Abstract

Sedimentological analysis plays a vital role in hydrocarbon exploration and production. The present study focuses on the effects of diagenetic changes and clay mineralogy on the reservoir quality of Mandapeta Pay sands. A detailed investigation of 50 wells has been carried out. Sand-I and Sand-II have grain-coated illite and chlorite as the dominant clay matrix. Sand-III and Sand-IV consist essentially of illite, chlorite, and mixed-layer chlorite-illite with occasional kaolinite, smectite, and smectite-illite. The proportion of kaolinite and smectite decreases with increasing depth. The presence of authigenic illite severely reducing the permeability by blocking the pore throats, while kaolinite does not seriously affect the reservoir's permeability. The presence of smectite and smectite-illite tends to damage the reservoir owing to their swelling property. The diagenetic model suggests that latestage calcite cementation has severely deteriorated the porosity and permeability of Sand-I and Sand-II. Calcite cementation is more towards the southeastern part as compared to the western part, suggesting an increase of alkalinity towards the dip direction (Southeast). Diagenetic changes like dissolution of unstable grains in Sand-III and Sand-IV have created optimum secondary porosity, and significant microporosities are also retained within the pore filling illite, chlorite, and kaolinite matrix, which might account for its higher productivity.