Nature fractures in shales of the Lianggaoshan Formation in northern Sichuan Basin: Fracture development characteristics and fracture formation and evolution model

doi:10.13745/j.esf.sf.2024.6.16

Abstract

Abstract:

The Jurassic Lianggaoshan Formation in Yingshan-Pingchang region, northeastern Sichuan has great exploration potential for shale oil and gas resources. To better understand the control of natural fractures on oil and gas enrichment and reservoir production, this paper utilizes multi-scale fracture characterization methods such as field observation, imaging, core logging, thin section, and CT scanning, combined with vein inclusion analysis, carbon-oxygen isotope analysis, acoustic emission testing, and U-Pb dating of veins. The characteristics of the multiscale fracture system in the study area are revealed; the formation history of natural fractures of different origins are clarified; and the fracture evolution model is constructed. The Jurassic shale reservoir mainly develops mechanical types of fractures, such as shear fractures, bedding slip fractures, bedding fractures, and fluid overpressure fractures. The tectonic deformation zone is dominated by NW-/NE-striking, high-angle parallel shear fractures and reticulate tension-shear hybride fractures, with high fracture density and large-scale vertical interlayers. The stable zone is characterized by bedding fractures, fluid overpressure fractures, and a small number of near-EW-/NNE-striking planar shear fractures, with low fracture density and high fracture filling. The Jurassic mainly experienced four episodes of fracture development : the early-middle Yanshanian (170-140 Ma), with the formation of near SN/NE-striking hydraulic overpressure fractures fully filled with fibrous calcite; the late Yanshanian (100-80 Ma), with the development of NNE-/near-EW-striking conjugate planar shear fractures, NE-striking expansion fractures, and hydrocarbon generation overpressure fractures; the early-middle Himalayan (67-32 Ma), with the continued development of planar shear fractures, NW-treding parallel shear fractures, longitudinal tension fractures, and bedding fractures; and the late Himalayan (15-6 Ma) with the activation of pre-existing fractures and formation of small number of NNE-striking tensile fractures. This study provides important support for quantitative prediction of effective fractures under multistage tectonic superposition in the Jurassic and identification of favorable areas for shale oil and gas enrichment.

Key words: shale reservoir, natural fractures, formation and evolution, effectiveness, Lianggaoshan Formation, Yingshan-Pingchang area

CLC Number:

P618.13

HE Jianhua, CAO Hongxiu, DENG Hucheng, YIN Changhai, ZHU Yanping, LI Chang, LI Yong, YIN Shuai. Nature fractures in shales of the Lianggaoshan Formation in northern Sichuan Basin: Fracture development characteristics and fracture formation and evolution model[J]. Earth Science Frontiers, 2024, 31(5): 17-34.

Figures/Tables 16

Fig.1 Structural map of the northeastern Sichuan basin (a), comprehensive stratigraphic column of the Lianggaoshan Formation in well PA1 (b), and structural cross-section map of the study area (c)

Table 1 Classification and characteristics of natural fractures in shale of the Langgaoshan Formation

类型	亚类	裂缝特征	裂缝尺度	尺度类型
构造裂缝	剪切缝	由平行于裂缝面的剪应力引起,具有共轭组系、缝面平直、剖面型或平面型擦痕、切割颗粒、纵向穿层等特征	长度以5~15 cm为主,开度主要为0.2~1.5 mm,θ>60°	中大尺度
	张性缝	在页岩受拉张应力作用破裂而形成,表现为垂直平行组系、缝面粗糙、绕颗粒、纵向终止等特征	长度以5~10 cm为主,开度0.2~1 mm,θ>75°	中小尺度
	层面滑移缝	在挤压构造作用下,沿着泥页岩层面顺层滑动的剪切应力产生的裂缝,常见擦痕或光滑摩擦镜面	长度以7~10 cm为主,开度主要为0.2~0.5 mm,θ<15°	中大尺度
非构造裂缝	层理缝	主要为成岩作用或上覆应力卸载所导致,发育于页理、纹层等力学薄弱面附近的水平缝	长度以5~10 cm为主,开度0.1~0.2 mm	中小尺度
	流体超压缝	生烃初期的水力超压缝为快速埋藏排水不充分造成水力超压所致	长度以8~10 cm为主,开度2~5 mm	中小尺度
	流体超压缝	后期达到生烃高峰以后的生烃超压缝主要由大规模生烃增压而形成的微破裂	长度以5~15 μm为主,开度多分布在0.05~0.2 μm	微观尺度

Fig.2 Macroscopic characteristics of different types of mechanical fractures in the Langgaoshan Formation

Fig.3 Distribution of core fracture orientations and filling characteristics of key wells in Yinshan-Pingchang area

Fig.4 Cross-well comparison of fracture characteristics in Jurassic Yingshan structure

Fig.5 Characteristics of microfractures in the Langgaoshan Formation under SEM and CT scanning

Fig.6 Characteristics of fracture development in outcrops of the Langgaoshan Formation

Fig.7 Fracture strikes based on image log interpretation from outcrops (a) and gentle terrain (b)

Fig.8 Cross-cutting relationship between fractures in the study area

Fig.9 Photomicrographs of fluid inclusions in veins in the study area

Fig.10 Histograms of homogenization temperatures of fracture filling inclusions in structural (a) and non-structural (b) fractures in the study area

Fig.11 Cross-plots of δ13C vs. δ18O of fracture fillings in structural (a) and non-structural (b) fractures in the study area

Fig.12 Acoustic emission test results for rocks in the study area

Fig.13 Tera-Wasserburg Concordia diagram of U-Pb data of calcite samples

Fig.14 Fracture evolution in Jurassic shale reservoirs

Fig.15 Evolution mode of various genetic natural fractures in Jurassic shale reservoirs

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