Influence of Pore-Water Chemistry and Burial Depth on Diagenetic Pathways of Lower Gondwana Sands, Pranhita-Godavari Basin, Andhra Pradesh

Petrographic examination of 106 sandstone samples belonging to Lower Gondwana Group of Bellampalli-Chinnur area, Pranhita-Godavari Basin were carried out to evaluate the effects of interstitial water chemistry and burial depth on detrital and authigenic mineralogy of these sandstones. Except Basal Talchir sandstones there was no significant alteration in the framework mineralogy. Minor variation in the Q F R ratio of these sandstones was brought about by variable relief-ratio of the watershed and changing climatic during the Permian. Overall sequence of the mineral authigenesis can he divided into two stages. The early stage was characterised by eogenetic formation of kaolinite and chlorite under the influence of climate-controlled groundwater chemistry. Chlorite was formed in times of relative aridity (as in Talchii Barren Measures and Lower Kamthi times) when cation concentration in groundwater was high. During the times of high precipitation (as during Barakar and Lower Kamthi times) pore water was dilute, resulting in the flushing out of the cations and consequent formation of kaolinite. The later stage of diagenetic sequence involves variation in Eh-pH condition of pore fluids as a function of thermocatalytic degradation of kerogen and depth of burial. This stage is characterised by mica authigenesis, precipitation of calcite and iron Oxide cement and dissolution of early formed calcite. Alteration of potash feldspar into hydromuscovite in the Barakar and Talchir strata shows Signatures of depth-controlled Process of illitization.