Depositional facies and reservoir characteristics of the Early Cretaceous Lower Goru Formation, Lower Indus Basin Pakistan: Integration of petrographic and gamma-ray log analysis

Abstract

The sand intervals of the Early Cretaceous Lower Goru Formation are a conventional reservoir, generally distributed in the Middle and Lower Indus Basin of Pakistan. Lithostratigraphically formation is classified into two parts; the upper parts are predominantly composed of shale, siltstone, and thin layers of alternate shale and sandstone, while the lower parts are composed of sandstone with interlayering of shale and limestone. The sandstone of the Lower Goru Formation has been further divided into A, B, C, and D sand intervals based on reservoir quality. Detailed depositional facies and reservoir characteristics are essential for the evaluation of hydrocarbon exploration and development. This paper aims to evaluate the depositional environment and reservoir characterization of the siliciclastic reservoir of the Early Cretaceous Lower Goru Formation by integrating the gamma-ray log patterns and petrographic analysis and scanning electron microscopic (SEM) analysis. Petrographic characterization of the sand intervals and Gamma-ray log signatures were used for the interpretation of the depositional environment of the reservoir intervals. Petrographic analysis reveals that the sandstone of the Lower Goru Formation is fine-to medium-grained, well-sorted, arkose or feldspathic arenite. Primary intergranular macroporosity, secondary intragranular macropores, and Intercrystalline micropores were identified within the sandstone by the SEM analysis. The diagenetic analysis suggests that the sandstone possesses high porosity, low permeability, and has undergone significant alterations such as compaction, quartz cementation, feldspar dissolution, and clay minerals alteration. Five electrofacies are interpreted based on gamma-ray log patterns including (1) funnel shape (FA); (2) bell shape (FB); (3) cylindrical shape (FC); (4) bow shape (FD); and (5) serrated shape (FE) patterns. The interpreted facies results reveal shoreface environment for A-sand, Tidal flat for B-sand, mixed tidal flat for C sand, Tide dominated mixed for D-sand, and transgressive shelf for E−sand. The present study will be helpful for the assessment of the reservoir quality of the Early Cretaceous Lower Goru Formation for further exploration and development in the Indus Basin of Pakistan.