Geochemical Evaluation of Campanian Sirte Shale Source Rock, Arshad Area, Sirt Basin, Libya

Abstract

The present geochemical study deals with evaluation of Sirte Shale (Campanian) source rock in Arshad area which is located in the central Sirt Basin in north central Libya. Sirte Formation is composed mainly of shale, sandy shale, being partially sandy and calcareous facies indicating presence of possible source rocks. It contains enough amount of organic matter (>0.5 wt. %) to generate and expel hydrocarbons under suitable thermal maturity level in the area. The recorded kerogens belong to types III and IV. The pyrolysis results indicate deficient of hydrogen content and poor potential source rocks in all samples. The HI values of the analyzed samples prove predominance of type III (gas-prone) and type IV (non-hydrocarbon potential) kerogens. In general, pyrolysis results and derived parameters indicate poor generating potential for the Sirte Formation (Campanian) in the area and mature kerogen in the oil-window zone. The thermal burial history curves of selected wells in Arshad area indicate that Sirte Shale had been buried in the top peak oil generation or oil window (Late maturity, Ro: 0.9-1.35%) since 4 to 13.5 M. Y. ago at depth range 9100'-9600'. Time of hydrocarbon expulsion has started in the area since 15 M. Y. nearly during the Middle Miocene time (Serravallian age). In Sirt Basin, oil has migrated from the Campanian Sirte Shale source rock which has reached the oil-window at western flank of the South Ajdabiya Trough to the Mesozoic and Paleozoic reservoirs in Zaltan Platform and to the Upper Cretaceous Arshad sandstone reservoir in Arshad oil field as well. The migrated hydrocarbons most likely are entrapped in the structural closures of the different structural traps (folds and fault-blocks) in Zaltan Platform. Likewise, westward up-dip direction of gas migration took place from the Campanian Sirte Shale source rock as buried in the gas-window in the deep Wadayet (NW Ajdabiya Trough) to the Mesozoic and