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Abstract

Determination of the depositional architecture of carbonate hydrocarbon reservoirs is a necessary prerequisite for pre- and syn-development modelling and simulation. Such reservoirs are typically too thin for internal bedding geometries to be confidently determined from seismic and wireline log evidence, and this aim is best achieved by integrated palaeontological and sedimentological studies. Of these, micropalaeontological analysis and biofacies interpretation of closely spaced core samples can reveal cryptic, high-frequency depositional cycles and three-dimensional palaeoenvironmental information.

In Saudi Arabia, the Shu’aiba Formation is the main reservoir in the Shaybah field, and has been subdivided into a number of biofacies (ShBf-1 to ShBf-10) within four main depositional units, of which the lower unit (Sh1) is regionally extensive, moderately deep marine and of non-reservoir significance. A thicker second unit (Sh2) displays significant lateral and vertical differentiation of a rudist-rimmed shallow carbonate platform in which discrete lagoonal, rudist bank complex and fore-bank biofacies have been determined. Deep platform sediments characterize the uppermost stacked unit (Sh3) of the main buildup, and are of early Aptian age. Flank carbonates onlap the former three units and are assigned a late Aptian age (Sh4).

This fully integrated palaeontological approach to over 50 cored wells from the Shaybah Field has significantly provided a detailed stratigraphic and palaeoenvironmental interpretation that is currently being used to successfully develop this field with 14 billion barrels of crude oil and 25 trillion cubic feet of gas in place. A satisfactory sequence stratigraphic interpretation of the Shu’aiba Formation at Shaybah remains elusive, as neither detailed seismic evidence nor correlatively valuable wireline logs are available.

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