Pore Structure Characteristics and Evaluation of Carbonate Reservoir: A Case Study of the Lower Carboniferous in the Marsel Exploration Area, Chu-Sarysu Basin

The pore structure of a carbonate reservoir determines the fluidity and storage characteristics of hydrocarbon, and it is vital for reservoir evaluation and oil and gas production. The Visean and Serpukhovian carbonate reservoirs in the Marsel exploration area of the Chu-Sarysu Basin possess a complex lithology and pore structure, which significantly affect production efficiency. The pore structure of the reservoirs was investigated in this study via X-ray diffraction, conventional petrophysical measurements, cast thin section analysis, scanning electron microscopy (SEM), imaging logging, and high-pressure mercury injection (HPMI). The findings show that the lithology in this area mainly comprises fine-grained limestone, bioclastic limestone, and silty limestone; moreover, the mineral assemblage in this region is markedly different. The types of carbonate reservoir pore spaces are complex and diverse. Primary pores are mainly biological cavity pores with small pore size, mostly isolated individuals, poor connectivity, and weak percolation capacity. Secondary dissolution pores, which are dominated by intergranular dissolution pores, intragranular dissolution pores, and microfractures, are the dominant storage spaces in the Carboniferous carbonate reservoirs. Through an innovative approach of combining imaging logging and cast thin section analysis, it was concluded that mainly three types of fractures exist, namely conductivity fractures, fissures, and resistive fractures. Moreover, structural fractures in this area are relatively developed, which substantially increases the percolation capacity of the reservoirs and it is conducive to oil and gas flow. However, some structural fractures are filled with calcite, destroying the effectiveness of the fractures and reducing the percolation capacity. Based on capillary pressure curves, the samples were divided into three types, of which the physical properties decrease in the order of type I > type III > type II. The fractal dimensions obtained from HPMI and SEM data characterize the complexity of the reservoir microstructure in the study area. The formation of high-porosity and high-permeability zones was caused by quasi-syngenetic dolomitization and syngenetic–quasi-syngenetic dissolution. In this study, the pore structure of carbonate reservoirs in the area was evaluated quantitatively, which is highly significant to improving oil and gas recovery.