Petrography and diagenesis of Neoproterozoic Bhander Sandstones, Vindhyan Supergroup, Southeastern Rajasthan, India: Implications for provenance and reservoir characteristics

The Neoproterozoic Bhander Group, the youngest and most widely distributed group of Upper Vindhyans, consists of about 1000 m thick succession of sandstone, shale, and limestone. Petrographic investigations reveal that the Bhander Sandstones are mineralogically mature and classified as quartzarenite and sublitharenite type, which is composed of varieties of quartz with scarcity of feldspar, lithic fragments, micas, and heavy minerals. The average framework composition of the Lower Bhander Sandstone is Qt(98.68)F(0.10)L(1.22,) and Upper Bhander Sandstone is Qt(95.92)F(0.12)L(3.96). The provenance, tectonic setting and diagenesis of Bhander Sandstones are evaluated using integrated petrographic studies. Analysis reveals detrital derivation from granitic and metamorphic Precambrian basement source rocks of a craton interior with a minor quartzose recycled sedimentary rock. A scarcity of feldspar and lithic fragments suggests intense chemical weathering in a warm and humid paleoclimate. The diagenetic processes recognised include compaction, cementation, and dissolution, affecting the sandstone porosity, thereby directly affecting reservoir quality. Mechanical compaction, cements, authigenic clays, and dissolution and modification of unstable clastic grains are the major diagenetic components identified based on the framework grain-cement relationships. Compaction was more effective than cementation in affecting primary porosity. Cementation decreased porosity and permeability drastically. Kaolinite and silica (quartz) overgrowth are found as pore-filling and lining cements. Kaolinite fills pore spaces, reducing the porosity and permeability. Secondary porosity developed as a result of partial to complete feldspar dissolution. The diagenetic signatures observed in the Bhander Sandstones are suggestive of deep burial conditions. The reservoir quality of the studied sandstones is degraded by authigenic clay minerals and cementations but enhanced by the alteration and dissolution of unstable grains.