Abstract
The Longmaxi and Niutitang Formations are typical shale reservoirs in southeastern Chongqing. Since the Paleozoic, southeastern Chongqing has experienced multistage tectonic movement and diagenesis, resulting in the formation of a large number of natural fractures. In shale reservoirs, fractures not only provide seepage channels for oil and gas migration but also play an important role in the oil and gas reservoir space. Natural fractures provide a flow path from source rock to reservoir during oil filling and connect hydraulic fractures, matrix pores, and boreholes during production. Therefore, identifying the main factors that control the development of natural fractures in the Longmaxi and Niutitang Formations in southeastern Chongqing has a guiding significance for the efficient development of shale reservoirs in this area. By considering the Longmaxi and Niutitang Formation shales as case studies, using field outcrops, drilling cores, and other data, and conducting X-ray diffraction (XRD) analysis as well as total organic carbon (TOC) measurements, the quantitative parameter characteristics and main factors that control the development of natural fractures in reservoir shales were examined in detail. The results obtained showed that there are three types of fractures in the lower Paleozoic shale in southeastern Chongqing, which are structural, diagenetic, and abnormal high-pressure fractures. Among them, the fractures in the Longmaxi Formation shale, which are relatively abundant, predominantly consist of low-angle and high-angle inclined fractures, while the Niutitang Formation predominantly consists of high-angle fractures. Additionally, the investigation of fracture size and fracture density, as well as correlation analyses, showed that the fractures of the Lower Paleozoic shale are predominantly micro fractures that play a key role in improving reservoir seepage. It was also noted that the development of fractures is affected by several factors, including tectonic stress, mineral composition, organic carbon content, and rock thickness. The degree of fracture development was found to be positively correlated with brittle mineral and quartz contents, and high organic matter contents also significantly favored the formation micro fractures.
Highlights
Since the 21st century, owing to a significant increase in the demand for energy, interest in unconventional oil and gas resources has increased significantly [1,2,3,4,5,6]
By using field outcrops, drilling cores, and other data and conducting X-ray diffraction (XRD) analysis as well as total organic carbon (TOC) measurements, this study focuses on the main factors that control shale fracture development in the Longmaxi and Niutitang Formations in southeastern Chongqing, to provide a new reference for marine shale gas exploration, prediction, and optimization in South China
This type of structure refers to the fracture caused by regional or local tectonic stress, which is mainly controlled by in situ stress and is closely related to fracture tectonic deformation
Summary
Since the 21st century, owing to a significant increase in the demand for energy, interest in unconventional oil and gas resources has increased significantly [1,2,3,4,5,6]. Fractures are common in some shales, which may be the local factors affecting the growth of hydraulic fracture height [9]. Fractures are mainly concentrated in the shale with low clay content, and the abundance and size of outcrop fractures vary greatly [9]. To evaluate the distribution of shale oil and gas sweet spots, it is necessary to study the development of natural fractures [10,11,12]. This is of great significance and may help to minimize drilling and completion costs
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