The enrichment patterns of natural gas in the tight sandstone reservoirs of the Xujiahe Formation in the northeastern Sichuan Basin are complex, and the fault system exerts a crucial control on hydrocarbon accumulation. To clarify the structural characteristics of the faults and their impact on natural gas enrichment in this area. Based on three-dimensional seismic data and drilling data, integrated with artificial intelligence fault identification technology and structural analysis, this study identifies a NE-SW trending en echelon strike-slip fault zone in the Xujiahe Formation of the Wubaochang Area, northeastern Sichuan Basin. The structural deformation characteristics, evolutionary history, and reservoir-controlling and accumulation-controlling effects of this fault zone are investigated. The results indicate that the fault zone exhibits vertically layered deformation characteristics: it manifests as thrust faults in the strata from the Upper Ordovician to the Lower Triassic Jialingjiang Formation, while in the Upper Triassic Xujiahe Formation, it appears as a strike-slip fault zone composed of a series of small NNW-SSE trending thrust faults arranged in a right-stepping en echelon pattern. The fault zone underwent two stages of tectonic evolution. During the Indosinian-Yanshanian period, under continuous SE-NW oriented tectonic compression, it exhibited thrust fault activity. In the Himalayan period, the regional tectonic stress field shifted, and under a NE-SW oriented horizontal compressional stress field, the pre-existing major faults underwent sinistral strike-slip movement, forming a series of right-stepping en echelon strike-slip faults in the Xujiahe Formation. The faults in the Xujiahe Formation are interconnected with the deep-seated major faults, linking the Xujiahe Formation reservoir, the Longtan Formation source rocks, and deep fluids. This connection establishes a “strong below and weak above” transport system and a “dual-source hydrocarbon supply” model. Simultaneously, it exerts a dual effect on the tight sandstone reservoirs of the Xujiahe Formation, namely “dissolution and porosity enhancement” and “cementation and destruction,” providing favorable dynamic conditions for the differential evolution of tight reservoirs and the development of sweet spot reservoirs. Consequently, the fault zone serves as a favorable site for natural gas enrichment and preservation in the tight sandstone reservoirs of the Xujiahe Formation. The research demonstrate that strike-slip fault zones are advantageous enrichment belts for natural gas in Triassic tight sandstone reservoirs in the northeastern Sichuan Basin, and greater emphasis should be placed on the evaluation and exploration of such target areas.

Natural fractures in tight sandstone reservoirs play an important role in hydrocarbon migration and accumulation. Fracture identification remains challenging due to the scarcity of labeled data and the complex logging responses of fractures. To address these problems, we propose a novel hybrid deep learning framework (CNN-Attention-BiLSTM) for labeled data balancing. First, labeled fracture classification based on full waveform sonic logs (FWS) characteristics is employed to screen unlabeled data, replacing sampling algorithms for data balancing. This approach provides conventional logging with more fracture labels that align with authentic geological information, thereby enhancing the reliability of fracture labels. Subsequently, one-dimensional convolution is applied to construct multi-dimensional fracture logging response patterns that characterize fracture development. A Channel Self-Attention (CSA) mechanism is introduced to assign optimal weights to response patterns across different dimensions, achieving an optimized pattern combination and thereby offering clearer response pattern guidance for subsequent identification models. A double-layer BiLSTM (DL-BiLSTM) is then utilized to mitigate the impact of sedimentary cycles on logging identification, while capturing both short- and long-term dependencies of fracture responses across different network layers. Ultimately, intelligent fracture identification is realized. The identification method is applied to the H1 member of the Lower Shihezi Formation in the Hangjinqi area, China. The test set accuracy is higher than 90%, and blind wells verification demonstrates an improvement of over 8% in accuracy compared to conventional methods. The identification results reveal that fractures are the most developed in H1-2 interval, followed by H1-1 and H1-3 intervals, while H1-4 interval is the least developed. The fracture distribution pattern is evidently controlled by both sedimentary rhythms and reservoir properties, resulting in complex storage and flow capabilities. The findings can provide guidance for the migration, accumulation and efficient development of tight sandstone gas.

Carbonate reservoirs in the Sinian Dengying Formation, central Sichuan Basin, are a hot spot for hydrocarbon exploration and development in deep and ultra-deep sequences in China. Multi-type and multi-scale natural fractures are widely seen in the Dengying reservoirs, and of significant impacts on the seepage flow pattern and well productivity. In this study, the genetic types and developmental characteristics of natural fractures in the fourth member of Dengying Formation (Deng 4 Member), Gaoshiti-Moxi area, are clarified utilizing the cores, thin sections, image logs, experimental tests and production testing data. Major geological factors controlling the development of natural fractures are used to analyze the impact of natural fractures on well productivity. Results show that natural fractures in the Deng 4 Member reservoirs in Gaoshiti-Moxi area are mainly of tectonic and diagenetic types. The tectonic fractures can be divided into shear fractures and tension fractures, and the diagenetic fractures are mainly bedding fractures and stylolites, among others. The shear fractures dominate the Deng 4 Member reservoirs, and are mainly of high-angle ones and striking in the NNW-SSE, nearly EW, NE-SW and nearly SN directions. Factors controlling the shear fracture effectiveness include the timing of fracture formation, cementation, dissolution, fracture occurrence and current in-situ stress. The shear fractures in NNW-SSE and nearly EW orientation are more effective than others. The development degree of shear fractures is closely related to the lithology, mechanical layer thickness and faults. The shear fractures are more highly developed in micritic dolomites, and their density decreases and scale enlarges along with the increasing thickness of mechanical layers. The shear fractures near the main strike-slip faults, in particular within the tip and superimposed part of faults, are better developed. The development degree and effectiveness of fractures jointly determine gas well productivity. The effective fractures in different occurrences and scales can form fracture network, and get relatively isolated pores interconnected, which greatly improves the seepage capacity of reservoirs and elevate well productivity. In the Deng 4 Member reservoirs of Gaoshiti-Moxi area, the shear fractures in NNW-SSE and nearly EW orientation are of stronger ability to enhance reservoir permeability, and make greater contribution to the improvement of well productivity.

Natural fractures serve as effective storage spaces and primary seepage pathways in deep to ultra-deep tight reservoirs, affecting the hydrocarbon migration and enrichment, single-well productivity, and exploitation methods and outcomes of the reservoirs. Based on the summary of latest research results and literature review on fractures in tight reservoirs, this study delves into the distribution characteristics and developmental patterns of natural fractures in deep to ultra-deep tight reservoirs. The results show that the natural fractures are of large, meso, small, and micro scales, following a power law distribution. In other words, a larger scale corresponds to a smaller number of fractures, and vice versa. Large- and meso-scale fractures primarily facilitate seepage; small-scale ones mainly enable seepage and storage; and micro-scale ones principally serve as storage spaces. The type, occurrence, and mechanical properties of the natural fractures formed across different periods are determined by the evolution of stress regime during stratigraphic burial. The formation, distribution, and developmental degree of multi-scale fractures are subjected to the magnitude of tectonic stress, the mechanical properties of rock mechanical stratigraphy, and the thickness differences in mechanical layers. Structural deformation results in varied local stress and strain distribution at different structural locations, increasing fracture heterogeneity. Thrust faults control the distribution of faulted fracture zones by controlling the deformation of strata on the hanging walls. The combination style and movement mode of strike-slip faults, along with rock mechanical stratigraphy, jointly dictate the three-dimensional spatial distribution of related fractures. Furthermore, the crack-seal patterns of the fractures during formation and evolution determine their storage spaces and record the evolutionary history of their effectiveness.