Development characteristics and controlling factors of bedrock weathering crust reservoirs in the northern belt of the Dongying sag

doi:10.13745/j.esf.sf.2023.2.65

Abstract

Abstract:

The discovery of the Wangzhuang buried-hill reservoir in the western section of the northern belt of the Dongying sag highlights the significant oil and gas exploration potential of Archean bedrock reservoirs. Based on data from outcrops, cores, thin sections, logging, imaging logging, porosity, permeability, and oil testing, this paper summarizes the development characteristics of Archean bedrock weathering crust reservoirs in the Dongying sag. It also delves into the main controlling factors. The results indicate: (1) Archean bedrocks mainly consist of monzogranite and granodiorite, with locally developed granite gneiss. These rocks are brittle and prone to fracturing, providing a lithological basis for high-quality reservoir formation. (2) Vertically, the bedrock weathering crust reservoirs can be categorized into three types: disintegration reservoirs, dissolution-disintegration reservoirs, and eluvial-disintegration reservoirs. (3) Disintegration reservoirs are primarily composed of structural fractures, dissolution fractures, and joint fractures. They exhibit poor physical properties with an average porosity of 2.2% and permeability of 0.25 mD, mainly concentrated in the upper part of the structure. Dissolution-disintegration reservoirs display a distinct double-layer structure. The overlying dissolution layer is 50-150 m thick with good physical properties, including dissolution pores and enhancement fractures, resulting in an average porosity of 4.7% and permeability of 1.3 mD. The underlying disintegration reservoirs have poor physical properties and are dominated by fractures, primarily located in the middle and lower parts of the structure. Eluvial-disintegration reservoirs are characterized by a thin upper layer and a thick lower layer. The overlying eluvium is 10-43 m thick, loose, and fragmented. The reservoir spaces are mainly composed of dissolution cavities, with an average porosity of 4.5% and permeability of 1.7 mD. The underlying disintegration layer is dense with a massive structure, and the reservoir spaces are primarily fracture-dominated, mainly distributed in the gentle slope belt near the higher part of the structure. (4) The physical properties and distribution of bedrock weathering crust reservoirs are controlled by four main factors: the presence of high brittle minerals in rocks influences fracture development, tectonic stress affects fracture formation and distribution, a developed fault system accelerates the formation of effective reservoirs, and paleo-elevation determines the distribution of bedrock weathering crust reservoirs. These research findings are crucial for evaluating bedrock weathering crust reservoirs and guiding oil and gas exploration in the Dongying sag and other faulted basins.

Key words: Dongying sag, Archean, bedrock, weathering crust reservoir, controlling factors

CLC Number:

P618.130.21

ZHU Maolin, LIU Zhen, LIU Huimin, ZHANG Pengfei, ZHAO Zhen. Development characteristics and controlling factors of bedrock weathering crust reservoirs in the northern belt of the Dongying sag[J]. Earth Science Frontiers, 2024, 31(3): 324-336.

Figures/Tables 10

Fig.1 Comprehensive geologic map of Dongying sag

Fig.2 Lithologic characteristics of Archean bedrock in the study area

Fig.3 Longitudinal development types of bedrock weathering crust reservoirs in the study area

Fig.4 Spatial characteristics of the bedrock weathering crust reservoirs in the study area

Fig.5 Relationship between porosity and permeability of the bedrock weathering crust reservoirs in the study area

Fig.6 Imaging logging characteristics of dissolution-disintegration reservoirs in the study area

Table 1 Statistics on the development of fractures in main rock types of the Archean in the Jiyang depression (adapted from [35])

岩石类型	统计井数/口	薄片数量/片	见裂缝薄片比例/%	最大裂缝数量/条	最小裂缝数量/条	平均裂缝数量/条
钾长花岗岩	5	28	46.4	125	2	15
二长花岗岩	15	149	66.4	110	1	26
闪长岩类	9	95	44.1	60	1	11

Table 2 Statistics on the development of reservoir fractures in the Archean during different tectonic periods

资料来源	裂缝数量/ 条	裂缝倾角	裂缝走向	构造时期
鲁西露头	34	低角度	北西向	印支期
鲁西露头	74	高角度	北东向	燕山—喜山期
Sg6井	128	低角度	北西向	印支期
Sg6井	249	高角度	北东向	燕山—喜山期
Zg1井	1	低角度	北西向	印支期
Zg1井	196	高角度	北东向	燕山—喜山期

Fig.7 Development characteristics of compressional torsional faults and fractures of medium-grained monzogranite of the Songshan unit in the rock formation of Aolai Mountain in Sheduo area, Zhangqiu

Fig.8 Control of paleo-elevation on the evolution and distribution of bedrock weathering crust reservoirs

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