Characterization of Porosity Within a Microporous Reservoir, Shuaiba Formation of Oman

Although carbonate reservoirs often have high total pore volumes, permeability often doesn’t 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 sub-micron to millimetre 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 CT and 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 analyzed. Within the sample set, over 70% of the total pore volume is < 1 m diameter. The 3D equivalent pore radii within individual samples ranges from <0.1m to >100 m, the size of the X-ray imaged samples being limited to 1 mm. The average aspect ratios of all pores was < 2, and was highest in micropores (<1 m pore radii). Mean co-ordination number was < 3 in all samples, and was highest within micropores. Since most pore throat radii are < 1 m, this most likely reflects the higher resolution needed to image micropores. Multivarient analysis shows that permeability prediction is improved when pore topological parameters are known. The highest measured permeability within the dataset occurs in the sample with the highest volume of resolved porosity, highest aspect ratio and highest co-ordination number. However, average permeability overall is highest in those facies associations with abundant macropores, the representative elemental volume of which 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 a connectivity of a low volume of large (>> 30 m) 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 datasets of 3D pore shape and size within this volumetrically important reservoir and provides insight into pore connectivity within microporous reservoirs on the Arabian Plate. The results provide good evidence that the >1 m fraction of these rocks contributes to single phase flow, but demonstrates the complexity of pore shape even at the micron-scale