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Diagenetic Controls from Reservoir Sands of Nannilam Formation, Ramnad Sub Basin, Cauvery Basin, India

Syamalendu Mahanti
KDMIPE, ONGC Ltd, Dehradun, India

Ramnad sub basin located in the southern most part of the Cauvery basin extends from the on land part to the offshore part of Gulf of Mannar in south- east and Palk bay in the north- east. The sands in Nannilam Formation (Santonian to Maastrichtian) in the eastern part of the sub basin have produced commercial gas from several reservoirs. Conventional cores have been studied to know the lithological characters, sedimentation processes and diagenetic changes in sandstones. The sands are medium to coarse grained, moderately to occasionally well sorted, massive to thickly bedded with thin layers of gray and red shale. Clasts of claystone and quartz along with calcareous, sideritic and ferruginous concretions are observed within massive sandstone. Petrographic and mineralogical investigations through x-ray diffractometer and scanning electron microscope were carried out to identify the facies types and the diagenetic changes. The sandstones of Nannilam Formation include arkosic arenite, arkosic wacke and calcareous arenite. Modal analysis of sandstone shows quartz, feldspar, rock fragment (granite, claystone) and mica as the detrital components and chlorite and kaolinite clay as matrix. The sandstones have good porosity and permeability but at times are affected by various diagenetic changes, while some led to porosity reduction others led to its partial generation. Feldspar is dominated by potash feldspar showing various stages of alteration ranging from partly altered to highly altered cloudy grains to sericitised and vacuolised grains. Detrital grains are partially coated with authigenic chlorite. The cements represented mainly by blocky calcite followed by authigenic kaolinite, silica, chert, pyrite and hematite led to partial to significant occlusion of porosity. The extensive alterations of K-feldspars have given rise to minor secondary porosity.

Presentation GEO India Expo XXI, Noida, New Delhi, India 2008©AAPG Search and Discovery