Depositional set-up, age and reservoir properties of the heavy oil bearing subsurface Infra-Cambrian-Cambrian sedimentary succession of the Marwar Supergroup, Bikaner–Nagaur Basin (Western Rajasthan), India

Detailed sedimentological and biostratigraphic studies on the Infra-Cambrian-Cambrian Marwar Supergroup in the key well BGW-A (depth-interval 481–1123 m) in Bikaner–Nagaur Basin have provided a fresh geological insight for depositional facies, reservoir properties and precise age of the different lithounits of this Infra-Cambrian-Cambrian sequence of the well. Jodhpur Formation (1123–1103 m), the lowermost lithounit of the sequence with heavy oil reservoir in this well and Lophosphaeridium-Leiosphaeridia rich acritarch assemblage of Early Ediacaran (635–590 Ma), mainly comprises reddish brown to ash-grey coarse to medium grained sandstones and grey shale. Lithological features and associated acritarch flora suggest a littoral to marginal marine environment for the Jodhpur Formation in a foreshore depositional set-up. The succeeding Bilara Formation in this well (1103–1086 m) mainly includes finely laminated bioturbated dolostones and thin black shales, and is marked by the Lophosphaeridium and Vandalosphaeridium- rich assemblage, indicating Late Ediacaran (550–541 Ma) age with inner neritic depositional environment in a supra-tidal set-up. The overlying depth interval of 1086–920 m is represented by the Hanseran Evaporite Formation and characterised by interbedded dolostone, anhydrite/halite and reddish-brown mottled claystone. This lithounit is marked by the Dictyotidium, Asteridium, Comasphaeridium and Annulum -dominated acritarch assemblages of Early Cambrian, indicative of an inner neritic depositional environment. The reddish-brown sandstone mainly represents the overlying Nagaur Formation (920–727 m) and intercalated siltstones/shales with poor microfossil contents. It is inferred to be of the upper Early Cambrian age. Upper Carbonate Formation (727–481 m), the youngest lithounit of the Marwar Supergroup, includes a thick succession of dolostone and dolomitic limestone. Lower (727–634 m) and middle (634–574 m) parts of this unit are marked by the Cristallinium and Cymatiosphaera dominated acritarch assemblage of Middle Cambrian, while the upper part (574–481 m) of this lithounit shows the notable presence of Late Cambrian-Tremadocian marker acritarch taxa, like Buedingiisphaeridium papillatum, Striatotheca rarirrugulata and Dorsennidium minutum , suggesting a similar age. Petrographic and SEM studies reveal that the sandstones in the Jodhpur Formation show moderate-to-good intergranular porosity. However, Bilara and Hanseran Evaporite formations showed tightness of the sandstone/siltstone reservoirs, which are cemented mainly by anhydrite, giving rise to destruction of the porosity. Nagaur Formation also has very poor porosity, while moderate intercrystalline porosity is observed in the Upper Carbonate Formation. Associated grey shales/siltstones of Jodhpur and Bilara formations in this well have moderate-to-rich sapropelic organic matters, with TAI varying from 2.50 to 2.60, and were in the generative oil window during Permian to Jurassic with local hydrocarbon migrations.