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Diagenetic Processes and Hydrocarbon Accumulation in Low Resistivity Shales in Cambay Basin, Gujarat, India

Kamal K. Das, J. K. Srivastava, Nilam Niranjan, Bharati Jagtap, Jyoti Borthakur, and Sanjeev Satyarthi
Oil & Natural Gas Corp. Ltd., Regional Geoscience Laboratories., Vadodara, India

The Early Eocene Chhatral Member in Cambay Formation has a number of hydrocarbon occurrences within the marginal marine to inter tidal shale in Ahmedabad and Mehsana tectonic Block. The reservoir facies is discontinuous, very thin having very low resistivity. The overall resistivity of the Chhatral Member is very low and it is difficult to pick the pay zones on conventional electro logs.

The main objective of the study is to understand the mechanism of selective development of the reservoir facies in the shale and possible causes of their low resistivity. Detailed microfacies study was carried out to demarcate such zones and understand the diagenetic transformation of the clay minerals. The study indicates selective diagenetic transformation of chlorite clay minerals to fine grained authigenic kaolinite pellets in an alkaline pH in humid environment during early stage of diagenesis is responsible for development of the reservoir facies in shale. The floating kaolinite pellets / pseudomorphs and chlorite in argillaceous matrix form the framework grains and define an unconventional heterogeneous insitu reservoir in shale.

The Pore Throat Sorting (PTS) and Pore Size Distribution (PDS) pattern suggests a moderate sorting (PTS=1.54 - 2.57) and dominance of micropores of less than 1 µm size.The irreducible water saturation (Sirw) range from 35% to 84%. The diagenetically formed reservoir units thus have relatively better flowing characteristics and pore geometry.

The reservoir facies is heterogeneous and its thickness is normally less than the minimum resolution of the conventional electro logs which make their identification very difficult on conventional resistivity logs.
The main cause of the low resistivity is attributed to high capillary bound (or irreducible) water resulting from the small pores associated with the clays. The dispersed clays provide additional conductive pathway through the pore system, adding conductivity and reducing resistivity via cationic exchange mechanism and increasing the pore surface area.

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