Fracture stages and distribution patterns in Cambrian fractured dolomite reservoirs, Sandaoqiao gas field

doi:10.13745/j.esf.sf.2024.6.23

Abstract

Abstract:

The Sandaoqiao gas field in northern Tarim Basin, southern slope zone of Tabei Uplift, develops typical fractured dolomite reservoirs in deep Cambrian. The dolomite reservoirs show weak dissolution where structural fractures are the main controlling factors of reservoir storage space and physical properties. Through comprehensive analysis of fracture intersections using core logs, microscope, and micro CT, and analysis of the strength of ancient structures through acoustic emission experiments, combined with regional diagenesis and burial history, the characteristics of differential development of two main fracture stages are clarified. The first stage, mostly between the late Caledonian to early Hercynian, forms a wide, high-angle echelon fracture, with rough fracture surface and severe filling and cementation. The second stage, considered between the late Hercynian and Indosinian, forms finer, medium- to high-angle fractures, with straight fracture surface, mostly semi-filled. The intersection between the two stages results in significant local displacement. Through statistical analyses of core fracture densities and single-well fracture development parameters the degree of fracture development in the dolomite reservoirs at different structural locations is quantitatively characterized. The top parts of the limbs of anticlines in the Cambrian dolomite buried hill have the highest degree of fracture development while the lower parts show relatively low fracture development. A fracture development model for asymmetrical anticlines in the research area is established. The understanding of the differences and distribution pattern of fracture stages in this study can provide a reference for the regional fracture development stages, identification/characterization of effective fractures, quantitative prediction of fracture distribution, and evaluation of favorable fracture zones in the study area.

Key words: fracture period, quantitative evidence of fractures, distribution pattern of fractures, dolomite reservoir, Cambrian System, Sandaoqiao gas field

CLC Number:

P618.130.21

MENG Qingxiu, CAO Zicheng, DING Wenlong, YANG Debin, MA Hailong, DIAO Xindong, WANG Ming, HAN Pengyuan, WANG Huanhuan. Fracture stages and distribution patterns in Cambrian fractured dolomite reservoirs, Sandaoqiao gas field[J]. Earth Science Frontiers, 2024, 31(5): 247-262.

Figures/Tables 13

Fig.1 Maps showing regional tectonic units of the northern Tarim Basin, structural location of the Sandaoqiao gas field, and fracture distribution in the study area

Fig.2 Seismic amplitude profile perpendicular to the fault direction through well QG1 (a) and structural interpretation result (b)

Fig.3 Charateristics of fracture development from core observation

Fig.4 Characteristics of fracture development under thin sections and scanning electron microscopy

Fig.5 Fracture intersection patterns in cross-section of well QG1 core (a, b), revealing two-stage fractures (c)

Fig.6 Identifying characteristics of fracture stages under microscope from well QG1

Fig.7 Sampling points in 3D micro CT experiment and identifying characteristics of fracture stage by CT imaging

Fig.8 Characteristics of two-stage fracture by 3D segmentation and feature extraction through micro CT data processing

Table 1 Paleotectonic stresses in different periods of tectonic movements in the study area by acoustic emission experiment

记忆的主要构造运动期(幕)次	下石炭统(C₁)	泥盆系上统(D₃d)	奥陶系(O)	寒武系(XCC符号新.png;)	震旦系(Z)
喜马拉雅运动中期— 晚期( $T 1 0$ , $T 2 0$ )	14.7	16.7	14.8	13.0	12.7
燕山运动中、晚期( $T 3 0$ , $T 3 2$ )— 喜马拉雅运动早期( $T 2 2$ )	26.6	27.7	31.8	18.0	17.6
燕山运动早期( $T 4 0$ )	31.2	32.6	41.1	24.7	24.7
印支运动期( $T 4 6$ )	37.0	38.8	52.9	35.2	33.6
海西运动末期( $T 5 0$ )	50.7	52.0	75.1	44.3	42.5
海西运动中期—晚期( $T 5 1$ , $T 5 4$ )	58.3	61.8	102.8	62.3	64.2
加里东运动晚期( $T 6 1$ )— 海西运动早期( $T 6 0$ )			123.9	73.5	75.3
加里东运动中期Ⅰ幕、Ⅱ幕 ( $T 7 0$ , $T 7 4$ )			135.1	82.8	87.0
加里东运动早期( $T 9 0$ )					104.9

Table 1 Paleotectonic stresses in different periods of tectonic movements in the study area by acoustic emission experiment

记忆的主要构造运动期(幕)次	下石炭统(C₁)	泥盆系上统(D₃d)	奥陶系(O)	寒武系(XCC符号新.png;)	震旦系(Z)
喜马拉雅运动中期— 晚期( $T 1 0$ , $T 2 0$ )	14.7	16.7	14.8	13.0	12.7
燕山运动中、晚期( $T 3 0$ , $T 3 2$ )— 喜马拉雅运动早期( $T 2 2$ )	26.6	27.7	31.8	18.0	17.6
燕山运动早期( $T 4 0$ )	31.2	32.6	41.1	24.7	24.7
印支运动期( $T 4 6$ )	37.0	38.8	52.9	35.2	33.6
海西运动末期( $T 5 0$ )	50.7	52.0	75.1	44.3	42.5
海西运动中期—晚期( $T 5 1$ , $T 5 4$ )	58.3	61.8	102.8	62.3	64.2
加里东运动晚期( $T 6 1$ )— 海西运动早期( $T 6 0$ )			123.9	73.5	75.3
加里东运动中期Ⅰ幕、Ⅱ幕 ( $T 7 0$ , $T 7 4$ )			135.1	82.8	87.0
加里东运动早期( $T 9 0$ )					104.9

Fig.9 Schematic burial history and diagenetic evolution history of Cambrian strata, central and western section, Yakla Fault Convex. Modified from [32-33].

Fig.10 Quantitative identification of the location of single well fracture development segments based on variable scale fractal method and imaging logging verification

Fig.11 Statistical comparison characteristics of multiple quantitative characterization parameters of cracks between different structural parts in the study area

Fig.12 The development model of compression anticline fractures proposed by predecessors and this study

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