Analysis of the developmental characteristics and major regulating factors of fractures in marine–continental transitional shale-gas reservoirs: A case study of the Carboniferous–Permian strata in the southeastern Ordos Basin, central China

Abstract

Fractures play an important role in the formation of shale-gas reservoirs because they can enlarge the transport channels and aggregation spaces and increase the specific surface area of the gas shale. For artificial hydraulic fracturing of these reservoirs, the natural fracture system must be fully integrated with the artificial fracture system to form an intact fracture system. In this study, we first comprehensively examined the fractures in 42 shale-gas wells using several approaches, including a systematic examination and description of the cores and the casting of thin sections, a compilation of the statistics of fracture feature parameters, and observation of various analytical and test data, such as the mineral composition, the organic carbon contents, and the rock mechanics properties for specimens from the corresponding fractured intervals. The data enabled us to thoroughly explore the developmental features and major factors affecting organic-rich shale fractures in the upper Paleozoic Carboniferous–Permian marine–continental transitional coal-bearing formation, which is located in the southeastern Ordos Basin. Our results reveal that, in comparison with the Paleozoic marine shale in the United States and southern China, as well as the Paleozoic basin transitional shale in northern China, the upper Paleozoic black shale in the Ordos Basin is primarily characterized by a relatively low content of brittle minerals and a high content of clay ingredients. The total content of brittle minerals, e.g., quartz, feldspar, and siderite, was approximately 33%, which included 27% quartz and 0.3% K-feldspar but did not include carbonate. The total content of clay minerals reached 64% and was dominated by mixed-layer illite–smectite (I/S), which accounted for more than 41% of the total clay ingredients. The shale accommodated the widespread development of various types of macro- and microfractures. In the core specimens, medium-angle slip fractures and horizontal bedding cracks were the most common types of fractures, whereas vertical and high-angle fractures and horizontal bedding cracks were underdeveloped. In the thin sections, microfractures arising in organic matter laminations or at their edges as well as those of tectonic origin were the predominant type of fractures, and they were mainly short, narrow, and open. Overall, the surface/fracture ratios of the thin sections were concentrated in a range of approximately 0.1–0.3%. The developmental level of the fracture was influenced by various factors, including tectonism, lithology, rock mechanics, and organic matter and mineral content. Thus, increased developmental level of fractures was correlated with higher paleostructural elevation and increased sand content, whereas the developmental level of microfractures was correlated with high lamellation development, high level of organic carbon (leading to more pronounced laminations), and high contents of quartz, mixed-layer I/S, and illite (leading to low levels of kaolinite). These findings were corroborated by other data generated in this study, including the rock mechanics results for the upper Paleozoic black shale and silty shale, the observation results from the cores and from the thin sections sampled from more than 40 shale-gas wells, and the anomalies of gas logging.