Abstract: Diagenetic Controls on Porosity Development in Bassein Pay, Mumbai Offshore Basin, India

Peters, James and K.C.Das* - Keshava Deva Malaviya Institute of Petroleum Exploration, Oil & Natural Gas Corporation Ltd., India

Western continental shelf of India is an Atlantic type, passive continental margin, where sedimentation initiated with deposition of non-marine to paralic siliciclastics of Paleocene to Lower Eocene as syn-rift sediments. Post-rift sediments have been marked by both clastics and carbonate deposition, out of which two episodes of widespread carbonate growth during Middle-Late Eocene (Bassein Formation) and Lower Miocene (Bombay Formation) are significant from hydrocarbon point of view. Among the major hydrocarbon discoveries in Mumbai Offshore Basin, Bombay High is an established giant oil field which produces from Lower Miocene Limestone reservoirs. Besides this, sizable production comes from satellite fields like Mukta, Panna, Bassein, Neelam and Heera from Middle-Upper Eocene Limestone reservoirs. These fields are located east and southeast of Bombay High in the Panna-Bassein structural block.

The Bassein Limestone having average thickness of about 3OOmts. in the platform area is dominated by deposition of mud rich carbonate facies which occasionally grades into grain rich ones. Fossil components of the carbonates are mainly represented by forams (miliolids), peloids, bioclasts having a low diversity nature and suggests its deposition in restricted platforms (facies belt 7 & 8 of Wilson, 1978) with varying carbonate productivity and localised shoaling.

Petroleum habitat in the limestone sequence has been related to diagenetically controlled porosity, development due to unconformities and diastems. Integrated data from core studies, biostratigraphy, electrologs and eustatic sea level curves has established a major unconformity of varying durations (maximum of 3.4 m.y.) at the top of Bassein Formation (Fig. 1).

Different diagenetic processes and their products include; (a) Micritization of fossil grains in marine phreatic water during syndepositional/early diagenetic stage leading to the formation of most of the peloids; (b) Meteroic diagenesis in vadose and phreatic zones creating vugs, channels, molds by dissolution, and drusy, grannular and neomorphic calcite by cementation; and (c) Burial diagenesis with compaction and pressure solution activity leading to the formation of stylolamination, stylonodular bedding and stylolites. Localized solution porosity, patches of coarse calcite cement and small tensional fractures perpendicular to sylolites are related to the process of stylolitisation (Fig.2). Selective dolomitization around argillaceous rich layers may be due to compactional clay diagenesis.

Depositionally, less abundance of grain dominated facies and diagenetically preferential cementation of this facies wherever present have given rise to poor development of primary porosity. It is the vadose related secondary porosity such as large scale solution vugs, channels and molds are the main contributor towards reservoir development. These porosities generated are mainly of: (a) Eogenetic nature due to localized diastems in the lower and middle part; (b) Telogenetic at the top with maximum effect in the eastern margin where extensive leaching and karstification have helped in major hydrocarbon accumulation; and (c) Mesogenetic in form of localized solution porosity and small unfilled fractures. Some of the anomalous fluid flow behavior observed in structures like Mukta and Neelam may be due to small scale spatial and temporal variation in porosity distribution that are controlled by diagenesis.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil