A Petroleum System in Block 18, Marib-Jawf Basin, Yemen

M. Sturgess, R. J. Wharton, I. D. Maycock

The Meem-Lam-Alif-Safer(!) petroleum system within Block 18, Marib-Jawf Basin, Republic of Yemen involves Upper Jurassic to lowermost Cretaceous clastics and evaporites. Early to Late Cretaceous halokinesis and rejuvenation of basement-related faults control the trapping elements, while a mid-Tertiary heating event results in the peak expulsion of both oil and gas.

The Meem and Lam Formations are comprised of shales, organic-rich laminated claystones associated with turbiditic calcareous sandstones, and minor deep-water carbonates. These were deposited in a developing rift, associated with the break up of Gondwana. Total organic carbon content typically ranges from 1.27% to 5%+, within source beds commonly less than 25 cm thick. Characterization of reservoired hydrocarbons and source organic matter shows generation to be from mixed Type II/III kerogen with expulsion at moderately high maturities.

The Alif Formation sandstone reservoir was deposited in environments ranging from alluvial braid-plain to deltaic, and typically has excellent porosities (16-21%) and permeabilities (>500md). The overlying Safer evaporites form both lateral and vertical seals.

Trapping is a combination of subtle normal fault closures and an unusual gravity induced separation of the reservoir unit. The voids created between blocks of the reservoir were filled by salt from the overlying Safer evaporites.

Due to increased subsidence and sedimentation rates generation of hydrocarbons began in the Early Cretaceous (~120 mya) and expulsion in the late Cretaceous (~70 mya). Peak expulsion occurred 30 mya as a result of higher regional heat flow associated with Gulf of Aden and Red Sea rifting.

Migration of hydrocarbons into the Alif Formation from the underlying Lam and Meem source units was facilitated by faults and microfractures.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995