SEQUENCE STRATIGRAPHY OF MIDDLE EOCENE TST IN BROACH-JAMBUSAR BLOCK, CAMBAY BASIN, INDIA

Abstract

Sedimentation and sequence building pattern of the Middle Eocene Hazad Member of the Cambay Basin, Western India constitute the core theme of this thesis. The present thesis is aimed for bringing out an integrated depositional model through facies characterization, sediment dispersal and Process response modeling of Hazad sandstones to understand tectonics through sedimentation and basin evolution in Broach-Jambusar Block of Cambay Basin for providing insight in to complex reservoir distribution and resultant hydrocarbon prospectivity through high resolution sequence stratigraphy. Observations and conclusions made in this course of the study further encouraged into a venture that also focuses on the key elements of petroleum system. The Hazad Member (Middle Eocene) of the Ankleswar Formation in Cambay Basin, India, is traditionally reported as deltaic system. Present work documents three major facies associations, namely, (i) sandstone-rich upper delta plain (FA-1) deposits, (ii) sandstone-mudstone heterolithic lower delta plain (FA-2) deposits, and (iii) shale-dominated prodelta (FA-3) deposits, in an overall coarsening-up to fining up succession. The sandstone-rich facies association (FA-1) consists of three facies, viz. massive poorly sorted sandstone (1a), medium to large cross-stratified sandstone (1b) and normally graded sandstone (1c), predominantly occurring in lower part of the succession. Overall facies architecture suggests a fluvial depositional system. The sandstone-mudstone heterolithic facies association (FA-2) consists of shale with sand lenticles (2a), coarsening- to fining-up sandstone (2b), argillaceous sandstone (2c) and sandstone-shale heterolith (2d). All the facies types show varied sand: mud ratio with systematic change from sand to mud and mud to sand in vertical successions. The coarsening up sandstone possibly represents distributary mouth bar in a lower delta plain regime. All the facies types are characterized by development of different tidally-generated structures, like flaser, lenticular and wavy beddings, tidal bundles with quick sand-shale alternations, and abundant bioturbations, which indicates strong tidal influence within the lower delta plain depositional system. The sand-dominated facies signify deposition within the Page i distributary channels, while the mud-dominated and heterolithic facies indicate deposition in intertidal-subtidal flats within the inter-distributary bays/plains. The shale-dominated facies association (FA-3) is represented by grey, carbonaceous shale (3a) and shale with sand lenticles (3b). Dearth of coarser clastics, rarely preserved tidal features and abundant bioturbations within silty-shale indicates deposition in relatively deeper and seaward part of the basin. Overall facies character with predominance of grey, carbonaceous shale and sand lenticles in the upper part signify a prodelta depositional setup. Thus, the lithofacies and their association indicate sedimentation took place in laterally adjacent sub-environments, viz. upper delta plain, lower delta plain and prodelta. The overall coarsening-up successions with profound evidences of bioturbation in fluvial-tidal interactive system suggest deposition in a tide-affected deltaic environment. The Middle Eocene Hazad Member 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) suggesting a wide spread transgression during the Middle Eocene. However, such a TST is characterized by several short term fining-up and coarsening-up successions, within an overall fining up trend. Thus, identification of these high frequency lower order cycles and corresponding sequence stratigraphic surfaces is felt necessary to bring out the changes in sedimentation pattern in the Hazad Member within a high resolution sequence stratigraphic framework. 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 basin-ward to form six coarsening-up 4th order cycles (parasequence sets), which are further subdivided into twenty one 5th order cycles Page ii (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 3rd order TST-HST TST units of the Hazad Member may be interpreted as the result of varying rates of rising RSL and sedimentation. The absence of RSL falls may have been favoured by continuous subsidence. In order to understand the key factors that control the petroleum systems of the Broach-Jambusar Block, a Basin and Petroleum Systems Modeling study have been carried out. The Paleocene, Early to Middle Eocene – Middle Eocene TST petroleum system are proven by this study. Multiple source rocks at different levels of Cambay Shale charge the main Middle Eocene reservoir. Hydrocarbon generation and expulsion timing varies over the main low in Broach-Jambusar Block. The primary aim of this petroleum system modeling study through high resolution facies maps of Hazad sandstones is to get a better understanding of the various elements of Petroleum System along with the evaluation of thermal maturity and timing of hydrocarbon generation.