This paper is a progress report on geochemical studies of the Pre-Khuff Paleozoic Sequence in Saudi Arabia, summarizing current laboratory results from both Saudi Aramco and U.S. Companies and their preliminary interpretations. Oil-oil and oil-source rock correlations indicate that Pre-Khuff Paleozoic oils and condensates are closely related to each other because they have a common source, the "hot shale" at the base of the Qusaiba shale Member of the Qalibah Formation. Oils and condensates in the Hawtah trend have been generated from highly mature Qusaiba shales. Since Paleozoic source rock maturities are no more than marginal in the producing trend, hydrocarbons must have migrated generally westward from the presently overmature Udaynan depocenter of the Rub-Al-Khali Embayment. Indeed, maturity measurements and computer-assisted maturity modeling clearly indicate that oil generation and updip migration commenced as long as about 160 million years ago (MA). Residual oil was cracked into condensate and gas when the depocenter passed into a higher temperature regime beyond the oil window, approximately 90 MA. Thus, the light hydrocarbons produced in the Hawtah trend can be considered the result of a "natural refinery" process at depths in excess of approximately 13,000 ft. (3962m) and temperatures around 350°F (177°C). Late stage non-hydrocarbon gases should be co-generated in the very overmature depocenter (over 2% Ro) at present. The likely source for such gases is pyrobitumen, the residue of cracked oil. Pyrobitumen is found in many deep wells where it has a pore-plugging effect. Advantageously all of the hydrocarbons are sulfur-poor because the initial kerogen was sulfur-poor. This difference between Saudi Arabian Paleozoic and Mesozoic oils is extremely important economically!
An integrated geochemical model was developed to reconstruct the history of expulsion, migration and entrapment of Paleozoic oil and gas in the main regional Permian Unayzah Sandstone Reservoir in Central Saudi Arabia. The model indicates that by the Late Jurassic, approximately 140 million years ago (Ma), the principal Paleozoic source rock, the Lower Silurian Qusaiba “hot” shale, was mature in the deepest hydrocarbon “kitchens”. Hydrocarbon expulsion started during the Aptian and Albian (late Early to early Middle Cretaceous, 100 to 120 Ma). Expulsion of oil and gas is linked to three geochemical events. Primary kerogen cracking led to a first episode of expulsion about 120 Ma. Secondary heavy component and oil cracking resulted in a second episode of expulsion at approximately 100 Ma. Between 20 to 10 Ma, later uplift, and the resulting pressure drop in the source rock, led to a third expulsion phase. The first two expulsion episodes were gradual, whereas the third was more rapid and related to uplift of the Arabian Arch, opening of the Red Sea and the Zagros Orogeny during the Miocene. Expulsion of oil nearly terminated after the Late Cretaceous, while gas continued to be expelled, though at a lower rate, in the Tertiary. Peak gas expulsion occurred post Early Eocene with significant gas generation from secondary cracking of oil retained in the source rock. Gas was sourced either directly from kerogen, or from secondary cracking of heavy absorbed components or non-migrated oils. The expulsion of gas coincides with oil expulsion for the first two episodes because the gas and oil formed as a single phase. As a result of Tertiary Uplift, gas separated from the oil and re-migrated in the final episode (20 to 10 Ma).
The major Paleozoic petroleum system of Saudi Arabia is qualitatively characterized by a proven Silurian (Qusaiba Member, Qalibah Formation) source rock, Devonian (Jauf Formation), Permian and Carboniferous (Khuff and Unayzah formations) reservoirs, a laterally extensive, regional Permian seal (basal Khuff clastics and Khuff evaporites), and four-way closed Hercynian structures. Hydrocarbons found in these systems include non-associated gas in Eastern Arabia and extra light oil in Central Arabia. A basin modeling approach was used to quantify important aspects of the petroleum system. Firstly, seventeen regional wells were selected to establish a reference tool for the three-dimensional (3-D) basin model using multiple one-dimensional (1-D) models. This was accomplished by studying core material from source rocks and other lithologies for thermal maturity and kerogen quality. The major emphasis was on the Silurian section, other Paleozoic intervals and to a lesser extent on the Mesozoic cover from which only few samples were studied. Although vitrinite macerals, solid bitumen, and other vitrinite-like particles were not abundant in most of the investigated samples, enough measured data established valid maturity-depth trends allowing for calibrated models of temperature history. Sensitivity analyses for maturity support the view that thermal boundary conditions and Hercynian uplift and erosion did not greatly influence the Paleozoic petroleum systems. Secondly, a 3-D basin model was constructed using major geologic horizon maps spanning the whole stratigraphic column. This model was used to gain insight into the general maturity distribution, acquire a better control of the model boundary conditions and investigate charge, drainage, migration and filling history of the main Paleozoic reservoirs. The 3-D hydrocarbon migration simulation results qualitatively account for the present gas accumulations in the Permian-Early Triassic Khuff and Carboniferous-Permian Unayzah reservoirs in the Ghawar area. This kind of study illustrates the importance of basin modeling when used with other geologic data to describe petroleum systems. It provides a predictive exploratory tool for efficiently modeling hydrocarbon distribution from known fields. Real earth models can only be described in 3-D as pressure variations and fluid movements in the subsurface are impossible to address in 1-D and 2-D domains.
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