Fracture and Stress Orientation from Borehole Image Logs: A Case Study from Cambay Basin, India

摘要

The Deccan trap basalt, laid down by multiple lava flows during upper Cretaceous to Paleocene times forms the basement of current study in Cambay basin. As such, there is great interest and value in fracture detection and evaluation of fractured basement reservoirs in the Cambay basin. The procedure for identification and evaluation of natural as well as induced fractures in basaltic basement of the Cambay basin is presented in this work. In this study formation micro-imager (FMI) and extended range micro-imager (XRMI) log data for fracture identification is used. The Deccan trap basaltic basement of the study area, comprising five wells in the Tarapur-Cambay block, has potential for holding commercial hydrocarbon due to the presence of fractures and weathered basement. Both image logs (FMI, XRMI) identify three types of fracture including open (conductive), partially open and closed (resistive) fractures, of which open and partially open fractures are important for hydrocarbon accumulation. Fracture dip ranges from 10 degrees to 80 degrees. Image logs have also identified washout, breakout and drilling-induced fracture zones. The strike direction of the open natural fractures for four wells varies from N60 degrees E to N30 degrees E whereas the strike direction of most natural fracture in the fifth well is oriented towards N20 degrees W. The orientations of drilling-induced fractures and breakouts may be interpreted for the insitu stress direction over the logged interval. Drilling-induced tensile fractures, identified over the depth interval of 1969-1972 m, and borehole breakouts over the interval of 1953-1955 m in one well, suggest an orientation of maximum in-situ horizontal compressive stress (S-H) lies in the north-south direction. The azimuths of open natural fractures in the same well vary from north-south to N30 degrees E. It is expected that the direction of fluid flow will be controlled by open natural fractures and therefore would be in a direction parallel to the SH direction, which is orthogonal to the minimum horizontal stress (S-h) direction. The orientations observed are consistent with the present day S-H direction in the study area of Cambay basin.