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Falicitation of VP APG, Chennai Chapter, Dr. K. S Bhushan, ED Basin Manager, Cauvery Basin superannuating in September 2020 by APg Executive Body, Southern Chapter, Chennai
A field trip to Gulcheru / Vempalle / Pulivendula Formations of Cuddapah Basin was organized by APG Mumbai Regional Chapter from 01.03.2020 to 02.03.2020 under the expert guidance of Dr. D K Dasgupta, Ex ED-Head KDMIPE and Dr. Anand Kale, Consultant Geologist, Bharat PetroResources Ltd. Geoscientists from Western Offshore Basin, Western Onshore Basin, KG Basin, Cauvery Basin, Frontier Basin, KDMIPE, GEOPIC, E&D Directorate and also from BPRL attended the same.
As per the Petroliferous Basin classification, the Cuddapah Basin is a Type-III Basin. Globally, Type-III basins are also hydrocarbon producers. Recent discovery of hydrocarbon in the Vindhyan Basin would definitely give impetus for further exploration. Keeping same in view and predominant carbonate reservoir of Mumbai offshore, the limestone/dolomite deposits of Vempalle Formation was chosen for field training with a view to better visualization and understanding
The tour started with a visit to the Precambrian Crystallines with intrusions of Pegmatitic veins of three different episodes clearly demonstrated by the discordant relationships (Photo-1). The granites are highly weathered at places giving an impression of sandstone in megascopic view. The Proterozoic sediments of Cuddapah unconformably overlie the Precambrian Crystallines. The Papaghni Group comprising the Gulcheru and Vempalle Formations are the oldest Group within the Cuddapah Supergroup. The contact of the Gulcheru Formation (bottom most unit of Cuddapah) and the Precambrian Crystallines known as “Eparchaean Unconformity” is distinctly visible in the road / nallah cutting at different places (Photos at 2A & 2B).
The Vempalle Formation lies conformably over the Gulcheru Formation. The contact is gradual with alternations of clastics and carbonates, gradually passing into thick carbonate deposits consisting of dominantly stromatolitic and oolitic facies with minor mudstones (Photo-5). The carbonates are occasionally chertified. The contact of both Vempalle Formation and Gulcheru Formation is distinct in the river section (Photo-3). The Vempalle Formation is best developed in the Papaghni river section near the Vempalle village (Photo-4). The karst topography and the sink holes are also seen in the section. The Vempalle formation exposed in the Kadri road cut was also visited where development of large oncolites could be seen (Photo-7) indicating a very conducive environment for the growth of stromatolites. Frequently interbedded stromatolitic and oolitic facies indicate fluctuating water depths.
The Vempalle Formation of Cuddapah preserves a record of deposition of carbonates in a peritidal complex in a ramp setting. The Formation is approximately 1900m thick and numerous sections across it are available for study with excellent quality of exposure. The typical paraseqence in muddy peritidal settings starting from the subtidal laminated mudstones, grading into the intratidal settings and ending in the supratidal current affected complex is ideally observed at numerous locations. Variants of this normal cycle help decipher the spatial and temporal settings for the carbonate deposition and their relation with local sub-environments. Stacking patterns and their variations in the parasequences help decipher the short term and long term cyclicity and sequence tracts within the carbonates. In the present case demonstration of 4th and 5th order sequence cycles and their lateral correlation could be achieved due to the continuous exposed sections available. For any worker in carbonate sedimentology, relation between sunlight, nutrient supply and energy on carbonate sedimentation is ideally demonstrated in the Vempalle dolomites. Further the demonstration of catch-up and keep-up phases of carbonate growth in a base level cycle is also ideally demonstrated. The correlation of these examples created by non-carbonate precipitating cyano-bacteria with Phanerozoic organically fixed carbonates, is ideally demonstrated by the exposures of the Vempalle Formation. The structures (Folds and Faults of 3rd and 4th order) in Vempalle is very well visible in a canal cut at Parnapalli which depict the amount of stress the sediments have undergone (Photo-7 & 8).
The group as a whole utilized the trip as an extensive learning experience. It was observed that the complete sequence of a hypothetical “Petroleum System” i.e. the Source, Reservoir and Trap seems to co-exist in the area. Hydrocarbon discoveries in Proterozoic basins from different parts of the world have demonstrated that Proterozoic marine biomass had potential for generation, entrapment and accumulation of hydrocarbons. The possibility of hydrocarbons being generated could be high as the depositional conditions for a source, reservoir and seal are present in the exposed sections, while greater than 6,000m of the sediments remain unexposed.
The Vempalle sediments being quite older ones may be tight so far as porosity is concerned. But it was noticed that visual porosity in some of the weathered oolitic dolomite and stromatolitic facies is moderately high (Photo-9). The utility of Proterozoic Carbonates in developing an understanding of base level cycles and their controls on depositional sediment facies in comparison with Phanerozoic cycles is very well demonstrated.
The resource assessment carried out two years back estimates a figure of 5 MMTOE only which seems to be low as compared to the potential of such basins worldwide. Reconnaissance surface geochemical survey for adsorbed soil gas analysis depict linear trend indicating that all the gases are derived from the same source & suggests thermogenic origin for these gases.
The recent seismic data acquisition under National Seismic Program would help unravel the mystery. Considering the global experience of Proterozoic sediments and the experience gathered during the field trip, it is concluded that further exploration efforts is required in this basin for realising the hydrocarbon dream.
