According to the classification of Zhu et al. (2010), we rearranged the lithofacies of volcanic rocks in Junggar Basin based on rock type, lithology, texture, structure and occurrence. By the occurrence and depth of volcanic rocks, the area from the near-crater zone to the far-crater zone is divided into volcanic conduit facies, explosive facies, effusive facies and volcanic sedimentary facies (Table 1), so that the occurrence of volcanic rocks is highlighted, thereby providing basis for the theory of facies, reservoir, and physical properties controlled by volcanic architectures.
Facies Subfacies Depth Major rock types Typical lithologies Texture Structure Occurrence Remarks Effusive Upper Surface Lava Basalt, basaltic andesite, andesite, rhyolite Interlaced, intergranular, and intersertal Fumarolic, amygdaloidal, lithophysa, rhyolitic Lava flow, lava sheet; rope-like, dreg-like, columnar, pillow-like lavas Products of volcanic effusion and overflow Middle Lower Explosive Near-crater Surface Volcaniclastic rock Tuffaceous breccia, breccia, agglomerate, welded tuff Volcaniclastic Massive Fallout, volcaniclastic flow, near-crater spattering Products of volcanic explosion Far-crater Tuff Tuffaceous Massive Floating, far-crater spattering Volcanic conduit Subvolcanic Subsurface Intrusive rocks Granite porphyry, diorite- porphyritic, dolerite Porphyritic texture Massive Near surface, super-shallow, shallow strata Produced at the volcanic architecture and its periphery Volcanic sedimentary Continental Surface Sedimentary volcaniclastic rock, volcaniclastic sedimentary rock, sedimentary rocks, etc. Lacustrine mudstone, marine mudstone, tuffaceous sandstone, tuffaceous glutenite, sedimentary tuff Rounded agglomerate, volcaniclastic/terrigenous sedimentary texture Layered Fluvial-lacustrine sediments, marine sediments; layered and lenticular sediments Products in the volcanic explosive intermittence and in the low ebb Marine
Table 1. Lithofacies and characteristics of Carboniferous volcanic rocks in Junggar Basin (modified after Zhu et al., 2010)
With reference to the classification of pore space by Wang et al. (2007), and based on the data of 37 reserves reports and the 3 200 reservoir test data, we divide the pore space of volcanic reservoirs in the basin into three types: primary pores, secondary pores, and microfractures (Table 2). The primary pores are subdivided into primary vesicle, residual vesicle, intergranular (inter-gravel) pore, and intercrystalline/intracrystalline pore, the secondary pores are subdivided into phenocryst dissolved pore, amygdaloidal dissolved pore, intra-matrix dissolved pore, and inter-breccia dissolved pore, and the microfractures are subdivided into primary condensing shrinkage fracture, structural fracture, weathering fracture, and dissolved fracture (Fig. 3).
Type Formation mechanism Characteristics Lithologies Primary pores Primary vesicle Generated by gas expansion and overflow in diagenesis Frequently distributed at the top and bottom of rock flow layer, with varying sizes and morphologies Volcanic breccia, lava Residual vesicle Residual pores under the condition that secondary minerals don't fully fill the vesicle Semi-filled pores Basalt, volcanic breccia Intergranular (inter-gravel) pore Residual pores formed by the compaction of void space between clastic particles Frequently observed in volcaniclastic rocks Volcanic breccia, agglomerate, volcanic sedimentary rocks Intercrystal/ intracrystal pore Pores in the framework of phenocryst minerals such as pyroxene and plagioclase with cleavages, which are intracrystal pores in nature Frequently distributed in the middle of rock flow layer, with small void space Lava, volcaniclastic rock Secondary pores Phenocryst dissolved pore Pores formed by the dissolution of phenocryst in the flow process (usually) along the cleavage fractures Irregular pore morphology, frequently in estuary distribution, with intracrystal pore in dominance Andesite, rhyolite Amygdaloidal dissolved pore Formed by the metasomatic dissolution of fillings in the vesicles Irregular pore morphology, with poor connectivity Basalt, lava Intra-matrix dissolved pore Formed by devitrification of vitric in the matrix or dissolution of micro-crystal feldspar Tiny pores, with dissolved pores in dominance, certain connectivity Welded tuff, tuff, dolerite, Inter-breccia pore Formed in epigenesis such as weathering, leaching, and dissolution Developed along the fractures, clastic rock belts, and structural highs Basalt, andesite, breccia Microfractures Primary condensing shrinkage fracture Shrinkage microfractures formed in the magma condensation and crystallization Columnar joint, showing opening mode and sheet fracturing without evident displacement Volcanic breccia, andesite, trachyte Structural
Microfractures formed in volcanic rocks under the action of tectonic stress Developed near the fault, relatively flat and straight, mostly high-angle fractures Basalt, andesite Weathering fracture Frequently intersected with dissolved pores, fractures and structural fractures, which cut the rocks into various fragments Connected with dissolved pores, fractures and vugs, and with structural fractures Volcaniclastic rock, volcanic breccia Dissolved fracture Formed by leaching and dissolution Amygdaloidal andesite, volcanic breccia
Table 2. Types and characteristics of pore space of Carboniferous volcanic reservoirs in Junggar Basin
Existence of primary pores is fundamental for the formation of effective volcanic reservoirs, but the primary pores are mostly filled or dissolved to different extents during the subsequent burial evolution. Obviously, the late alteration of the Carboniferous volcanic rock is pivotally important to the formation of reservoir (Zhang et al., 2014). According to the type of pore space, the volcanic rock is a dual-media reservoir with typical dissolved pores and microfractures (Li et al., 2010; Lin et al., 2009). The Carboniferous reworked weathering crust volcanic reservoirs generally demonstrate the pore space in three combinations: structural fracture+dissolved fracture+dissolved pore, primary vesicle+structural fracture+dissolved fracture+dissolved pore, and intercrystalline pore+primary pore+structural fracture+ dissolved pore. The last combination is mainly observed in volcaniclastic rocks. Various types of pore spaces can be found in the Lower Carboniferous volcanic reservoirs in east-western Junggar. Primary vesicles and dissolved pores exist in the near-crater explosive facies and explosive facies belts, implying the possibility of high-quality reservoirs. Secondary pores are mostly developed in far-crater facies belts, where the development degree of weathering and structural fractures determines the alteration degree and storage capacity of reservoir. The reservoirs are dominantly fracture-pore type (interconnected fractures, microfractures and pores) and fracture type (microfractures and fractures) (Zhang et al., 2018).
The Carboniferous volcanic reservoirs in the basin are diverse and tight in lithology, and the lithology, lithofacies and pore space determine their strong heterogeneity, exclusively being homogeneous reservoirs with medium-low porosity and low-extra-low permeability (Jin et al., 2018; Song et al., 2016). The physical properties are extremely variable for different lithologies and lithofacies in horizontal and vertical directions. As a kind of in-situ accumulation, volcanic reservoirs are widespread very differently from region to region (Wang et al., 2012). Generally, the volcanic reservoirs show better physical properties in the Upper Carboniferous than in the Lower Carboniferous, in volcaniclastic rocks than in lava, and in intermediate acidic lava than intermediate basic lava (Zhu et al., 2012). The Lower Carboniferous is dominant at the northwestern margin of the basin, while the Upper Carboniferous is dominant in eastern Junggar (Wu et al., 2011).
According to 37 reserves reports and 3 200 reservoir test data, the lithofacies include explosive facies→effusive facies→near-crater sedimentary facies→volcanic conduit facies→sub-volcanic facies→far-crater sedimentary facies, in an ascending order of distance to the crater (Fig. 4a). By lithologies, the volcanic reservoirs with the best physical properties are welded (vesicle) basalt, amygdaloidal (vesicle) andesite, volcanic breccia, welded (vesicle) tuff, rhyolitic dacite, amygdaloidal basaltic andesite, and vesicle basaltic andesite, followed by agglomerate, andesite basalt, basaltic andesite, dolerite, rhyolitic breccia, rhyolite, and andesite; the lithofacies with the worst physical properties are tight basalt, tight andesite, tuff, dolerite, and sandy tuff (Fig. 4b). For the Carboniferous volcanic rocks in the Di'nan uplift zone in the eastern segment of the Luliang uplift, the rocks of explosive facies include agglomerates, volcanic breccia, tuff, breccia-bearing tuff, tuffaceous volcanic breccia and rhyolitic breccia (with a porosity of 8.91%); the rocks of effusive facies include amygdaloidal andesite, rhyolitic dacite, amygdaloidal basaltic andesite, basaltic andesite, and andesite (with a high porosity of 7.12%); the rock of volcanic conduit facies is andesitic tuff lava (with a porosity of 6.07%); the rocks of volcanic sedimentary facies include crystal debris tuff (with a porosity of 7.33%) and sedimentary tuff (with a porosity of 5.45%).
2.1. Lithology and Lithofacies
2.2. Pore Space and Physical Properties
2.2.1. Pore space
2.2.2. Physical properties