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Volume 32 Issue 4
Aug.  2021
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Zhiguo Mao, Rukai Zhu, Jinghong Wang, Jinglan Luo, Ling Su. Characteristics of Diagenesis and Pore Evolution of Volcanic Reservoir: A Case Study of Junggar Basin, Northwest China. Journal of Earth Science, 2021, 32(4): 960-971. doi: 10.1007/s12583-020-1366-y
Citation: Zhiguo Mao, Rukai Zhu, Jinghong Wang, Jinglan Luo, Ling Su. Characteristics of Diagenesis and Pore Evolution of Volcanic Reservoir: A Case Study of Junggar Basin, Northwest China. Journal of Earth Science, 2021, 32(4): 960-971. doi: 10.1007/s12583-020-1366-y

Characteristics of Diagenesis and Pore Evolution of Volcanic Reservoir: A Case Study of Junggar Basin, Northwest China

doi: 10.1007/s12583-020-1366-y
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  • Unconventional volcanic reservoir is different from conventional reservoir in reservoir space, diagenesis, pore formation and evolution. The Carboniferous volcanic reservoir was selected in Junggar Basin, Northwest China because based on sediment/rock cores and outcrop data, diagenesis and pore evolution were studied by elemental measurements, thin section observations, and diagenetic analyses. These analyses shows that the reservoir lithology is predominantly intermediate-basic volcanic, and the reservoir storage space is composed mainly of secondary dissolved pores and fractures. The reservoir displays great heterogeneity, and has experienced a great variety of diagenetic alteration during various diagenetic stages including: (1) eruption fragmentation, crystallization differentiation and condensing consolidation at consolidated diagenetic stage; (2) metasomatic alteration, filling, weathering and leaching, dissolution by formation fluids and tectonism at the epigenetic modifications stage. The formation and evolutionary process of the pores is extremely complicated. The primary pores were formed during the consolidated diagenetic stage, and laid a foundation for the late development and alteration of effective reservoir. During the epigenetic modifications stage secondary reservoir storage space was developed via the formation of secondary pores and the development of fractures through weathering and leaching, dissolution by formation fluids and tectonism.
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    Wang, G., Qin, Y., Shen, J., et al., 2018. Dynamic-Change Laws of the Porosity and Permeability of Low- to Medium-Rank Coals under Heating and Pressurization Treatments in the Eastern Junggar Basin, China. Journal of Earth Science, 29(3): 607-615. https://doi.org/10.1007/s12583-017-0908-4 doi:  10.1007/s12583-017-0908-4
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Characteristics of Diagenesis and Pore Evolution of Volcanic Reservoir: A Case Study of Junggar Basin, Northwest China

doi: 10.1007/s12583-020-1366-y

Abstract: Unconventional volcanic reservoir is different from conventional reservoir in reservoir space, diagenesis, pore formation and evolution. The Carboniferous volcanic reservoir was selected in Junggar Basin, Northwest China because based on sediment/rock cores and outcrop data, diagenesis and pore evolution were studied by elemental measurements, thin section observations, and diagenetic analyses. These analyses shows that the reservoir lithology is predominantly intermediate-basic volcanic, and the reservoir storage space is composed mainly of secondary dissolved pores and fractures. The reservoir displays great heterogeneity, and has experienced a great variety of diagenetic alteration during various diagenetic stages including: (1) eruption fragmentation, crystallization differentiation and condensing consolidation at consolidated diagenetic stage; (2) metasomatic alteration, filling, weathering and leaching, dissolution by formation fluids and tectonism at the epigenetic modifications stage. The formation and evolutionary process of the pores is extremely complicated. The primary pores were formed during the consolidated diagenetic stage, and laid a foundation for the late development and alteration of effective reservoir. During the epigenetic modifications stage secondary reservoir storage space was developed via the formation of secondary pores and the development of fractures through weathering and leaching, dissolution by formation fluids and tectonism.

Zhiguo Mao, Rukai Zhu, Jinghong Wang, Jinglan Luo, Ling Su. Characteristics of Diagenesis and Pore Evolution of Volcanic Reservoir: A Case Study of Junggar Basin, Northwest China. Journal of Earth Science, 2021, 32(4): 960-971. doi: 10.1007/s12583-020-1366-y
Citation: Zhiguo Mao, Rukai Zhu, Jinghong Wang, Jinglan Luo, Ling Su. Characteristics of Diagenesis and Pore Evolution of Volcanic Reservoir: A Case Study of Junggar Basin, Northwest China. Journal of Earth Science, 2021, 32(4): 960-971. doi: 10.1007/s12583-020-1366-y
  • Recently volcanic rocks have become an important frontier for oil and gas exploration (Mao et al., 2015; Zou et al., 2010). By the end of 2019, numerous oil and gas fields have been discovered in volcanic strata in the Songliao, Bohai Bay, Junggar and Santanghu basins in China. A series of methods and techniques for the exploration of volcanic reservoirs have been developed simultaneously (Zhao et al., 2009; Kang et al., 2008; Zou et al., 2008).

    Volcanic rocks in sedimentary basins are usually widely distributed and are thick (Mao et al., 2015). Therefore, the scale of volcanic activity is an important factor controlling reservoir quality. From this point of view, the reservoir is the key to volcanic oil and gas exploration (Zhu et al., 2010). The research on volcanic reservoirs mainly focuses on lithology and reservoir space type, but less on diagenesis and pore evolution. The former Soviet Union scholars summarized the technology and method of volcanic reservoir research (Yi et al., 1998). In the South Nagaoka gas field in Niigata, Japan, Gamma ray logging, compensated formation density and compensated neutron logs were used together to successfully identify reservoirs in Miocene "Green tuff"—rhyolite volcanic rocks in the Qigu Formation (Yagi et al., 2009). In the 2000s, many scholars put forward the concept of volcanic reservoir geology (Zhu et al, 2010; Zhao et al, 2009; Chen et al, 2003), which studies the macro distribution, internal structures, reservoir parameter distribution, and pore structures in volcanic rocks, together with dynamic changes of reservoir parameters during development of fields in volcanic rocks, for the purpose of guiding exploration and development of oil and gas fields.

    As an unconventional reservoir, volcanic reservoirs are different from conventional reservoir in pore space, diagenesis, pore formation, and evolution. The volcanic reservoirs universally evolved from eruption, condensation, consolidation, diagenesis, burial and fluid-rock interactions, and as well as underwent complicated modifications and superimposition process (Hou et al., 2012; Wu et al., 2005; Qiu et al., 2000).

    Compared with clastic reservoirs and carbonate reservoirs, volcanic reservoirs evaluation and prediction is more difficult. Therefore, it needs more comprehensive research. On the basis of analyzing the reservoir characteristics and diagenesis of the Carboniferous volcanic rocks in the Junggar Basin, Northwest China, this paper attempts to document the diagenetic sequence of different types of volcanic rocks, determine the diagenetic stages of volcanic reservoirs, study its diagenetic modifications process and pore evolution, unravel the development mechanism of multiphase alteration and superimposition of the Carboniferous volcanic reservoirs, and provide an assessment on volcanic reservoirs and some insight for exploring volcanic reservoirs.

  • Located in the northwest region of China, the Junggar Basin is a multi-cycle superimposed basin formed in key parts of the Central Asia accretionary orogenic belt at the junction of Kazakhstan Micro-Plate, Tarim Micro-Plate, and Siberia Micro-Plate (Zhu et al., 2016; Qu et al., 2009; Chen et al., 2005; Wang et al., 2004) (Fig. 1a). With Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Tertiary and Quaternary formations deposited, the total thickness of sedimentary rocks is over 10 000 m (Wang et al., 2018).

    Figure 1.  Geological background of the Junggar Basin.(a) Location map of the Junggar Basin; (b) simplified structural units of the Junggar Basin; (c) the Carboniferous stratigraphic column of the Junggar Basin; (d)geological cross section in the Junggar Basin.Units: DEN.g/cm3; Klogging, Kcorc.10-3um2.

    Late Paleozoic Period is a key turning point for tectonic system of the Junggar Basin, when the region experienced diminishing of the ancient Asian Ocean and collision of continents, and the generation of several inter-continental rifts or faulted basins (Cai et al., 2000). The Carboniferous system was a transitional stage, when intensive tectonic activities induced volcanic activities in multiple stages and multiple craters (Mao et al., 2010). The Early Carboniferous Period is the stage of relic sea enclosure with interlude periods of collisions and continent- continent collision, with volcanic rocks developing along the suture zone; in the later Carboniferous Period, extensional collapse occurred after collision when volcanic rocks extensively developed along the fracturing zone, forming alternating volcanic rocks and sedimentary rocks architecture, i.e., a package of regional Carboniferous volcanic rock structures, which distributes mainly in the Batamayineishan Formation and the Dishuiquan Formation of the Carboniferous System. Showed by the drilling in the Luliang uplift and outcrop in the northeastern basin, the thickness of the Carboniferous System is over 1 000 m (Fig. 1).

    The change in tectonic environments between the Early and the Late Carboniferous System resulted in the angular unconformity between the Lower Carboniferous and Upper Carboniferous. Due to tectonic compression in Late Carboniferous Period, the Northern Xinjiang area was uplifted on the whole, leading to the exposure and thus weathering and leaching of large areas of volcanic rocks. Due to impacts of structural features of alternating uplift and faults in Carboniferous–Permian, different zones had differences in time of exposure, weathering and leaching. The belt from the northwest part of the Junggar Basin to the Dishuiquanbei in the northeast is characterized by unconformable overlapping of Medium and Upper Triassic formations on the Carboniferous System. Later on, in the Mesozoic and Cenozoic Period, the entire area was a long-term stable depression dominated subsidence, accepting new deposits above (Zhao et al., 2019; Li et al., 2006). A regional unconformity exists between the Carboniferous formation and above formations. Prolonged exposure provides favorable conditions for weathering and leaching of volcanic rocks. In this way, the porosity and permeability of volcanic rocks were increased significantly. Finally, volcanic reservoirs are formed with certain thickness and scale distribution. In recent years, a large number of reservoirs have been discovered in the Carboniferous volcanic rocks of the Junggar Basin (He et al., 2010; Kang et al., 2008) (Fig. 1).

  • Systematic samples were collected from Carboniferous volcanic rocks in Junggar Basin, including cores and outcrops. A series of analytical methods and instruments are used to analyze reservoir characteristics and diagenesis.

    Elemental analyses: The cores and outcrops samples are crushed into 200 mesh-size for major and trace element analyses. All elemental analyses were carried out at the Analytical Laboratory Beijing Research Institute of Uranium Geology. Major elements were analyzed by Axios mAX High-power WDXRF spectrometer (Malvern Panalytical Ltd, Netherlands).

    Porosity and permeability analyses: The samples were made into a cylindrical shape (diameter is 2.54 cm; height is 5 cm). All samples were dried at constant temperature (105 ℃) before testing. The analyses were carried out with the oil and gas industry standard of the people's Republic of China (Practices for core analysis, SY/T 5336-2006) in the State Key Laboratory of Enhanced Oil Recovery, Beijing, China. Helium porosity meter (CAT 113) and high and low permeability meter (CAT 112) are used to measure porosity and permeability respectively.

    Scanning electron microscope (SEM) analyses: The SEM analyses were performed at the Key Laboratory of Oil & Gas Reservoir (KLOGR) of CNPC, Beijing. The test instrument was Quanta 650F thermal field emission SEM (FEI, USA). The samples were cut into small pieces, and polished to the size of 1 cm×1 cm×1 cm using P50 coarse and P500 fine sandpapers. The observed surface must be the natural section (and not the polished). The ready-made rock fragments were glued to the sample pile using a conductive adhesive, and then were kept for 1 day until the adhesive was dried completely. The samples were then coated with gold and observed under SEM.

  • The volcanic rocks in the Junggar Basin mainly distribute along three faulted zones: The Kewu zone at the northwest margin, the Kelameili zone at the northeast margin, and the Sangequan zone at the southeast margin (Cui et al., 2013). Petrographic and geochemical analyses show that the Carboniferous volcanic rocks are composed of a variety of rock types (Fig. 2a). They are dominated by intermediate and basic rock types, such as basalt, basaltic andesite and andesite with a small amount of rhyolite with intermediate-low potassium content (Fig. 2b) (Chen et al., 2003). The volcanic rock type is predominantly lava, which is mostly hemicrystalline with fine mineral grains and porphyritic and vitrophyric textures (Wang et al., 2008). There are also secondary volcaniclastic lava, volcaniclastic rock, and sedimentary pyroclastic rock developed. Lava and volcaniclastic rock are mostly interbedded, and are intercalated in the terrigenous clastic rock. Most of the rocks experienced late dissolution, deformation and minor alteration, but are not metamorphosed.

    Figure 2.  Characteristics of the major elements of the Carboniferous volcanic rock in the Junggar Basin (anhydrous treated data). (a) TAS graph (78 samples), rock borderline type after Le et al. (1989); (b) SiO2-K2O graph (78 samples), rock borderline type after Rickwood (1989).

    The volcanic reservoirs are found in the northeast and northwest margins of the basin. The volcanic rocks in the Ludong- Wucaiwan area at the northeast margin are primarily within the Carboniferous Batamayineishan Formation, which consists of basalt, andesite, dacite, rhyolite, volcanic breccia and tuff. The volcanic rocks at the northwest margin are mainly within the Carboniferous Baogutu Formation and the Permian Jiamuhe Formation, with primarily andesite, basalt, andesitic basalt, volcanic breccia, tuff breccia, welded breccia, tuff and agglomerate.

  • The pore type of the Carboniferous volcanic reservoir in the Junggar Basin is more complex and diversified, including different types of primary pores and fractures, such as vesicle, intergraular (inter-gravel) pore/fracture, intercrystalline and intracrystalline pores/fractures, joint and shrinkage joint formed during the process of condensing and consolidation. Various secondary pores/fractures are also present including: amygdaloid dissolved pores, phenocryst dissolved pores, matrix dissolved pores, secondary mineral dissolved pores, intercrystalline and inter-gravel dissolved pores/fractures, devitrified pores/fractures, structural fractures and dissolved fractures formed by late hypergene weathering and leaching and rock-formation fluid interaction and tectonic stress during burial (Wu et al., 2012).

    A total of 53 Carboniferous volcanic rock drill-core samples from the Ludong area were selected to document the pore types under microscope and determine the visual porosity and pore types. The result shows that the reservoir space of the volcanic rocks in the area is dominated by different types of secondary dissolved pores (average: 32.8%) and fractures (average: 31.1%), and subordinate vesicles (average: 21.8%), intergranular (inter-gravel) pores and fractures (average: 11.8%), and shrinkage joints (average: 2.5%).

    Secondary dissolved pores account for the most important reservoir space, and are observed in all types of volcanic rocks (Figs. 3c, 3d, 3e, 3f and 3g). The amount of secondary dissolved pore is the highest in the subvolcanic rock (orthophyre) (average: 3.0%), which is followed by tuff (average: 1.35%), sedimentary volcanic breccia (average: 3.0%) and andesite (average: 1.35%). There are relatively few secondary dissolved pores in basalt, volcanic breccia lava, rhyolite and volcanic breccia, which account for 0.8%, 0.54%, 0.5% and 0.2%, respectively. Fractures account for a fraction of the reservoir storage space. The fractures in the volcanic rock in Ludong area consist of tectoclase (Figs. 3h, 3i) and weathered and dissolved fractures (Figs. 3i, 3j), which are seen in different volcanic rocks, and are mostly developed in the volcanic breccia lava and tuff (average: 1.9% and 1.7%) and then in the volcanic breccia (average: 1.3%). Vesicles are most developed in rhyolite (average: 3.2%), followed by andesite and basalt (average: 1.8% and 1.0%). There are also a few vesicles in lava breccia. Volcanic breccia is characterized by the development of pores and fissures (Figs. 3g, 3h) (average: 2.5%) between breccias.

    Figure 3.  Pore types in the carboniferous volcanic rocks in the Ludong area of the Junggar Basin. (a) Vesicle pores in basalt, Well Dixi17, 3 634.79 m; (b) intergranular pores in volcanic breccia, Well Dixi5, 3 649.2 m; (c) dissolved fractures and intra-phenocryst dissolved pores in basalt, Well Dixi173, 3 665.21 m; (d) dissolved enlarged pores among volcanic rubbles, Well Dixi1824, 3 535.8 m; (e) intragranular dissolved pores of weathered and leached feldspar, Well Dixi1824, 3 700.1 m; (f) matrix dissolved pores of andesite, Well Dixi172, 3 499.78 m; (g) intracrystalline dissolved pores of feldspar by weathering and leaching, Well Dixi182, 3 639.25 m; (h) phase 2 tectoclase in basalt, Well Dixi172, 3 502.52 m; (i) tectoclase and weathered dissolving fracture in volcanic breccia tuff, Well Dixi10, 3 028.11 m; (j) weathered and leached dissolved pores and fractures in volcanic breccia tuff, Well Di403, 3 818.55 m. The blue part in the photos is the reservoir spaces.

  • Volcanic reservoirs are controlled by volcanic eruptive mode, cyclicity and volcanic edifice size. They are characterized by small sizes but large and rapid physical property variations. Due to the heterogeneity of the rock textures and mineral compositions during magma condensing consolidation, the reservoir storage space is extremely complex with most of the volcanic reservoirs being characterized by dual pore structures with both fractures and pores, exhibiting extreme heterogeneity (Zhu et al., 2010; Qiu et al., 2000).

    In this study 3 420 Carboniferous core samples were analyzed from 127 wells, of which there were 22 sub-lithotypes (10 sub-types of volcanic rocks, 3 sub-types of intrusive rocks and 9 sub-types of sedimentary rocks) identified from the Carboniferous System of the Junggar Basin. The 10 sub-types of volcanic rocks can be formed effective reservoirs with the maximum porosity of 32.0% and the minimum porosity of 0.61% (Fig. 4). The permeability is low overall, mostly less than 1×10-3 μm2 with a significant part < 0.01×10-3 μm2. Overall the volcanic rock features moderate-high porosity and low-moderate permeability, with strong heterogeneities and poor correlation between porosity and permeability (Fig. 4).

    Figure 4.  Porosity-permeability cross plot of the Carboniferous volcanic rocks in the Junggar Basin (4 191 core samples).

    It is shown by the available data analysis that the porosity and permeability of different volcanic rocks vary due to the substantial difference of lithology. Even with the same lithology, the porosity and permeability are also not exactly the same in different wells. Lithology is a factor affecting the physical property of the volcanic reservoir, but not the major controlling factor, so lithology has a limited effect on the physical properties of the volcanic reservoir.

  • In reference to the diagenesis model of sedimentary rock (Taylor et al., 2010), the consolidated diagenesis of volcanic rock can be defined as the physical and chemical diagenesis processes occurred during the period when the magma condenses, consolidates and finally forms volcanic rock from the deep underground upward to the shallow subsurface or surface. The epigenetic diagenesis can be defined as "all physical and chemical diagenesis processes occurred during the period from the volcanic rock formation prior to the formation of sedimentary rock or metamorphic rock". The diagenesis of the Carboniferous volcanic rocks in the Junggar Basin mainly includes eruption fragmentation, crystallization differentiation, condensing consolidation, weathering and leaching, tectonism, filling and cementation, dissolving, metasomatic alteration and devitrification.

  • The diagenesis, diagenetic environment and diagenetic evolutionary process of volcanic rocks have apparent phase characteristics. The different diagenetic stages of volcanic rocks should therefore be further subdivided into different phases (Table 1).

    Diagenetic stage Diagenesis Diagenetic mechanism Diagenetic marker Pore type
    Stage Phase
    Consolidated diagenetic stage Crystallization phase Crystallization differentiation Magmatic differentiation, fractional crystallization Volcanic rock with different crystalloid and mineral components Vesicle, intergranular pore, exploded fracture, shrinkage joint, and intercrystal pore
    Condensing phase Condensing consolidation Condensing shrinkage Volcanic shrinkage joint
    Epigenetic diagenetic stage Hydrothermal phase Metasomatic alteration Temperature fluctuation due to the ascending of thermal fluid from deep strata Chloritization, zeolitization, etc. Clay mineral intercrystalline micro-pore, intra-amygdaloid pore, residual vesicle, dissolved pore, and dissolved fracture
    Packing action Crystallization and deposition of mineral matters carried by volcanic hydrothermal solution Filled by chlorite and zeolite
    Dissolution Dissolution and metasomasis of volcanic hydrothermal solution Early chlorite and zeolite dissolved pores
    Modification phase Weathering and leaching Temperature caused expansion or contraction of rock, weathering and leaching Weathered fractures and a great deal of intergranular and intragranular dissolved pores and fractures Weathered fracture, dissolved pore, and dissolved fracture
    Compaction Burial compaction Contact variation among clastic particles and crystalline
    Tectonism Tectonic stress High-angle fracture, subhorizontal fracture, and reticular fracture Tectoclase
    Dissolution Formation water and organic acid dissolution Substantive secondary dissolved pores Matrix dissolved pore, phenocryst dissolved pore, interparticle dissolved pore, and dissolved fracture
    Metasomatic alteration Burial temperature, pressure buildup and formation fluid action Zeolitization, chloritization and associated minerals like clay
    Fill and cementation Deposition of formation fluid dissolved minerals Fill and cementation of zeolite, chlorite and associated minerals like clay
    Devitrification Burial temperature, pressure buildup Devitrified felsitic texture and cryptocrystalline texture

    Table 1.  Division of diagenetic stage of volcanic rocks

  • The consolidated diagenetic stage refers to the stage from melted magma erupting to the surface or intruding into shallow subsurface to its condensation, consolidation and finally forming rocks. With respect to the Paleozoic volcanic reservoirs in the Junggar Basin, this stage is extremely ephemeral. Carboniferous– Permian volcanic rocks were formed over multiple periods of intermittent eruption with an overall thickness of 5 000 m. Based on the calculation using geologic thermometer and barometer, the magma could completely consolidate and crystallize in less than 10 thousand years. Although the consolidated diagenetic stage only experienced a short geologic period, it is the stage for the formation and development of primary pores with associated major diagenesis of fragmentation, differentiation, resorption, condensing crystallization and welding. Substantive primary vesicle (Fig. 3a), intergranular pores (Fig. 3b), exploded fractures and condensing shrinkage joints were also developed.

  • Thermal fluids including hot gas were frequently produced at the last stage of volcanism. Affected by the hydrothermal activity, many rock-forming minerals in the volcanic rock experienced alteration. For instance, pyroxene and amphibole altered into chlorite; alkaline plagioclase altered into kaolinite, sericite and chlorite; olivine replaced by iddingsite; chlorite altered into zeolite and carbonate; and there is also tuff altered into laumontite or carbonate minerals. With the proceeding of alteration and mineral conversion, substantive minerals, like chlorite, zeolite, calcite and quartz, carried by thermal fluid crystallized under proper conditions and further infilling developed. The metasomatic alteration and filling of volcanic rocks mainly occurred in this phase.

  • During the subsequent prolonged geological process after being erupted to the surface, the volcanic rock had been, either directly exposed to the surface, in contact with lake water or sea water, or exposed by late uplift, or directly buried by the overlying formation. They would have experienced a great variety of geological alteration such as weathering and leaching, compaction, tectonism, dissolution, filling, cementation and devitrification. All these processes led to the breaking off of volcanic rock, the chemical element dissolution migration, dispersion or enrichment, formation of oxide, silicoide, sulfate mineral and a large amount of dissolved pores and fractures. Effective hydrocarbon reservoirs were consequently formed. This phase is the main developmental phase of secondary reservoir storage space in volcanic reservoirs (Figs. 3c3j).

  • The volcanic reservoirs were formed by the combined action of volcanism, diagenesis and tectonism, with extremely complicated evolutionary process of formation, development, plugging and modifications of reservoir space (Qiu et al., 2000). With respect to the conventional sedimentary rocks, volcanic rocks are comparatively rigid, tight, so the primary pores, such as intercrystal and shrinkage pores, formed during the cooling procedure are less affected by compaction. However, the lithology, texture, structure and mineral composition of the volcanic rocks are more complicated than those of sedimentary rocks. During the late diagenetic process, besides varying with the change of volcanic texture itself, the reservoir space is apt to vary from being subjected to modifications by processes such as; volcanic hydrothermal solution, formation fluid and tectonic stress, e.g., devitrification of volcanic glass in volcanic rock, vesicular filling, dissolution of matrix and minerals like feldspar. This directly affected the formation and evolution of pores, and played a destructive or a constructive role in the development of reservoir.

    After the volcanic eruption in the Junggar Basin in the Carboniferous, relatively strong intracontinental reversalcompressional tectonism occurred in the whole area. Uplifting denudation became predominated in the Permian, so the volcanic rocks were exposed and experienced a long-term weathering and leaching. A relatively thick sedimentary cover was developed in the Triassic–Jurassic. The volcanic reservoirs were then buried for a long period (Zhao et al., 2008; Li et al., 2006; Chen et al., 2005). Over the entire process, the volcanic rock experienced superposition of multiple actions over multiple periods/stages of diagenesis (Fig. 5).

    Figure 5.  Diagenetic sequence and pore evolution of Carboniferous volcanic rock in the Junggar Basin (modified after Mao et al., 2015).

  • Carboniferous is a period of intense volcanic activity in the Junggar Basin. The high-temperature magma erupted out of surface or intruded the shallow layer through deep faults or volcanic conduits. Due to the eruption and detonation of volcanos, a large amount of exploded fractures and intergranular pores formed in the pyroclastics (Fig. 3b). Simultaneously, with the rapid decrease of temperature and pressure, the magma and pyroclastics cooled down swiftly and formed rocks. In this process, the volatile constituents in the magma escaped and formed a lot of vesicles (Fig. 3a). In addition, numerous intercrystalline pores, intergranular pores and condensing shrinkage joints were formed in the course of condensation and crystallisation of magma and pyroclastics (Fig. 5). These primary reservoir pores formed before the Early Permian, which laid a good foundation for the formation and modifications of reservoirs subsequently.

  • The residual heat of magma resulted in temperature increase of formation water, which took effect on silicate minerals and tephros. The following ions were released Fe2+, Mg2+, Ca2+, K+, Na+ and Si4+ by this process and formed diagenetic minerals such as illite, carbonate minerals, chlorite, zeolite, anhydrite and gypsum, which replaced the original minerals and filled some pores (e.g., vesicle, condensing shrinkage joint, intra-phenocryst cleavage crack, irregular microcrack, inter-gravel pore and fracture, etc.), so parts of the pores in the volcanic rock were lost (Fig. 6).

    Figure 6.  Micro characteristics of diagenesis of Carboniferous volcanic rock. (a) Fillings in pores, andesite, Well Dixi172, 3 487.1 m; (b) fillings in pores, andesite, Well Dixi1824, 3 605.5 m; (c) fillings in pores, andesite, Well Dixi182, 3 641.45 m; (d) fillings in pores, rhyolite, Well Dixi10, 3 026.1 m; (e) porous space formed by matrix dissolution, Well Shidong-8, 2 852.5 m; (f) porous space formed by phenocryst dissolution, Well Shidong-8, 2 852.5 m.

    Core observation and petrographic analysis indicate that hydration, metasomatic alteration and packing mainly occurred during the hydrothermal phase of the Carboniferous volcanic rocks in the Junggar Basin (Fig. 6). For instance, pyroxene and amphibole were altered into chlorite, alkaline plagioclase was altered into kaolinite, sericite and chlorite, olivine was iddingsitized, chlorite was altered into zeolite and carbonate, and the tuff matrix underwent carbonatization and lomontitization. With the proceeding of alteration and mineral conversion, a mass of minerals as chlorite, zeolite, calcite and quartz carried by thermal fluid crystallized and precipitated out under proper conditions, filling the reservoir space and largely reducing the reservoir quality of volcanic rock.

    This study found that the attendant phenomena of chlorite and zeolite are apparent and their distribution characteristics are basically consistent, but they have different forming sequence. The formation of chlorite was earlier than zeolite at the alteration stage. However, the chlorite precipitating in the vesicles and vugs was obviously later than the zeolite. The pores filled by minerals exceed the residual pores in most of the samples. Mineral filling also occurred over many stages (Fig. 6). The fill process is the leading cause resulting in the reduction of reservoir quality of the Carboniferous volcanic reservoirs in the Junggar Basin (Fig. 5). However, because most of the minerals are lyotropic, they provide a soluble material for subsequent dissolution.

  • 5.2.2.1 Epidiagenesis (mainly weathering and leaching)

    Multiphase volcanic eruption cycles were developed in the Carboniferous of the Junggar Basin associated with a major depositional break, and several sets of weathering and leaching surfaces were formed. The Carboniferous volcanic rock formed in the Hercynian underwent some violent compressional faulting process and prolonged weathering and denudation, and formed a large-scale weathered and denuded unconformity across the entire area. Above this unconformity, the Carboniferous volcanic weathered crust is overlain directly by the various younger strata which become caprocks for the Carboniferous volcanic reservoirs. The longer the time difference between the basement and overlying caprock, the longer exposure of the rock, and the stronger the suffering of weathering and leaching, and consequently the more abundant pores and fractures developed (Gao et al., 2018; Hou et al., 2012; Zou et al., 2012; Zhao et al., 2008).

    For the moment, the vast majority of the Carboniferous volcanic reservoirs in the Junggar Basin were developed underneath the weathered crust. Core analysis data indicates that the weathering and leaching played an apparent role in the improvement of the reservoir porosity. Simultaneously, the more intense the weathering, the more porosity developed (Fig. 7). The average original porosity of basalt without weathering and alteration is 7.6%. Basalt with weak weathering and alteration has porosity of 8.7%. Basalt with strong weathering and alteration has porosity of 15.3%.

    Figure 7.  Impact of weathering and leaching on the porosity of volcanic rock.

    The weathering and leaching in this area can be demonstrated by the following aspects: (A) Favorable dissolution spaces exist at both the top and bottom of each effusion cycle. Dissolution firstly starts from the top and bottom due to weathering and leaching. An alternative development of erosion-collapsing belt and tight lava zone is formed. This phenomenon is extremely apparent in the continuous coring interval. Only when the weathering is thorough or the tectoclase is connected with the volcanic massif, can the rock exhibit a phenomenon of large set of broken slag. (B) When the weathering and leaching is combined with the fault, the leaching fluid will infiltrate over 500 m beneath the weathered crust. The Dixi-14 gas reservoir is a typical secondary weathering volcanic reservoir type, and the reservoir space is composed of massive of dissolved pores and fractures formed by weathering and leaching. (C) The leaching is also one of the major causes resulting in the dissolution of zeolite in this area. The zeolite is stable under a high pH value (9.1–9.9) environment. However, with weathering and leaching, the infiltrated surface water carries away the dissolved elements and changes the pH value of the formation water, which in turn accelerates the dissolution of zeolite. The zeolite is the major filler in pores and fractures of the volcanic rocks in the area, and the zeolite dissolved pore is the major reservoir space for volcanic reservoirs. Therefore, the dissolution of zeolite is a key to improve the volcanic reservoir quality. It is shown by the analysis on the volcanic reservoirs at the top of the Upper Carboniferous that dissolution of top zeolite mainly results from the leaching of surface water; therefore, the scope scale affected by the leaching beneath weathered crust should be the target for searching for effective volcanic reservoirs.

  • The Junggar Basin has experienced multiphase tectonic movement and faults since Late Carboniferous. Numerous fractures and broken belts formed in the volcanic rocks distributed along the faulted zone, e.g., the Shinan-1 and Cai-27, wells in the course of faulting, which connected the originally isolated vesicles, increased the net porosity of volcanic reservoirs, and thus enhanced the reservoir quality of hydrocarbons. In addition, the dissolving, leaching, infiltration, deposition and packing processes of the subsurface water controlled by faults play a leading role in the filling of amygdaloid in the vesicle or the formation of dissolved pore.

  • The volcanic minerals formed in the high-temperature and high-pressure environment often underwent secondary changes and formed stable hydrous minerals. Such secondary change, on the one hand, caused the mineral volume to expand and block the pores, on the other hand, created favorable conditions for subsequent dissolution. The Carboniferous volcanic rocks in the Junggar Basin underwent dissolution by formation water and organic acid fluids for a long period during the burial process, and this is one of the most important mechanisms for the forming of the reservoir space in this area. Redissolution of early fillers and altered products are the main diagenesis process in the basalt and andesite. For example, early filled chlorite and calcite are usually partially dissolved in the samples from Well Shinan-3, Shinan-4 and Madong-2. Dissolution of hornblende, feldspar phenocryst and matrix are the main diagenesis process in dacite, such as the Carboniferous dacite in Block Shixi-1 of the Shixi oilfield. The plagioclase phenocryst was often dissolved with dissolved pores or dissolution enlarged fractures well developed in the matrix. As for the alkaline rocks like trachite and phonolite, they are more sensitive to the acid fluids. Once the environment changes from alkalinity to acidity, a great deal of alkaline feldspar phenocryst and matrix were dissolved, with a lot of dissolved pores as seen in the tephriphonolite and trachyandesite cores taken from Well Shidong-8 (Figs. 6e, 6f). The acidic fluid can also be divided into inorganic acid and organic acid. They may act separately or jointly in different areas. Inorganic acid possibly dominated in the vicinity of fault or the place where late volcanic eruption exists, while the dissolution of the organic acid may be stronger in the area near the oil source. The occurrence of dissolution in a large area is vitally related to the developmental conditions of faults. With regard to different kinds of minerals, especially the major sacrificial mineral-feldspar, whether it is dissolved by organic acid or by inorganic acid, the dissolution mechanism is quite different.

    It is shown that diagenesis can play both constructive and destructive roles in volcanic reservoirs. The space-time relationship between filling and dissolution, and diagenetic intensity directly affect the quality of the reservoir modifications (Fig. 5). In the diagenetic process, weathering and leaching is the key to the improvement of reservoir quality, while the weathering degree has an apparent impact on it. The development of reservoir is also affected by the lithofacies, rock types and so on, and the coupling of these factors is the controlling factor to form an ideal volcanic reservoir.

  • (1) Carboniferous volcanic reservoirs are well developed in the Junggar Basin. The reservoir lithology is complex, and mainly composed of basalt, basaltic andesite and andesite with small amounts of rhyolites. The rock lithotype is predominantly lava with subordinate volcaniclastic lava, volcaniclastic rock and sedimentary volcanic rock.

    (2) The reservoir storage space types of the Carboniferous volcanic reservoirs in the Junggar Basin are diverse, overall dominated by secondary dissolved pores and fractures. Having experienced multiple modifications at the late stage, the reservoir storage space is of dual structure with both fractures and pores and the storage space is extremely heterogeneous.

    (3) The formation and evolution of the Carboniferous volcanic reservoirs in the Junggar Basin are extremely complicated. The volcanic reservoirs experienced a great variety of diagenesis alteration including eruption fragmentation, crystallization differentiation and condensing consolidation during the consolidated diagenetic stage, and as well as metasomatic alteration, filling, weathering and leaching, formation fluid dissolution and tectonism during the epigenetic modifications stage. Different diagenesis occurred at each evolutionary stage. The consolidated diagenetic stage is the phase for the development of volcanic primary pores, which laid a foundation for the late development and modifications of effective reservoirs. During the epigenetic modifications stage secondary pores were developed as a result of weathering and leaching, formation fluid dissolution and tectonism. The Carboniferous volcanic reservoirs had experienced at least two stages of fill and dissolution. The space-time relationship between filling and dissolution, and diagenetic intensity directly affect the quality of the reservoir modification.

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