Gas Hydrate Occurrence in the Krishna–Godavari Offshore Basin Off the East Coast of India
Figure 3: Typical Multichannel Seismic Sections Depicting Bottom-simulating Reflections and Several Faults of Varied Dimensions
NW
NW 400 312 Seabed 350 Slump 300 BSR Slide 250 200
AD-94-11
1,500 2,000 BSR 3,000 BSR 2,500 4,000 SE 152 SE 198 NW Seabed
29 69 109 149 189 229 269 309 349 389 429 469 509 Slump
CR
AD-94-17
BSR
SE 1,000
5,000
These faults are surmised to be the fluid migration pathways.
Figure 4: Seismic Line GDSW-16 (Orientation is North-west–South-east) Crossing Drill Site NGHP-01-21
NW Trace
1.3 1.4 1.5 1.6 1.7 1.8 1.9
2.0 2.1
200 300 400 500 600 SE 700 SW 690 600 Recent sediments 1,500
Acoustic pipe
Gas saturation 500 BSR 400 300 Drill well
GDSW-46
200 NE 100
Free gas
Fault 2,000 MD
Note the occurrence of deep-seated high-amplitude reflectors implying the occurrence of free gas as well as potential shallow debris flow. Also, the seismic section where NGHP-01-10 well drilled is shown along GD9W-45 (refer to Figure 2 for locations).
occur sporadically throughout the sediment. Possible fossil chemosynthetic communities composed of a variety of mollusc shells with some encrusted by carbonatic worm tubes were also recovered during NGHP expedition 01. Recent investigations on-board M/V Marion Dufresne have shown the presence of chemosynthetic clams, directly indicating the presence of cold seeps at subsurface levels.17 Concentrations of authigenic carbonate nodules of varying size (a few millimetres to over 5cm) are also observed near these fossil assemblages. In addition to these concentrated zones, authigenic carbonates are observed in all holes below 15–20mbsf and occur as both small (1–3cm) and large (7–8cm) nodules and thin (2–3cm) fine-grained horizontal bands. However, at a few locations a variety of nanofossil-, volcanic glass-, pyrite- and authigenic-bearing clay was encountered.
In the KG offshore most of the conventional hydrocarbon gases from deeper horizons are of microbial origin, and the stable isotope studies of hydrate core samples recovered under NGHP expedition 01 also indicate that the methane gas present in the hydrate samples is derived from microbial sources, and that the estimated gas hydrate saturation is as high as 50% of the pore space at NGHP-01 site 10. The generation of huge quantities of hydrocarbon gases at the deeper horizons and their migration is an important factor in controlling the formation and distribution of natural gas hydrate at the shallower depths. Therefore,
EXPLORATION & PRODUCTION – VOLUME 8 ISSUE 1
Figure 5: Massive and Disseminated Form of Gas Hydrate Recovered from Krishna–Godavari Offshore During NGHP Expedition
prior knowledge of the fluid migration pathways apart from the geology is required to assess the gas hydrate potential of this area. The terrestrial organic matter occurs in trace from 3 to 5% in both smear slides and, rarely, in coarse fraction.15
The seismic sections show the faults of different dimensions. This situation prevails more or less in the KG offshore, indicating the presence of pathways for migration of methane gas either from the deeper horizons or from the shallow horizons. It is well established that most discoveries of hydrocarbons in the KG offshore have been within the sands deposited by turbidity flows.12,18,19
The Pliocene
27
Two-way travel time (s)
Two-way travel time (ms) Two-way travel time (ms)
Two-way travel time (ms)
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