*3.1. Structural Characteristics*

During the Middle-Late Permian, a strong compressional orogenic activity occurred in the western mountains of the Junggar Basin and the Hong-Che Fault Zone was characterized by a large-scale thrust nappe due to compression. The structural type of the Hong-Che Fault Zone mainly consists of a thrust nappe structure. This fault zone is generally characterized by the development of multiple N-S thrust faults and fault terrace belts uplifted from east to west [30,31]. The fault combination is mainly an imbricate thrust fault combination (Figure 3).

**Figure 3.** Uninterpreted (**top**) and interpreted (**bottom**) seismic profiles across the Hong-Che Fault Zone. The location of the profile is marked in Figure 1c (modified from [23]).

The strike direction of the thrust fault is nearly north-south and its dip is westward. The dip angle of the upper part is 50–70◦ and the dip angle of the lower part is 20–30◦ [17]. The fault strata date is from the Carboniferous to the upper Jurassic boundary. The Hongche Fault has a variable strike and extends approximately 120 km in an arc shape along the plane and can be divided into three sections: South, middle, and north. Among them, the north section tends to trend NNE, the middle section trends nearly NS, and the south section trends NW [23].

#### *3.2. Characteristics of Volcanic Reservoirs*

Drilling has shown that the Carboniferous lithology of Hong-Che Fault Zone is dominated by igneous rocks with small amounts of sedimentary rocks [32]. Igneous rocks can be divided into igneous extrusive rocks and igneous intrusive rocks depending on the type of volcanism. Based on the volcanism products, the Carboniferous igneous rocks in the study area can be divided into five types: Volcanic lava, pyroclastic lava, pyroclastic rock, sedimentary pyroclastic rock, and pyroclastic sedimentary rock (Table 3). The volcanic lavas include basalt and andesite and pyroclastic rocks include tuff and volcanic breccia.


**Table 3.** Lithology statistics of Carboniferous in the Hong-Che Fault Zone.

Data Source: This table is modified from [32].

Liu (2013) collected thin section data for 404 sample points from 36 wells of the Carboniferous in the Hong-Che Fault Zone and constructed a Carboniferous lithology statistical map (Figure 4) [32]. It can be seen from Figure 4 that igneous rocks mainly developed in this area, the percentage of normal sedimentary rocks is below 10%, and intrusive rocks rarely developed. Among the igneous rocks, pyroclastic rocks and volcanic lavas are most developed, which account for 42% and 26%, respectively, and are followed by pyroclastic sedimentary rocks, which account for 12%.

**Figure 4.** Lithology statistics map of the Carboniferous in the Hong-Che Fault Zone.

The Carboniferous volcanic rocks in the Hong-Che Fault Zone are characterized by multiple stages and intermittent eruptions. There were at least three eruption periods in the Carboniferous and there were multiple eruption cycles in each eruption period. The lithofacies distributions of each eruption period exhibit both similarities and differences [29,32]. The lithofacies of the Carboniferous volcanic rocks gradually changed from volcanic eruptive facies to overflow facies to volcanic sedimentary facies and later to pyroclastic facies (Figure 5). Carboniferous volcanic eruptions mainly occurred along the Hongche Fault, Guaiqian Fault, and other large-scale boundary faults, such as the Che 47, Che 43, Che 46, and Che 72 well volcanic eruption centers. The volcanic eruptions occurred along the faults and were linear fissure eruptions [33].

Oil testing and well logging interpretation data show that the lithofacies of the Carboniferous reservoirs in the Hong-Che Fault Zone are mainly eruptive facies, overflow facies, clastic sedimentary facies, and volcanic sedimentary facies. Among the 46 reservoir samples collected, eruptive facies account for 39.1% and are followed by clastic sedimentary facies, which account for 28.3%. The reservoir samples that developed in the overflow facies account for 23.9% and a small amount (8.7%) developed in volcanic sedimentary facies (Figure 6).

Through core analyses of the volcanic rocks, the relationship between volcanic lithofacies and porosity and permeability parameters was obtained (Table 4) [29,32]. The porosity and permeability of each lithofacies are quite different. The physical properties of the eruptive and clastic sedimentary facies are most favorable, while those of the volcanic sedimentary facies are least favorable.

The reservoir spaces of the Carboniferous reservoirs in the Hong-Che Fault Zone have dual pore media, which include pores and fractures. The primary pores are generally not developed in the Carboniferous and are mainly secondary pores, which include intragranular dissolved pores, intragranular intercrystalline pores, zeolite dissolution pores, and residual intergranular pores, as well as other dissolved pores [34]. There are various types of fractures with complex characteristics in the Carboniferous volcanic reservoirs in the Hong-Che Fault Zone, which include structural fractures, diagenetic fractures, dissolution fractures, and induced fractures. Among these fracture types, structural fractures are the main fracture types. The structural fractures of the Carboniferous volcanic rocks mainly consist of oblique and reticular fractures (Figure 7). The fracture tendency is disorderly and the strikes are NNW and NNE. After the formation of Carboniferous volcanic rocks, the Hong-Che Fault Zone experienced four major tectonic activities, which correspond to the four stages of fracture formation. Vertically, the Carboniferous volcanic rock fractures are widely developed from the top of the Carboniferous downward to a depth of approximately 250 m but the fractures rarely developed below depths of 250 m and are only found in individual wells [35,36].

**Figure 5.** Lithofacies plane distribution map of the Carboniferous volcanic rocks in the Hong-Che Fault Zone (modified from [32]).

**Figure 6.** Lithofacies of Carboniferous reservoir in the Hong-Che Fault Zone (modified from [32]).



**Figure 7.** Photos showing the fracture characteristics of the core. (**a**) The tuffaceous sandstone in the Che 211 well at 1176.67–1177.09 m. (**b**) The andesite in the Chefeng 7 well at 1350.22–1350.35 m.

Taking the Che 210 well block as an example, based on core observations and casting thin section data analysis, the pore types of the Carboniferous reservoirs in this area are mainly dissolution pores and micro-fracture pores. By microscopic analysis, the core at 1180.55 m in the Che 222 well consists of tuffaceous fine sandstone with intragranular dissolved pores, which account for 50% of the total pores and micro-fractures, which also account for 50%. There is fine-grained sandstone at 1334.08 m in the Che 222 well and intragranular dissolved pores account for 100% of the total pores (Figure 8).

**Figure 8.** Casting thin section photos of Che 222 well. (**a**) The tuffaceous fine sandstone at 1180.55 m. (**b**) The fine-grained sandstone at 1334.08 m.

According to the analysis of casting thin section data, the tuffaceous sandstone in the area of the Che 210 well mainly developed dissolution pores, which are dominated by intragranular dissolved pores, matrix dissolved pores, and micro-fractures. Volcanic breccias mainly developed dissolution pores and microfractures, tuff mainly developed fractures, and basaltic andesite mainly developed dissolution pores dominated by phenocryst-dissolved pores (Figure 9).

**Figure 9.** Statistical figure of different lithology pore types in the Che 210 well block. (**a**) The pore types of tuffaceous sandstone. (**b**) The pore types of volcanic breccia.

#### *3.3. Reservoir Characteristics and Types*

The Carboniferous reverse faults in the Hong-Che Fault Zone are developed and can be divided into two groups: One group is a nearly north-south trending fault system, which extends farther in the plane direction and is the main fault of the Hong-Che Fault Zone. The other group is nearly an EW trending fault system with short plane extensions and small fault distances. The two groups of faults cut each other to form a fault block group, which formed a series of fault block traps such as the Che 23 well, Che 210 well, Che 91 well, and Che Feng 6 well. The Hong-Che Fault Zone has a variety of favorable fault block traps of di fferent sizes and there are many oil and gas producing locations, which easily formed fault block oil and gas reservoirs. Along the plane, they are mainly distributed along the fault zone in strips and along the profile, they are mainly distributed in the ascending wall of the Hong-Che Fault Zone but there are also some favorable traps in the footwall. The main types of traps are fault block and fault-lithology. The Carboniferous in the Hong-Che Fault Zone mainly developed fault block reservoirs, lithologic reservoirs, and fault-lithologic reservoirs [37].

Taking the reservoir of the Chefeng 3 well block as an example, the Carboniferous fault block in this area can be divided into three secondary structures: The Che 91 well fault block, Che 63 well west fault block, and Che 24 well fault block. It is believed that this area is controlled by fault blocks.

The Carboniferous reservoir in the area of the Che 91 well consists mainly of volcanic rock and its lithology is mostly volcanic breccia, basalt, andesite, and tu ff. The physical properties of the explosive facies breccia are most favorable and are followed by broken basalt, andesite, and tu ff, which are least favorable. This reservoir is a pore-fracture dual-medium reservoir. Due to the strong tectonic activity, the fractures in the study area are relatively well-developed and are mainly structural fractures, which more e ffectively change the physical properties of igneous reservoirs in this section.

The reservoir type of the Carboniferous in the area of the Chefeng 3 well is a fault block reservoir, which is controlled by volcanic lithofacies (Figure 10). The reservoir is controlled by faults and volcanic lithofacies along the plane and is controlled by ancient volcanic eruption sequences in the vertical direction. The reservoir lithology is mainly composed of volcanic breccia from eruptive facies, broken basalts of overflow facies, and the upper sedimentary tu ff is a good caprock.

**Figure 10.** Lithology profile of the Carboniferous reservoir in the Chefeng 3 well area ("CF" represents Chefeng).

Taking the Che 210 well area as an example, two groups of reverse faults developed mainly in the Carboniferous. One group is a near east-west fault with a short plane extension distance, such as Che 212 well north fault, Che 210 well south fault, Che 213 well south fault, and Guai 2 well south fault. There is a group of nearly north-south trending faults with long plane extension distances, such as the Che 36 well east fault, Che 211 well west fault, and Che 11 well west fault, which control the stratigraphic distribution of the Hong-Che Fault Zone and gradually rise from east to west and have resulted in serious denudation of the strata. The two groups of faults cut each other to form multiple fault block traps. The Carboniferous reservoir in the area of the Che 210 well developed in four fault block traps, which are Che 210 well, Che 211 well, Chefeng 7 well, and Che 228 well block trap (Figure 11).

**Figure 11.** Contour line of the top structure of Carboniferous Formation of the Che 210 well block.

According to a statistical analysis of thin section identifications and core observation data, three types of reservoirs developed in the Carboniferous strata in the area of the Che 210 well: Tuffaceous sandstone, tuff and volcanic breccia, and basaltic andesite in overflow facies (Figure 12). Among them, tuffaceous sandstone is widely distributed in this area and forms the main reservoir, which is followed by tuff and volcanic breccia, while basaltic andesite is less commonly distributed. These properties can be seen in the oil-bearing property statistical histogram of cores from different lithologies of the Carboniferous in the Che 210 well area. Relatively high oil-bearing grades are present in the tuffaceous sandstone and volcanic breccia, and basaltic andesite has poor grades (Figure 13). According to the oil test results and lithology analysis of the Carboniferous in the Che 210 well area, the main lithology of the oil-producing section is tuffaceous sandstone. The oil test results confirm that commercial oil flow can also be obtained from tuff, volcanic breccia, and basaltic andesite but there is only local development in this area.

**Figure 12.** Photos showing the lithology of Carboniferous Formation in the Che 210 well block. (**a**) The volcanic breccia in the Che 210 well at 1447.5–1447.6 m. (**b**) The volcanic breccia in the Che 22a well at 1866.3–1866.5 m. (**c**) The tuff in the Chefeng 7 well at 1267.5–1267.7 m. (**d**) The tuff in the Che 40 well at 1377.7–1378.0 m. (**e**) The tuffaceous sandstone in the Che 211 well at 1176.6–1177.0 m. (**f**) The tuffaceous sandstone in the Che 44 well at 1600.9–1601.1 m. (g) The andesite in the Chefeng 7 well at 1350.2–1350.3 m. (**h**) The basalt in the Chefeng 7 well at 1350.4–1350.6 m.

**Figure 13.** The statistical figure of different lithologies and core oil contents in the Che 210 well block. ("Rich oil bearing" means crude oil can be seen in more than 75% of the observed core section, "oil immersion" means crude oil can be seen in more than 40% of the observed core section, "oil spot"

"means crude oil can be seen in 40%–5% of the observed core section, "oil trace" means crude oil can be seen in less than 5% of the observed core section, and "fluorescence" means the crude oil is not visible to the naked eyes, but the fluorescence detection shows it.).

The Carboniferous in the Che 210 well area was exposed at the surface for a long period, experienced long-term weathering and leaching, and was then directly covered by the Jurassic Badaowan Formation, which lacked Permian and Triassic sediments. Weathering and leaching have further transformed the top of the Carboniferous bedrock into a reservoir, which macroscopically, is the bedrock reservoir controlled by unconformity. The Carboniferous in the area of the Che 210 well mainly contains oil. Reservoir oil layers mainly developed in the leaching zone at the top of the Carboniferous. The widely distributed tu ffaceous sandstone has large numbers of matrix pores. In addition, with later leaching, transformation, and fracture communication, various lithologies developed secondary pores and micro-fractures such as intragranular dissolved pores and matrix dissolved pores. Tu ffaceous sandstone, tu ff, volcanic breccia, and basaltic andesite can all form good volcanic reservoirs and among these, volcanic breccia reservoirs have the best physical properties. The area of the Che 210 well is located in the middle of the Hong-Che Fault Zone, which is associated with violent tectonic movements and well-developed faults. There are two groups of faults in the entire area, which have cut the Carboniferous oil reservoirs into four fault blocks.

The Carboniferous reservoirs in area of the Che 210 well are massive reservoirs, which are controlled by faults and physical properties. The oil reservoir is controlled by fault blocks and lithology along the plane. The reservoir is divided into four fault blocks. Vertically, the oil layers are distributed within 350 m from the top of the Carboniferous and their oil-bearing properties are a ffected by weathering and leaching.

Due to the complex volcanic lithology and the scattered rock mass, the oil recovery e ffect after large-scale fracturing is good. The method of supplementing energy does not work well, and neither water injection nor steam injection works.

#### **4. Analysis of Main Controlling Factors of Carboniferous Reservoir**
