Petroleum geochemistry of Lower Indus Basin, Pakistan: I. Geochemical interpretation and origin of crude oils

The study focused on the petroleum geochemistry of crude oils produced from Cretaceous reservoirs. Geochemical portrayal of crude oils has been carried out by means of diagnostic biomarker parameters like relative distribution of steranes (C27-C28-C29 ααα-20R steranes), C19 and C23 tricyclic terpanes (TT), C24 tetracyclic terpanes (TeT) and hopanes. These parameters suggest that the crude oils contain terrigenous organic matter (OM) mixed with small input of marine OM. The OM of the source rocks was deposited in oxic depositional environment. Maturity parameters, C32 22S/(22S+22R) homohopanes and sterane isomerization ratios [20S/(20S+20R), αββ/(αββ+ααα) for C29 steranes] indicate that these crude oil are produced from the source rocks at early mature stage to mature stage.