High resolution sequence stratigraphy of Middle Eocene Hazad Member, Jambusar-Broach Block, Cambay Basin, India

Abstract

Abstract The Middle Eocene Hazad Member of the Ankleswar Formation in the Cambay Basin is characterized by 13 sandstone units (GS-0 to GS-12), representing progradational pulses within an overall fining-up 2nd order transgressive systems tract (TST). The overall facies architecture of the Hazad Member implies deposition in a tide affected marginal marine (deltaic) environment. High resolution sequence stratigraphic analysis based on core and electrolog data allows to subdivide the 2nd order TST into three 3rd order systems tracts, namely, (i) basal shaly transgressive systems tract (TST), followed by (ii) a highstand systems tract (HST) incorporating four intervening parasequence-sets (GS-0 to GS-2, GS-3, GS-4 to GS-6 and GS-7 to GS-9) separated by 4th order flooding surfaces, and (iii) a transgressive systems tract (TST) at the top consisting of parasequence-sets (GS-10 to GS-12) capped by the Kanwa Shale that includes the maximum flooding surface (MFS). The amalgamated sands of the 3rd order sequences found landward split basinward to form six coarsening-up 4th order cycles (parasequence-sets), which are further subdivided into twenty one 5th order cycles (parasequences), separated by minor events of marine flooding. The depositional model and paleogeographic maps using 3rd, 4th and 5th order cycles reveal a minimal sand supply at the beginning of 3rd order HST and a significant increase in fluvial sand supply thereafter, which resulted in progradation leading to constructive delta building till GS-9. The topmost transgressive parasequence-sets constituting GS-10 to GS-12 mark landward retreat due to a paucity of sand supply and/or a rise in relative sea-level, which resulted in back-stepping of the paleoshoreline and development of estuaries over the older deltaic system. The model helps to explain the extent of short-term transgressive-regressive cycles, improving elucidation of the hydrocarbon-bearing sand units of the Cambay Basin in future exploration.