Abstract

The large negative free-air gravity anomaly observed over the Mumbai High oil field resting on a huge anticlinal Precambrian basement high (Figure 1) is intriguing as such a structural feature would normally be expected to show a positive anomaly. This paper presents, for the first time, a hypothesis explaining the anomaly based primarily on the interpretation of results from drilled wells in the Mumbai High oil field. Results of several wells drilled into the basement across the field reveal the basement composed of varied types of rocks consisting of Paleocene trap basalts, Archean granites, metamorphics, phyllites, schists and quartzites. Significantly, the Deccan traps are encountered in the basement in the structurally highest northern part bypassing the structurally lower southern part, which is covered mainly by Archean granites and metamorphics (Figure2). This led to the crucial inference that the basalts encountered in Mumbai High field are not the Deccan trap flood basalts but are intrusive basalts, younger than the flood basalts that form the basement in the Mumbai offshore basin. The massive intrusion of the basalts from a shallow source of magma pond below the Moho aided partly by its downwarp explains the large negative anomaly (Figure 3).