Origin and Occurrence of Lower Cretaceous High Gamma-High Resistivity (Raghavapuram) Shale -a Key Stratigraphic Sequence for Hydrocarbon Exploration in Krishna-Godavari Basin, A. P.

Most of the commercial hydrocarbon occurrences in West Godavari Sub-basin and Mandapeta-Endamuru areas in Krishna-Godavari basin are associated with the High Gamma-High Resistivity Shale unit of Lower Cretaceous age which is bounded by unconformities at the top and the bottom. This unit is a potential source for liquid and gaseous hydrocarbons. Hitherto this unit has been called Raghavapuram Shale in Mandapeta-Endamuru area and has been considered as lower part of Raghavapuram Shale in West Godavari sub-basin. In the present study, the HG-HR Shale unit of West Godavari sub-basin is correlated with Raghavapuram (HG-HR) Shale of Mandapeta-Endamuru area. The study covers the entire area of the basin to the North of Matsyapuri-Palakollu fault and was carried out systematically by integrating all the available data from 103 wells and regional seismic profiles. This sequence has been mapped defining its distribution and limits.

High Gamma-High Resistivity (Raghavapuram) Shale sequence was the resultant of post-rift (Late Jurassic-Early Cretaceous) marine transgression into the NE plunging rift grabens (half grabens), forming a restricted shallow marine environment. Stagnant water conditions and slow rate of sedimentation in this shallow sea led to the deposition of micaceous silty shale rich in organic matter. The provenance (Archaean basement) being in close proximity resulted in the richness of radioactive elements (Th, K&U). These two reasons are attributed for the high resistivity and high gamma of the sequence respectively. This sequence represents the particular transition stage, in the geological history of the basin, when the palaeoslope was changing from 'towards northwest' to 'towards southeast'. In view of its distinctive characters and its significance in the geological history, the authors suggest to give this unit the deserved identity of a 'sequence' in the stratigraphy of Krishna-Godavari basin. By virtue of its log characters, this serves as a good 'marker sequence' and can resolve many a correlation problem.