The Deccan Traps large igneous province (LIP) comprises one of the largest continental flood basalt provinces on Earth with the main phase of volcanism spanning the Cretaceous-Palaeogene boundary. The oldest volcanism of the province is encountered in the northwest of modern-day India where Deccan stratigraphy is often buried beneath thick Cenozoic sedimentary sequences. The Raageshwari Deep Gas (RDG) Field, located onshore in the central Barmer Basin, NW India, produces gas from the early Deccan Raageshwari Volcanics which are subdivided into two members, the Agni Member and the overlying Prithvi Member. The RDG comprises a globally important example of a producing volcanic reservoir whilst also offering unique insights into the early volcanism of the Deccan with the aid of extensive high quality sub-surface data. Within this study, the volcanic facies of the RDG sequences are investigated from five cored intervals (total 160 m). Core-based facies determinations are compared with geochemical analyses, petrophysical analyses of the cores (density, porosity and permeability), and wireline data including micro-resistivity borehole images (FMI) and Nuclear Magnetic Resonance (NMR) data. A wireline based volcanic lithofacies scheme is developed and applied to the uncored parts of the sequence which in turn is compared to 3D seismic data. Results of the study reveal the Agni Member to comprise a compositionally bimodal (basalt through to trachyte), dominantly alkaline series with mixed volcanic facies including spectacular felsic ignimbrites, basic-intermediate simple lava flows, volcaniclastic units and newly identified shallow intrusions. The Prithvi Member in contrast is dominated by tholeiitic basalt compositions with less common basic-intermediate alkaline compositions and comprises a sequence dominated by classic tabular lava flow facies inter-digitated with boles, volcaniclastic units, rare compound braided lava facies and evolved tuffaceous ash layers. In one interval of the Prithvi Member, evidence for agglutinated spatter is recorded inferring potential proximity to a palaeo-eruption site within the area. Comparison between core data and volcanic facies reveals a first order control of volcanic facies on reservoir properties highlighting the importance of volcanic facies appraisal in the development of volcanic reservoirs.
Bibliographical noteFunding Information:
Cairn Oil and Gas along with Raageshwari Deep Gas Field joint venture partners ONGC are thanked for permission to publish this study. Anne Jay and Raymond Duraiswami are thanked for detailed reviews which substantially improved the paper and Craig Magee is thanked for editorial handling. The authors would also like to thank Sudeep Bhat, Amlan Das, Steve Goodlad of Cairn team for their support during the study. DAJ and SP are partly funded by the Research Council of Norway Centre of Excellence funding scheme (project 223272).
Data Availability StatementData that support the findings of this study are available in the online appendix.
- Barmer Basin
- Deccan Traps
- volcanic facies
- volcanic reservoir