Provenance Control of Diagenesis in Clayey Sediments in the Northern Part of Western Continental Shelf of India

Recent marine sediments coupled with sea-bottom cores, parting shales and insoluble clay residues of carbonate reservoirs in northeastern continental shelf of the Arabian Sea were investigated for studies of clay mineral diagenesis to evaluate respective roles of source conditions and environment of deposition. The material was also compared with those from selected weathering profiles on basalts and/or diabases. The data indicate an uniform distribution of montmorillonite and expandable mixed-layers with little kaolinite over the continental shelf diluted by sporadic occurrences of degraded illite and chlorite off the Saurashtra coast and beyond in the north and northwest upto the international boundary. These clays were derived by weathering from basalts/diabases and so have not responded to diagenetic changes even at depth. Occurrence of illite and chlorite mixed with montmorillonite and expandable mixed-layers could have resulted from material brought in by the Indus river. The factors of source and inheritance seem to have been the governing criteria for structural failure of these clays against diagenetic processes. However, clay mineral diagenesis indicates that hydrocarbons discovered in this area are independent of the associated clay mineral characters with regard to their genesis and entrapment.