Reversal of Basement-Block Motions in Cambay Basin, India, and its Importance in Petroleum Exploration

Abstract

The study of the nature of the basement and the reversals of the sense of movements along basement fault blocks are of great significance to petroleum exploration in the Cambay graben basin of western India. Oil accumulation in the basin is controlled primarily by basement relief and fault-block tectonics. The Cambay basin is a composite basin characterized by a two-stage structure. The Precambrian basement of the basin, composed of igneous and metamorphic rocks, is an extension of the Aravalli-Delhi, Satpura, and Dharwar orogenic belts trending, respectively, northeast-southwest, east-northeast-west-southwest, and north-south. The present basin apparently first began to form during the Mesozoic (Jurassic) when the region was an undisturbed shelf bounded on the east by the Indian shield. An intracratonic graben came into existence on this shelf during or immediately following the formation of the latest Cretaceous Deccan traps. A thick sequence of Tertiary sediments was deposited directly above the trap. During Paleocene-early Eocene time, the Olpad Formation was deposited on a Deccan t ap floor characterized by narrow fault blocks--horsts and grabens. Some of these fault blocks reversed their structural relations during the deposition of the early Eocene Cambay Shale. Such a reversal is exemplified by (1) the north-south Sanand-Nawagam-Dholka block, parallel with the Dharwar strike and (2) the east-northeast-striking Kosamba-Olpad block parallel with the Satpura strike. During the subsequent, post-Cambay Shale evolution of the basin, drape-fold structures were formed over fault scarps and/or horsts. Early Tertiary volcanism is conspicuous along the west coast of India and is manifested by flood basalts, dike swarms, and eruptive centers. The volcanic activity and the incipient faulting which led to the subsidence of the Cambay graben basin probably are related to the opening of the northwestern Indian Ocean along the Central Indian and Carlsberg Ridges. As a result of the subsidence of the basin floor, compressional stresses (stressing) were developed in the crustal layers. These stresses were relieved by the uplift of many fault blocks in the sagging basin floor. The location of the Cambay basin at the junction of three ancient Precambrian tectonic trends--the Aravalli-Delhi, Satpura, and Dharwar--is significant. The reversal of the fault blocks is believed to have been caused by unstressing. This stage may have been controlled by the rejuvenation and interaction of the three ancient tectonic trends. The unstressed blocks then were readjusted in response to forces such as gravity, isostasy, loading, and unloading. The history of fault-block movement and rejuvenation of the old Precambrian lineation can be applied directly to petroleum exploration. The relative position of a fault block in the basin controls the deposition of source materials and reservoirs within it. Furthermore, the time of reversal of basement blocks is of significance, both from the standpoint of dating the deposition of lenticular reservoir sandstones, and the development of unconformity traps.