Abstract
•Risha gas and light oil is mainly sourced from the Lower Silurian source rocks.•Cross biomarker correlations and Rock-Eval are used for maturity assessment.•Burial and thermal histories are simulated by Schlumberger's PetroMod (1D) model.•Silurian and Ordovician shales contain Type II kerogen from biomarker analysis.•Middle-Upper Ordovician shales are overmature with negligible residual hydrocarbons.
Detailed geochemical analyses of the Lower Silurian-Cretaceous shales in the Risha Field were carried out to determine their source input (marine or terrigenous), paleoenvironment, maturity, and hydrocarbon generation potential for their contributions to the gas field. The Lower Silurian shales are believed to be effective source rocks for oil within the Paleozoic petroleum system in the Risha field. Furthermore, the Upper Cretaceous shales have high hydrocarbon potentials and are considered as a promising source for oil and gas. The Upper Ordovician, Lower Silurian and Upper Cretaceous shales in the study area were deposited in a marine anoxic environment, resulting in the accumulation of highly liptinitic organic matters derived from algae and bacteria (Type II kerogen). The maturity of the Lower Silurian source rocks increases from the oil window to the gas window. Ordovician shales have reached the over maturity stage. Current hydrogen index (HI) values for the Silurian organic matter are relatively low after significant hydrocarbon production and expulsion. Two oil types are found from the standard petroleum geochemical analyses of the studied shales: naphthenic oil from immature Cretaceous black shales and high maturity paraffinic oils from Lower Silurian and Ordovician layers. The thermal history model of the Lower Paleozoic source rocks reveals that the Lower Silurian shales reached the dry gas generation stage 280–250 million years ago during Early Triassic, suggesting the large volume of gas accumulated in the Risha field mainly from the Silurian source rocks.