Research On Oil And Gas Reservoir Spaces Within Rhyolitic Volcanic Rocks

Volcanic reservoirs,a unique type of hydrocarbon reservoir,are drawing great attention worldwide.Rhyolitic volcanic rocks represent an important component of volcanic reservoirs;unfortunately,the characteristics,capacity,formation mechanisms,and main controlling factors of reservoir spaces developed in rhyolitic volcanic rocks have received little attention.The lower Cretaceous Shankuli Formation,located in the western boundary of the Hailar Basin,NE China,contains a succession of continuously exposed asphalt-bearing volcanic rocks,which can be considered an analog for volcanic reservoirs exposed at the surface.This area provides many new geological phenomena and represents an ideal natural laboratory in which to conduct a systematic study on volcanic reservoirs from macroscopic-to microscopic-scale.Based on detailed field study and integrating of techniques including fluorescence image analyzer,color image analysis system,scanning electron microscopy,electron probe microanalyzer,laser-scanning confocal microscopy,geochemical analysis,40Ar/39 Ar isotopic dating,and structural analysis,this dissertation provides a systematic study on types,characteristics,formation mechanisms,and main controlling factors of oil and gas reservoir spaces within rhyolitic volcanic rocks.A series of new findings are as follows.1.Geological features of volcanic rocksRhyolitic volcanic rocks of the lower Cretaceous Shankuli Formation,situated on the western boundary of the Hailar Basin,have complete lithologies and lithofacies,and showing as complex combinations.As for lithology,the rhyolitic volcanic rocks mainly include lava,pyroclastic lava,pyroclastic rock,tectonite,and their sub-types.The most dominant lithology is rhyolitic lava and glassy lava(70.78%)with high viscosity(?Rhyolite = 6.34 × 108–2.37 × 107 Pa·s).Regarding lithologies,there are explosive facies of the early stage of volcanic eruptions,effusive facies of the intermediate stage of volcanic eruptions,extrusive and conduit facies of the late stage of volcanic eruptions,and their sub-facies.Volcanic edifices are dominated by small-scale cinder cones,followed by lava domes which are produced by extrusion through parasitic cones.The formation of these volcanic edifices are clearly controlled by the Erguna deep-large fault,they successively outcrop along the NE-SW-trending in a beaded pattern,and belong to fissure eruptions.For geochemistry,the rhyolitic volcanic rocks meet the criteria of typical A-type granite.They were most probably derived from magmatic underplating and partial melting of quartz-feldspathic lower crust,with the lithospheric mantle material involved due to the extensional deformation of the Erguna deep-large fault.2.Types,characteristics,and formation mechanisms of oil and gas reservoir space,and effective reservoir systems of rhyolitic volcanic rocks12 sub-type primary and 12 sub-type secondary diagenetic processes are recognized.These processes have close relationships with the reservoir spaces.Primary processes,which lead to high porosity and permeability,mainly include(isovolumetric)solidification and contraction,gas expansion and release,deuteric dissolution,flow fragmentation,autoclastic brecciation,fragmentation of phenocrysts,and high-temperature devitrification.These processes control the development of corresponding primary porosity such as intraspherulite microfractures,interspherulite pores,interlayer fissure,primary joint,lithophysa cavity void,vesicle,deuteric dissolution pore,interflow laminar pore,flow joint,interclast pore,shattered crystal fracture,radiating micropore,intercrystallite pore,and intracrystallite micropore.Primary porosity commonly makes a large contribution to enhanced reservoir quality,and also promotes migration,alteration,and filling by inorganic fluids and petroleum,which provides a foundation for the development of secondary porosity.Secondary processes,which result in high porosity and permeability,mainly include hydration,secondary dissolution,recrystallization,quench fragmentation,explosion,hydrothermal brecciation,tectonism,and low-temperature devitrification.These processes control the development of corresponding secondary porosity such as perlitic fracture,secondary dissolution pore,inter-recrystal pore,quench fracture,explosive fracture,hydraulic fracture,drusy breccia pore,tectonic fracture,and low-temperature devitrification pore.Devitrification,secondary dissolution,and tectonism commonly widely influence all types of rhyolitic volcanic rocks,pores and fractures produced by these processes have considerable effective surface porosity and fairly good connectivity,making them the most important reservoir space types or channels for migration of fluids.Different types of reservoir space types combine and connect with each other to develop three-dimensional pore-fracture nets,forming effective reservoir systems in rhyolitic volcanic rocks.3.Devitrification and associated devitrification poresRhyolitic volcanic rocks commonly underwent wide devitrification and numerous devitrification pores were developed.The primary high-temperature devitrification produced spherulites and lithophysae.Spherulites occur as individuals and within clusters.Clustered spherulites have small diameters(< 1 mm)and poor intraspherulitic porosity,but exhibit well-developed interspherulite pores.Interspherulite pores typically have large diameters(50–400 ?m)and display very good connectivity.In contrast,isolated spherulites have large diameters(> 1 mm)and contain well-developed intraspherulitic radiating micropores with diameters of 3–8 ?m.The development of intraspherulitic porosity is positively correlated with spherulite diameter.Lithophysae commonly comprise spherical cavities and layers of fine-grained feldspar and/or Si O2 polymorphs.These crystallites are mainly skeletal with long axes of 30–150 ?m.Abundant pore spaces are preserved and form intercrystallite pores with large diameters(30–200 ?m)and very good connectivity.Numerous sieve-like intracrystallite micropores occur within the crystallites and have diameters of 1–7 ?m.The secondary low-temperature devitrification produced flow-banded crystal fibers within glassy lavas.Abundant devitrification micropores(micrometers wide)occur between crystal fibers.Nucleation density and the morphology of crystals that form during devitrification are dependent on ?T,which governs the formation of devitrification pores.With increasing ?T,skeletal crystallites in lithophysae,isolated spherulites,clustered spherulites,and flow-banded crystal fibers in glassy lava form sequentially and are associated with distinct devitrification pores.The mineralogy of newly formed crystals is also controlled by ?T.With increasing ?T,the proportion of Si O2 polymorphs decreases and that of feldspar(or zeolite)increases.Gaussian distribution analyses indicate that the effective storage of NSO-bearing complexes requires a pore diameter of >(4.8–6.8)?m.4.The controlling of lithology,diagenetic process,lithofacies,and volcanic edifice to oil and gas reservoir spaceThe quality of volcanic reservoirs,represented by variables such as porosity,permeability is governed by factors such as lithology,lithofacies(volcanic edifice),diagenetic process,and tectonism.Diagenetic processes determine the types and textures of such rocks,and also govern their pore types and reservoir quality.Lithological variation results in differences in the type and intensity of the developed porosity.The lithologies which have high reservoir capacities mainly are pyromeride,autobreccia,glassy lava,lapilli/breccia lava,and cryptoexplosive breccia,then followed by lithophysa rhyolite,vesicular rhyolite,and non-welded pyroclastic rock.Petrophysical properties of volcanic oil and gas reservoirs are also controlled by lithofacies.The level of reservoir quality of different lithofacies is in the order of extrusive facies > conduit facies > extrusive facies > effusive facies.Effusive facies also can be optimized when tectonic fractures are well developed.The level of reservoir quality of volcanic edifice is in the order of proximal zone > medial zone > distal zone.5.The controlling of tectonism to volcanic reservoirThe Erguna deep-large fault is the first order fault controlling the basin formation and volcanism of the study area.The fault is a NE-SW-striking large-scale ductile shear zone.The ductile shear zone records a top-to-the-NW sense of shear and is a lengthening-thinning ductile shear zone.The 40Ar/39 Ar plateau ages of biotite from the granitic mylonites are 106.16 ± 0.79 Ma and 111.55 ± 0.67 Ma,indicating the fault experienced an intense extensional activity during the Early Cretaceous.The regional extension of the Erguna deep-large fault triggered the basement extension and strong depression of the Hailar Basin.Besides,it also controlled the fissure eruptions of viscous rhyolitic magma in a beaded pattern and formed a series small-scale cinder cones and lava domes along the NE-SW-trending.After diagenetic processes,the present-day lithology,lithofacies,and volcanic edifice in the study area were finally formed.Within volcanic reservoirs,reservoir quality is often positively correlated with the size and density of tectonic fractures.Faults are a necessary condition for the formation of effective reservoir spaces and for the migration,permeation,and accumulation of oil and gas.Areas close to fault zones are commonly preferred targets.Pyromeride,autobreccia,glassy lava,lapilli/breccia lava,cryptoexplosive breccia,lithophysa rhyolite,vesicular rhyolite,non-welded pyroclastic rock,and tectonic breccia(the breccias consist of these rocks)located near fault zones may be considered the most favorable targets for volcanic oil and gas exploration in the study area or other similar volcanic reservoirs worldwide.