ABSTRACT
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!
