Quantitative Characterization and Determination of the Main Factors That Control Fracture Development in the Lower Paleozoic Shale in Southeastern Chongqing, China

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.

Numerical Modeling of Natural Fracture Distributions in Shale

“…5). Assuming that the borehole is a vertical straight line in the center, the intersection point in the model is the number of fractures in the core and the number of fractures per unit length is the vertical linear density of the model (Zhang et al, 2021).…”

Favorable tectonic patterns for shale gas preservation and enrichment in lower Paleozoic, southeastern Chongqing, China

“…There are abundant NNE-oriented folds and faults, as well as natural fracture systems and bedding slip tectonic deformation, forming a NNE-oriented dominant structural configuration (Figure 1). These tectonic factors have an important influence on the preservation, enrichment and percolation of shale gas in southeastern Chongqing (Ding et al, 2012; Guo et al, 2014a ; Hu et al, 2014a ; Wang et al, 2016; Zeng et al, 2016; Zhang et al, 2021).…”