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ABSTRACT: Burial porosity reservoirs in Bassein Formation, Bombay Basin, offshore India

Minero, Charles J.1, Mateu Esteban2, E. Raul Sarmiento3, and Hari Kumar4
(1) Enron Global Exploration and Production, Inc, Houston, TX
(2) Carbonates International Iberia, S. L 
(3) Enron Global Exploration and Production, Inc 
(4) Enron Oil & Gas India, Ltd

Eo-Oligocene Bassein limestones produce oil and gas in Panna-Mukta fields in the Bombay Basin. Reservoir rock is highly heterogeneous with relict nodular fabric preserved as decimeter-scale interweaving of reservoir (30+% porosity) and non-reservoir (<6% porosity) elements. Compaction, pressure dissolution and cementation during burial destroyed nearly all early porosity. Associated cements include calcite, dolomite, ankerite and siderite.

Porosity re-developed after stylolitization, and commonly is limited to highly pressure-solved internodular areas that retained minor permeability, and localized initial dissolution. It was fabric selective for micrite, resulting in extensive microporosity, and advanced into nodules. Advanced dissolution produced moderate moldic, vuggy and fracture (tension gashes) porosity, and collapse breccias. Minor dickite and quartz cement postdate reservoir formation. Kv/Kh ratio is high.

Circulation of hot, H2S- and CO2-bearing, low-salinity groundwater, as presently reside in the Bassein Formation, driven by regional hydraulic head from onshore is the proposed mechanism for sustained porosity creation. Source rock maturation and hydrocarbon migration since Late Mio-Pliocene time appear contemporaneous with reservoir formation.

Reservoir quality is seismically mapped. Stratiform development of enhanced porosity adjacent to faults implies a genetic relation between porosity creation and fluid flow up faults. Reservoir quality varies with stratigraphic architecture and meteoric fluid access. Panna East fault zone and Eocene unconformity are primary conduits. Tight transgressive limestones focussed flow through intervening highstand limestones. Collapse breccia pipes, mapped from 3D seismic, are conduits for vertical fluid migration.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia