Print this pageDiagenesis and Reservoir Characterization of Miocene Carbonate (Ratnagiri Formation) in Deep Continental Shelf Area of Mumbai Offshore Basin, India
Mumbai Offshore Basin, a passive margin basin of India formed due to break up of Madagascar and Seychelles from the Indian sub-continent during Upper Cretaceous. This basin, a NNW-SSE trending shelfal horst graben complex, comprises six major tectonic blocks namely Tapti-Daman, Diu, BH-DCS, Heera-Panna- Bassein, Ratnagiri and Shelf Margin. The study area, a broad monoclinal platform surrounding the Bombay High exhibits thick Tertiary sediment over Deccan Trap. In this area a thick Miocene carbonate (Ratnagiri Formation) hosts important hydrocarbon bearing pools with limited regional extension.Cores were studied to understand the sedimentary fabric and diagenetic imprints to find reasons for reservoir heterogeneity. Petrography, clay mineralogy and SEM-EDS studies of the limestones from selected wells were carried out and integrated with log data.
The limestones of Ratnagiri Formation are dirty white, buff also tan in color with abundant larger foraminifera, algae, corals and bioclasts as grains. Argillaceous matters are common and occur as lamina and partings. The limestones are moderately hard, chalky with scattered large solution vugs and are interbedded with thin to thick beds of shale. The lithology and biotic associations indicate deposition in a shallow inner shelf condition and exhibit rhythmic sedimentation. Sedimentary structures include parallel and stylolaminations. The microfacies comprise mudstone, wackestone, packstone with occasional grainstone and boundstone and indicate mainly low energy during deposition in a shelf. Primary porosity is poor and includes matrix and intergranular types. Secondary porosity is fair to good at times and occurs as solution vugs, channels and molds. The porosity distribution is non uniform and is not facies controlled. The important diagenetic changes include compaction, matrix and grain sparitisation, dolomitisation and cementation. The pores have been partially occluded by granular calcite formed during late stage (burial) cementation. The presence of interbedded shales has made a permeability barrier within the limestone reservoirs. Dolomitisation is common and increases with depth. Pyrite occurs as disseminated crystals and within foram chambers. The occurrence of shale interbeds and partings along with late stage calcite cementation and dolomitisation led to reduction in permeability, porosity and thus forming heterogeneous and regionally discontinuous reservoirs.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil