Reservoir Characterisation of Gollapalli Gas Pay Sands of Mandapeta - Endamuru Areas of Krishna Godavari Basin, India

A.v.v.s. Kamaraju, Dn Prasad, and Manmohan Meka
ONGC, Jorhat, India

Mandapeta and Endamuru gas fields of Krishna Godavari Basin, Andhra Pradesh, India are producing gas from the oldest reservoirs of Gondwana sediments ranging in age from Permo-Triassic ( Mandapeta sand stone ) to Lower Cretaceous (Gollapalli sand stone).

These fields were put on production 20 years ago. The Mandapeta sand stone reservoirs occurring at 2.9 Kilometers depth show meager production rates, produced cumulatively 20 % of total gas of these fields, while the Gollapalli sand stone reservoirs occurring at 2.4 Kilometers depth show good production rates and cumulatively produced 80 % of gas.

An attempt was made in this paper in understanding the nature of the Gollapalli gas reservoirs of these fields integrating all the available G&G data. The study reveals that the Gollapalli pay sands belong to the lower part of Lower cretaceous shales, called Raghavapuram shales. These gas sands appear to have been deposited under restricted shallow marine conditions during transgression of sea into the NE plunging rift grabens (half grabens), forming a restricted and shallow water environment. The sedimentation took place at a very slow rate probably with no classical delta build up except for minor bar sands along the shore deposited during transgressive phase.
The hydrocarbon entrapment mechanism for the reservoirs is of stratistructural nature. The bar sands that crept over the palaeohighs (raising flanks of the grabens) during the transgressive phase formed updip pinchouts due to the post Raghavapuram SE tilt during upper cretaceous. These sands were well preserved by the transgressive shales and retained original reservoir characters. These reservoirs are genetically correlatable with the gas reservoirs of Endamuru area.This helped in identifying new areas.

Presentation GEO India Expo XXI, Noida, New Delhi, India 2008©AAPG Search and Discovery