Pore structure and fractal characteristics of Ordovician Majiagou carbonate reservoirs in Ordos Basin, China

Fractal analysis was performed on carbonate core plug samples from the Ordovician Majiagou carbonate reservoirs in the Ordos Basin to improve our understanding of the pore structure characteristics, using mercury intrusion capillary pressure (MICP), nuclear magnetic resonance (NMR) and the X-ray computed tomography (CT) measurements. The relationships between pore structure parameters and the fractal dimensions were investigated. The pore systems are dominated by secondary intercrystalline pores and enlarged dissolution pores as well as microfractures. The fractal curves from MICP analysis break into two segments at the Swanson’s parameter. The small pore throat systems can be described by the fractal theory, while pores connected by relatively large throats (>rapex, pore throat radius at the Pittman’s hyperbola’s apex) are not cylindrical in shape and can’t be described by a capillary tube model and tend to have apparent fractal dimensions larger than 3.0. The fact that the entirety of the capillary curve cannot be fit by a single fractal dimension implies that there are multiple pore systems present with different fractal dimensions. CT analysis shows that the pores are dispersed in the 3-D spaces mainly with elliptical shapes. NMR measurements are sensitive to pore body size and MICP probes pore throat dimensions, the latter being complimentary to pore body size distribution. None of the CT, MICP, and NMR techniques provides “right” or “wrong” answers to the pore throat systems but probe different aspects of the pore systems. This study assumes the pore shapes to be spherical in general, and then the fractal dimension is calculated from the NMR T2 (transverse relaxation time) spectrum. The fractal dimensions of all the samples are calculated, and the accuracy of the fractal model is verified by the high regression coefficients. Almost all the pore systems can be described by fractal theory, and the fractal dimensions are strongly correlated with T2cutoff (T2 separating the immovable fluid and the free fluid) values. Microfractures may bias T2 toward larger values, making it hard to derive fractal dimension from NMR measurements. The coexistence of small pores (pore radius50μm) results in a heterogeneous pore distribution and a high fractal dimension. Reservoir quality increases with the complexity degree of the microscopic pore structure. Conversely, samples which are dominated by small pores systems, tend to have a lower fractal dimension and a less complex pore structure.