Multi-scale fractures formation and distribution in tight sandstones—a case study of Triassic Chang 8 Member in the southwestern Ordos Basin

doi:10.1007/s11707-022-0990-5

Front. Earth Sci.

2022, Vol. 16

Issue (2) : 483-498 https://doi.org/10.1007/s11707-022-0990-5

RESEARCH ARTICLE

Multi-scale fractures formation and distribution in tight sandstones—a case study of Triassic Chang 8 Member in the southwestern Ordos Basin

Gaojian XIAO¹, Ling HU², Yang LUO¹(

), Yujing MENG¹, Ali Bassam Taher AL-SALAFI³, Haoran LIU⁴

¹. Key Laboratory of Tectonics and Petroleum Resources, China University of Geosciences, Wuhan 430074, China
². Wenhua College, Wuhan 430074, China
³. Faculty of Petroleum and Natural Resources, Sana’a University, Sana’a, Yemen
⁴. Louisiana State University, Baton Rouge LA 7080, USA

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Abstract

Fracture system is an important factor controlling tight oil accumulation in the Triassic Chang 8 Member, southwestern Ordos Basin, China. A systematic characterization of the multi-scale natural fractures is a basis for the efficient tight oil production. Based on outcrops, seismic reflections, well cores, well logs (image and conventional logging), casting thin sections, and scanning electron microscope observation, the multi-scale fractures occurrences and their influences on Chang 8 tight sandstone reservoirs are revealed. The results show that three periods of strike-slip faults and four scales of natural fractures developed, namely mega-scale (length > 7 × 10 ⁷ mm), macro-scale (3.5 × 10⁵ < length < 7 × 10 ⁷ mm), meso-scale (10 < length < 3.5 × 10 ⁵ mm), and micro-scale (length < 10 mm) fractures. The mega- and macro-scale fractures developed by strike-slip faults are characterized by strike-segmentation and lateral zonation, which connect the source and reservoir. These scale fractures also influence the distribution and effectiveness of traps and reservoirs, which directly influence the hydrocarbon charging and distribution. The meso fractures include the tectonic, diagenetic, as well as hydrocarbon generation-related overpressure types. The meso- and micro-scale fractures improve the sandstone physical properties and also the tight oil well production performance. This integrated study helps to understand the distribution of multi-scale fractures in tight sandstones and provides a referable case and workflow for multi-scale fracture evaluation.

Keywords natural fractures characteristics geological significance tight sandstone reservoir Upper Triassic Yanchang Formation

Corresponding Author(s): Yang LUO

About author: Tongcan Cui and Yizhe Hou contributed equally to this work.

Online First Date: 20 July 2022 Issue Date: 26 August 2022

Cite this article:

Gaojian XIAO,Ling HU,Yang LUO, et al. Multi-scale fractures formation and distribution in tight sandstones—a case study of Triassic Chang 8 Member in the southwestern Ordos Basin[J]. Front. Earth Sci., 2022, 16(2): 483-498.

URL:

https://academic.hep.com.cn/fesci/EN/10.1007/s11707-022-0990-5
https://academic.hep.com.cn/fesci/EN/Y2022/V16/I2/483

Fig.1 Structural division map of the study area, outcrops, wells, and fault distribution in the Ordos Basin. (a) Location of the Honghe oilfield and tectonic units of the Ordos Basin. (b) Distribution of faults, outcrops, and wells in the Chang 8 layer of the Honghe Oilfield.

Fig.2 The composite stratigraphic columns of the Triassic Yanchang Formation in the Ordos Basin (Ch represents Chang).

Fig.3 Different scales strike-slip faults in 3 periods of the Honghe Oilfield on seismic profile.

Fig.4 Photos of fractures exposed in the Nashuihe outcrops. (a) Fractures with NW and NE trending in gray fine sandstones were 2–9 m in length and present scratches on the surface. (b) Rose diagram of fracture strikes from Nashuihe outcrops (N = 985, photos, and data from the China North Branch of Sinopec, collected by Zeng LB).

Fig.5 Response characteristics of natural fractures in the HH90 Well of the Chang 8 layer. In the track of Full-bore Microscan Images (FMI) and core images, the arrows show fractures with high dip-angle and nearly 90°.

Tab.1 Fracture response characteristics of conventional well logs in Chang8 tight sandstones

Fig.6 Pictures of different types of fractures. (a) HH100 well, 2428.72 m, fine sandstones, showing high dip-angle tectonic shear fractures (yellow arrow) without filling. (b) HH87 well, 2462.48 m, dark gray sandstone, showing high dip-angle fractures (yellow arrow) filled with calcite. (c) HH158 well, 2142.67 m, fine sandstones, showing vertical shear fractures with the oil patch. (d) HH1057-3 well, 2229.61 m, gray fine sandstones, showing two parallel vertical overpressure-related fractures. (e) HH29 well, 2195.81 m, brown muddy siltstone, showing a set of vertical conjugate tectonic fractures (yellow arrow). (f) HH42 well, 1784.50 m, showing conjugate scratches. (g) ZJ4 well, 2049.9 m, dark gray silty mudstone, showing diagenetic fracture. (h) HH87 well, 2463.40 m, dark brown silty mudstone, showing horizontal shrinkage fractures filled with bitumen.

Fig.7 Characterization of fractures from cores in the Chang 8 oil-bearing layer of the Upper Triassic Yanchang Formation.

Fig.8 Statistics of fracture filling distribution from cores of the Upper Triassic Yanchang Formation. (a) Fractures filled with non-calcareous cementation. (b) Fractures filled with calcareous cementation.

Fig.9 Overpressure-related fractures (yellow arrows) presented as wormlike forms in HH16 well (2092.63 m), filled with bitumen.

Fig.10 Characteristics of micro and nanoscale fractures in the Chang 8 tight sandstones of the Upper Triassic Yanchang Formation. (a) Tectonic micro-fractures (yellow arrows) in a thin section from HH2 well (2062.85 m) under cross-polarized light, partly filled with bitumen and calcite (A-bitumen; B-calcite); (b) open tectonic micro-fractures (yellow arrows) in a thin section from HH39 well (2387.96 m) under cross-polarized light, cutting mineral grains; (c) overpressure-related micro-fracture (yellow arrows) in a thin section from ZJ9 well (2270.79 m) under cross-polarized light, filled with bitumen; (d) diagenetic micro-fracture (yellow arrows) in a thin section from HH9 well (1079.17 m) under cross-polarized light, filled with calcite; (e) open tectonic micro-fractures (yellow arrows) in a thin section without mineral fillings from the ZJ3 well (1586.46 m) under cross-polarized light; (f) overpressure-related micro-fracture (yellow arrows) in a thin section from ZJ17 well (2261.56 m) under cross-polarized light, partly filled with bitumen; (g) nanoscale fracture (yellow arrows) between grains in a SEM image from HH31 well (2196.41 m), partly filled with illite and increases the space by connecting intergranular pores; (h) nanoscale fracture (yellow arrows) and chlorite cladding over grains in a SEM image from HH44 well (2115.17 m); (i) nanoscale fracture (yellow arrows) appears in authigenic quartz particle in a SEM image from HH2 well (2071.41 m).

Fig.11 The micro-fractures characteristic from thin sections of the Chang 8 sandstones. (a) Micro-fractures length. (b) Micro-fractures width.

Fig.12 The result of carbon/oxygen isotopes analysis in the fractures with mineral-filled of the Chang 8 tight sandstones. The data of carbon/oxygen isotope samples in the quartz and calcite-cemented were from Chen et al. (2018), and the carbon/oxygen isotope data were used to calculate the fractures formation temperature based on the temperature measurement equation. The fractures formation periods were deduced by correspondence between fractures formation depth and burial depth based on fractures formation temperature and geothermal gradients from Liu (2013).

Fig.13 Geotemperature history, burial history, and fluid-inclusion homogenization temperature of calcite and quartz deposits in tectonic fractures in the Chang 8 tight sandstones, south-western Ordos Basin. The burial history was derived from the stepwise stratigraphic back-stripping technique according to compaction correlation, regional denudation data from Wang et al.(2017b). The data of temperature was from Bai (2013). The fluid inclusions data of the calcite and quartz deposits in the tight gas sandstones of the Chang 8 oil-bearing layer of the Upper Triassic Yanchang Formation were from Chen et al. (2018). The “J₂” is the first period of fractures development in the Yanshannian Period, and the “E” is the second period of fractures development is in the Himalayan Period. (a) The first hydrocarbon charging period with fluid-inclusion homogenization temperatures from 60°C to 90°C in the middle-late Jurassic. (b) The second hydrocarbon charging period with fluid-inclusion homogenization temperatures from 90°C to 125°C, was from the end of the Jurassic to the middle Cretaceous. (c) The third hydrocarbon charging period with fluid-inclusion homogenization temperatures from 125°C to 150°C, was from the middle Cretaceous to the Early Paleogene. T-Triassic; J-Jurassic; K-Cretaceous; E-Eogene; N-Neogene; Q-Quaternary.

Tab.2 Characteristics of multi-scale fractures in the Chang 8 tight sandstones

Fig.14 The conceptual model of strike-slip fault band (Torabi et al., 2019).

Fig.15 Distribution of multi-scale fracture band and the oil saturation of tight sandstones.

Fig.16 The distribution diagram of fractures porosity and permeability. The fractures porosity and permeability were calculated by the Monte Carlo method from cores, casting thin sections, and SEM images observations.

Fig.17 The scatter plot of fractures development index from well logs (the ratio of fractured layers thickness to statistical subzone thickness) and distance of fractures to faults in the fault number one and number two.

Fig.18 Production capacity distribution sketch map in different faults and fractured regions. The production capacity in NEE and NNE fractured regions is significantly better than the production capacity in NNW fractured region, and the high-yield wells (> 9 t/d) are distributed mainly in the intersection area of NWW, NNE and NEE fractured regions.

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