupted material (Ali-Zade et al., 1984; Guliev 1992). Although eruptions are usually not associated with seismic activity, earthquakes may act as a triggering mechanism for mud volcanism (Banerjee, 1975; Ridd, 1970; Yassir, 1987). Opinions differ as to the main driving forces responsible for the eruption of deep-seated muds. However, all mud volcanoes seem to have at least a common set of observations attached to their presence: • generally associated with thick, rapidly deposited, overpressured Tertiary and younger sequences; • exotic rock fragments and faunal assemblage from deeper strata are brought to surface; • a deep (several km) structural focus; • generally associated with hydrocarbon gases, particularly methane; • active tectonism (very often compressive tectonism). There is a correlation between active plate margins and the presence of mud volcanoes. This association has led several authors to propose that tectonic stress is the most important driving force for their eruption (Goubkin, 1934;

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Higgins & Saunders, 1974; Khalilov & Kerimov, 1981; Mekhtiyev et al., 1985; Sokolov, Buniat-Zade, Goedekian & Dadashev, 1968; Yassir, 1987, 1989). Others focus on the presence of methane, sometimes in abundant quantities and believe that gas expansion/buoyancy is the most likely mechanism (Ali-Zade et al., 1984; Guliev, 1992; Hedberg, 1974; Hovland, Hill & Stokes, 1997; Langseth et al., 1988). Nine active mud volcanoes can be identified from a seabed dip-map generated from the 3D seismic data cube (Fig. 1). These appear as 1–2 km diameter circular expressions. The linear features are active faults reaching the surface and the numerous small dots are pock marks. The mud volcanoes are located in two separate clusters. The four in the central area overly a deeper roll-over anticline (Area 1) and the cluster to the SW are located above a shale diapir (Area 2). At least five abandoned mud volcanoes have also been identified in the data set in addition to those seen in Fig. 1. Mud volcanoes seem to be related to the presence of underlying structures. Their shape and seismic characteristics seem to vary from one structural style to another. These differences are described in the text and are explained by the relationship between eruption styles and kinetic variables. Areas 1 and 2 are therefore treated separately. Three of the active mud volcanoes in Area 1 (see Fig. 1) were sampled with gravity cores and analysed for grain size distribution, biostratigraphy and organic geochemistry.

3. Regional setting The Tertiary Niger Delta, a wave and tidal dominated delta, is composed of an overall regressive clastic sequence which reaches a maximum thickness of 10–12 km (Doust & Omatsola, 1990). The onshore and shelf part of the delta is extremely sand-rich with average net to gross sand ratios of 50–60% in the paralic facies and 80–90% in the continental deposits (Allen, 1965). The progradation of the deltaic sequence has been controlled by syn-sedimentary growth faults and the interplay between subsidence and sediment supply. The delta can be divided into a number of major growth-fault bounded sedimentary units or “depobelts”, which, as the delta prograded, succeeded one another in a basin-ward direction (Fig. 2). Older depobelts are fossilised once sedimentation has shifted to a younger (seaward) depobelt. One of the most important structural components of the depobelts are the down-to-continent faults (counter-regional faults) that separate depobelts from one another and are oriented sub-parallel to the coastline. Each depobelt extends almost across the entire delta, but tend to merge at the flanks where sedimentation rates are lower. The “Offshore Depobelt” (no. VII in Fig. 2), being the active depocentre today, separates the shelf from the deepwater area by an extensive set of counter-regional faults. The present shelf edge is located approximately 50 km from the shoreline. The structural picture of the deepwater

area is different from the onshore and shelf part of the delta. Elongated compressional folds are seen at the lower slope of the delta. The lower to middle slope is dominated by toe thrusts and ramp anticlines. The structures in the middle to upper slope are mainly formed by extensional faulting and shale diapirism. Both the extensional and compressional structures in the deepwater area are genetically linked to growth of the active depobelts. The sedimentary succession of the slope and deepwater area is believed to consist predominantly (⬎80%) of marine shale with interbedded sandstones deposited as sliders, debris flows and turbidites. Rapid sedimentation of dominantly shaly sediments causes undercompaction and overpressuring of the buried deposits. Ductile deformation of undercompacted shales into diapirs and thrust systems generates a variety of structural culminations that will be the focus for migrating formation fluids. These factors combined, with the low mechanical strength of the overburden, are probably the most critical elements for mud volcanoes to form in this environment.

4. Seabed cores Eleven gravity cores were collected in the study area during 1995 (see Fig. 1 for location). Five were targetted at the mud volcanoes and six at the seabed background. The cores were subjected to visual description, grain-size analysis, biostratigraphy and geochemical analysis. The results in Table 1 represent averaged values for each core; the highest and lowest values are not presented. All the cores appeared to consist of grey, sticky mud without any apparent lamination or textural variation. However, when probed with a needle it was observed that all the cores from mud volcanoes contained variable amounts of shale fragments from 0.2–2 cm in size. The cores were sampled at regular intervals for grain size analysis and biostratigraphy. Material left after sampling was washed through a 450-mm mesh in order to separate shale clasts for nannofossil analysis. None were found in the seabed background cores. The collected shale clasts showed various degrees of consolidation from hard calcareous nodules to soft semi-consolidated granules that easily deformed on the mesh. Similar nodules have also been found in samples taken from mud volcanoes in the Black Sea (Ivanov, Konyukhov, Kul’nitskii & Musatov, 1989) and off the Barbados Ridge (Langseth et al., 1988). A coarse fraction, from silt to coarse sand, was present in all the mud volcano cores, comprising between 3 and 8 wt% (Table 1). No sand laminae were observed, thus the sand grains appear to be dispersed in the clay matrix. The cores from the seabed background consisted entirely (100%) of clay. All the mud volcano cores are shorter than the others by an average of 30% (36 cm), suggesting a correlation between sediment composition and core recovery. Reduced core recovery probably reflects a denser nature for the mud volcano sediments relative to the background. Grain size

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Table 1 Core recovery, grain size analyses and biostratigraphy of seabed cores. The lowest core recoveries correlate well with the sandiest and oldest sediments. The grain size distributions represent averaged values from 2 to 3 samples per core. A total of 23 samples were analysed. Mud volcano samples numbers 1–4 and 9 Core

Rec. (m)

Clay

Silt

vf sst

f sst

m sst

c sst

Forams

Nanno

No. 1 No. 2 No. 3 No. 4 No. 9 No. 5 No. 6 No. 7 No. 8 No. 10 No. 11

0.75 1.00 0.84 0.94 0.97 1.41 1.11 1.05 1.40 1.30 1.27

94.85 95.35 94.29 95.02 94.54 100.0 100.0 100.0 100.0 100.0 100.0

4.21 0.92 2.40 1.44 1.43 – – – – – –

0.46 1.75 2.40 2.26 1.54 – – – – – –

0.21 1.15 0.65 0.81 0.95 – – – – – –

0.23 0.59 0.28 0.36 1.11 – – – – – –

– 0.16 – 0.10 – – – – – – –

L.Plioc E.Plioc E.Plioc E.Plioc Plio-Pleistoc E.Pleistoc E.Pleistoc E.Pleistoc E.Pleistoc L.Pleistoc L.Pleistoc

– L.Mioc E.Plioc E.Plioc E.Pleistoc E.Pleistoc – L.Pleistoc L.Pleistoc L.Pleistoc –

analyses of mud volcanoes in Trinidad and Taiwan show predominantly silt and sand contents with only 30–60% shale (Higgins & Saunders, 1974; Yassir, 1989). Similar measurements in samples from the Gulf of Mexico show shale contents front 50–80% (Kohl & Roberts, 1994). The most important conclusion from this grain size analysis, however, is the difference between samples from the mud volcanoes and the seabed background. Exotic blocks frequently occur in the mud volcanoes in Trinidad; generally between 3 and 11 cm but specimens up to 1 m have been reported (Yassir, 1989). Clasts seem to be frequent in Azerbaijan, where violent mud volcano eruptions seem to be always associated with shale breccias. Ejecta up to 5 m in diameter have been encountered. The cores analysed in this study contained few clasts. The apparent lack of textures and sedimentary structures is taken as an indication of a total collapse of the clay fabric and sediment remoulding during transport. Carbonate material from areas supporting chemosynthetic biota has been reported over seeps and mud volcanoes in the Gulf of Mexico (Kohl & Roberts, 1995; Neurauter & Roberts, 1994; Sassen, Brooks, Kennucutt, MacDonald & Guinasso, 1993). No such fragments or material were found in the studied samples.

Nannofossil and foraminiferal analyses have allowed ages to be ascribed to the cores based on the occurrence of the oldest marker fossils. The foraminifera assemblage of the shale matrix suggests an Early Pliocene age for cores 1–4. The most significant markers are: Sphaeroidinellopsis subdehiscens, Globorotalia scitula, Ga. miocenica, Ga. crassula, Ga. margaritae primitiva and Ga. aff margaritae. The ages from foraminifera are generally in agreement with the nannofossil content of the shale clasts, except from core no. 2 which is possibly slightly older (Late Miocene–Early Pliocene). Marker fossils are: Sphenolithus abies and Discoaster quinqueramus. Material sampled from mud volcano vents in the Gulf of Mexico has been dated as Miocene/Pliocene (Kohl & Roberts, 1994, 1995). Geochemical analyses (Table 2) show that the amount of extracted organic material (EOM) is 10–15 times higher in the mud volcano samples compared to the seabed background. Gas chromatography of the extracts show high contents of heavily biodegraded oil (Fig. 3a). From these results, it is concluded that the oils were matured in the early oil window and thereafter transported/migrated to surface. The measured headspace gas was in agreement with these results with high gas levels (particular methane) from all the mud volcano samples. Oxygen isotope values indicate a

Table 2 Geochemical analyses of seabed cores. The values represent average values for 3 samples per core. A total of 33 samples were analysed. Mud volcano samples numbers 1–4 and 9 Core

C1 (mg/g sed)

C2–C4 (mg/g sed)

C23 (mg/g sed)

C23 (mg/g sed)

LOM (mg/g sed)

No. 1 No. 2 No. 3 No. 4 No. 9 No. 5 No. 6 No. 7 No. 8 No. 10 No. 11

630.8 20.4 70.7 83.1 492.1 8.8 9.3 91.1 10.2 11.7 5.0

4.8 1.0 1.9 1.3 2.4 0.7 0.6 2.3 0.9 0.9 0.3

39.6 84.7 57.6 79.0 84.4 1.3 1.2 1.9 1.6 2.3 1.2

29.4 46.9 51.4 50.5 45.9 6.1 4.8 6.0 6.3 5.1 4.5

92.1 142.1 122.5 141.6 152.2 11.7 9.7 13.3 13.7 12.1 9.3

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Fig. 3. Characteristic gas chromatograms from (a) mud volcanoes and (b) background. Graph a (core no. 4) shows an unresolved complex mixture of biodegraded oil. Graph b (core no. 10) shows recent organic matter.

bacterial origin for the gas, probably due to breakdown of the surface oil. Some of this may have taken place in the cans before analyses were performed. None of the seabed background cores contained detectable amounts of migrated oil. The modest quantities of EOM found in the seabed background cores show typical signatures of recent organic material (Fig. 3b). Methane levels of 90–99% have been reported from active mud volcano vents in Azerbaijan (Guliev, 1992; Sokolov, Buniat-Zade, Goedekian & Dadashev, 1968), Trinidad (Yassir, 1989) and Sumatra (Banerjee, 1975). These measurements were made on samples from active vents in onshore mud volcanoes and are therefore interpreted as deep thermogenic gas. Although the results from this study also show methane levels of 95–99%, it should be

stressed that these samples were stored in frozen cans for several months before analysis, without biocides added. The measured methane may therefore represent a combination of thermogenic gas and biogenic gas from the bacterial breakdown of oil. Thermogenic gas and crude oil are reported from mud volcanoes in the Gulf of Mexico (Neurauter & Bryant, 1989, 1990; Neurauter & Roberts, 1994). Based on the above analyses, it is evident that the material from the mud volcanoes is distinctly different from the seabed background and shows similarities with material sampled from published mud volcanoes in other deepwater areas. It is concluded therefore that material has been brought to surface from deeper in the section.

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5. Mud volcanoes in area 1 Fig. 4a shows a seismic line over the largest mud volcano here (Fig. 1). The high amplitude of the seabed anomaly overlies a vertical chimney on the seismic data with an almost complete data wipe-out. This is typical of how the mud volcanoes generally appear on seismic data in Nigeria and as described elsewhere (Kohl & Roberts, 1994; Neurauter & Bryant, 1990; Neurauter & Roberts, 1994; Roberts, 1996). This lack of seismic resolution has given room for a variety of explanations. The three seabed cores taken from this mud volcano (see Figs. 1 and 5) show that the material contains 3–8% sand and exotic shale clasts. The high seismic amplitude is believed to result mainly from the higher density of these surface sediments, possibly in combination with gas saturated sediments underneath. The lack of seismic resolution underneath the seabed amplitude is probably caused by a combination of seismic scaling, gas in the section and chaotic geology that combine to make the seismic data acoustically amorphous. The seismic resolution improves toward the edges of the seabed

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features, as seen on line b (Fig. 4), located approximately 1 km to the north and away from the highest amplitudes. On this line, it is possible to distinguish details, such as collapse into the vent through a set of normal faults. These faults have a concentric shape on the side-scan sonar images in Fig. 5a, and are almost identical to similar images from the Gulf of Mexico (Prior et al., 1989). The root of this mud volcano is interpreted to be close to 2500 ms TWT, where the seismic data are extremely chaotic. This also conforms to a mapped structural closure at this level. The degree of disturbance seems to diminish away from the centre and gradually passes into more coherent reflectivity on the flanks. The overburden, down to 2 s TWT, seems to be relatively undisturbed, apart from the collapse into the vertical vent. A set of prominent growth faults, with a curved shape at the seabed, are intimately connected to the mud volcanoes. In Fig. 1, it can be seen that one curved fault “embraces” the two easternmost mud volcanoes and another fault connects the two westernmost. Both faults reach the surface on the upslope side of the mud volcanoes. Consequently the question

Fig. 4. Seismic expression of a mud volcano in Area 1 (see Fig. 1 for location): (a) seismic line over the centre of the mud volcano. Note the high amplitude, the data wipe-out, multiples and structural closure at depth where the fault terminates; (b) located 1 km to the north where the amplitude is lower and seismic resolution is better. Note strong rotation on the listric faults and caldera-like collapse into the vent. See also the abandoned mud volcano.

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Fig. 5. Sidescan sonar images of two mud volcanoes. See Fig. 1 for location. Numbers represent the sampling stations for seabed gravity cores.

arises whether the mud volcanoes are triggered by the activity of these faults, or vice versa, or whether there is any connection at all. The following observations suggest that fault activity occurs as a result of mud volcano eruption:

A simplified model that may explain the above observations is presented in Fig. 6.

1. The faults terminate directly underneath the root system. 2. The degree of roll-over does not conform with the apparent extension in the area. 3. The section fails to restore in a structural balancing exercise. 4. The faults are shaped as a “half funnel” with an extreme cuspate shape near the root.

The critical elements believed to be responsible for the mud volcano illustrated in Fig. 4 are severely overpressured gas-charged carrier beds overlain by low integrity seals. Overpressure is generated by rapid deposition of shaly sediments. These prevent decompaction water from escaping from the section during burial. Overpressure is further enhanced with depth by clay diagenesis releasing excess

5.1. Model for the origin of the mud volcanoes

Fig. 6. Model for the formation of mud volcano shown in Fig. 4. Not to scale.

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pore water, by trapped thermal and biogenic gas, by the focussing of decompaction water, by migrated hydrocarbons and by tectonic loading. Focussed migration of decompacted water and hydrocarbons from the subsiding basin may eventually build up pressures that exceed the fracture pressure at the structural closure and cause a seal failure. Because of the compressibility of gas (particularly methane) the seal failure will be accompanied by a gas expansion that provides the explosive power of the system and enables vertical penetration through overburden, as also suggested by Hovland et al. (1997). The sudden pressure release associated with eruptions will cause an implosion at the root, shaking the undercompacted shale such that the poorly consolidated clay fabric disintegrates into liquefied mud with a complete loss of mechanical strength. The liquefied mud subsequently flows up the vent together with gas, liquids, rock fragments and sand from the carrier bed. This provides a plausible explanation for the extremely distorted seismic image in the root zone. The rapid kinetics of such eruptions are also likely to cause a rapid depletion of pressure to a level below that of the fracture pressure of the seal, which is the point when the roof collapses into the vent and plugs the conduit to surface. This collapse is believed to account for the concentric faults seen in Fig. 5a and b and the dipping section seen on the seismic data (Fig. 4). The overburden, labelled “bypass section” in Fig. 6, is relatively little disturbed apart from the vent itself and nearby caldera collapse. This indicates that activity of the mud volcanoes had a limited impact on the stratigraphic section other than in the immediate surroundings of the vent. Material removed from the stratigraphic section needs to be compensated for in the mass balance. In the model presented in Fig. 6, it is proposed that eruptions trigger spontaneous activity along the listric faults. Contraction occurs at the base, with corresponding fault displacement up section. The extreme curved shape of these faults near the root, as mapped from seismic data, is thus caused by removal of a confined volume at depth. The faults exhibit a gentler curvature at the surface (Fig. 1), due to stress being distributed over a larger area away from the foci of the root, hence giving a half-funnel shape to the faults. The gravity gradient of the slope explains the up-slope position and down-to-the-basin throw of the faults. The expanded stratigraphic section along the growth faults indicates periodic activity through the same system over time. It is therefore likely that mud volcanoes, once formed, represent weak spots that are subject to later reactivation. The first eruption of a mud volcano is believed to be more powerful than subsequent ones due to the threshold energy required to breach the seal for the first time. The apparent lack of seabed cones indicates that the erupted material has been removed from the vent probably through a combination of injection into the water column and mass transport downslope as debris flows. Some of the erupted material can, however, be accounted for by subsidence into

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the caldera. The general lack of cones at sub-sea mud volcanoes has been recognised by several authors (e.g. Barber et al., 1986; Guliev, 1992; Jakubov et al., 1971). Guliev (1992) described the characteristics of eruptions from violent (class 1) mud volcanoes in Azerbaijan. Eruptions are usually initiated by ejection of relatively small amounts of solid material and breccia that is believed to represent material from the vent itself, possibly from a former eruption. The main stage is dominated by a violent gas plume accompanied by solid material from mm to metre size clasts. The gas usually ignites after a while (sub-aerial conditions) and fine grained material settles like volcanic ash over a large area. The final stage of an eruption, when the gas plume trails off, is succeeded by a viscous flow of mud and breccia deposited as debris flows on the cone and neighbouring areas. The gaseous explosive mud volcanoes are the most violent ones, but eruption usually lasts for only a short period, such as a day or two (Guliev, 1992). The fanshaped amplitudes seen on the down-slope side of the mud volcano in Fig. 6 are believed to represent debris flows derived from the mud volcano, possibly the viscous tail of the last eruption. These particular features were also classified as debris flows by Heggland et al. (1996). A similar looking debris flow was reported from the Gulf of Mexico by Prior et al. (1989), showing a ca 1 km crater bounded by concentric faults. The seabed craters of the mud volcanoes in Area 1 have diameters of 0.5–1.5 km (Figs. 1 and 5); the one illustrated in Fig. 5a is the largest. There is little published material to compare these with, since most of the measurements refer to the cone diameters of onshore mud volcanoes. However, Reed, Silver, Tagudin, Shipley and Vrolijk (1990) showed side-scan sonar images of several mud volcanoes from offshore Panama in water depths in excess of 2000 m that also measure 0.5–2.0 km across. Ivanov et al. (1989) described one in the Black Sea with a diameter close to 2 km. Most of the mud volcanoes described in the Gulf of Mexico range in size from a few meters up to 0.5 km (Kohl & Roberts, 1994, 1995 with the exception of the 1 km large crater described by Prior et al., (1989). The volume of material captured in the debris flows in Fig. 7 can only account for a fraction of the material needed to explain the fault activity. However, if these mud volcanoes erupt as violent explosions, material would be sent as a jet into the water column and transported down slope as mass-flows. Hence, limited material would be expected to settle at or near the active vent. Neurauter and Roberts (1994) showed pictures taken by a submersible device of a small erupting mud volcano on the Louisiana slope, showing a plume of suspended sediments in the water. 5.2. Structural impact of mud volcano activity The structure underlying the four mud volcanoes in Fig. 1 is mapped as a large roll-over anticline, with a depression at the point where the structural crest should have been. The

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Fig. 7. Reflection intensity map produced over a 50 ms interval below seabed in Area 1. Dark colours represent high intensity and light colours low intensity. Sediments interpreted as debris flows are seen on the downslope side of the mud volcano.

four mud volcanoes seen on the seabed dip map project vertically down to a mapped closure in a circle around this depression (Fig. 8). The roots of mud volcanoes 5 and 8 are believed to correspond approximately to the mapped level in Fig. 8, whilst numbers 6 and 7 appear to be sourced from a slightly shallower level. The seismic data over the entire closure and especially within the crestal depression is extremely chaotic and mapping of the area is difficult. Dipmaps produced on shallow levels in areas of less chaotic seismic have highlighted the presence of several additional calderas that are not seen on the present seabed. These are confirmed on time slices from the 3D seismic cube. The dipmap presented in Fig. 9 is made on a reflector 200–500 ms TWT below seabed. In addition to the four active mud volcanoes, one additional circular feature appears in the SE corner of the picture. The seismic data beneath this feature are completely chaotic and there are no surface amplitudes that may provide an alternative explanation for poor seismic imaging. Mud volcanoes 1–3 (Figs. 8 and 9) are even older mud volcanoes, identified as circular outlines on deeper maps and time slices. The mud volcanoes are numbered in age order. The oldest mud volcanoes are located at the centre of the structure, with younger generations progressively shifting toward the flank. Mud volcano 3 is the same as the abandoned mud volcano seen in Fig. 4b. Eight mud volcanoes have been mapped confidently in this area, however, since the seismic definition is poor at the structural crest, there may be additional ones. Fig. 10 shows an attempt to capture the interplay between structure and mud volcano evolution through time. The deeper section (6–8 Ma) was deposited in an

unconfined slope setting. Activity along a major down-tothe-basin fault, to the east of the study area, marked the onset of the roll-over anticline and a thick expanding sedimentary wedge was deposited to the east. Subsequent burial resulted in release of decompacted water and maturation of hydrocarbons, which migrated to charge the crest of the

Fig. 8. Simplified structure map of the rollover anticline in Area 1. Structural closure in grey. Solid circles are active mud volcanoes reaching seabed today. Hatched circles are various generations of abandoned mud volcanoes. Note the cluster of abandoned mud volcanoes in the depression at the centre of the structure. Line A-B shows tentative location of the schematic profile in Fig. 10.

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Fig. 9. Dip-map generated on a shallow reflector (200–500 ms below seabed). The four active mud volcanoes are seen as calderas (5–8). The arrow points to a recently abandoned mud volcano. The dotted circles are projected locations of the abandoned mud volcanoes seen in Fig. 8. Numbers are ascribed to sequence of activity. See also additional curved listric faults not reaching seabed.

structure. The first mud volcano is believed to have erupted from the structural apex, in the area that today represents a depression with poor seismic resolution. (Stage 1, Fig. 10). As a result of the eruption, or series of eruptions, a depression began to form around the root and the fluid/ hydrocarbon focus shifted to closures on the flanks. In the meantime, continuous sedimentation resulted in further subsidence and shallower reservoirs tapped into the plumbing system of the drainage basin. Hence new mud volcanoes formed on the flank closures of the abandoned system, but in stratigraphically shallower levels. As indicated in Fig. 10, stage 2, a set of listric faults formed on the up slope side to accommodate the material removed from the root. Eventually these mud volcanoes also exhausted their supplies and the decompaction water and hydrocarbon focus shifted to the edges of the expanding depression. Stage 3 in Fig. 10 represents what is considered to be the present-day situation. Several generations of mud volcanoes have through time removed solid material from the root to the surface and progressively created a depression over an area that, according to the structural model, should have been the crest of a rollover anticlinal. The older mud volcanoes are buried in the section in an area with extremely chaotic seismic. 5.3. Volume of erupted material Published figures for erupted volumes are normally based on surface calculations. An indirect approach has been

attempted on this dataset by estimating the total volume loss from the structure through time. By extrapolating the reflectors across the depression near the root zone, using a shape normally associated with a rollover anticline, a reconstructed map was generated (see hatched area in Fig. 10, stage 3). The volume difference between these maps is of the order of 9.7 km 3. Uncertainties in the extrapolation, compaction and the amount of fault activity, that could have taken place unrelated to mud volcanoes, are not accounted for in the calculation. However, the estimate is believed to give an indication of the magnitude of material removed through the life span of this structure. No attempts have been made to estimate volumes ejected from individual mud volcanoes due to insufficient data resolution. However, if the total removed volume is divided by the number of mapped mud volcanoes in the area (8), the average yield per mud volcano is 1.2 km 3. It is difficult to compare these figures with previously published estimates, since most authors have only calculated material from single eruptions. Earlier eruptions have generally been washed away by rainfall in onshore areas or dispersed and buried in submarine conditions. Prior et al. (1989) estimated the volume of a debris flow from a mud volcano in Gulf of Mexico to be 2-million m 3. Langseth et al. (1988) calculated 2 km 3 of extruded material from a mud volcano off Barbados. A total of 5 × 106 m3 material has been reported for one single eruption in Azerbaijan, with totals for the lifetime of a mud volcano up to 11.4 km 3 (Guliev 1992).

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Fig. 10. Schematic development of the Area 1 structure with tentative position of mud volcanoes projected into the line. Vertical lines represent mud volcanoes with stars indicating position of the root. Numbers refer to sequence of activity as indicated in Figs. 8 and 9. Hatched area in stage 3 represents the total volume of sediments ejected from the mud volcanoes.

6. Mud volcanoes in area 2 Mud volcanoes in the SW corner of this area have a different character from the Area 1 mud volcanoes. They are all clustered in a geographically restricted area and their seabed expressions are more irregular. The largest mud volcano (at the line intersection in Fig. 1) represents an aggregate of several smaller circular features with a common, complex root system. The others show a pronounced positive seabed relief (Fig. 11). A total of five active mud volcanoes are identified on the seabed dip-map. These mud volcanoes were not sampled by seabed cores, so lithology, age and hydrocarbon contents are not known.

Seismic resolution below the seabed is not as distorted as for the mud volcano in Fig. 4. Although the seabed amplitudes are also high in this area, no data wipe-out zone with multiples can be seen. The surface relief is, however, more pronounced. More importantly, a sedimentary wedge is seen at surface. This clearly defines a circular wedge as illustrated by the two crossing lines in Fig. 11. The entire section, from seabed down to 2500 ms, plunges into a trough-shaped depression, with an image directly analogous, both in scale and seismic expression, to a mud volcano seen on proprietary seismic from the Caspian Sea. Similar trough-shaped depressions are also seen on seismic examples of mud volcanoes from the Black Sea (Ivanov et al.

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Fig. 11. Seismic expression of a mud volcano complex in the SW corner of the 3D area (see Fig. 1 for location). Note the sediment wedge at surface, the caldera and chaotic reflectivity from 2.5 to 3.0 s. TWT. The N–S oriented line b shows two mud volcanoes of which the southernmost has a pronounced seabed relief.

1989). These structures are, according to the authors, widely recognised on seismic data in Azerbaijan where they are called “depression synclines”. Mud volcanoes with large seabed relief are also seen in the Gulf of Mexico (Roberts & Carney, 1997). Vertically beneath the seabed anomaly, from 2500 ms and deeper, the seismic data are completely chaotic with no coherent reflectivity at all. This seismic character is believed to be the root system of this mud volcano and corresponds to a mapped focus of a shale diapir. The seismic character changes away from the root zone through a transition zone to a time equivalent section with fairly clean and coherent reflectivity. The section from the seabed and down to 2500 ms TWT is interpreted as a bypass zone through which ejected material has been transported. The collapse of section into the vent is thus believed to be the result of collapse after the eruption was completed, partially to compensate for material removed from the vent, but also to accommodate for the volume that has been displaced at depth. The wedge at the surface is assumed to be ejected material. Even though only a minor positive relief can be seen at surface, the seismic

data indicate that a significant volume can be accounted for by subsidence into the caldera. A distinct positive seabed relief can, however, be seen on the N–S line in Fig. 11b to the south of the large mud volcano. Listric faults are not as abundant near these mud volcanoes as in Area 1 (Fig. 1). The concentric faults, however, are clearly seen. These appear to be better developed on the up-slope side. The seabed dip-map (Fig. 1) indicates the presence of several circular seabed features, within the larger circle of this mud volcano. By analogy to the Azerbaijan terminology, these may represent salses and gryphons, which are known as parasitic features to the master system. 6.1. Suggested model A growing shale diapir deforms the rocks in its core to a ductile shale mass that loses its initial cohesive strength and also exerts a continuous stress to the overburden, which deforms through a series of crestal collapse faults. The combined effect of ductile deformation, weak overburden and a focus of migrated water and hydrocarbons make the

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crest of the shale diapir a natural spot for mud volcanoes to form. Since the mechanical strength of the overburden is likely to be lower than elsewhere, the threshold pressure for eruption should also be lower. It is therefore proposed that this type of mud volcano erupts more frequently than the explosive ones, is less violent, and probably also carries a larger portion by ductile flow, hence also a higher fraction of shale breccia. This set of circumstances may explain the seabed relief seen on the mud volcano in Fig. 11b. The volume between the dotted line and seabed in Fig. 11 is calculated to be 1.0 km 3. Mud volcanoes have been described from Azerbaijan that erupt mostly through ductile flow with only minor release of gas (Guliev, 1992). This type of eruption suggests that seal failure of a gas-charged reservoir may not always be the most important mechanism. However, since gas seems always to be associated with the eruption of mud volcanoes, it is suggested that the gravitational buoyancy of gascharged muds is an important mechanism, as also suggested by Hedberg (1974), Yassir (1989) and Hovland et al. (1997). Less violent eruptions allow for a better seismic resolution near the vent of these mud volcanoes, supporting the theory of ductile deformation rather than explosive damage.

7. Discussion The majority of reported mud volcanoes occur at active plate margins, such as the Alpine–Himalayan trend and the Pacific plate boundaries (Higgins & Saunders, 1974). This association has led many authors to suggest that tectonic stress, mainly compressional, is the main driving mechanism for mud volcanoes (Birchwood, 1965; Goubkin, 1934; Higgins & Saunders, 1974; Khalilov & Kerimov, 1981; Mekhtiyev et al., 1985; Sokolov et al., 1968; Yassir, 1987, 1989). They suggest that tectonic stress, mainly in compressional regimes, deforms the rocks into folds and thrusts, associated with a significant horizontal shear along the structural axis. Tectonic stress causes an increase in pore pressure, with a consequent decrease in effective stress of the sediments. The sediment structure eventually breaks down associated with a dramatic loss of strength and the sediment flows in a ductile manner towards areas of less pressure, usually at the crests of folds and diapirs. Where mud volcanoes occur in areas dominated by a compressional regime, they usually show a linear distribution corresponding to anticlinal trends (Ali-Zade et al., 1984; Banerjee, 1975; Barber et al., 1986; Higgins & Saunders, 1974; Treves, 1985). The intimate relationship between methane and mud volcanoes is also acknowledged by the same authors, but although contributing to overpressure and sediment buoyancy, and possibly also to the mobility of sediments, methane is said not to be sufficient on its own to drive the formation of mud volcanoes. The presence of mud volcanoes over diapirs and extensional structures both in

the Gulf of Mexico and Nigeria, suggests that compressional tectonics alone cannot explain the genesis of all mud volcanoes. The role of methane was given more emphasis by AliZade et al. (1984), Guliev (1992), Hedberg (1974), Langseth et al. (1988), Prior et al. (1989) and Reed et al. (1990). The largest mud volcanoes in Azerbaijan are generally the most violently explosive ones, with emissions of large quantities of gas. It has been estimated that one eruption released 495million m 3 of gas. The gas plumes reach heights of hundreds of meters and usually ignite spontaneously. An estimated average of 20-million m 3 gas is emitted in the quiet salse/ gryphon stage per year in the Azerbaijan area. By including the gas expelled to atmosphere during the eruptions as well, the estimated annual average for the area reaches an astonishing 350 × 106 m3 (Ali-Zade et al., 1984; Guliev, 1992; Jakubov et al., 1971; Sokolov et al., 1968). Sokolov et al. (1968) estimated the average eruption periodicity of mud volcanoes in Azerbaijan to be of the order of 50–60 years. Hence, when millions of m 3 of gas are released through violent eruptions, such as those occuring in Azerbaijan, a reservoir must be present to store the gas. Thus, it is concluded that the presence of a reservoir is required to explain the numbers and periodicity of mud volcanoes observed in deepwater Nigeria, especially the gaseous explosive ones (class 1 of Guliev, 1992). Ali-Zade et al. (1984) stated that mud volcanoes are the natural agents through which the Earth’s interior is being degassed and that the periodicity is directly related to gas charge. This interpretation is a fascinating description that may also fit with the observations in this study, in particular for those in Area 1. The culmination of the rollover anticline taps into a huge drainage basin to the NE, which is likely to have expelled hydrocarbons and compaction water for a long time. The entire culmination can therefore be described as a giant “pressure valve” through which excess pressure is released at periodic intervals. Some oil will probably always be associated with the eruptions, as seen from the core samples, but the rheology of the oil is probably not favourable for large quantities to flow during the short bursts of the eruptions. The focussing of decompaction water is believed to be an important contributor to high formation pressures, in many cases the most important. Focussed water could possibly also cause seal fracture and leakage, but it is unlikely that water alone would penetrate a vertical section of more than 1 km. Because of the near zero compressibility of water, combined with the hydraulic flow properties, leakage would cause an immediate pressure drop when small amounts of water escapes, causing the seal to heal. It is believed, therefore, that there is not sufficient explosive power in water alone to account for the vertical penetration of mud volcanoes. Due to the apparent lack of compressional forces in this area, horizontal tectonic stress is rejected as a mechanism for these mud volcanoes. The main drive mechanism is

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believed to be due to the trapping and focusing of hydrocarbons and decompacted water. It is therefore proposed that the Area 1 mud volcanoes belong to the gaseous explosive end member, and the area two mud volcanoes are characterised by ductile flow and would fall into class 2 or 3 of the Guliev (1992) classification. Neurauter and Bryant (1990) indicated that a similar range is present in the Gulf of Mexico, from gaseous explosive mud volcanoes to plastic flow from mud diapirs. Based on the large differences observed in shape, size and eruption styles of the various mud volcanoes, it is clear that there is no unique model that can explain them all. Mud volcanoes probably owe their existence to a set of kinetic variables of which the relative importance changes from one province to another and also within an area. The mud volcanoes in deepwater Nigeria belong to a passive margin setting such as those described from deepwater Louisiana. Furthermore, the mud volcanoes in Area 1 overlie a rollover anticline, with no apparent compressional tectonics. A set of variables that needs to be present in combination with at least another are suggested: 1. Rocks with low mechanical strength. Usually provided through rapid deposition with associated under-compaction and low degree of consolidation. 2. Compressive tectonic stress, to provide ductile deformation and increased pore pressures of shaly sediments. 3. Structural culminations that focus decompacted water and hydrocarbons. 4. Carrier beds for migrating fluids and gases.

8. Conclusions Seabed cores taken from some 1–2 km diameter seabed craters in deepwater Nigeria contain exotic clasts, Pliocene faunal assemblages, 3–8% sand and significant quantities of live oil and gas. Cores taken from the seabed background areas contain 100% clay and minor quantities of recent organic material. These differences indicate that both liquid and solid materials have been brought to seabed from subsurface. The seabed anomalies are thus interpreted to be mud volcanoes. Two types of mud volcanoes are described from the study area. Four active ones are located over a roll-over anticline (Area 1) and five are located over a shale diapir (Area 2). The seismic data in Area 1 show a relationship between mud volcanoes and poor seismic imaging, particularly at around 2–3 s TWT where the root-systems are believed to be located. A gradual data improvement is seen some 2– 3 km away from the centre. The mud volcanoes are associated with large listric faults that reach the seabed on the up-slope side and terminate directly underneath the root. It is proposed that the material removed during eruptions has triggered activity along these faults to compensate for the

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volume loss. The circular calderas seen at the surface were formed when the eruption trailed off and material collapsed into the vent. The shallowest 1–2 km of the overburden, regarded as a bypass section, express such a caldera collapse. Lack of seabed cones at the surface indicates that relatively small amounts of ejected material have been deposited near the vent. It is hence believed that these mud volcanoes are of an explosive origin and that most of the material has been ejected into the water column and transported down-slope as turbidites and debris flows. Gas under pressure, particularly methane, is proposed to be the main driving force, due to its expansion when subject to lower pressures. The chaotic seismic seen near the root is believed to result from the shock during eruption, which probably had the same effect to the rocks as an implosion. Four active mud volcanoes are located at seabed in a vertical position above the regional culmination of a rollover anticlinal. A depression is formed at the centre of the structure, in an area where the seismic character is extremely chaotic and where at least four abandoned mud volcanoes of various generations have been identified. It is therefore proposed that this depression is formed by repetitive activity of mud volcanoes in the area, creating a depression through their activity and causing hydrocarbon focus to be shifted towards flank closures. The total amount of ejected material during the lifetime of the structure has been estimated to be 9.7 km 3. The repetitive nature and long-lived record of mud volcanoes in this area indicate an efficient plumbing system and charge of compaction water and hydrocarbons. The mud volcanoes in Area 2 are located above a shale diapir to the SW in the study area. The largest of these show several active outlets within a 3 km large circular caldera. The four other mud volcanoes in the immediate vicinity express a 100–200 ms TWT seabed relief with steep flanks. It is proposed that these mud volcanoes are less violent and erupt more frequently through ductile flow of mud, water and gas. The underlying diapir has, through vertical growth, deformed the rocks and reduced the integrity of the overburden, hence reducing the threshold pressure required to breach the seal. Expansion of gas may play a role also during eruption of these mud volcanoes, but is probably inferior to the mechanical stress component provided by the diapirism.

Acknowledgements The author would like to thank the managements of Statoil (Nigeria) Ltd, Texaco Outer Shelf Limited and the Nigerian National Petroleum Corporation for permission to publish this paper. A special thanks to Martin Hovland for constructive discussions during the initial stages and for later reviews. I also thank Anthony Spencer, Richard Seaborne, Stephen Hay, Olujide A. Ojo and Phil Townsend for critically reviewing this manuscript. The views

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