A New Regional Co-Source Rock for Paleozoic Khuff Gas Condensates in the Arabian Gulf

The Silurian Qalibah Formation “hot shale” has been known as the principal source of hydrocarbons in the main Paleozoic reservoirs of the Arabian Gulf. These shales geochemically correlate with the generated and expelled hydrocarbons during the different generation stages of its maturation history. This is attributed to the “hot shale” thickness, organic content and presence in many Paleozoic wells. Recent regional evidence indicates that the thinly bedded algal mats deposited within the different layers of the Khuff Formation may contribute wet gas into the Paleozoic reservoirs in addition to that originating from the Qalibah “hot shale”. These algal mats are much thinner than the Qalibah source rock and have variable organic richness which makes them more difficult to map and detect on logs.

New regional data indicates that not all the Paleozoic hydrocarbons in the Arabian Gulf are generated from the “hot shale” alone. Instead, co-sourcing from the Khuff algal mats may explain the variable differences in thermal maturity levels between the source rocks and the generated hydrocarbons. Residual Khuff oil, with a maturity corresponding to the onset of oil generation, correlates positively with the maturity and character of the Khuff algal source extracts. Alternatively, Unayzah hydrocarbons have a much higher maturity level of about 1.2-2.0 vitrinite reflectance equivalent. This observation is also supported by biomarker and carbon-13 data. If the Silurian shales were the only source, it would be very difficult to explain why the Khuff liquids are less mature than the Unayzah liquids. The Khuff algal mats are proposed as a co-generating source of the liquid hydrocarbons found in the different Khuff reservoirs, whereas the Qalibah “hot shale” is the source of the Khuff dry gas and the Unayzah condensate. A mixing of both is very likely and can only be verified by geochemical correlations.

Khuff co-sourcing has important implications for hydrocarbon resource development, with the potential for more unconventional liquid hydrocarbons from the Khuff algal mats being present in addition to the liquids from the known Qalibah “hot shale.”

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California