Diagenetic Imprints and Its Role in Reservoir Development in Upper Cretaceous Nannilam Sands of Ramnad Sub Basin, Cauvery Basin, India
Cauvery Basin located in the south eastern part of India formed due to fragmentation of East Gondwanaland in Late Jurassic. It hosts thick Cretaceous with overlying Tertiary sediments. A series of horst and grabens constitute the basin tectonics and clastic sediments as basin fill. Ramnad sub basin, the southern most depression has thick Cretaceous sediment. Nannilam Formation (Santonian-Maastrichtian) occurring as thick sandstone interbedded with thin shale is the main reservoir. Present study is to understand the diagenetic changes and its effect on reservoirs. Cores were studied to understand the sedimentary fabric, sedimentary structures and diagenetic imprints. Sedimentological studies including petrography, clay typing and reservoir petrographic analysis of the sand spread over the sub basin were carried out.
The sandstones of Nannilam Formation are dirty white, medium to coarse grained, at places fine grained with normal and inverse grading. Sedimentary structures include parallel laminations, ripples and bioturbations. Clasts of different sizes and composition are random and at places in preferred orientation within sandstone. The sandstones are friable to highly indurated at places. Patches of calcite, glauconite and pyrite nodules are abundant. Horizontal burrows are frequent in the fine sand. The sedimentary structures suggest deposition of the sands in deep water conditions due to debris flow, bottom current reworking and slump processes. Petrographically the sandstones are poor to moderately sorted arkosic to sub arkosic arenite, arkosic calc arenite and minor arkosic wacke. The framework minerals include monocrystalline quartz with subordinate poly crystalline grains. Feldspar content is high and constitute mainly of K-feldspar with sub ordinate plagioclase and perthite. Feldspar grains are generally altered and show alteration varying from partly altered to highly altered grains. The matrix includes chlorite and subordinate kaolinite and seldom exceeds 15%. The cements comprise dominantly calcite followed by quartz overgrowth and authigenic kaolinite. Minor chlorite, pyrite, hematite and glauconitic cements are present. Calcite occurs as blocky and poikilotopic cement. Detrital grains are coated with thin rim of authigenic chlorite at places. Calcite cementation which formed in two stages highly affected the primary porosity at several places. Extensive feldspar alteration has how ever created minor secondary porosity.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009