INTEGRATED SEQUENCE STRATIGRAPHIC SYNTHESIS, DEPOSITIONAL HISTORY AND REGIONAL DIAGENETIC HETEROGENEITIES OF COMPLEX RESERVOIR QUALITY SANDS OF LOWER GORU INTERVALS, MIDDLE INDUS BASIN, PAKISTAN

The Lower Goru sands are the most prolific hydrocarbon producing reservoirs of Indus Basin, Pakistan. Seismic resolution cannot be used exclusively to distinguish the numerous regressive and transgressive parasequence cycles and distribution of complex reservoir quality sands of Lower Goru. The integrated high resolution sequence stratigraphic framework in conjunction with cores, log data and petrography were used for hybrid investigation of sequence stratigraphy, facies analysis, depositional models and regional diagenetic controls. In ‘B’ sand interval of Lower Goru four sequences have been identified. Reservoir sandstone lies within unit 40 transgressive system tract (TST), representing tidally influenced estuary deposits. Lowstand system tract (LST) characterized by non-barred, linear siliciclastic shorelines and transgressive system tract (TST) by transgressive, shallow marine shelf sediments. The diagenetic sequence includes calcite, siderite and chlorite grain coating/pore filling as main authigenic clay. Facies type consist of tidal inlet, lower to middle shoreface, offshore transition zone, transgressive lag, bioturbated shelf and condensed sediment fluctuating energy levels facilitate the generation of authigenic clays. The lower most unit of ‘E’ sand (E6-E3) comprise prograding high sand supply shoreline system, passing up from offshore muds with the storm sands (E6) into tidal inlet sands (E3) along with subtidal estuarine channel sands and lags development in ‘E2’ unit. Reservoir quality of ‘E’ sand units are poor to good, reduced by combination of quartz and authigenic chlorite. In ‘C’ sand interval lower most part comprises of middle shoreface, shelfal to offshore deposits and middle part represents proximal delta facies and very heterogenous sand dominated delta plains. Upper most part of ‘C’ interval represents shelfal to offshore environment together with some part corresponding to transgressive condensed section. Glauconitic grains, chomositic grains, bioturbation, siderite cement, pore lining iron chlorite rims and late iron calcite cements are considered important diagenetic overprints. Rio Grande river delta in Brazil, Willapa Bay in U.S, Saloum delta in Senegal and Dutch coastal islands in Netherland are some of the modern analogues for Lower Goru deposition.