Devitrification Pores and Their Contribution to Volcanic Reservoirs

Marine and Petroleum Geology 98 (2018) 718–732

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Marine and Petroleum Geology

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Research paper Devitrification pores and their contribution to volcanic reservoirs: Acase T study in the Hailar Basin, NE China ∗ Han Zhenga, Xiaomeng Suna, , Jiping Wangb, Defeng Zhub, Xuqing Zhanga a College of Earth Sciences, Jilin University, Changchun, 130061, China b Exploration and Development Research Institute of Daqing Oilfield Company Ltd., Daqing, 163712, China

ARTICLE INFO ABSTRACT

Keywords: Volcanic rocks represent important unconventional hydrocarbon reservoirs; however, devitrification pores, Devitrification which are ubiquitous in volcanic rocks, remain mostly unstudied. In this study, we use fluorescence image Pore analyzer (FIA), scanning electron microscopy (SEM), electron probe microanalyzer (EPMA), and laser-scanning Volcanic rock confocal microscopy (LSCM) techniques to determine the types, characteristics, formation mechanisms, and Reservoir contributions to volcanic reservoirs of various devitrification pores in typical oil-bearing volcanic rocks. Primary Volcanic glass high-temperature devitrification produced clustered and individual spherulites and lithophysae. Clustered Spherulite spherulites have small diameters (< 1 mm) and poor intraspherulitic porosity, but exhibit well-developed interspherulite pores. In contrast, isolated spherulites have larger diameters (> 1 mm) and well-developed intraspherulitic radiating micropores. Lithophysae contain spherical cavities and layers comprising skeletal crystallites. Abundant intercrystallite pores and sieve-like intracrystallite micropores occur between and within the crystallites, respectively. Secondary low-temperature devitrification generated flow-banded crystal fibers within glassy lavas. Abundant devitrification pores occur between crystal fibers. Nucleation density and morphology of crystals that formed during devitrification are dependent on the degree of supercooling (ΔT), which governs the formation of devitrification pores. At a constant pressure, increasing ΔT results in the systematic formation of lithophysae, isolated spherulites, clustered spherulites, and flow-banded crystal fibers, each corresponding to distinct devitrification pore types. Interspherulite pores, intercrystallite pores, and lithophysa cavities commonly have wide diameters and very good connectivity, which define them as good reservoir spaces. Radiating, intracrystallite, and flow-banded micropores have small diameters and yet they represent significant reservoir spaces due to their high abundance. We conclude that devitrified volcanic rocks such as pyromeride, lithophysa rhyolite, and flow-banded glassy lava are favorable targets for volcanic oil and gas exploration.

1. Introduction Volcanic rocks cover ∼8% of the Earth's present-day land surface (Wilkinson et al., 2009). Rapid cooling of magma generally produces Volcanic rocks may contain unconventional oil and gas reservoirs, volcanic glass, which can form during pyroclastic eruptions and the and have generated increasing attention in recent years (Luo et al., extrusion or shallow intrusion of magma (Marshall, 1961; Rowe et al., 2005; Sruoga and Rubinstein, 2007; Lenhardt and Götz, 2011; Chen 2012). Most magma compositions can produce glass but rhyolitic et al., 2016; Wang and Chen, 2015; Zheng et al., 2018). Hydrocarbon glasses such as obsidian and perlite are especially common (McPhie basins containing commercial volcanic reservoirs have been discovered et al., 1993; Rafferty, 2012). Volcanic glass is thermodynamically un- and developed in many parts of the world (Schutter, 2003), such as the stable and typically undergoes crystallization, a process known as de- Qinshen gas field in China (3 ×1011 m3 gas reserves; Feng, 2008), Scott vitrification (Marshall, 1961; Lofgren, 1971a; McPhie et al., 1993). Reef gas field in Australia (11.98 ×10 bbl oil reserves and Rhyolitic and basaltic glasses are significantly less dense than their 3.8 × 1011 m3 gas reserves; Zou, 2013), Yabase oil field in Japan corresponding minerals (Table 1), and as a consequence the crystals (5 × 107 bbl oil reserves; Magara, 2003), and Medanito-25 de Mayo that form during devitrification occupy less volume than the original field in Argentina (4.4 ×108 bbl oil reserves; Sruoga and Rubinstein, glass. The open space created during devitrification is known as devi- 2007). trification porosity (e.g., Zhao et al., 2009; Zheng et al., 2018).

∗ Corresponding author. College of Earth Sciences, Jilin University, No.2199, Jianshe Street, Changchun, 130061, China. E-mail address: [email protected] (X. Sun). https://doi.org/10.1016/j.marpetgeo.2018.09.016 Received 10 July 2018; Received in revised form 11 September 2018; Accepted 14 September 2018 Available online 20 September 2018 0264-8172/ © 2018 Elsevier Ltd. All rights reserved. H. Zheng et al. Marine and Petroleum Geology 98 (2018) 718–732

Table 1 Paleozoic (Xiao et al., 2003; Wu et al., 2011)(Fig. 1a). The Erguna Density (g/cm3) of rhyolitic and basaltic glasses and minerals from their de- Block in the northwest comprises basement of a variety of khondalitic vitrification (data are from Zhao et al., 2010; Rafferty, 2012). rocks that formed between 1680 and 1060 Ma (Wu et al., 2011). The Glass or mineral Density Mean Great Xing'an Range and Hailar Basin together comprise the Xing'an Block to the south (Fig. 1a). The Great Xing'an Range is composed Rhyolitic glass Obsidian 2.13–2.42 2.36 mainly of large volumes of Mesozoic volcanic rocks and granitoids (Wu Pitchstone 2.22–2.51 2.37 et al., 2011). Basement lithologies include amphibolite-to greenschist- Perlite 2.23–2.39 2.31

SiO2 polymorphs 2.65 2.65 facies metamorphic rocks that formed from Paleozoic protoliths (A K-feldspar 2.55–2.63 2.59 et al., 2013). Basaltic glass 2.50–2.99 2.75 The stratigraphic succession of the Hailar Basin comprises the Lower Plagioclase 2.61–2.76 2.69 Jurassic Budate Group, the Middle Jurassic–Lower Cretaceous Pyroxene 3.21–3.56 3.39 Xing'anling and Zhalainuoer groups, and the Upper Cretaceous–Paleocene Beierhu Group (Wan, 2006; A et al., 2013) (Fig. 1c). The Budate Group is considered to represent the basement to Extrusive and explosive rocks commonly contain devitrification pores, the basin and contains three distinct lithological units: interlayered which can serve as an important reservoir space due to their great volcanic rocks, Paleozoic igneous rocks, and low-grade metamorphic abundance and high degree of connectivity (Zhao et al., 2009; Zheng rocks (Wan, 2006). The Xing'anling Group comprises a syn-rift succes- et al., 2018). For example, in the Songliao Basin of NE China, devi- sion, divided from base to top into the Tamulangou, Tongbomiao, and trification pores comprise ∼70% and ∼30% of pores in nonwelded tuff Nantun formations. The Tamulangou Formation is the least extensive of and ignimbrite, respectively (Zhao et al., 2009), and three wells yield these and is characterized by bimodal volcanic rocks that formed high porosities comprising 7%–13% devitrification pores (Feng et al., mainly during the Late Jurassic (Wan, 2006; Chen et al., 2007). The 2008). dominant Tongbomiao and Nantun formations (together also known as The devitrification of volcanic rocks has been studied by petrolo- the Shangkuli Formation) consist mainly of interbedded conglomerate, gists and chemists for more than a century. In recent decades, numerous sandstone, siltstone, and mudstone deposited in alluvial and lacustrine studies on crystal morphology (Keith and Padden, 1963; Lofgren, facies, which are also interlayered with volcanic rocks (Wan, 2006). 1971a, 1974), growth kinetics (Lofgren, 1971b; Gránásy et al., 2005; The overlying Zhalainuoer Group is divided into the Damoguaihe and Castro et al., 2008; Rowe et al., 2012), and growth rates (Marshall, Yimin formations, which comprise typical fluvial–deltaic and lacustrine 1961; Swanson, 1977; Watkins et al., 2009) have constrained the facies deposits. The uppermost Beierhu Group includes alluvial and conditions and processes associated with the nucleation and growth of fluvial conglomerate and mudstone of the Late Cretaceous Qingyuan- crystals from volcanic glass. Despite several investigations into the gang and Paleocene Huchashan formations (Wan, 2006)(Fig. 1c). devitrification mechanisms of volcanic glasses, the porosity produced The Hailar Basin has a NE–SW-trending axis and consists of 16 by devitrification remains poorly studied. Most devitrification experi- contiguous sags that formed synchronously in an extensional tectonic ments report significant void spaces due to volume contraction asso- setting (Sun et al., 2011; A et al., 2013)(Fig. 1b). The tectonic evolution ciated with crystallization. However, such spaces have generally been of the basin involved early syn-rift, middle post-rift, late inversion, and ignored during observations and in calculations (Lofgren, 1971a, 1974; sag stages, with deposits of each stage being separated by un- Watkins et al., 2009). Furthermore, many studies of volcanic reservoirs conformities (e.g., A et al., 2013)(Fig. 1c and d). Early syn-rift sub- have reported the pervasiveness and significance of devitrification but sidence and sedimentation were controlled by NE–SW-trending normal systematic studies of devitrification pores are rare and typically donot faults, and this phase was accompanied by Early Cretaceous volcanism include direct observations (Liu et al., 2010; Wang and Chen, 2015; (A et al., 2013). In contrast, post-rift deposition was characterized by Jiang et al., 2017). Previous optical microscopy investigations were broad subsidence and the widespread deposition of fine-grained clastic unable to properly characterize devitrification pores, which are typi- rocks in lacustrine and fluvial–deltaic environments (A et al., 2013). cally microns to sub-microns in size (Zhao et al., 2009; Feng et al., The study area is located along the western boundary of the Hailar 2008). Therefore, many fundamental questions remain regarding the Basin (Fig. 1b), within the Hulun Nur Sag. Dark mudstone (i.e., source types of devitrification pores, their characteristics, formation mechan- rock) that accumulated in the sag has a thickness of > 792 m and a total isms, and their effect on reservoir quality. organic carbon (TOC) content of > 2% (Feng et al., 2004). Oil and gas The purpose of this study is to answer these questions by providing are contained primarily within volcanic rocks of the Lower Cretaceous qualitative and quantitative data from devitrification pores of oil- Shangkuli Formation (Fig. 1c). The NE–SW-trending basin-controlling bearing volcanic rocks from the Hailar Basin, northeast (NE) China. Erguna Fault, which links the volcanic rocks to the source rocks, acted Detailed images of various devitrification pores are presented using a as a pathway for oil and gas migration (Fig. 1b and d). Oil-bearing combination of fluorescence image analyzer (FIA), scanning electron volcanic rocks with zircon U–Pb ages of 136–125 Ma (Li et al., 2014) microscopy (SEM), electron probe microanalyzer (EPMA), and laser- are widely exposed in the study area, and consist mainly of pyroclastic scanning confocal microscopy (LSCM) techniques. A clear under- and rhyolitic rocks (i.e., rhyolite and rhyolitic glass) (Fig. 1d). Three standing of devitrification pores may aid in the prediction of the quality major types of devitrified rhyolitic glass are recognized: pyromeride, of reservoirs containing devitrified volcanic rocks, thereby improving lithophysa rhyolite, and flow-banded glassy lava. Glassy shards with oil and gas exploration and development in many parts of the world. diameters of several millimeters to several centimeters commonly occur in other volcanic rocks, especially pyroclastic and autoclastic deposits. 2. Geological setting 3. Materials, methods, and terms used The late Mesozoic to Cenozoic Hailar Basin is located ∼1000 km north of Beijing, China, in the northern part of the late Mesoproterozoic 3.1. Materials and terms used to Carboniferous Central Asian Orogenic Belt. The Paleozoic tectonic development of this region was dominated by the subduction of the To develop a better understanding of devitrification pores and paleo-Asian oceanic lithosphere between the Siberian and North China constrain their effect on oil and gas production, the samples arere- cratons (e.g., Şengör et al., 1993; Xiao et al., 2003). Several discrete quired that (1) contain features that are typical of devitrified volcanic micro-continental blocks surround the Hailar Basin and its adjacent rocks, and (2) clearly illustrate the relationship between devitrification areas, and these are interpreted to have been accreted during the pores and oil or gas.

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Fig. 1. (a) Tectonic subdivisions of northeastern China and the location of the Hailar Basin (modified from Wu et al., 2011). 1, Tayuan–Xiguitu Fault; 2, Hegen- shan–Heihe Fault; 3, Solonker–Xar Moron–Changchun Suture; 4, Chifeng–Kaiyuan Fault; 5, Jiayi Fault; 6, Dunmi Fault; 7, Mudanjiang Fault. (b) Geological sketch map showing the location of the study area and distribution of sags within the Hailar Basin (after Zheng et al., 2018). (c) Stratigraphic column for the Hailar Basin (interior and periphery) showing the main lithologies, petroleum system elements, and tectonic events (after Wan, 2006; Chen et al., 2007; Zheng et al., 2018). The chronostratigraphic correlation of the Shangkuli Formation (basin periphery) with the Nantun and Tongbomiao formations (basin interior) is based on U-Pb zircon dating (Li et al., 2014) and palynological flora (Wan, 2006). Plh, Huchashan Formation (Fm.); K2q, Qingyuangang Fm.; K1y, Yimin Fm.; K1d, Damoguaihe Fm.; K1yl,

Yiliekede Fm.; K1sh, Shangkuli Fm.; K1n, Nantun Fm.; K1t, Tongbomiao Fm.; J2–K1t, Tamulangou Fm. (d) Cross section of the basin fill drawn through A–B (modified from Chen et al., 2007).

Volcanic glass commonly forms from rhyolitic magmas because its banded glassy lavas. chemical composition (high silica) induces a high degree of viscosity Lithological definitions used in this paper are based on Wyatt and melt polymerization. The inhibition of atomic diffusion through (1986), McPhie et al. (1993), and McGraw-Hill (2003), as follows. (1) this highly viscous and polymerized magma explains the lack of crystal Spherulite: a group of mostly spherical masses in an igneous rock growth and glass formation (e.g., Marshall, 1961; Rafferty, 2012). Ba- comprising radiating acicular crystals, which often also show con- saltic glass typically occurs as thin selvages and scattered fragments of centric banding. These are found dominantly in glassy, devitrified, or ash. In contrast, rhyolitic glass commonly occurs as thick flows. Con- hemicrystalline rocks, and are most common in rhyolitic rocks. (2) sequently, rhyolitic glass generally undergoes devitrification and con- Pyromeride: a conspicuous nodular spherulite present in certain rhyo- tains a range of devitrification-pore types. Therefore, oil-bearing litic rocks. (3) Lithophysa: a spherical or lenticular volcanic rock con- rhyolitic glass is considered an ideal sample material to characterize sisting of multiple concentric layers of fine-grained crystalline material devitrification pores and their contribution to volcanic reservoirs. separated by voids. Some lithophysae only contain one thick layer The continuous exposure of oil-bearing rhyolitic glass makes the surrounding a central void. (4) Flow-banded glassy lava: a dominantly study area an ideal natural laboratory. A total of 354 oil-bearing vitreous, devitrified, or hemicrystalline rock mass with well-developed rhyolitic glass samples were collected for analysis from surface out- flow foliations formed by the consolidation of magma effused from crops, including 284 pyromerides, 35 lithophysa rhyolites, and 35 flow- volcanic vents and fissures, consisting primarily of magnesium silicate.

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Table 2 Products and associated pores of devitrification of volcanic glass.

aLithophysa cavities and interspherulite pores are not shown in the sketches.

3.2. Methods maps of key regions were obtained using an electron probe microanalyzer (EPMA; JXA8230, JEOL Ltd.) at the EDD. These points and Thin sections of oil-bearing rhyolitic glass samples were prepared maps were used to confirm the presence of organic matter andde- using standard grinding and polishing methods, and the sections were termine the compositions of crystals that formed during devitrification. analyzed using a petrographic optical microscope (Olympus BX51). The The samples contain carbon concentrations that are several orders of morphology, size, and distribution of devitrification pores were pre- magnitude higher than the carbon coating, and therefore the acquisi- liminarily determined through a combination of field, hand-specimen, tion of the C composition was unaffected by the presence of the latter. and thin-section observations. Analysis was conducted using typical operating conditions: accelerating Aromatic components and nitrogen, sulfur, and oxygen (NSO)- voltage of 15 kV, probe current of 1 × 10−10 A, circular beam shape, bearing complexes (also known as resins and asphaltenes) within crude dwell time of 10 ms, and 300 × 300-point map areas. Analytical un- oil are fluorescent (Teichmuller and Wolf, 1977; Khorasani, 1987), and certainties were 2%–5%. The detailed procedures follow the standard therefore devitrification pores filled by crude oil can be recognized and SY/T 6027-2012 of the Chinese petroleum industry and the GB/T 4930- measured by analyzing fluorescence images. A fluorescence image 2008 National Standard of the People's Republic of China. analyzer (FIA; ZEISS Axio Imager M1m, ZEISS Group) was used for Following element mapping, three-dimensional (3D) reconstruc- fluorescence observations and imaging of thin sections at the Explora- tions of devitrification pores within representative samples were pro- tion and Development Research Institute of Daqing Oilfield Company duced using laser-scanning confocal microscopy (LSCM; Leica TCS Ltd., Daqing, China (EDD). A mercury source with a peak intensity of SP5II, Leica Microsystems) at the EDD, which is a nondestructive 360–365 nm and UV excitation filters were used to excite the samples. imaging technique that may be used to reconstruct 3D structures within Emitted fluorescence was obtained over the range of 450–700 nm.A samples (Andersson et al., 2010; Bagherzadeh et al., 2013). An argon total of 1512 high-resolution fluorescence images (2584 × 1936 pixels) ion laser source was used to excite the samples. The laser was operated were obtained and the major types of devitrification pore were de- with a wavelength of 480–630 nm (peak 520 nm) for hydrocarbons, and termined. Due to their differing fluorescence-emission spectra 600–800 nm (peak 670 nm) for NSO-bearing complexes. The LSCM (Khorasani, 1987), the light (aromatics), intermediate (resins), and images were acquired as a series of 30 slices with a size of 1024 × 1024 heavy (asphaltenes) components of the oil display short-wavelength pixels and a spatial resolution of 0.18 μm. Each image was then im- (blue–green), medium-wavelength (yellow–orange), and long-wave- ported into the Imaris 7.2.3 software package and the 3D structure of length (brown–dark-brown) colors, respectively. the pore system was reconstructed and quantitative calculations were Selected fluorescence images of various devitrification pores were performed to determine the values of the associated pore parameters. examined using a color image analysis system (CIAS-2004; Sichuan University), housed at College of Earth Sciences, Jilin University, 4. Types and characteristics of devitrification pores Changchun, China. Following hue-, luminance-, and saturation (HLS)- based scanning and calculations, the effective surface porosity (ESP) of Six major types of devitrification pore were identified (Table 2). The each devitrification-pore type was obtained from each image. The bulk various types of devitrification pore exhibit distinct characteristics and ESP for each lithology was then calculated as the arithmetic mean of at their distributions are dependent on lithology. least 6 fluorescence images. The analytical errors were less than 10%. The development of porosity in volcanic rocks depends on primary The detailed procedures and calculation principles are based on the and secondary processes that occur under closed- and open-system standard SY/T 6103-2004 of the Chinese petroleum industry. The ob- conditions, respectively (Sruoga and Rubinstein, 2007; Lenhardt and tained ESPs are used for the quantitative evaluation of reservoir quality Götz, 2011; Wang and Chen, 2015; Zheng et al., 2018). Spherulites and for the various types of devitrification pore and their corresponding lithophysae comprising fine-grained SiO2 polymorphs and feldspar are lithologies. characteristic products of the high-temperature (high-T) devitrification A scanning electron microscope (SEM; JXA8230, JEOL Ltd.) hosted of rhyolitic glass (Lofgren, 1971a; Kirkpatrick, 1975; McPhie et al., at the EDD was used to determine the morphology and size of devi- 1993). High-T devitrification occurs under closed-system conditions trification pores. Samples were mounted on SEM stubs and coated with during cooling and is therefore regarded as a primary process. carbon (thickness < 20 nm) to provide a conductive surface layer. The Experimental and petrological studies indicate that the growth rate SEM was operated at an accelerating voltage of 20 kV, a current value of high-T devitrification products becomes prohibitively slow at tem- of 1 × 10−8 A, and a working distance of 1 mm. A total of 497 thin peratures of < 400 °C (Watkins et al., 2009) and, as such, temperatures sections were analyzed and SEI images were obtained by a secondary of < 400 °C are defined as low-temperature (low-T) in this study. De- electron detector. Following initial SEM imaging, element points and vitrification can also occur at low temperatures, but requires alonger

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Fig. 2. Spherulites and associated devitrification pores. (a) Spherulite bands aligned along flow foliations. (b) Clustered spherulites. (c) Interspherulite poresfilled with asphaltenes (brown fluorescence). (d) Isolated spherulite in a glassy matrix. (e) Radiating micropores filled with aromatic hydrocarbons (blue fluorescence)and resins (orange fluorescence). (f) Radiating micropores and mosaic “microparticles”. (g) Alternating micropores and crystal fibers, and “microparticles” thatlack pores. (h) Micropores and crystal fibers. time (Lofgren, 1971a, 1974). During the late cooling of magma, low-T primary pores are referred to as interspherulite pores (Fig. 2b). The devitrification may form axiolite under closed-system conditions release of volatiles during magma cooling may enhance interspherulite (Lofgren, 1971a, 1974). Furthermore, the atomic arrangement of vol- porosity. Interspherulite pores typically have diameters of 50–400 μm canic glass is highly unstable and, under open-system conditions, the and display good connectivity, and are therefore favorable for the diffusion of water and/or secondary heating can result in theinsitu storage of oil and gas. Fluorescence image analyses show that inter- hydrothermal alteration of glass with a negligible loss of material spherulite pores are commonly filled with large quantities of crude oil (Marshall, 1961; Gimeno, 2003). This process is known as secondary (Fig. 2c). Seventy-one clustered spherulite-bearing pyromeride samples low-T devitrification and may produce flow-banded crystal fibersin yield ESPs of 4.6%–19.7%, with a mean of 11.2%. Notably, only minor glassy lavas. amounts of crude oil are observed within the clustered spherulites (Fig. 2c). 4.1. Spherulites and associated devitrification pores Isolated spherulites display significant differences relative to those that occur in clusters. Numerous intraspherulitic radiating-devitrifica- Spherulites are typically produced during high-T devitrification of tion micropores occur along crystal-fiber boundaries (Fig. 2e–h). Pore volcanic glass and comprise a radiating array of individual crystal fi- diameters are typically homogeneous and similar to those of the fibers. bers, with only minor changes in crystallographic orientation between In Fig. 2e–h, the crystal fibers and radiating micropores have diameters adjacent crystals (Fig. 2). In rhyolitic glass, the fibers are mainly K- of 3–8 μm. feldspar and/or SiO2 polymorphs (e.g., Watkins et al., 2009). Spher- Fluorescence and SEM imaging reveal two significant phenomena ulites in pyromerides occur in megascopic light-grey bands that are that are discussed in detail below. Firstly, in contrast to homogeneous aligned along the flow foliation, which records the shear flow ofhot devitrification features observed under cross-polarized light (e.g., ex- magma (Fig. 2a). tinction patterns), the distribution of radiating micropores is strongly Spherulites may nucleate preferentially to form clusters (Fig. 2b) or heterogeneous. Micropores are common in the middle layers of in- occur randomly and isolated (Fig. 2d). Clustered spherulites generally dividual spherulites and rare/absent in the center and outermost layers impinge on each other and form irregular polyhedrons with diameters (Fig. 2e). The middle layers also contain mosaic “microparticles” scat- of < 1 mm, whereas isolated spherulites are commonly spherical and tered among crystal-fiber bundles (Fig. 2f). A SEM image (Fig. 2g) have diameters of > 1 mm. Some spherulites also exhibit a complex shows that the radiating micropores alternate with arrays of crystal morphology that resemble sheaves of wheat tied at the center (“bow-tie fibers, resembling a series of pipes. In contrast, the adjacent “micro- spherulites” in Lofgren, 1971a). particles” lack pores. Secondly, crude oil occurs heterogeneously within Significant void space is created between clustered spherulites due the radiating micropores (Fig. 2e and f). Aromatic hydrocarbons (with to the volume contraction associated with devitrification. These blue–green fluorescence) and resins (yellow–orange fluorescence) occur

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Fig. 3. EPMA element maps and points of a region within a pyromeride sample (see Fig. 2g). The coloured scale bar at the right shows relative concentration. The C compositional map confirms that the radiating micropores are filled with organic matter. The primary mineral phases are mainly oblate cristobalite2 masses(SiO ) and

K-feldspar fibers (K[AlSi3O8]). The identification of mineral phases is based on Anthony et al. (2015).

723 H. Zheng et al. Marine and Petroleum Geology 98 (2018) 718–732 within micropores, whereas asphaltenes (brown–dark-brown fluores- Thus, the effective storage of NSO-bearing complexes is inferred to cence) are rare. Furthermore, some areas of the micropores contain require a pore diameter of > 4.8–6.8 μm, whereas pores with diameters only aromatic hydrocarbons. of 22 nm to 4.8–6.8 μm can store hydrocarbons and gas. It should be Element points and maps were used to investigate the hetero- noted that more precise data are required to confirm this hypothesis. geneous distribution of radiating micropores. Element points and maps Such data may be acquired through quantitative petrophysical methods (Fig. 3) were obtained from the area shown in Fig. 2g. Homogeneous such as mercury injection capillary pressure (MICP) measurements and bands of high K and Na concentrations transect the analyzed region. In nuclear magnetic resonance (NMR) spectroscopy. contrast, the other elements display a heterogeneous distribution. Si is As mentioned above, only small amounts of crude oil occur within relatively enriched in the “microparticles”, depleted in the crystal fi- clustered spherulites with diameters of < 1 mm (Fig. 2c). In contrast, bers, and exhibits subtle banding. O displays clear banding in the abundant crude oil occurs in the larger isolated spherulites with dia- crystal fibers, whereas in the “microparticles” it displays an oblate meters of > 1 mm (Fig. 2e and f). Fluorescence images of 213 spher- shape. Al, Ca and Fe are relatively enriched in the crystal fibers and ulites indicate a strong correlation between spherulite diameter and the occur in bands. The distributions of these elements suggest that the development of radiating micropores (Fig. 4b). Larger spherulites primary mineral phases within individual spherulites are arranged contain larger crystal fibers and thus exhibit wider pore diameters and heterogeneously. Feldspars comprise dominantly homogeneous ra- better connectivity. The 178 analyzed pyromeride samples (spherulite diating fibers of K-feldspar (Fig. 3). Radiating micropores occur along diameter > 1 mm) yield ESPs of 4.8%–28.5%, with a mean of 16.8%. crystal fiber boundaries. Devitrification also produced abundant cris- In summary, both clustered and isolated spherulites can form fa- tobalite crystallites (microlites), which occur as oblate masses and are vorable reservoir spaces. Clustered spherulites have small diameters locally intergrown with the K-feldspar fibers within radiating micro- and poor intraspherulitic porosity, but contain well-developed inter- pores, forming mosaic “microparticles” (Fig. 3). Ca and Fe were not spherulite pores. In contrast, isolated spherulites have large diameters incorporated into the cristobalite structure during its growth and well-developed intraspherulitic porosity (i.e., radiating micro- (Ca < 0.02% and Fe < 0.09%; Anthony et al., 2015), and thus most of pores). Despite their small size, radiating micropores represent im- them are separate from cristobalite and included in K-feldspar (Fig. 3). portant reservoir spaces due to their high abundance. Therefore, the distribution of devitrification-related fibrous K-feldspar and oblate cristobalite masses constrained the growth of radiating micropores and produced the texture shown in Fig. 2e–h. 4.2. Lithophysae and associated devitrification pores The heterogeneous distribution of oil within radiating micropores results from the lower limit on the diameter of pores that can effectively Lithophysae are another typical product of the high-T devitrification store crude oil components. The molecular diameter of hydrocarbons of volcanic glass. They begin to grow during the early stages of cooling (saturates and aromatics) is significantly lower than that of NSO- (McPhie et al., 1993). Lithophysae in the collected samples have dia- bearing complexes (resins and asphaltenes). For example, the molecular meters of generally 2–4 cm with a maximum of 8 cm (Fig. 5). The diameters of methane, benzene, normal alkane, and cycloalkane are diameters of the spherical cavities vary from several millimeters to 0.38, 0.47, 0.48, and 0.54 nm, respectively, which are 10–20 times several centimeters and are positively correlated with lithophysa dia- smaller than those of asphaltenes (5–10 nm) (Tissot and Welte, 1984). meter (Zheng et al., 2018). The cavities are commonly filled with More importantly, viscosity increases and fluidity decreases in the order abundant crude oil within concentric rings (Fig. 5a), or, more rarely, of hydrocarbons–resins–asphaltenes (Selley and Sonnenberg, 2014). with syngenetic melts or deuteric fluids that form solid cores. Therefore, during oil emplacement, the various hydrocarbon com- Lithophysae are considered a distinct form of spherulite (McPhie ponents are stored in pore spaces of different diameters. Hydrocarbons et al., 1993). In the analyzed samples, some lithophysa display ra- and resins can effectively permeate into radiating micropores with diating devitrification similar to spherulites. This type of lithophysa small diameters, whereas large asphaltenes molecules do not. In con- represents a large spherulite with concentric cavities. Furthermore, trast, all oil components can enter pores with large diameters, such as large spherulites can also contain cavities (Zheng et al., 2018). How- interspherulite pores (Fig. 2c). The lower limit of pore diameter that ever, most lithophysae exhibit devitrification textures and porosity can effectively adsorb methane molecules is 22 nm(Du et al., 2017). characteristics that differ from those of spherulites. Crystallites (or Based on the statistics obtained from the integration of fluorescence and crystals) hosted in lithophysae are well-formed with clear crystal faces SEM images, the lower limit of pore diameter that can effectively store (Fig. 5b). Most crystallites are skeletal but scarce subhedral and eu- NSO-bearing complexes is estimated to exceed 4.8–6.8 μm (Fig. 4a). hedral grains also occur. Crystallite diameters (measured along the long axis) are typically 30–150 μm, with a maximum of 500 μm.

Fig. 4. (a) Gaussian distribution of diameters for pores containing hydrocarbons (red) and NSO-bearing complexes (blue). The effective storage of NSO-bearing complexes requires a pore diameter of > 4.8–6.8 μm. μ = mean; σ = standard deviation. (b) Effective surface porosity (ϕ) versus spherulite diameter (D) for individual spherulites. With increasing diameter, the effective surface porosity of spherulites is enhanced. (For interpretation of the references to colour inthisfigure legend, the reader is referred to the Web version of this article.)

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Fig. 5. Typical lithophysa and associated devitrification pores. (a) Concentric spherical cavities filled with crude oil (black). (b) Fine-grained skeletalSiO2 polymorphs and K-feldspar that formed through devitrification. (c) Intercrystallite pores filled with asphaltenes (brown fluorescence) and intracrystallite micropores filled with aromatic hydrocarbons (blue–green fluorescence). (d) Intercrystallite pores and sieve-like intracrystallite micropores.

volcanic glass commonly contain three types of devitrification pore (Fig. 8): (1) lithophysa cavities, (2) intercrystallite pores, and (3) intracrystallite micropores. Individual lithophysae therefore represent excellent reservoir spaces; however, natural lithophysae are typically isolated and surrounded by coherent glassy or rhyolitic matrix. Twenty- three lithophysa rhyolite samples yield ESPs of 1.8%–14.4%, with a mean of 7.7%.

4.3. Flow-banded glassy lavas and associated devitrification pores

Diffusion of water and/or secondary heating can trigger thesec- ondary low-T devitrification of volcanic glass. Secondary low-T devitrification is typically observed in flow-banded glassy lava, which contains well-developed primary interflow laminar pores along flow ϕ Fig. 6. Effective surface porosity ( ) versus crystallite diameter (D) for inter- foliations that provide permeable channels for the influx of hydro- crystallite pores. thermal fluids and/or formation water (Fig. 9)(Sruoga and Rubinstein, 2007; Zheng et al., 2018). Secondary low-T devitrification produces Lithophysae typically form during isovolumetric cooling of volatile- numerous crystal fibers along the foliation, as well as abundant devi- rich magma. Crystallites commonly grow in wide spaces and are in trification micropores that form between parallel crystal fibers(Fig. 9b contact only along their crystallographic long axes. Intercrystallite and c). Crystal fibers grow around pre-existing primary phenocrysts pores are common and have large diameters (30–200 μm) with very resulting in the formation of voids (Fig. 9d and e). good connectivity, forming favorable reservoir spaces. Such pores can The crystal fibers typically display a homogeneous size distribution be readily filled by large quantities of oil(Fig. 5c). FIA-based statistical with diameters of 3–8 μm. Individual flow-banded micropores have analyses indicate that intercrystallite pore diameters are dependent on diameters of several micrometers (Fig. 9c, f, h); however, they com- the length of the crystallographic long axes of crystallites (Fig. 6). In- monly overlap with large interflow laminar pores, enlarging the effec- tercrystallite pores within 25 lithophysae yield ESPs of 16.2%–34.4%, tive pore diameter to tens or hundreds of micrometers (Fig. 9g). Iso- with a mean of 25.6%. lated flow-banded micropores are generally filled with light oil Abundant hydrocarbons occur within the crystallites and are pre- components with blue–green fluorescence, whereas those that overlap sent in dense clusters. SEM imaging (Fig. 5d) indicates that crystal with interflow laminar pores contain all oil components and exhibit surfaces contain numerous intracrystallite micropores, forming a sieve- yellow–dark-brown fluorescence (Fig. 9b). like texture with pore diameters of 1–7 μm. Flow-banded micropores represent favorable reservoir spaces due to According to the EPMA element points and maps, the primary mi- their high abundance and very good connectivity. Such pores are ubi- neral phases of the crystallites are skeletal cristobalite and K-feldspar quitous and distributed homogeneously in banded volcanic glass and (Fig. 7). The C compositional map indicates that large amounts of or- other rhyolitic lavas. As it is problematic to distinguish between flow- ganic matter occur within intercrystallite pores, with lesser amounts in banded micropores and interflow laminar pores, their ESPs were mea- intracrystallite micropores. The O compositional map clearly illustrates sured collectively. Thirty flow-banded glassy lava samples yield ESPs of the sieve-like morphology of the intracrystallite micropores. Notably, 2.7%–31.8%, with a mean of 14.3%. lithophysae contain less K-feldspar and more SiO2 polymorphs than The C compositional map suggests that large quantities of organic spherulites. matter occur within flow-banded micropores and interflow laminar In summary, lithophysae that formed through devitrification of pores (Fig. 10). The primary mineral phases are dominantly fibrous K-

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Fig. 7. EPMA element maps and points of a region within a lithophysa rhyolite sample (see Fig. 5d). The coloured scale bar at the right shows relative concentration. The C compositional map confirms that the intercrystallite and intracrystallite micropores are filled by organic matter. The primary mineral phases areskeletal cristobalite (SiO2) and K-feldspar (K[AlSi3O8]). The identification of mineral phases is based on Anthony et al. (2015).

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crystals. For example, crystals in lithophysae and flow-banded glassy lavas have distinct morphologies (Figs. 5 and 9), and crystals in clustered and isolated spherulites have similar morphologies but distinct nucleation densities (Fig. 2), resulting in a wide range of devitrification pore types (Table 2). Both the nucleation density and the morphology of newly formed crystals are dependent on the degree of supercooling (ΔT), which represents the difference between the liquidus temperature (Tl) and the crystallization (growth) temperature (Tc) of the melt (Swanson, 1977; Di Lorenzo, 2003). With increasing ΔT, the nucleation density increases from zero to a maximum at a specific ΔT (Swanson, 1977; Di Lorenzo, 2003). Furthermore, with increasing ΔT, the crystallographic instability Fig. 8. Sketch of typical lithophysae and associated devitrification pores. Blue increases and thus the crystal morphology is expected to change sys- areas represent effective pore spaces. (For interpretation of the references to tematically from euhedral–subhedral crystals to skeletal crystallites, colour in this figure legend, the reader is referred to the Web version ofthis dendritic crystallites, spherical spherulites, bow-tie spherulites, and fi- article.) nally to axiolitic fibers (fibrils) (Fig. 11)(Keith and Padden, 1963; Lofgren, 1971a, 1974; Kirkpatrick, 1975; Swanson, 1977; Di Lorenzo, feldspar and SiO2 polymorphs. As the crystal habit of SiO2 polymorphs 2003; Castro et al., 2008; Watkins et al., 2009). For example, experi- in flow-banded lavas is poorly presented, it is problematic to identify ments performed by Fenn (1977) revealed that feldspar grown in the their specific phases. Consequently, such2 SiO fibers are collectively NaAlSi3O8–KAlSi3O8H2O system form (1) isolated tabular crystals at referred as SiO2 polymorphs. Notably, flow-banded fibers contain more low ΔT (< 40 °C), (2) coarse open spherulites at moderate ΔT K-feldspar and less SiO2 polymorphs than lithophysae and spherulites. (75°C–145 °C), and (3) fine closed spherulites at high ΔT (245°C–395 °C). The cooling of lithophysae is continuous and the temperature drops 5. Formation of devitrification pores slowly. Under these conditions, the temperature of crystal growth ap- proximates the liquidus temperature (low ΔT). Consequently, the newly The formation of devitrification pores is dependent on the number formed crystallites (or crystals) have a skeletal morphology with a low of nuclei (i.e., nucleation density) and the morphology of newly formed

Fig. 9. Flow-banded glassy lava and associated devitrification pores. (a) Glassy lava showing a subtle flow foliation. (b) Isolated flow-banded micropores filledwith aromatic hydrocarbons (blue–green fluorescence). Overlapping flow-banded micropores and interflow laminar pores filled with asphaltenes (brown fluorescence). (c) Typical flow-banded micropores filled with organic matter. (d) Secondary crystal fibers that formed through low-T devitrification are aligned withmagmaticflow foliations and surround K-feldspar phenocryst that grew during crystallization of the magma. (e) Fibrous flow-banded micropores filled mainly with aromatic hydrocarbons (blue–green fluorescence); the large void is filled with asphaltenes (brown fluorescence). (f) Zone of crystal fibers. (g) Overlapping flow-banded micropores and interflow laminar pores. (h) Typical crystal fibers formed by secondary low-T devitrification.

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Fig. 10. EPMA element maps and points of a region in a flow-banded glassy lava sample (see Fig. 9g). The coloured scale bar at the right shows relative concentration. The C compositional map confirms that the flow-banded micropores and interflow laminar pores are filled by organic matter. The primary mineralphases are K-feldspar (K[AlSi3O8]) and SiO2 polymorphs fibers. The identification of mineral phases is basedon Anthony et al. (2015).

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Fig. 11. Sketch showing the evolution of crystal morphology with increasing degree of supercooling (ΔT). nucleation density. Many pore spaces are preserved and form well-de- feldspar increases (Figs. 3, 7 and 10). For example, cristobalite is veloped intercrystallite pores (Fig. 5b–d). As the melt within the li- dominant in lithophysae, whereas flow-banded glassy lavas contain thophysa cools slowly, the newly produced skeletal crystallites are mainly feldspar. commonly corroded by deuteric fluids to form sieve-like intracrystallite Al–O bonds in volcanic glasses are much weaker than Si–O bonds micropores (Sruoga et al., 2004; Sruoga and Rubinstein, 2007) (Kirkpatrick, 1975). At high ΔT, the breaking, stretching, and reforming (Fig. 5d). of Al–O bonds to form feldspar crystals is much easier than that in- At lower ΔT, isolated spherulites with diameters of > 1 mm form volving Si–O bonds (Marshall, 1961; Kirkpatrick, 1975). Therefore, new first, and are characterized by fewer individuals, regular geometry, and crystals that form by low-T devitrification (corresponding to a large ΔT) large crystal fiber diameters, corresponding to a relatively low nu- typically contain more feldspar and fewer SiO2 polymorphs. cleation density and large openness between crystal fibers (Fig. 2d–h). Clustered spherulites with diameters of < 1 mm form at lower ΔT and are characterized by more individuals, irregular geometry, and small 6. Contribution to volcanic reservoirs crystal fiber diameters, due to a relatively high nucleation density and small openness between crystal fibers (Fig. 2b and c). Consequently, The contribution of devitrification pores to volcanic reservoirs can radiating micropores decrease in abundance, eventually becoming occur via two processes: (1) the formation of favorable reservoir spaces negligible, whereas abundant void spaces are produced between the within typical devitrified volcanic rocks (as discussed above), or (2) the faces of clustered spherulites, forming interspherulite pores. enhancement of bulk porosity and permeability in rhyolites, pyroclastic Water and/or secondary heating have significant effects on sec- rocks, autoclastic deposits, and other volcanic rocks. ondary low-T devitrification (Marshall, 1961; Rowe et al., 2012; Devitrification pores are commonly small (micron scale) andyet Kirkpatrick, 1975). Flow-banded glassy lavas contain well-developed they are present in large quantities and display very good connectivity. primary interflow laminar pores that provide channels for the influxof They may form high-quality reservoir spaces and can act as channels for hydrothermal fluids and/or formation water (Fig. 9b, d, e). They are the flow of inorganic and organic fluids. The reservoir capacity ofeach therefore favorable for secondary heating and the diffusion of water, devitrification-pore type analyzed in the present study is shownin and thus secondary low-T devitrification. Water and heat are con- Fig. 12a. These data may be used to predict the quality of reservoirs centrated at laminar interflow pore faces, allowing numerous crystal containing such pores. fibers to form parallel to the flow foliation, and thus large quantitiesof As clustered spherulites and flow-banded crystal fibers form ata devitrification micropores are produced between the fibers(Fig. 9g and high nucleation densities, they are numerous and display a homo- h). These flow-banded crystal fibers are analogues of the primary ax- geneous distribution. Therefore, the ESPs of interspherulite pores and iolitic fibers that form at very large ΔT. flow-banded micropores (including interflow laminar pores) maydi- The various devitrified volcanic rocks examined in this study show rectly represent the ESPs of the corresponding pyromerides and flow- similar whole-rock major element compositions (Supplementary banded glassy lavas, respectively. In contrast, isolated spherulites and Table 1), indicating that the various devitrification pores formed under lithophysae that are produced at low nucleation densities are scattered isochemical conditions. In contrast, element points and maps of the within a coherent glassy or rhyolitic matrix. Thus, the ESPs of radiating various newly formed crystals are distinct (Figs. 3, 7 and 10). With micropores and intercrystallite pores do not directly represent those of increasing ΔT, devitrification of volcanic glass produce lithophysae, the corresponding pyromerides and lithophysa rhyolites. then spherulites, and finally flow-banded crystal fibers. The mineralogy The reservoir capacity of each lithology is shown in Fig. 12b. Ra- of the newly formed crystals also shows a coupled evolution. With in- diating micropores display variable ESPs due to variations in spherulite diameter; however, the corresponding pyromeride yields relatively creasing ΔT, the proportion of SiO2 polymorphs decreases and that of homogeneous ESPs. A similar phenomenon occurs in intercrystallite

Fig. 12. Box-and-whisker plots showing the variation in effective surface porosity (ESP) in various (a) devitrification pores and (b) lithologies. Pyromeride AandB represent pyromerides containing isolated and clustered spherulites, respectively. Q1, first quantile; Q3, third quantile.

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Fig. 13. 3D reconstructions of devitrification pores obtained using laser-scanning confocal microscopy (LSCM). (a) Radiating micropores. (b) Intercrystallite pores and intracrystallite micropores. (c) Flow-banded micropores. NSOCs represents NSO-bearing complexes; HN ratio is the ratios of hydrocarbons and NSO-bearing complexes in devitrification pores. pores and corresponding lithophysa rhyolite. The sizes of pores within dissolution when they are permeated by hydrothermal fluids, formation isolated spherulites and lithophysae change with local physical condi- water, or crude oil. The dissolved primary components and precipitated tions but the effect of such variation is limited during whole-rock secondary minerals (e.g., kaolinite and montmorillonite that are loosely porosity calculations. In summary, all of the studied lithologies re- packed and easily transported) are commonly mechanically and/or present favorable targets for volcanic oil and gas exploration. chemically removed by the fluids (Sruoga and Rubinstein, 2007; Zheng LSCM 3D reconstructions of devitrification pores in representative et al., 2018). Therefore, this process significantly enhances the porosity oil-bearing samples show the distribution of hydrocarbons and NSO- of the rock. For example, Fig. 14 shows a spherulite that was subjected bearing complexes within various devitrification pores (Fig. 13). Large to intense secondary dissolution. The original devitrification texture of quantities of oil fill the examined pore spaces. Reconstructed volumes the spherulite is locally dissolved, and the pore spaces are thus en- of an isolated spherulite, lithophysa, and flow-banded glassy lava yield hanced and are filled by large quantities of crude oil. The dissolution bulk effective porosities of 11.1%, 35.7%, and 10.7%, respectively, and intensity is positively correlated with the development of pre-dissolu- ratios of hydrocarbons and NSO-bearing complexes of 1.4, 1.2, and 2.1, tion porosity, which provides a channel for fluid flow (Zheng et al., respectively. Lithophysae contain the most NSO-bearing complexes due 2018). The devitrification of volcanic glass typically produces abundant to their large intercrystallite pores that have a mean volume of 4835 μm3. Consequently, oil in macropores such as intercrystallite and interspherulite pores is extracted first during the exploitation of volcanic reservoirs. Light oil components and gas in small pores (e.g., radiating and intracrystallite pores and flow-banded micropores) are then slowly released. This process ensures a stable hydrocarbon yield and recovery from volcanic reservoirs.

Rhyolitic glass, SiO2 polymorphs, and K-feldspar have densities of 2.13–2.51, 2.65, and 2.55–2.63 g/cm3, respectively (Table 1). There- fore, complete devitrification of low-density (2.133 g/cm ) rhyolitic glass to crystals with high densities (2.65 g/cm3) would result in a porosity increase of 17.7%. However, several of our samples yield ESPs of > 17.7%. This excess porosity was produced during the secondary dissolution of glass and minerals under open-system conditions. Sec- ondary dissolution is a key process during porosity evolution and can Fig. 14. Fluorescence image showing the effects of secondary dissolution. The accompany all post-eruptive processes (McPhie et al., 1993; Chen et al., arrows indicate dissolved crystal fibers and resulting pore spaces that are filled 2016, 2017; Zheng et al., 2018). Pore spaces undergo secondary with asphaltenes (brown fluorescence).

730 H. Zheng et al. Marine and Petroleum Geology 98 (2018) 718–732 pore spaces, which are then enhanced through secondary dissolution, During the exploitation of oil and gas from volcanic reservoirs, oil is making devitrified volcanic rocks favorable reservoirs. It should be first extracted from macropores, after which light oil and gas within noted that secondary dissolution involves complex physical, chemical, small pores are slowly released, ensuring a stable yield and re- and geological processes that will be examined in future research. covery. Devitrified volcanic rocks such as pyromeride, lithophysa The devitrification of volcanic glass can also produce less-common rhyolite, and flow-banded glassy lava are favorable targets for orb and micropoikilitic textures comprising fine-grained feldspar and volcanic oil and gas exploration. SiO2 polymorphs (Lofgren, 1971b; McPhie et al., 1993). As the trans- formation of volcanic glass to crystals commonly generates consider- Acknowledgements able porosity due to isovolumetric contraction, these textures may also contain abundant devitrification pores that might host oil and gas. We gratefully acknowledge the constructive comments of two However, further work is required to verify this hypothesis. anonymous reviewers. We also thank Xianda Sun for helping with the EPMA and LSCM analyses, and Jingxiong Tian for assistance with the 7. Conclusions collection of samples. This study was funded by the National Natural Science Foundation of China (Grants 41790453 and 41472304), the The integration of FIA, SEM, EPMA, and LSCM analyses illustrates National Basic Research Program of China (Grant 2009CB219305), and the complexity of devitrification pores in volcanic rocks. The main the China Scholarship Council (Grant 201806170211). findings of this study are as follows. Appendix A. 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Res. 40, developed intraspherulitic radiating micropores with diameters of 21–42. https://doi.org/DOI 10.1016/j.cretres.2012.05.005. 3–8 μm. The distribution of radiating micropores within individual Andersson, L., Jones, A.C., Knackstedt, M.A., Bergström, L., 2010. Three-dimensional spherulites is heterogeneous due to the intergrowth of fibrous K- structure analysis by X-ray micro-computed tomography of macroporous alumina templated with expandable microspheres. J. Eur. Ceram. Soc. 30, 2547–2554. feldspar and oblate cristobalite masses during devitrification. The https://doi.org/10.1016/j.jeurceramsoc.2010.05.003. development of intraspherulitic porosity is positively correlated Anthony, J.W., Bideaux, R.A., Bladh, K.W., Nichols, M.C., 2015. Handbook of Mineralogy. with spherulite diameter. 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