Origin of natural sulphur-bearing immiscible inclusions and H2S in oolite gas reservoir, Eastern Sic

242Science in China: Series D Earth Sciences 2006 Vol.49 No.3 242- -257DOI: 10.1007/s11430-006-0242-7Origin of natural sulphur-bearing immiscible inclusionsand H2S in oolite gas reservoir, Eastern SichuanLIU Dehan, XIAO Xianming, XIONG Yongqiang, GENG Ansong, TIAN Hui,PENG Ping'an, SHEN Jiagui & WANG YunpengState Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences,Guangzhou 510640, ChinaCorrespondence should be addressed to Liu Dehan (email: liudh@gig.ac.cn)Received June 3, 2005; accepted August 11, 2005Abstract Based on results of microscopic observation and laser Raman analysis about fluid in-clusions, multiple special forms of immiscible inclusions that contain sulphur, liquid hydrocarbon, bi-tumen, etc. were discovered in samples collected from the H2S gas reservoir-containing carbonates inthe Lower Triassic Feixianguan Formation in the Jinzhu-Luoja area, Kai County, Sichuan Province.Based on the lithology and burial history of the strata involved as well as measurement results ofhomogenization temperature of fluid inclusions, bitumen reflectivity, etc., it is concluded that the H2S in .the gas reservoir resulted from the thermal reaction between hydrocarbons in reservoir and CaSO4 inthe gypsum-bearing dolostone section at the high temperature (140C一170C) oil-cracked gas for-mation stage in Late Cretaceous. Thereafter, research on a great number of immiscible inclusions inthe reservoir reveals that elemental sulphur resulted from oxidation of part of the earlier-formed H2Sand further reaction between sulphates, hydrocarbons and H2S in geological fluids in H2S-bearing gasreservoir at a temperature of 86C- 89°C and a pressure of 340x10*Pa and during the regional upliftstage as characterized by temperature decrease and pressure decrease in Tertiary. Meanwhile, gyp-sum, anhydrite and calcite formed at this stage would trap particles like elemental sulphur and resultin a variety of special forms of immiscible inclusions, and these inclusions would contain informationconcerning the complexity of the fluids in the reservoir and the origin of H2S and natural sulphur in thegas reservoir.Keywords: Sichuan Basin, immiscible inclusion, origin of hydrosulfide, natural sulphur, oil-cracking gas,microscopic laser Raman analysis.Fluid inclusions as captured in homogeneous fluids amount of research has already been conducted towardin rocks and minerals have been extensively studiedthe immiscible inclusions and“boilling” inclusions inand successfully applied to exploring the metalloge- CO2-H2O system, only study on immiscible inclusionsnetic temperature and pressure of metallic ore depositsof other formation mechanisms is obviously lacking.and in investigating hydrocarbon generation, migra-At present, fluid inclusion study in the field of hydro-tion, etc." 0. In regard to multiple forms of immis-carbon geological research is mainly focused on ho-cible inclusions in rocks and minerals, a significantmogenization temperature, freezing point temperature,www.scichina.com www. springerlink.com中国煤化工MYHCNMH G.Origin of natural sulphur-bearing immiscible inclusions and HzS in oolite gas reservoir243etc. of fluid inclusions as trapped in homogenized tion of sapropel-type pelitic carbonate source rocks ofsystem, while few research on immiscible inclusionstrough facies in Permian and Triassic, and the layouthas been published so far. As a matter of fact, multiple of the Ertan gas reservoir in the Feixianguan Forma-forms of immiscible inclusions might occur in sometion in the two sides of the trough. Here in Easternhydrocarbon reservoirs and pervious layers, only not Sichuan, the marine facies strata were mainly devel-much focus was put on their research. Recently theoped from Upper Permian to Lower Triassic, and theauthors discovered the occurrence of polyphase im-Lower Triassic Feixianguan Formation mainly con-miscible inclusions that contain natural sulphur and sists of pelitic carbonates of marine facies-tidal-flatH2S in the Ertan gas reservoir of the Triassic Feixian-facies and saltish lagoon facies and deposits of gyp-guan Formation in Sichuan Basin, which can unsum-salt facies. The pelitic carbonates of trough faciesdoubtedly yield key information concerning the originin the Upper Permian Changxing Formation has a relicof hydrogen sulfide and natural sulphur. Gas from theorganic carbon concentration being as high as 0.4%-Ertan reservoir is highly enriched in H2S, and on De-1.1%, while the pelitic carbonates of trough facies incember 23, 2003 a blowout accident occurred in the Feixianguan Formation has a relic organic carbonLuojia Village 16 gas mine in Kai County, Sichuan,concentration of 0.4%一0.5%, hence becoming majorwhich resulted in large area H2S-related poisoning.hydrocarbon source rocks in the study area. On theTherefore, study on the origin of hydrogen sulfideswo sides of the NE-trending Kaijiang-Liangpingand natural sulphur in gas reservoirs has become a keytrough, carbonate rocks of oolitic beach facies charac-issue that has attracted extensive attention in gas ex- terized by abundant porosity were developed, and be-ploration and development.came important hydrocarbon reservoirs. The multiplelayers of gypsum- salt and pelitic rocks in the TriassicJialingjiang Formation and Feixianguan Formation1 Samples and petroleum geologic settingserved as the direct covers for the Ertan gas reservoirI.1 Petroleum geologic settingin the Feixianguan Formation and the Cretaceous redThe oolite gas reservoir in the Feixianguan Forma-beds as the regional cover for the gas reservoir.tion is located in Kaijiang, Liangping, Kaixian andAn analysis about the burial history and thermalWanyuan in northeast Sichuan, and geotectonicallyevolution history of the hydrocarbon source rocks inbelongs to the northwestern margin of the Yangtzethe Permian System and the Triassic System in thePlate. Its basement is composed of Proterozoic lovstudy area was carried out, and the results indicate thatmetamorphic rocks and small amount of acidic erup-at the end of Triassic, the vitrinite reflectivity Ro is astive rocks, and on this basement, Sinian to Tertiaryhigh as 0.5% -0.7%, signaling the beginning of oilstrata ca.12,000 m thick were deposited, among which generation stage, while at the sedimentation stage ofthe Sinian system to Middle Triassic Leikoupo Forma-the Lower Shaximiao Formation in the Early Juras-tion mainly consist of carbonates of shallow sea faciessic-Mid-Jurassic system, the vitrinite reflectivity Rand small a amount of deposits of oceanic-continentalcan be as high as 0.7% - - 1.0%. Thereafter, sedimenta-transitional facies and have a thickness of ca. 4,000-tion and settlement were speeded up, and at the sedi-7,000 m, while the Middle Triassic Xujiahe Formationmentation stage of Late Jurassic Suining Formation,and its overlying strata mainly consist of clastic rocksthe vitrinite reflectivity Ro of hydrocarbon sourceof trrestrial facies'.rocks became 1 .0%一1.3%, signaling the peak periodPetrographical and paleogeographical study andfor oil generation. At the sedimentation stage of Latepalaeotectonic stress field analysis indicate that multi-Jurassic Penglai Formation, the maturity Ro for hy-ple syneclise, depression and anteclise structures weredrocarbon source rocks was 1.3%- 2.0%， signalingdeveloped due to the pullapart extension of Sichuan the stage for condensate oil and wet gas formation;Basin ever since Paleozoic. In the study area, theCretaceous to Early Miocene is the stage for maxi-NE-trending Kajjiang-Liangping trough formed in mum settlement, at this stage the thermal evolutionLate Permian to early Triassic controlled the deposi-degree for sour中国煤化工was as highYHCNMH G ..244Science in China: Series D Earth Sciencesas 2.5%一3%, signaling the beginning of the stage for12 POL S and LEICA DMR XP, were used respec-the formation of oil-cracked gas and dry gas. At thistively. Here the conditions for fluorescence observa-stage the study area shows an average geothermal gra-tion are: 100W Hg lamp， excitation wavelength:dient of 3.01C/100 m, the maximum Tertiary burialH3-420- 490 nm, dichromatic reflection mirror:depth of ca. 6,400 m and the highest geothermal tem-RKB-510 nm, protection filter wavelength: 515 nm.As to the shapes, dimensions and gas/liquid ratiosperature of ca. 220C. After this stage, the hydrocar-bon reservoir in the study area was at the process ofof fluid inclusions, LEICA DC350 digital camera sys-pressure decrease and temperature decrease due to thetem and QWIN STANDRD image analysis softwarewere used together with LEICA DMR XP-NiKONextensive uplift of Sichuan Basin in Pliocene8.The oolite limestone gas reservoir in Lower TriassicDXM 1200F digital imaging system.In regard to measurement of homogenization tem-Feixianguan Formation is mainly of the struc-tural-lithological trap type and consists of ooliticperatures and freezing temperatures of fluid inclusions,dolostones. The reservoir is characterized by devel-USGS FLUID INI gas flow stage and LINKAMopment of pores and fissures, and shows high gas ma-THMS-G600 stage were respectively used for bothturity, CH4 content of 73.7%- 84.9%, CH2 content ofheating and freezing, together with objectives L32/0.49 P and H50/0.50 of long focal lengths, and at a0.03%- 0.11%，CH3 content of 0.0% 一0.05%, H2Sheating rate of 1- 5"C/m.content of 8.28%一17.06%, CO2 content of 0.46% .In regard to measurement of reflectivity of bitumen10.41%, N2 content of 0.42%- 1.11%, H2 content ofin samples, LEITZ MPV 3 microphotometer was used0.002%- 3.45%, He content of 0.013%- 0.03%, and and the operating conditions involve a wavelength ofδ "3C= -32.35%o- -29.52%0, δ "C2 = -33.8%o -514nm, objective of 50X/0.85 OIL P, immersion oil-32.39%8. Gas compositions and carbon isotope re-with N=1 .515, and reflectivity calibration standard ofsults for methane and ethane reveal that the gas is ofNR1149 optical glass with Ro=1.24%.In regard to microscopic laser Raman analysisthe crude-cracked type. Presently the pressure coeffi-about inclusions, RENISHAN (R) RM-2000 modelcient for the gas reservoir is 1.03- 1.37.Raman spetrophotometer was used, and the operatingconditions are Art laser generator with laser wave-2 Samples and experimental conditionslength of 514 nm, power of 25.2 mw, grating slit of 202.1 Samplesμm, objective N PLAN 50X/0.75 P, measurement di-Research was carried out toward nine boreholes lo-ameter of 5 um, and scanning time of 10 s.cated at Luojiazhai-Jinzhu, Dukouhe and Tieshan ar-eas on the two sides of Kaijiang-Liangping trough, and3 Types and characteristics of occurrence of fluiddozens of samples were collected from the carbonateinclusionsreservoirs and source rocks in the Triassic and Per-Fluid inclusions are primary samples of geologi-mian strata, while our focus was placed on the carbon-cal fluids sealed in authigenic minerals, enlargementate reservoir samples showing development of poresedges of minerals or healing surfaces of secondaryand fissures in Lower Triassic (Tf 3-) and Upperfissures in minerals during the petrogenetic evolutionPermian (P2). Distribution of the samples and de- of rocks and migration-charge processes of hydrocarscriptions of the optical properties of the reservoir bi-bons. However, the formation and distribution oftumen are shown in Fig. 1 and Table 1.various types of fluid inclusions in the reservoir wereconstrained by multiple geological and geochemical2.2 Experimental conditionsconditions, as the reservoir is characterized by strongFor microscopic observation about inclusions andpetrogenesis, remarkable recrystallization. Hence fluidsolid bitumen in reservoir samples, polarizing, reflec-inclusions were well developed along interfacestion and fluorescence systems of LEITZ LABORLUXamong oil, gas中国煤化工gas reservoirMHCNMH G ..Origin of natural sulphur-bearing immiscible inclusions and HzS in oolite gas reservoir245WanyuanChengkouTongjiangZhenpingPol。。∞。。。Jinzhul CDu2。PatOYihanKaixian8&( Dachuan 。9 °KaijangFengjic38QuxianTieshanYunyangDazhu50 km[。] Borchole .团Facies boundaryFig. 1. Plane map about Jinzhu-Luojia region in the Kaijiang-Liangping trough in Sichuan Basin.Table 1 Descriptions about fluid inclusions in minerals and measurement of reflectivity of reservoir bitumen in samplesBitumen reflectivityObservation of fluidSample Bore No. Bore depth (m) AgeBitumen type(BR。%)inclusionsContaining immiscibleJinzhu 12969Tif2lCarbonate reservoirCd862977Tf2ca5Po13953.56Tf312.40(2.4- 3.10)Pyrobitumen in inhomogenizedCd9Po24034.9yostructural reservoir2.42(2.4- 2.45)Cd20Po33617.54.15(5.87-4.26) .Cd24Luojia 13477.2.72(2.67- 2.76) Containing fluid inclusionspyrobitumen in inhomogenized3477.7Tf212.72(2.67- 2.76)3494Tf33.552.95 -4.5)Cd42Du 24374Tyf-1 Carbonate reservoir and carbonatesContaining fluid inclusionsCd-60 Tieshan52874Tf2.52(2.33 - 2.76)Cd-61Tieshan52876T2.45(2.19- 2.88)111Pbitumen in inhomogenized structural2.63<2.55-2.83)reservoirCd-74 Huanglong 140192.25(2.05 - 2.45)Cd-76Huanglong 436202.14(1.78- 2.32)中国煤化工MHCNMH G.246Science in China: Series D Earth Sciencesand oil reservoir the development of fluid inclusionsand the results are shown in Table 2 and Fig. 2.was constrained to a great extent.Meanwhile，measurement of homogenization tem-Due to the complexity of petrogenesis undergoneperature and PVTX simulation and calculation wereby the reservoirs and formation and evolution of hy-conducted toward oil inclusions that were rarely seendrocarbon reservoirs in the study area, fluid inclusions and show yellow fluorescence. As shown in Fig. 2, theshow great inhomogeneity in both type and extent ofhomogenization temperature for fluid inclusions indevelopment. Fluid inclusions were well developed inminerals can be approximately classed into threesparite and dolomite that show apparent recrstalliza-groups: 80C - 120C， 130°C - 160°C, and 170°C-tion, in oolitic carbonate cement, in authigenic calcite,200C. Here the homogenization temperature for ele-dolomite, quartz in carbonate solution holes, and inmental sulphur-bearing immiscible inclusions in gyp-secondary gypsum-anhydrite in pores of rocks, whilesum-anhydrite and calcite that were formed at laterfluid inclusions that can be used for temperaturestage, and for small quantity of oil inclusions mainlymeasurement were hard to find in finely crystallizedvaries in the range of 89°C一105°C.carbonate rocks, or in coarse -grained milk-white cal-The bitumen reflectivity of Triassic and Permiancite and dolomite veins. The inclusions distributed inthe reservoirs show a variety of types, which includecarbonates is shown in Fig. 3.single phase pure liquid inclusions, pure gas inclusions,solid bitumen inclusions, and two-phase gas-liquid 4 Types and characteristics of immiscible inclu-inclusions as well as three-phase CO2- bearing inclu-sionssions, etc. However, it should be noted that inclusionsImmiscible inclusions in minerals and rocks can beformed in the early stage were seldom preserved indirectly trapped from inhomogenized systems duringperfect conditions due to dissolution and deuterogene-either petrogenic or metallogenic process, and can alsosis in reservoirs. The inclusions that currently occur inbe captured from the originally homogenized system,the reservoirs are mainly comprised of early stage in-only in the latter case the inclusions thus capturedclusions resulted from formation of oil-cracked gas inYanshannian and late stage inclusions formed duringwould be transformed into secondary immiscible in-the uplift in Himalayan. In order to investigate the ori-clusions due to a variety of later stage geologicalgins of both gas and H2S, this paper will be focused onprocesses. An extensive investigation about hydrocar-inclusions that can reflect the temperature required forbon reservoir samples collected from the study areaformation and evolution of gas and H2S and immis-indicates that fluid inclusions occur in multiple forms,cible inclusions that contain elemental sulphur andand our research was focused on immiscible fluid in-H2S, etc. In regard to the former type of inclusions,clusions, which contain hydrocarbon, natural sulphur,they are mainly hosted in minerals like recrysallizedH2S, HS-' and the results are listed in the followingcalcite and dolomite, and it was found that bitumensections (Fig. 4-1 - 10).inclusions and gas inclusions were generally par-4.1 Liquid hydrocarbon-bearing protogenic immis-agenetic with high temperature fluid inclusions. Incible fluid inclusionsregard to the latter type of inclusions, they are mainlyhosted in minerals like gypsum-anhydrite and calciteSince oil and natural gas both show rather lowthat were formed at later stage, and it was found thatsolubility in water and petroleum hydrocarbons aregas inclusions, low temperature salt water inclusions strongly hydrophobic, systems characterized by theand a small quantity of oil inclusions were generallyimmiscible coexistence of oil, gas and water wouldparagenetic with immiscible inclusions.generally occur in hydrocarbon reservoir traps andIn regard to two-phase salt water inclusions thapersist during the oil generation and migration proc-were well preserved and show big sizes among fluid esses. Multiple forms of immiscible fluid inclusionsinclusions in reservoirs, measurement of their homog-would be easily formed in transition zones between oilenization temperature， initial melting temperature， and water and between gas and water, in milky-cloudyeutectic temperature and freezing point was performed,systems and i中国煤化Iate.YHCNMH G ..Origin of natural sulphur-bearing immiscible inclusions and HzS in oolite gas reservoir247Table 2 Measurement results of homogenization temperature, initial meling temperature, eutectic temperature and freezing point of fluid inclusionsGas/iquidInitial meltingEutectic temp.Sample No. | Drill No. Drill depth(m)| AgeHost mineralTh(C)| Freezing point(%)temp.(C)(C8995-5-6.5-20-10Cd-5Po13459.9| Tf 3.1171 175 176182 184+3.261158 176Cd-42Du24374Tf3IDolomite 161 16411-15Cd-426if3:|Dolomite151 12712- -161401621021056-9Cd-85| Jinzhu 12969Tf315411-16Cd-24| Luojia 13473.3T/3Calcite118 1206-8-5.5- -20-901615-20859497 102 10584.5 89 907.595 94 97 101 1058.Cd-86| Jinzhu 13012Tf3l109 112 115 105-5.-24.582Anhydrite 102 114138 140148 14813150 157 158 1551081109.3Cd-66 Tieshan 53094P218018218Cd-59| Tieshan 43114126 194seen and only a small quantity of immiscible oil inclu-营sions of special forms can be found in gypsum thatwas formed at later stage. As can be seen under trans-mission microscopy (Fig. 4-2,3), yellow spheroids inthe form of double rings and black spheroids occur in&2the big spherical inclusions, and when H3 blue la-ser-excited fluorescence was applied to this field of80 100 120 140 160 180 200 220vision, the spheroids in the form of double rings in theHomogenization temp. (C)inclusions would show remarkable yellow fluores-Fig. 2. Histogram showing frequency distribution of homogenizationcence and refer to liquid hydrocarbon, the peripheraltemperature of fluid inclusions in Triassic System(Tif ).parts of the inclusions show weak yellowish greenfluorescence and refer to water phases (containing56Fsoluble hydrocarbon), while the black spheroids show48no fluorescence. When LIKAM-600 model stage was40-used for further temperature measurement, the innerrings of the double-ring spheroids would disappear atg 24-g 16t89C, hence referring to gaseous hydrocarbon phases,while the black spheroids show no remarkable varia-8tions. Laser Raman detection yields no positive results.01.5 2.0 2.53.0 3.5 4.0 4.5 5.0since the fluorescence shown by the inclusions is tooBitumen reflectivity (R%)strongFig. 3. Histogram showing frequency distribution of rflectivity of.2 Elemental sulphur. bearing immiscible inclusionsTriassic and Permian solid bitumen.The solid phases in an inclusion trapped from aIn the study area primary immiscible fluid inclu-homogenized system generally refer to saline mineralssions that contain liquid hydrocarbons were rarelyclosely related中国煤化工However, aYHCNM HG.Origin of natural sulphur-bearing immiscible inclusions and HzS in oolite gas reservoir249Fig. 4. llustrations on inclusion photomicrographs. 1, 5, 6. The polyphase immiscible fluid inclusions trapped in authigenic gypsum in the T1f3-1carbonate reservoir in Jinzhu 1 well contain two separate spheroids, one is gaseous H2O, while the other was determined to be solid natural sulphur bylaser Raman spectrometry. Transmitted light, 63x10. 2, 3. The hydrocarbon-bearing, polyphase immiscible fluid inclusions trapped in calcite in theT1f3-1 carbonate reservoir in Jinzhu 1 well are spheroidal in shape and big in size. The spheroidal, big-sized inclusion consists of one yellow, dou-ble-ring-shaped spheroid and one black spheroid. Under fluorescent microscope, the double-ring shaped spheroid shows yellow fluorescence andrefers to liquid hydrocarbon and its peripheral water phase shows weak yellowish green fluorescence (indicating occurrence of soluble hydrocarbon),the blackpheroid shows no fluorescenceence.(2) Transmitted light, 63x10; (3) Fluorescent light, 25x10. 4. Solid bitumen and bitumen bearingimmiscible inclusions distributed in calcite in the TIf3-1 carbonate reservoir in Luojia 2 well. Transmitted light, 20x10. 7, 8, 10. Double-spheroidalimmiscible fluid inclusion groups distributed in authigenic gypsum in the T1f3-1 carbonate reservoir in Jinzhu 1 well. Transmited light, 63x10. 9.The two phase fluid inclusions trapped in authigenic gypsum in the T1f3-1 carbonate reservoir in Jinzhu 1 well show a homogenization temperatureTh=98C. Transmited light, 32x10. 11. Gas inclusions contained in elemental sulphur (orthorhombic sulphur) in the T1f3-1 carbonate reservoir inJinzhu 1 well. Transmited light, 63x10. 12. High temperature fluid inclusions trapped in calcite in the T13-1 carbonate rervoir in Jinzhu 1 wellshow a homogenization temperature Th= 161.2C. Transmitted light, 32x10. 13. Reservoir bitumen with strongly inhomogeneous spherulitic inter-phase structure trapped in calcite of the T1f3-1 limestone in Tieshan 5 well. Reflected light with crossed nicols, 20x10. 14. Reservoir bitumen withspheroidal structure flld in calcite of the T1f3-1 carbonate geode in Tieshan 5 well. Reflected light, 20x10.very special type of immiscible inclusions in the formthe host mineral gypsum, peak at 2917 cm-' is CH4of double-spheroid can be found in authigenic gypsum (B2 in Fig. 5); Point C (water phase in the inclusion)in gas reservoirs of the study area. As can be seenshows not only a remarkable water peak at 3432 cm~from Fig. 4-1, 5, 6, 7, 7, 9, 10, two spheroids separatedbut also a peak at 2589 cm characteristic of HS- irfrom each other occur in inclusions of various shapes.water. In addition, as shown in Fig. 6, cd86-47 sampleFurther observation indicates that one of the twoshows a peak at 2917 cm^ that is characteristic ofspheroids can be mobile and got homogenized into methane in the gas inclusion in gypsum (Fig. 6A).liquid and finally disappeared when being heated on aCd86-7 sample shows peaks characteristic of elemen-stage, suggesting that it is a kind of gas bubble. Nev- tal sulphur in inclusions trapped in calcite (Fig. 6B);ertheless, the other spheroid showed no change whencd86- 14 and cd86-15 samples both show two distinctbeing heated even to a very high temperature on the types of laser Raman peaks that correspond to thestage, and laser Raman spectrometric analysis suggestsdouble-spheroidal immiscible inclusion in gypsum,that it is the solid phase of elemental sulphur. For de-here one of the test points shows peaks representativetails please see Fig. 5, Fig. 6 and Fig. 7. For example,of elemental sulphur (Fig. 6C), while the other testlaser Raman analyses were carried out separately atpoint corresponds to gaseous H2O, but only showsthe A, B, and C points as marked on the dou-peaks characteristic of the host mineral gypsum (Fig.ble-spheroidal immiscible inclusion in CD8660-1-56D). Similar to the case shown in Fig. 6, in Fig. 7 onesample (Fig. 5), and it can be seen from the laser Ra- of the small spheroids refers to elemental sulphur,man spectra corresponding to A1, A2, B1, B2,Cl, C2 while the other test point coresponds to gasecous H2O.that the compositions at the three points are quite dif-Being paragenetic with elemental sulphur-bearingferent from one another, here point A (big spheroid in immiscible inclusions as trapped in gypsum, salt-waterthe inclusion) shows strong peaks at 218, 472， 150inclusions and CH4-bearing pure gas inclusions, etc.cm , which are characteristic of sulphur(A1 in Fig. 5),show a homogenization temperature of ca.100C.Identification of laser Raman spectra as made aboveand peak at 2569 cm-， which is representative of H2S(A2inFig.5).Astothepeakat2497cm,itishardfor fluid inclusions was mainly based on measurementof calibration samples and literature results. The Ra-to assign to a particular compound. When the resultsman peaks at 216.4- 217 cmi，472- 473 cm~were presented at the symposium dedicated to inclu-sions held in Beijing in 2004, this special peak at-149- 152 cm~', 436- 437 cm~' are characteristic oftracted attention from Prof. Jacques Pironon, who re-amorphous elemental sulphur-orthorhombic sulphur,ported that they had also found it in their latest re-while the peaks at 2497- 2590 cm-' are characteristicsearch. Point B (small spheroid in the inclusion) basi-of HS and H2S in water'); the peak at 2917 cm-' iscally shows no peaks that represent sulphur (B1 in Fig.characteristic of CH49; the peaks at 999- 1000 cmi5), except peaks at 1000, 622 cm-' representative ofand at 1016中国煤化工aks referringYHCNMH G.250Science in China: Series D Earth Sciences8000018Cd8660-330000 12569 cmr' C86660228000 |70000 t60000-471000 cm-126000+249750000 -i 240005022000 |40000-30000-1800020000 8010000-242910931156.16000 I622E1188140000120000 200 400 600 800 1000 1200140016002000 2200 2400 2600 2800 3000X axis titeX axis title70000 I1000 cm-l Cd8661-3Cd8661-5B1600032/60000 -5000040000 I2917 cm300002000010000s 6287 150127456{ .655 Jt 2299580000-0200 400 600 800 1000 1200 14001600 1800600022002400 2600 2800 3000 3200Cd8661-422000 -Cd8661-120000 .3432 cm-1C17500-18000-16000-700014000-12000-65008000-2589) 2917/55004000-5000-2000-1500 2000 2500 3000 3500 400002100 2200 2300 2400 2500 2600 2700 2800 2900 30000+十年10umCd8660-1-5C08660-1-5Fig. 5. Microscopic laser Raman spectra at points A, B, and C of an in中国煤化工TYHCNMH G.Origin of natural sulphur-bearing immiscible inclusions and HzS in oolite gas reservoir5160000999 cm-1219Cd681 iu-04620000Cd86-75000040000150000 153、1086 cm-172百1000029175000 ;111000001000 2000 3000 40000 200 400 600 800 1000 120014001600Xaxis title .X axis title60002917 cm-Cd86-1iu047160001086 cm-'14000Cd86-55500+1200000010000-8000-45006000-40006382473.712129835002000-3000-2400 2600 2800 3000 3200400 800 1200 1600 2000Elemental|CH,sulphur14000 t999.5 cmr1Cd86-158000- 1000 cmr'Cd86-148000 t218.8,460.74000-6000 t 149.6|4604000 t440.62000622.820↓637|1155 .0 200 400 600 800 10001200 140016000 500 10001500 20020030005004000(DGaseous H2O| sulphur。 Cd86-16Cd86-I7Fig. 6. Microscopic laser Raman spectra at points of an immisc中国煤化工YHCNMH G.252Science in China: Series D Earth Sciences8000 ]999.34 cm-lCd86-1612000 t21999.34 cm-1Cd86-1710000 -460.78000十15040006000 -日300042 |2000.601000 -622 ,6552000622. ,6s51156(- 10000 200 400 600 800 1000 1200 1400 1600X axis titleGaseous H2OElementalsulphurFig. 7. Microscopic laser Raman spectra at points of an inmiscible fluid inclusion.respectively to the host minerals gypsum and anhy-perature (> 170- 180"C) thermal evolution in its geo-drite containing fluid inclusions; the peaks at 1086,logical history, a great amount of thermogenic solid711, 154 cm are characteristic of the host minerals bitumen was formed at the stage for formation ofcalcite containing fluid inclusions.oil-cracked gas, and generally filled in pores of car-Because sulphur shows low solubility in water,bonate reservoirs, in anhedral filling form or spher-elemental sulphur particles can be easily captured inidal shape. The solid bitumen typically shows inho-minerals like gypsum that grow in milky immisciblemogeneous structures characteristic of pyrobitumen-system. Currently a comparative analysis by usinganthraxolite and their intermediate phaseslho. Fig.laser Raman spectroscopy and X-ray diffraction tech- 4- 14 demonstrates that massive reservoir bitumen dis-nique was performed and the results indicate that thetributed in carbonate reservoirs shows a structureelemental sulphur filled in pores in reservoirs in thetypical of intermediate phases of remarkable, highstudy area has been changed into relatively stable or-temperature thermogenic origin， while Fig. 4-13thorhombic sulphur from its original amorphous form. shows that pyrobitumen filled in pores of carbonateThe elemental sulphur filled in reservoir pores shows areservoirs is in vein wall form or spherulitic form.Raman peak at 471 cm-, which is higher than theAmong the samples studied, the reservoir bitumenpeak at 217 cm，indicating that the measured Ramanshows a reflectivity BR。as high as 2.14% - 4.5% andpeaks are different from the characteristic spectra for remarkable double reflection. Results of determinationelemental sulphur in immiscible inclusions. Moreover,are shown in Table 1 and Fig. 3.the orthorhombic sulphur filled in reservoir pores mayAs can be seen from Fig. 4-4， not only calcitepossibly contain gas inclusions (Fig. 4-11).formed during the high temperature thermal evolutionstage contains solid bitumen in impurity forms, but4.3 Solid bitumen-bearing immiscible inclusionsalso fluid inclusions contain solid bitumen in immis-Because the oolite reservoir underwent high tem-cible forms.中国煤化工g immiscibleYHCNMHG.Origin of natural sulphur-bearing immiscible inclusions and HzS in oolite gas reservoir253inclusions records abundant information concerning sions may be captured by minerals which were newlycrude oil at the high temperature pyrolysis stage.formed in oil-water-gas transition zones and duringprocesses of hydrocarbon migration and pressure de-4.4 Epigenetic immiscible inclusionscrease.Since calcite， dolomite and anhydrite generallyshow fragile and soft properties, both organic and in-5 Trapping temperature and pressure of fluidorganic fluid inclusions originally captured from ainclusionshomogenized system would undergo pyrolysis, pclymerization, loss or scragging deformation, etc. inThe trapping pressure of oil inclusions is an impor-later stage thermal metamorphism and tectonic defor- tant parameter that can reflect the depth and pressuremation, which would change the compositions andfor formation of hydrocarbon reservoirs, but is diffi-phases of the original system and transform the origi-cult to determine. Presently extrapolation of trappingnal fluid inclusions into epigenetic immiscible inclu-pressure can be performed with multiple PVTX simu-sions. As is shown from Fig. 4-4, black solid bitumenlation and calculation software and related meth-occurs in the walls of part of the high temperature in-ods12- 10. In regard to the method employed for thisclusions that show relatively higher gas/liquid ratio ofwork, the first step is to simulate and calculate thecalcite, this is because liquid hydrocarbon in thesecompositions of and saturation pressure for oil inclu-inclusions would undergo thermal metamorphism atsions with PVTSim software and the method proposedthe stage of oil cracking in reservoirs, and be trans- by Aplin et al. in 1999，while the second step is toformed into gas, and a great amount of gas would befurther simulate and derive the isometric equation forleft in the gaseous state, while a small amount of gas oil inclusions and their paragenetic salt water inclu-would be condensed into solid bitumen that wouldsions and to calculate the trapping temperature andoccur on the walls of high temperature inclusions.pressure for various types of oil inclusions, with4.5 Relationship between immiscible inclusions andPVTsim software and the method adopted by Liu et al.in 200314.homogeneous fluid inclusionsBasic parameters utilized for PVTsim simulationImmiscible inclusions are coexistent with homoge-and calculation for oil inclusions from Jinzhu 1 wellneous fluid inclusions in samples studied. Homoge-are: homogenization temperature for oil inclusions =neous fluid inclusions of gasliquid phases predomi-98.1C, gas/liquid ratio = 11.6%; (Since oil inclusionsnate in calcite, while immiscible inclusions are mainlyshow spheroidal shapes similar to those of bubbles,concentrated locally or distributed in a sporadic form their two dimensional image analytical data shall bein gypsum, which demonstrates that the fluids in hy-similar to the CIL .SM-determined results). The compo-drocarbon reservoirs are remarkably distinct from thesitions simulated and calculated for inclusions (mol% )ordinary petrogenic fluids and the melallogenic fluids are: (Nz=0.544, CO2=3.450, C1=49.605, C2=5.326,for metallic hydrothermal ore deposits. Macroscopi-C3=3.596, iC4=0.680, nC4=1.604, iCs=0.710, nCs=cally, fluid in an oil/gas structure is a relatively com-0.846, C6=1.115, C7=2.653, Cg=3. 169, Cg=2.080,plicated inhomogenized system, while in some local-C1o-C13=6.532，C14- -C16= 4.383, C17- C2o=4.63,ized sections or microscopic parts fluids can be of aC21-C23= 2.821, C24-C26=2.146, C27-C31=2.509,single-phase that is at its temperature and pressureC32一C36=1.600).equilibrium. As a result, fluid inclusions may possiblyThe saturation pressure for trapped oil inclusionsbe captured in newly formed minerals, at the enlarge-through simulation and calculation is P=274.15x10'Pa,ment edges or on pore-healing planes of minerals in aand the isometric equation A derived for the oil inclu-reservoir, and hence can be used as tools for meas-sions is: P=4.43t- 159.99. The salt-water inclusionsurement and estimate of the temperature and pressureparagenetic with trapped oil inclusions yield a hoconditions of the reservoir, while immiscible inclu-mogenization temperature =105 C, a freezing point =sions enriched in hydrocarbons and“boiling” inclu--5.5C, andi 中国煤化1-38.2-3972.TYHCNMHG.254Science in China: Series D Earth SciencesWhen the two equations A and B are solved together, between sulphate minerals in gypsum-bearing carbon-the temperature and pressure for trapped oil inclusionsate rocks and hydrocarbons (TSR)17.18].are: Tr=112.88C, Pr=340.07x10'Pa. Fig. 8 is a dia-Based on the close relationship between the H2S-gram that shows the evolution of phases in oil inclu-bearing gas reservoirs and the anhydrite-bearing car-sions under different P-T conditions and the isometricbonate rocks in the study area, the characteristic δtslines for both oil inclusions and salt water inclusionsof H2S in natural gas (+12%o-+13%o CDT) and the(The isometric line for salt water inclusions as showncharacteristic δ S of sulphate minerals (+11%o-in the diagram refers to the line of simulation of the+21%o，with a mean of +15%o CDT), and geologi-unsaturated system of methane in water)!4- 10. As thecal-geochemical data such as the highest geothermaloil inclusions are paragenetic with immiscible inclu-temperature of > 130C suffered by reservoir rock, etc.,sions that contain elemental sulphur in the samples,previous scholars like Wang et al. (2002)8, Cai e1 al.，the P-T conditions derived from oil inclusions can also(2003)21, Cai et al. (2004)22 , and so on held that H2Sbe regarded as conditions for formation of the im-in natural gas reservoirs was mainly of TSR origin. Inmiscible inclusions.this paper the temperature, pressure and geological-geochemical conditions during the TSR processes of6 Discussion on the origin of hydrogen sulphideH2S in natural gas reservoirs will be further discussed,In regard to the origin of H2S in natural gas reser-based upon reflectivity of reservoir bitumen and ho-voirs, extensive research has been conducted bymogenization temperature of fluid inclusions in thenumerous scholars based on sulphur isotope composi-samples, and in particular the occurrence and distribu-tion data and experimental results of thermal reducing tion characteristics of immiscible inclusions that arereactions between sulphates and hydrocarbonsh7 23. Itenriched in elemental sulphur, H2S and other compo-is generally agreed that H2S in gas reservoirs in dif-nents.ferent areas could be related to three types of geologi-Formation of H2S and elemental sulphur in naturalcal-geochemical processes, i.e, (1) thermal disintegra- gas reservoirs in the study area requires some basiction of sulphur- bearing organic matter in kerogen; (2) conditions to be satisfied: In Luojiazhai, Dukouhe andbacterial reduction of sulphate minerals (BSR) ingyp-Tieshanpo at the two sides of the Kajjiang-Liangpingsum-bearing carbonate at a temperature of <60- 80"C;trough, the Ertan gas reservoir in the Lower Triassicand (3) high temperature thermochemical reductionFeixianguan Formation contains 12%- 17% of H2S,lsometric line forPr-340.07X 10% Paoil inclusinosTr-ll2.88C300250豆200CritP_lsomtric line for150◆salt water inclusions蓖10050 t-100.00 .0.00100.00200.00 .300.00400.00500.00Temperature(C)Fig. 8. Diagram showing evolution of phases in oil i中国煤化工YHCNMH G.Origin of natural sulphur-bearing immiscible inclusions and HzS in oolite gas reservoir255and the gas reservoir at Jinzhu is filled locally byand pressure conditions are listed as follows:natural sulphur (orthothombic sulphur) of light yellow(1) The oil/gas reservoir underwent settlement andcolor. The burial history and thermal evolution history burial, at this stage H2S was formed through the highof gas reservoirs in the study area demonstrate that thetemperature thermal fluid reaction as follows:greatest burial depth is more than 6,000 m and the120C- 180Chighest paleogeothermal temperature is above 200Ch.CaSO4+HC+H2S+CO2+CaCO3+bitumen+gaseousThe measured reservoir bitumen in samples shows a hydrocarbonreflectivity as high as 2.4% - 4.1% and remarkableAlso at this stage, the newly generated calcitedouble reflection, and structures of pyrobitumen-an- trapped high temperature fluid inclusions, which con-thraxolite of pyrometamorphic origin and their specialtain H2S-bearing gaseous hydrocarbons and solid bi-intermediate phases, and platy mosaic structures; The tumen.carbonate minerals in the reservoirs show three groups(2) During the later stage of uplift of reservoirs andof homogenization temperatures for fluid inclusions: decrease in both temperature and pressure, elementalthe homogenization temperature of fluid inclusionssulphur was formed according to the following reac-can be as high as 160C- 180°C in the stage for for- tions: .mation of oil-cracked gas, varies from 100C- 110°C(95"C - 105"C) (340x10'Pa)in calcite formed at late stage after the reservoirs were(A) 2H2S+O2-→2S+2H2Ouplifted; and mainly changes from 98C- 108C in(95"C - 105*C) (340x10'Pa)authigenic gypsum minerals. Multiple layers anc(B) CaSO4+ HC+H2S-→S+CaCO3+CO2+H2Oblocks of anhydrite were developed in gas-bearingAt this second stage, not only elemental sulphursections; calcite, anbydrite, gypsum, natural sulphur was commonly generated in cracks and pores in res-(orthorhombic sulphur) are extensively parageneticervoirs but also multiple types of sulphur-bearing im-with one another in reservoirs like Jinzhu 1 well; mul-miscible inclusions were trapped in newly generatedtiple types of immiscible inclusions that contain ele-and recrystallized calcite and gypsum crystals.mental sulphur and H2S, etc. occur in late stage gyp-The genetic models as described above demonstratesum -anhydrite and calcite crystals.that in gas-bearing strata with gypsum and anhydriteWorden et al. (1995)19] and Machei (1998)18]interlayers, generation of H2S can possibly continue inthought that the temperature for TSR to occur in gasareas with burial depth greater than 5,000- 4,000 mreservoirs is 140 C，while Worden and Smalleyand temperature higher than 110°C- 120"C, which(2001)20) held that in some other hydrocarbon pro-will surely result in higher concentration of H2S in theducing basins TSR may occur in 120- 130C. Still,gas reservoir. However, in areas with no gypsum andCai et al. (2003)22] and Cai et al. (2004)21 deemedanhydrite interlayers and burial depth less than 3,000that TSR mainly occurred at 130- 140°C in the studym, the H2S concentration will generally be lower.area. The measured results of homogenization tem-perature of fluid inclusions and the characteristics of7 Conclusionsoccurrence of immiscible inclusions that contain ele-mental sulphur and H2S in gas reservoirs in the studyResults of laser Raman spectral analysis and fluo-area further demonstrate that H2S in the reservoirs wasrescence detection indicate that multiple special formsmainly originated from thermochemical reaction beof immiscible inclusions that contain elemental sul-tween hydrocarbons and sulphates at the oil-crackedphur, liquid hydrocarbons and bitumen, etc. occur ingas formation stage in Yanshannian and that elementalthe carbonate reservoir samples collected from Jinzhusulphur was derived from partial oxidation of H2S andrea. Fluid inclusions of multiple formation stages thatfurther reactions among sulphates, hydrocarbons andshow homogenization temperatures of 89°C - - 120C,H2S at the reservoir uplift stage in Himalayan. Majo130C - 160C, and 170C - 180C occur in dolomitegenetic models involved and associated temperature and calcite mi中国煤化工and multipleCNMHG.256Science in China: Series D Earth Sciencestypes of immiscible inclusions showing homogeniza-4. Lu Huanzhang, Fan Hongrui, Ni Pei et al., Fluid Inclusions (intion temperature of 85C- -95*C occur in the laterChinese), Beijing: Science Press, 2004, 17- 202.stage anhydrite-gypsum and calcite. Based on the ref-5. Liu Bin, Shen Kun, Thermodynamics of Fluid Inclusions (inChinesc), Bijing: Geological Publishing House, 1999, 12- 16.lectivity of reservoir bitumen (2.4% - 4%), the stage6. 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