Abstract

Silurian organic-rich (hot) shales were the source of large amounts of hydrocarbons in northern Gondwana, with supergiant and giant fields in the Middle East and North Africa. A study of these black shales has been conducted in Jordan, where they represent the source for the Risha gas field. Two organically enriched horizons occur in the Silurian in Jordan, termed the lower hot shale and the upper hot shale. Deposition of the transgressive lower hot shale occurred during the early Llandoverian and was restricted to the earliest Silurian paleodepressions. Three lower hot shale depocenters have been identified in Jordan; they are located in the western Risha, eastern Wadi Sirhan, and Jafr areas. The eastern Risha area was part of a larger scale paleohigh covering northeast Jordan, most of Syria and Iraq, and north-central Saudi Arabia (Qusaiba area). At least in Jordan, the high coincides with the depocenter of the latest Ordovician glaciation. Sedimentation of the upper hot shale occurred around peak sea level during the late Llandoverian and/or early Wenlockian and was limited to the distal parts of the shelf (Risha area), in front of the prograding silty-sandy deltaic front.

Thermal maturity increases from immature in the Southern Desert outcrops to late or postmature in northern Jordan. Organic richness and pyrolysis data deteriorate significantly with increasing thermal maturity because of hydrocarbon generation. Prior to maturation, maximum organic richness is interpreted to have considerably exceeded 10% with good S2 (remaining generative potential) yields, as reflected in the values of the immature lower hot shale in the shallow borehole BG-14 in the Southern Desert outcrop area and exploration well Hunt JF-1 in the Jafr area.

Identification and isopach mapping of the lower hot shale is based on its characteristic uranium enrichment. The gamma-ray/total organic carbon correlation coefficient increases with increasing maturity, suggesting that uranium is less vulnerable to maturity effects than the organic matter itself. The gamma-ray log, therefore, can be used as a proxy for the prematuration organic richness, representing a suitable basis for cross-basin comparisons of original organic richness trends.

A two-phase generation of hydrocarbons is interpreted for both the Risha and Wadi Sirhan areas, with the first as a pre-Hercynian phase in the Carboniferous and the second as a less intense generation phase during the Mesozoic to early Tertiary. In contrast, gas generation of the Silurian-sourced giant fields in the central part of the Arabian Peninsula and Iran was restricted to the post-Hercynian. In Jordan, both structural and stratigraphic play concepts are envisaged, including fault, paleohigh, and channel plays. Key elements that need attention in future exploration of the Silurian plays in Jordan are Ordovician reservoir quality and timing of maturation. Poor permeabilities have been reported from wells in both the Risha and Wadi Sirhan areas.

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