The Lower Miocene L-III limestone reservoir of Bombay High South, Western offshore, India, is a multi-layered reservoir. It has been divided conventionally from top to bottom into major layers A, B, C, D, E, and F separated by intervening shale and argillaceous tight limestone. A' layer is further subdivided into Al, A2-I, A2-lI, A2-III, A2-lV, A2-V, A2-VI and A2-VII and along with layer B constitutes the main producing horizon. Identification and mapping of individual layers and facies distribution in them has important implications in the effective reservoir management as well as performance prediction. The present paper attempts to work out a depositional model for these layers through integration of log, core and reservoir data in order to explain the spatial and temporal variations in the distribution of poro-perm characteristics of flow units.
Based on log characters and core data, a tidal depositional model has been evolved. It has been envisaged that the carbonate layers have been deposited in a tidal regime comprising supratidal, intertidal and subtidal environments. Though the deposition has taken place in shallow lagoonal conditions, the reworking effect of the tidal waves in a regressive regime has given rise to the carbonate deposits in the tidal channels and adjacent levees, and shale in the marshes. These layers have also witnessed subaerial exposure. Starting from C and B layers, during their deposition, the environmental conditions were uniform without much facies and thickness variation and the area had an open marine influence as indicated by the presence of pelecypod shells and other associated fauna. These layers, along with the overlying N shale, have been designated as Sequence I. The overlying A2 -VII and A2-VI layers, deposited in a subtidal regime are termed as Sequence 2. The layers A2-V and A2-IV represent an intertidal regime and constitute the Sequence 3. Finally, the carbonate cycle ends with intertidal environment having influence of supratidal regime comprising the layers A2-IIl, A2-lI, A2-I and Al. The occurrence of microdolomite/ dolomitic wackestone in this sequence supports the presence of supratidal environments. The sequences are identifiable on GR log showing diagnostic log features.
In general the area has been under the influence of a regressive cycle during the deposition of sequences 1 to 4. At micro level, however, each stratigraphic sequence has witnessed deposition of different facies due to fluctuations in bathymetry. In the ultimate analysis, the model will provide significant leads about reservoir facies changes in time and space and help in recognizing the horizontal and vertical poroperm variations in the different flow units of the prolifically producing L-III horizon.
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