Bedding-parallel fractures in ultradeep tight sandstone reservoirs in Jurassic and Cretaceous of Yongjin Oil Field, Junggar Basin, China

doi:10.1007/s11707-022-0999-9

Front. Earth Sci.

2022, Vol. 16

Issue (4) : 975-988 https://doi.org/10.1007/s11707-022-0999-9

RESEARCH ARTICLE

Bedding-parallel fractures in ultradeep tight sandstone reservoirs in Jurassic and Cretaceous of Yongjin Oil Field, Junggar Basin, China

Hongping LIU¹(

), Changmin ZHANG¹(

), Li ZHANG¹, Yang LUO²

¹. School of Geosciences, Yangtze University, Wuhan 430100, China
². Key Laboratory of Tectonics and Petroleum Resources (Ministry of Education), China University of Geosciences, Wuhan 430074, China

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Abstract

Bedding-parallel fractures are fractures that are parallel to rock bedding structure planes and have been widely accepted as a key factor for oil and gas production in tight sandstone and shale reservoirs. However, the formation mechanisms of these parallel-bedding fractures are still under debate. In this study, bedding-parallel fractures in Yongjin Oil Field were analyzed using methods including core and microscopic observations, element geochemistry, and carbon and oxygen isotope analysis. Their origin and relations to reservoir quality, faults, and rock mechanical properties were examined. The discovery of bedding-parallel fractures in both the Upper Jurassic and Lower Cretaceous formations indicates that the BPFs are generated later than the early Cretaceous. The filling state of bedding-parallel fractures that with no bitumen and carbonate cement indicate that they formed after oil charging and carbonate cementation. The tensile fracture characteristics in core and thin section observations, and the fact that overburden stress exceeds the pore pressure indicate that the bedding-parallel fractures were neither generated from tectonic compression nor overpressure. The most likely generation mechanism is stress relief during core drilling under high in situ stress conditions. High in situ stress and low tensile strength lead to thinner fracture spacing. The existence of high bedding-parallel fracture density is an indicator of good reservoir quality and result in high oil/gas production.

Keywords Bedding-parallel fractures ultradeep reservoirs tight sandstone fracture origin Junggar Basin

Corresponding Author(s): Hongping LIU,Changmin ZHANG

Online First Date: 11 October 2022 Issue Date: 11 January 2023

Cite this article:

Hongping LIU,Changmin ZHANG,Li ZHANG, et al. Bedding-parallel fractures in ultradeep tight sandstone reservoirs in Jurassic and Cretaceous of Yongjin Oil Field, Junggar Basin, China[J]. Front. Earth Sci., 2022, 16(4): 975-988.

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https://academic.hep.com.cn/fesci/EN/10.1007/s11707-022-0999-9
https://academic.hep.com.cn/fesci/EN/Y2022/V16/I4/975

Fig.1 (a) Major tectonic units of the Junggar Basin (modified from Bian et al., 2010). (b) Cross-section through the Junggar Basin from south to north (modified from Yang, 2009). (c) Structure image of the top of the Jurassic units.

Fig.2 Stratigraphic names and ages, lithologies, sedimentary environments, and tectonic evolution of the Junggar Basin, China (modified from Hu et al., 2020).

Fig.3 (a) Ternary diagram illustrating the framework compositions of the Upper Jurassic tight sandstones in the Yongjin Oil Field. Q = quartz; F = feldspar; and R = rock fragment. (b) Rock fragment ternary plot of the Upper Jurassic tight sandstones. VRF = volcanic rock fragments; MRF = metamorphic rock fragments; and SRF = sedimentary rock fragments.

Fig.4 (a) Porosity and (b) permeability histograms of the Qigu Formation.

Fig.5 Photography of blue epoxy resin-impregnated thin sections illustrates the diagenetic types. (a) Intensive compaction with line contact, Well Y1, 5826.65 m. (b) Line-point contact sandstone with both residual intergranular pores and dissolution pores, Well Y1, 5821 m. (c) Point-contact sandstone with both residual intergranular pores and dissolution pores, Well Y301, 5549 m. (d) Kaolinite and chlorite cements, Well Y1, 5882 m. (e) Quartz overgrowth, Well Y3, 5619 m. (f) Calcite cements after bitumen charging, Well Y6, 6048.59 m. (g) Dolomite cement in pores with partial dissolution, Well Y6, 6028.52 m. (h) Calcite and dolomite cementation showing calcite and dolomite colors of red and purple using alizarin red, respectively. (i) Lithic and feldspar partial dissolution, Well Y1, 5821 m.

Fig.6 Photography of the cores with different fracture types. (a) Bedding-parallel fractures (BPFs) (core discing) in the Qigu Formation, Well Y1. (b) Bedding-parallel fractures (core discing) and vertical fractures in the Qigu Formation, Well Y301. (c) and (d) Bedding-parallel fractures in the Qigu Formation, Well Y2. (e) Bedding parallel fractures (BPFs) in Qigu Formation, Well Y2. (f) High dip-angle fractures in the Qigu Formation, Well Y7.

Fig.7 Photography of blue epoxy resin-impregnated thin sections illustrating the bedding-parallel fractures. (a) Bedding-parallel fractures in coarse grains, Qigu Formation, Well Y301, 5549.2 m. Section a1 is magnified to show the pore-filling state of the BPFs. (b) Bedding-parallel fractures in coarse grains, Qigu Formation, Well Y301, 5544.7 m. Section b1 is magnified to show the pore-filling state of the BPFs. (c) Bedding-parallel fractures in coarse grains that pass through quartz overgrowths, Qigu Formation, Well Y1, 5827.7 m. (d) Bedding-parallel fractures in tight rocks. No horizontal displacement, filling, or dissolution in the fracture. Qigu Formation, Well Y3, 5865.6 m.

Fig.8 Bedding-parallel fracture surface (P1, P3) and fresh surface (P2, P4) element geochemistry analysis using the portable element analyzer. (a), (b), and (c) are the cores used for the element geochemistry analysis and (d) is the element mass content. Ba and S elements are the main difference between BPF surfaces and fresh surfaces. The Ba and S come from the invasion of the oil drilling mud.

Fig.9 Porosity and permeability cross-plot with and without bedding-parallel fractures. Samples with bedding-parallel fractures display higher porosity and permeability.

Fig.10 Bedding-parallel fracture (BPF) density and high dip-angle fracture (HDA) density with respect to the faults on the top of Jurassic formation. The wells with high BPF density can be either close to faults or far from faults. The HDA fractures are mainly close to the faults.

Fig.11 (a) The δ¹³C and δ¹⁸O values of calcite cement in tight sandstones of the Qigu Formation. (b) Histogram of the oxygen isotope temperature distribution in tight sandstones of the Qigu Formation.

Fig.12 Burial history of Well Y1 and the carbonate cementation formation time. Bedding-parallel fractures should have formed after the carbonate cements (Modified from Shi et al. (2009)).

Fig.13 Bedding-parallel fractures in (a) core and (b) thin sections in the Yanchang Formation in the Ordos Basin, China. The fractures were filled by bitumen, which was jointly affected by the overpressure and tectonic compression.

Fig.14 Core discing formation mechanisms during drilling (modified from Zheng et al. (2020)).

Fig.15 The relationship among core disc thickness (t/D), horizontal maximum in situ stresses (σ₁), and tensile strengths (σ_t) in rocks from Ertan Hydropower Station, Sichuan, China (Hou, 1985).

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