--> --> A New Mesozoic Siliciclastic Fairway with Coaly Source Rock in the Rub’ Al Khali, Saudi Arabia

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A New Mesozoic Siliciclastic Fairway with Coaly Source Rock in the Rub’ Al Khali, Saudi Arabia

Abstract

A recent exploration well tested gas at significant rates from siliciclastic sands in the Middle Jurassic lower Dhruma Formation in the Rub’ al-Khali, opening a new gas play. The lower Dhruma Formation consists of a succession of fine grained, well-sorted quartz arenites interbedded with mature paleosols, carbonaceous shales and oolitic ironstones deposited in shallow marine, shoreface and fluvio-deltaic settings. These strata occur towards the top of a several hundred meter thick succession of siliciclastics, sandy carbonates and anhydrites that span a range of ages from Middle Triassic (Jilh Formation) to Middle Jurassic (Dhruma Formation). This Rub’ al-Khali sequence correlates to a succession of blocky bedded calcareous sandstones, shales, gypsiferous mudstones and tree-bearing paleosols that outcrop in extensive escarpments west of Riyadh. The outcrop includes laterally accreting sandwave complexes up to 30 meters thick and 100s of meters to kilometers (km) in width. Siliciclastic sedimentation was terminated across the basin by a major transgression recorded by the end-Bajocian Dhibi Limestone. The siliciclastics were probably sourced from the Hadramaut and Mukalia Arches to the south and west. The sediments infilled accommodation space that was created by crustal-scale arches and basins that are in the order of 100 km in wavelength and 100s of meters in amplitude and developed in the late Triassic and early Jurassic. These intrabasinal highs were finally overstepped by the Dhruma Formation. The highs contain deep, distributed systems of steep fault structures representing a gentle phase of transpression. This stratigraphy has previously been drilled many times but does not stand out in petrophysical analysis due to the thinly bedded and often tight nature of the sand units, and test data are rare. Although the tested zone is sub-seismic resolution the gross architecture of the interval is clearly imaged and shows clinoformal geometries and a significant degree of onlap onto the Triassic structured surface (top Jilh Formation). Geochemistry of the tested gas and associated condensate indicates a Type III kerogen source. The simplest interpretation is that the system is self-sourcing from interbedded coaly material that is observed in the wells and at outcrop. There is appreciable potential in this fairway due to the number of untested structures and onlapping geometries that give rise to combined structural stratigraphic traps.