2D Petroleum System Modeling of the Early Cretaceous Sembar Formation in the Southern Indus Basin, Pakistan: Insights for Petroleum Generation, Migration, and Accumulation Histories

Abstract

In Pakistan's southern Indus Basin, the Early Cretaceous Sembar Shale Formation is a principal source rock, but the petroleum system with its essential elements and processes has not been defined yet. This study presents the first 2D petroleum system modeling of the Sembar source rock system, and its essential elements were studied, in order to determine the timing of hydrocarbon generation, migration, and accumulation using Schlumberger PetroMod software. The 2D model integrates bulk geochemical data and a depth-converted 2D seismic line across three exploration wells. The organic-rich shale of the Sembar Formation has been identified as the most important petroleum-source rock, with total organic content (TOC) between 0.76 and 3.07 wt.% and hydrogen index (HI) values in the range of 47–463 mg HC/g TOC. The Sembar shale source rock reached the main oil window (0.7–1.1%VRo) during the Late Cretaceous to early Paleocene (90–60 Ma), and the wet-gas window thereafter occurred with VRo of up to 1.23%. 2D modeling results also indicate that the transformation ratio (TR) of kerogen reached 80%–100% in the depocenter, and the oil and gas migration and accumulation began at ∼90 Ma, with lateral migration through microfractures into Sembar sandstone and vertical migration along faults upward to the Lower Goru sandstone reservoirs. The Upper Goru limestone and shale units provide effective seal capacity. These findings reduce exploration risk and highlight the remaining potential in deeper stratigraphic levels of the Southern Indus Basin.