Controls on the Regional Distribution of Khuff Gas Fields, and a Conceptual Model for Lateral Hydrocarbon Migration into the Khuff Reservoirs

Afifi, Abdulkader M.; Bhullar, Abid G.; Rwiai, Mohammad A.; Jenden, Peter D.
Saudi Aramco, Dhahran, Saudi Arabia.

The Permo-Triassic Khuff Formation contains some of the world's largest conventional gas accumulations, which are clustered in the Arabian Gulf surrounding the giant North Field/South Pars megadome. Most of the Khuff gas fields exist in a sweet spot in the central Arabian Gulf region where two areas overlap: (1) a large Carboniferous basin located under the central and southern Arabian Gulf, where the Silurian source rocks were saved from erosion during the Hercynian orogeny, and (2) the Ediacaran Hormuz salt basins, where the growth of high-relief salt domes breached regional seals under the Khuff reservoirs through fractures, augmented by reduction of capillary entry pressure due to expanding gas columns.

Basin modeling indicates that generation of wet gas from the Silurian source rocks started in the central Arabian Gulf kitchen (NW of Qatar) during the Late Cretaceous, and continued through the Tertiary until deeper burial under the Zagros foredeep pushed the source rocks into the dry gas window. Gas migrated vertically to the Khuff above the rising salt domes in the central Arabian Gulf, but due to low net reservoir thickness, it then migrated laterally by filling and spilling from the smaller salt domes into progressively higher structures such as basement-cored anticlines (e.g., Ghawar), the Zagros anticlines, and the North Field megadome. For example, in the Ghawar field, lateral charge from the north is supported by the lateral variation in gas maturities indicated by carbon isotopic compositions.

This model is significant to exploration for additional Khuff hydrocarbon accumulations, particularly in stratigraphic or diagenetic traps. For example, oil/wet gas accumulations are predicted to occur more distally from the north-central Gulf kitchen, which is consistent with observations to date. The strong enrichment of non-hydrocarbon gases in some fields, including nitrogen, is attributed to thermochemical sulfate reduction following entrapment.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012