Evolution of Krishna Godavari Basin and Hydrocarbon Prospects in Offshore: Elementary Account for Non-explorationists

GNN | May 25, 2017, 6:00 PM IST|

The KG-PG (Krishna Godavari and Pranahita Godavari) sedimentary Basin’s diverse hydrocarbon prospects from pure biogenic gas (generated due to bacterial activity at shallow depths) to metagenic thermal gas (oil is converted to gas because of high temperature) makes it unique. The potential of the basin is not yet unravelled completely. Depression, basinal side and graben mean where sediments are buried deeper. Ridge, horst and high denote where sediments are uplifted. The crux for the existence of petroleum system is the presence of three characteristics: source rock which generates hydrocarbons (large organic matter converted into petroleum), reservoir rock where generated petroleum is stored after migration (sand or fractured formations) and a cap rock above reservoir which keeps the petroleum in place.

The KG area has a polycyclic (dual-rift) evolution history, with extensional pericratonic (the one that extends into sea as well) margin superimposed on the orthogonal intracratonic (within the land) Pranahita Godavari Graben in the eastern continental margin of Indian plate. The northeast and southwest of KG basin are lined by two pericratonic basins, Mahanadi and Cauvery. KG area comprises a wide array of sedimentary facies from Early Permian (oldest formation in the area) through Cenozoic with the analogous outcrops, defining the basin limits, along the north-western part of the basin.

Early Permian has not yielded commercial hydrocarbons. Nonetheless, Late Permian sediments comprising of coal-shale sequence with intermittent sand sections in fluvio-lacustrinal (lake like environment where movement and availability of oxygen are not present) setting has contributed to hydrocarbon generation. The Permian sediments are unconformably overlain by arenaceous (sandy) Triassic formation. It is believed that major hiatus (a period when no deposition occurred) at this stage and invasion of meteoric (rain) waters have reduced the porosity and permeability of the Formation considerably resulting in very tight reservoirs. The Triassic is overlain unconformably by Late Triassic to Middle or Late Jurassic in many areas.

KG-PG area’s sedimentation started in Permian age while the huge shale gas prospects in America belong to a much older Devonian time. Over Permian sediments, other depositions of various ages took place. Penetrating a certain formation for hydrocarbon prospects depends on various aspects, such as perceived hydrocarbon potential, burial depths, formation temperature and pressures, reservoir permeability etc. Thus it is not possible and economically unviable to reach the bottom most sedimentary rock in every well. If hydrocarbon bearing formations are sufficiently uplifted to say 3000-5000 meters depth from surface, drilling a hole and monetization of the find are more likely. HTHP wells mean high temperature and high pressure wells where hydrocarbon prospects are quite deep and formation temperatures and pressures are very high. KG-OSN-2001/3 (Deendayal Field) of GSPC is one such area.

KG offshore presents three types of hydrocarbon reservoirs: (i) Conventional oil and gas reservoirs from tertiary finds (Pliocene to Palaeocene age) (ii) Predominantly biogenic to partially thermal gas reservoirs (Pliocene to at best Oligocene in some areas) and (iii) HTHP gas reservoirs (Cretaceous and older, called Mesozoic reservoirs).

It is relatively easier to drill and monetize first kind of hydrocarbon prospects located mostly in shallow to moderately deep bathymetry (depth of sea water) between the northern most river-sea confluence of Godavari branch (nearer to Yanam) to Southern most branch at Antarvedi-Kesavadsapalem. It consists of peak mature waxy oil with accompanying gas albeit unevenly from area to area and Ravva is the most prolific find.

Second type of bio-genic to partially thermogenic gas, spread in the entire KG offshore (north of Poduru-Yanam high to Avanigadda Cross trend in South) has deep offshore structures. Our KG-OSN-2001/3, in my opinion is likely to have shallow gas as it is very near to spit area where a lot of sediments are dumped in to the sea from Godavari branch. Assuming the river took this shape about 4-5 million years ago, shallow gas reservoir needs to exist and the hypothesis is supported by the presence of such reservoirs in ONGC and RIL. Estimating the actual potential of these reservoirs before drawing up the development plan is difficult.

GSPC’s working interests lie in the third type, i.e. HTHP reservoirs. Due to the formation temperature being high (over 180oC), the initially produced oil undergoes a lot of cracking before giving high pressured gas. Such gases contain methane almost up to 85% and most of these discoveries in KG contain carbon dioxide from 6 to 12%. These reservoirs pose a host of problems which include difficulties in drilling, proper cementation, overcoming low permeability by hydro fracturing if required etc. Considering the enormity of difficulties, GSPC has done commendably well and let all of us hope that Deendayal prospect would do wonderfully well for a long period.
Courtesy: Urja Setu
Dr. Penukonda Ramabrahmam
Senior Manager Chemistry (Offshore) – KG Asset