Evaluating the paleo-depositional environment of productive reservoir sand of Lower Goru Formation: an integrated stratigraphic and diagenetic study

The early Cretaceous sand intervals of Lower Goru Formation (LGF) are significant reservoir for hydrocarbons and are situated in study area of Sawan gas field, Middle Indus Basin, Pakistan. This integrated study focuses on the development of stratigraphic traps and reservoir geometries in paleo-depositional environment through sequence stratigraphic and diagenetic analysis of productive Lower Goru sandstone. The datasets of well-logs from five wells (Sawan-1, Sawan-2, Sawan-3, Judge-1, and Nara-1), core samples from two wells (Sawan-1 and Sawan-2) and 2D seismic section (line: PSM96-133) are used in this study. The key system tracts including lowstand, transgressive, and highstand system tracts are identified with several progradational and retrogradational parasequences. The bounding stratigraphic surfaces are identified as sets of onlaps and downlaps unveiling different episodes of rise and fall in sea level with different sedimentation conditions. A depositional model is generated to map the paleo-environment of the reservoir by integrating the results of stratigraphic analysis. The log trends and seismic stratigraphic analysis showed the thickening trend of productive C-sand interval towards the eastern direction followed by gradual thinning in the middle part and shale-out trend towards the south and west part of the area. This indicates that the paleo-depositional direction was from east to west in shallow marine settings. The petrography and diagenesis results reveal that reservoir sands are iron chlorite-cemented sublitharenites to lithic arenites. The porosity and permeability of the reservoir is preserved due to a high amount of early diagenetic pore-lining iron chlorite, which commonly coats the surface of the quartz grains. The facies depositional environment is a wave dominated lowstand shelf edge delta system in which proximal delta front sands constitute the best reservoir. The outcomes of this research hold significant promise for advancing the comprehension of diagenetic processes and their influence on reservoir properties within the Lower Goru sandstone and its surrounding regions.