CHANDRA,AVINASH, Directorate General of Hydrocarbons, New Delhi, India
Abstract: New Geophysical Data Reveals Presence of Large Deep Water Prospects along East Coast of India
The east coast of India comprises several offshore sedimentary basins, encompassing an area of 295,000 sq. km., (including deep water beyond 400m), and is relatively unexplored. Currently, exploration is being carried out by the two National Oil Companies ONGC and OIL as well as by private companies, in the shallow water areas.
A number of commercial hydrocarbon discoveries have been established up to a water depth of 100m. Further, limited exploratory drilling in water depth beyond 300m has established the presence of oil and gas in G I and G2 structures in the Krishna-Godavari basin. Nevertheless, most of the deep water areas with great potential are yet to be explored.
In order to understand the regional geological setting in deep water areas, Directorate General of Hydrocarbons (DGH), India has, for the first time, prepared satellite derived Bouguer gravity maps (jointly with Petroscan, Sweden) along the east coast which brought out interesting gravity anomalies in deep water areas.To corroborate the gravity anomalies, DGH has acquired 7430 LKM of 2D seismic and gravity-magnetic data (jointly with Western Geophysical, UK) with a general grid of 40km x 40km.
Interpretation of the newly acquired ship-borne gravity-magnetic data has indicated that more than 8 km of sediments are present in the depressions flanked by structural highs in the shallow as well as deep water areas (Fig. 1). Further, the southern part of the east coast (Cauvery - Palar area) is tectonically more disturbed than the basins to the north. This is reflected in the formation of a number of horst-graben features.
The east coast of India represents a major part of the Indian Continental margin - a passive margin that evolved through an extended period of rifting and drift following the break-up of Gondwanaland. The east coast margin separated from Antarctica at around 120 Ma (Aptian-Albian) which resulted in the formation of the Cauvery, Palar, Krishna-Godavari, Mahanadi and the Bengal basins.
The structural style of the east coast basins is marked by horsts and grabens along the NE-SW Eastern Ghat tectonic trend. These ancient faults were reactivated into major strike-slip faults as India separated from Antarctica and related anticlockwise rotation during its northward drifting. The structural fabric is controlled by this wrench system oblique to the rift alignment. These basins are sensu stricto pull apart basins.
Source Rock Potential
These sedimentary basins are characterised by the presence of rich source rocks ranging in age from Gondwana (Upper Carboniferous to Lower Jurassic) to Tertiary. The principal source rocks occur in the coal and carbonaceous shales of Cretaceous and Paleogene age as established from interpretation of geochemical data from wells in the onshore and shallow offshore areas. The source rock kerogen is of Type II and Type III suggesting their potential for generating oil and gas. The maturity data suggests the source rocks to be within oil window.
For the deep water areas, maturity modeling was carried out based on VITRIMAT. The Cretaceous and Paleocene source rocks have attained the maturity required for hydrocarbon generation (Fig.2). The top of the oil window occurs in the depth region of 2500-3000m. Dry gas is expected to be generated below a depth of about 6000m in this region. A major part of the oil was generated and expelled during Oligocene-Miocene period. The principal migration routes appear to be updip towards north-west and west.
Hydrocarbon Play Types
The interpretation of the newly acquired geophysical data in the deep water areas of the east coast has indicated presence of a variety of new hydrocarbon plays, as well as evidence of direct hydrocarbon indicators. Several large structural features varying in size from more than 200 sq.km to 2400 sq.km. have been identified (Fig.3).
In the Cauvery area, Eocene carbonates and sands over Cretaceous highs, Paleocene wedge-out prospects, plays within gravity slumps/Miocene turbidite fans constitute the main play types. Large structural traps within Tertiary sequence, drape-over features/rollover anticlines within Mio-Pliocene sequences, Eocene fringe reefs and Paleogene channels with bright spots constitute the principal play types in the Krishna-Godavari basin (Fig.4). In the Mahanadi area, large structural traps within Tertiary sequence, Paleocene carbonate build-ups, Paleogene wedge-outs against basement highs, turbidites, channel fills and submarine fans are some of the prominent play types. In addition to the above play types, the presence of direct hydrocarbon indicators such as bright spots, gas chimneys, pock-marks, velocity sags etc. have been observed.
There are proven petroleum
systems with producing fields in onshore and in the shallow offshore areas
adjacent to deep water regions of the east coast. The geoscientific studies
carried out recently by DGH have shown that all geological conditions exist
in the deep offshore for significant hydrocarbon generation and entrapment
besides the evidence of direct hydrocarbon indicators. This would justify
further aggressive exploration activity in the area.
Fig. 1. Structural interpretation of spec. G-M data.
Fig. 3. Map showing marginal basin and various play types in deep offshore, east coast, India.
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