Assessment of Paleocene to lower Oligocene formations and basement to estimate the potential hydrocarbon reservoirs using seismic inversion: a case study in the Upper Assam Shelf, India

Neha Rai,Dip Kumar Singha,Rima Chatterjee

doi:10.1007/s13202-021-01357-1

Abstract

The upper Assam shelf is a self-slope basin in north-east India, filled with nearly 7 km of sedimentary rocks of tertiary period with the granite basement and various oil fields along the border of the Naga thrust. The major producing fields are structural and strati-structural. The study area is placed in between the Mikir hills and Naga thrust. The objective of the study is to identify potential hydrocarbon reservoir zones in the geologically complex south upper of the Assam shelf using estimates of acoustic impedance and porosity derived by 3D post-stack seismic inversion. Well data, such as sonic velocity and density logs, from two wells (namely, KA and TE) are used in the inversion and validation of results. Inversion results are used to build a geological model in the form of acoustic impedance from which we derive 3D porosity cube which are used for hydrocarbon potential in the Paleocene to lower Oligocene sands, and the Precambrian basement. Although the amplitude maps provide an indication of potential reservoirs, the extent of these zones are much better identified in the inverted impedance maps and the corresponding estimated high-porosity zones. The analysis predicted the potential reservoir rocks in the Sylhet, Kopili and Barail formations, in which the Sylhet and Kopili appear to have good potential zones. Near the vicinity of the Naga thrust belt, the proximity of potential reservoir is predicted in the Kopili, Sylhet formation and in the fractured basement, respectively.

Highlights

Derivation of elastic parameters such as acoustic and shear impedance and density is the primary task of seismic inversion (e.g. Sen 2006)
Acoustic impedances are closely related to rock properties such as porosity; seismic inversion results can be used to obtain lateral information on petrophysical properties (Latimer et al 2000; Dolberg et al 2000; Helgesen et al 2000; Marion and Jizba 1997)
The prime objective of this paper is to evaluate the hydrocarbon potential in the upper Assam basin by accurate estimation of acoustic impedance and porosity for Sylhet, Kopili and Barail formations and the granite basement in the study area (Zamanek et al 1970; Hussain et al 2017a,b)

Summary

Introduction

Derivation of elastic parameters such as acoustic and shear impedance and density is the primary task of seismic inversion (e.g. Sen 2006) It involves iterative updating of elastic properties while reducing the misfit between the observed and synthetic data assuming that the source wavelet is known (normally derived from the seismic trace) (Latimer et al 2000). Prospect evaluation and determination of the quality of a hydrocarbon reservoir require estimation of petrophysical properties of a reservoir (Pandey et al 2020; Chatterjee et al 2016; Das and Chatterjee 2018) This is generally achieved by first establishing a relationship between the petrophysical properties and inverted seismic attributes at the well locations—this relationship is later used to estimate petrophysical properties in the entire volume (Rijks and Jauffred 1991; Lefeuvre et al 1995; Russell 2004; Sancevero et al 2005; Soubotcheva 2006). This commonly used method known as ‘seismic reservoir characterization’ or ‘quantitative seismic interpretation’ requires careful implementation of each step in areas of complex subsurface geology such as the upper Assam shelf, India—the area of our study

Objectives

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Conclusion