Abstract

The paper is based on integrated studies of geoscientific data including high quality 3D seismic data over a gas bearing carbonate-reservoir field in Bombay Offshore Basin off West Coast of India. The studies cover seismic attribute analysis along with structural interpretation. Paleogene carbonate sequence is a major lithological entity in Bombay Offshore Basin. It harbours most of the satellite hydrocarbon fields in the basin. Most of the hydrocarbon discoveries show that only the upper part of the sequence comprising Mukta formation and upper part of Bassein Formation is oil and/or gas bearing. Due to the predominance of carbonate facies and presence of a very thin transgressive shale layer at the Bassein-Mukta interface the seismic section in general is transparent through this interface. The presence of gas in the sequence has increased the contrast in acoustic impedance at the interface and hence given a weak but recognisable reflection. Present study is based on the recognition of these reflection, computing various attributes based on this and demarcating hydrocarbon boundary over the entire field. The seismic attributes computed were found to be closely following the structural trends. The reflection character corresponding to top of Bassein unit changes at gas oil contact. The limit of the hydrocarbon boundary is not as obvious on the amplitude sections as it is on Average Instantaneous Phase & Average Instantaneous Frequency. It is observed that the seismic attribute maps follow the structural trend that establishes an interdependency of the two. This has given confidence to believe that the gas pool boundary interpreted from seismic attributes is more realistic one. Gridding and contouring of attributes have reduced the inherent noise present in raw attribute data. The same is manifested in the paper. Instantaneous Phase attribute has given the best exhibition of the gas pool boundary. The present study has also given an exploration lead in other adjoining area to probe on the basis of seismic attributes. Seismic attributes can effectively help in demarcating hydrocarbon boundary in carbonate reservoirs. It may also help in refining the velocity model where the hydrocarbon accumulation is structurally controlled.

Introduction

The Bassein field, having an areal extent of about 210 sq.km is a major gas producing carbonate-reservoir field in Bombay Offshore Basin off West Coast of India. The study involves an interpretation of more than 800 sq km high quality 3D seismic data and petrophysical evaluation of the reservoir at more than seventy well locations including forty-six development wells. The study was conducted using a network of state of the art geophysical and geological analysis software running on UNIX computer workstation. Signatures on the seismic sections were identified, as direct hydrocarbon indications (DHI). Later the DHI were correlated all over the field.

A carbonate sequence of Paleogene age is the main reservoir for the gas accumulation. The carbonate sequence comprising Bassein and Mukta Formation is a major lithological entity in Bombay Offshore Basin in West Coast of India. It harbours most of the satellite hydrocarbon fields in the basin.

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