- Email: [email protected]
Multiple resolution seismic imaging of a shallow hydrocarbon plumbing system, Woolsey Mound, Northern Gulf of Mexico
Marine and Petroleum Geology 38 (2012) 128e142
Contents lists available at SciVerse ScienceDirect
Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo
Multiple resolution seismic imaging of a shallow hydrocarbon plumbing system, Woolsey Mound, Northern Gulf of Mexico Leonardo Macelloni a, *, Antonello Simonetti b, James H. Knapp b, Camelia C. Knapp b, Carol B. Lutken a, Laura L. Lapham c a b c
Mississippi Mineral Resources Institute, 111 Brevard Hall, University of Mississippi. University, MS 38677, USA Department of Earth and Ocean Sciences, University of South Carolina, Columbia, SC, USA Chesapeake Biological Lab, University of Maryland Center for Environmental Science, P.O. Box 38, 1 Williams St., Solomons, MD 20688, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 October 2011 Received in revised form 22 May 2012 Accepted 7 June 2012 Available online 24 July 2012
The northern Gulf of Mexico is dominated by salt tectonics, resulting fracturing and numerous seafloor seeps and vents. Woolsey Mound, site of the Gulf of Mexico Hydrates Research Consortium’s seafloor observatory, has been investigated extensively via surveys, direct sampling and seafloor instrument systems. This study presents an innovative approach to seismic data interpretation, integrating three different resolution datasets and maximizing seismic coverage of the complex natural hydrocarbon plumbing system at Woolsey Mound. 3D industry seismic data reveal the presence of a salt body at in the shallow subsurface that has generated an extended network of faults, some extending from the salt body to the seafloor (master faults). Higher resolution seismic data show acoustic wipe-out zones along the master faults with expulsion features e seafloor pockmarks and craters e located immediately above them and associated, in the subsurface, with high-amplitude, negative anomalies at constant depth of 0.2 s TWTT b.s.f., interpreted as free gas. Since pockmarks and craters provide pathways for hydrocarbons to escape from depth into the water column, related sub-surface seismic anomalies may indicate free gas at the base of the gas hydrates stability zone (GHSZ). Fluid flow and gas hydrates formation are segmented laterally along faults. Gas hydrates formation and dissociation vary temporally in the vicinity of active faults, and can temporarily seal them as conduits for thermogenic fluids. Periodic migrations of gases and other fluids may perturb the GHSZ in terms of temperature and pressure, producing the observed lack of classical BSRs. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Hydrocarbon plumbing system Gas hydrates Amplitude anomalies Seismic resolution
1. Introduction Naturally occurring gas hydrates have been documented in continental slope sediments worldwide. In temperate climes gas hydrates form naturally in water depths greater than a few hundred meters in a zone extending from the seafloor to sub-bottom depths determined primarily by temperature gradient, pressure regime, pore-water salinity, and availability of hydrocarbon gases (Ruppel et al., 2005). Gas hydrates research is particularly important in the northern Gulf, a major oil and gas exploration/exploitation province, because drilling is now routinely conducted on the continental slope. Drilling through gas hydrates presents hazards concerns related to slope stability, well safety, pipelines, and platforms (Hovland and Gudmestad, 2001), making it critically important to establish their distribution. On the Gulf’s continental * Corresponding author. Tel.: þ1 662 915 7320; fax: þ1 662 915 5625. E-mail address: [email protected] (L. Macelloni). 0264-8172/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.marpetgeo.2012.06.010
slope, faults and fractures, radiating from salt bodies, produce natural conduits that facilitate migration of hydrocarbon fluids from deep oil reservoirs into the Hydrates Stability Zone (HSZ). Natural hydrocarbon seepage is so common that the seafloor is populated by a profusion of mounds and seeps (Shedd and Roberts, 2009), where hydrates outcrop at the seabed. Furthermore, salt produces a high, laterally variable geothermal gradient that perturbs the thermobaric stability field, making the HSZ difficult to define (lack of BSRs, presence of multiple stability boundaries, etc.). Woolsey Mound, Mississippi Canyon Lease Block 118 (MC118), has been designated by the Bureau of Ocean Energy Management (BOEM) as the Gulf’s only Research Reserve and the site of a Seafloor Observatory (Fig. 1) because it represents a mature example of a hydrates/carbonates complex with a fault migrationconduit system overlying shallow salt, directly connected to deeply-buried source rocks for gas and oil (Sassen et al., 2006; Knapp et al., 2010). Submersible missions to the seafloor have identified large gas hydrates outcrops, authigenic carbonate
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
129
Figure 1. The sea floor observatory at MC118. The cartoon illustrates the sensors and devices being installed at Woolsey Mound (MC118). PFA Pore Fluid Array, VLA Vertical Line Array, HLA Horizontal Line Array, CSA Chimney Sampler Array, FOC Fiber Optic Cable, AUV Autonomous Underwater Vehicle, MSR Multibeam Sonar Rotator, RCS Rotating Camera System, BBLA, Benthic Boundary Layer Array, IDP Integrated Data Power unit.
mounds, gas vents, and chemosynthetic communities, including ice worms (Sassen and Roberts, 2004; Woolsey et al., 2005). In order to define a geological and geophysical model of the mound’s subsurface and characterize the underlying plumbing system, an innovative approach has been adopted through the integration of three different resolution seismic datasets: an oil exploration 3D Volume collected in 2000, a pseudo-3D high resolution surface-source deep-receiver (SSDR) single channel seismic dataset collected in 2006, and a 2D very high resolution autonomous underwater vehicle (AUV)-borne chirp dataset, collected in 2005. The penetration depth and vertical seismic resolution, different for each dataset, span from thousands of meters (penetration) and 50 m (vertical resolution) to 50 m and a few centimeters resolution. The SSDR system, which images up to hundreds of meters below the seafloor with a vertical resolution of about 1 m, bridges the gap between the 3D volume and chirp data, exactly where the gas hydrates stability zone is present (Macelloni et al., 2011). This multi-scale approach better defines the upward thermogenic gas-fluid migration process, including transiting a seafloor mound and the HSZ. Here, we present the qualitative/comparative analysis of the three seismic datasets to: 1) define the source of hydrocarbon fluids; 2) identify fluid-flow pathways; 3) understand the structural controls and geological mechanisms of fluid migration within the HSZ; 4) formulate hypotheses on the temporal evolution of the system.
2. Site location MC118 is located offshore approximately 150 km south of Pascagoula (Ms) and 100 km east of the Mississippi Canyon in w890 m of water (Fig. 2). It is sited on the eastern flank of the main Mississippi Canyon, in a gently seaward dipping portion of the continental slope. A fault-controlled canyon, flanking the Whiting dome and a slamp structure to the east is the only relevant morphological feature present in the area, as shown in Figure 2. Extended discussions of the MC118 complex system are given in Sassen et al. (2006), Lapham et al. (2008), Ingram et al. (2010) and Macelloni et al. (2010). Salt domes in the nearby shallow subsurface appear to dominate the MC118 upper slope as revealed by the bathymetric map (Fig. 2). Visible outcrops of gas hydrates, faulted carbonate “hard-grounds” and pockmark features, cover approximately 1 km2 of the seafloor (e.g., Sassen et al., 2006; Sleeper et al., 2006; Macelloni et al., 2010). The seaward slope across the study area typically ranges from 3 to 4 , but slopes of 10 e12 are present locally across the pockmark. The supply of hydrocarbons (natural gas and petroleum) to the seafloor supports an active biological seep community and microbial chemolithotrophy in the immediate vicinity of active gasefluid seepage (Macelloni et al., 2010). The mound has been subdivided in macro areas, and following the description of Macelloni et al. (2010), we recognize, the south-east complex, the north-west complex, and the south-west complex.
130
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
Figure 2. Geographic location of MC118. Woolsey Mound is located in 900 m water depth on the northern continental slope of the Gulf of Mexico. The slope here is highly discontinuous, intersected by slumping, folding and faulting mainly due to the tectonics derived from the underlying salt and the sediment load delivered by the Mississippi River.
3. Data and methods The MC118 hydrates mound subsurface has been imaged using three different seismic datasets: a high quality 3D Volume from TGS-Nopec; a Surface-Source Deep-Receiver (SSDR) single channel system pseudo 3D volume and an AUV-borne Chirp Sub-bottom Profiler. Survey geometries, seismic data specifications, acquisition parameters and digital signal processing flow, for each dataset are given in Appendix A.
These three datasets embrace a wide spectrum of penetration depths and vertical resolutions, providing an innovative multiscale imaging of the geological setting beneath the hydrates mound (Fig. 3). The initial step in data analysis was the creation of a fully integrated Kingdom Suite 8.3 seismic interpretation Project. Accurate spatial integration of the multiple datasets required calibration of the reflection points’ spatial distribution and normalization of the dataset seafloor reflection time. The first was accomplished via statistical analysis of misfits among cross points
Figure 3. Multiple resolution seismic data set. Woolsey Mound at MC118, has been investigated using three different datasets: high quality 3D oil industry multichannel seismic data from TGS-Nopec (3D Volume); surface-source deep-receiver (SSDR) single channel system pseudo-3D volume and AUV-borne CHIRP sub-bottom profiler. Their different vertical resolutions provide a multi-scale image of the sub-surface beneath the mound (Credits: 3D Volume TGS-Nopec).
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
and the second, by referring all the seafloor reflection point times to the AUV seafloor bathymetry. Further, in order to provide evidences and visual control of possible hydrocarbon leakage at the seafloor, high-resolution mound bathymetry and reflectivity as well as the spatial distribution at the seafloor of the bio-geological facies, commonly related to hydrocarbon seeping and venting (i.e. bacterial mats, authigenic carbonate, chemosynthetic community, etc.), as presented by Macelloni et al. (2010), have been integrated into the seismic volume (Fig. 4). 3.1. Seismic data interpretation Two main targets were identified in our seismic interpretation, (1) to reconstruct the general geological setting of the mound subsurface, and (2) to image the architecture of the plumbing system and related leakage structures. For target 1 we have carefully picked all of the seismic horizons (with particular effort to trace seismic horizons that represent the same stratigraphic level in different datasets) and the first order faults. For target 2, instead, we have first defined the leakage-related seismic anomalies (i.e. discontinuity zones, wipeout zones, bright spots and phase reversals) adopting the classification of Løseth et al. (2009). We then mapped the spatial distribution of these anomalies across the mound. This time-consuming process was
131
essential to map and geometrically define the geological and structural framework of the mound subsurface. 4. Results and discussion 4.1. Geological setting and hydrocarbon source Seismic data interpretation reveals a scenario dominated by a salt body (Fig. 5) whose top extends to 0.6 s TWTT beneath the seafloor (b.s.f.). Salt tectonics generates an extensive network of faults and fractures; however, only few directly connect the salt body with the seafloor carbonate/hydrate mound. We define these faults as “master faults” and color code them in Figure 5 as blue, red, yellow and magenta faults. The “master faults” are normal faults, the yellow and magenta dip west, the blue north-east, and the red south-west. All are rooted in the salt diapir’s upslope side and appear to originate very deep in the stratigraphic column (the 3D volume is truncated at 3 s). The main mound craters (south-east, north-west and southwest) form where the “master faults” intersect the seafloor. In particular, the south-east crater complex lies at the top of the yellow master fault, the south-west crater complex is related to the magenta master fault, and the north-west complex coincides with the top of the blue and red master faults (Fig. 5). Features related to hydrocarbon leakage are present at the seafloor and include pockmarks, gas hydrates, authigenic carbonates, active vents and
Figure 4. Woolsey Mound seafloor bio-geological processes. Seafloor back-scatter map, relevant biological habitat and sediment distribution of the mound. A) Undisturbed pelagic mud typical of the mound’s periphery. B) Area covered by chemoautotrophic clams; C) authigenic carbonate nodules; D) bacterial mat (Beggiatoa sp.); E, F, G area paved by authigenic carbonate crusts and slabs; H) gas venting from the seafloor; I) the sleeping dragon, the largest complex carbonate hydrates outcrop ever imaged on the seafloor; L) hydrate dike (Macelloni et al., 2010; modified).
132
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
Figure 5. Salt dome and master faults from 3D standard seismic data. The south-east crater complex lies atop the yellow master fault and the south-west crater complex is related to the magenta master fault, while the north-west complex lies above of the blue and red master faults (Credits: 3D Volume TGS-Nopec).
chemosynthetic communities (Figs. 4 and 5). Geochemical analyses from gas hydrates samples outcropping at the mound surface (Sassen et al., 2006), revealed high percentages of propane (15.9%) and ethane (7.5%), suggesting a thermogenic source. Further proof of a deep source for hydrocarbon gases at the site has been provided, just recently, by d13C isotope analysis (Chanton et al., 2011). 4.2. Faults system The master faults are the relevant tectonic motif detectable in the industry dataset, and clearly show the close relationship between salt tectonics and mound formation; it is not a coincidence that the main craters occur where master faults intersect the seabed. However, a much more complex system of faults emerges from the study of the higher resolution seismic data (SSDR and Chirp). In fact, carefully picking all the horizons small offset in these two seismic datasets enabled us to map a fine-scale faults pattern exhibiting radial orientation and reflecting upward movement of the underlying salt body (Fig. 6). We suggest that this radial faultefracture complex is an integral part of the plumbing system at MC118 and two evidences prove it. The first one is presented in Figure 7: in the southern part of the mound, in an area distant from the main morphological structures, we have imaged a fault which appears, in the 3D Volume data, to be blind; however, in the SSDR data, this fault can be seen to continue to displace sediment and in the very high resolution CHIRP data is shown to reach the seafloor. On the multibeam bathymetric image, a satellite pockmark is present above this fault. Another example of the complexity of the plumbing system recorded in the SSDR data is illustrated in Figure 8. Two north-south seismic profiles (3D Volume and SSDR) are positioned off the mound on the west side. The SSDR profile depicts a system of small-scale faults dislocating a gas charged horizon (see paragraph 4.3.3), which appears continuous and unbroken in the 3D Volume lines. SSDR data also show several acoustic wipeout zones (4.3.2) in the proximity of the faults evidence that gas is being diffused into the sediments above. 4.3. Seismic anomalies Once the fault network was established, effort was focused on defining seismic anomalies indicative of hydrocarbon leakage. We
used mainly the SSDR data because they show clearly two different vertical anomalies: the discontinuity zone and the wipeout zone (sensu Løseth et al., 2009). 4.3.1. Discontinuity zones The discontinuity zones are defined by Løseth et al. (2009) as areas on the seismic section where reflections are more discontinuous than in adjacent areas and can be interpreted as gas migration pathways through sediments (generally fine-grained). SSDR data present dense patterns of such a discontinuity zone (Fig. 9a). We observe that the spatial distribution of a majority of them follows the faults previously identified on the SSDR data (Fig. 6). Therefore, the discontinuities provide seismic evidence of
Figure 6. Small scale faults network from SSDR data. The tectonic framework beneath the mound is very complex; however, meticulous interpretation of high-resolution data reveals a radial fault complex (shown in red) that is undetectable on 3D Volume (Credits: 3D Volume TGS-Nopec). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
133
Figure 8. Imaging fault displacement at different seismic resolutions. A small-scale network of active faults is a vital component of the hydrocarbon plumbing system, and serves as a hydraulic connection between stratigraphic reservoirs. On the western part of the mound, away from the main seafloor morphological unit, 3D Volume data show the continuous and unbroken 0.4 s TWTT bright spot, that, in the SSDR data is highly segmented with numerous degassing features (Credits: 3D Volume TGS-Nopec).
Figure 7. Imaging fault displacement at different seismic resolutions. The integration of multiple-resolution datasets has been key to image the real tectonic regime. In fact, comparing one particular fault across the three datasets, mapped on a line located in the southern portion of the mound, has shown that the fault, which appears to terminate (blind) at 1.30 s TWT in the 3D Volume, continues up to 1.240 s TWT in the SSDR and even reaches the seafloor in the chirp data (Credits: 3D Volume TGSNopec).
sediment deformation accommodating the numerous faults. Alternatively, these discontinuity zones may represent upward migration conduits for gas and fluids through an area of lithological weakness. However, this latter interpretation is still hypothetical in the absence of additional hydrocarbon indicators, though the two processes can coexist.
4.3.2. Wipeout zone A wipeout zone is an area on the seismic section where the reflections from stratigraphic layers are deteriorated such that the primary reflections are either absent or very weak (Løseth et al., 2009). In our data, they mainly occur along faults and directly underneath seafloor pockmarks and craters (Fig. 9b) since these features are generally formed by intermittent escape of fluids or gas into the water-column and by violent episodic gas blowouts (Hovland and Judd, 1988; Løseth et al., 2001). This anomalous seismic pattern results from processes that promote mobilization of fluids through layers, and may also indicate, areas where upward migration of gas-fluids occurred (Hovland and Judd, 1988; Løseth et al., 2001; Gay et al., 2007; Huuse et al., 2010). 4.3.3. “Bright spots” polarity reverse In addition to vertical seismic anomalies, other anomalies have been identified in the MC118 sub-surface. They are high-amplitude
134
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
Figure 9. Seismic discontinuity zone and wipe-out zone. 3D Volume data show remarkable lack of detail when compared to the high-resolution SSDR data. The latter show a dense pattern of pipe-like discontinuity zones, which in literature are often referred as hydrocarbon conduits. The spatial distribution of these anomalies seems to follow the radial fault complex. Since the occurrence of discontinuity zones may arise from a loss of signal coherence close to the fault planes, they do not necessarily indicate hydrocarbon conduits. In the absence of other indicators, we interpret them as potential hydrocarbon migration pathways, b) SSDR data also highlight how the seismic signal underlying seafloor pockmarks and craters is always wiped out or extremely chaotic. Since pockmarks and craters are generally formed by gas/fluids escaping into the water column, the wipeout zones can be interpreted as the outcome of these upward hydrocarbon migration processes, which produce sediment mobilization through the sedimentary layering. Hence, the wipeout zones must represent areas where hydrocarbon transit is more likely than in discontinuity zones (Credits: 3D Volume TGS-Nopec).
“bright spots” (Allen and Peddy, 1993), in some cases with evident phase reversal (or polarity reversal with respect to the seafloor reflection). Bright spots are very clear on both the 3D Volume and the SSDR (Fig. 10). In both datasets, the bright spots occupy the same stratigraphic levels and, despite the differences in seismic systems and signal frequencies, they present very similar seismic signatures (Fig. 10). The bright spots are always associated with a distinctive seismic facies which is characterized by (1) moderate/high amplitude, (2) moderate/high continuity, and (3) presence of several lenticular and concave-up geometries (Fig. 11). We interpret this facies as sandy lobe complexes or partially channelized e turbidite lobe complexes, according to the seismic facies model proposed by Galloway and Buffler (2004). The first one occurs at about 0.4 s TWTT b.s.f. (BS-1) at the top of the above mentioned sandy facies; the second one occurs at shallower depth, at about 0.2 s TWTT b.s.f. (BS-2) and is related to smaller sand bodies top-sealed by low permeability hemipelagic sediments and laterally bounded by fault planes. We interpret the bright spots as seismic evidence of free gas accumulation in the sandy sediments (Fig. 12). The two gas horizons are in hydraulic communication, via the faults. In fact we observe that the gas is mainly trapped in the 0.4 TWTT sand deposits, and only when the faults intersect this horizon, the gas is able to migrate upward and charge the shallower sand deposit (Fig. 12). If we display the planar distribution of the BS-1 and BS-2 (Fig. 13), it is possible to observe that the deeper one
(Fig. 13a) occupies a larger area than the shallower. It has two main sacks: one located off the mound to the north-west and the other beneath the mound, slightly north the south-east complex (Fig. 13a, box 1 and 2). The shallower gas horizon, instead, is geographically localized beneath the mound including beneath the two peripheral pockmarks in the north and in the south (Fig. 13b). The BS-2 is discontinuous and, not homogenously present beneath the mound; it is strong beneath the north west-area, moderate beneath the south-west, but almost absent beneath south-east (Fig. 13b). Macelloni et al. (2010) have found that mound bio-geological processes are completely different in the three complexes; in fact they have seen that the south-east complex appears to be an extinct or quiescent vent, the south-west a mature and well established vent, while the north-west appears to be a young vent with more vigorous seeping. The seismic data confirm that hydrocarbon flux activity must vary within the mound and that it is controlled by the master faults and by the dynamic of the 0.2 TWTT b.s.f. gas horizon. In fact, since pockmarks and craters are clear evidence for episodic gas and fluid expulsion into the water column, we have to conclude that the gas originating the bright spot, is trapped by a sealing mechanism different than the difference between the lithology (porosity) of the sandy deposit and the overlying sediments and, most importantly, a time variant mechanism. Otherwise the gas would vent continuously at the seabed without accumulating in the subsurface and forming the seismic anomaly.
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
135
Figure 10. Synoptic image of seismic bright spots between TGS and SSDR data. Reverse polarity bright spots are imaged, clearly, in both 3D Volume and SSDR data. Although the two datasets were collected with different parameters (Appendix A), the seismic anomalies share the same stratigraphic position and a very similar signature.
4.4. Hydrates stability zone thermobaric model We speculate that the shallower gas anomalies (BS-2) indicate the base of the GHSZ, a transient thermobaric boundary over which gas hydrates may form and act as a temporary seal for the underlying
free gas. The transient thermobaric boundary represents a very peculiar bottom simulating reflector (BSR), discontinuous and segmented along the base of the hydrates stability zone. Such peculiar BSRs have been hypothesized by Wood et al. (2008) and by Shedd et al. (2012) for the Gulf of Mexico region, which generally
Figure 11. Sandy lobe complexes or partially channelized e turbidite lobe complexes. Bright spot anomalies are associated with a peculiar seismic facies characterized by 1) moderate/high amplitude, (2) moderate/high continuity, and (3) presence of several lenticular and concave-up geometries. We interpret this facies as sandy lobe complexes or partially channelized e turbidite lobe complexes, according to the seismic facies model proposed by Galloway and Buffler (2004).
136
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
Figure 12. Hydraulic connection between BS-1 and BS-2. The BS-1 and BS-2 gas horizons are in hydraulic communication, via the faults system. In fact we observe that the gas is mainly trapped in the 0.4 TWTT sand deposits, and only when the faults intersect this horizon, the gas is able to migrate upward and charge the shallower sand deposit.
shows a paucity of a classic BSR. To support this hypothesis we have derived a possible thermobaric model of the hydrates stability zone at MC118. In first approximation we used the temperature data from the only well available in the area (Arco Oil and Gas archives, well position is reported in Fig. 13), the hydrates chemical composition analysis from the samples recovered at the seafloor and the program CSMHYD, by Sloan (1998). The Arco well totally bypassed the upper sediment cover and the first temperature data is 25 C at 1234 m below seafloor. Below this point, temperature increases at a rate of 17.2 C/km (R2 ¼ 0.96), and the value is similar to other thermal gradients reported in the Gulf of Mexico (Milkov and Sassen, 2000). Hydrates at MC118 is found to be 70% CH4, 7.5% ethane, 15.9% propane, 4.4% i-butane, and 4.4% n-butane (Sassen et al., 2006). Using the CSMHYD program, the above mentioned thermal gradient, the hydrates composition and assuming a salinity of 3.5% (seawater) we
have derived hydrates equilibrium pressures. The results were used to derive the PT plot in Figure 14A. At MC118, the depth of hydrates stability is 1167 m below seafloor. These results, however, apply only to the locations of the well, that is located some distance from the mound in an area moderately affected by salt tectonic. Adopting the same geothermal gradient and hydrates compositions, we have tried to extend the thermobaric model at the mound, using the geophysical evidence to locate the depth of salt. Therefore we calculated the hydrates stability zone assuming a salt gradient from 3.5% seawater salt at the seafloor surface to 100% saturated salt at the depth of the top of the diapir or, more specifically, at the top of a possible cap rock, which is lithologically different from the salt but 100% saturated. Results from this model are displayed in Figure 14B and C; where we found that, due to the salt effect, the stability moves up between 185 m and 295 m b.s.f. This window in the depth of the
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
137
Figure 13. Horizontal distribution of the BS-1 and BS-2. BS-1 (a) and BS-2 (b) planar distribution map displayed as Root Mean Square (RMS) of the manually picked negative amplitude reflections. The two gas horizons have different distributions in the mound subsurface. BS-1 is wider and mainly localized in the north-west, while BS-2 is present mainly beneath the mound. Note that BS-2 is discontinuous and segmented, with a strong presence beneath the north-west crater, while it is moderate and almost absent beneath the south-west and south-east craters respectively.
stability zone is due to the approximation in locating the cap rock on seismic data, and to the uncertainty of the p wave velocity in the shallow sediments. These depths are slightly higher than the depth of the shallower gas horizon which is, assuming a velocity of 1500 m/ s about 150 m b.s.f, though the two depth are comparable. Nevertheless, in the mound we expect a thinner hydrates stability zone (even closer to the seafloor) than the thermobaric model predicts, because advecting warm hydrocarbon fluids, transiting along the faults, is expected to perturb the P-T boundary defining the hydrates stability zone upward (Wood et al., 2002). 4.5. Transient hydrates system The amplitude anomalies associated with seafloor pockmarks and craters and with the master faults, support the hypothesis that hydrates formation and dissociation is significantly controlled by the dynamic of the BS-2 and by the fluids migration along the fault. In particular, hydrocarbons migrating upward from deep sources perturb hydrates stability in terms of pressure and temperature and promote dissociation. Further, such processes provide supplies of hydrocarbon gases into the GHSZs. This hypothesis is supported by the comparison of some of these pockmarks/craters-related gas anomalies, that are present on the 3D Volume data (collected in 2000) but is absent on the SSDR data collected in 2006 (Fig. 15). The two datasets have completely different acquisition parameters and signal spectra (Appendix A), however in Figure 10, it has been shown that the bright spots, associated with gas horizons, occupy the same stratigraphic position and display a very similar seismic signature. Most important, although the SSDR data have been acquired with a much higher nominal signal frequency (about 1500 Hz) than the 3D Volume (about 10e100 Hz), the 0.4 TWTT b.s.f. bright spots clearly imaged in both datasets. McGee and Macelloni (2006) have calculated the SSDR signal transmission coefficient for the entire survey.
They have found that from the seafloor to the 0.4 s. TWTT bright spot the transmission coefficient is 0.99, therefore we have to assume very low or totally absent absorption of the SSDR pulse. Figure 15 shows that in the SSDR, the bright spot below the pockmark (BS-2) has disappeared, while the deeper anomaly (BS-1) is still present in the two datasets. Our hypothesis is that between 2000 and 2006 the gas trapped in the BS-2 has escaped at the seafloor thus forming the pockmark (Fig. 15). This is a key finding, because it: 1) confirms that the 0.2 TWTT gas horizon is a thermobaric boundary; 2) suggests that the system is transient at a temporal scale of as little as 6 years; 3) similar processes could be triggered by stress/failure mechanisms due to upward migration of thermogenic gases and fluids, and not necessarily requiring movement along the fault planes and therefore a seismic event sensu strictu; 4) the HSZ is segmented along the faults ergo laterally variable within the mound. Our key findings reinforce the recent evidences of strong geological controls on focused fluid flow associated with seafloor seeps and vents in complex geological setting presented by Gay et al. (2007) for the lower Congo Basin, by Reilly and Flemings (2010) for the Gulf of Mexico, by Andresen et al. (2011) for the Angola offshore and summarized by Huuse et al. (2010) in a general conceptual model for continental margin. In particular, Woolsey Mound scenario confirms that thermogenic gases undergo to several stages of residence in the subsurface before venting and the seafloor. Stages of residence have different timing according with the time spent in the various “reservoirs” (i.e. buried turbidite channels, gas hydrates) but it can be a very fast and dynamic process since we have seen changes within 6 years. Faults are a relevant geological control either providing the physical conduits where fluids transits or perturbing the thermobaric regime with tectonic activity, however, we believe that seismic events sensu strictu play a more general and long period role providing the hydraulic connection and the fracture where fluids can transit and
138
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
Figure 14. Hydrates stability zone thermobaric model. A) Theoretical hydrates stability zone depth using pressure and temperature data from Arco well, hydrates composition from Sassen et al. (2006), and thermobaric model from Sloan (1998). Since the well is located off the mound in an area not influenced by the salt body, the hydrates stability zone results very thick (about 1200 m b.s.f.) Instead in the mound, where the salt is very shallow affecting the saturation of the pore water, the stability zone is shallower (about 200e300 m b.s.f. e B and C).
Figure 15. Temporal evolution of hydrocarbon fluid migration. The amplitude anomalies showed in Figure 11 are associated with seafloor pockmarks and craters, as well as with the master faults leading to the hypothesis that hydrate formation and dissociation might be severely controlled by tectonic quiescence and activity, respectively. In particular, during tectonic activity, hydrocarbons migrating upward from deep sources perturb the hydrates stability in terms of pressure and temperature and promote dissociation.
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
hydrates can form, but that the short period variability is mainly due to the “poorly understood” dynamic of fluids migration within the shallow sediments in deep water (Huuse et al., 2010). Two evidences support our hypothesis, observing the very small scale movement of the master faults in the chirp data, the faults appear to be all active and coeval in the very recent geological time; but in opposite, the crater complexes originated by the master faults have completely different morphology, mineralogical products, and biological assemblage leading to the conclusion that different fluid flux regimes are present at the site (Macelloni et al., 2010). The local segmented BHSZ, determined by different advecting mechanism along the faults can produce different fluid flux regime within the same Mound. We do not exclude that peculiar micro seism might be associated with fluids moving in the subsurface, but is more a side consequence that a driving force, and further study should be pursued to understand this phenomenon. 5. Conclusions Geological and geophysical characterization of the shallow subsurface in the vicinity of Woolsey Mound at MC118, where the thermobaric model predicts the presence of the hydrates stability field, has been obtained by integrating three multiple resolution seismic datasets. Particular effort has been made to define, describe and spatially map seismic expressions of hydrocarbon leakage, the main cause of the mound’s formation. Seismic data show that the subsurface geology of Woolsey Mound is dominated by the presence of a salt body, “rooted” to depths consistent with Gulf of Mexico models of hydrocarbon generation, and rising isostatically as additional sediments are loaded onto the dome. Upward salt movement generates a radiating pattern of faults and fractures that provides the hydraulic conduits for hydrocarbon fluid migration. The complexity of the plumbing system can be fully revealed only when observed over the three datasets at different resolution. The 3D Volume industry data display few master faults nucleating at the salt dome and intersecting the seafloor. Where these intersections occur, craters and pockmarks form. Amplitude anomalies, or “bright spots,” at multiple stratigraphic levels occur in close spatial association with the master fault system, suggesting that hydrocarbon fluids are migrating from the deep oil reservoirs via the faults and that they can and do accumulate in the shallow subsurface where favorable hydraulic conditions are met. The SSDR data also suggest that small faults networks, not resolvable in the oil industry data, contribute to the hydraulic communication within the hydrates stability zone. A direct comparison of coincident profiles from the 3D Volume and the SSDR seismic survey suggests that the geophysical signature of the subsurface at MC118 exhibits transience on the scale of years. These two seismic surveys were collected with significantly different acquisition parameters (particularly frequency and sample rate) approximately six years apart. Although the seismic response and corresponding images look considerably different, stratigraphic and structural geometries can readily be correlated between the two datasets. While the 0.4 s. TWTT high amplitude, negative bright spot within the 3D volume exhibits a similar identifiable amplitude anomaly on the SSDR data, there appears to be other areas where the 0.2 s. TWTT bright spot is present on the 3D Volume (collected in 2000) but is absent on the SSDR data (collected in 2006). Our interpretation is that the shallower bright spot is the seismic signature of free hydrocarbon gas. Faults, fractures and evident vertical seismic anomalies (discontinuity and wipeout zones) indicative of hydrocarbon leakage, hydraulically connect the gas with the seafloor where pockmarks and craters are present. Therefore a time variant sealing mechanism must be acting to trap e and alternatively release e the gas within the sediment. We suggest
139
that the mechanism is the formation/dissociation of gas hydrates in the faults and in the fractures associated with them. The shallower bright spot, therefore, represents an anomalous discontinuous “BSR” marking the base of the HSZ. Most importantly, integrating both mound surface and subsurface observations, we speculate that the mechanisms through which fluids and gas transit along the shallow fault systems are different within the mound and directly guide the morphology and the biological evolution of the mound.
Acknowledgments The authors are deeply grateful to TGS-Nopec for the 3D seismic volume and to SMT to support educational licenses of Kingdom Suite. The research has benefited by numerous inputs and suggestions from colleagues attending the 2010 Gordon Conference, where preliminary results of this work were presented. For this reason, we would like to thank conference Chairs C. Ruppel and P. Flaming who have provided support to A. Simonetti. A special thanks to the MMRI Shop, SDI personnel and the R/V Pelican Crew for their always-outstanding job at sea, and to Paul Mitchell, Marco D’Emidio and Marco Pizzi of MMRI for graphic and GIS support. This project has been funded through the Gulf of Mexico Hydrates Research Consortium with funds jointly provided by U.S. DOE-NETL, DOI-BOEM, DOC NOAA-NIUST.
Appendix A. Seismic data specs High quality 3D oil industry multichannel seismic data from TGSNopec (3D Volume) Standard oil industry data have been acquired in 1999e2000 by TGS-Nopec, and data specs and relevant seismic processing steps are reported in Table A1. The 3D seismic volume was truncated at 3sec. to respect constraints imposed by proprietary conditions. Table A1 TGS NOPEC Acquisition Parameter and processing flowchart. Acquisition parameters Acquisition date: Data acquired by: Shooting orientation: Recording instrument: Streamer type: Source/streamer positioning: Airgun source: Gun depth: Shotpoint interval: CMP crossline separation: Group interval: Recording channel: Streamer depth: Streamer length: Record length: Sample interval: Nominal fold:
10/23/99e07/06/2000 CGG NortheSouth Syntron 480 Syntron GPS/DGPS 4180 cubic inches 7.5 m 1 m 62.5 m per CMP line 40 m 25 m 288 per streamer 9 m 1.5 m 7200 m 12.288 s 2 ms 57.6
Surface-Sources Deep-Receiver (SSDR) SSDR data were collected by the CMRET during two different cruises in April and June, 2006. The SSDR recording geometry is shown in Figure A1. SSDR data processing strategy, the benefit of the system and the imaging capability are presented extensively in McGee (2000), Macelloni (2005), Battista et al. (2007) and Macelloni et al. (2011).
140
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
Figure A1. SSDR geometry and survey grid.
AUV-born chirp sub-bottom profiler Chirp sub-bottom profiler data were collected with the C&C Technology AUV Hugin 3000 in 2005 aside high-resolution swath bathymetry and Side-Scan Sonar. A sub-bottom profiler (DW216) manufactured by Edgetech was utilized for seismic imaging of the near-seabed sediments. The system is frequency modulated
between 2 and 8 kHz. A record length of 300 ms and a 63-ms sampling interval were used for seismic data recording. The subbottom profiles were output in SEG-Y format. Static offsets for the AUV depth are added in post-processing to eliminate as much of the water column as possible from the dataset. A total of 25 lines running east-west and 180 m spacing plus two control lines running north-south were collected, as reported in Figure A2.
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
141
Figure A2. Chirp sub-bottom survey grid.
Appendix B. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.marpetgeo.2012. 06.010. References Allen, J.L., Peddy, P.G., 1993. Amplitude Variation with Offset e Gulf Coast Studies. Society of Exploration Geophysicists. Andresen, K.J., Huuse, M., Schodt, N.H., Clausen, L.F., Seidler, L., 2011. Hydrocarbon plumbing system of salt minibasins offshore Angola revealed by threedimensional seismic analysis. AAPG Bulletin 95 (6), 1039e1065. Battista, B.M., Knapp, C., McGee, T.M., Goebel, V., 2007. Application of the empirical mode decomposition and HilberteHuang transform to seismic reflection data. Geophysics 72, H29eH37. Chanton, J., Lapham, L.L., Wilson, R., Martens, C., Mendlovitz, H., 2011. Geochem Studies at MC118 Seafloor Station. Information Transfer Meeting in New Orleans, LA. March 22e24th, 2011. Galloway, W.E., Buffler, R.T., 2004. Gulf Basin Depositional Synthesis Project. Available at: http://www.ig.utexas.edu/research/projects/gbds/gbds.htm. Gay, A., Lopez, M., Berndt, C., Séranne, M., 2007. Geological controls on focused fluid flow associated with seafloor seeps in the Lower Congo Basin. Marine Geology 244, 68e92. Hovland, M., Gudmestad, O.T., 2001. Potential influence of gas hydrates on seabed installations. In: Paull, C.K., Dillon, W. (Eds.), Natural Gas Hydrates: Occurrence, Distribution and Detection. American Geophysical Union Monograph, vol. 124. AGA, Washington, D.C, pp. 307e315. Hovland, M., Judd, A.G., 1988. Seabed Pockmarks and Seepages. Impact on Geology, Biology and the Marine Environment. Graham and Trotman, Ltd., London. Huuse, M., Jackson, C.A., Van Rensbergen, P., Davies, R., Flemings, P.B., Dixon, R.J., 2010. Subsurface sediment remobilization and fluid flow in sedimentary basins: an overview. Basin Research 22, 342e360. Ingram, W.C., Meyers, S.R., Brunner, C.A., Martens, C.S., 2010. Evaluation of Late PleistoceneeHolocene sedimentation surrounding an active seafloor gas hydrate and cold seep field on the Northern Gulf of Mexico Slope. Marine Geology 278, 43e53.
Knapp, J.H., Knapp, C.C., Macelloni, L., Simonetti, A., Lutken, C.B., 2010. Subsurface Structure and Stratigraphy of a Transient, Fault-Controlled Thermogenic Hydrate System at MC-118, Gulf of Mexico AAPG Annual Meeting Transactions, New Orleans (LA) April 11eApril 14 2010. Lapham, L.L., Chanton, J.P., Martens, C.S., Woolsey, J.R., 2008. Microbial activity in surficial sediments overlying acoustic wipe-out zones at a Gulf of Mexico cold seep. Geochemistry, Geophysics, Geosystems 9 (6). Løseth, H., Wensaas, L., Arntsen, B., Hanken, N., Basire, C., Graue, K., 2001. 1000 M Long Gas Blow-Out Pipes. EAGE, Amsterdam. Løseth, H., Gading, M., Wensaas, L., 2009. Hydrocarbon leakage interpreted on seismic data. Marine and Petroleum Geology 26, 1304e1319. Macelloni, L., 2005. High-Resolution Marine Digital Seismic Method: Theoretical Constrains, Digital Processing, New Developments. PhD dissertation, pp. 71. In PADIS e Pubblicazioni Aperte Digitali Interateneo Sapienza [PHDe2005-187] http://padis.uniroma1.it/. Macelloni, L., Caruso, S., Lapham, L., Lutken, C., Brunner, C., Lowrie, A., 2010. Spatial distribution of seafloor bio-geological and geochemical processes as proxy to evaluate fluid-flux regime and time evolution of a complex carbonate/hydrates mound, northern Gulf of Mexico. Gulf Coast Association of Geological Societies Transactions 60, 461e480. Macelloni, L., Batista, B.M., Knapp, C.C., 2011. Optimal filtering high-resolution seismic reflection data using a weighted-mode empirical mode decomposition operator. Journal of Applied Geophysics 75 (4), 603e614. McGee, T.M., 2000. Pushing the limits of high-resolution marine seismic profiling. European Journal of Environmental and Engineering Geophysics 5, 43e53. McGee, T.M., Macelloni, L., 2006. Seismo-Acoustic Imagery of a Carbonate/Hydrate Mound in the Gulf of Mexico. American Geophysical Union Fall Meeting San Francisco, California 11e15 December 2006. Milkov, A.V., Sassen, R., 2000. Thickness of the gas hydrate stability zone, Gulf of Mexico continental slope. Marine and Petroleum Geology 17, 981e991. Reilly, M.,J., Flemings, P.,B., 2010. Deep pore pressure and seafloor venting in the Auger Basin, Gulf of Mexico. Basin Research 22, 380e397. Ruppel, C., Dickens, G.R., Castellini, D.G., Gilhooly, W., Lizarralde, D., 2005. Heat and salt inhibition of gas hydrate in the northern Gulf of Mexico. Geophysical Research Letters, 32. Sassen, R., Roberts, H.H., 2004. Site Selection and Characterization of Vent Gas, Gas Hydrate, and Associated Sediments. In DOE Technical Report, pp. 28e227. Sassen, R., Roberts, H.H., Jung, W., Lutken, C.B., DeFreitas, D.A., Sweet, S.T., Guinasso Jr., N.L., 2006. The Mississippi Canyon 118 gas hydrate site: a complex
142
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
natural system. In: OTC Paper #18132; Offshore Technology Conference, Houston, TX. Shedd, W., Roberts, H.H., 2009. Use of 3-D seismic to characterize seafloor hydrocarbon, brine, and sediment expulsion in the slope of the northern Gulf of Mexico. Environmental Geoscience 16 (2), 106e117. Shedd, W., Boswell, R., Frye, M., Godfriaux, P., Krame, K., 2012. Occurrence and nature of “bottom simulating reflectors” in the northern Gulf of Mexico. Marine and Petroleum Geology 34, 31e40. Sleeper, K., Lowrie, A., Bosman, A., Macelloni, L., Swann, C.T., 2006. Bathymetric mapping and high resolution seismic profiling by AUV in MC 118 (Gulf of Mexico). In: OTC Paper # 18133, Offshore Technology Conference, Houston, TX. Sloan, E.D., 1998. Clathrate Hydrates of Natural Gases, second ed. Dekker, New York, 705 pp.
Wood, W.T., Gettrust, J.F., Chapman, N.R., Spence, G.D., Hyndman, R.D., 2002. Decreased stability of methane hydrates in marine sediments owing to phaseboundary roughness. Nature 420, 656e660. Wood, W., Hart, P.E., Hutchinson, D.R., Dutta, N., Snyder, F., Coffin, R.B., Gettrust, J.F., 2008. Gas and gas hydrate distribution around sea floor seeps in Mississippi Canyon, northern Gulf of Mexico, using multi-resolution seismic imagery. Marine and Petroleum Geology 25 (9), 952e959. Woolsey, J.R., Higley, P., Lapham, L.L., Chanton, J.P., Lutken, C.B., Sleeper, K., Culp, R., Sharpe, S., Ross, D., 2005. Operations Report of Cruise GOM2-05-MC118: Deployment of the Initial Components of the Sea Floor Monitoring Station e The Pore-Fluid Array and the Geophysical Line Array e Via the Sea-Floor Probe System and Collection of Core Samples, Mississippi Canyon 118. In Gulf of Mexico Hydrates Research Consortium Cruise Report.
Contents lists available at SciVerse ScienceDirect
Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo
Multiple resolution seismic imaging of a shallow hydrocarbon plumbing system, Woolsey Mound, Northern Gulf of Mexico Leonardo Macelloni a, *, Antonello Simonetti b, James H. Knapp b, Camelia C. Knapp b, Carol B. Lutken a, Laura L. Lapham c a b c
Mississippi Mineral Resources Institute, 111 Brevard Hall, University of Mississippi. University, MS 38677, USA Department of Earth and Ocean Sciences, University of South Carolina, Columbia, SC, USA Chesapeake Biological Lab, University of Maryland Center for Environmental Science, P.O. Box 38, 1 Williams St., Solomons, MD 20688, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 October 2011 Received in revised form 22 May 2012 Accepted 7 June 2012 Available online 24 July 2012
The northern Gulf of Mexico is dominated by salt tectonics, resulting fracturing and numerous seafloor seeps and vents. Woolsey Mound, site of the Gulf of Mexico Hydrates Research Consortium’s seafloor observatory, has been investigated extensively via surveys, direct sampling and seafloor instrument systems. This study presents an innovative approach to seismic data interpretation, integrating three different resolution datasets and maximizing seismic coverage of the complex natural hydrocarbon plumbing system at Woolsey Mound. 3D industry seismic data reveal the presence of a salt body at in the shallow subsurface that has generated an extended network of faults, some extending from the salt body to the seafloor (master faults). Higher resolution seismic data show acoustic wipe-out zones along the master faults with expulsion features e seafloor pockmarks and craters e located immediately above them and associated, in the subsurface, with high-amplitude, negative anomalies at constant depth of 0.2 s TWTT b.s.f., interpreted as free gas. Since pockmarks and craters provide pathways for hydrocarbons to escape from depth into the water column, related sub-surface seismic anomalies may indicate free gas at the base of the gas hydrates stability zone (GHSZ). Fluid flow and gas hydrates formation are segmented laterally along faults. Gas hydrates formation and dissociation vary temporally in the vicinity of active faults, and can temporarily seal them as conduits for thermogenic fluids. Periodic migrations of gases and other fluids may perturb the GHSZ in terms of temperature and pressure, producing the observed lack of classical BSRs. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Hydrocarbon plumbing system Gas hydrates Amplitude anomalies Seismic resolution
1. Introduction Naturally occurring gas hydrates have been documented in continental slope sediments worldwide. In temperate climes gas hydrates form naturally in water depths greater than a few hundred meters in a zone extending from the seafloor to sub-bottom depths determined primarily by temperature gradient, pressure regime, pore-water salinity, and availability of hydrocarbon gases (Ruppel et al., 2005). Gas hydrates research is particularly important in the northern Gulf, a major oil and gas exploration/exploitation province, because drilling is now routinely conducted on the continental slope. Drilling through gas hydrates presents hazards concerns related to slope stability, well safety, pipelines, and platforms (Hovland and Gudmestad, 2001), making it critically important to establish their distribution. On the Gulf’s continental * Corresponding author. Tel.: þ1 662 915 7320; fax: þ1 662 915 5625. E-mail address: [email protected] (L. Macelloni). 0264-8172/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.marpetgeo.2012.06.010
slope, faults and fractures, radiating from salt bodies, produce natural conduits that facilitate migration of hydrocarbon fluids from deep oil reservoirs into the Hydrates Stability Zone (HSZ). Natural hydrocarbon seepage is so common that the seafloor is populated by a profusion of mounds and seeps (Shedd and Roberts, 2009), where hydrates outcrop at the seabed. Furthermore, salt produces a high, laterally variable geothermal gradient that perturbs the thermobaric stability field, making the HSZ difficult to define (lack of BSRs, presence of multiple stability boundaries, etc.). Woolsey Mound, Mississippi Canyon Lease Block 118 (MC118), has been designated by the Bureau of Ocean Energy Management (BOEM) as the Gulf’s only Research Reserve and the site of a Seafloor Observatory (Fig. 1) because it represents a mature example of a hydrates/carbonates complex with a fault migrationconduit system overlying shallow salt, directly connected to deeply-buried source rocks for gas and oil (Sassen et al., 2006; Knapp et al., 2010). Submersible missions to the seafloor have identified large gas hydrates outcrops, authigenic carbonate
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
129
Figure 1. The sea floor observatory at MC118. The cartoon illustrates the sensors and devices being installed at Woolsey Mound (MC118). PFA Pore Fluid Array, VLA Vertical Line Array, HLA Horizontal Line Array, CSA Chimney Sampler Array, FOC Fiber Optic Cable, AUV Autonomous Underwater Vehicle, MSR Multibeam Sonar Rotator, RCS Rotating Camera System, BBLA, Benthic Boundary Layer Array, IDP Integrated Data Power unit.
mounds, gas vents, and chemosynthetic communities, including ice worms (Sassen and Roberts, 2004; Woolsey et al., 2005). In order to define a geological and geophysical model of the mound’s subsurface and characterize the underlying plumbing system, an innovative approach has been adopted through the integration of three different resolution seismic datasets: an oil exploration 3D Volume collected in 2000, a pseudo-3D high resolution surface-source deep-receiver (SSDR) single channel seismic dataset collected in 2006, and a 2D very high resolution autonomous underwater vehicle (AUV)-borne chirp dataset, collected in 2005. The penetration depth and vertical seismic resolution, different for each dataset, span from thousands of meters (penetration) and 50 m (vertical resolution) to 50 m and a few centimeters resolution. The SSDR system, which images up to hundreds of meters below the seafloor with a vertical resolution of about 1 m, bridges the gap between the 3D volume and chirp data, exactly where the gas hydrates stability zone is present (Macelloni et al., 2011). This multi-scale approach better defines the upward thermogenic gas-fluid migration process, including transiting a seafloor mound and the HSZ. Here, we present the qualitative/comparative analysis of the three seismic datasets to: 1) define the source of hydrocarbon fluids; 2) identify fluid-flow pathways; 3) understand the structural controls and geological mechanisms of fluid migration within the HSZ; 4) formulate hypotheses on the temporal evolution of the system.
2. Site location MC118 is located offshore approximately 150 km south of Pascagoula (Ms) and 100 km east of the Mississippi Canyon in w890 m of water (Fig. 2). It is sited on the eastern flank of the main Mississippi Canyon, in a gently seaward dipping portion of the continental slope. A fault-controlled canyon, flanking the Whiting dome and a slamp structure to the east is the only relevant morphological feature present in the area, as shown in Figure 2. Extended discussions of the MC118 complex system are given in Sassen et al. (2006), Lapham et al. (2008), Ingram et al. (2010) and Macelloni et al. (2010). Salt domes in the nearby shallow subsurface appear to dominate the MC118 upper slope as revealed by the bathymetric map (Fig. 2). Visible outcrops of gas hydrates, faulted carbonate “hard-grounds” and pockmark features, cover approximately 1 km2 of the seafloor (e.g., Sassen et al., 2006; Sleeper et al., 2006; Macelloni et al., 2010). The seaward slope across the study area typically ranges from 3 to 4 , but slopes of 10 e12 are present locally across the pockmark. The supply of hydrocarbons (natural gas and petroleum) to the seafloor supports an active biological seep community and microbial chemolithotrophy in the immediate vicinity of active gasefluid seepage (Macelloni et al., 2010). The mound has been subdivided in macro areas, and following the description of Macelloni et al. (2010), we recognize, the south-east complex, the north-west complex, and the south-west complex.
130
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
Figure 2. Geographic location of MC118. Woolsey Mound is located in 900 m water depth on the northern continental slope of the Gulf of Mexico. The slope here is highly discontinuous, intersected by slumping, folding and faulting mainly due to the tectonics derived from the underlying salt and the sediment load delivered by the Mississippi River.
3. Data and methods The MC118 hydrates mound subsurface has been imaged using three different seismic datasets: a high quality 3D Volume from TGS-Nopec; a Surface-Source Deep-Receiver (SSDR) single channel system pseudo 3D volume and an AUV-borne Chirp Sub-bottom Profiler. Survey geometries, seismic data specifications, acquisition parameters and digital signal processing flow, for each dataset are given in Appendix A.
These three datasets embrace a wide spectrum of penetration depths and vertical resolutions, providing an innovative multiscale imaging of the geological setting beneath the hydrates mound (Fig. 3). The initial step in data analysis was the creation of a fully integrated Kingdom Suite 8.3 seismic interpretation Project. Accurate spatial integration of the multiple datasets required calibration of the reflection points’ spatial distribution and normalization of the dataset seafloor reflection time. The first was accomplished via statistical analysis of misfits among cross points
Figure 3. Multiple resolution seismic data set. Woolsey Mound at MC118, has been investigated using three different datasets: high quality 3D oil industry multichannel seismic data from TGS-Nopec (3D Volume); surface-source deep-receiver (SSDR) single channel system pseudo-3D volume and AUV-borne CHIRP sub-bottom profiler. Their different vertical resolutions provide a multi-scale image of the sub-surface beneath the mound (Credits: 3D Volume TGS-Nopec).
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
and the second, by referring all the seafloor reflection point times to the AUV seafloor bathymetry. Further, in order to provide evidences and visual control of possible hydrocarbon leakage at the seafloor, high-resolution mound bathymetry and reflectivity as well as the spatial distribution at the seafloor of the bio-geological facies, commonly related to hydrocarbon seeping and venting (i.e. bacterial mats, authigenic carbonate, chemosynthetic community, etc.), as presented by Macelloni et al. (2010), have been integrated into the seismic volume (Fig. 4). 3.1. Seismic data interpretation Two main targets were identified in our seismic interpretation, (1) to reconstruct the general geological setting of the mound subsurface, and (2) to image the architecture of the plumbing system and related leakage structures. For target 1 we have carefully picked all of the seismic horizons (with particular effort to trace seismic horizons that represent the same stratigraphic level in different datasets) and the first order faults. For target 2, instead, we have first defined the leakage-related seismic anomalies (i.e. discontinuity zones, wipeout zones, bright spots and phase reversals) adopting the classification of Løseth et al. (2009). We then mapped the spatial distribution of these anomalies across the mound. This time-consuming process was
131
essential to map and geometrically define the geological and structural framework of the mound subsurface. 4. Results and discussion 4.1. Geological setting and hydrocarbon source Seismic data interpretation reveals a scenario dominated by a salt body (Fig. 5) whose top extends to 0.6 s TWTT beneath the seafloor (b.s.f.). Salt tectonics generates an extensive network of faults and fractures; however, only few directly connect the salt body with the seafloor carbonate/hydrate mound. We define these faults as “master faults” and color code them in Figure 5 as blue, red, yellow and magenta faults. The “master faults” are normal faults, the yellow and magenta dip west, the blue north-east, and the red south-west. All are rooted in the salt diapir’s upslope side and appear to originate very deep in the stratigraphic column (the 3D volume is truncated at 3 s). The main mound craters (south-east, north-west and southwest) form where the “master faults” intersect the seafloor. In particular, the south-east crater complex lies at the top of the yellow master fault, the south-west crater complex is related to the magenta master fault, and the north-west complex coincides with the top of the blue and red master faults (Fig. 5). Features related to hydrocarbon leakage are present at the seafloor and include pockmarks, gas hydrates, authigenic carbonates, active vents and
Figure 4. Woolsey Mound seafloor bio-geological processes. Seafloor back-scatter map, relevant biological habitat and sediment distribution of the mound. A) Undisturbed pelagic mud typical of the mound’s periphery. B) Area covered by chemoautotrophic clams; C) authigenic carbonate nodules; D) bacterial mat (Beggiatoa sp.); E, F, G area paved by authigenic carbonate crusts and slabs; H) gas venting from the seafloor; I) the sleeping dragon, the largest complex carbonate hydrates outcrop ever imaged on the seafloor; L) hydrate dike (Macelloni et al., 2010; modified).
132
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
Figure 5. Salt dome and master faults from 3D standard seismic data. The south-east crater complex lies atop the yellow master fault and the south-west crater complex is related to the magenta master fault, while the north-west complex lies above of the blue and red master faults (Credits: 3D Volume TGS-Nopec).
chemosynthetic communities (Figs. 4 and 5). Geochemical analyses from gas hydrates samples outcropping at the mound surface (Sassen et al., 2006), revealed high percentages of propane (15.9%) and ethane (7.5%), suggesting a thermogenic source. Further proof of a deep source for hydrocarbon gases at the site has been provided, just recently, by d13C isotope analysis (Chanton et al., 2011). 4.2. Faults system The master faults are the relevant tectonic motif detectable in the industry dataset, and clearly show the close relationship between salt tectonics and mound formation; it is not a coincidence that the main craters occur where master faults intersect the seabed. However, a much more complex system of faults emerges from the study of the higher resolution seismic data (SSDR and Chirp). In fact, carefully picking all the horizons small offset in these two seismic datasets enabled us to map a fine-scale faults pattern exhibiting radial orientation and reflecting upward movement of the underlying salt body (Fig. 6). We suggest that this radial faultefracture complex is an integral part of the plumbing system at MC118 and two evidences prove it. The first one is presented in Figure 7: in the southern part of the mound, in an area distant from the main morphological structures, we have imaged a fault which appears, in the 3D Volume data, to be blind; however, in the SSDR data, this fault can be seen to continue to displace sediment and in the very high resolution CHIRP data is shown to reach the seafloor. On the multibeam bathymetric image, a satellite pockmark is present above this fault. Another example of the complexity of the plumbing system recorded in the SSDR data is illustrated in Figure 8. Two north-south seismic profiles (3D Volume and SSDR) are positioned off the mound on the west side. The SSDR profile depicts a system of small-scale faults dislocating a gas charged horizon (see paragraph 4.3.3), which appears continuous and unbroken in the 3D Volume lines. SSDR data also show several acoustic wipeout zones (4.3.2) in the proximity of the faults evidence that gas is being diffused into the sediments above. 4.3. Seismic anomalies Once the fault network was established, effort was focused on defining seismic anomalies indicative of hydrocarbon leakage. We
used mainly the SSDR data because they show clearly two different vertical anomalies: the discontinuity zone and the wipeout zone (sensu Løseth et al., 2009). 4.3.1. Discontinuity zones The discontinuity zones are defined by Løseth et al. (2009) as areas on the seismic section where reflections are more discontinuous than in adjacent areas and can be interpreted as gas migration pathways through sediments (generally fine-grained). SSDR data present dense patterns of such a discontinuity zone (Fig. 9a). We observe that the spatial distribution of a majority of them follows the faults previously identified on the SSDR data (Fig. 6). Therefore, the discontinuities provide seismic evidence of
Figure 6. Small scale faults network from SSDR data. The tectonic framework beneath the mound is very complex; however, meticulous interpretation of high-resolution data reveals a radial fault complex (shown in red) that is undetectable on 3D Volume (Credits: 3D Volume TGS-Nopec). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
133
Figure 8. Imaging fault displacement at different seismic resolutions. A small-scale network of active faults is a vital component of the hydrocarbon plumbing system, and serves as a hydraulic connection between stratigraphic reservoirs. On the western part of the mound, away from the main seafloor morphological unit, 3D Volume data show the continuous and unbroken 0.4 s TWTT bright spot, that, in the SSDR data is highly segmented with numerous degassing features (Credits: 3D Volume TGS-Nopec).
Figure 7. Imaging fault displacement at different seismic resolutions. The integration of multiple-resolution datasets has been key to image the real tectonic regime. In fact, comparing one particular fault across the three datasets, mapped on a line located in the southern portion of the mound, has shown that the fault, which appears to terminate (blind) at 1.30 s TWT in the 3D Volume, continues up to 1.240 s TWT in the SSDR and even reaches the seafloor in the chirp data (Credits: 3D Volume TGSNopec).
sediment deformation accommodating the numerous faults. Alternatively, these discontinuity zones may represent upward migration conduits for gas and fluids through an area of lithological weakness. However, this latter interpretation is still hypothetical in the absence of additional hydrocarbon indicators, though the two processes can coexist.
4.3.2. Wipeout zone A wipeout zone is an area on the seismic section where the reflections from stratigraphic layers are deteriorated such that the primary reflections are either absent or very weak (Løseth et al., 2009). In our data, they mainly occur along faults and directly underneath seafloor pockmarks and craters (Fig. 9b) since these features are generally formed by intermittent escape of fluids or gas into the water-column and by violent episodic gas blowouts (Hovland and Judd, 1988; Løseth et al., 2001). This anomalous seismic pattern results from processes that promote mobilization of fluids through layers, and may also indicate, areas where upward migration of gas-fluids occurred (Hovland and Judd, 1988; Løseth et al., 2001; Gay et al., 2007; Huuse et al., 2010). 4.3.3. “Bright spots” polarity reverse In addition to vertical seismic anomalies, other anomalies have been identified in the MC118 sub-surface. They are high-amplitude
134
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
Figure 9. Seismic discontinuity zone and wipe-out zone. 3D Volume data show remarkable lack of detail when compared to the high-resolution SSDR data. The latter show a dense pattern of pipe-like discontinuity zones, which in literature are often referred as hydrocarbon conduits. The spatial distribution of these anomalies seems to follow the radial fault complex. Since the occurrence of discontinuity zones may arise from a loss of signal coherence close to the fault planes, they do not necessarily indicate hydrocarbon conduits. In the absence of other indicators, we interpret them as potential hydrocarbon migration pathways, b) SSDR data also highlight how the seismic signal underlying seafloor pockmarks and craters is always wiped out or extremely chaotic. Since pockmarks and craters are generally formed by gas/fluids escaping into the water column, the wipeout zones can be interpreted as the outcome of these upward hydrocarbon migration processes, which produce sediment mobilization through the sedimentary layering. Hence, the wipeout zones must represent areas where hydrocarbon transit is more likely than in discontinuity zones (Credits: 3D Volume TGS-Nopec).
“bright spots” (Allen and Peddy, 1993), in some cases with evident phase reversal (or polarity reversal with respect to the seafloor reflection). Bright spots are very clear on both the 3D Volume and the SSDR (Fig. 10). In both datasets, the bright spots occupy the same stratigraphic levels and, despite the differences in seismic systems and signal frequencies, they present very similar seismic signatures (Fig. 10). The bright spots are always associated with a distinctive seismic facies which is characterized by (1) moderate/high amplitude, (2) moderate/high continuity, and (3) presence of several lenticular and concave-up geometries (Fig. 11). We interpret this facies as sandy lobe complexes or partially channelized e turbidite lobe complexes, according to the seismic facies model proposed by Galloway and Buffler (2004). The first one occurs at about 0.4 s TWTT b.s.f. (BS-1) at the top of the above mentioned sandy facies; the second one occurs at shallower depth, at about 0.2 s TWTT b.s.f. (BS-2) and is related to smaller sand bodies top-sealed by low permeability hemipelagic sediments and laterally bounded by fault planes. We interpret the bright spots as seismic evidence of free gas accumulation in the sandy sediments (Fig. 12). The two gas horizons are in hydraulic communication, via the faults. In fact we observe that the gas is mainly trapped in the 0.4 TWTT sand deposits, and only when the faults intersect this horizon, the gas is able to migrate upward and charge the shallower sand deposit (Fig. 12). If we display the planar distribution of the BS-1 and BS-2 (Fig. 13), it is possible to observe that the deeper one
(Fig. 13a) occupies a larger area than the shallower. It has two main sacks: one located off the mound to the north-west and the other beneath the mound, slightly north the south-east complex (Fig. 13a, box 1 and 2). The shallower gas horizon, instead, is geographically localized beneath the mound including beneath the two peripheral pockmarks in the north and in the south (Fig. 13b). The BS-2 is discontinuous and, not homogenously present beneath the mound; it is strong beneath the north west-area, moderate beneath the south-west, but almost absent beneath south-east (Fig. 13b). Macelloni et al. (2010) have found that mound bio-geological processes are completely different in the three complexes; in fact they have seen that the south-east complex appears to be an extinct or quiescent vent, the south-west a mature and well established vent, while the north-west appears to be a young vent with more vigorous seeping. The seismic data confirm that hydrocarbon flux activity must vary within the mound and that it is controlled by the master faults and by the dynamic of the 0.2 TWTT b.s.f. gas horizon. In fact, since pockmarks and craters are clear evidence for episodic gas and fluid expulsion into the water column, we have to conclude that the gas originating the bright spot, is trapped by a sealing mechanism different than the difference between the lithology (porosity) of the sandy deposit and the overlying sediments and, most importantly, a time variant mechanism. Otherwise the gas would vent continuously at the seabed without accumulating in the subsurface and forming the seismic anomaly.
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
135
Figure 10. Synoptic image of seismic bright spots between TGS and SSDR data. Reverse polarity bright spots are imaged, clearly, in both 3D Volume and SSDR data. Although the two datasets were collected with different parameters (Appendix A), the seismic anomalies share the same stratigraphic position and a very similar signature.
4.4. Hydrates stability zone thermobaric model We speculate that the shallower gas anomalies (BS-2) indicate the base of the GHSZ, a transient thermobaric boundary over which gas hydrates may form and act as a temporary seal for the underlying
free gas. The transient thermobaric boundary represents a very peculiar bottom simulating reflector (BSR), discontinuous and segmented along the base of the hydrates stability zone. Such peculiar BSRs have been hypothesized by Wood et al. (2008) and by Shedd et al. (2012) for the Gulf of Mexico region, which generally
Figure 11. Sandy lobe complexes or partially channelized e turbidite lobe complexes. Bright spot anomalies are associated with a peculiar seismic facies characterized by 1) moderate/high amplitude, (2) moderate/high continuity, and (3) presence of several lenticular and concave-up geometries. We interpret this facies as sandy lobe complexes or partially channelized e turbidite lobe complexes, according to the seismic facies model proposed by Galloway and Buffler (2004).
136
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
Figure 12. Hydraulic connection between BS-1 and BS-2. The BS-1 and BS-2 gas horizons are in hydraulic communication, via the faults system. In fact we observe that the gas is mainly trapped in the 0.4 TWTT sand deposits, and only when the faults intersect this horizon, the gas is able to migrate upward and charge the shallower sand deposit.
shows a paucity of a classic BSR. To support this hypothesis we have derived a possible thermobaric model of the hydrates stability zone at MC118. In first approximation we used the temperature data from the only well available in the area (Arco Oil and Gas archives, well position is reported in Fig. 13), the hydrates chemical composition analysis from the samples recovered at the seafloor and the program CSMHYD, by Sloan (1998). The Arco well totally bypassed the upper sediment cover and the first temperature data is 25 C at 1234 m below seafloor. Below this point, temperature increases at a rate of 17.2 C/km (R2 ¼ 0.96), and the value is similar to other thermal gradients reported in the Gulf of Mexico (Milkov and Sassen, 2000). Hydrates at MC118 is found to be 70% CH4, 7.5% ethane, 15.9% propane, 4.4% i-butane, and 4.4% n-butane (Sassen et al., 2006). Using the CSMHYD program, the above mentioned thermal gradient, the hydrates composition and assuming a salinity of 3.5% (seawater) we
have derived hydrates equilibrium pressures. The results were used to derive the PT plot in Figure 14A. At MC118, the depth of hydrates stability is 1167 m below seafloor. These results, however, apply only to the locations of the well, that is located some distance from the mound in an area moderately affected by salt tectonic. Adopting the same geothermal gradient and hydrates compositions, we have tried to extend the thermobaric model at the mound, using the geophysical evidence to locate the depth of salt. Therefore we calculated the hydrates stability zone assuming a salt gradient from 3.5% seawater salt at the seafloor surface to 100% saturated salt at the depth of the top of the diapir or, more specifically, at the top of a possible cap rock, which is lithologically different from the salt but 100% saturated. Results from this model are displayed in Figure 14B and C; where we found that, due to the salt effect, the stability moves up between 185 m and 295 m b.s.f. This window in the depth of the
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
137
Figure 13. Horizontal distribution of the BS-1 and BS-2. BS-1 (a) and BS-2 (b) planar distribution map displayed as Root Mean Square (RMS) of the manually picked negative amplitude reflections. The two gas horizons have different distributions in the mound subsurface. BS-1 is wider and mainly localized in the north-west, while BS-2 is present mainly beneath the mound. Note that BS-2 is discontinuous and segmented, with a strong presence beneath the north-west crater, while it is moderate and almost absent beneath the south-west and south-east craters respectively.
stability zone is due to the approximation in locating the cap rock on seismic data, and to the uncertainty of the p wave velocity in the shallow sediments. These depths are slightly higher than the depth of the shallower gas horizon which is, assuming a velocity of 1500 m/ s about 150 m b.s.f, though the two depth are comparable. Nevertheless, in the mound we expect a thinner hydrates stability zone (even closer to the seafloor) than the thermobaric model predicts, because advecting warm hydrocarbon fluids, transiting along the faults, is expected to perturb the P-T boundary defining the hydrates stability zone upward (Wood et al., 2002). 4.5. Transient hydrates system The amplitude anomalies associated with seafloor pockmarks and craters and with the master faults, support the hypothesis that hydrates formation and dissociation is significantly controlled by the dynamic of the BS-2 and by the fluids migration along the fault. In particular, hydrocarbons migrating upward from deep sources perturb hydrates stability in terms of pressure and temperature and promote dissociation. Further, such processes provide supplies of hydrocarbon gases into the GHSZs. This hypothesis is supported by the comparison of some of these pockmarks/craters-related gas anomalies, that are present on the 3D Volume data (collected in 2000) but is absent on the SSDR data collected in 2006 (Fig. 15). The two datasets have completely different acquisition parameters and signal spectra (Appendix A), however in Figure 10, it has been shown that the bright spots, associated with gas horizons, occupy the same stratigraphic position and display a very similar seismic signature. Most important, although the SSDR data have been acquired with a much higher nominal signal frequency (about 1500 Hz) than the 3D Volume (about 10e100 Hz), the 0.4 TWTT b.s.f. bright spots clearly imaged in both datasets. McGee and Macelloni (2006) have calculated the SSDR signal transmission coefficient for the entire survey.
They have found that from the seafloor to the 0.4 s. TWTT bright spot the transmission coefficient is 0.99, therefore we have to assume very low or totally absent absorption of the SSDR pulse. Figure 15 shows that in the SSDR, the bright spot below the pockmark (BS-2) has disappeared, while the deeper anomaly (BS-1) is still present in the two datasets. Our hypothesis is that between 2000 and 2006 the gas trapped in the BS-2 has escaped at the seafloor thus forming the pockmark (Fig. 15). This is a key finding, because it: 1) confirms that the 0.2 TWTT gas horizon is a thermobaric boundary; 2) suggests that the system is transient at a temporal scale of as little as 6 years; 3) similar processes could be triggered by stress/failure mechanisms due to upward migration of thermogenic gases and fluids, and not necessarily requiring movement along the fault planes and therefore a seismic event sensu strictu; 4) the HSZ is segmented along the faults ergo laterally variable within the mound. Our key findings reinforce the recent evidences of strong geological controls on focused fluid flow associated with seafloor seeps and vents in complex geological setting presented by Gay et al. (2007) for the lower Congo Basin, by Reilly and Flemings (2010) for the Gulf of Mexico, by Andresen et al. (2011) for the Angola offshore and summarized by Huuse et al. (2010) in a general conceptual model for continental margin. In particular, Woolsey Mound scenario confirms that thermogenic gases undergo to several stages of residence in the subsurface before venting and the seafloor. Stages of residence have different timing according with the time spent in the various “reservoirs” (i.e. buried turbidite channels, gas hydrates) but it can be a very fast and dynamic process since we have seen changes within 6 years. Faults are a relevant geological control either providing the physical conduits where fluids transits or perturbing the thermobaric regime with tectonic activity, however, we believe that seismic events sensu strictu play a more general and long period role providing the hydraulic connection and the fracture where fluids can transit and
138
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
Figure 14. Hydrates stability zone thermobaric model. A) Theoretical hydrates stability zone depth using pressure and temperature data from Arco well, hydrates composition from Sassen et al. (2006), and thermobaric model from Sloan (1998). Since the well is located off the mound in an area not influenced by the salt body, the hydrates stability zone results very thick (about 1200 m b.s.f.) Instead in the mound, where the salt is very shallow affecting the saturation of the pore water, the stability zone is shallower (about 200e300 m b.s.f. e B and C).
Figure 15. Temporal evolution of hydrocarbon fluid migration. The amplitude anomalies showed in Figure 11 are associated with seafloor pockmarks and craters, as well as with the master faults leading to the hypothesis that hydrate formation and dissociation might be severely controlled by tectonic quiescence and activity, respectively. In particular, during tectonic activity, hydrocarbons migrating upward from deep sources perturb the hydrates stability in terms of pressure and temperature and promote dissociation.
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
hydrates can form, but that the short period variability is mainly due to the “poorly understood” dynamic of fluids migration within the shallow sediments in deep water (Huuse et al., 2010). Two evidences support our hypothesis, observing the very small scale movement of the master faults in the chirp data, the faults appear to be all active and coeval in the very recent geological time; but in opposite, the crater complexes originated by the master faults have completely different morphology, mineralogical products, and biological assemblage leading to the conclusion that different fluid flux regimes are present at the site (Macelloni et al., 2010). The local segmented BHSZ, determined by different advecting mechanism along the faults can produce different fluid flux regime within the same Mound. We do not exclude that peculiar micro seism might be associated with fluids moving in the subsurface, but is more a side consequence that a driving force, and further study should be pursued to understand this phenomenon. 5. Conclusions Geological and geophysical characterization of the shallow subsurface in the vicinity of Woolsey Mound at MC118, where the thermobaric model predicts the presence of the hydrates stability field, has been obtained by integrating three multiple resolution seismic datasets. Particular effort has been made to define, describe and spatially map seismic expressions of hydrocarbon leakage, the main cause of the mound’s formation. Seismic data show that the subsurface geology of Woolsey Mound is dominated by the presence of a salt body, “rooted” to depths consistent with Gulf of Mexico models of hydrocarbon generation, and rising isostatically as additional sediments are loaded onto the dome. Upward salt movement generates a radiating pattern of faults and fractures that provides the hydraulic conduits for hydrocarbon fluid migration. The complexity of the plumbing system can be fully revealed only when observed over the three datasets at different resolution. The 3D Volume industry data display few master faults nucleating at the salt dome and intersecting the seafloor. Where these intersections occur, craters and pockmarks form. Amplitude anomalies, or “bright spots,” at multiple stratigraphic levels occur in close spatial association with the master fault system, suggesting that hydrocarbon fluids are migrating from the deep oil reservoirs via the faults and that they can and do accumulate in the shallow subsurface where favorable hydraulic conditions are met. The SSDR data also suggest that small faults networks, not resolvable in the oil industry data, contribute to the hydraulic communication within the hydrates stability zone. A direct comparison of coincident profiles from the 3D Volume and the SSDR seismic survey suggests that the geophysical signature of the subsurface at MC118 exhibits transience on the scale of years. These two seismic surveys were collected with significantly different acquisition parameters (particularly frequency and sample rate) approximately six years apart. Although the seismic response and corresponding images look considerably different, stratigraphic and structural geometries can readily be correlated between the two datasets. While the 0.4 s. TWTT high amplitude, negative bright spot within the 3D volume exhibits a similar identifiable amplitude anomaly on the SSDR data, there appears to be other areas where the 0.2 s. TWTT bright spot is present on the 3D Volume (collected in 2000) but is absent on the SSDR data (collected in 2006). Our interpretation is that the shallower bright spot is the seismic signature of free hydrocarbon gas. Faults, fractures and evident vertical seismic anomalies (discontinuity and wipeout zones) indicative of hydrocarbon leakage, hydraulically connect the gas with the seafloor where pockmarks and craters are present. Therefore a time variant sealing mechanism must be acting to trap e and alternatively release e the gas within the sediment. We suggest
139
that the mechanism is the formation/dissociation of gas hydrates in the faults and in the fractures associated with them. The shallower bright spot, therefore, represents an anomalous discontinuous “BSR” marking the base of the HSZ. Most importantly, integrating both mound surface and subsurface observations, we speculate that the mechanisms through which fluids and gas transit along the shallow fault systems are different within the mound and directly guide the morphology and the biological evolution of the mound.
Acknowledgments The authors are deeply grateful to TGS-Nopec for the 3D seismic volume and to SMT to support educational licenses of Kingdom Suite. The research has benefited by numerous inputs and suggestions from colleagues attending the 2010 Gordon Conference, where preliminary results of this work were presented. For this reason, we would like to thank conference Chairs C. Ruppel and P. Flaming who have provided support to A. Simonetti. A special thanks to the MMRI Shop, SDI personnel and the R/V Pelican Crew for their always-outstanding job at sea, and to Paul Mitchell, Marco D’Emidio and Marco Pizzi of MMRI for graphic and GIS support. This project has been funded through the Gulf of Mexico Hydrates Research Consortium with funds jointly provided by U.S. DOE-NETL, DOI-BOEM, DOC NOAA-NIUST.
Appendix A. Seismic data specs High quality 3D oil industry multichannel seismic data from TGSNopec (3D Volume) Standard oil industry data have been acquired in 1999e2000 by TGS-Nopec, and data specs and relevant seismic processing steps are reported in Table A1. The 3D seismic volume was truncated at 3sec. to respect constraints imposed by proprietary conditions. Table A1 TGS NOPEC Acquisition Parameter and processing flowchart. Acquisition parameters Acquisition date: Data acquired by: Shooting orientation: Recording instrument: Streamer type: Source/streamer positioning: Airgun source: Gun depth: Shotpoint interval: CMP crossline separation: Group interval: Recording channel: Streamer depth: Streamer length: Record length: Sample interval: Nominal fold:
10/23/99e07/06/2000 CGG NortheSouth Syntron 480 Syntron GPS/DGPS 4180 cubic inches 7.5 m 1 m 62.5 m per CMP line 40 m 25 m 288 per streamer 9 m 1.5 m 7200 m 12.288 s 2 ms 57.6
Surface-Sources Deep-Receiver (SSDR) SSDR data were collected by the CMRET during two different cruises in April and June, 2006. The SSDR recording geometry is shown in Figure A1. SSDR data processing strategy, the benefit of the system and the imaging capability are presented extensively in McGee (2000), Macelloni (2005), Battista et al. (2007) and Macelloni et al. (2011).
140
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
Figure A1. SSDR geometry and survey grid.
AUV-born chirp sub-bottom profiler Chirp sub-bottom profiler data were collected with the C&C Technology AUV Hugin 3000 in 2005 aside high-resolution swath bathymetry and Side-Scan Sonar. A sub-bottom profiler (DW216) manufactured by Edgetech was utilized for seismic imaging of the near-seabed sediments. The system is frequency modulated
between 2 and 8 kHz. A record length of 300 ms and a 63-ms sampling interval were used for seismic data recording. The subbottom profiles were output in SEG-Y format. Static offsets for the AUV depth are added in post-processing to eliminate as much of the water column as possible from the dataset. A total of 25 lines running east-west and 180 m spacing plus two control lines running north-south were collected, as reported in Figure A2.
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
141
Figure A2. Chirp sub-bottom survey grid.
Appendix B. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.marpetgeo.2012. 06.010. References Allen, J.L., Peddy, P.G., 1993. Amplitude Variation with Offset e Gulf Coast Studies. Society of Exploration Geophysicists. Andresen, K.J., Huuse, M., Schodt, N.H., Clausen, L.F., Seidler, L., 2011. Hydrocarbon plumbing system of salt minibasins offshore Angola revealed by threedimensional seismic analysis. AAPG Bulletin 95 (6), 1039e1065. Battista, B.M., Knapp, C., McGee, T.M., Goebel, V., 2007. Application of the empirical mode decomposition and HilberteHuang transform to seismic reflection data. Geophysics 72, H29eH37. Chanton, J., Lapham, L.L., Wilson, R., Martens, C., Mendlovitz, H., 2011. Geochem Studies at MC118 Seafloor Station. Information Transfer Meeting in New Orleans, LA. March 22e24th, 2011. Galloway, W.E., Buffler, R.T., 2004. Gulf Basin Depositional Synthesis Project. Available at: http://www.ig.utexas.edu/research/projects/gbds/gbds.htm. Gay, A., Lopez, M., Berndt, C., Séranne, M., 2007. Geological controls on focused fluid flow associated with seafloor seeps in the Lower Congo Basin. Marine Geology 244, 68e92. Hovland, M., Gudmestad, O.T., 2001. Potential influence of gas hydrates on seabed installations. In: Paull, C.K., Dillon, W. (Eds.), Natural Gas Hydrates: Occurrence, Distribution and Detection. American Geophysical Union Monograph, vol. 124. AGA, Washington, D.C, pp. 307e315. Hovland, M., Judd, A.G., 1988. Seabed Pockmarks and Seepages. Impact on Geology, Biology and the Marine Environment. Graham and Trotman, Ltd., London. Huuse, M., Jackson, C.A., Van Rensbergen, P., Davies, R., Flemings, P.B., Dixon, R.J., 2010. Subsurface sediment remobilization and fluid flow in sedimentary basins: an overview. Basin Research 22, 342e360. Ingram, W.C., Meyers, S.R., Brunner, C.A., Martens, C.S., 2010. Evaluation of Late PleistoceneeHolocene sedimentation surrounding an active seafloor gas hydrate and cold seep field on the Northern Gulf of Mexico Slope. Marine Geology 278, 43e53.
Knapp, J.H., Knapp, C.C., Macelloni, L., Simonetti, A., Lutken, C.B., 2010. Subsurface Structure and Stratigraphy of a Transient, Fault-Controlled Thermogenic Hydrate System at MC-118, Gulf of Mexico AAPG Annual Meeting Transactions, New Orleans (LA) April 11eApril 14 2010. Lapham, L.L., Chanton, J.P., Martens, C.S., Woolsey, J.R., 2008. Microbial activity in surficial sediments overlying acoustic wipe-out zones at a Gulf of Mexico cold seep. Geochemistry, Geophysics, Geosystems 9 (6). Løseth, H., Wensaas, L., Arntsen, B., Hanken, N., Basire, C., Graue, K., 2001. 1000 M Long Gas Blow-Out Pipes. EAGE, Amsterdam. Løseth, H., Gading, M., Wensaas, L., 2009. Hydrocarbon leakage interpreted on seismic data. Marine and Petroleum Geology 26, 1304e1319. Macelloni, L., 2005. High-Resolution Marine Digital Seismic Method: Theoretical Constrains, Digital Processing, New Developments. PhD dissertation, pp. 71. In PADIS e Pubblicazioni Aperte Digitali Interateneo Sapienza [PHDe2005-187] http://padis.uniroma1.it/. Macelloni, L., Caruso, S., Lapham, L., Lutken, C., Brunner, C., Lowrie, A., 2010. Spatial distribution of seafloor bio-geological and geochemical processes as proxy to evaluate fluid-flux regime and time evolution of a complex carbonate/hydrates mound, northern Gulf of Mexico. Gulf Coast Association of Geological Societies Transactions 60, 461e480. Macelloni, L., Batista, B.M., Knapp, C.C., 2011. Optimal filtering high-resolution seismic reflection data using a weighted-mode empirical mode decomposition operator. Journal of Applied Geophysics 75 (4), 603e614. McGee, T.M., 2000. Pushing the limits of high-resolution marine seismic profiling. European Journal of Environmental and Engineering Geophysics 5, 43e53. McGee, T.M., Macelloni, L., 2006. Seismo-Acoustic Imagery of a Carbonate/Hydrate Mound in the Gulf of Mexico. American Geophysical Union Fall Meeting San Francisco, California 11e15 December 2006. Milkov, A.V., Sassen, R., 2000. Thickness of the gas hydrate stability zone, Gulf of Mexico continental slope. Marine and Petroleum Geology 17, 981e991. Reilly, M.,J., Flemings, P.,B., 2010. Deep pore pressure and seafloor venting in the Auger Basin, Gulf of Mexico. Basin Research 22, 380e397. Ruppel, C., Dickens, G.R., Castellini, D.G., Gilhooly, W., Lizarralde, D., 2005. Heat and salt inhibition of gas hydrate in the northern Gulf of Mexico. Geophysical Research Letters, 32. Sassen, R., Roberts, H.H., 2004. Site Selection and Characterization of Vent Gas, Gas Hydrate, and Associated Sediments. In DOE Technical Report, pp. 28e227. Sassen, R., Roberts, H.H., Jung, W., Lutken, C.B., DeFreitas, D.A., Sweet, S.T., Guinasso Jr., N.L., 2006. The Mississippi Canyon 118 gas hydrate site: a complex
142
L. Macelloni et al. / Marine and Petroleum Geology 38 (2012) 128e142
natural system. In: OTC Paper #18132; Offshore Technology Conference, Houston, TX. Shedd, W., Roberts, H.H., 2009. Use of 3-D seismic to characterize seafloor hydrocarbon, brine, and sediment expulsion in the slope of the northern Gulf of Mexico. Environmental Geoscience 16 (2), 106e117. Shedd, W., Boswell, R., Frye, M., Godfriaux, P., Krame, K., 2012. Occurrence and nature of “bottom simulating reflectors” in the northern Gulf of Mexico. Marine and Petroleum Geology 34, 31e40. Sleeper, K., Lowrie, A., Bosman, A., Macelloni, L., Swann, C.T., 2006. Bathymetric mapping and high resolution seismic profiling by AUV in MC 118 (Gulf of Mexico). In: OTC Paper # 18133, Offshore Technology Conference, Houston, TX. Sloan, E.D., 1998. Clathrate Hydrates of Natural Gases, second ed. Dekker, New York, 705 pp.
Wood, W.T., Gettrust, J.F., Chapman, N.R., Spence, G.D., Hyndman, R.D., 2002. Decreased stability of methane hydrates in marine sediments owing to phaseboundary roughness. Nature 420, 656e660. Wood, W., Hart, P.E., Hutchinson, D.R., Dutta, N., Snyder, F., Coffin, R.B., Gettrust, J.F., 2008. Gas and gas hydrate distribution around sea floor seeps in Mississippi Canyon, northern Gulf of Mexico, using multi-resolution seismic imagery. Marine and Petroleum Geology 25 (9), 952e959. Woolsey, J.R., Higley, P., Lapham, L.L., Chanton, J.P., Lutken, C.B., Sleeper, K., Culp, R., Sharpe, S., Ross, D., 2005. Operations Report of Cruise GOM2-05-MC118: Deployment of the Initial Components of the Sea Floor Monitoring Station e The Pore-Fluid Array and the Geophysical Line Array e Via the Sea-Floor Probe System and Collection of Core Samples, Mississippi Canyon 118. In Gulf of Mexico Hydrates Research Consortium Cruise Report.