Source-reservoir characteristics and coupling evaluations for the Lower Jurassic lacustrine shale gas reservoir in the Sichuan Basin

doi:10.13745/j.esf.sf.2020.5.24

Abstract

Abstract:

Continental shale gas in the Sichuan Basin has good exploration prospects with obtainable industrial gas flow in the Lower Jurassic formation of the northeastern Sichuan Basin. Compared with marine shale, the characteristics of continental shale include rapid lithofacies variation, multiple types of lithofacies association (often interlayers), low total organic matter content (TOC), and complex source-reservoir relationship. Based on the idea of “source-reservoir coupling”, using macro-micro reservoir characterization methods, we analyzed and evaluated the source-reservoir characteristics and coupling relationship for the continental shale in the Da’anzhai Member of the Low Jurassic Ziliujing Formation. The results can be summarized into four main areas. (1) Lithofacies combinations. The Da’anzhai Member is a set of shallow to semi-deep lake deposits with dark shale and shell limestone interlayers of varying thickness. The main lithofacies combinations of 4 lithofacies and 3 thicknesses are mainly thin (< 1 m) interlayers of shell limestone between high TOC argillaceous shale, thin to medium-thin (< 3 m) interlayers of shell limestone between medium TOC argillaceous/silty shale, and medium-thick interlayers of shell limestone with a small amount of fine-silty sandstone between low-medium TOC silty shale. Shale is the source rock and main reservoir; shell limestone serves as secondary reservoir and interlayer; and tight sandstone with poor physical properties mainly functions as interlayer. (2) Source-reservoir characteristics. The thickness of shale in the northeastern Sichuan Basin is between 30-40 m, with TOC content generally between 1.0%-2.0%. Organic matter types are mainly types II₂ and III. The shale reservoir space types are organic and inorganic pores and macro-micro fractures. The most developed pores are marginal and interparticle pores in clay minerals, followed by organic pores, while interconnected micro fractures are also well developed. In the shell limestone interlayers, the inter and intraparticle pores and micro fractures are the main types of reservoir space. (3) The macro source-reservoir configuration and micro source-storage coupling mechanism of continental shale strata in the Da’anzhai Member. Macroscopically, both TOC and porosity decreased with increasing carbonate content. The condition of source-reservoir coupling is better in shale than in interlayers, and the argillaceous shale is the best shale. Microscopically, there are a integrated source-reservoir coupling between organic matter and organic pore and a migration coupling within nm to μm distance between organic matter/inorganic pore and micro-fracture in shale strata; between shale and interlayer there is a migration coupling within mm to cm distance; and in the interlayer, migration coupling occurs within cm to m distance between organic matter (from the adjacent shale)/inorganic pore and micro-fracture. (4) Coupling evaluation. The corresponding source-reservoir coupling evaluation parameters are established for shale strata and interlayers, while coupling conditions are evaluated for two typical wells, and a preferred horizontal well’s target window has been suggested based on the evaluation results.

Key words: shale gas, source rock, reservoir, source-reservoir coupling mechanism, lacustrine shale, interlayer, Lower Jurassic, Sichuan Basin

CLC Number:

HU Zongquan, WANG Ruyue, LIU Zhongbao, LIU Guangxiang, FENG Dongjun, YANG Zhenheng, WANG Pengwei. Source-reservoir characteristics and coupling evaluations for the Lower Jurassic lacustrine shale gas reservoir in the Sichuan Basin[J]. Earth Science Frontiers, 2021, 28(1): 261-272.

Figures/Tables 12

Fig.1 Sedimentary and distribution characteristics of the Da’anzhai Member in the Sichuan Basin

Fig.2 Major lithofacies types and characteristics of the Da’anzhai Member in the Sichuan Basin

Fig.3 Distribution of shale and interlayer thicknesses in the Da’anzhai Member, Sichuan Basin

Fig.4 TOC content of different lithofacies in the Da’anzhai Member of the Yuanba and Fuling areas

Table 1 Macropore types and characteristics of the Da’anzhai Member

岩性	孔隙类型	孔隙载体	孔隙分布	孔隙形态	孔隙发育程度
页岩	有机孔	迁移有机质(固体沥青)	固体沥青内部	球状为主	较高
	有机孔	原地有机质(生物碎屑等)	有机质内部、边缘	不规则状	低
	无机孔	无机矿物	黏土及碎屑矿物颗粒缘及晶间	线状、三角形、不规则状	高
			碳酸盐等不稳定矿物内部、边缘	针孔状、不规则状	较低
			粉砂质碎屑粒间、边缘	针孔状	低
灰岩	有机孔	迁移有机质	固体沥青内部	球状为主	低
	无机孔	无机矿物	碳酸盐矿物粒间	不规则状	低
	无机孔	无机矿物	碳酸盐矿物粒内	针孔状、不规则状、线状解理	中等

Fig.5 Macropore characteristics of shale strata in the Da’anzhai Member

Fig.6 Fracture characteristics of shale strata in the Da’anzhai Member

Fig.7 Physical properties of shale strata in the Da’anzhai Member of the Yuanba and Fuling areas

Fig.8 Pore size distributions of different types of shale reservoirs in the Sichuan Basin

Fig.9 Schematic diagram illustrating the source-reservoir coupling mechanism for continental shale in the Da’anzhai Member

Fig.10 Cumulative frequency distribution of source-reservoir coupling coefficient in the Da’anzhai Member

Fig.11 Source-reservoir characteristics and coupling evaluation of shale strata in the Da’anzhai Member of key wells in Yuanba and Fuling areas

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