# Characteristics of Carboniferous Volcanic Reservoirs in Beisantai Oilfield, Junggar Basin

Authors: Hanlie Cheng; Peng Ma; Guofeng Dong; Shun Zhang; Jianfei Wei; Qiang Qin
Journal: Mathematical Problems in Engineering
 (2022)

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## Abstract

In recent years, petroleum exploration in the Carboniferous volcanic rock reservoirs in the Junggar Basin has been the focus of important petroleum energy development in western China. The lithologic identification of volcanic rock reservoirs seriously restricts the accuracy of reservoir prediction and affects the success rate of oil exploration. Different types of volcanic rocks have different petrological characteristics and mineral assemblages, especially affected by the depositional environment. The volcanic rocks in different regions have their own uniqueness. This paper takes the Carboniferous volcanic reservoirs in Xiquan block, Beisantai Oilfield, Junggar Basin as the research target. Through a large number of core observations, casting slices, scanning electron microscopy, and X-ray diffraction methods, the Carboniferous volcanic rocks are analyzed. The petrology, pore characteristics, physical properties, and diagenetic evolution history of the reservoir are analyzed. The study shows that the volcanic facies in the Xiquan block can be divided into explosive facies, overflow facies, and volcanic sedimentary facies, among which the explosive facies is subdivided into empty subfacies (volcanic breccia-breccia tuff combination) and thermal base wave subfacies (tuff). The lithology of the reservoir is pyroclastic rock and volcanic lava, belonging to medium-porous and ultralow permeability reservoirs, and the storage space can be divided into three types: primary pores, secondary pores, and fractures. The lithology of key exploration is breccia tuff, followed by breccia tuff and volcanic breccia.


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## Body

## 1. Introduction

Beisantai Oilfield is located in the southern margin of the Junggar Basin, under the jurisdiction of Fukang City, Changji Hui Autonomous Prefecture. The Permian and Carboniferous areas are the main oil-bearing layers in the region. Since oil drilling began in 1989, the oil production in the shallow Permian in Beisantai oilfield has been greatly reduced, and the water production of oil wells has gradually increased. Deep oil drilling in the region has become the future development direction [1, 2]. The Carboniferous basement in the Xiquan block is a nearly north-south-trending paleo-uplift, in which the Carboniferous strata on the top of the west wing are denuded, and the strata on the east wing dipped eastward. The proven oil reserves of the Carboniferous stratum in the Xiquan block in 2017 are 2010.15 × 104t. The oil reservoir is mainly controlled by structure and rock mass, and the lithology of the reservoir is mainly pyroclastic rocks, including breccia tuff and volcanic breccia [3, 4]. The porosity of Carboniferous reservoirs is between 13.2 and 22.9%, and the permeability distribution ranges from 0.10 to 1.85 mD. The fracture porosity is generally low, mainly microfractures, and belongs to mesoporous and ultralow permeability reservoirs. Carboniferous reservoirs are characterized by high displacement pressure, small median radius, poor sorting, and poor pore structure [5].The Xiquan block is located in the Beisantai uplift, which is an inherited paleo-uplift formed in the Hercynian period, with a relatively high altitude. Due to the closed uplift of the ocean trough in the midlate Hercynian period, a strong compressive and torsional stress field was generated, forming the Beisantai uplift extending from north-northwest to south-southeast [6, 7]. The Carboniferous stratum has experienced long-term uplift and erosion, resulting in the development of internal fractures in the Carboniferous stratum, with uneven top surfaces and large changes in the thickness of the depressions. In the middle and late stages, the sedimentary range expanded continuously, the distribution range was wide and continuous, and the variation of sediment thickness was small. After uplifting again during the Yanshanian period, the Jurassic and Triassic strata were severely damaged during this period. The residual Lower Triassic Jiucaiyuan Formation was in the low-lying area, and the Wutonggou Formation in the uplift also experienced partial denudation and pinching out. [8, 9]. The Carboniferous stratum is directly covered by Cretaceous stratum in most areas of the Beisantai uplift. The current exploration shows that the strata developed from the bottom to top in the Xiquan block include the Carboniferous Batamayineshan Formation (C2b), the Permian Wutonggou Formation (P3wt), the Cretaceous Tugulu Group (K1tg), and the Paleogene (E), Neogene (N), and Quaternary (Q) formations . Both the Wutonggou formation and its upper and lower strata are in contact with unconformity [10, 11].The volcanic reservoir oil and gas fields discovered in China are mainly developed in the Mesozoic and Cenozoic strata in the east, and the rock types are mainly intermediate-acid volcanic rocks. The west is mainly developed in the Paleozoic stratum, and the rock types are mainly intermediate-basic volcanic rocks. Some researchers have analyzed the Carboniferous volcanic rock cores in the Junggar Basin, showing that tectonic movement, lithology and lithofacies, and weathering and leaching are the main factors controlling the Carboniferous volcanic rock reservoirs [12–14]. The types of volcanic rock reservoir space are divided into four categories: primary fractures, primary pores, secondary fractures, and secondary pores. Other researchers believe that the Carboniferous volcanic rock reservoirs in the Junggar Basin have experienced three diagenetic evolution stages, syngeneic stage, epigenetic stage, and burial diagenetic stage, and developed early sintering, condensation shrinkage and plate tectonic processes, weathering leaching, dissolution, zeoliteization and chloriteization, and other diagenesis [15–17]. The main factors affecting the development of volcanic rock reservoirs are complex and diverse. Lithology and lithofacies are the direct factors affecting the reservoir performance of volcanic rock. According to the different genetic types of volcanic rock fractures, their effects on volcanic rock reservoirs are also different. Based on the study of volcanic rock lithology, lithofacies, porosity characteristics, permeability characteristics, and pore structure,. this paper conducts classification and evaluation of reservoirs to establish classification and evaluation standards for volcanic reservoirs in the Xiquan block, which provides a reference for the carboniferous oil exploration in this block.


## 2. Materials and Methods

### 2.1. Regional Stratigraphic Characteristics

From the top to bottom, the drilled strata in Beisantai uplift include Quaternary, Neogene, and Paleogene formations, Cretaceous Tugulu Group, Jurassic Qigu Formation, Toutunhe Formation, Sangonghe Formation, Badaowan Formation, Triassic Huangshanjie Formation, Karamay Formation, Shaofangou Formation, Jiucaiyuan Formation, Permian Wutonggou Formation, Pingdiquan Formation, and Carboniferous Formation. The missing strata from the top to bottom are Jurassic Kalaza Formation, Xishanyao Formation, Triassic Haojiagou Formation, and Permian Jiangjunmiao Formation (Table1).Table 1
Brief stratigraphic table of Xiquan block.
FormationSymbolThickness mBrief description of lithology and lithofaciesTieSystemGroupQuaternary——Q410∼440Mainly gray-yellow clay, mixed with colored gravel and sandy gravelNeogene——N738∼933The middle and upper parts are mainly composed of gray-yellow mudstone, sandy mudstone, and gravel-bearing mudstone with gypsum mudstone and argillaceous siltstone, and the lower part is mainly composed of gray-yellow mudstone, sandy mudstone, and sandy mudstone mixed with brown-gray mudstone and small mudstone conglomeratePaleogene——E115∼227Red-brown, gray mudstone, and sandy mudstone are mainly intercalated with argillaceous siltstone, siltstone, gravel-bearing siltstone, and gluteniteCretaceousLowerTugu LuqunK1tg275∼441The middle and upper parts are mainly composed of brown-gray, gray-brown mudstone, and sandy mudstone interbedded with brown-gray argillaceous siltstone; the lower part is interbedded with gray and brown argillaceous siltstone, siltstone and brown and gray silty mudstoneTriassicLowerShaofanggou groupT1s0∼117Red-brown, gray-brown mudstone, and sandy mudstone are mainly mixed with gray, brown-gray, variegated gravel-bearing fine sandstone, gravel-bearing medium sandstone, and gluteniteJiucai Garden groupT1j42∼169Red-brown, gray-brown mudstone, and sandy mudstone are mainly mixed with gray, brown-gray, variegated gravel-bearing fine sandstone, gravel-bearing medium sandstone, and glutenitePermianUpperWutonggou FormationP3wt8∼136Taupe, gray mudstone, silty mudstone, and even gray argillaceous siltstoneCarboniferousUpperBatamayC2b48∼907Gray, brown-gray, and brown volcanic breccia with green-gray, gray-green, gray-black andesite, tuff, and carbonaceous mudstoneThe lithology and contact relationship are as follows [18]:Quaternary (Q): yellow, khaki undiagenetic clay, gray, and variegated glutenite layers. It is in unconformity contact with the underlying formation.Neogene (N): light grayish-yellow and maroon mudstone and argillaceous siltstone interbed; the bottom is a variegated fine conglomerate. It is in pseudo-conformity contact with the underlying formation.Paleogene (E): the middle and upper parts are green-gray and reddish-brown mudstone intercalated with argillaceous siltstone; the lower part is reddish-brown, variegated argillaceous siltstone, siltstone, fine sandstone, and fine conglomerate. It is in unconformity contact with the underlying formation.Cretaceous Tugulu Group (K1tg): the upper part is gigantic thick brown mudstone and brownish-gray sandy mudstone; the bottom is a gigantic thick gray conglomerate, sandstone, and conglomerate. It is in unconformity contact with the underlying formation.Jurassic Qigu Formation (J3q): huge thick layers of purple-red and brown-red mudstone intercalated with thin layers of gray fine sandstone and siltstone. It is in integrated contact with the underlying formation. This formation is missing in the Xiquan block.Toutunhe Formation (J2t): medium-thick layers of gray, gray-green, brown fine sandstone, siltstone, and medium-thick layers of brown, and light-purple mudstone with slightly equal thickness interbeds. It is in unconformity contact with the underlying formation. This formation is missing in the Xiquan block.Sangonghe Formation (J1s): medium-thick layers of gray, gray-white sandstone, siltstone interbed with gray, gray-green mudstone, and sandy mudstone. It is in integrated contact with the underlying formation. This formation is missing in the Xiquan block.Badaowan Formation (J1b): the upper part is thick gray, dark gray sandy mudstone, and the mudstone is sandwiched with thin coal lines or coal seams; the middle and the lower part is a huge thick layer of fine sandstone, and the bottom is gray sandy mudstone and gray sandstone interbedded. It is in unconformity contact with the underlying strata. This formation is missing in the Xiquan block.Triassic Huangshanjie Formation (T3h): mainly gray and green mudstone and argillaceous siltstone, interspersed with gray fine sandstone and siltstone. Integrate contact with the underlying formation. This formation is missing in the Xiquan block.Karamay Formation (T2k): medium and thick layers of variegated mudstone, siltstone, and fine sandstone are interbedded with unequal thickness. It is in integrated contact with the underlying formation. This formation is missing in the Xiquan block.Shaofangou Formation (T1s): the upper part is dominated by thick-to-thick layers of brown-gray, yellow-brown mudstone, and sandy mudstone, interspersed with thin-medium-thick layers of brown-gray, yellow-brown argillaceous siltstone, and medium sandstone; the lower part is gray siltstone, medium sandstone, gravel-bearing siltstone, and tawny mudstone are interbedded with unequal thickness. It is in integrated contact with the underlying formation.Jiucaiyuan Formation (T1j): the middle and upper parts are a set of huge dark brown pebble-bearing mudstone, dark brown mudstone, and sandy mudstone; the lower part is thick to very thick gray and dark gray mudstone, with medium-thick layers of brown-gray and gray mudstone and muddy siltstone. It is in unconformity contact with the underlying formation.Permian Wutonggou Formation (P3wt): it is an interbed of dark gray, gray-brown mudstone and light gray fine sandstone, siltstone, and pebbly anisotropic sandstone. It is in unconformity contact with the underlying formation.Pingdiquan Formation (P2p): it is an interbed of dark gray, gray-brown mudstone and light gray fine sandstone, siltstone, and pebbled anisotropic sandstone. It is in unconformable contact with the underlying strata. This formation is missing in the Xiquan block.Carboniferous (C2b): the Carboniferous Batamayineshan Formation is mainly developed, and its lithological distribution is relatively complex, including both pyroclastic rocks and volcanic lava. According to data such as drilling coring and cuttings logging, the lithology of pyroclastic rocks is mainly volcanic breccia, breccia tuff, and tuff; the lithology of volcanic lava is mainly andesite.


### 2.2. Experimental Materials and Equipment

Experimental materials: several cores were obtained from the drilling of the Carboniferous Reservoir in the Xiquan block of the Beisantai Oilfield, and a large number of core samples were made and labeled for standby [19, 20].Experimental instruments used were X-ray diffractometer (RigakuD/Max-2500), ZHM-1B grinding machine (ZHM-2), sample press (ZHY-505), rock porosity casting instrument (JS-5), Quattro ESEM scanning electron microscope (Thermo Scientific), full diameter core permeability tester (DYX-1), core cathodoluminescence electron microscope (CL8200 MK5), rock cutter (DQ1-6), automatic specific surface area and pore size distribution tester (MiniX-1), Zeiss Primotech polarizing microscope, core holder, etc. [21, 22].Auxiliary materials include emery, epoxy resin, curing agent, dyeing agent, and fir glue.


## 2.1. Regional Stratigraphic Characteristics

From the top to bottom, the drilled strata in Beisantai uplift include Quaternary, Neogene, and Paleogene formations, Cretaceous Tugulu Group, Jurassic Qigu Formation, Toutunhe Formation, Sangonghe Formation, Badaowan Formation, Triassic Huangshanjie Formation, Karamay Formation, Shaofangou Formation, Jiucaiyuan Formation, Permian Wutonggou Formation, Pingdiquan Formation, and Carboniferous Formation. The missing strata from the top to bottom are Jurassic Kalaza Formation, Xishanyao Formation, Triassic Haojiagou Formation, and Permian Jiangjunmiao Formation (Table1).Table 1
Brief stratigraphic table of Xiquan block.
FormationSymbolThickness mBrief description of lithology and lithofaciesTieSystemGroupQuaternary——Q410∼440Mainly gray-yellow clay, mixed with colored gravel and sandy gravelNeogene——N738∼933The middle and upper parts are mainly composed of gray-yellow mudstone, sandy mudstone, and gravel-bearing mudstone with gypsum mudstone and argillaceous siltstone, and the lower part is mainly composed of gray-yellow mudstone, sandy mudstone, and sandy mudstone mixed with brown-gray mudstone and small mudstone conglomeratePaleogene——E115∼227Red-brown, gray mudstone, and sandy mudstone are mainly intercalated with argillaceous siltstone, siltstone, gravel-bearing siltstone, and gluteniteCretaceousLowerTugu LuqunK1tg275∼441The middle and upper parts are mainly composed of brown-gray, gray-brown mudstone, and sandy mudstone interbedded with brown-gray argillaceous siltstone; the lower part is interbedded with gray and brown argillaceous siltstone, siltstone and brown and gray silty mudstoneTriassicLowerShaofanggou groupT1s0∼117Red-brown, gray-brown mudstone, and sandy mudstone are mainly mixed with gray, brown-gray, variegated gravel-bearing fine sandstone, gravel-bearing medium sandstone, and gluteniteJiucai Garden groupT1j42∼169Red-brown, gray-brown mudstone, and sandy mudstone are mainly mixed with gray, brown-gray, variegated gravel-bearing fine sandstone, gravel-bearing medium sandstone, and glutenitePermianUpperWutonggou FormationP3wt8∼136Taupe, gray mudstone, silty mudstone, and even gray argillaceous siltstoneCarboniferousUpperBatamayC2b48∼907Gray, brown-gray, and brown volcanic breccia with green-gray, gray-green, gray-black andesite, tuff, and carbonaceous mudstoneThe lithology and contact relationship are as follows [18]:Quaternary (Q): yellow, khaki undiagenetic clay, gray, and variegated glutenite layers. It is in unconformity contact with the underlying formation.Neogene (N): light grayish-yellow and maroon mudstone and argillaceous siltstone interbed; the bottom is a variegated fine conglomerate. It is in pseudo-conformity contact with the underlying formation.Paleogene (E): the middle and upper parts are green-gray and reddish-brown mudstone intercalated with argillaceous siltstone; the lower part is reddish-brown, variegated argillaceous siltstone, siltstone, fine sandstone, and fine conglomerate. It is in unconformity contact with the underlying formation.Cretaceous Tugulu Group (K1tg): the upper part is gigantic thick brown mudstone and brownish-gray sandy mudstone; the bottom is a gigantic thick gray conglomerate, sandstone, and conglomerate. It is in unconformity contact with the underlying formation.Jurassic Qigu Formation (J3q): huge thick layers of purple-red and brown-red mudstone intercalated with thin layers of gray fine sandstone and siltstone. It is in integrated contact with the underlying formation. This formation is missing in the Xiquan block.Toutunhe Formation (J2t): medium-thick layers of gray, gray-green, brown fine sandstone, siltstone, and medium-thick layers of brown, and light-purple mudstone with slightly equal thickness interbeds. It is in unconformity contact with the underlying formation. This formation is missing in the Xiquan block.Sangonghe Formation (J1s): medium-thick layers of gray, gray-white sandstone, siltstone interbed with gray, gray-green mudstone, and sandy mudstone. It is in integrated contact with the underlying formation. This formation is missing in the Xiquan block.Badaowan Formation (J1b): the upper part is thick gray, dark gray sandy mudstone, and the mudstone is sandwiched with thin coal lines or coal seams; the middle and the lower part is a huge thick layer of fine sandstone, and the bottom is gray sandy mudstone and gray sandstone interbedded. It is in unconformity contact with the underlying strata. This formation is missing in the Xiquan block.Triassic Huangshanjie Formation (T3h): mainly gray and green mudstone and argillaceous siltstone, interspersed with gray fine sandstone and siltstone. Integrate contact with the underlying formation. This formation is missing in the Xiquan block.Karamay Formation (T2k): medium and thick layers of variegated mudstone, siltstone, and fine sandstone are interbedded with unequal thickness. It is in integrated contact with the underlying formation. This formation is missing in the Xiquan block.Shaofangou Formation (T1s): the upper part is dominated by thick-to-thick layers of brown-gray, yellow-brown mudstone, and sandy mudstone, interspersed with thin-medium-thick layers of brown-gray, yellow-brown argillaceous siltstone, and medium sandstone; the lower part is gray siltstone, medium sandstone, gravel-bearing siltstone, and tawny mudstone are interbedded with unequal thickness. It is in integrated contact with the underlying formation.Jiucaiyuan Formation (T1j): the middle and upper parts are a set of huge dark brown pebble-bearing mudstone, dark brown mudstone, and sandy mudstone; the lower part is thick to very thick gray and dark gray mudstone, with medium-thick layers of brown-gray and gray mudstone and muddy siltstone. It is in unconformity contact with the underlying formation.Permian Wutonggou Formation (P3wt): it is an interbed of dark gray, gray-brown mudstone and light gray fine sandstone, siltstone, and pebbly anisotropic sandstone. It is in unconformity contact with the underlying formation.Pingdiquan Formation (P2p): it is an interbed of dark gray, gray-brown mudstone and light gray fine sandstone, siltstone, and pebbled anisotropic sandstone. It is in unconformable contact with the underlying strata. This formation is missing in the Xiquan block.Carboniferous (C2b): the Carboniferous Batamayineshan Formation is mainly developed, and its lithological distribution is relatively complex, including both pyroclastic rocks and volcanic lava. According to data such as drilling coring and cuttings logging, the lithology of pyroclastic rocks is mainly volcanic breccia, breccia tuff, and tuff; the lithology of volcanic lava is mainly andesite.


## 2.2. Experimental Materials and Equipment

Experimental materials: several cores were obtained from the drilling of the Carboniferous Reservoir in the Xiquan block of the Beisantai Oilfield, and a large number of core samples were made and labeled for standby [19, 20].Experimental instruments used were X-ray diffractometer (RigakuD/Max-2500), ZHM-1B grinding machine (ZHM-2), sample press (ZHY-505), rock porosity casting instrument (JS-5), Quattro ESEM scanning electron microscope (Thermo Scientific), full diameter core permeability tester (DYX-1), core cathodoluminescence electron microscope (CL8200 MK5), rock cutter (DQ1-6), automatic specific surface area and pore size distribution tester (MiniX-1), Zeiss Primotech polarizing microscope, core holder, etc. [21, 22].Auxiliary materials include emery, epoxy resin, curing agent, dyeing agent, and fir glue.


## 3. Results

### 3.1. Rock Types and Division

Volcanic rock refers to the rock formed by the gradual condensation of magma deep underground, driven by the tectonic movement of the crust, rising to the upper part of the crust or ejecting from the crust along the fragile zone of the crust. Volcanic rocks can erupt from the surface, or they may intrude into shallower rock necks, rock walls, bedrock, and other volcanic channel facies or subvolcanic lithofacies rocks. Volcanic rocks are relatively poorly crystalline, very poorly identifiable minerals, mostly cryptocrystalline, and vitreous magmatic rocks. Volcanic lava can usually reflect the magma composition better, while the chemical composition of pyroclastic rocks cannot represent the magma composition due to the mixing of foreign debris [23, 24].The types of rocks developed in the Carboniferous stratum in the Xiquan block are pyroclastic rocks and volcanic lava. Among them, pyroclastic rocks include volcanic breccia, breccia tuff (including breccia tuff identified by thin sections, tuffaceous breccia, and breccia vitreous tuff) and tuff [25, 26]. The main rock types are shown in Table 2.Table 2
Rock types in the Xiquan block.
Rock categoryMain rock nameMineral compositionRemarkPyroclastic rocksVolcanic brecciaThe main components of the rock are igneous metamorphic rock blocks with a diameter of 3 mm–10 mm, the lava fragments are mostly tuff, and the glass fragments and whole stones are less.Reservoir rockBreccia tuffThe igneous metamorphic rock blocks with the main composition of 3 mm–5 mm contain lava fragments mostly tuff and fewer glass fragments and whole stones.Reservoir rockTuffThe main components of the rock are volcanic debris and terrigenous debris. The volcanic debris is mainly composed of volcanic ash, wave debris, crystal debris, and a small amount of rock debris, and the particle diameter is less than 2 mm.Nonreservoir rockVolcanic lavaAndesiteThe rock composition is equivalent to diorite, and the phenocrysts are mainly plagioclase and dark minerals (common dark minerals are pyroxene, amphibole, and biotite). The matrix is mainly composed of plagioclase crystallites, pyroxene, chlorite, and andesite glass.Reservoir rock


#### 3.1.1. Pyroclastic Rocks

The Xiquan block Carboniferous pyroclastic rocks are encountered in all wells, and volcanic breccia, breccia tuff, and tuff are developed.(1)Volcanic Breccia. Brown-gray, maroon, and gray volcanic breccia are composed of volcanic breccia (breccia particle size 3–64 mm, content >50%) and are tuffaceous with volcanic breccia structure. The volcanic breccia is mainly composed of andesite debris, tuff debris, and a small amount of feldspar. The crystal debris is composed of feldspar, quartz, and a small amount of pyroxene. The lithology of the 2161.7–2164.57 m coring section in Well Xiquan 104 is volcanic breccia (Figure 1). By observing the rock slices of all the coring wells in the whole area, four kinds of slices, namely, tuffaceous volcanic breccia, crystalline detrital volcanic breccia, volcanic breccia, and andesite crystalline detrital volcanic breccia, are observed. The lithology is classified as volcanic breccia (Figure 2).Figure 1
Volcanic breccia core samples of Well Xiquan 104 (2052.6 m–2052.9 m).Figure 2
Cast thin sections of Carboniferous core samples in the Xiquan block: (a) tuffaceous volcanic breccia; (b) crystalline detrital volcanic breccia.
(a)(b)(2)Breccia Tuff. Gray, brown-gray breccia tuff (tuff grade particle size <2 mm, content >50%, breccia particle size 2–64 mm, content <50%), tuff material, and volcanic breccia are cemented by pozzolan, with a breccia tuff structure. The tuff material is mainly glass debris, crystal debris, and a small amount of rock debris. The glass debris is distributed in the shape of chicken bones or cambers. Some of them have plastic deformation glass debris, and some have been altered into the zeolite. The crystal debris is mainly quartz and feldspar, and the debris is mainly andesite and tuff debris. The volcanic breccia is mainly andesite and tuff; the cement is dusty volcanic ash and chlorite, and most of them have been devitrified into fine felsic minerals. The lithology of the coring section at 2219.5–2220.0 m in Well Xiquan 3047 is breccia tuff (Figure 3). Through the observation of rock slices from all coring wells in the whole area, andesite breccia lava and almond-shaped cuttings are condensed. Twenty-three thin lithologies such as lime lava are classified as breccia tuff (Figure 4).Figure 3
Breccia tuff core samples of 2219.5–2220.0 m in Well Xiquan 3047.Figure 4
Cast thin sections of Carboniferous core samples in the Xiquan block: (a) andesite breccia lava; (b) almond-shaped detrital tuff lava.
(a)(b)(3)Tuff. Gray and dark gray tuff (tuff grade particle size < 2 mm, content >50%) are made of tuff material cemented by pozzolan. The rock is mainly composed of chicken bone-like glass debris, feldspar crystal debris, rock debris, and volcanic ash. Most of the cement has been devitrified into fine felsic minerals. The lithology of Well Xiquan 103 in the 2156.35–2156.95 m coring section is tuff (Figure 5). Through the observation of the rock slices of all the coring wells in the whole area, the glass debris fused tuff, glass debris tuff, and porosity glass debris tuff are observed. Three thin-sliced lithologies are classified as tuffs (Figure 6).Figure 5
Tuff core samples of 2156.35–2156.95 m in Well Xiquan 103.Figure 6
Cast thin sections of Carboniferous core samples in Xiquan block: (a) vitreous tuff; (b) vitreous sintered tuff.
(a)(b)


#### 3.1.2. Volcanic Lava

Andesite is gray, brownish-gray, with interwoven structure, porphyritic structure, massive structure, and almond-shaped structure. In an oriented arrangement, glassy chlorite and granular magnetite are distributed among the thin lath like plagioclases. Near the top of the lava layer, almonds and stomata are more developed, almond structures are elliptical and irregular, mostly covered with siliceous and chlorite filling. The lithology of the 2267.3–2267.6 m coring section in Well Xiquan 103 is andesite (Figure7). By observing the rock slices of all the coring wells in the whole area, two kinds of thin rocks, andesite breccia lava and almond-shaped andesite, are found. It is classified as tuff (Figure 8).Figure 7
Andesite core samples of 2267.3 m–2267.6 m in Well Xiquan 103.Figure 8
Cast thin sections of Carboniferous core samples in the Xiquan block: (a) andesite breccia lava; (b) almond-shaped andesite.
(a)(b)


### 3.2. Characteristics of Volcanic Lithofacies

A clear volcanic eruption pattern is the basis for the establishment of volcanic institutions, and it is also closely related to the lithofacies distribution of volcanic rocks. Generally speaking, there are three types of volcanic eruptions: central, fractured, and mixed.The central vent eruption is a kind of eruption of magma along the neck-shaped pipeline, and the eruption channel is point-shaped on the plane, also known as the point eruption. It is characterized by an obvious crater, the formation of a volcanic cone, and strong eruption energy. The interior of the volcano is characterized by a chaotic mound-like structure in the seismic profile, usually accompanied by the appearance of a large amount of pyroclastic material.Fissure Eruption. Large fractures (fissures) or groups of fractures where magma rises in one direction and erupts to the surface. It is characterized by weak eruption energy, often beading along the fissure zone, showing no obvious crater, weak eruption energy, good continuity of the event axis on the seismic section, moderate strong reflection, and magma flowing out along the fissure and then along the ground.The mixed eruption mainly includes the mixed eruption form of central vent eruption and fissure eruption. The Junggar Basin is located in the carboniferous period of the three plates of Tarim, Kazakhstan, and Northwest China. It has experienced three tectonic stages of plate (continent) block subduction-collision, oceanic closure orogeny, postcollision extensional rift environment, and overall uplift and denudation. Carboniferous volcanic rocks are developed in the Xiquan block of the Zhundong uplift belt, and the main rock types are volcanic lava-type andesite, pyroclastic rock type volcanic breccia, breccia tuff, and tuff. The volcanic structures in this area have been damaged to varying degrees due to denudation and differential tectonic uplift, making it difficult to identify them.At present, there is no unified standard for lithofacies division of volcanic rocks, and the division scheme has regional characteristics. In this paper, the study area has the characteristics of fractured eruption in the main part and the central eruption in the east. Therefore, a comprehensive plan for the division of Carboniferous volcanic lithofacies in the Xiquan block is formulated, as shown in Table3.Table 3
Classification and characteristics of Carboniferous volcanic lithofacies in the Xiquan block.
Volcanic lithofaciesSubphaseGenesis mechanism and division markRock typeStructureVolcanic channel facies—The lava flow stagnates and fills the volcanic channel, the crater collapses, and fillsMainly andesiteGlassy structure, fine-grained structure, spotted structure, etc.Overflow phase—The magma containing crystal exudates and contemporaneous breccia flows along the surface under the combined action of the subsequent ejecta and its own gravityMainly andesiteGlassy structure, fine-grained structure, spotted structure, etc.Outburst phaseHeat base and wave subphaseThe gas-solid-liquid multiphase turbidity flow system under the action of a gas jet acts as a suspended mass transport near the surface under neutral action, and the debris-carrying vapor flowsMainly crystalline tuff, etc.Pyroclastic structureEmpty subphaseAerodynamic solid and plastic ejecta (under the influence of wind) in free fallMainly volcanic breccia, breccia-bearing tuff, breccia crystalline tuff, crystalline tuff, etc.(Mainly crystalline tuff structure)Volcanic sedimentary facies—Pyroclastics are transformed by water flowCarbonaceous mudstone, tuff, etc.Agglomerated structure, breccia structure, tuff structure, etc.


### 3.3. Reservoir Physical Properties and Reservoir Space Types

#### 3.3.1. Reservoir Physical Property Statistics

The physical property analysis data of 5 coring wells are statistically analyzed, and the petrophysical properties of the Carboniferous volcanic rocks in the Xiquan block are summarized. 406 samples are analyzed in the breccia tuff reservoir, with the tested porosity of 6.3–32% and the average porosity of 19.6%; the tested permeability is 0.01–101.48 mD, and the median permeability is 0.307 mD. 308 samples are analyzed in the oil layer, the tested permeability is distributed in the range of 15.2–32.1%, and the average porosity is 20.8%; the tested permeability is between 0.01 and 101.48 mD, and the median permeability is 0.33 mD.29 samples were analyzed in the volcanic breccia reservoir, with a test porosity of 6.6–22.9% and an average porosity of 16.2%. 25 samples of porosity were analyzed in the oil layer, with a porosity distribution of 13.2–22.9% and an average porosity of 18.4%. The permeability of 29 samples ranges from 0.01 to 1.848 mD, and the median permeability is 0.209 mD. For the 25 samples in the oil layer, the permeability is mainly distributed between 0.103–1.848 mD, and the median permeability is 0.225 mD.Generally speaking, the Carboniferous reservoirs in the Xiquan block belong to medium-porosity and ultralow permeability reservoirs.


#### 3.3.2. Types of Storage Space

The reservoir space of volcanic rocks can be divided into three types: primary pores, secondary pores, and fractures. The primary pores are mainly stomata, intra-almond pores, intracrystalline pores, and matrix pores, and secondary pores are mainly dissolved pores, intercrystalline micropores, and fractures. The main ones are structural fractures, dissolution fractures, and condensation shrinkage fractures (Table4).Table 4
Pore types of volcanic rock reservoirs.
Pore typeCauseDistribution characteristicsFilling situationPrimary poresStomataGas expansion and contractionPartially covered by chlorite and turbiditePartially covered by chlorite and turbiditeAmygdala, inner holeRemaining pores in amygdala formed by mineral-filled stomata that are not filledCalcite fillCalcite fillIntracrystalline pores, matrix poresDiagenetic compaction among pyroclastic particlesPartially filled with turbidite, followed by chlorite and calcitePartially filled with turbidite, followed by chlorite and calciteSecondary poresDissolution poresPores formed by dissolution and dissolution under the action of hydrothermal fluidFilling, poor connectivity, little contribution to reservoir physical propertiesFilling, poor connectivity, little contribution to reservoir physical propertiesIntergranular poresTiny pores formed by clay mineralization, recrystallization, and dissolutionUnfilled and oilyUnfilled and oilyCrackTectonic seamGround stressHalf-filledHalf-filledDissolution seamWeathering leaching erosionHalf-filled and communicating matrix poresHalf-filled and communicating matrix poresCondensation shrinkage jointsCold shrink differential movementCalcite full to half fillingCalcite full to half fillingAccording to the analyses of casting and fluorescent thin section data, the pore types of Carboniferous reservoirs in the Xiquan block are mainly intragranular dissolved pores, followed by phenocryst dissolved pores, pores, microcracks, and a small amount of intergranular dissolved pores.Intragranular dissolution pores generally refer to the pores formed after the soluble components in crystal cuttings. The pulp cuttings are dissolved, mainly including dissolved pores in crystal cuttings, dissolution pores in cuttings, and dissolution pores in pulp cuttings. Dissolution pores in crystal debris are mainly found in tuff and crystal debris tuff, which are formed by dissolution in the diagenetic stage and leaching dissolution after diagenesis and are generally developed along the edge of crystal debris and cleavage fractures. Dissolution pores in cuttings are mainly found in volcanic breccias, volcanic agglomerates, and tuffs, which are the spaces left by the soluble components in the cuttings, such as feldspar, after being dissolved. Dissolution pores in the magma are mainly found in the fused tuff or breccia lava developed in the magma. The pore space formed by the dissolution of the soluble components in the magma is generally distributed along the lava strip.The composition of volcanic rocks is composed of phenocrysts and matrix, and phenocryst dissolution pores refer to the pores formed by the dissolution of relatively large lava phenocrysts and soluble components in pyroclastics. The pore shape is irregular and is mainly developed in rhyolite, fused breccia, breccia lava, and volcanic breccia, followed by dacite, tuff, and volcanic clastic rock, including the dissolution pores in the chips, the dissolution pores in the crystal chips, and the mold holes.The stomata are the pore spaces left after the volatile components escape due to the reduced pressure when the molten lava is ejected from the surface. The characteristics of the stomata are related to the composition and volatile content of the lava. The pore size is mostly between 0.1 and 5 mm, and it is mostly developed on the top of the overflow phase.Microfractures include lithologic fractures controlled by sedimentation, structural fractures controlled by tectonic action, shrinkage fractures, grain-edge fractures, and elongated pores. In thin sections, real microfractures often have a certain extension direction and may gradually narrow or disappear with the increase of extension distance. Some microfractures formed in the early stage may deform, fully fill, or disappear under the influence of diagenesis.


## 3.1. Rock Types and Division

Volcanic rock refers to the rock formed by the gradual condensation of magma deep underground, driven by the tectonic movement of the crust, rising to the upper part of the crust or ejecting from the crust along the fragile zone of the crust. Volcanic rocks can erupt from the surface, or they may intrude into shallower rock necks, rock walls, bedrock, and other volcanic channel facies or subvolcanic lithofacies rocks. Volcanic rocks are relatively poorly crystalline, very poorly identifiable minerals, mostly cryptocrystalline, and vitreous magmatic rocks. Volcanic lava can usually reflect the magma composition better, while the chemical composition of pyroclastic rocks cannot represent the magma composition due to the mixing of foreign debris [23, 24].The types of rocks developed in the Carboniferous stratum in the Xiquan block are pyroclastic rocks and volcanic lava. Among them, pyroclastic rocks include volcanic breccia, breccia tuff (including breccia tuff identified by thin sections, tuffaceous breccia, and breccia vitreous tuff) and tuff [25, 26]. The main rock types are shown in Table 2.Table 2
Rock types in the Xiquan block.
Rock categoryMain rock nameMineral compositionRemarkPyroclastic rocksVolcanic brecciaThe main components of the rock are igneous metamorphic rock blocks with a diameter of 3 mm–10 mm, the lava fragments are mostly tuff, and the glass fragments and whole stones are less.Reservoir rockBreccia tuffThe igneous metamorphic rock blocks with the main composition of 3 mm–5 mm contain lava fragments mostly tuff and fewer glass fragments and whole stones.Reservoir rockTuffThe main components of the rock are volcanic debris and terrigenous debris. The volcanic debris is mainly composed of volcanic ash, wave debris, crystal debris, and a small amount of rock debris, and the particle diameter is less than 2 mm.Nonreservoir rockVolcanic lavaAndesiteThe rock composition is equivalent to diorite, and the phenocrysts are mainly plagioclase and dark minerals (common dark minerals are pyroxene, amphibole, and biotite). The matrix is mainly composed of plagioclase crystallites, pyroxene, chlorite, and andesite glass.Reservoir rock


### 3.1.1. Pyroclastic Rocks

The Xiquan block Carboniferous pyroclastic rocks are encountered in all wells, and volcanic breccia, breccia tuff, and tuff are developed.(1)Volcanic Breccia. Brown-gray, maroon, and gray volcanic breccia are composed of volcanic breccia (breccia particle size 3–64 mm, content >50%) and are tuffaceous with volcanic breccia structure. The volcanic breccia is mainly composed of andesite debris, tuff debris, and a small amount of feldspar. The crystal debris is composed of feldspar, quartz, and a small amount of pyroxene. The lithology of the 2161.7–2164.57 m coring section in Well Xiquan 104 is volcanic breccia (Figure 1). By observing the rock slices of all the coring wells in the whole area, four kinds of slices, namely, tuffaceous volcanic breccia, crystalline detrital volcanic breccia, volcanic breccia, and andesite crystalline detrital volcanic breccia, are observed. The lithology is classified as volcanic breccia (Figure 2).Figure 1
Volcanic breccia core samples of Well Xiquan 104 (2052.6 m–2052.9 m).Figure 2
Cast thin sections of Carboniferous core samples in the Xiquan block: (a) tuffaceous volcanic breccia; (b) crystalline detrital volcanic breccia.
(a)(b)(2)Breccia Tuff. Gray, brown-gray breccia tuff (tuff grade particle size <2 mm, content >50%, breccia particle size 2–64 mm, content <50%), tuff material, and volcanic breccia are cemented by pozzolan, with a breccia tuff structure. The tuff material is mainly glass debris, crystal debris, and a small amount of rock debris. The glass debris is distributed in the shape of chicken bones or cambers. Some of them have plastic deformation glass debris, and some have been altered into the zeolite. The crystal debris is mainly quartz and feldspar, and the debris is mainly andesite and tuff debris. The volcanic breccia is mainly andesite and tuff; the cement is dusty volcanic ash and chlorite, and most of them have been devitrified into fine felsic minerals. The lithology of the coring section at 2219.5–2220.0 m in Well Xiquan 3047 is breccia tuff (Figure 3). Through the observation of rock slices from all coring wells in the whole area, andesite breccia lava and almond-shaped cuttings are condensed. Twenty-three thin lithologies such as lime lava are classified as breccia tuff (Figure 4).Figure 3
Breccia tuff core samples of 2219.5–2220.0 m in Well Xiquan 3047.Figure 4
Cast thin sections of Carboniferous core samples in the Xiquan block: (a) andesite breccia lava; (b) almond-shaped detrital tuff lava.
(a)(b)(3)Tuff. Gray and dark gray tuff (tuff grade particle size < 2 mm, content >50%) are made of tuff material cemented by pozzolan. The rock is mainly composed of chicken bone-like glass debris, feldspar crystal debris, rock debris, and volcanic ash. Most of the cement has been devitrified into fine felsic minerals. The lithology of Well Xiquan 103 in the 2156.35–2156.95 m coring section is tuff (Figure 5). Through the observation of the rock slices of all the coring wells in the whole area, the glass debris fused tuff, glass debris tuff, and porosity glass debris tuff are observed. Three thin-sliced lithologies are classified as tuffs (Figure 6).Figure 5
Tuff core samples of 2156.35–2156.95 m in Well Xiquan 103.Figure 6
Cast thin sections of Carboniferous core samples in Xiquan block: (a) vitreous tuff; (b) vitreous sintered tuff.
(a)(b)


### 3.1.2. Volcanic Lava

Andesite is gray, brownish-gray, with interwoven structure, porphyritic structure, massive structure, and almond-shaped structure. In an oriented arrangement, glassy chlorite and granular magnetite are distributed among the thin lath like plagioclases. Near the top of the lava layer, almonds and stomata are more developed, almond structures are elliptical and irregular, mostly covered with siliceous and chlorite filling. The lithology of the 2267.3–2267.6 m coring section in Well Xiquan 103 is andesite (Figure7). By observing the rock slices of all the coring wells in the whole area, two kinds of thin rocks, andesite breccia lava and almond-shaped andesite, are found. It is classified as tuff (Figure 8).Figure 7
Andesite core samples of 2267.3 m–2267.6 m in Well Xiquan 103.Figure 8
Cast thin sections of Carboniferous core samples in the Xiquan block: (a) andesite breccia lava; (b) almond-shaped andesite.
(a)(b)


## 3.1.1. Pyroclastic Rocks

The Xiquan block Carboniferous pyroclastic rocks are encountered in all wells, and volcanic breccia, breccia tuff, and tuff are developed.(1)Volcanic Breccia. Brown-gray, maroon, and gray volcanic breccia are composed of volcanic breccia (breccia particle size 3–64 mm, content >50%) and are tuffaceous with volcanic breccia structure. The volcanic breccia is mainly composed of andesite debris, tuff debris, and a small amount of feldspar. The crystal debris is composed of feldspar, quartz, and a small amount of pyroxene. The lithology of the 2161.7–2164.57 m coring section in Well Xiquan 104 is volcanic breccia (Figure 1). By observing the rock slices of all the coring wells in the whole area, four kinds of slices, namely, tuffaceous volcanic breccia, crystalline detrital volcanic breccia, volcanic breccia, and andesite crystalline detrital volcanic breccia, are observed. The lithology is classified as volcanic breccia (Figure 2).Figure 1
Volcanic breccia core samples of Well Xiquan 104 (2052.6 m–2052.9 m).Figure 2
Cast thin sections of Carboniferous core samples in the Xiquan block: (a) tuffaceous volcanic breccia; (b) crystalline detrital volcanic breccia.
(a)(b)(2)Breccia Tuff. Gray, brown-gray breccia tuff (tuff grade particle size <2 mm, content >50%, breccia particle size 2–64 mm, content <50%), tuff material, and volcanic breccia are cemented by pozzolan, with a breccia tuff structure. The tuff material is mainly glass debris, crystal debris, and a small amount of rock debris. The glass debris is distributed in the shape of chicken bones or cambers. Some of them have plastic deformation glass debris, and some have been altered into the zeolite. The crystal debris is mainly quartz and feldspar, and the debris is mainly andesite and tuff debris. The volcanic breccia is mainly andesite and tuff; the cement is dusty volcanic ash and chlorite, and most of them have been devitrified into fine felsic minerals. The lithology of the coring section at 2219.5–2220.0 m in Well Xiquan 3047 is breccia tuff (Figure 3). Through the observation of rock slices from all coring wells in the whole area, andesite breccia lava and almond-shaped cuttings are condensed. Twenty-three thin lithologies such as lime lava are classified as breccia tuff (Figure 4).Figure 3
Breccia tuff core samples of 2219.5–2220.0 m in Well Xiquan 3047.Figure 4
Cast thin sections of Carboniferous core samples in the Xiquan block: (a) andesite breccia lava; (b) almond-shaped detrital tuff lava.
(a)(b)(3)Tuff. Gray and dark gray tuff (tuff grade particle size < 2 mm, content >50%) are made of tuff material cemented by pozzolan. The rock is mainly composed of chicken bone-like glass debris, feldspar crystal debris, rock debris, and volcanic ash. Most of the cement has been devitrified into fine felsic minerals. The lithology of Well Xiquan 103 in the 2156.35–2156.95 m coring section is tuff (Figure 5). Through the observation of the rock slices of all the coring wells in the whole area, the glass debris fused tuff, glass debris tuff, and porosity glass debris tuff are observed. Three thin-sliced lithologies are classified as tuffs (Figure 6).Figure 5
Tuff core samples of 2156.35–2156.95 m in Well Xiquan 103.Figure 6
Cast thin sections of Carboniferous core samples in Xiquan block: (a) vitreous tuff; (b) vitreous sintered tuff.
(a)(b)


## 3.1.2. Volcanic Lava

Andesite is gray, brownish-gray, with interwoven structure, porphyritic structure, massive structure, and almond-shaped structure. In an oriented arrangement, glassy chlorite and granular magnetite are distributed among the thin lath like plagioclases. Near the top of the lava layer, almonds and stomata are more developed, almond structures are elliptical and irregular, mostly covered with siliceous and chlorite filling. The lithology of the 2267.3–2267.6 m coring section in Well Xiquan 103 is andesite (Figure7). By observing the rock slices of all the coring wells in the whole area, two kinds of thin rocks, andesite breccia lava and almond-shaped andesite, are found. It is classified as tuff (Figure 8).Figure 7
Andesite core samples of 2267.3 m–2267.6 m in Well Xiquan 103.Figure 8
Cast thin sections of Carboniferous core samples in the Xiquan block: (a) andesite breccia lava; (b) almond-shaped andesite.
(a)(b)


## 3.2. Characteristics of Volcanic Lithofacies

A clear volcanic eruption pattern is the basis for the establishment of volcanic institutions, and it is also closely related to the lithofacies distribution of volcanic rocks. Generally speaking, there are three types of volcanic eruptions: central, fractured, and mixed.The central vent eruption is a kind of eruption of magma along the neck-shaped pipeline, and the eruption channel is point-shaped on the plane, also known as the point eruption. It is characterized by an obvious crater, the formation of a volcanic cone, and strong eruption energy. The interior of the volcano is characterized by a chaotic mound-like structure in the seismic profile, usually accompanied by the appearance of a large amount of pyroclastic material.Fissure Eruption. Large fractures (fissures) or groups of fractures where magma rises in one direction and erupts to the surface. It is characterized by weak eruption energy, often beading along the fissure zone, showing no obvious crater, weak eruption energy, good continuity of the event axis on the seismic section, moderate strong reflection, and magma flowing out along the fissure and then along the ground.The mixed eruption mainly includes the mixed eruption form of central vent eruption and fissure eruption. The Junggar Basin is located in the carboniferous period of the three plates of Tarim, Kazakhstan, and Northwest China. It has experienced three tectonic stages of plate (continent) block subduction-collision, oceanic closure orogeny, postcollision extensional rift environment, and overall uplift and denudation. Carboniferous volcanic rocks are developed in the Xiquan block of the Zhundong uplift belt, and the main rock types are volcanic lava-type andesite, pyroclastic rock type volcanic breccia, breccia tuff, and tuff. The volcanic structures in this area have been damaged to varying degrees due to denudation and differential tectonic uplift, making it difficult to identify them.At present, there is no unified standard for lithofacies division of volcanic rocks, and the division scheme has regional characteristics. In this paper, the study area has the characteristics of fractured eruption in the main part and the central eruption in the east. Therefore, a comprehensive plan for the division of Carboniferous volcanic lithofacies in the Xiquan block is formulated, as shown in Table3.Table 3
Classification and characteristics of Carboniferous volcanic lithofacies in the Xiquan block.
Volcanic lithofaciesSubphaseGenesis mechanism and division markRock typeStructureVolcanic channel facies—The lava flow stagnates and fills the volcanic channel, the crater collapses, and fillsMainly andesiteGlassy structure, fine-grained structure, spotted structure, etc.Overflow phase—The magma containing crystal exudates and contemporaneous breccia flows along the surface under the combined action of the subsequent ejecta and its own gravityMainly andesiteGlassy structure, fine-grained structure, spotted structure, etc.Outburst phaseHeat base and wave subphaseThe gas-solid-liquid multiphase turbidity flow system under the action of a gas jet acts as a suspended mass transport near the surface under neutral action, and the debris-carrying vapor flowsMainly crystalline tuff, etc.Pyroclastic structureEmpty subphaseAerodynamic solid and plastic ejecta (under the influence of wind) in free fallMainly volcanic breccia, breccia-bearing tuff, breccia crystalline tuff, crystalline tuff, etc.(Mainly crystalline tuff structure)Volcanic sedimentary facies—Pyroclastics are transformed by water flowCarbonaceous mudstone, tuff, etc.Agglomerated structure, breccia structure, tuff structure, etc.


## 3.3. Reservoir Physical Properties and Reservoir Space Types

### 3.3.1. Reservoir Physical Property Statistics

The physical property analysis data of 5 coring wells are statistically analyzed, and the petrophysical properties of the Carboniferous volcanic rocks in the Xiquan block are summarized. 406 samples are analyzed in the breccia tuff reservoir, with the tested porosity of 6.3–32% and the average porosity of 19.6%; the tested permeability is 0.01–101.48 mD, and the median permeability is 0.307 mD. 308 samples are analyzed in the oil layer, the tested permeability is distributed in the range of 15.2–32.1%, and the average porosity is 20.8%; the tested permeability is between 0.01 and 101.48 mD, and the median permeability is 0.33 mD.29 samples were analyzed in the volcanic breccia reservoir, with a test porosity of 6.6–22.9% and an average porosity of 16.2%. 25 samples of porosity were analyzed in the oil layer, with a porosity distribution of 13.2–22.9% and an average porosity of 18.4%. The permeability of 29 samples ranges from 0.01 to 1.848 mD, and the median permeability is 0.209 mD. For the 25 samples in the oil layer, the permeability is mainly distributed between 0.103–1.848 mD, and the median permeability is 0.225 mD.Generally speaking, the Carboniferous reservoirs in the Xiquan block belong to medium-porosity and ultralow permeability reservoirs.


### 3.3.2. Types of Storage Space

The reservoir space of volcanic rocks can be divided into three types: primary pores, secondary pores, and fractures. The primary pores are mainly stomata, intra-almond pores, intracrystalline pores, and matrix pores, and secondary pores are mainly dissolved pores, intercrystalline micropores, and fractures. The main ones are structural fractures, dissolution fractures, and condensation shrinkage fractures (Table4).Table 4
Pore types of volcanic rock reservoirs.
Pore typeCauseDistribution characteristicsFilling situationPrimary poresStomataGas expansion and contractionPartially covered by chlorite and turbiditePartially covered by chlorite and turbiditeAmygdala, inner holeRemaining pores in amygdala formed by mineral-filled stomata that are not filledCalcite fillCalcite fillIntracrystalline pores, matrix poresDiagenetic compaction among pyroclastic particlesPartially filled with turbidite, followed by chlorite and calcitePartially filled with turbidite, followed by chlorite and calciteSecondary poresDissolution poresPores formed by dissolution and dissolution under the action of hydrothermal fluidFilling, poor connectivity, little contribution to reservoir physical propertiesFilling, poor connectivity, little contribution to reservoir physical propertiesIntergranular poresTiny pores formed by clay mineralization, recrystallization, and dissolutionUnfilled and oilyUnfilled and oilyCrackTectonic seamGround stressHalf-filledHalf-filledDissolution seamWeathering leaching erosionHalf-filled and communicating matrix poresHalf-filled and communicating matrix poresCondensation shrinkage jointsCold shrink differential movementCalcite full to half fillingCalcite full to half fillingAccording to the analyses of casting and fluorescent thin section data, the pore types of Carboniferous reservoirs in the Xiquan block are mainly intragranular dissolved pores, followed by phenocryst dissolved pores, pores, microcracks, and a small amount of intergranular dissolved pores.Intragranular dissolution pores generally refer to the pores formed after the soluble components in crystal cuttings. The pulp cuttings are dissolved, mainly including dissolved pores in crystal cuttings, dissolution pores in cuttings, and dissolution pores in pulp cuttings. Dissolution pores in crystal debris are mainly found in tuff and crystal debris tuff, which are formed by dissolution in the diagenetic stage and leaching dissolution after diagenesis and are generally developed along the edge of crystal debris and cleavage fractures. Dissolution pores in cuttings are mainly found in volcanic breccias, volcanic agglomerates, and tuffs, which are the spaces left by the soluble components in the cuttings, such as feldspar, after being dissolved. Dissolution pores in the magma are mainly found in the fused tuff or breccia lava developed in the magma. The pore space formed by the dissolution of the soluble components in the magma is generally distributed along the lava strip.The composition of volcanic rocks is composed of phenocrysts and matrix, and phenocryst dissolution pores refer to the pores formed by the dissolution of relatively large lava phenocrysts and soluble components in pyroclastics. The pore shape is irregular and is mainly developed in rhyolite, fused breccia, breccia lava, and volcanic breccia, followed by dacite, tuff, and volcanic clastic rock, including the dissolution pores in the chips, the dissolution pores in the crystal chips, and the mold holes.The stomata are the pore spaces left after the volatile components escape due to the reduced pressure when the molten lava is ejected from the surface. The characteristics of the stomata are related to the composition and volatile content of the lava. The pore size is mostly between 0.1 and 5 mm, and it is mostly developed on the top of the overflow phase.Microfractures include lithologic fractures controlled by sedimentation, structural fractures controlled by tectonic action, shrinkage fractures, grain-edge fractures, and elongated pores. In thin sections, real microfractures often have a certain extension direction and may gradually narrow or disappear with the increase of extension distance. Some microfractures formed in the early stage may deform, fully fill, or disappear under the influence of diagenesis.


## 3.3.1. Reservoir Physical Property Statistics

The physical property analysis data of 5 coring wells are statistically analyzed, and the petrophysical properties of the Carboniferous volcanic rocks in the Xiquan block are summarized. 406 samples are analyzed in the breccia tuff reservoir, with the tested porosity of 6.3–32% and the average porosity of 19.6%; the tested permeability is 0.01–101.48 mD, and the median permeability is 0.307 mD. 308 samples are analyzed in the oil layer, the tested permeability is distributed in the range of 15.2–32.1%, and the average porosity is 20.8%; the tested permeability is between 0.01 and 101.48 mD, and the median permeability is 0.33 mD.29 samples were analyzed in the volcanic breccia reservoir, with a test porosity of 6.6–22.9% and an average porosity of 16.2%. 25 samples of porosity were analyzed in the oil layer, with a porosity distribution of 13.2–22.9% and an average porosity of 18.4%. The permeability of 29 samples ranges from 0.01 to 1.848 mD, and the median permeability is 0.209 mD. For the 25 samples in the oil layer, the permeability is mainly distributed between 0.103–1.848 mD, and the median permeability is 0.225 mD.Generally speaking, the Carboniferous reservoirs in the Xiquan block belong to medium-porosity and ultralow permeability reservoirs.


## 3.3.2. Types of Storage Space

The reservoir space of volcanic rocks can be divided into three types: primary pores, secondary pores, and fractures. The primary pores are mainly stomata, intra-almond pores, intracrystalline pores, and matrix pores, and secondary pores are mainly dissolved pores, intercrystalline micropores, and fractures. The main ones are structural fractures, dissolution fractures, and condensation shrinkage fractures (Table4).Table 4
Pore types of volcanic rock reservoirs.
Pore typeCauseDistribution characteristicsFilling situationPrimary poresStomataGas expansion and contractionPartially covered by chlorite and turbiditePartially covered by chlorite and turbiditeAmygdala, inner holeRemaining pores in amygdala formed by mineral-filled stomata that are not filledCalcite fillCalcite fillIntracrystalline pores, matrix poresDiagenetic compaction among pyroclastic particlesPartially filled with turbidite, followed by chlorite and calcitePartially filled with turbidite, followed by chlorite and calciteSecondary poresDissolution poresPores formed by dissolution and dissolution under the action of hydrothermal fluidFilling, poor connectivity, little contribution to reservoir physical propertiesFilling, poor connectivity, little contribution to reservoir physical propertiesIntergranular poresTiny pores formed by clay mineralization, recrystallization, and dissolutionUnfilled and oilyUnfilled and oilyCrackTectonic seamGround stressHalf-filledHalf-filledDissolution seamWeathering leaching erosionHalf-filled and communicating matrix poresHalf-filled and communicating matrix poresCondensation shrinkage jointsCold shrink differential movementCalcite full to half fillingCalcite full to half fillingAccording to the analyses of casting and fluorescent thin section data, the pore types of Carboniferous reservoirs in the Xiquan block are mainly intragranular dissolved pores, followed by phenocryst dissolved pores, pores, microcracks, and a small amount of intergranular dissolved pores.Intragranular dissolution pores generally refer to the pores formed after the soluble components in crystal cuttings. The pulp cuttings are dissolved, mainly including dissolved pores in crystal cuttings, dissolution pores in cuttings, and dissolution pores in pulp cuttings. Dissolution pores in crystal debris are mainly found in tuff and crystal debris tuff, which are formed by dissolution in the diagenetic stage and leaching dissolution after diagenesis and are generally developed along the edge of crystal debris and cleavage fractures. Dissolution pores in cuttings are mainly found in volcanic breccias, volcanic agglomerates, and tuffs, which are the spaces left by the soluble components in the cuttings, such as feldspar, after being dissolved. Dissolution pores in the magma are mainly found in the fused tuff or breccia lava developed in the magma. The pore space formed by the dissolution of the soluble components in the magma is generally distributed along the lava strip.The composition of volcanic rocks is composed of phenocrysts and matrix, and phenocryst dissolution pores refer to the pores formed by the dissolution of relatively large lava phenocrysts and soluble components in pyroclastics. The pore shape is irregular and is mainly developed in rhyolite, fused breccia, breccia lava, and volcanic breccia, followed by dacite, tuff, and volcanic clastic rock, including the dissolution pores in the chips, the dissolution pores in the crystal chips, and the mold holes.The stomata are the pore spaces left after the volatile components escape due to the reduced pressure when the molten lava is ejected from the surface. The characteristics of the stomata are related to the composition and volatile content of the lava. The pore size is mostly between 0.1 and 5 mm, and it is mostly developed on the top of the overflow phase.Microfractures include lithologic fractures controlled by sedimentation, structural fractures controlled by tectonic action, shrinkage fractures, grain-edge fractures, and elongated pores. In thin sections, real microfractures often have a certain extension direction and may gradually narrow or disappear with the increase of extension distance. Some microfractures formed in the early stage may deform, fully fill, or disappear under the influence of diagenesis.


## 4. Conclusion

(1)
The thickness of the Carboniferous drilling in the Xiquan block is 48 m–907 m, with an average of 336 m. The Carboniferous volcanic rock body and the overlying Permian strata are in contact with the regional unconformity, and the east-dipping volcanic rock strata and the west-dipping overlying classic rock strata form a “roof-like” structure, which is easy to form stratigraphic traps. The types of rocks developed in the Carboniferous are pyroclastic rocks and volcanic lava, and the lithology is mainly breccia tuff and volcanic breccia.(2)
The volcanic lithofacies are divided into single wells according to the lithologic combination. The single wells in the Xiquan block mainly encounter explosive facies, overflow facies, and volcanic sedimentary facies. The volcanic breccia and breccia tuff are the corresponding lithologies of the eruptive facies cavitation subphase. The corresponding lithology of the overflow facies is andesite. The lithology corresponding to the volcanic sedimentary facies is tuff, volcanic breccia, and a breccia tuff combination.(3)
Carboniferous oil reservoirs are medium-porous and ultralow permeability reservoirs, and the storage space can be divided into three types: primary pores, secondary pores, and fractures.



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*Source: 7800630-2022-04-25.xml*