Characterization of organic content, brittleness index, and geomechanical properties of the Eocene Cambay Shales — Insights from the Ankleshwar oil field in western India

Abstract

Shale resource assessment involves a detailed characterization of organic and geomechanical parameters for better insights on the reservoir properties and classifying areas of economic yield. To assess the Eocene Younger Cambay Shale (YCS) Group of the Ankleshwar field, western India, for feasible shale resource play, we have applied a multistage screening methodology that combines an estimation of organic richness, brittleness index (BI), and geomechanical analyses. The estimated thermal maturity and average total-organic-carbon (TOC) contents are within the range of 0.8–1.0 and 1.8 wt%, respectively. These estimates are comparable to the reported core-based measurements. BI based on the mineralogical composition reveals that the YCS intervals of marine origin fall into the “less-ductile” to “brittle” zone, whereas the elastic property-based estimated BI falls into the “less-brittle” to “high brittle” zone. We have established a field relationship between the BI and the shale volume, and we also deciphered the effect of TOC content on the rock-elastic properties. Pore pressure in the shales is slightly above the hydrostatic gradient (10.5–11.5 MPa/km). The estimated average fracture pressure of 18.5 MPa/km, together with the BI of moderately ductile to less-brittle behavior suggests that the studied shales are capable of withstanding substantial strain while hydrofracturing for effective production. We demonstrate an expedient example to characterize a potential shale unit within a producing hydrocarbon field using the drilled wells with limited or no core data.