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ABSTRACT

The Cauvery basin is a pericratonic rift basin along the east coast of India. A thick succession of sedimentary rocks of Early Cretaceous to Holocene age are known in the basin. The Cretaceous sedimentary sequences are the dominant habitat for petroleum in the basin.

The relationships among sea level changes, anoxic conditions, organic matter richness, and petroleum source rock potential have been investigated, especially for the Cretaceous sediments of the basin. Foraminiferal data have been used to evaluate sea level changes. Anoxic conditions have been assessed based on (1) localized occurrence of agglutinated foraminifera, (2) presence of pyritized tests, and (3) absence of benthonic foraminifera and presence of planktonic foraminifera or very high planktonic-benthonic ratios. Organic contents (TOC), hydrogen index (HI), and oxygen index (OI) are utilized to evaluate source potential. The results of thermal maturation studies based on Tmax and vitrinite reflectance (Ro) are included to qualify “immature” (Tmax less than 430°C and Ro less than 0.5%) and “mature” (Tmax of 430–465°C and Ro of 0.5 to 1.3%) source rock sequences.

Five major cycles of sea level changes are described for the Cauvery basin. In each cycle usually two or more microcycles of sea level changes are observed. Anoxic conditions are found to be a characteristic usually of sediments deposited in a paleobathymetric regime of >80 m. The observed anoxic conditions do not coincide necessarily with the Global Anoxic Events of Jenkyns (1980) at any of the studied locales in the basin.

Organic matter enrichment and petroleum source rock development are observed in each transgressive event. The sediments associated with basal transgressions, having no distinct foraminiferal evidence of anoxic conditions, show relatively better organic matter enrichment compared with the post-basal transgressive phases having foraminiferal evidence of anoxic conditions. Peaking of organic matter enrichment often is observed to be coincident with the peaking of the transgression. At any given locale in the basin, the hydrogen richness of organic matter, as revealed by hydrogen indices, is generally higher (HI=1.5 to 2 times higher) for the sediments of the very first transgressive event from pre-Albian to Santonian ages.

Relatively better organic matter richness coupled with hydrogen enrichment in the early transgressive events is due to higher organic matter flux in the paleobathymetric regimes of 50 to 80 m, as well as better preservation of organic matter. The decrease in hydrogen richness in terms of hydrogen indices in most of the younger transgressive events associated with anoxic conditions at each studied locale is most likely due to lesser organic matter flux in the paleobathymetric regime of >80 m, preburial alteration, and early diagenetic transformations.

The anoxic conditions and organic matter enrichment in any single transgressive event within any major chronostratigraphic sequence are not observed to be synchronous in different structural locales within subdepressions. Variations in paleobathymetric regimes are considered the likely factors to have controlled the level of organic enrichment. The distribution and volume of shallow marine transgressive pre-Albian-Santonian sediments may be of use in prioritizing generative depocenters in the basin.

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