Класс Месторождения: Крупное
Тип Месторождения: Газовое
Местоположение: Море
Местность:
Стадия разработки: Добыча
Год открытия: 2003
Источник информации:
Метод открытия:
Площадь: 229.8 км²
Bozhong 19–6
The Bozhong 19–6 Condensate gas field is the first giant gas field discovered in the Bohai Bay Basin in recent years. The gas field reservoir has diverse reservoir space and strong heterogeneity.
The major types of tectonic fractures in study area are shear fractures, followed by tensile fractures. Study area mainly developed high-angle fractures and completely filled fractures, fractures are commonly filled with carbonate and clay cement. Four groups of tectonic fractures were found in the study area, which corresponds to the four fracture formation period. Tectonic movement controls the fractures development in study area, the multi-stage tectonic movement had caused a complex fractures network system. The faults control the development of associated fractures formation in the fractures zone, different faults control the fractures formation and orientations in the vicinity. Rock type and minerals content is the basis of forming different fractures, such as the high felsic content is the basis of developed fractures in metamorphic rocks, and rock layer thickness, reservoir porosity and permeability are important factors of the fractures development heterogeneity longitudinally.
In recent years, a large condensate gas field has been found in the deep Archean metamorphic buried hill in the Bozhong 19–6 structural belt, southwestern Bozhong sag, which is the largest gas field discovered so far in the Bohai sea basin. (Wang et al., 2019; Ye et al., 2021; Wang et al., 2022). The permeability of such reservoirs is significantly improved by the tectonic fractures and their associated secondary pores (Lyu et al., 2019), it provides a effective pathways for oil and gas migration and reservoir space for oil and gas accumulation (Cui et al., 2013). In addition, tectonic fractures connect “geological sweet spots” and “engineering sweet spots” together (Wang et al., 2017; Gong et al., 2021), therefore, the study of tectonic fractures in metamorphic buried hill reservoirs should run through the whole exploration and development process.
Many scholars have studied the tectonic fractures of metamorphic buried hill reservoirs in Bozhong 19–6 gas field, Bohai Bay Basin (Yu and Koyi, 2016; Ye, Chen, et al., 2020; Wang et al., 2021; Zeng et al., 2022). However, the main research is the important role of fractures in the process of oil and gas migration and accumulation, there is still a lack of analysis and research on the comprehensive characterization of reservoir tectonic fractures and the influencing factors of reservoir fracture development in the area. Therefore, the observation and statistics of imaging logging, core and thin section tectonic fractures are carried out to quantitatively characterize the tectonic fractures of metamorphic buried hill reservoirs in Bozhong 19–6 gas field, combining with the whole rock mineral analysis, core porosity and permeability analysis, and clarify the control factors of structural fracture development in the study area. These results provide a guidance for the prediction of tectonic fractures in the study area, and provide reliable basic data for the further exploration and development of the Bozhong 19–6 gas field.
Geological setting
Bozhong 19–6 condensate gas field is located in the southwest of Bozhong sag, surrounded by Bozhong sag, Shanan sag and Huanghekou sag (Zhao et al., 2015a; Xu et al., 2019) (Figure 1A).
Fig.1 (A) Regional location of the BZ19-6 condensate gas field; (B) Comprehensive stratigraphic column of the BZ19–6 condensate gas field.
From the top to the bottom, the stratum is Pingyuan Formation, Minghuazhen Formation, Guantao Formation, Dongying Formation, Shahejie Formation, Kongdian Formation, local Mesozoic stratum and Archean metamorphic rock basement (Xu et al., 2019; Feng et al., 2020) (Figure 1B). Mesozoic rock reservoirs are mainly developed in the Minghuazhen Formation, Guantao Formation and Kongdian Formation, the lithology is mainly glutenite and tuff (Li et al., 2012).
Fig.2 Vertical zonation characteristics of metamorphic reservoirs in Bozhong Sag
Archean is the main gas-bearing interval of Bozhong 19–6 condensate gas field, and it is buried more than 3,800 m (Wang et al., 2021). Its lithology is metamorphic granite, mainly including monzonitic gneiss, plagioclase gneiss, locally developed cataclastic mixed granite (Liu et al., 2023). The rock components are mainly albite, quartz and potassium feldspar, followed by chlorite, calcite, illite, muscovite and dolomite (Ye et al., 2022).
The core physical property test shows that the average porosity of Archean metamorphic rock buried hill reservoir is 5.22%, and the average permeability is 0.37 mD (Pei et al., 2022). The reservoir space is mainly intergranular pore and dissolution pore (Cao et al., 2015; Wang et al., 2018).
Fig. 3 Reservoir prediction map of weathered fracture zone-weathered fracture zone based on azimuth anisotropy
The Bozhong 19–6 condensate gas field has experienced multistage tectonic activities since Indosinian (Liu et al., 2022; Yi et al., 2022). During the Indosinian period, the South China plate subducted and collided with the North China plate, forming a regional compressive stress field from south to north. A series of south to north thrust faults were formed, and the fractures were mainly distributed in the near EW direction (Zhou et al., 2012).
During the Yanshanian period, the study area was affected by the NW compressive stress, due to the NW subduction of the ancient Pacific Ocean. At the same time, the Tanlu Fault was left-laterally squeezed, forming a large number of NE strike-slip faults. The largescale faults of the Indosinian period were reversed during the formation of the buried hill, and the original continuous Indosinian uplift zone was cut into a series of fragments of different sizes. At the same time, the internal part of the buried hill block was broken (Chen, 1998; Huang et al., 2003).
The early formation of fractures in the study area was subjected to the northnorthwest tensile stress of the Himalayanian period, while it was subjected to the superimposed modification of the right-lateral tensile action of the Tanlu Fault. The early NNW and N-E faults were reactive, and the overall fracture subsided. Finally, the tectonic appearance of the buried hill was shaped (Zhao et al., 2015).
Fracture development characteristics
The tectonic fractures of metamorphic buried hill reservoir in Bozhong 19–6 condensate gas field are mainly shear fractures. High angle fractures (45° < θ < 75°) and vertical fractures (θ > 75°) are mainly developed, and a small number of low angle fractures (θ < 45°) are locally developed (Figure 4A). The scratches on the shear fractures surface are obvious. The fractures length varies greatly (0–30 cm). The filling degree of fractures is high, and most of them are filled with mud (Figure 4B). The development of tensile fractures is less, which is mainly developed in granitic gneiss, and the fractures filling degree is high. It is mostly filled with clay, calcite and other minerals, accompanied by a small amount of mineral dissolution phenomenon (Figure 4C). According to the core fractures cutting relationship, combined with the regional stress evolution process. It is believed that there are at least four phases of tectonic fractures in the study area: the near EW tectonic fractures in the early Indosinian period, the NW tectonic fractures in the late Indosinian period, the NWW tectonic fractures in the Yanshanianian period, the NE tectonic fractures in the Himalayan period.
Fig. 4 (A) Well BZ19-6-C, 4538.05–4538.90 m, exhibits numerous shear and tensile fractures with significant variations in orientations. The distribution forms a mesh-like intersection pattern, with many fractures being partly or completely filled. (B) Well BZ19-6-I, 4429.00–4429.90 m, displays a highangle shear fracture and a weathered and dissolved fractured zone in the lower part. (C) Well BZ19-6-C, 4678.06–4678.59 m, shows multiple shear and tensile fractures. The filling minerals within the fractures have undergone dissolution.
The microfractures of metamorphic rock buried hill reservoir in Bozhong 19–6 condensate gas field are developed. It can be observed that the early fractures are cut by the later fractures, and the later fractures development is restricted by the early fractures.
Shear fractures are mainly developed, and the fracture surface is relatively straight. The fracture aperture is between 2 and 7 mm and the fracture length is between 1 and 10 mm. The fracture filling degree is high, and the fractures are mainly filled with clay and carbonate minerals. Fractures mostly cut through quartz, feldspar and other mineral particles, and are accompanied by dissolution, pores enlarged by dissolution are developed along microfractures. In addition, the network fracture system intersected by fractures can also be observed, it is significant for improving the storage and permeability properties of metamorphic rock reservoirs.
The fracture filling degree of metamorphic buried hill reservoir in Bozhong 19–6 condensate gas field is high, it is mainly completely filled fractures, accounting for 89.13%, followed by partly filled fractures, accounting for 10.51%, open fractures only accounts for 0.36% (Figure 5A). The fractures are mainly filled with caly, followed by carbonate fillings, quartz fillings and asphaltene account for a small proportion of fractures (Figure 5B). The filling degree of low angle fractures is higher, and the filling degree of high angle fractures and vertical fractures is lower (Figure 5C). The fractures filling degree of glutenite is high, and the fractures effectiveness is low. A certain number of partly filled fractures are developed in granitic gneiss, and the fractures effectiveness is high (Figure 5D).
Fig. 5 A) Types of structural fracture filling; (B) Types of materials filling tectonic fractures; (C) Filling degree of tectonic fractures at different angles; (D)The Filling degree in tectonic fractures for different lithologies
Lithology
The difference of rock mineral composition and rock fabric of different lithologies determines the difference of structural fracture development, which is the internal factor determining the difference of fracture development (Ding et al., 2012; Ju and Sun, 2016). There are various types of metamorphic buried hill reservoir rocks in Bozhong 19–6 condensate gas field, and the main reservior rocks are metamorphic rocks and intrusive dikes. The tectonic fractures in the study area are mainly developed in plagioclase gneiss and monzonitic gneiss, which density is high, followed by fractures density in the granite, and the fracture density in diorite porphyrite and diabase is low (Figure 6A). The content of brittle minerals in plagioclase gneiss, monzonitic gneiss and granite is more than 80%, and diorite porphyry and diabase is less than 80% (Figure 6B).
Fig. 6 (A) Connection about density and lithology of structural fracture; (B) Distribution of mineral content in diverse lithologies.
The rock of lower plastic minerals content has a better brittleness. Under the action of tectonic stress, it is easy to development fractures, and the degree of fractures development is
higher (Wang et al., 2023). It is common for plagioclase gneiss, monzonitic gneiss and granite to undergo migmatization (Lander and Laubach, 2015). Recrystallization makes the rock grains become larger from small, as the mineral grains increase, the particle size becomes longer (Maréchal et al., 2004). Under the same tectonic stress, the moment exerted on the same kind of mineral increases with larger grain size. Larger mineral grains are more likely to exceed the stress-bearing strength and generate fractures (Li et al., 2017).The observation results of core and thin section show that the rock with relatively high content of brittle minerals has a high degree of composition and structural heterogeneity. The rock contains minerals of various scales, components and shapes, and the transmission speed of each grain to force and its own deformation are different, which leads to the uneven distribution of stress field in the rock. Under the action of stress, it is more likely to development fractures due to the influence of rock composition and structural heterogeneity (Li et al., 2022). Therefore, rocks with a high degree of heterogeneity are more likely to development fractures.
Tectonic movement
The thrust extrusion in the Indosinian period is the key factor for the reservoir fractures development in the study area. The metamorphic rock basement is subjected to strong compression and thrusting, forming a large number of large scale faults in the N-W oriented. Especially, the core of the extrusion is the stress concentration development area, and a large number of tectonic fractures are formed inside the buried hill (Cheng et al., 2018) (Figure 7A).
Fig. 7 (A) Structural fracture development patterns in different tectonic periods within BZ19-6 condensate gas field.
At the same time, the N-E oriented thrust extrusion caused the study area to uplift sharply, forming an anticline uplift structure. The Archean metamorphic rock basement exposed the surface to weathering and leaching, and atmospheric water leached along the fault to the interior of the buried hill, forming a thick weathering crust. Due to the early formation time, these fractures experienced strong diagenesis in the later deep burial process, and most of the fractures were filled (Tong et al., 2012).
During the Yanshanian period, it was mainly affected by the N-W trending low angle subduction of the Paleo Pacific Ocean. A large number of N-E oriented trending strike-slip faults were formed in the Bozhong sag, which retransformed the early fractures and further expanded the scale of fracture development (Huang et al., 2003) (Figure 7B).
Fig. 7 (B) Structural fracture development patterns in different tectonic periods within BZ19-6 condensate gas field.
The influence of the Himalayan tectonic movement on the reservoir in the study area is mainly reflected in the reactivation of early fractures and the generation of new fractures. Most of the fractures formed in the Indosinian and Yanshanian periods are filled with argillaceous, iron and carbonate minerals (Wang et al., 2022). A large number of high angle fractures are formed inside or on the edge of the early filled fractures, and a small number of new fractures are generated.
The Himalayan period is the main period for the formation of effective fractures in the reservoir of the study area. Although the ability of the Himalayan period to development new fractures is not as good as that of the Indosinian and Yanshanian periods, the Himalayan tectonic movement can re-activate the early fractures (Wang et al., 2018)(Figures 7C,D).
Fig. 7 (C,D) Structural fracture development patterns in different tectonic periods within BZ19-6 condensate gas field.
In general, The formation of buried hill reservoir fractures in the study area has experienced the initial formation of buried hill caused by strong extrusion in Indosinian period, the superimposed transformation and destruction of compression-torsion strike-slip in early Yanshanian period, the reformation of fault depression into mountains in middle Yanshanian period, and the multi-stage structural transformation and superimposed transformation process of reactivation of tension-torsion extension fractures in Himalayan period. The strong compressional tectonism in Indosinian period laid the reservoir foundation of buried hill, the sinistral strike slip in Yanshanian period further expanded the scale of fractures, and the dextral strike slip in Himalayan period reactivated the early fractures and formed a super large scale fractures system.
Control of faults on fractures
Studies have shown that although there are differences in the characteristics and formation mechanism of tectonic fractures in the fault zone under different mechanical properties, it is a common law that the farther the distance from the fault zone is, the less developed the tectonic fractures are (Lavenu et al., 2013). According to the rose diagram of fracture trends in 11 wells (Figure 8), it can be seen that the fractures orientation is parallel or nearly parallel to the orientation of adjacent faults, which indicates that faults play an important role in the development of tectonic fractures. In addition, near the fault or at the inflection point and intersection of the fault zone, the stress is concentrated, and the tectonic fractures are developed. BZ19-6-I, BZ19-6-J, and BZ19-6-K are located at the intersection of multiple different sequence faults, and the fracture density is high. In addition, the cataclastic zone in these three wells is extremely developed and has a certain direction, which is the result of the combined action of shear stress and tensile stress.
Fig. 8 Major larger fault distribution map, modified from (Dai et al., 2021), the fracture orientations are obtained from the data of 11 wells.
Data source: Characteristics and controlling factors of tectonic fractures within the buried hill reservoirs from the Archaean metamorphic basement: a case study in the Bozhong 19–6 condensate gas field, Bohai Bay Basin. Zongbing Liu, Qi Cheng, Xinwu Liao, Lei Zhang, Wenchao Liu and Guanjie Zhang, 2024.
https://html.rhhz.net/ZGSYKT/html/2021/6/20210611.htm#b23
Следующее Месторождение: Lufeng 17-2 1