Controls on near-surface and burial diagenesis of a syn-rift siliciclastic rock succession: A study from Permian Barren Measures Formation, Southern India

Abstract

Distinct processes during the syn-depositional sedimentation and the post-depositional diagenesis determine the fabric of a clastic sedimentary rock. Temporal and spatial variations in any of these processes cause development of specialized characters in the sedimentary rocks. The Permian Barren Measures Formation in the Pranhita-Godavari Valley in southern peninsular India, is characterized by signatures of early to late diagenetic cementations within coarse-grained sandstones and mudstones. An integrated petrographic-geochemical study is carried out to understand the post-depositional near-surface and burial processes. Facies architecture manifests sedimentation in fluvial (channel, bar and overbank) to tide-influenced fluvio-tidal mixed depositional condition. Post-depositional near-surface early diagenesis changes include – (i) partial dissolution of detrital grains, (ii) biotite kaolinization, (iii) localized extensive ferruginous cementation, (iii) calcite cementation, and (iv) development of rhizocretions observed at different stratigraphic levels. These point to the role of meteoric water during the pre-burial stage that changed the pore water chemistry within the fluvial channel and bar sediments, along with onset of pedogenic (calcretization) processes in the vadose zone within the fine-grained overbank deposits. Marine (tidal) water influx in fluvial regime under a semi-arid climate modified the pore water chemistry. Shallow burial diagenetic changes, under elevated temperature and pressure include – (i) extensive calcite cementation in intergranular pore spaces and fractures, and (ii) late phase partial dissolution producing secondary porosity. A predictive model is derived to decipher the role of post-depositional changes in developing potential reservoir heterogeneity in a fluvio-tidal mixed depositional system.