Sedimentology and Sequence Stratigraphy of Hydrocarbon-Bearing Mandapeta Pays: a Braided Fluvial Reservoir, Krishna-Godavari Basin

Investigation of nearly 200 m of conventional cores from Mandapeta Formation recovered from 14 wells in Mandapeta field reveals that the sediments are sandy braided fluvial deposits laid on incised valley floors. Sequence stratigraphic concepts are applied to analyze this fluvial depositional setting. Sedimentary features and facies patterns indicate mixed flow regime of both bed load and suspension, giving rise to three genetic units viz., (a) channel lag, (b) in-channel, and (c) overbank accretions, These are subdivided into six depositional facies having a definite relationship with varying changes of fall or rise in stratigraphic base level. Hydrocarbon-bearing sands fall within the in-channel facies. Texturally immature sandstones are dominantly quartz wackes, feldspathic wackes with subordinate feldsarenitel lithic wackes. Garnet is conspicuous in these sands indicating a metamorphic provenance. Drill core, electron scanning and porosimeter data indicate good primary intergranular porosity and secondary porosity with an average of 15 per cent. Extremely low permeabilities caused by excess matrix comprising boxwork type smectite, interlayers of illitesmectite, filamentous illite, chlorite and early quartz cement collectively cause them to become low permeability reservoirs. On the combined strength of stratal relations, lack of fauna and bioturbation, grain surface textures, statistical parameters, stacked fining up electrolog motifs and ferruginization, a distal braided river model of Donjek type is envisaged for these sands.

The critical constraints in the exploration of Mandapeta pays is their reservoir heterogeneity due to extreme grain size variation and vertico-lateral discontinuity caused by high matrix acting as permeability barrier within them. These sandstone reservoirs are essentially controlled by sedimentary process with excess diagenetic overprint along with structural complexities. Multiple gravity faults originating from deeper source facies (Kommugudem Formation) have acted as vertical conduits for hydrocarbon entrapment in the Mandapeta sands. Borehole invasion is mainly ascribed to clays like smectite having a high degree of swelling. A synergic approach combining geologic and reservoir engineering would help augment better production from these low permeability reservoirs. The area to the west and southwest of Mandapeta may have better prospectivity as oil/gas reservoirs.