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Occurrence and nature of “bottom simulating reflectors” in the northern Gulf of Mexico
Marine and Petroleum Geology 34 (2012) 31e40
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Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo
Occurrence and nature of “bottom simulating reflectors” in the northern Gulf of Mexico William Shedd a, *, Ray Boswell c, Matthew Frye b, Paul Godfriaux a, Kody Kramer a a
U.S. Bureau of Ocean Energy Management, New Orleans, LA 70123, USA U.S. Bureau of Ocean Energy Management, Herndon, VA, USA c U.S. Department of Energy, National Energy Technology Laboratory, Morgantown, WV, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 May 2011 Received in revised form 10 August 2011 Accepted 11 August 2011 Available online 22 August 2011
Subsurface interpretation, utilizing a database of more than 450,000 km2 (175,000 mi2) of threedimensional (3-D) seismic in the northern Gulf of Mexico (GoM), reveals 145 discrete areas, totaling 4450 km2 (1.1 million acres) where the base of gas hydrate stability (BGHS) can be confidently inferred from seismic data. Unlike many other areas of the world, the majority of these features are not Bottom Simulating Reflectors (BSRs) in the “classic” sense, meaning continuous coherent events that cross-cut primary stratigraphy. Those typical, or continuous BSRs, are noted in only 24% of the features identified within this study. In contrast, the most common seismic manifestation of the BGHS in the GoM (59%) is the discontinuous “BSR”, delineated by widely separated anomalous seismic events that align in general conformance with seafloor bathymetry. A third type of seismic feature, pluming “BSRs”, are continuous events that are not bottom-simulating, but are bowed toward the seafloor and represent areas where large, but areally-limited increases in heat flow (linked to strong vertical fluid flux), perturb the BGHS. The limited nature of continuous BSRs and the relative abundance of discontinuous and pluming forms are attributed to the strong lithologic and structural heterogeneity of the northern GoM basin. This lithologic and structural complexity has served to disrupt and localize regionally pervasive and homogeneous gas flux that is consistent with the formation of large, continuous BSRs noted across other less complex continental margins. The various BSR forms identified in this study are shown to be closely associated (125 of 145) with the occurrence of seafloor amplitude anomalies, which are in turn usually associated with the flanks and crests of salt-cored ridges. These associations are interpreted to reflect the co-dependence of BSRs and seafloor reflectivity along the migration pathways that typify this geologic setting. Published by Elsevier Ltd.
Keywords: Bottom simulating reflectors Gas hydrates Gulf of Mexico Seafloor anomalies
1. Introduction Seismic reflection data from deepwater continental margins commonly exhibit anomalous, shallow seismic events called “bottom-simulating reflectors” (BSRs). BSRs are anomalous in that their orientation shows no relationship to sedimentary layering; instead, they mimic the general bathymetry of the seafloor, with gradually increasing sub-seafloor depth with increasing water depth. As a result, BSRs are inferred to be caused by physical property changes in marine sediments that are driven by depth dependent phenomena such as pressure, or temperature, or both. Two primary phenomena are responsible for the majority of BSRs. The first includes BSRs associated with diagenetic boundaries,
* Corresponding author. Tel.: þ504 736 2497; fax: þ504 736 2905. E-mail address: [email protected] (W. Shedd). 0264-8172/$ e see front matter Published by Elsevier Ltd. doi:10.1016/j.marpetgeo.2011.08.005
such as the transformation of opal-A to opal-CT, which tends to occur at fixed subsea depth in siliceous sediments (Hein et al., 1978). These “diagenetic” BSRs mark horizons where sediment density increases with depth. Such BSRs have polarity equivalent to the seafloor reflection. Other less common diagenetic processes, such as opal-CT-to-quartz and illite-to-smectite transitions, may also produce BSRs in some areas (Berndt et al., 2004), although data are inconclusive. A second phenomenon, which produces the majority of documented BSRs, is related to the negative impedance caused by the transition from gas-hydrate-bearing sediments (higher compressional velocity (Vp)) to water- or free-gas-bearing sediments (lower Vp) at the base of gas hydrate stability zone (BGHS). The resulting BSRs are characterized by having a reflection polarity opposite that of the seafloor. Kvenvolden and Lorenson (2001) provide a list of 53 global locations where BSRs inferred to be related to gas hydrate had been reported prior to 2001. All BSRs noted in the Gulf of
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Mexico to date (Dai et al., 2004; Kou et al., 2007; Hutchinson et al., 2008) are interpreted to be related to the occurrence of gas hydrate. BSRs were first attributed to gas hydrate where they occur broadly and conspicuously along the Blake Ridge, offshore eastern North America (ex. Bryan, 1974; Tucholke et al., 1977). Shipley et al. (1979) extended this interpretation to similar features noted on continental shelves world-wide. Subsequently, BSRs of appropriate polarity and sub-seafloor depth have been considered to be reliable indicators of the position of the BGHS and of the occurrence of gas hydrate (Hyndman and Spence, 1992). Most BSRs coincide with the BGHS, but situations do develop in which the top of free gas occurs below the BGHS and also where the base of gas hydrate occurs well within the gas hydrate stability zone (GHSZ). This leads to potential offsets between observed BSRs and the BGHS (Xu and Ruppel, 1999). Also, gas hydrate has been observed occurring below BSRs where sufficient heavier hydrocarbon gases are available to produce structure II or structure H hydrates (Hadley et al., 2008). Consequently, the acronyms BSR and BGHS are not interchangeable terms. BGHS describes an inferred phase boundary (often indicated by a BSR, but also commonly not), while a BSR is any anomalous seismic event which has an orientation that mimics seafloor bathymetry without regard to sediment depositional fabric. Such seismic reflections may or may not be related to gas hydrate. Much work has been conducted to discern the utility of a BSR as a gas hydrate exploration tool. Attempts to quantify the
concentration of gas hydrate associated with BSRs have relied primarily on advanced AVO analyses (ex. Ecker et al., 1998). However, most workers agree that the seismic attributes of BSRs are primarily influenced by the occurrence of free gas (Singh et al., 1993). Given that even very low free-gas saturations produce significant changes in Vp (Domenico, 1976), attempts to obtain meaningful quantification of gas hydrate phase saturations from BSRs remain challenging (see Chen et al., 2007). In addition, the nature and appearance of BSRs depend on the nature and frequency of the seismic data in which they are expressed. For example, reflection amplitude decreases with the frequency content of the seismic data (Chapman et al., 2002). Furthermore, drilling programs which have tested multiple locations of varying geophysical character within regional BSR areas, have concluded that BSRs provide little useful information related to the occurrence of gas hydrate, either at the BSR or within the GHSZ above a BSR (Tsujii et al., 2009). Despite the difficulties in discerning various phase saturations from geophysical attributes of BSRs, we suggest that differences in the overall seismic expression of the BGHS may provide important information on the nature of the associated gas hydrate petroleum system. This report presents the results to date of a review of extensive 3-D seismic datasets to document the occurrence and distribution of gas-hydrate-related BSRs in the northern Gulf of Mexico. In addition, we also report the occurrence of over 21,000 seafloor
Figure 1. Seafloor morphology of the northern Gulf of Mexico. Super-imposed are outlines of 145 areas in which features inferred to mark the BGHS are observed in seismic data. The color indicates the dominant morphology (many areas show elements of more than one form) of these BSRs as continuous (yellow), discontinuous (red), and pluming (green). The dashed blue line indicates the area of uninterrupted 3-D seismic data coverage available for the study. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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amplitude anomalies which appear to define areas of past or current hydrocarbon leakage at the seafloor, and we compare the distribution of these features to mapped BSRs. We conclude with some observations which attempt to relate the unique geology of the northern Gulf of Mexico slope to the distribution and nature of the observed BSRs. 2. Seismic data Our study was conducted by accessing more than 450,000 km2 (175,000 mi2) of 3-D seismic data in water depths greater than 200 m
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(656 ft) in the possession of the U.S. Bureau of Ocean Energy Management, Regulation, and Enforcement (BOEMRE). These data cover the vast majority of the northern Gulf of Mexico shelf and slope (Fig. 1). Much of the data is recent vintage and high quality, and in most regions, multiple overlapping datasets were available for review. However, 3-D data coverage for the area seaward of the Sigsbee Escarpment is not yet complete, and although 2-D datasets are available in that region, the difficulty in delineating BSRs in its flatlying sediments dictated that minimal effort was spent in searching for BSRs within the abyssal plain. Confidence in the identification of
Figure 2. Example discontinuous BSR from Walker Ridge 313. (a) Un-interpreted. (b) Interpreted. (c) Red dashed line indicates inferred BSR and BGHS. DataÓ 2011 WesternGeco. Used by permission. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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each geophysical feature was qualitatively rated, with only those assigned as “fair” (14% of the total identified), “good” (38%), “very good (31%)” and “excellent (17%)” being included. Seafloor amplitude anomalies are derived from the same seismic dataset and include both high positive anomalies (hard-grounds associated with seafloor authigenic carbonate precipitation and sometimes gas hydrate (Roberts et al., 2006)) as well as low-positive or negative anomalies (currently active high flux hydrocarbon venting). Given the immensity of the available seismic data, this analysis is not exhaustive or complete and is considered a report of work in progress.
3. Results Bottom simulating reflectors are, as the phrase implies, associated with coherent and continuous reflectors which mimic the seafloor bathymetry. The majority of features recognized to date in regions outside of the GoM that are labeled as BSRs are of this nature. We call these “classic” features continuous BSRs. However, Vanneste et al. (2001) at Lake Baikal and McConnell and Kendall (2002) in the northern GoM reported “BSRs” which were not manifested as actual reflectors, but were a virtual subsurface horizon delineated by the alignment of terminations of separate high-amplitude reflections within a succession of stratigraphic units. These reflection terminations can be closely spaced, as observed in the Lake Baikal data, or widely separated as was noted in the northern GoM. In either case, there was no perceptible change in the character of the seismic data within the section between the anomalous reflectors (Fig. 2). We call this seismic expression of the BGHS a discontinuous “BSR”, which is consistent with prior usage of the term. Furthermore, in areas of strong vertical gas and fluid flux, local variations in lateral temperature gradients of sediments (and probably in pore-water salinity as well) can create discordant and continuous reflectors which mark the BGHS but which are clearly not “bottom-simulating” (Wood et al., 2002). We term these seismic features pluming “BSRs”. This review identified 145 areas across the northern Gulf of Mexico in which the BSR can be identified in seismic data (Fig. 1). Over half (55%) of the mapped features include elements of more than one BSR type (see Fig. 3 for an excellent example from Garden Banks 859). In such cases, each BSR is categorized based on its dominant form. Overall, discontinuous “BSRs” account for 85 of the 145 (58.6%) identified BSRs in the northern GoM. Continuous BSRs (ex. Fig. 4) account for 35 features (24.2%), while the remaining 25 features (17.2%) have a “pluming” form (Fig. 5). As shown in Figure. 1, BSRs are observed throughout the minibasin salt province but appear to be most common in the central and western GoM, away from the Mississippi Fan coincident with the region of most extensive salt diapirism. Within the Mississippi Fan, most deep-seated faults are buried by rapid, recent sedimentation and, cannot act as conduits to the shallow, hydrate-prone stratigraphic section. BSRs are also less common in the western portions of the Alaminos Canyon and East Breaks protraction areas. BSRs of any type are conspicuously absent in the abyssal plain, with the exception of a few small features noted in Atwater Valley and Mississippi Canyon protraction areas. This latter observation may be attributed in part to the difficulty of recognizing BSRs in flatlying strata. Cumulatively, the 145 mapped BSRs in the GoM encompass roughly 4450 km2 (1.1 million acres (Fig. 6)). The largest feature is a discontinuous “BSR” which occurs around the East Breaks block 992 area and is roughly 453 km2 (112,000 acres). Individually the features are generally small in comparison to many BSRs noted in other basins, with a mean size of 30.9 km2 (7634 acres). Pluming “BSRs”, which by their highly discordant nature are easily
Figure 3. Example of an extensive BSR from Garden Banks 859 showing pluming, discontinuous, and continuous components. (a) Un-interpreted profile. (b) Interpreted profile with red dashed line indicating inferred BSR and BGHS. DataÓ 2011 WesternGeco. Used by permission. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
recognized across small areas, account for most of the smaller features (Fig. 6). Each BSR was also categorized with respect to its general geologic setting. The majority (66.9%) of BSRs are located above diapiric salt structures. Of those, there is an equal portion of BSRs which occur directly over shallow salt (within 0.5 s of seafloor) compared to above salt flanks (Fig. 7). BSRs which occur over deeper salt bodies account for 31% of the data. Only 2% of BSRs occur entirely within the interior of mini-basins. Notably, 125 of the 145 of the mapped BSRs (86.2%) are associated (spatially) with anomalies in seafloor reflectivity. The shared occurrence between BSRs and seafloor reflection amplitude anomalies with salt bodies and salt margins is inferred to reflect preferential pathways for hydrocarbon migration into the shallow section afforded both by sediment dip (upwards away from mini-basin interiors) and more importantly by faulting associated with diapiric salt bodies (Fig. 8). Although a zone of gas hydrate stability occurs throughout the northern GoM wherever water depth exceeds w400 m (1300 ft), we find no conclusive seismic expressions of the BGHS in water depths less than 606 m (1989 ft; Fig. 9). The deepest feature mapped (in Alaminos Canyon 818) is in 2790 m (9151 ft) of water. The average water depth of the 154 features is 1416 m (4646 ft). Although the general rule is that the depth of the GHSZ below the seafloor increase with water depth, a plot of GHSZ thickness below the seafloor and water depth shows a great degree of scatter (Fig. 10). This scatter is attributed to the significant heterogeneity found in the shallow, gas hydrate-prone GoM subsurface stratigraphic section (thermal, pressure, and geochemical) that results from the basin’s salt tectonism which produced extensive structural and stratigraphic complexity. 4. Discussion The early perception of gas hydrate occurrence in the GoM basin was pessimistic, due largely to the lack of recognition of large,
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Figure 4. Example of continuous BSR from Garden Banks 982. (a) Un-interpreted profile. (b) Interpreted profile with red dashed line indicating inferred BSR and BGHS. DataÓ 2011 WesternGeco. Used by permission. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Figure 5. Example of pluming BSR (left side of image) from Green Canyon 475. Continuous BSRs are present to the center and right. (a) Un-interpreted profile. (b) Interpreted profile with red dashed line indicating inferred BSR and BGHS. DataÓ 2011 WesternGeco. Used by permission.
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Figure 6. (a) Histogram of GoM BSRs size (in acres). Over 80% of BSRs are 12,000 acres or smaller. (b) BSR size (in acres) color-coded by type. Discontinuous BSRs account for most of the larger features, while pluming BSRs are generally small. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Figure 7. Map showing the relationship between mapped BSRs and depth to top of salt. More than 66% of the mapped features are associated with the crest or flanks of shallow salt features.
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Figure 8. Map showing the relationship between seafloor bathymetry (gray), seafloor seismic amplitude anomalies (red), and mapped BSRs (yellow). The inset shows an area typical of the GoM upper slope in which seafloor reflection amplitudes are concentrated along salt-cored highs and ridges which form the margins of salt-withdrawal mini-basins. Seafloor reflectivity anomalies and BSRs are rare in the interior of mini-basins, probably because of the general focusing of hydrocarbon migration toward mini-basin flanks. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Figure 9. Range of water depths plotted with BSR type noted. Discontinuous and continuous BSRs occur through the full range of water depths, whereas pluming BSRs are concentrated in water depths of w1200 m (w4000 ft) or less, and none occur in water depths greater than w1500 m (w5000 ft).
Figure 10. Scatter plot of 145 GoM BSRs with respect to sub-seafloor depth (thickness of subsurface portion of the GHSZ) and water depth. The scatter in the data is a function of extreme heterogeneity of the basin resulting from salt tectonism.
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regional BSRs of the type that had been previously described on the Blake Ridge and in many basins around the world (see Boswell et al., 2012). A number of studies correctly pointed out that the prospects for widespread occurrences of gas hydrate in the basin were limited, due primarily to the thermal and geochemical heterogeneities which significantly reduce the total volume of the GHSZ (Paull et al., 2005; Ruppel et al., 2005; Johnson and Smith, 2006). However, these same heterogeneities, compounded by the basin’s noted lithologic heterogeneity and structural complexity, as well as by the existence of an underlying world-class petroleum system, result in two considerations relevant to the occurrence of gas hydrate and the subsequent development of localized BSRs.
These impacts, while rendering the basin unfavorable for the formation of large and continuous gas hydrate occurrences like the Blake Ridge, make it more favorable for the formation of relatively small, concentrated gas hydrate deposits which may have relevance as a producible hydrocarbon resource. 4.1. Occurrence of reservoir controls Most continental margins receive significant coarse-sediment supply. In many passive margins, most of this sand bypasses the slope and is deposited in the deep basin. However, seafloor bathymetry related to salt tectonics in the northern GoM produces
Figure 11. Seismic data from Green Canyon 781 showing a discontinuous BSR with phase reversals at two horizons. (a) Un-interpreted profile. (b) Partially interpreted profile. DataÓ 2011 WesternGeco. Used by permission.
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conditions suitable for significant sand deposition within “ponded” mini-basins on the slope (Prather et al., 1998). The permeability inherent in these sands provides suitable conditions for the formation of highly concentrated gas hydrate occurrences (as reviewed in Collett et al., 2009). 4.2. Occurrence of pathways for focusing gas migration and sourcing Areas with generally homogeneous sediment overall (typically muds in deepwater settings occurring in thin stratigraphic sections over active salt diapirs) are predisposed to continuous BSRs. This outcome is primarily due to the lack of differential local permeability, which can serve to focus gas migration and accumulation. Instead, a regionally pervasive and diffuse gas migration can occur, with continuous BSRs being a likely result. Where fluid flux in such settings does become concentrated (and typically vertical), BGHS may be deflected upwards (reflecting higher temperatures), producing a pluming “BSR”. In contrast, sediment systems with greater lithologic heterogeneity (interbedded sands and shales), are more likely to focus the migration and accumulation of gas into coarser-grained sediments. Structural deformation, such as that related to the salt tectonics in the GoM, can provide significant structural dip to such units, making them viable pathways for extended migration with significant lateral components (Frye, 2008). Furthermore, the extensive faulting that typifies the flanks of diapiric salt structures provides additional high-permeability pathways for the migration of gas into the shallow sedimentary section. The efficacy of these pathways is demonstrated by the correlation between seafloor reflectivity anomalies, the occurrence of BSRs of all types, and the margins of salt bodies (Figs. 7 and 8). In addition to the previously mentioned abyssal plain, we recognize several areas where BSRs are largely absent. The lower slope of southern Keathley Canyon (KC) and Walker Ridge (WR) is characterized by a thick, continuous salt canopy which is fed by a combination of numerous vertical salt stocks and the
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allochthonous lateral movement of extensive salt canopies (e.g., Pilcher et al., 2011). The absence of an extensive through-going fault network, such as observed near the flanks of diapiric salt features on the upper slope, reduces the opportunity for significant point-sourced gas accumulations in the GHSZ. Additionally, the stratigraphic section above these salt canopies is often quite thin and high heat flow and salinities may inhibit hydrate formation. The western half of protraction areas East Breaks (EB) and Alaminos Canyon (AC) are also characterized by a limited vertical salt movement and a complete absence of BSRs. Note that both of these areas (southern KC and WR; western EB and AC) have few seismic amplitude reflection anomalies on the seafloor (Fig. 8). A key finding of this study is not only the gas hydrate-relevant features identified, but also the strong predisposition for the BGHS to manifest as discontinuous “BSRs” in the northern GoM. Joint Industry Project (JIP) Leg II drilling at WR block 313 was designed to test the hypothesis that seismic amplitude phase reversals (McConnell and Zhang, 2005) associated with discontinuous “BSRs”, which suggest the buoyant separation of gas and water within discrete units, provide good evidence of the gas charge (and gas hydrate occurrence) within reservoir facies. The results provided strong confirmation of this model (Shedd et al., 2010; Boswell et al., 2012). Of the 85 discontinuous “BSRs” identified, good evidence of phase reversals in 45 of the features was noticed. Figure. 11 (GC block 781) provides an additional example of such a system, which although not drilled by the JIP Leg II program, must now be considered highly prospective for gas hydrate in sands. The correlation between the occurrence of BSRs and the locations of discovered oil and gas fields is less than one to one (Fig. 12), although we do observe some significant relationships. BSRs are found in close proximity to many of the oil and gas fields of eastern EB, AC, Garden Banks (GB), and Green Canyon (GC), yet BSRs are mostly absent from the western halves of these same four protraction areas, where discovered oil and gas fields number far fewer. Additionally, oil fields on the lower slope of southern KC and WR are deeply buried (up to 30,000 ft below sea level) and are located underneath extensive salt canopies. We do not expect these
Figure 12. Distribution of mapped BSRs and discovered oil and gas fields in the deepwater Gulf of Mexico.
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oil fields to have a direct correlation with a dense BSR network. Many of the BSRs in the oil and gas rich Mississippi Canyon protraction area are located near discovered fields, but the inverse relationship of a BSR associated with every oil or gas field does not hold true. 5. Summary By accessing BOEMRE managed 3-D seismic data covering more than 450,000 km2 (175,000 mi2) of the deepwater northern Gulf of Mexico, more than 145 locations were identified and mapped where the BGHS is delineated by anomalous seismic features. These BSRs are assigned to three classes based on their gross morphology. The most common are discontinuous “BSRs” which reflect areas where a variable lithology consists of alternating sediments of fine and relatively coarse grain size. The sands traverse the BGHS in an area of sufficient gas charge to saturate the sand with gas hydrate above the BSR and to accumulate some volume of free gas below. Less common are the through-going, continuous BSRs (similar to those seen in other basins, although typically much smaller) which have been interpreted as forming primarily in areas with more widely distributed gas flux and generally homogeneous and fine-grained sediments. A third category known as pluming “BSRs”, in which continuous BSRs are warped upward, represent areas of concentrated methane and warm brine vertical flux. This variety in seismic expression of the BGHS is a reflection of the heterogeneous geologic setting of the northern Gulf of Mexico. The distinctions in BSR type provide important clues into the nature of the local gas hydrate petroleum system. Of the three various forms, discontinuous “BSRs” are recognized as a strong indicator of the confluence of both source and reservoir at the base of the gas hydrate stability zone. Confirmation of this concept provided by Joint Industry Project Leg II drilling (Shedd et al., 2010; Boswell et al., 2012) and the prevalence of these forms in the northern GoM suggests strong potential for the occurrence of concentrated gas hydrates in the basin. Acknowledgments The authors wish to thank all the sources of the seismic data used in this study: WesternGeco, TGS, PGS, CGGVeritas, Diamond, Geophysical Pursuit, Jebco, Seitel, and especially WesternGeco for granting permission to use the images of their seismic crosssections. References Berndt, C., Bunz, S., Clayton, T., Mienert, J., Saunders, M., 2004. Seismic character of bottom simulating reflectors: examples from the mid-Norwegian margin. J. Mar. Pet. Geol. 21, 723e733. Boswell, R., Collett, T., Frye, M., Shedd, B., McConnell, D., Shelander, D., 2012. Subsurface gas hydrates in the northern Gulf of Mexico. J. Mar. Pet. Geol. 34, 4e30. Bryan, G., 1974. In situ indications of gas hydrate. Marine Sci. 3, 299e308. Chapman, N., Gettrust, J., Walia, R., Hannay, D., Spence, G., Wood, W., Hyndman, R., 2002. High-resolution, deep-towed, multichannel seismic survey of deep-sea gas hydrates off western Canada. Geophysics 67 (4), 1038e1047. Chen, M., Riedel, M., Hyndman, R., Dosso, S., 2007. AVO inversion of BSRs in marine gas hydrate studies. Geophysics 72 (2), 31e43. Collett, T., Johnson, A., Knapp, C., Boswell, R., 2009. Natural gas hydrates e a review (Chapter 1). In: Collett, T., et al. (Eds.), Natural Gas Hydrates e Energy Resource Potential and Associated Geologic Hazards. AAPG Memoir, vol. 89, p. 74.
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Seismic evidence for widespread possible gas hydrate horizons on continental slopes and rises. AAPG Bull. 63, 2204e2213. Singh, S., Minshull, T., Spence, G., 1993. Velocity structure of a gas hydrate reflector. Science 260, 204e207. Tsujii, Y., Fujii, T., Hayashi, M., Kitamura, R., Nakamizu, M., Ohbi, K., Saeki, T., Yamamoto, K., Namikawa, T., Inamori, T., Oikawa, N., Shimizu, S., Kawasaki, M., Nagakubo, S., Matsushima, J., Ochiai, K., Okui, T., 2009. Methane-hydrate occurrence and distribution in the eastern Nankai trough, Japan: Findings of the Tokai-oki to Kumano-nada methane-hydrate drilling program. In: Collett, T., Johnson, A., Knapp, C., Boswell, R. (Eds.), Natural Gas Hydrates e Energy Resource Potential and Associated Geologic Hazards: AAPG Memoir, vol. 89, pp. 228e246. Ch 3. Tucholke, B., Bryan, G., Ewing, J., 1977. Gas-hydrate horizons detected in seismicprofiler data from the western North Atlantic. AAPG Bull. 61, 698e707. Vanneste, M., De Batist, M., Golmshtok, A., Kremlev, A., Versteeg, W., 2001. Multifrequency seismic study of gas hydrate-bearing sediments in Lake Baikal, Siberia. Marine Geol. 172, 1e21. Wood, W., Gettrust, J., Chapman, N., Spence, G., Hyndman, R., 2002. Decreased stability of methane hydrates in marine sediments due to phase boundary roughness. Nature 420, 656e660. Xu, W., Ruppel, C., 1999. Predicting the occurrence, distribution, and evolution of methane gas hydrate in porous marine sediments from analytical models. J. Geophys. Res. 104, 5081e5096.
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Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo
Occurrence and nature of “bottom simulating reflectors” in the northern Gulf of Mexico William Shedd a, *, Ray Boswell c, Matthew Frye b, Paul Godfriaux a, Kody Kramer a a
U.S. Bureau of Ocean Energy Management, New Orleans, LA 70123, USA U.S. Bureau of Ocean Energy Management, Herndon, VA, USA c U.S. Department of Energy, National Energy Technology Laboratory, Morgantown, WV, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 May 2011 Received in revised form 10 August 2011 Accepted 11 August 2011 Available online 22 August 2011
Subsurface interpretation, utilizing a database of more than 450,000 km2 (175,000 mi2) of threedimensional (3-D) seismic in the northern Gulf of Mexico (GoM), reveals 145 discrete areas, totaling 4450 km2 (1.1 million acres) where the base of gas hydrate stability (BGHS) can be confidently inferred from seismic data. Unlike many other areas of the world, the majority of these features are not Bottom Simulating Reflectors (BSRs) in the “classic” sense, meaning continuous coherent events that cross-cut primary stratigraphy. Those typical, or continuous BSRs, are noted in only 24% of the features identified within this study. In contrast, the most common seismic manifestation of the BGHS in the GoM (59%) is the discontinuous “BSR”, delineated by widely separated anomalous seismic events that align in general conformance with seafloor bathymetry. A third type of seismic feature, pluming “BSRs”, are continuous events that are not bottom-simulating, but are bowed toward the seafloor and represent areas where large, but areally-limited increases in heat flow (linked to strong vertical fluid flux), perturb the BGHS. The limited nature of continuous BSRs and the relative abundance of discontinuous and pluming forms are attributed to the strong lithologic and structural heterogeneity of the northern GoM basin. This lithologic and structural complexity has served to disrupt and localize regionally pervasive and homogeneous gas flux that is consistent with the formation of large, continuous BSRs noted across other less complex continental margins. The various BSR forms identified in this study are shown to be closely associated (125 of 145) with the occurrence of seafloor amplitude anomalies, which are in turn usually associated with the flanks and crests of salt-cored ridges. These associations are interpreted to reflect the co-dependence of BSRs and seafloor reflectivity along the migration pathways that typify this geologic setting. Published by Elsevier Ltd.
Keywords: Bottom simulating reflectors Gas hydrates Gulf of Mexico Seafloor anomalies
1. Introduction Seismic reflection data from deepwater continental margins commonly exhibit anomalous, shallow seismic events called “bottom-simulating reflectors” (BSRs). BSRs are anomalous in that their orientation shows no relationship to sedimentary layering; instead, they mimic the general bathymetry of the seafloor, with gradually increasing sub-seafloor depth with increasing water depth. As a result, BSRs are inferred to be caused by physical property changes in marine sediments that are driven by depth dependent phenomena such as pressure, or temperature, or both. Two primary phenomena are responsible for the majority of BSRs. The first includes BSRs associated with diagenetic boundaries,
* Corresponding author. Tel.: þ504 736 2497; fax: þ504 736 2905. E-mail address: [email protected] (W. Shedd). 0264-8172/$ e see front matter Published by Elsevier Ltd. doi:10.1016/j.marpetgeo.2011.08.005
such as the transformation of opal-A to opal-CT, which tends to occur at fixed subsea depth in siliceous sediments (Hein et al., 1978). These “diagenetic” BSRs mark horizons where sediment density increases with depth. Such BSRs have polarity equivalent to the seafloor reflection. Other less common diagenetic processes, such as opal-CT-to-quartz and illite-to-smectite transitions, may also produce BSRs in some areas (Berndt et al., 2004), although data are inconclusive. A second phenomenon, which produces the majority of documented BSRs, is related to the negative impedance caused by the transition from gas-hydrate-bearing sediments (higher compressional velocity (Vp)) to water- or free-gas-bearing sediments (lower Vp) at the base of gas hydrate stability zone (BGHS). The resulting BSRs are characterized by having a reflection polarity opposite that of the seafloor. Kvenvolden and Lorenson (2001) provide a list of 53 global locations where BSRs inferred to be related to gas hydrate had been reported prior to 2001. All BSRs noted in the Gulf of
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Mexico to date (Dai et al., 2004; Kou et al., 2007; Hutchinson et al., 2008) are interpreted to be related to the occurrence of gas hydrate. BSRs were first attributed to gas hydrate where they occur broadly and conspicuously along the Blake Ridge, offshore eastern North America (ex. Bryan, 1974; Tucholke et al., 1977). Shipley et al. (1979) extended this interpretation to similar features noted on continental shelves world-wide. Subsequently, BSRs of appropriate polarity and sub-seafloor depth have been considered to be reliable indicators of the position of the BGHS and of the occurrence of gas hydrate (Hyndman and Spence, 1992). Most BSRs coincide with the BGHS, but situations do develop in which the top of free gas occurs below the BGHS and also where the base of gas hydrate occurs well within the gas hydrate stability zone (GHSZ). This leads to potential offsets between observed BSRs and the BGHS (Xu and Ruppel, 1999). Also, gas hydrate has been observed occurring below BSRs where sufficient heavier hydrocarbon gases are available to produce structure II or structure H hydrates (Hadley et al., 2008). Consequently, the acronyms BSR and BGHS are not interchangeable terms. BGHS describes an inferred phase boundary (often indicated by a BSR, but also commonly not), while a BSR is any anomalous seismic event which has an orientation that mimics seafloor bathymetry without regard to sediment depositional fabric. Such seismic reflections may or may not be related to gas hydrate. Much work has been conducted to discern the utility of a BSR as a gas hydrate exploration tool. Attempts to quantify the
concentration of gas hydrate associated with BSRs have relied primarily on advanced AVO analyses (ex. Ecker et al., 1998). However, most workers agree that the seismic attributes of BSRs are primarily influenced by the occurrence of free gas (Singh et al., 1993). Given that even very low free-gas saturations produce significant changes in Vp (Domenico, 1976), attempts to obtain meaningful quantification of gas hydrate phase saturations from BSRs remain challenging (see Chen et al., 2007). In addition, the nature and appearance of BSRs depend on the nature and frequency of the seismic data in which they are expressed. For example, reflection amplitude decreases with the frequency content of the seismic data (Chapman et al., 2002). Furthermore, drilling programs which have tested multiple locations of varying geophysical character within regional BSR areas, have concluded that BSRs provide little useful information related to the occurrence of gas hydrate, either at the BSR or within the GHSZ above a BSR (Tsujii et al., 2009). Despite the difficulties in discerning various phase saturations from geophysical attributes of BSRs, we suggest that differences in the overall seismic expression of the BGHS may provide important information on the nature of the associated gas hydrate petroleum system. This report presents the results to date of a review of extensive 3-D seismic datasets to document the occurrence and distribution of gas-hydrate-related BSRs in the northern Gulf of Mexico. In addition, we also report the occurrence of over 21,000 seafloor
Figure 1. Seafloor morphology of the northern Gulf of Mexico. Super-imposed are outlines of 145 areas in which features inferred to mark the BGHS are observed in seismic data. The color indicates the dominant morphology (many areas show elements of more than one form) of these BSRs as continuous (yellow), discontinuous (red), and pluming (green). The dashed blue line indicates the area of uninterrupted 3-D seismic data coverage available for the study. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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amplitude anomalies which appear to define areas of past or current hydrocarbon leakage at the seafloor, and we compare the distribution of these features to mapped BSRs. We conclude with some observations which attempt to relate the unique geology of the northern Gulf of Mexico slope to the distribution and nature of the observed BSRs. 2. Seismic data Our study was conducted by accessing more than 450,000 km2 (175,000 mi2) of 3-D seismic data in water depths greater than 200 m
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(656 ft) in the possession of the U.S. Bureau of Ocean Energy Management, Regulation, and Enforcement (BOEMRE). These data cover the vast majority of the northern Gulf of Mexico shelf and slope (Fig. 1). Much of the data is recent vintage and high quality, and in most regions, multiple overlapping datasets were available for review. However, 3-D data coverage for the area seaward of the Sigsbee Escarpment is not yet complete, and although 2-D datasets are available in that region, the difficulty in delineating BSRs in its flatlying sediments dictated that minimal effort was spent in searching for BSRs within the abyssal plain. Confidence in the identification of
Figure 2. Example discontinuous BSR from Walker Ridge 313. (a) Un-interpreted. (b) Interpreted. (c) Red dashed line indicates inferred BSR and BGHS. DataÓ 2011 WesternGeco. Used by permission. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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each geophysical feature was qualitatively rated, with only those assigned as “fair” (14% of the total identified), “good” (38%), “very good (31%)” and “excellent (17%)” being included. Seafloor amplitude anomalies are derived from the same seismic dataset and include both high positive anomalies (hard-grounds associated with seafloor authigenic carbonate precipitation and sometimes gas hydrate (Roberts et al., 2006)) as well as low-positive or negative anomalies (currently active high flux hydrocarbon venting). Given the immensity of the available seismic data, this analysis is not exhaustive or complete and is considered a report of work in progress.
3. Results Bottom simulating reflectors are, as the phrase implies, associated with coherent and continuous reflectors which mimic the seafloor bathymetry. The majority of features recognized to date in regions outside of the GoM that are labeled as BSRs are of this nature. We call these “classic” features continuous BSRs. However, Vanneste et al. (2001) at Lake Baikal and McConnell and Kendall (2002) in the northern GoM reported “BSRs” which were not manifested as actual reflectors, but were a virtual subsurface horizon delineated by the alignment of terminations of separate high-amplitude reflections within a succession of stratigraphic units. These reflection terminations can be closely spaced, as observed in the Lake Baikal data, or widely separated as was noted in the northern GoM. In either case, there was no perceptible change in the character of the seismic data within the section between the anomalous reflectors (Fig. 2). We call this seismic expression of the BGHS a discontinuous “BSR”, which is consistent with prior usage of the term. Furthermore, in areas of strong vertical gas and fluid flux, local variations in lateral temperature gradients of sediments (and probably in pore-water salinity as well) can create discordant and continuous reflectors which mark the BGHS but which are clearly not “bottom-simulating” (Wood et al., 2002). We term these seismic features pluming “BSRs”. This review identified 145 areas across the northern Gulf of Mexico in which the BSR can be identified in seismic data (Fig. 1). Over half (55%) of the mapped features include elements of more than one BSR type (see Fig. 3 for an excellent example from Garden Banks 859). In such cases, each BSR is categorized based on its dominant form. Overall, discontinuous “BSRs” account for 85 of the 145 (58.6%) identified BSRs in the northern GoM. Continuous BSRs (ex. Fig. 4) account for 35 features (24.2%), while the remaining 25 features (17.2%) have a “pluming” form (Fig. 5). As shown in Figure. 1, BSRs are observed throughout the minibasin salt province but appear to be most common in the central and western GoM, away from the Mississippi Fan coincident with the region of most extensive salt diapirism. Within the Mississippi Fan, most deep-seated faults are buried by rapid, recent sedimentation and, cannot act as conduits to the shallow, hydrate-prone stratigraphic section. BSRs are also less common in the western portions of the Alaminos Canyon and East Breaks protraction areas. BSRs of any type are conspicuously absent in the abyssal plain, with the exception of a few small features noted in Atwater Valley and Mississippi Canyon protraction areas. This latter observation may be attributed in part to the difficulty of recognizing BSRs in flatlying strata. Cumulatively, the 145 mapped BSRs in the GoM encompass roughly 4450 km2 (1.1 million acres (Fig. 6)). The largest feature is a discontinuous “BSR” which occurs around the East Breaks block 992 area and is roughly 453 km2 (112,000 acres). Individually the features are generally small in comparison to many BSRs noted in other basins, with a mean size of 30.9 km2 (7634 acres). Pluming “BSRs”, which by their highly discordant nature are easily
Figure 3. Example of an extensive BSR from Garden Banks 859 showing pluming, discontinuous, and continuous components. (a) Un-interpreted profile. (b) Interpreted profile with red dashed line indicating inferred BSR and BGHS. DataÓ 2011 WesternGeco. Used by permission. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
recognized across small areas, account for most of the smaller features (Fig. 6). Each BSR was also categorized with respect to its general geologic setting. The majority (66.9%) of BSRs are located above diapiric salt structures. Of those, there is an equal portion of BSRs which occur directly over shallow salt (within 0.5 s of seafloor) compared to above salt flanks (Fig. 7). BSRs which occur over deeper salt bodies account for 31% of the data. Only 2% of BSRs occur entirely within the interior of mini-basins. Notably, 125 of the 145 of the mapped BSRs (86.2%) are associated (spatially) with anomalies in seafloor reflectivity. The shared occurrence between BSRs and seafloor reflection amplitude anomalies with salt bodies and salt margins is inferred to reflect preferential pathways for hydrocarbon migration into the shallow section afforded both by sediment dip (upwards away from mini-basin interiors) and more importantly by faulting associated with diapiric salt bodies (Fig. 8). Although a zone of gas hydrate stability occurs throughout the northern GoM wherever water depth exceeds w400 m (1300 ft), we find no conclusive seismic expressions of the BGHS in water depths less than 606 m (1989 ft; Fig. 9). The deepest feature mapped (in Alaminos Canyon 818) is in 2790 m (9151 ft) of water. The average water depth of the 154 features is 1416 m (4646 ft). Although the general rule is that the depth of the GHSZ below the seafloor increase with water depth, a plot of GHSZ thickness below the seafloor and water depth shows a great degree of scatter (Fig. 10). This scatter is attributed to the significant heterogeneity found in the shallow, gas hydrate-prone GoM subsurface stratigraphic section (thermal, pressure, and geochemical) that results from the basin’s salt tectonism which produced extensive structural and stratigraphic complexity. 4. Discussion The early perception of gas hydrate occurrence in the GoM basin was pessimistic, due largely to the lack of recognition of large,
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Figure 4. Example of continuous BSR from Garden Banks 982. (a) Un-interpreted profile. (b) Interpreted profile with red dashed line indicating inferred BSR and BGHS. DataÓ 2011 WesternGeco. Used by permission. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Figure 5. Example of pluming BSR (left side of image) from Green Canyon 475. Continuous BSRs are present to the center and right. (a) Un-interpreted profile. (b) Interpreted profile with red dashed line indicating inferred BSR and BGHS. DataÓ 2011 WesternGeco. Used by permission.
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Figure 6. (a) Histogram of GoM BSRs size (in acres). Over 80% of BSRs are 12,000 acres or smaller. (b) BSR size (in acres) color-coded by type. Discontinuous BSRs account for most of the larger features, while pluming BSRs are generally small. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Figure 7. Map showing the relationship between mapped BSRs and depth to top of salt. More than 66% of the mapped features are associated with the crest or flanks of shallow salt features.
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Figure 8. Map showing the relationship between seafloor bathymetry (gray), seafloor seismic amplitude anomalies (red), and mapped BSRs (yellow). The inset shows an area typical of the GoM upper slope in which seafloor reflection amplitudes are concentrated along salt-cored highs and ridges which form the margins of salt-withdrawal mini-basins. Seafloor reflectivity anomalies and BSRs are rare in the interior of mini-basins, probably because of the general focusing of hydrocarbon migration toward mini-basin flanks. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Figure 9. Range of water depths plotted with BSR type noted. Discontinuous and continuous BSRs occur through the full range of water depths, whereas pluming BSRs are concentrated in water depths of w1200 m (w4000 ft) or less, and none occur in water depths greater than w1500 m (w5000 ft).
Figure 10. Scatter plot of 145 GoM BSRs with respect to sub-seafloor depth (thickness of subsurface portion of the GHSZ) and water depth. The scatter in the data is a function of extreme heterogeneity of the basin resulting from salt tectonism.
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regional BSRs of the type that had been previously described on the Blake Ridge and in many basins around the world (see Boswell et al., 2012). A number of studies correctly pointed out that the prospects for widespread occurrences of gas hydrate in the basin were limited, due primarily to the thermal and geochemical heterogeneities which significantly reduce the total volume of the GHSZ (Paull et al., 2005; Ruppel et al., 2005; Johnson and Smith, 2006). However, these same heterogeneities, compounded by the basin’s noted lithologic heterogeneity and structural complexity, as well as by the existence of an underlying world-class petroleum system, result in two considerations relevant to the occurrence of gas hydrate and the subsequent development of localized BSRs.
These impacts, while rendering the basin unfavorable for the formation of large and continuous gas hydrate occurrences like the Blake Ridge, make it more favorable for the formation of relatively small, concentrated gas hydrate deposits which may have relevance as a producible hydrocarbon resource. 4.1. Occurrence of reservoir controls Most continental margins receive significant coarse-sediment supply. In many passive margins, most of this sand bypasses the slope and is deposited in the deep basin. However, seafloor bathymetry related to salt tectonics in the northern GoM produces
Figure 11. Seismic data from Green Canyon 781 showing a discontinuous BSR with phase reversals at two horizons. (a) Un-interpreted profile. (b) Partially interpreted profile. DataÓ 2011 WesternGeco. Used by permission.
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conditions suitable for significant sand deposition within “ponded” mini-basins on the slope (Prather et al., 1998). The permeability inherent in these sands provides suitable conditions for the formation of highly concentrated gas hydrate occurrences (as reviewed in Collett et al., 2009). 4.2. Occurrence of pathways for focusing gas migration and sourcing Areas with generally homogeneous sediment overall (typically muds in deepwater settings occurring in thin stratigraphic sections over active salt diapirs) are predisposed to continuous BSRs. This outcome is primarily due to the lack of differential local permeability, which can serve to focus gas migration and accumulation. Instead, a regionally pervasive and diffuse gas migration can occur, with continuous BSRs being a likely result. Where fluid flux in such settings does become concentrated (and typically vertical), BGHS may be deflected upwards (reflecting higher temperatures), producing a pluming “BSR”. In contrast, sediment systems with greater lithologic heterogeneity (interbedded sands and shales), are more likely to focus the migration and accumulation of gas into coarser-grained sediments. Structural deformation, such as that related to the salt tectonics in the GoM, can provide significant structural dip to such units, making them viable pathways for extended migration with significant lateral components (Frye, 2008). Furthermore, the extensive faulting that typifies the flanks of diapiric salt structures provides additional high-permeability pathways for the migration of gas into the shallow sedimentary section. The efficacy of these pathways is demonstrated by the correlation between seafloor reflectivity anomalies, the occurrence of BSRs of all types, and the margins of salt bodies (Figs. 7 and 8). In addition to the previously mentioned abyssal plain, we recognize several areas where BSRs are largely absent. The lower slope of southern Keathley Canyon (KC) and Walker Ridge (WR) is characterized by a thick, continuous salt canopy which is fed by a combination of numerous vertical salt stocks and the
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allochthonous lateral movement of extensive salt canopies (e.g., Pilcher et al., 2011). The absence of an extensive through-going fault network, such as observed near the flanks of diapiric salt features on the upper slope, reduces the opportunity for significant point-sourced gas accumulations in the GHSZ. Additionally, the stratigraphic section above these salt canopies is often quite thin and high heat flow and salinities may inhibit hydrate formation. The western half of protraction areas East Breaks (EB) and Alaminos Canyon (AC) are also characterized by a limited vertical salt movement and a complete absence of BSRs. Note that both of these areas (southern KC and WR; western EB and AC) have few seismic amplitude reflection anomalies on the seafloor (Fig. 8). A key finding of this study is not only the gas hydrate-relevant features identified, but also the strong predisposition for the BGHS to manifest as discontinuous “BSRs” in the northern GoM. Joint Industry Project (JIP) Leg II drilling at WR block 313 was designed to test the hypothesis that seismic amplitude phase reversals (McConnell and Zhang, 2005) associated with discontinuous “BSRs”, which suggest the buoyant separation of gas and water within discrete units, provide good evidence of the gas charge (and gas hydrate occurrence) within reservoir facies. The results provided strong confirmation of this model (Shedd et al., 2010; Boswell et al., 2012). Of the 85 discontinuous “BSRs” identified, good evidence of phase reversals in 45 of the features was noticed. Figure. 11 (GC block 781) provides an additional example of such a system, which although not drilled by the JIP Leg II program, must now be considered highly prospective for gas hydrate in sands. The correlation between the occurrence of BSRs and the locations of discovered oil and gas fields is less than one to one (Fig. 12), although we do observe some significant relationships. BSRs are found in close proximity to many of the oil and gas fields of eastern EB, AC, Garden Banks (GB), and Green Canyon (GC), yet BSRs are mostly absent from the western halves of these same four protraction areas, where discovered oil and gas fields number far fewer. Additionally, oil fields on the lower slope of southern KC and WR are deeply buried (up to 30,000 ft below sea level) and are located underneath extensive salt canopies. We do not expect these
Figure 12. Distribution of mapped BSRs and discovered oil and gas fields in the deepwater Gulf of Mexico.
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oil fields to have a direct correlation with a dense BSR network. Many of the BSRs in the oil and gas rich Mississippi Canyon protraction area are located near discovered fields, but the inverse relationship of a BSR associated with every oil or gas field does not hold true. 5. Summary By accessing BOEMRE managed 3-D seismic data covering more than 450,000 km2 (175,000 mi2) of the deepwater northern Gulf of Mexico, more than 145 locations were identified and mapped where the BGHS is delineated by anomalous seismic features. These BSRs are assigned to three classes based on their gross morphology. The most common are discontinuous “BSRs” which reflect areas where a variable lithology consists of alternating sediments of fine and relatively coarse grain size. The sands traverse the BGHS in an area of sufficient gas charge to saturate the sand with gas hydrate above the BSR and to accumulate some volume of free gas below. Less common are the through-going, continuous BSRs (similar to those seen in other basins, although typically much smaller) which have been interpreted as forming primarily in areas with more widely distributed gas flux and generally homogeneous and fine-grained sediments. A third category known as pluming “BSRs”, in which continuous BSRs are warped upward, represent areas of concentrated methane and warm brine vertical flux. This variety in seismic expression of the BGHS is a reflection of the heterogeneous geologic setting of the northern Gulf of Mexico. The distinctions in BSR type provide important clues into the nature of the local gas hydrate petroleum system. Of the three various forms, discontinuous “BSRs” are recognized as a strong indicator of the confluence of both source and reservoir at the base of the gas hydrate stability zone. Confirmation of this concept provided by Joint Industry Project Leg II drilling (Shedd et al., 2010; Boswell et al., 2012) and the prevalence of these forms in the northern GoM suggests strong potential for the occurrence of concentrated gas hydrates in the basin. Acknowledgments The authors wish to thank all the sources of the seismic data used in this study: WesternGeco, TGS, PGS, CGGVeritas, Diamond, Geophysical Pursuit, Jebco, Seitel, and especially WesternGeco for granting permission to use the images of their seismic crosssections. References Berndt, C., Bunz, S., Clayton, T., Mienert, J., Saunders, M., 2004. Seismic character of bottom simulating reflectors: examples from the mid-Norwegian margin. J. Mar. Pet. Geol. 21, 723e733. Boswell, R., Collett, T., Frye, M., Shedd, B., McConnell, D., Shelander, D., 2012. Subsurface gas hydrates in the northern Gulf of Mexico. J. Mar. Pet. Geol. 34, 4e30. Bryan, G., 1974. In situ indications of gas hydrate. Marine Sci. 3, 299e308. Chapman, N., Gettrust, J., Walia, R., Hannay, D., Spence, G., Wood, W., Hyndman, R., 2002. High-resolution, deep-towed, multichannel seismic survey of deep-sea gas hydrates off western Canada. Geophysics 67 (4), 1038e1047. Chen, M., Riedel, M., Hyndman, R., Dosso, S., 2007. AVO inversion of BSRs in marine gas hydrate studies. Geophysics 72 (2), 31e43. Collett, T., Johnson, A., Knapp, C., Boswell, R., 2009. Natural gas hydrates e a review (Chapter 1). In: Collett, T., et al. (Eds.), Natural Gas Hydrates e Energy Resource Potential and Associated Geologic Hazards. AAPG Memoir, vol. 89, p. 74.
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