Hydrocarbon Source and Reservoir Rock Potential of the Paleocene Hangu Formation in the Himalayan Foreland Basin, North West Pakistan: Insight from Geochemical and Diagenetic Study

Abstract

The detail study of the Paleocene Hangu Formation consisting of sandstone, carbonaceous shale, coal, and laterite has been carried out for its source and reservoir rock potential in the Salt Range, Surghar Range, and Attock-Cherat Ranges. The TOC values of the shales range from 0.33-11.19 (2.97 wt. %) and are characterized as good to very good quality source rock except the samples from Attock-Cherat Ranges. Similarly, the free (S1) and cracked hydrocarbons (S2) amount are very small suggesting Hangu Formation as a poor source rock for free and cracked hydrocarbons except the samples from the Lumshiwal Nala. The generative potential, type of kerogen and thermal maturity calculated on the basis of TOC, S1, S2, HI, PI and Tmax all characterized Hangu Formation as fair to excellent gas or oil source, type III and mixed type III/II kerogen and immature source rock. The Hangu Formation sandstone is brownish to yellowish brown, fine to coarse grained, medium to thick bedded and massive in places. The major diagenetic changes observed in a sandstone of the Hangu Formation are; compaction, cementation, replacement and grain fracturing. The effect of mechanical compaction is more evident than that of chemical compaction. Grain contact ranges from pointed to long through sutured. The type of cement present includes silica-cement, calcite-cement, dolomite-cement, and iron-oxide cement. Silica-cement is present as both overgrowth and pore-filling cement. Clay rim is present around few grains. The process of early calcite cementation, mechanical compaction, silica, and iron oxide cementation destroys the reservoir properties of the Hangu Formation sandstone. There is no visible porosity observed except the dissolution of few grains at their margins. However, during the process of uplifting such porosity usually filled by the iron- oxide cementation. Hence, Hangu Formation is an immature source rock with a poor reservoir potential.