Source Rock Evaluation and Depositional Environment of Early Cretaceous Sediments, Pranahita-Godavari Graben, East Coast of India

Samuel J. Bantu
Geology Laboratory, Oil and Natural Gas Corporation Limited, Rajahmundry, India

The study attempts to analyze the shallow borehole samples between 410-480 ft depth at southern flank of Pranhita-Godavari Graben near Gopalapuram village in West Godavari district, Andhra Pradesh for Source rock Palynology , depositional environment and age.

The carbonaceous clay section (below 470 ft) recorded diagnostic and diversified high altitude Gymnosperms taxa viz. Callialasporites dampieri, Callialasporites turbatus and low altitude Pteridophytic palynomorphs such as Cyathidites, Contignisporites and Cicatricosisporites etc. Rich assemblages of dinoflagellate cysts viz., Cribroperidinium and Oligosphaeridium species which reasonably define Early to Middle Cretaceous (Valanginian to Albian) age. The recovered palyno-assemblage occurs in close association with conifers of Late Jurassic to Early Cretaceous age. Similar palynomorph assemblage has been well documented in Gollapalli Formation of Mandapeta and Kaikalur areas of Krishna- Godavari Basin.

The abundance of dinoflagellate assemblage suggests strong marine affinity during sedimentation. Further, the Gymnosperm and Pteridophytic assemblage also suggests well developed lagoonal and lacustrinal environment surrounded by high relief lush green habitants which contributed to the sediments.

The carbonaceous sediments are represented by biodegraded and woody organic matter. TOC data on these sediments has yielded 1.94 -27.31 mg/g range. TAI values around 2.5 amply indicates source rocks are matured enough to generate gaseous hydrocarbon. The sediments appears to be subsurface analogue to the Mandapeta and Kaikalur areas of Krishna-Godavari Basin and points to favorable existence of source and reservoir facies.

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