Characterization of natural fractures in deep-marine shales: a case study of the Wufeng and Longmaxi shale in the Luzhou Block Sichuan Basin, China

doi:10.1007/s11707-022-1021-2

Front. Earth Sci.

2023, Vol. 17

Issue (1) : 337-350 https://doi.org/10.1007/s11707-022-1021-2

RESEARCH ARTICLE

Characterization of natural fractures in deep-marine shales: a case study of the Wufeng and Longmaxi shale in the Luzhou Block Sichuan Basin, China

Shasha SUN¹, Saipeng HUANG^2,³(

), Enrique GOMEZ-RIVAS³, Albert GRIERA⁴, Βο LIU², Lulu XU⁵, Yaru WEN⁵, Dazhong DONG¹, Zhensheng SHI¹, Yan CHANG¹, Yin XING⁶

¹. PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China
². Key Laboratory of Continental Shale Hydrocarbon Accumulation and Efficient Development, Ministry of Education, Northeast Petroleum University, Daqing 163318, China
³. Departament de Mineralogia, Petrologia i Geologia Aplicada, Facultat de Ciències de la Terra, Universitat de Barcelona (UB), c/ Martí i Franquès s/n, Barcelona 08028, Spain
⁴. Departament de Geologia, Universitat Autònoma de Barcelona, Bellaterra 08193, Cerdanyola del Vallès, Spain
⁵. Hubei Geological Survey, Wuhan 430034, China
⁶. School of Geography Science and Geomatics Engineering, Suzhou University of Science and Technology, Suzhou 215009, China

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Abstract

Natural fractures are of crucial importance for oil and gas reservoirs, especially for those with ultralow permeability and porosity. The deep-marine shale gas reservoirs of the Wufeng and Longmaxi Formations are typical targets for the study of natural fracture characteristics. Detailed descriptions of full-diameter shale drill core, together with 3D Computed Tomography scans and Formation MicroScanner Image data acquisition, were carried out to characterize microfracture morphology in order to obtain the key parameters of natural fractures in such system. The fracture type, orientation, and their macroscopic and microscopic distribution features are evaluated. The results show that the natural fracture density appears to remarkably decrease in the Wufeng and Longmaxi Formations with increasing the burial depth. Similar trends have been observed for fracture length and aperture. Moreover, the natural fracture density diminishes as the formation thickness increases. There are three main types of natural fractures, which we interpret as (I) mineral-filled fractures (by pyrite and calcite), i.e., veins, (II) those induced by tectonic stress, and (III) those formed by other processes (including diagenetic shrinkage and fluid overpressure). Natural fracture orientations estimated from the studied natural fractures in the Luzhou block are not consistent with the present-day stress field. The difference in tortuosity between horizontally and vertically oriented fractures reveals their morphological complexity. In addition, natural fracture density, host rock formation thickness, average total organic carbon and effective porosity are found to be important factors for evaluating shale gas reservoirs. The study also reveals that the high density of natural fractures is decisive to evaluate the shale gas potential. The results may have significant implications for evaluating favorable exploration areas of shale gas reservoirs and can be applied to optimize hydraulic fracturing for permeability enhancement.

Keywords marine shale natural fracture filled fracture tortuosity

Corresponding Author(s): Saipeng HUANG

About author:

^* These authors contributed equally to this work.

Online First Date: 24 October 2022 Issue Date: 03 July 2023

Cite this article:

Shasha SUN,Saipeng HUANG,Enrique GOMEZ-RIVAS, et al. Characterization of natural fractures in deep-marine shales: a case study of the Wufeng and Longmaxi shale in the Luzhou Block Sichuan Basin, China[J]. Front. Earth Sci., 2023, 17(1): 337-350.

URL:

https://academic.hep.com.cn/fesci/EN/10.1007/s11707-022-1021-2
https://academic.hep.com.cn/fesci/EN/Y2023/V17/I1/337

Fig.1 (a) Isopach map of the Longmaxi Formation in the southern Sichuan Basin and location of the Luzhou block (modified from Chen et al. (2011)). (b) Sedimentary and petrophysical logs of the target interval of well LZ4 in the Luzhou block.

Fig.2 (a) Sketch of a natural fracture in shale. (b) Diagram showing some of the key parameters used to describe fractures in this study.

Fig.3 The total number of natural fractures in the vertical dimension for wells LZ1 to LZ5. The red rectangles indicate the part with a higher fracture density.

Fig.4 Fracture density vs. reservoir depth in the Luzhou block.

Fig.5 Distribution of natural fractures in well LZ1.

Fig.6 (a−e) Example of Type I natural fractures (i.e., veins). (a) Fracture filled with calcite vein from LZ1 well (3935.21−3935.53 m), (b) fracture formed in calcite vein from LZ1 well (3951.07−3951.22 m), (c) fracture network filled with calcite veins from LZ1 well (3946.48−3946.69 m), (d) fracture with pyrite filled from LZ2 well (3799.55−3799.86 m), (e) fracture formed in pyrite vein from LZ2 well (3798.55−3798.80 m), (f) pyrite nodules from LZ2 well (3795.10−3795.42 m).

Fig.7 Example of type II fractures. (a) Conjugate strike-slip faults from LZ2 well (3842.70−3842.90 m depth), (b) from LZ1 well (3968.46−3968.64 m), (c) parallel strike-slip faults from LZ1 well (4034.47−4034.66 m), (d) sub-vertical normal fault from LZ1 well (4022.20−4022.37 m).

Fig.8 Several examples of typical fractures formed by other processes than tectonic stress (so-called non-structural by Zhang et al. (2019)). (a) Bedding-parallel fractures from LZ1 well (4035.38−4035.55 m), (b) bedding-parallel fractures from LZ4 well (3782.75−3782.95 m), (c) induced fracture from LZ4 well (3772.17−3772.32 m) and (d) induced fracture from LZ1 well (3983.72−3983.99 m).

Fig.9 Frequency of fracture (a) dip type, (b) aperture, and (c) filled cement.

Fig.10 (a) Burial depth vs. natural fracture aperture. (b) Burial depth vs. natural fracture length.

Fig.11 Formation thickness vs. fracture density.

Fig.12 (a−h) Dip (red) and dip direction (blue) of natural fractures from LZ1, LZ2, LZ3, and LZ4 wells, respectively.

Fig.13 (a−c) 3D CT scans in the horizontal and vertical directions with a diameter of 25 mm. The digitalised fractures (black lines) and envelopes (dashed lines) are indicated. (d−e) Summary of the natural fracture orientation, (d) fracture dip directions and (e) dips respect to bedding.

Fig.14 Tortuosity of the natural fractures. (a) horizontal direction, (b) vertical direction.

Fig.15 Natural fracture (a) dips (red) and (b) dip directions (blue) compared to present-day maximum horizontal principal stress (σ_H) orientations (green) for the Luzhou block.

Tab.1 Detailed information of the orientation of the present-day maximum horizontal principal stress (σ_H) near the Luzhou block. Data have been taken from Heidbach et al. (2016a, 2016b).

Fig.16 Spider diagram evaluating the main four parameters for predictive shale gas obtained from the four studied wells.

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