Fault-related overpressure in the Krishna-Godavari Basin, India

Abstract

Drilling and reaching to deeper target zones through an overpressured overburden formation in a structurally complex geologic setting requires robust geologic and geomechanical analysis to mitigate risk and control operational costs. These types of geologic conditions are present in the Krishna-Godavari Basin, where a series of horst and grabens defined by deep-seated faults and persistent high sedimentation rates through geologic time, result in the development of challenging conditions for exploratory drilling. We have developed possible overpressure mechanisms across the central part of the Krishna-Godavari Basin and its interplay through fault-related lateral pressure transfer. The basin sits over a horst, which is one of the many northeast–southwest-trending en echelon horst and graben structures comprising sediments from the lower Cretaceous to Holocene. In the study area, Paleocene formations in the horst are overpressured (12–12.2 ppg). Three wells were drilled through this formation and reached the target without any drilling issues in the central and eastern part. However, the same formation in the western part of the horst (adjacent to the graben) has higher overpressure of approximately 14 ppg, which complicates the drilling operations because it requires an additional intermediate casing to reach the target reservoir safely. A detailed analysis of the overpressure mechanisms across the horst area to the adjacent deep graben revealed that the disequilibrium compaction signatures are related to the burial history and overburden thickness. The major difference between horst and graben area is the magnitude of overpressure, with an average of 16 ppg across the graben area. The larger overpressures experienced toward the western part of the horst indicate a secondary source of pressure from the adjacent deep graben. The fault stress analysis in this region presents a feasible lateral pressure transfer through critically stressed faults/fractures from the deep graben to the western part of the horst structure. The current model accounts the common pore pressure estimation method along with other critical geologic information to predict such overpressure related challenges in the upcoming future wells in a similar geologic setup to plan safe and cost-effective wells.