Petrography and Diagenesis of Neoproterozoic Bhander Sandstones of Bundi-Indergarh area, Vindhyan Supergroup, Southeastern Rajasthan, India: Implication for Provenance and Reservoir characteristics

Abstract

The Neoproterozoic Bhander Group, the youngest and most widely distributed group of Upper Vindhyans, consists of about 1000m 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 ultra-scarcity of feldspar, lithic fragments, micas, and heavy minerals. Average framework composition of the Lower Bhander Sandstone is Qt98.68F0.10L1.22 and Upper Bhander Sandstone is Qt95.92F0.12L3.96. Quartz is a more abundant mineral, as shown by X-ray Diffraction Analysis. Petrofacies reveal that the Bhander Sandstones were derived mainly from the source rocks of a craton interior setting with a minor quartzose recycled sedimentary source material. A scarcity of feldspar and rock fragments suggests intense chemical weathering in a warm and humid paleoclimate. 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. Kaolinite and silica (quartz) overgrowth are found as pore-filling and lining cements. Compaction was more effective than cementation in affecting primary porosity. Cementation decreased porosity and permeability drastically. Kaolinite fills pore spaces, reducing the sandstone's 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. The reservoir quality of the studied sandstones is degraded by authigenic clay minerals and cementations, but enhanced by alteration and dissolution of unstable grains.