Controls on Reservoir Heterogeneity of a Shallow-Marine Reservoir in Sawan Gas Field, SE Pakistan: Implications for Reservoir Quality Prediction Using Acoustic Impedance Inversion

Umar Ashraf,Saiq Shakeel Abbasi,Hassan Nasir Mangi,Hucai Zhang,Muhammad Ali,Xiaonan Zhang,Aqsa Anees

doi:10.3390/w12112972

Abstract

The precise characterization of reservoir parameters is vital for future development and prospect evaluation of oil and gas fields. C-sand and B-sand intervals of the Lower Goru Formation (LGF) within the Lower Indus Basin (LIB) are proven reservoirs. Conventional seismic amplitude interpretation fails to delineate the heterogeneity of the sand-shale facies distribution due to limited seismic resolution in the Sawan gas field (SGF). The high heterogeneity and low resolution make it challenging to characterize the reservoir thickness, reservoir porosity, and the factors controlling the heterogeneity. Constrained sparse spike inversion (CSSI) is employed using 3D seismic and well log data to characterize and discriminate the lithofacies, impedance, porosity, and thickness (sand-ratio) of the C- and B-sand intervals of the LGF. The achieved results disclose that the CSSI delineated the extent of lithofacies, heterogeneity, and precise characterization of reservoir parameters within the zone of interest (ZOI). The sand facies of C- and B-sand intervals are characterized by low acoustic impedance (AI) values (8 × 106 kg/m2s to 1 × 107 kg/m2s), maximum sand-ratio (0.6 to 0.9), and maximum porosity (10% to 24%). The primary reservoir (C-sand) has an excellent ability to produce the maximum yield of gas due to low AI (8 × 106 kg/m2s), maximum reservoir thickness (0.9), and porosity (24%). However, the secondary reservoir (B-sand) also has a good capacity for gas production due to low AI (1 × 107 kg/m2s), decent sand-ratio (0.6), and average porosity (14%), if properly evaluated. The time-slices of porosity and sand-ratio maps have revealed the location of low-impedance, maximum porosity, and maximum sand-ratio that can be exploited for future drillings. Rock physics analysis using AI through inverse and direct relationships successfully discriminated against the heterogeneity between the sand facies and shale facies. In the corollary, we proposed that pre-conditioning through comprehensive petrophysical, inversion, and rock physics analysis are imperative tools to calibrate the factors controlling the reservoir heterogeneity and for better reservoir quality measurement in the fluvial shallow-marine deltaic basins.

Highlights

Seismic inversion methods have been widely acknowledged for the delineation of reservoir facies in the exploration and production sectors (E&P) [1]
We aimed to provide new facts related to the narrow Cretaceous sections, as well as low-impedance sweet-spot zones to exploit for future drillings
Since all the logs were not available in S-14, it was not incorporated in the analysis

Summary

Introduction

Seismic inversion methods have been widely acknowledged for the delineation of reservoir facies in the exploration and production sectors (E&P) [1]. Due to the increasing demand for hydrocarbons in the E&P, geoscientists continually aim to produce feasible solutions to critical problems to estimate the maximum reserves and reservoir parameters [2,3]. These problems include the reuse of water during the drilling processes [4], which will solve the water scarcity problem [5]. The primary objective of seismic inversion is to convert the seismic reflection data into a layer rock property that describes the behavior of the reservoir quantitatively with high resolution that results in correct interpretations [10]. A dependable calculation of the reservoir properties is critical for the policy-making and decision-making process throughout the production stage [13]

Results

Discussion

Conclusion