Abstract

The Jurassic sedimentary sequence of eastern Saudi Arabia contains one of the most impressive petroleum systems in the world. This petroleum system contains two excellent petroleum source rock intervals, the Hanifa and the Tuwaiq Mountain Formations; excellent carbonate reservoirs, the Arab Formation; and excellent seals, the overlying evaporites. With the presence of large, low relief closures and the almost juxtaposed and regionally extensive source to reservoir to seal relationships, this area has effectively trapped enormous volumes of oil and gas.

Jurassic source rocks belonging to the Hanifa and Tuwaiq Mountain Formations were evaluated for source quality (richness and potential). Results indicated that the best source rock is the Tuwaiq Mountain, which attains thicknesses exceeding 152 m and is extremely rich, with total organic carbon (TOC) averaging 3.5% and excellent pyrolysis yields (S₂ Rock-Eval yields) averaging 22.2 mg hydrocarbons/g rock. This source rock has extremely oil-prone hydrogen indices exceeding 600 mg hydrocarbons/g TOC. The Hanifa source rocks are thinner than the Tuwaiq Mountain and not as source rich. Thicknesses are generally less than 30 m and TOC average 2.6%. Hydrogen indices average <400, indicating mixed oil and gas proneness.

Oils from the Arab and Hanifa Formation reservoirs correlate well with source rock extracts from the Tuwaiq Mountain and Hanifa Formations. Correlations were based on biomarker and chromatographic fingerprints.

By using empirical and mathematical modeling results, the Tuwaiq Mountain and Hanifa source rocks attain peak oil generation and associated expulsion at 120°C (0.60-0.70% VRe). These source rocks have essentially generated and expelled most of their liquid products by 0.85-0.90% VRe. Based on established generation and expulsion criteria, maturity trends for the source rocks have been regionally mapped.