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ABSTRACT

The Shu’aiba reservoir that is the focus of this study is one of a family of reservoirs in the Middle East that produces from a carbonate ramp-margin rudist complex along the margin of the Lower Aptian Bab Basin (field referred to here as North Oman Ramp Margin Field or RMF). This margin depositional setting imparts a level of petrophysical heterogeneity that adds greatly to the production complexities of the field. Accordingly, a series of detailed reservoir studies have been undertaken to better understand the stratigraphic and petrophysical architecture of the reservoir. As part of this field development study for the RMF, regional and outcrop data along with field analogues were reviewed and integrated to refine geological models and concepts for the field. Newly acquired well data (core, high-resolution resistivity image logs, dipmeter and conventional electric logs) and seismic data were integrated with the available production performance data for input into 3-D static reservoir models.

Of the six stratigraphic sequences defined for North Oman, three third-order composite sequences of the Lower Shu’aiba Formation (LSh1, LSh2, LSh3) and one third-order composite sequence of the Upper Shu’aiba Formation (USh1) are recognised in the field.

The Lower Shu’aiba 1 Sequence (LSh1) is the initial transgressive sequence comprised of laterally extensive shallow-marine to intertidal deposits, including the argillaceous limestones of the Hawar Member. The Lower Shu’aiba 2 Sequence (LSh2) is typically more mud-rich and characterised by Bacinella-Lithocodium wackestones alternating with Planolites-burrowed skeletal wackestones and less common mollusk-skeletal mud-dominated packstones. This sequence represents continued deepening following the initial transgression recorded by the Lower Shu’aiba 1 Sequence. The Lower Shu’aiba 3 Sequence (LSh3) is comprised of aggradational to slightly progradational rudist buildup complexes with a multifaceted lateral and vertical arrangement of high-energy caprinid rudist banks and interbank channels and shoals. The LSh3 Sequence is by far the most complex in terms of facies distribution, stratal geometry and flow-unit architecture within the Shu’aiba of Oman. Towards the northwest of the Lower Shu’aiba ramp margin, a wedge of Upper Shu’aiba rocks (USh1) is preserved. Comprised of carbonate grainstone/packstones wedges, these strata reflect a shift towards lower accommodation within the Bab Basin, where individual clinoforms pinch out into tight outer-ramp/basin carbonate mudstones.

Reservoir properties are strongly related to depositional facies and facies-related diagenesis. Grain-dominated lithofacies of the buildup/flank/channel facies tract in the LSh3 Sequence constitute the main reservoir rocks with porosity of about 18% and permeability of 100s to 1,000s of millidarcies. Carbonate mud-dominated lithofacies of the buildup core and lower transgressive sequences have higher porosity but only a few to 10s of millidarcy permeability. In general, the vertical permeability profile is one of decreasing permeability with depth, with significant flow baffles (Kh < 1mD) present in the LSh3 reservoir units enhancing the permeability contrast. Based on field and simulation experience, the permeability contrast has a large impact on field performance and ultimate recovery.

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