Understanding Clastic-Carbonate Interplay in Distal Part of Tapti-Daman Sector of the Mumbai Offshore Basin and Its Implications on Hydrocarbon Prospectivity

Abstract

The prolific gas prone Tapti-Daman Block of the Mumbai Offshore Basin is one of the thrust areas for exploration and development along the west coast of India. Hydrocarbon accumulations generally occur in clastic and carbonate reservoirs of Early Oligocene Mahuva Formation and Late Oligocene Daman Formation. During Early Oligocene, the basin experienced the maximum subsidence that led to the deposition of Mahuva Formation. In the proximal part, the Mahuva Formation comprises of thick under-compacted claystone relating to the prograding delta from the northeast which has also resulted in the formation being overpressure in many instances. However, towards the distal part, the clastic influence diminishes and carbonates become more predominant. The close of Early Oligocene is marked by a minor period of non-deposition in Tapti-Daman area. During Late Oligocene, Tapti-Daman Block witnessed reduced subsidence resulting in a regressive coastline. During this period, a fluvial system with distributary channels, coastal bars, tidal deltas and other transitional environments encased in marginal silty and carbonaceous shale known as Daman Formation formed in the proximal part. The depositional regime changes from clastic to carbonate in the distal part particularly towards the southern and western part of the B9 area. Sea-level changes due to regional-tectonic and climatic parameters during Oligocene formed a major controlling factor for sediment deposition. The westerly tilt of the basin created additional accommodation space. The sediment supply during this period in the proximal part was controlled by increasing input of clastic material from northeast. However, in the distal part, a thick carbonate succession was formed due to low sediment supply, adequate rate of subsidence and conducive climatic conditions. As a result, the carbonate factory in the subsiding distal part of the basin was influenced by this interplay which further complicates the understanding of the distribution of reservoir facies. However, in the proximal part, the carbonate build-up was restricted by increasing input of silici-clastics and eventually led to the termination of the carbonate factory during Late Oligocene. The formation of a potential reservoir for hydrocarbon accumulation and seal was significantly influenced by this interplay, particularly along the clastic-carbonate transition areas in the distal part of the basin. Through this study, an attempt has been made to understand: 1. The basic litho-facies deposition, their disposition in time and space and the physical properties associated with these litho-facies that can aid in the prediction of reservoir distribution and in the refinement of geologic models. 2. Its implication on hydrocarbon accumulation through the integrated interpretation of seismic and well data and previous studies in the area. About 20 wells from proximal to distal set-up along B12 and B9 areas have been analysed to develop an understanding of the distribution of potential reservoir rocks and thickness variation. Well correlation is carried out and a number of geological sections were prepared to understand the interplay between the clastic and carbonate litho-facies and its implications on hydrocarbon accumulation. An attempt has been made to delineate the sand, shale and carbonate litho-facies through integrated interpretation of 3D seismic data. Key seismic attributes have been generated to understand the limit of clastic input and carbonate distribution patterns.