Xiohong Liu ,Yiqin Zhuo ,Mingyou Feng ,*,Benjin Zhng ,Molong Xi ,Xingzhi Wng
a School of Geoscience and Technology,Southwest Petroleum University,Chengdu,610500,China
b Natural Gas Geology Key Laboratory of Sichuan Province,Chengdu,610500,China
c Northwest Sichuan Mining District,PetroChina Southwest Oil and Gas Filed Company,Jiangyou,621709,China
d PetroChina Southwest Oil and Gas Field Company Exploration and Development Research Institute,Chengdu,610041,China
Keywords:Permian Pyroclastic rock Eruption environment Hydrothermal fluid Sichuan basin Southwest China
ABSTRACT A breakthrough has been recently made in the hydrocarbon exploration of the Permian pyroclastic rocks in the Jianyang Area,western Sichuan Basin,China.With an aim to decipher the impacts of the eruption environment,the temperature of hydrothermal fluids,and the paleo-salinity on the formation of authigenic mineral assemblage and pores,this study implements comprehensive petrographic and geochemical studies through the integrated core and thin section analyses.The data presented demonstrate that the Permian volcanic intervals are intensively affected by an event of Emei taphrogeny.During basaltic magma upward migration,fractional crystallization of anorthose results in slightlyalkaline magma.The specific pyroclastic rocks are formed by the eruption of slightly-alkaline magma in the sea or a salt lake and subsequent hydrothermal alteration.During deposition and diagenesis,the authigenic mineral association is constrained jointly by the sodium-rich and high salinity water environment,and mid-high temperature,high-salinity hydrothermal fluid.Specifically,the sodium-rich hydrothermal fluid,which may sustain till the late diagenesis stage,caused pervasive albitization of pyroclastic rocks,then leading to mineral transformation and formation of a series of mineral associations.Therefore,zeolitization of volcanic glass and vesicle-infillings of zeolite is an essential condition for later mineral transformation and dissolution.Albitization of analcite,recrystallization induced by deep hydrothermal fluids,and both meteoric and deep burial dissolution expanding the micro-pore space ultimately formed porous pyroclastic reservoirs.
As an important type of unconventional oil and gas reservoirs,pyroclastic rocks are widely distributed all over the world(e.g.the U.S.,Cuba,Argentina,Georgia,and Japan) [1,2].Specifically,these rocks have drawn tremendous attention from the geoscience community in multiple petroliferous basins in China (e.g.the Songliao,Bohai Bay,Erlian,Santanghu and Junggar Basins) [3-8].Previous studies demonstrate the fundamental effects of volcanic eruption environment,lithology,and lithofacies on the formation of volcanic reservoirs,as well as influences of subsequent diagenetic alterations caused by hydrothermal activities,weathering leaching,and tectonic movement [9,10].In general,pyroclastic rocks (especially tuff) feature fine particles and tight lithology.However,they have relatively high porosity that is maintained even with the burial depth over 5000 m,which is closely related to the reservoir modification by hydrothermal activity-associated devitrification and recrystallization,alteration,clay mineral transformation,and dissolution [11].Secondary pores,resulting from devitrification and recrystallization of vitric fragments,are important contributors to oil and gas accumulation in pyroclastic rocks.Meanwhile,it is also worth investigating the pores that are dissolved by inorganic acids resulting from hydrothermal activities and clay mineral transformation,and the organic acid generated during the maturation of organic matter [12-15].
Fig.1.Simplified geological map (a),location of the study area (b),and distribution of the Permian volcanic rocks (c) in the Sichuan Basin.
Besides leading to secondary dissolved pores,the interaction between subsurface thermal fluids and rocks can result in precipitation of various minerals,depending on temperatures,pressures,fluid compositions,and relative ion activities [16,17].In particular,the devitrification products and authigenic minerals in pyroclastic rocks mainly include quartz,kaolinite,albite,smectite,chlorite,illite,zeolite,and calcite.Researches upon their occurrences,mineralogical characteristics,and paragenetic associations can help investigate various post-volcanism fluid activities(such as meteoric water dissolution,hydrothermal activity,and early hydrocarbon charging),and more importantly,clarify the genetic mechanism behind the formation of high-quality volcanic reservoir rocks about fluid-rock interaction [18,19].Nevertheless,insufficient attention has been paid to understand the properties of lava during the volcanic eruption,the properties of homologous volcanic rocks that erupt,cool down,and solidify earlier,and the dynamic diagenesis of surrounding and basement rocks with different lithologies,due to the complexity of the provenance and differential diagenetic evolution paths of pyroclastic materials.
Our main objectives are:(1) to focus on the occurrences,types and paragenetic associations of authigenic minerals in the pyroclastic rock of Well YT1 in the Jianyang Area,Sichuan Basin,China,based on detailed petrographic and geochemical analysis;(2) to enunciate the regularity behind petrologic and geochemical variations in the pyroclastic rock interval of Well YT1;and (3) to investigate the rock genesis,volcanic eruption environment,and genesis of authigenic minerals and pores in the pyroclastic rocks.Results of this study are expected to provide an important basis for further hydrocarbon exploration in pyroclastic reservoirs.
The study area(Jianyang Area)is situated in the western Sichuan Basin (SW China) (Fig.1(a)).Several complex basement structures are presented in this area,such as the Longquanshan and Longmenshan Faults with approximate NW-SE strikes (Fig.1(b)).After the uplifting of the Caledonian orogeny during the Late Carboniferous,siltstones of the Lower Permian Liangshan Formation and carbonate rocks of the Middle Permian Qixia and Maokou (P2m)Formations are deposited,which then experience modification by multi-episodes of tectonic movements such as the Emei taphrogeny (Dongwu movement) and the Indonesian movement.Specifically,the Emei taphrogeny during the Early Hercynian results in crust uplifting and tensile cracking,which leads to extensive faults and fractures and the massive eruption of magmatic rocks that form the Emeishan large igneous province.Specifically,the study area is located in the “mid-outer zone” of the Emeishan large igneous province(Fig.1(c)).Subsequently,the Middle-Late Permian Emeishan Taphrogeny results in the large-scale central eruption of the Emeishan basalt (Fig.2(a)).
Recently,a major breakthrough has been made in the key riskexploration well targeting the pyroclastic reservoir,Well YT1(Fig.2(b)).Previously,pyroclastic reservoirs with similar high porosity and high permeability are also encountered in Well YS1,which is 21 km distant from Well YT1.These indicate the high gas exploration prospects of the Permian pyroclastic rocks in the Sichuan Basin [20].Subsequently,comprehensive studies have been implemented on these pyroclastic rocks,including detailed gas reservoir characterization,well logging evaluation,seismic facies identification [21,22],and lithofacies and reservoir characteristic analyses[23].Results show that the Jianyang Area is in the proximity of the hydrocarbon generation center of the high-quality source rocks,with effusive Permian pyroclastic rocks as the main reservoir rocks [24].To date,the Jianyang Area is considered a favorable area for natural gas exploration in pyroclastic rocks.The Permian pyroclastic reservoir rocks of Well YT1 are mostly slightlyalkaline mafic pyroclastic rocks of the explosive facies,with a thickness of about 100 m.However,a diabase-basalt interval,which is tight and thus a non-reservoir interval,develops in the lower portion of the pyroclastic rock interval,and it presents huge differences from the fracture-dominated basalt reservoir rocks that are previously encountered during drilling in the Sichuan Basin.In addition,the pyroclastic reservoir is characterized by intensive alteration,high alkaline contents,extensively developed micronano pores,and poorly developed fractures [25,26],which imply the extreme complexity of the influential factors controlling the formation of reservoirs.It is suspected that the pyroclastic rocks in the Jianyang Area present characteristics that result from rapid cooling and solidification,which usually lead to low degrees of mineral crystallization and high vitric contents that form massive pores after devitrification [24,27].The understandings mentioned above to some extent point out the genesis of the high-quality pyroclastic reservoir rocks,and yet no deepened clarification has been made,regarding key controlling effects of the Permian pyroclastic rocks on paragenetic associations of authigenic minerals.
Twenty core samples from Well YT1 are employed for petrographic and eight core samples are measured for geochemical analyses (Fig.2(b)).Thin-section,scanning electron microscopy(SEM),and electron probe investigation are all accomplished at the Natural Gas Geology Key Laboratory of Sichuan Province,Southwest Petroleum University,China.The porosity and permeability are measured by PetroChina southwest oil and gas field company exploration and development research Institute.
Fig.2.Simplified geological section of the western Sichuan Basin (a) and the stratigraphic column of Well YT1 (b).
Fig.3.Macro photographs of the Permian pyroclastic rocks from Well YT1.(a) Grey-green breccia tuff,with volcanic bombs and plastic lithics,5649.47 m;(b) Breccia lava and plastic lithic filled with bitumen,5646.41 m;(c) Carbonatized breccia tuff,5649.72 m;(d) Albitized breccia lava,grey-white with fine vein networks,5645.76 m.The scale bar is 2.7 cm.
The quantitative analysis of chemical composition for minerals(8 samples,28 tested points in total)is carried out using a JXA-8230 electron probe micro-analyzer combined with a wavelength dispersive spectrometer (WDS).The micropore and authigenic mineral morphology are observed using a Quanta 650 FEG environmental scanning electron microscope (SEM).The bulk and clay mineral analyses based on X-ray diffraction(for a total of 4 samples)are carried out at the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Southwest Petroleum University,in which selected pyroclastic samples are ground into 200-mesh powders and tested for bulk mineral composition using an X-ray diffractometer (X Pert PRO MPD),and particles smaller than 2 μm are further extracted for separate determination of clay mineral types and contents.
Rare elements analysis (8 samples) is carried out by ALS Minerals-ALS Chemex (Guangzhou,China).The bulk element determination adopts the ME-XRF26D chemical analysis approach,in which lithium metaborate is used for fusion and an X-ray fluorescence spectrometer is used for the analysis.A total of 14 oxides(the content of Cr2O3is less than 0.01% and thus not listed) and their losses on ignition (LOIs) are tested,with a detection range of 0.01%-100% and RSD=0.1%-1%.It should be noted that some samples contain sulfur,and it is considered that 30% of SO3produced by ignition and decomposition is absorbed by CaO,which will be included in the final result of SO3.As for the trace element determination,the ME-ICP61 approach is used,in which four types of acids (HCl,HNO3,HF,and HClO4) are adopted for digestion and ICP-OES is used for chemical analysis.For rare elements,they are determined using the ME-MS81 chemical analysis approach,in which fusion adopts the lithium borate and the quantitative analysis uses the ICP-MS.Their LOIs are identified to range between 2.29% and 4.02%.For the convenience of comparison,contents of bulk elements are all implemented with normalization after deducting volatile contents.
4.1.Petrology and mineralogy
Pyroclastic reservoir rocks mostly develop within the depth interval of 5630-5750 m of Well YT1.They are mainly grey-green and grey in color,and they contain high contents of irregularshaped pyroclastic materials,among which larger ones may be lapilli(volcanic breccia)and smaller ones may be lithic,crystal,and vitric pyroclasts,and volcanic ash,with the local presence of volcanic bombs (Fig.3(a) and (b)).The pyroclastic rocks feature breccia,tuff,and welded textures as well as taxitic structures,with dark networked veins commonly observed on the rock surface and yet no apparent bedding(Fig.3(c)and(d)).Breccia mainly consists of basalt and andesite that cool down and solidify earlier and are then crushed,as well as plastic lithics (magma fragments) with clear boundaries and cooling-edge features [28],locally accompanied by a few carbonatized lithics(Fig.4(a)-(c)).Crystal pyroclasts such as quartz and feldspar are rarely seen,while massive unexploded porphyrotopic vitric pyroclasts (pumice) are occasionally observed(Fig.4(d)).Pumice is mainly seen with regular-shaped gas froth(such as circles and ellipses),which yet presents irregular edges composed of cambered surfaces and concave edges after crushing.
Fig.4.Photomicrographs of the Permian pyroclastic rocks from Well YT1.(a) Breccia lava,with plastic magma fragments (Pmf),plastic lithics,plastic lithic chloritization and carbonaceous bitumen (Cb) as the fillings,5646.64 m,under the plane-polarized light;(b) Breccia lava (Bl),obviously crushed,with dissolved pores and fractures filled with hematite,5645.98 m,under the plane-polarized light;(c)Breccia lava,with andesite lithics (Al),5646.41 m,under the plane-polarized light;(D)Vitritic pyroclastic tuff,seen with massive porphyrotopic vitritic pyroclasts (Pvp),and filling of titanite and chlorite (titanite shows dissolution;blue is dyed by epoxy resin),5649.72 m,under the plane-polarized light;(e) Albitization of feldspar phenocrysts,filled with chlorite and titanite,5645.76 m,under the plane-polarized light;(f) Plastic lithics transforming into albite and chlorite,with edges impregnated by bitumen,5645.98 m,under the plane-polarized light;(g) Hornblende phenocrysts transforming into chlorite and titanite,with matrix albitization,5646.64 m,under the plane-polarized light;(h)Dark mineral phenocrysts transforming into chlorite and rutile,with matrix albitization,5649.72 m,under the plane-polarized light;(i) Vesicles filled with chlorite,5649.72 m,under the plane-polarized light;(j) Residual Vesicles (Rv) filled with albite,chlorite and apatite,5649.72 m,under the plane-polarized light;(k) Breccia with distribution of titanite along its edge,5646.64 m,under the plane-polarized light;(l) Exploded fractures filled with albite and matrix with dissolved pores,5646.41 m,under the plane-polarized light.Pl-Plagioclase;Ab-Albite;Hbl-Hornblende;Bt-Biotite;Ttn-Titanite;Rt-Rutile;Chl-Chlorite;Hem-Hematite;Pore-P.
The pyroclastic rocks are seen with intensive alteration and filling.Feldspar phenocrysts,the matrix,plastic magma fragments,and vitritic pyroclasts are commonly albitized (Fig.4(e)-(h)),while dark minerals are commonly altered into chlorite and titanite,with a few into chlorite and rutile,accompanied by occasionally-observed complete crystal outline (Fig.4(g) and (h)).Vesicles (Fig.4(i) and (j)),inter-breccia pores (fractures) Fig.4(k),and pre-existing fractures(Fig.4(l))are all filled by albite,chlorite,and titanite.According to X-ray diffraction analyses of bulk and clay minerals and thin-section petrologic analysis under the microscope,the rocks are demonstrated to be mainly composed of albite and chlorite(together accounting for over 90%of the total),with a little amount of quartz (1%-3%) and a slightly more content of titanite (5%-7%) (Table 1).According to the relative content of chlorite and albite,the pyroclastic rocks can be divided into two sections,namely the upper breccia lava section (with albite ＞50%,5645.50-5646.24 m) and lower breccia tuff section (with chlorite ＞50%,5646.24-5651.13 m),respectively,corresponding to two different diagenetic processes,namely lava cementation and volcanic ash cementation.
Table 1 Electron probe-spectroscopy and X-ray data of secondary minerals of the Permian pyroclastic rocks from Well YT1 in the Jianyang Area.unit:(wt%).
The electron probe analysis shows that in most cases,anorthite(An) accounts for over 99% of the total elements in albite (Ab)(Table 1);chlorite is mostly iron-rich prochlorite and brunsvigite,with a few pycnochlorites that contain fewer irons;the Si/Al ratio is between 1.14 and 1.38(Table 1;Fig.5).The content of CaO in titanite lies within 27.73%-28.26%;TiO2,34.67%-34.09%;SiO2,30.75%-31.66%;and Al2O3(0.82%-2.16%),FeO (0.31%-1.76%),BaO (0.11%-0.31%) and fluorine element (0.13%-0.47%),which are less in contents.
4.2.Geochemical characteristics
The content of SiO2in pyroclastic rocks is 44.06%-57.09%;total alkaline materials (K2O+Na2O),4.45%-8.87%;Al2O3,15.41%-17.03%;TFe2O3,5.70%-20.61%;MgO,1.22%-3.18%;and TiO2,4.18%-4.97% (Table 2).With respect to classification by SiO2versus(K2O+Na2O) (Fig.6(a)),the pyroclastic rock lies in the alkali basanite-basaltic trachyandesite-trachyandesite zone.The content of Na2O(4.35%-8.30%)is far more than that of K2O(0.07%-0.12%),which indicates sodium-rich igneous rocks,accompanied by lowpotassium tholeiitic series in the SiO2-K2O chart (Fig.6(b)).Given that all rocks in the study area have to some extent experienced alteration,a rock classification scheme is developed,in accordance with ratios of elements inactive during alteration (Nb/Y-Zr/TiO2× 0.0001-Nb/Y) [30],and accordingly all samples are identified as alkaline basalts (Fig.6(c)).
Both pyroclastic rocks and basalts in Well YT1 present high ΣREE(＞150),and the REE patterns of pyroclastic rocks are similar to those of the basalts(Table 2).Compared with chondrite,pyroclastic rocks and basalts are enriched in light REE,with(La/Yb)Nvalues of 6.69-12.71,and have no apparent Eu anomaly Fig.7(a) and (b).In the primitive mantle-normalized trace element diagram,large ion lithophile elements such as Rb,K,Ba,Sr,and P present considerable deficiency in the pyroclastic rock samples and slight deficiency in the basalt samples(with one sample showing positive anomalies in Ba and Sr),while other elements show no considerable differences in contents(Fig.7(c)and(d)).The TiO2content is over 3%,and the Ti/Y ratio exceeds 500,which are consistent with the characteristics of the Emeishan Ti-rich basalts [32].
Table 2 Trace element data of the Permian igneous rocks from Well YT1 in the Jianyang Area,western Sichuan Basin.
5.1.Sources and eruption environment
The core samples from Well YT1 all share extremely similar trace element and REE distribution patterns with the ocean island basalt (OIB),which implies that they are both derived from the mantle and are not or only slightly contaminated by external substances.The pyroclastic rocks present an obvious negative Sr anomaly,indicating that they have been through the fractional crystallization of plagioclase or relatively intensive modification in the later period(Fig.7(c)).Furthermore,lower part diabase and fine basalts have no Sr anomaly,suggesting no extensive crystallization differentiation of plagioclase.The porphyritic basalt at the top of the igneous rock interval (YT1-9) is characterized by distinct plagioclase(albitized)phenocrysts and thus differentiated from the lower diabase and fine basalt(Fig.7(d)).Moreover,it shows positive Sr anomaly,which indicates the concentration of plagioclase conglomerates and also implies occurring of synchronous plagioclase fractional crystallization.
The volcanic eruption is associated with two types of environments,namely subaerial and subaqueous ones,which produce significant differences in rock types,textures and structures,alteration characteristics,occurrences,contact with underlying strata,development of pores and fractures,and oxidation coefficients[33-35].In terms of the petrologic characteristics,the pyroclastic rocks at the reservoir interval present relatively large thickness,complex material composition,and poor sorting of clasts with the inclusion of volcanic bombs and plastic lithics(magma fragments)showing large particle sizes;they commonly have the welded texture,with no obvious stratification;irregular-shaped cracks occur at the top of the pyroclastic rock part;dissolved pores and fractures are filled with hematite (Fig.4B).The presence of hematite in irregular-shaped fractures and pores implies the presence of a weathering crust at the top(meteoric water can oxidize Fe2+in basalt and change it into Fe3+,which enriches in pores and fractures and forms hematite)[36-38].Thus,it is clear that pyroclastic rocks in Well YT1 are mainly of the subaerial eruption.During the late stage of the Emeishan basalt eruption,some parts of the study area are subjected to marine transgression,and thus seawater may participate in the alteration of the pyroclastic rocks at this stage.Development of the vitric texture is an indicator for subaqueous-erupted volcanic rocks,while albitization is commonly recognized as the result of the pneumatolytichydrothermal alteration of the submarine eruption of lava or lava flowing from the land into sodium-rich seawater [39].In the lower portion of the pyroclastic interval,porphyrotopic vitritic pyroclasts(pumice) with the vitric texture develop,accompanied by high contents of albite among rock minerals,based on which previous studies conclude that part of these pyroclastic rocks is associated with the submarine eruption environment[24].
Previous research shows that the intermediate-mafic rocks lose calcium and yet obtain magnesium and water,due to hydrothermal alteration of seawater,associated with the formation of chlorite and yet unnecessary loss of alkaline [40-43].In the Jianyang Area,CaO and MgO contents of the Permian pyroclastic rocks are clearly positively correlated,with a correlation coefficient up to 0.97(Table 2),which demonstrates no considerable loss of CaO and gain of MgO.However,the mass fraction of Na2O is high among the components (＞4%),indicating that part of the sodium is possibly derived from seawater.In addition,the Sr/Ba ratio of sediments can represent the sedimentary environment (the Sr/Ba ratio of freshwater sediments is often below 1,while that of marine sediments generally surpasses 1).Nonetheless,it should be noted the Sr/Ba ratio that is able to identify the sedimentary environment mainly refers to the ratio of Sr and Ba elements that are in the free state and migrate directly from the transportation medium.In other words,Sr and Ba elements existing in the form of silicate are excluded.Sr is a highly-active large ion lithophile element,and feldspar is considered to be an important carrier of it[44].The pyroclastic rocks are found with obvious negative Sr anomaly,suggesting massive loss of Sr due to fractional crystallization of plagioclase or intensive modification in the late stage (Fig.7C and D).Therefore,the effects of Sr and Ba existing in the mineral structure state(e.g.feldspar)are minor,and the Sr/Ba ratio can still to some extent represent the sedimentary environment.The Sr/Ba ratios of the Permian pyroclastic rock is 2.87-3.95 (much higher than 1),indicating submarine or evaporation salt lake eruption(Fig.9A).
However,extensive pumice may also develop due to the early eruption of volatile-abundant viscous lava[45],the post-magmatic hydrothermal alteration may also generate albite [39],and the increased salinity in a lacustrine environment can elevate the Sr/Ba ratio of rocks [46].Fortunately,the participation of seawater components,deep hydrothermal fluids,and volatile components,as well as variation of water salinity can be directly captured from the paragenetic association and trace element composition of minerals [47,48].Hence,it is of necessity to further investigate altered and authigenic minerals that are able to represent the rock eruption and diagenetic environments,so as to further clarify the formation and evolution processes of volcanic rock reservoirs.
Fig.6.TAS classification (a),classification by the SiO2-K2O content (b) and classification by the Nb/Y-Zr/TiO2 content (c) of the Permian volcanic rock in Well YT1.
Fig.7.REE patterns (a,b) and trace element diagrams (c,d) for the volcanic rock from Well YT1 in the Jianyang area.A,C-pyroclastic rocks;B,D-basalt and diabase.
5.2.Paragenetic associations of authigenic minerals and their influential factors
5.2.1.Effects of the alkaline sodium-rich environment on mineral associations
In the upper part of the Permian pyroclastic reservoir,nearly all basaltic and andesitic breccia phenocrysts and matrices,feldspar crystal clasts of tuff,and feldspar minerals filling pores/fractures are high-purity albite,with contents of albite molecules (Ab) over 99% (Table 1).Given that these various types of breccias mainly come from homologous volcanic rocks that erupt earlier and already solidify,and that the feldspar in the earlier-erupted basalt is dominated by labradorite,the albite of these feldspars in question should be produced by metasomatism driven by sodium-bearing hydrothermal gas and water in the rocks.
Albite in the matrix is seen with fine particles,and frequently paragenetic with other minerals such as chlorite and titanite.It often presents itself as idiomorphic plate-like crystals with flat(straight)crystal surfaces and edges,and extensive development of inter-crystalline pores,which suggests authigenesis or relatively intensive recrystallization(Fig.8(a)-(c)).Moreover,albite with the occurrence of metasomatism and smaller albite molecule contents can also be observed in the lower porphyritic basalt,of which the edge seems to be the reaction rim of potassium feldspar,indicating production of K+-rich fluids after albite metasomatism.The reaction of plagioclase albitization is expressed as Formula (1):
Fig.8.Characteristics of authigenic minerals and reservoir space of Permian pyroclastic rocks from Well YT1 under SEM.
Zeolites can also be transformed into albite at the late stage of diagenesis,and analcite albitization is an important source of albite production [49].According to results of X-ray diffraction and electron probe analyses,no water-containing analcite and other zeolites are detected in the pyroclastic rocks.However,albite and chlorite are observed under the SEM.Specifically,albite is featured by the crystal shapes of analcite (NaAlSi2O6·H2O,combined cubic and tetragonal trisoctahedron) and gmelinite((Ca,Na2)[Al2Si4O12]·6H2O,rhombohedron)(Fig.8(a)),while chlorite is characterized by the crystal shape of stilbite ((Na2,Ca)[Al2Si7O18]·7H2O) (Fig.8(b)),which is similar to that of the orthorhombic class with parallel grouping.This implies early generation of zeolites that have been transformed into water-free stable minerals during the burialdiagenesis process that are accompanied by relatively high diagenetic temperatures.Early hydration of unstable mafic feldspar minerals and fine volcanic vitric materials in rocks are the main basis for generating zeolites.In the alkaline medium,the volcanic vitric material is converted into smectite and clinoptilolite(Na1.02K0.82Ca0.15Al2.26Si9.84O24·8H2O) by early hydration,associated with the generation of quartz.Then,clinoptilolite further transforms into various zeolites with low H2O contents and Si/Al ratios,and smectite evolves towards minerals such as illite and chlorite,during which released Ca2+is conducive to the formation of calcite [19].In view of the geochemical characteristics and mineral associations of rocks,the Permian pyroclastic rocks in the Jianyang area have extremely low contents of K2O (only 0.07%-0.12%) and almost no K+-rich minerals among their mineral composition (Table 2).The clay mineral X-ray diffraction analysis results show that clay minerals are all chlorite,with no K-rich illite,no mixed-layer clay minerals,and also no precipitation of quartz and calcite.Hence,it is implied that the volcanic vitric material does not go through the intermediate process involving the generation of smectite and clinoptilolite,and instead it directly forms zeolites such as analcite,chabazite,and stilbite at certain temperatures.
Besides,Ca2+that is released during conversion of plagioclase into albite and volcanic glass into analcite mainly forms titanite by being bonded with Ti4+produced by Ti-rich mineral decomposition(such as augite and ilmenite) and Si4+in pore water,instead of producing calcite.As analcite transforms towards other minerals,it preferably changes into albite,due to the dominance of Na+in water and Na+in analcite[50].The reaction formula from analcite into albite is shown as Formula (2):
The albitization temperature of analcite is about 120°C,while potassium-feldspar albitization requires higher diagenesis conditions[49,51].The plagioclase and analcite of the pyroclastic rocks in the study area have been through complete albitization(Fig.8(a)-(c)).Furthermore,later pores and fractures are filled by analcite(Fig.4l,8E and 8F),which all seem to be idiomorphic with the clean and flat crystal surface,and often coexist with bitumen(Fig.4(g)).This suggests that in the late diagenetic stage,there is still the activity of sodium-rich hydrothermal fluids,which migrate into rock pores and fractures together with hydrocarboncontaining fluids and subsequently precipitate and form analcite.
Fig.9.Cross plots of authigenic chlorite in the Permian pyroclastic rocks.(a) Si/Al ratio of chlorite and Sr/Ba ratio of rock versus depth;(b) Chlorite-based calculated temperature versus Si/Al ratio;(c) Chlorite-based calculated temperature versus depth [31];(d) Sr/Ba ratio of rocks versus Si/Al ratio of chlorite.
5.2.2.Effects of temperature variation on mineral associations
The albite-epidote association is typically the result of decalcification and feldspathization of plagioclase of intermediate-mafic magmatic rock under mid-high-temperature (about 400°C-600°C) hydrothermal effects [52].Titanite is the main calcium-rich and titanium-rich mineral in the pyroclastic rocks of the study area(with the local presence of rutile),and chlorite is the main magnesium-and iron-rich mineral.The albite-titanitechlorite association is the product of hydrothermal alteration,and thus is highly dependent upon environmental temperature variation,besides ion concentrations of each component in the medium.
The chlorite in the Permian pyroclastic rocks can be divided into three types,namely pycnochlorite,brunsvigite,and prochlorite(Fig.5).Pycnochlorite exists mainly in the form of metasomatic dark minerals in pyroclastic breccia,presenting pseudomorphs of dark minerals.Brunsvigite is mainly found in the upper pyroclastic rocks,filling various pores,and it is paragenetic with albite and authigenic titanite minerals.Prochlorite primarily exists in the lower pyroclastic rocks.Moreover,it fills dissolved pores of titanite as well as pores of other types,interacts with the matrix in the form of metasomatism,and is paragenetic with albite and apatite.The chlorite in the basalt is mainly ferroan clinochlore.
Chlorite can maintain its stability within a wide range of temperatures and occurs in numerous types of geological environments.The diagenesis chlorite has an increasing Al/Si ratio with the increasing burial depth or temperature,suggesting a decline of the Si content,growth of(Fe+Mg)contents,and rising of the quantity of ion-occupied positions of the octahedron,associated with loweringIVAl contents and climbingVIAl contents[53,54].In terms of the authigenic chlorite composition,an obvious linear correlation exists betweenIVAl contents and temperatures,and previous studies have discussed multiple aspects of the chlorite composition geothermometry[16].Prochlorite and brunsvigite of the pyroclastic rocks have the number of Si atoms no more than three and low Si/Al ratios (1.14-1.38) (Table 1),which indicate the origin of chlorite from smectite evolution or precipitation[55].As for the chlorite in the basalt,the number of Si atoms is 3.43-3.68,and the Si/Al ratio is relatively high (1.86-2.41),which demonstrates the origin of dark mineral alteration (Table 1).IVAl and the Fe/Fe+Mg ratio of the chlorite composition for the pyroclastic rock can be used to calculate the diagenesis temperature.Although large discrepancies are found between the formation temperatures of chlorite calculated using various empirical geothermometers [16],it is still certain that the authigenic chlorite of the pyroclastic rock in the study area forms at a temperature that is at least higher than 200°C(even surpassing 300°C);moreover,the formation temperature of prochlorite is higher than that of brunsvigite,and prochlorite should be the product of the mid-low-temperature hydrothermal effect.Besides,a clear positive correlation is seen between the Si/Al ratio of the chlorite in the study area and the burial depth,and a negative correlation between that and the temperature (Fig.9(a)and (b)),which are opposite to the Si/Al ratio variation trends of chlorite affected by increasing burial depth and temperature.It should also be noted that the calculated temperature difference can be up to 50°C-60°C across a depth span smaller than 1 m in the pyroclastic rock interval (Fig.9(c)),which implies that such temperature variation shall not be solely attributed to upward temperature variation along the vertical direction and other external factors may also play roles.
5.2.3.Effects of paleo-salinity on clay mineral evolution
Paleo-salinity is an effective proxy to reflect the paleo-condition of sediments.Owing to its clear positive correlation with the paleosalinity,the Sr/Ba ratio can be used as an important parameter to indicate the paleo-salinity[56].An increase in salinity can improve and accelerate clay mineral evolution.Paleo-salinity is found with great restraining effects upon chlorite components,in the case of a certain depth(range),paleo-salinity(Sr/Ba values no less than 0.4),and presence of NaCl minerals.As paleo-salinity climes up,the chlorite composition is seen with increasing values of Al,IVAl,Na,Na2O,and Al/Si and a correspondingly decreasing Si/Al ratio [57].
Besides,the euhedral halite is often observed in the pores of the pyroclastic rocks (Fig.8(d)).The Na2O content,the Sr/Ba ratio,and the Si/Al ratio are 4.35%-8.80%,＞3,and ＜1.5,respectively(Table 2).These,combined with the fact stated previously that all samples contain 100% of chlorite and that there are no unordered illitesmectite and chlorite-smectite mixed layers,indicate that these pyroclastic rocks are mostly associated with high paleo-salinity.As shown in Fig.9(d),the Sr/Ba ratio of the pyroclastic rocks of Well YT1 has an apparent negative correlation with the chlorite Si/Al ratio (with the correlation coefficient up to 0.92).Therefore,the clay mineral evolution of the pyroclastic reservoir rocks is also constrained by paleo-salinity.In particular,at the depth of 5649.72 m(YT1-8(b))in Well YT1,the Sr/Ba ratio can be up to 3.95,and the Si/Al ratio can be as small as 1.16,which corresponds to an average calculated temperature of 333.4°C,significantly higher than those of both the overlying and underlying strata(Fig.9C).The abnormally high salinity also suggests that the pyroclastic rocks may be products of the submarine or salt lake eruption.
In summary,the pore type of the Permian pyroclastic rocks in the western Sichuan Basin is dominated by inter-crystalline micropores of albite and chlorite(Fig.8(b)and(e)),followed by residual vesicles and matrix dissolved pores (Fig.4(d) and (j)) with locally developed dissolved vugs.Pores have good connectivity,leading to excellent reservoir performances.Cores examined in this study suggest that various types of primary and secondary pores coexist,with an average porosity of 13.76%and an average permeability of 2.446×10-3μm2.The porous reservoir is considered to be a highquality pore-dominated gas reservoir (Fig.8).Furthermore,the porosity of the upper part (high contents of sodium feldspar) is higher than that in the lower part (high contents of chlorite),indicating that albitization is positive for the formation of highquality reservoirs (Fig.2(d)).
The formation of pores is controlled by the eruption environment and hydrothermal alteration (Fig.10).The eruption of volatile-rich viscous alkaline magma results in extensive vesicles and vitric components,and zeolitization of volcanic vitric materials,and filling of vesicles by zeolites lay the foundation for later transformation among minerals and dissolution.As zeolites such as analcite transform into albite and chlorite,inter-crystalline pores are greatly expanded.This is attributed to the shrunk volume during the change of the crystal structure from the open framework structure into the compact framework structure[50].Furthermore,water-rich pyroclastic rocks can retain part of primary intercrystalline pores during compaction [58],which serve as the conduit for later hydrothermal fluids.The inter-crystalline pores created by hydrothermal recrystallization and the secondary dissolved pores from dissolution provide the main storage space,and they are also important factors for the formation of highquality pyroclastic reservoir rocks.
Fig.10.Development and alteration patterns of the Permian igneous rock in the Jianyang Area,western Sichuan Basin.
(1) Fractional crystallization of plagioclase occurs during the upward movement of basaltic magma to a shallow depth,forming slightly-alkaline magma.The eruption of magma in seawater or a salt lake and subsequent hydrothermal alteration result in the Permian pyroclastic rocks in the Jianyang Area of the western Sichuan Basin.
(2) The pyroclastic rocks are pervasively transformed by albitization during diagenesis.The authigenic mineral associations are affected jointly by the alkaline sodium-rich water environment,mid-high temperature,paleo-salinity,and hydrothermal fluid.The effect of the sodium-rich hydrothermal fluid may last till the late diagenesis stage.
(3) Zeolitization of volcanic glass and filling of vesicles by zeolites exert positive effects for later transformation and dissolution of minerals.The formation of high-quality pyroclastic reservoirs is the result of not only the analcite albitization,but also the coupling of recrystallization induced by upward migration of hydrothermal fluids with dissolution.
Author contributions
Conceptualization and Data curation,Y.Z.;Writing-original draft,X.L.;Writing-review &editing,M.F.;Resources,B.Z.;Investigation,M.X.;Supervision,X.W.
Declaration of competing interests
The authors declare no conflicts of interest.
Acknowledgments
This work was granted by the National Natural Science Foundation of China (Grant No.41202109),the National Major Science and Technology Projects of China(Grant No.2016ZX05007004001),the Innovation Foundation of PetroChina Carbonate Key laboratory(RIPED-HZDZY-2019-JS-695) and the China Scholarship Council.Thanks are expressed to our colleagues involved in igneous rocks researching in the Sichuan Basin,as well as to several anonymous reviewers from which this article has benefited.
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