Advanced Seismic Reservoir Characterization of Carbonate Reservoirs: A Case Study

Successful exploration for and effective exploitation of hydrocarbons from carbonate reservoirs necessitate accurate prediction of porosity pods and fluid distribution within an intricate maze of porosity permeability corridors. Conventional qualitative interpretation of G&G data to bring out gross depositional models and facies distribution maps do not provide adequate control in effective appraisal and exploitation of these discrete reservoirs. Often, such simplistic reservoir models lead to wrong well placement and inefficient appraisal and development plans. Quantitative interpretation of the seismic and well data, integrating a myriad of G&G data for reservoir characterization, is required to mitigate the risk and exploit the carbonate reservoirs in a cost-effective manner. A robust workflow for seismic-based reservoir characterization to elucidate the complexities in carbonate reservoirs involves high-frequency cycles mapping integrating well and seismic data with sediment logical data and image logs. This framework is subsequently calibrated with seismic attributes and P-impedance data to develop permeability, density and diagenetic models to bring out the cyclicity of porosity creation and destruction during deposition of multiple phases of carbonates corresponding to sea level oscillations. This improvised reservoir characterization workflow was adopted to evaluate the Eocene carbonate reservoirs of Bassein Formation in a field in Mumbai offshore basin, which will be discussed in detail. The study has explicitly established that Vadose zone diagenesis along the exposed geomorphic highs led to porosity generation. While the porosity in Upper Bassein has been largely preserved, it has been completely obliterated in Middle Bassein and is patchy in Lower Bassein. After integrating heterogeneity from nano- to seismic scale, a static model was developed which led to identification of upside potential beyond the established reservoir limits and infill development locations within the field area.