Gas Hydrate Occurrence in the Krishna–Godavari Offshore Basin Off the East Coast of India
Figure 2: Detailed Bathymetry of the Study Area (50m Contour Interval; A) and Seafloor Mosaic Derived from Swath Bathymetry Data (B)
16°30’N 16°20’N km 020 16°10’N 16°00’N 15°50’N 15°40’N 15°30’N
81°30’E 81°40’E 81°50’E 82°00’E 82°10’E 82°20’E 82°30’E 82°40’E Valley
DF SL Fan TC SR SC TTF Data gap m -2,000-1,600-1,200 -800 -400
The major geomorphic features identified are a delta-front (DF) with valleys and canyons, a toe-thrust fault (TTF, dashed line), turbidity channels (TC), a sedimentary ridge (SR), a fan, slumps/slides (SL) and scars (SC). The white patch at the top is a data gap resulting from this area being occupied by drilling rigs and locations of National Gas Hydrate Program (NGHP) drill wells.
subsurface geological conditions. The geoscientific data comprised geological sampling and under-way geophysical data. The geological samples were used to determine the textural characteristics of sediments and the pore water chemistry to infer sulphate and chloride anomalies, which are also proxy indicators. The geophysical data were used to generate a seafloor mosaic and to infer the subsurface geological environment and gas escape features. The seafloor mosaic generated from the swath bathymetry data obtained using the Hydrosweep system (M/s Atlas Elektronik GmBH, Germany) was used as a base map. A geo-acoustics deep tow system (Model 2000 Geo Acoustics, UK, comprising chirp sonar, side scan sonar and altitude sensor) was deployed to simultaneously record the seafloor images and higher solution subsurface information. The processed MCS data provided by the oil industry were re-examined to confirm BSRs. Aliquots of core material were analysed for fermenters, sulphate- reducing bacteria, nitrate-reducing bacteria and nitrifying bacteria.
Salient Findings
The seafloor mosaic (see Figure 2), shown between water depths of <400 and >2,000m, exhibits various geomorphic features such as a delta-front incised with valleys within the water depths of <400–600m, a deepwater fan beyond 1,200m water depth and an approximately west-north- west–east-south-east-trending sedimentary ridge in the south-western
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These features are the manifestation of the upward migration of high-pressure gas below the seafloor and subsurface strata. Faulting in the subsurface layers with a throw of about 5–20m and the presence of mud diapirs with a relief of 20–100m are common features, seen particularly in the BSR-prone areas (see Figure 3). The BSRs are continuous to discrete in character, and cross-cutting of lithology could be seen only in a few coast perpendicular (dip) tracks. Study of occurrence of gas hydrate deposits worldwide indicates various geological set-ups that hold these deposits. For example, in the Gulf of Mexico it occurs in muds; in Green Canyon, in rubble; in Blake Ridge, in muds; in the Niger delta, in clay; in the McKenzie delta, in sand and gravel; in the Black Sea, in clayey silt; in the Caspian Sea, in clayey silt; in Lake Baikal, in sand silt; in Costa Rica, in mud and muddy sands and microfractures; in the Middle American Trench (Mexico), in ash and mud; in Oregon, in silt; in Hydrate Ridge, in carbonate crust; in the Okhotsk Sea ooze, in silt; in the Japan Sea in sand with clay; and on the Makran coast, in sands.16
Some segments of the seabed over the BSR-prone zone in the study area are seen with pockmarks of variable dimensions (30–40m diameter), while the shallow subsurface sedimentary strata are associated with gas chimneys, gas masking/saturation and acoustic pipes.15
The widespread host sediments vary from mud/clay to sand and gravel. One important factor to be considered, apart from the shallow geological environment, is the continuous supply of methane gas or its generation due to microbial mediation. The predicted GHSZ thickness below the seafloor is around 300m, and most of the inferred BSRs occur within <200–300m in the KG offshore. Since gas hydrate occurs at shallow levels within the slope sediments of Quaternary age, knowledge of the shallow geological environment is an important factor in understanding the occurrence and genesis of gas hydrate. The shallow sediment is composed of nanofossil-bearing to nanofossil-rich clay, clay and silty clay that ranges in colour from dark brown to dark grey near the top of the core to very dark grey to greenish black to black. Major sediment components include non-biogenic grains composed of clay minerals, quartz, feldspar, mica, pyrite, authigenic carbonates and heavy minerals, and biogenic grains of calcareous nanofossils, foraminifera, mollusc shell fragments and sponge spicules. Few locales are filled with sand-rich sediments. Analyses of short cores (~5m) and long cores (>200m) show that the sediments are composed of nanofossil-bearing silty clay, and clay with thin beds of sand. At a few locations, mollusc shells (primarily bivalves) and visible foraminifera tests
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part of the study area. The fan-like feature gently deepens offshore and is bounded by an approximately north-north-west–south-south-east and east–west trending scarp/steep fault-like structural elements in the north and west, respectively. A north-east–south-west-trending toe-thrust fault is interpreted from a swath bathymetry mosaic under seismic constraints; this toe-thrust fault grazes the ~1,400m depth contour and appears to abut the north-north-west–south-south-east-trending boundary fault of the fan-like feature. The offshore area up to 1,600m water depth is seen with several mud flows caused by either slumping or sliding. The effect of slumping/sliding is seen in the form of sediment flows and sediment build-up in the mid-slope region. The sedimentary ridge, with an overall basal width of about 10km, rises to <950m above the seafloor at a water depth of 1,400m. West of this ridge, there are linear turbidity channels bounded by steep scarps. These geomorphic features have significantly contributed to the deposition of clays and sands and detrital flux.
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