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Although carbonate reservoirs often have high total pore volumes, permeability often does not show a strong correlation to total porosity. Carbonate pore networks are also widely recognized as being highly heterogeneous, with marked variability in pore size (from submicron to millimeter scale and above) within an individual core plug. It is perhaps for this reason that there has been relatively little quantification of carbonate pore size and shape, despite significant advances in our ability to image naturally porous media using electron microscopy and advanced X-ray imaging.

This study focuses on four samples of limestone from the uppermost Shuaiba Formation in northern Oman. These samples were selected for X-ray computerized tomography (CT) and environmental scanning electron microscope (ESEM) imaging and quantitative analysis following a detailed reservoir quality evaluation of the study interval across seven fields. This interval has been well studied sedimentologically, but the processes and timing of diagenetic modification, and the nature of the resultant pore network are less well understood. The samples represent a range of lithofacies associations that occur immediately beneath the Shuaiba–Nahr Umr unconformity, within an interval that is recognized for possessing higher permeability than the underlying reservoir. The samples were imaged at multiple scales, and their pore network was analyzed.

Within the sample set, over 70% of the total pore volume was <1 μm in diameter. The three-dimensional (3D) equivalent pore radii within individual samples ranged from <0.1 μm to >100 μm, with the size of the X-ray imaged samples being limited to 1 mm3. The average aspect ratios of all pores was <2, and it was highest in micropores (<1 μm pore radii). Mean coordination number was <3 in all samples, and it was highest within micropores. Since most pore throat radii are <1 μm, this most likely reflects the higher resolution needed to image micropores. Multivariant analysis shows that permeability prediction is improved when pore topological parameters are known. The highest measured permeability within the data set occurred in the sample with the highest volume of resolved porosity, highest aspect ratio, and highest coordination number. However, average permeability overall was highest in those facies associations with abundant macropores, where the representative elemental volume is greater than the sample size required for X-ray CT analysis and even routine core analysis. In these samples, high permeability is facilitated by the connectivity of a low volume of large (>>30 μm) pores embedded within a network of micropores. In these samples, sweep efficiency during hydrocarbon production is likely to be poor.

The results of this study provide one of the first detailed data sets of 3D pore shape and size within this volumetrically important reservoir and insight into pore connectivity within microporous reservoirs on the Arabian Plate. The results provide good evidence that the >1 μm fraction of these rocks contributes to single-phase flow, but they demonstrate the complexity of pore shape even at the micron scale.

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