Title : An organic investigation of source rocks and oils of the two upper cretaceous formations of the sirt basin, Libya
Abstract:
The source rocks and crude oils from two Upper Cretaceous Formations, the Sirte Shale, and Rachmat Formations of the Sirt Basin, have been investigated and characterized by a choice of geochemical techniques. The major aims of the study were to characterize the source rocks, thermal maturity, and depositional environments of the basin and attempt oil-source rock correlations and oil-oil correlations to improve our understanding of the origin of oil and gas in the basin. The dark gray and olive black shales of the Sirte Shale and Rachmat Formations that are deposited in a marine environment with varying water depths and are dominated by aquatic organic matter with minor contributions of plant materials. For both formations, the shales are rich in organic matter and of marginal, moderate to late middle maturity. The Sirte Shale source rock deposited in marine environments and the organic richness of this formation is higher than that of the Rachmat Formation. The Sirte Shale Formation also has a lower maturity level and fewer plant input than the Rachmat Formation and would therefore be expected to produce a high contribution to the oils found in the basin and is proposed to be more oil-prone than gas prone, while the Rachmat Formation has slightly more plant input could make a mixture of oil and gas prone. Oil-source and oil-oil correlation studies show that the crude oils discovered in both formations are produce from overlaying Sirte Shale Formation, while the Rachmat Formation has less produced mixtures of oil-gas and is the second source rocks in the basin. The petroleum system of the Sirt Basin is composed of impermeable sealing beds that well developed at the top and bottom of the Mesozoic and Cenozoic sequences, separating it from the Mesozoic and the lower Cenozoic strata to form an independent petroleum system. In this petroleum system, the source rocks are widely distribution of dark gray to olive black marine shales occurring in the Upper Cretaceous Campanian and Maastrichtian sequences, with a thickness of 150-700 m. The reservoir included Precambrian sandstone, Mesozoic sandstone, and Tertiary carbonate as carrier beds and reservoirs. The carbonate and clastic reservoirs have permeabilities of 0.2-900 md, 0.2-125 md, and porosities of 10-25%, and 5-30%, respectively. The regional cap rock is about 100-300 m thick of shale and evaporites in the Upper Cretaceous beds. Geochemical data showed that the migration was likely to have been over a short distance and that it occurred along both vertical and lateral pathways along the faults, in the Oligocene to Miocene, from the Sirte Shale and Rachmat source rocks to the reservoirs of the Upper Cretaceous-Tertiary sequences..
