The lithology of the volcanic rocks in the Junggar Basin is mainly medium-basic extrusive rocks, followed by acidic extrusive rocks and volcaniclastic rocks. The layout of intrusive rocks, dikes, and clastic lavas is visible. The main rock types developed in the basin are described as follows according to their color, material composition, structural structure, crack development and other characteristics.

(1) Volcanic lava

A type of rock formed by the condensation and solidification of molten magma that overflows the earth's surface. It has a volcanic lava structure. The mineral grains of this type of rock are fine, and most of the individual crystals cannot be identified with the naked eye. They often contain glass and usually have a porphyry structure. Common stomatal almond structures, rhyme structures, and massive structures are common.

1. Basalt

Basic volcanic lava with a SiO2 content of 45 to 52 is generally darker in color, with a color ratio of 35 to 65, dark black, and purple or dark brown on the weathered surface. It has a large specific gravity and has a spotty structure (Figure 3-5D), which is composed of phenocrysts and matrix. The matrix has an intergranular structure, an intermittent structure or a Raxian structure, and sometimes a spotless or less spotty fine-grained structure, intertwined structure, etc. The phenocrysts are mainly plagioclase, pyroxene, etc., with slight calcite alteration to sericitization and chloritization; the matrix is ??mainly microcrystalline plagioclase scattered randomly, filled with ilmenite, glassy, Chlorite, calcite and a small amount of zeolite, etc.; amygdaloid bodies are seen locally, which are irregular in shape, elongated and flattened, and are often filled with quartz, zeolite, calcite and chlorite, with fragmented structures and many cracks. It is filled in multiple stages by secondary minerals such as calcite, and porphyritic structures (Figure 3-5C) and stomatal almond structures (Figure 3-5A, B, D) are common. The phenocrysts and matrix minerals of the basalts in the Luliang area of ??the belly generally contain olivine in addition to pyroxene and plagioclase. The Wucaiwan basalts in the east also have a tholeiitic basalt structure in their matrix and no gap structure or interweaving structure, which is different from the Wucaiwan basalts in the west. Zone 8 basalt.

Figure 3-5 Basalt

2. Basaltic andesite

Medium-basic volcanic lava with SiO2 content of 52 to 63. Basaltic andesite is a transitional species between basalt and andesite. It mostly has a porphyritic structure (Figure 3-6A), with almond structures visible (Figure 3-6B). The phenocrysts are mainly plagioclase, with pyroxene and hornblende phenocrysts visible. The matrix includes phenocryst interlaced structure, tholeiitic basalt structure, vitreous structure and interstitial structure, etc.

Figure 3-6 Basaltic andesite

3. Andesite

Neutral volcanic lava, with SiO2 content of 52 to 63. Andesite is mostly gray, gray-green, and brown-gray (Figure 3-7A). The weathered surface is purple-red or gray-green, with a lighter color and a color ratio of about 30. Common plaque-like structures (Figure 3-7B) and stomatal almond structures (Figure 3-7C, D). The phenocrysts are mainly plagioclase, followed by hornblende, pyroxene and biotite; the matrix is ??fine, thin strips of plagioclase randomly distributed, partially arranged in parallel, filled with chlorite, magnetite, ilmenite, Vitreous and a small amount of amphibole, pyroxene, etc. (Figure 3-7E), with andesitic structure (Figure 3-7F); mostly pores and amygdala, the amygdala is irregular, and the edges are filled with authitic stones. Some are filled with chlorite and carbonate rocks, and partially calcite is filled in the cracks. Andesite is the most widely exposed lava type in the Batamayi Neishan Formation of the Carboniferous area in the Kelameili area. It is typically dark reddish brown on outcrops. Most of the rocks have varying degrees of alteration. Plagioclase and hornblende phenocrysts have large grains. , high content. The development of andesite types is slightly different in different regions. The Beisantai-Jimusar area of ??the Junggar Basin is dominated by andesite, the northwest edge of the Junggar Basin is dominated by basaltic andesite, and the Ludong-Wucaiwan area of ??the Junggar Basin has relatively few andesite types.

Figure 3-7 Andesite

4. Dacite

Medium acidic volcanic lava, with SiO2 content of 63 to 69. Porphyry structure, massive structure. The phenocrysts are mainly plagioclase, quartz, and a small amount of hornblende.

The matrix is ??mainly composed of fine feldspar crystallites, and some samples have fine structure, intertwined structure, and glassy structure (Figure 3-8A, 3-8B).

Figure 3-8 Dacite

5. Rhyolite

Acidic volcanic lava, SiO2 content >69. The color is lighter, the color rate is low, the specific gravity is small, and it has a spotty structure (Figure 3-9A), composed of phenocrysts and matrix. The phenocrysts are mainly alkaline feldspar and quartz, with occasional plagioclase and darkened biotite. Pyroxene is even rarer. The phenocryst plagioclase has a moderate degree of carbonate phenomenon, and some chlorite and fine feldspar are collected. The bodies are distributed in a band shape; the matrix is ??mostly fine structure, spherulite structure (Figure 3-9F) or vitreous structure. Rhyostructure (Fig. 3-9B, 3-9C) is common, and stomatal almond structure (Fig. 3-9D, 3-9E) and stone bubble structure sometimes develop.

6. Fine rock

Acidic volcanic lava, SiO2 content >69. There are usually porphyritic structures in acidic extrusive rocks, and there are also porphyritic structures. They are all composed of aphanitic crystals, which are fine rocks. Gray-yellow, gray-green, with fine structure (Figure 3-10A, 3-10B) and massive structure. It is mainly composed of microcrystalline plagioclase and quartz. Plagioclase is distributed randomly with a content of about 75. His-shaped quartz microcrystals are evenly distributed among plagioclase particles with a content of about 25. A small amount of dark minerals such as sericite and magnetite can also be seen. . Part of the feldspar phenocrysts have been replaced by iron III calcite. Hornblende phenocrysts can be seen locally, and most of the hornblende has been altered. A small amount of sericite, magnetite and other dark minerals can also be seen.

7. Perlite

Medium acidic volcanic lava, SiO2 content >63. Perlite is a vitreous rock formed by volcanic eruption and rapidly cooled. It is named after its pearl-like fragment structure (Wang Pujun et al., 2008). The color is variable, mostly with a greasy luster (Figure 3-11A). The most characteristic one is a concentric (spherical, ellipsoidal, polyhedral) and scroll-shaped crack structure, that is, the pearl structure (Figure 3-12B). Perlite is a vitreous rock with extremely poor stability. It is easily altered and shows varying degrees of devitrification or bentonite, and cryptocrystalline or spherulite microcrystals appear.

Figure 3-9 Rhyolite

Figure 3-10 Perlite

Figure 3-11 Perlite

8. Obsidian

Obsidian is a dense massive or slag-like acidic vitreous volcanic rock (Figure 3-12). Black, glassy structure, with glass luster and shell-like fracture, stone bubble structure (a native spherical structure more common in acidic lava. Its surface is due to the stone bubble structure in obsidian when it solidifies, gas escapes and the volume shrinks The resulting multi-layered concentric spheres with cavities are filled with fine secondary quartz, chalcedony and other minerals). The spherules are well developed and arranged in small amounts in layers.

(2) Pyroclastic rocks

Pyroclastic rocks are a transitional rock type between sedimentary rocks and igneous rocks. According to their material composition and diagenetic method, , can be further divided into pyroclastic lava, (normal) pyroclastic rocks (ignited pyroclastic rocks and ordinary pyroclastic rocks), volcano-sedimentary clastic rocks (sink pyroclastic rocks and pyroclastic sedimentary rocks) . Since volcaniclastic sedimentary rocks already belong to the category of typical sedimentary rocks, only the first three subcategories are discussed here.

Figure 3-12 Obsidian, Batamayi Neishan Formation in Laoshangou Section (field outcrop)

1. Pyroclastic lava type

Volcano Clastic lava is a transitional rock between volcaniclastic rock and lava. The lava matrix can contain 10 to 90% pyroclastic material. According to the size of the debris, it can be divided into three categories: agglomerate lava, breccia lava, and tuff lava.

From a distribution point of view, they are often limited to the vicinity of the crater. The debris is lava or rigid volcanic blocks thrown into the air by volcanic eruptions, which are cemented by the lava and formed into rocks after landing.

The formed rocks often have two characteristics: ① The fragments and cement are inconsistent in composition and structure, or the composition is similar but the structure is different, for example: volcanic bombs are cemented by dense lava, dense volcanic blocks are cemented by multiple The lava of the pores is cemented and complexed into volcanic blocks, including allogeneic clasts from the volcanic basement being cemented by dense or porous lava, such as Well Qi 8 being the most typical; ② The edges of fragments and clasts sometimes appear to be melted and baked. Signs such as edges and corners being corroded and rounded, pieces becoming smaller or redder, and bread-crust cracks appearing. Possible development scenarios for this type of rock include: ① Volcanic bombs and volcanic scoria fall from the air into the lava lake and are cemented near the volcanic neck; ② The eruption causes the channel wall rocks to break and are cemented by lava at the volcanic channel wall; ③ Volcanic cone Crevasse, pyroclastic material cemented into rocks by lava, surrounds the vicinity of a volcanic channel.

Figure 3-13 Andesitic tuff lava, Well Bai73, 1448.25m, ( )

(1) Tuff lava

Andesitic Tuff lava: A pyroclastic rock formed by cementing volcanic tuff material together with andesitic magma. The pyroclastic material accounts for 10 to 90% of the rock composition. The volcanic tuff materials here are rock fragments, crystal fragments or glass fragments with a particle size less than 2 mm, and volcanic dust materials (Figure 3-13). They are brown and have a clastic lava structure. Andesitic rock fragments contain more iron, and individual breccias are seen cemented by andesitic lava with andesitic structure. There are phenocrysts and matrix in the lava. The phenocrysts are plagioclase, and the matrix component is mainly plagioclase. Stone microcrystalline, chlorite, vitreous, with a small amount of epidote, and the vitreous is not devitrified.

Rhyolitic tuff lava: gray, composed of volcanic debris and cement, with a clastic lava structure. Volcaniclastic rocks are mainly composed of andesite, basaltic rock fragments and feldspar crystal fragments, cemented by fine-structured rhyolite with rhyolite structure.

(2) Volcanic breccia lava

Volcanic breccia lava (Figure 3-14A) belongs to the pyroclastic lava type and is between volcanic breccia and volcanic lava. A transitional rock type. The physical properties are slightly worse than those of volcanic breccia, but better than lava. Drilling cores describe that breccia lava develops pores, almond structure, and also has a porphyry structure. According to the chemical composition of the major elements of the rock, it can be named basaltic, andesitic, rhyolite, etc. (Figure 3-14B, 3-14C, 3-14D). In the Wuxia area, rhyolitic volcanic breccia lava can be divided into 4 types according to its origin (Table 3-3): reaccumulation type rhyolitic volcanic breccia lava (Well Q8, 4563.15~4565.55m), rock flow Autoclastic rhyolitic volcanic breccia lava (Well Xia 201, 4922~4925m; Well Fengnan 1, 4469.25~4472.46m) and foam lava type rhyolitic volcanic breccia lava (Well Madong 1, 4264.08~4266.78m ).

Figure 3-14 Volcanic breccia lava

Table 3-3 Classification of the genetic types of rhyolitic volcanic breccia lava in the Fengcheng Formation in the Wuxia area

Re The accumulation-type rhyolitic volcanic breccia lava spheroids are ellipsoid-spherical (Figure 3-15A). The matrix is ??gray-white and lighter in color than the spheroids. The composition is magma and is strongly welded with obvious directional arrangement of glass debris. There are obvious differences in the origin of spherical particles. It is believed that the consolidated ignited tuff formed by the early gas-magma eruption (the origin of pyroclastic flow) was exploded and broken by the later magma eruption, and was rapidly cooled and solidified during close transport with the subsequent magma and hydrothermal flow. Knots form.

The magma fragments in the autoclastic rhyolitic volcanic breccia lava are irregular in shape (Figure 3-15B), and crystal fragments or rock fragments are found inside in some cases, which should be multi-stage small-scale explosion phases. formed by stacking. The upper part of the accumulations in each phase is composed of flow autoclastic rhyolitic volcanic breccia lava, with semi-plastic slurries developed, but due to its small scale, pores are easy to escape and are not developed.

Foam-type rhyolitic volcanic breccia lava is rich in spheroids (Figure 3-15C). It is speculated that it is slag debris or volcanic bombs formed by strong explosions and fragmentation. There are few residual pores in the middle and lower parts and more in the upper part. ; The size of the spheroids ranges from 0.5 to 2.5cm. The pores are mostly seen in the larger particles, and the shape of an open smile is often seen. Filled or semi-filled amygdala can be seen in the pores, and the lower part is almost completely filled (fewer pores), also known as Foaming lava.

(3) Crypto-explosive volcanic breccia

It should be noted that crypto-explosive breccia belongs to the pyroclastic lava type (Figure 3-15D), but because of its It has special origin and lithofacies indication significance of volcanic channels, and is listed separately. Crypto-explosive breccia is a breccia formed by the magma before it erupts to the surface. Due to the large amount of volatile matter accumulated during the magma migration process, it was exploded and released underground, and was later cemented by the magma. The difference between the composition of lava breccia and cement lava is very small, and lava breccia is not easy to identify on the fresh surface.

Figure 3-15 Volcanic breccia lava

Crypto-explosive breccia can be used as the iconic rock type of volcanic channel phase. Cryptoexplosive breccia is also found in Well Baichong 12 near the Ke-Xia Fault in the Baikouquan area, which is speculated to be the product of a fissure-type eruptive volcanic mechanism. In Well Ke 301, which is far away from the fault in the Karamay area, the crypto-explosive breccia developed may be pyroclastic rocks formed by a central eruption volcanic mechanism.

2. Pyroclastic rocks

Pyroclastic rocks are volcanic clastics formed by volcanic processes (accounting for more than 90%). They are formed into rocks through compaction and are classified according to their particle size. Agglomerates, volcanic breccias and tuffs.

(1) Volcanic breccia

The clastic material of volcanic breccia is mainly composed of rock debris with a diameter of 2 to 64mm, with a small amount of volcanic ash and crystal debris (Figure 3-16) . The cement is volcanic ash or finer volcanic material. The structure of volcanic breccia, breccia and cuttings varies greatly in different wells, mainly depending on the properties of the magma. For example, in Well Ke 118, the breccia and cuttings are mainly composed of rhyolite, perlite, and perlite. The breccia and cuttings in Well Ke 113 are mainly composed of basalt, while the breccia and cuttings in Well Ke 120 are mainly composed of andesite. Both of them have the characteristics of single composition of breccia and cuttings. In the later stage, the breccia and cuttings have iron precipitation and are brown. The later volcanic ash is characterized by grape petrification, mudification, chlorite, silicification and calcite, and volcanic ash residue can be seen locally. The particle size of breccia is generally 0.5~8mm, with a maximum size of 10cm×32cm. It has obvious edges and corners, and local plastic slurry scraps are extruded, deformed and torn.

Figure 3-16 Volcanic Breccia

(2) Tuff

Tuff is the fine-grained volcanic clastic material that fell to the surface after the volcano erupted. Formed by accumulation and consolidation (Figure 3-17A), pyroclastic materials are mainly composed of glass debris, crystal debris, and a small amount of rock debris with a diameter of less than 2 mm. The diagenetic mode of tuff is mainly compaction and consolidation. According to the volcanic clastic components it contains, it can be further divided into crystalline tuff, glassy tuff and lithic tuff (Figure 3-17B, 3-17C, 3-17D).

Figure 3-17 Tuff

(3) Rhyolite breccia tuff (containing spheroids)

This type of rock belongs to welded volcanic debris For clastic rocks, the SiO2 content is generally greater than 63. In this area, the Xia 72 well area is the most typical, and it is also the main reservoir rock of volcanic oil and gas reservoirs in this area. This rock type was found in the cores of Xia 201 and Xia 202 wells. Among them, in the third coring (4932~4937.48m) of Well Xia 201, this type of rock is characterized by the development of irregular spherical spheroids and the development of pores inside the spheroids (Figure 3-18). From the internal structure, there are many deformed glass shards between the spheroids, showing a false rhyme structure. The pores can come from two aspects: one is the pores left by the escape of residual bubbles in the particles; the other is the particles themselves are pumice fragments with many pores. The former has large pores and a large number of pores.

Of course, the pores in the particles can also be partially filled with various minerals in later periods, and the pores seen today should be residual pores.

(4) Rhyolite ignimbrite (excluding spheroids)

This type of rock still belongs to the ignimbrite volcaniclastic rock type, but it does not contain spheroids and particles. It is small and belongs to the tuff grade. The typical identification feature is a certain degree of directional arrangement of glass chips or small-scale slurry chips under the microscope. Currently, more are found in the lower part of the second barrel coring section of Well Q8. Due to the fine grain size and poor reservoir physical properties, oil fields usually do not sample this type of rock for thin sectioning. In fact, similar cores are still found in many other wells and are suspected to be weak ignimbite (Figure 3-19).

Figure 3-18 Rhyolite ignimbrite

Figure 3-19 Chloritized weak ignimbrite

3. Sinking volcaniclastic rocks

Transitional rocks between volcaniclastic rocks and sedimentary rocks, formed under the dual effects of volcanism and sedimentary transformation. The pyroclastic material accounts for 90-50%, and the rest is normal sedimentary material, formed into rocks after compaction and water chemical cementation. Common ones include subsidence tuff and subsidence volcanic breccia (Figure 3-20).

Figure 3-20 Sinking pyroclastic rocks

(3) Subvolcanic rocks

The distribution of subvolcanic rocks in the Junggar Basin is small, but they are basic to acidic Drilling has revealed. Subvolcanic rocks are of the same origin as extrusive rocks, but they do not erupt to the surface. They are shallow to ultra-shallow intrusions formed by cooling and solidification near the surface. Its representative lithologies are porphyry and porphyry.

1. Granite porphyry

The SiO2 content is greater than 63, and the rock is flesh-red and cream-colored. The mineral composition is consistent with the corresponding plutonic rocks - granite and extrusive rocks - rhyolite. same. The difference is that it has a porphyry structure, indicating that it is epigenetic rock (Figure 3-21). The phenocryst content of granite porphyry is generally 15 to 20, mainly potassium feldspar and quartz, and sometimes biotite and hornblende. Quartz phenocrysts are often hexagonal bipyramidal, and potassium feldspar is orthoclase or feldspar. stone. Biotite and hornblende sometimes have darkened edges, and phenocrysts are usually eroded - the matrix is ??microcrystalline plagioclase, quartz and a small amount of biotite, and the quartz is mostly distributed in equiaxed grains among the feldspar grains. The matrix is ??microcrystalline plagioclase, quartz and a small amount of biotite. The quartz is mostly distributed in equiaxed granular shapes among the feldspar grains.

Figure 3-21 Granite porphyry

Outcrops are mainly distributed in the lower section of the Carboniferous Songkarsu Formation and the Batamayineishan Formation north of the Kelameili Fault, in the middle The Devonian Kelameili Formation is also scattered and produced in the form of rock branches and rock strains. An annular granite porphyry body is developed in the Batamayi Neishan Formation nearly 6km northwest of Baijiangou, which may be formed by magma emplacement along the fault channel of the ancient volcanic mechanism.

2. Monzonitic porphyry

Porphyry is a medium-basic (or weakly acidic, such as granodiorite porphyry) extrusive rock, epigenetic rock and The general term for ultra-superneophytic rocks. Plagioclase and dark minerals are the main phenocrysts, and the matrix is ??mostly cryptocrystalline-vitreous, such as diorite porphyry, andesite porphyry, diabase porphyry, etc. Diorite porphyry and andesite porphyry are common.

The SiO2 content is 53~59, (Na2O K2O) lt; 8. The mineral composition is the same as the corresponding plutonic rock-monzonite and extrusive rock-trachyandesite. The difference is that it has a porphyry structure, indicating that It is an epigenetic rock. The phenocryst content is generally 10 to 30, mainly tabular andesine and plagioclase. The surface of the phenocryst feldspar is deteriorated and muddy. The matrix is ??composed of fine columnar plagioclase and plate-like andesine, with other-shaped quartz and secondary chlorite embedded between the feldspars (Figure 3-22).

3. Diabase

Diabase is formed by the intrusion and crystallization of deep-source basaltic magma into the shallow part of the earth's crust. It is a basic epigenetic rock with a composition equivalent to gabbro (Fig. 3-23). Diabase is crystalline, fine to medium grained, often with diabase structure or sub-diabase structure.

The so-called pyroxene structure is composed of euhedral-semi-euhedral long strips of plagioclase (fine needle-like when viewed with the naked eye) forming a grid-like skeleton, with roughly equigranular pyroxene particles filling the gaps in the skeleton. Dark gray, gray-black, the mineral composition is mainly composed of pyroxene and basic plagioclase, with a small amount of olivine, biotite, quartz, apatite, magnetite, ilmenite, etc. Basic plagioclase is often altered into albite, zoisite, epidote and kaolinite; pyroxene is often altered into chlorite, amphibole and carbonate minerals. Due to the color of chlorite, the overall color is often gray-green.

Figure 3-22 Monzonite porphyry

Figure 3-23 Diabase

4. Diorite porphyry

Diorite Porphyry is a neutral epigenetic rock with the same mineral composition as the plutonic rock diorite. The main minerals are neutral plagioclase and ordinary hornblende. It has an obvious porphyry structure, and its phenocrysts are mostly euhedral, wide-plate plagioclase, followed by ordinary hornblende, and occasionally biotite (Figure 3-24A). Ring structures are often visible on neutral plagioclase phenocrysts. The matrix is ??fine-grained to cryptocrystalline, and the overall color of the rock is mostly gray and gray-green. Diorite porphyry can be produced alone as dykes or other small rock bodies, or it can become a local lithofacies of diorite rock bodies.

5. Andesite

Andesite is a type of magmatic rock and is a secondary rock of the neutral extrusive rock andesite. When andesite undergoes secondary changes, plagioclase often turns into chlorite, epidote, kaolinite, etc., losing its luster and turning green. This kind of changed andesite is called andesite, or porphyry (but porphyry is sometimes a general name for a type of rock) (Figure 3-24B).

Figure 3-24 Porphyry