Pore structure characteristics and evaluation of lacustrine mixed fine-grained sedimentary rocks: A case study of the Lucaogou Formation in the Malang Sag, Santanghu Basin, Western China

The lithology of the mixed fine-grained sedimentary rock reservoirs in the Permian Lucaogou Formation of the Santanghu Basin is very complex, and the reservoirs are dominated by light- and dark-colored laminated structures with extremely heterogeneous pore structure. In this study, the mineral compositions and pore types of the reservoirs were analyzed using thin sections, whole rock X-ray diffraction (XRD), scanning electron microscopy (SEM), and Quantitative Evaluation of Minerals by Scanning Electron Microscopy (QEMSCAN). This analysis was then combined with high-pressure mercury injection (HPMI), water-saturated nuclear magnetic resonance (NMR), and nano-CT scanning to quantitatively characterize the pore structure. In addition, the fractal dimensions obtained by NMR experimentation were used to comprehensively evaluate the pore structure and analyze the effect of volcanic ash on that structure. The results revealed that the reservoirs are characterized by fine-grained (<0.1 mm) minerals of tuffaceous material (felsic minerals) and carbonate, with light-colored lamination containing carbonate minerals and dark-colored lamination containing tuffaceous material. The petrophysical properties of the reservoir were found to be poor, with porosity ranging from 2% to 8% and permeability generally < 0.1 mD. Nanometer scale intergranular pores, intercrystalline pores, vugs, and microfractures are the main reservoir pore types. The NMR T-2 spectra of the dolomite reservoir are mainly characterized by the right unimodal type (1-100 ms), which was mainly caused by the dolomite intercrystal pores and intercrystal dissolution pores, with the larger T-2 components (>100 ms) mostly representing microfractures or vugs. In addition, the HPMI curves display a relatively low entry pressure (3-5 MPa), with medium sorting. The NMR T-2 spectra of the tuff and transitional lithology reservoirs are diverse, however, with the low T-2 components (0.01-1 ms) representing the intracrystal pores, intergranular pores that are mostly blocked by organic matter (OM), and some remaining intergranular pores during compaction, while the right peaks (1-100 ms) are similar to the dolomite reservoir. Also, the HPMI curves exhibit a higher entry pressure (>6 MPa) with poor sorting. Based on the T-2cutoff values, the fractal dimensions obtained by the NMR experiment could be divided into two distinct segments, representing different pore structure characteristics. The fractal dimension of the movable fluid pores (D-2) ranged from 2.493 to 2.973 (average 2.765); nevertheless, the fractal dimension of bound fluid pores (D-1) was determined to be unsuitable for fractal theory. Due to the influence of volcanic ash, there is a positive correlation among D-2, felsic mineral content, and TOC, while the D-2 displays a negative correlation with the movable fluid porosity, permeability, and dolomite content. Hence, D-2 increases with increasing felsic mineral content and decreasing dolomite content, indicating that the tuffaceous material can make the pore structure more complex. The relationship between the calcite content and D-2 is not obvious, however, and the clay mineral contents are so low that their effects on the pore structure are negligible. The results of this study indicate that the fractal dimension can comprehensively reflect the pore structure complexity of a mixed fine-grained sedimentary rock reservoir that affected by volcanic ash.