Back to Basics: The Critical Role of Fieldwork and Outcrop Samples in Mapping the Unexplored Neoproterozoic Play of NW Saudi Arabia

Paul G. Nicholson¹, Hendrik Sibon¹, and Dominique Janjou^2
1Area Exploration, Saudi Aramco, Dhahran, Saudi Arabia
²Bureau de Recherches Geologiques et Minieres, Orleans, France

Saudi Aramco has entered a new phase of exploration in the NW region of Saudi Arabia. Recently acquired 2D seismic data show a number of potential hydrocarbon traps in the deeper Neoproterozoic section. In neighboring Oman, Neoproterozoic reservoirs are major exploration targets and prolific hydrocarbon producers. However, because the Neoproterozoic play is completely unexplored across Saudi Arabia, there is no well calibration for our deeper seismic reflectors nor is there direct evidence whether a working petroleum system exists here. Fortunately, there are several excellent upper Neoproterozoic exposures in NW Saudi Arabia along the edge of the present-day basin margin, and these outcrops appear analogous in geometry and stratigraphical position to seismic reflectors in the subsurface. Fieldwork and outcrop sample studies are thus critical to mapping prospective Neoproterozoic hydrocarbon traps in this region.

Several seasons of fieldwork, combined with quantitative analyses of outcrop samples, have enabled us to identify potential Neoproterozoic play elements in the subsurface. Dark grey limestones of the Jibalah Group, deposited in a middle to outer marine ramp setting, have source rock potential with typical TOC values of 3%. These tight limestones, together with marine to paralic shales interbedded throughout the succession, comprise candidate seals. Fractured dolomites and cherts, and shallow marine sandstones with matrix porosity, all comprise potential reservoir targets. Structurally, the Jibalah Group was subjected to a distinct sequence of mappable events in the field, beginning with an initial phase of mild transpression and folding, followed by a major extensional collapse and culminating with Najd-trending shearing, folding and uplift. These events have produced a variety of potential hydrocarbon trapping geometries in the subsurface.

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