FORMATION EVALUATION OF BASEMENT ROCKS THROUGH TEXTURAL ANALYSIS AND ASSOCIATED GEOMECHANICAL PROPERTIES

Abstract

Unconventional reservoirs such as fractured basements have gained importance in India in the last two decades as significant amount of hydrocarbons have been encountered in them. However, their unconventional nature makes it difficult to characterize them. The development of reservoir properties such as porosity and permeability in the basement rocks depends on fracturing and weathering. In this study, we focus on evaluating the fractured/weathered basement and characterize the basement reservoirs of the western offshore basin of India with the help of well logs and core data. Petrographic studies were carried out on twenty-two core samples to identify the basement rock types and thin sections were prepared to evaluate the texture coefficient of the rocks. The porosity measurements were carried out on two available samples from the Deccan trap to understand their reservoir nature. Density, compressional, and shear wave velocities were carried out on twenty-four core samples and were used to calculate the dynamic Young’s modulus and Poisson’s ratio. Uniaxial compressive strength was measured in the laboratory for ten core samples and static Young’s modulus and Poisson’s ratio were measured for fourteen core samples. The study evaluates the basalt of the Deccan trap basement and identifies four reservoir facies based on their ability to hold and flow hydrocarbon. Standard log signatures are standardized in this study for various facies in the basalt and porous permeable zones are identified using conventional well log data. In the present study, texture is quantified for cores of igneous and metamorphic rocks and a texture coefficient (TC) is determined. An empirical relationship between TC and lab-derived uniaxial compressive strength (UCS) values was standardized, which provides a non-destructive method to calculate the UCS. Also, the new correlations between UCS, TC, and acoustic properties are proposed as new non-destructive methods to calculate TC and UCS. The static and dynamic values of E and ν were determined and new relations were standardized to calculate static values from dynamic. The study addresses the importance of rock texture in identifying rock strength and proposes correlations to determine the UCS and static elastic constants using non-destructive methods and will be helpful in wellbore instability studies, planning hydro fracturing designs, and rock strength analysis of basement rocks. The present work also identifies the variation of fracture and stress orientations across the Western Offshore basin using micro resistivity image data. The Kutch offshore and north of Mumbai High are observed to have different fracture and stress orientations than the south of Mumbai High and Heera field, which lies in the southern part of the Western Offshore Basin. This study helps in understanding the reservoir nature of igneous and metamorphic rocks of the Western Offshore Basin. The identification of reservoir facies in drilled basalt sections will help in identifying the interesting zones for the exploration and exploitation of hydrocarbon. The proposed correlations to determine UCS and static elastic parameters will be helpful in the primary rock strength analysis of basement rocks. The fracture and stress orientation analysis will help in solving a range of geomechanical problems including planning hydro fracturing jobs in the basement intervals. The present research work helps in reducing the uncertainties in the formation evaluation of the Basement reservoirs of the Western Offshore Basin of India.