Devitrification Pores and Their Contribution to Volcanic Reservoirs
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Marine and Petroleum Geology 98 (2018) 718–732 Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo 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 in- terspherulite pores. In contrast, isolated spherulites have larger diameters (> 1 mm) and well-developed in- traspherulitic radiating micropores. Lithophysae contain spherical cavities and layers comprising skeletal crys- tallites. 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, in- tracrystallite, 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.,