Rearch
Deep Matter &Energy—Review
Theoretical Progress and Key Technologies of Onshore Ultra-Deep Oil/Gas
Exploration
Xusheng Guo a ,⇑,Dongfeng Hu a ,Yuping Li a ,Jinbao Duan a ,Xuefeng Zhang b ,Xiaojun Fan a ,Hua Duan a ,Wencheng Li a
a SINOPEC Exploration Company,Chengdu 610041,China
b
Institute of Oil and Gas,School of Earth and Space Sciences,Peking University,Beijing 100871,China
黑白人物画
a r t i c l e i n f o Article history:
Received 28June 2018Revid 24October 2018Accepted 22January 2019
Available online 26April 2019
Keywords:
Oil/gas exploration Ultra-deep
Oil/gas sources Rervoir
Petroleum accumulation
Exploration and exploitation technologies
a b s t r a c t
Oil/gas exploration around the world has extended into deep and ultra-deep strata becau it is increas-ingly difficult to find new large-scale oil/gas rervoirs in shallow–middle buried strata.In recent years,China has made remarkable achievements in oil/gas exploration in ultra-deep areas including carbonate and clastic rervoirs.Some (ultra)large-scale oil and gas fields have been discovered.The oil/gas accu-mulation mechanisms and key technologies of oil/gas rervoir exploration and development are sum-marized in this study in order to share China’s experiences.Ultra-deep oil/gas originates from numerous sources of hydrocarbons and multipha charging.Liquid hydrocarbons can form in ultra-deep layers due to low geothermal gradients or overpressures,and the natural gas composition in ultra-deep areas is complicated by the reactions between deep hydrocarbons,water,and rock or by the addition of mantle-or crust-sourced gas.The oils/gas are mainly stored in the original high-energy reef/shoal complexes or in sand body diments.They usually have high original porosity.Secondary pores are often developed by dissolution,dolomitization,and fracturing in the late stage.The early pores have been prerved by retentive diagenes such as the early charging of hydrocarbons.Oil/gas accumulation in ultra-deep
areas generally has the characteristics of near-source accumulation and sustained prervation.The effective exploration and development of ultra-deep oil/gas rervoirs depend on the support of key technologies.U of the latest technologies such as ismic signal acquisi-tion and processing,low porosity and permeability zone prediction,and gas–water identification has enabled the discovery of ultra-deep oil/gas resources.In addition,advanced technologies for drilling,completion,and oil/gas testing have ensured the effective development of the fields.
Ó2019THE AUTHORS.Published by Elvier LTD on behalf of Chine Academy of Engineering and Higher Education Press Limited Company.This is an open access article under the CC BY-NC-ND licen
(creativecommons/licens/by-nc-nd/4.0/).
300字的日记
1.Introduction
After many years of petroleum exploration and exploitation,it is becoming increasingly difficult to find oil or gas in middle–shallow buried strata.Globally,with the development of theory and technology,more and more oil and gas have been found in deep (4500–6000m)to ultra-deep (>6000m)buried strata.Since 2008,production of oil and gas from rerves in ultra-deep buried strata
has incread greatly.
The potential for ultra-deep petroleum exploration is huge.Glob-ally,the six basins with the most enriched ultra-deep oil and gas rerves are the Mexican Bay Basin (USA),the Tarim Basin (China),the South Caspian Basin (Russia),the Arab Basin (Middle-East),the Santos Basin (South America),and the Sichuan Basin (China).In the basins,120ultra-deep oil or gas fields have been found with tremendous proven,probable,or possible deep rerves.
Ultra-deep strata are also important targets of petroleum exploration in China,and veral oil or gas fields have been found.In this paper,the discoveries are introduced with specific cas.In addition,key features of the oil/gas fields,including the oil/gas sources,rervoir,petroleum accumulation mechanisms,and major technologies in ultra-deep petroleum exploration and exploitation,are discusd.
2.Recent exploration discoveries in ultra-deep areas in China Ultra-deep petroleum exploration in China is mainly concen-trated in the Sichuan Basin and the Tarim Basin.In recent decades,veral ultra-deep giant oil or gas fields,such as the Yuanba
⇑Corresponding author.
E-mail address:guoxs. (X.Guo).
Gasfield in the Sichuan Basin,and the Tahe Oilfield and Keshen–Dabei Gasfield in the Tarim Basin,have been found.At prent,a total of 2.55Gt (oil equivalent)of rerves has been proven,accounting for 11.6%of the proven,probable,and possible rerves (Fig.1).The achievements were listed in Table 1as major pro-gress in science and technology on an annual basis over a ten-year period in China by the China National Petroleum Corporation (CNPC),Geological Society of China (GSC),and Chine Petroleum Society (CPS).However,considering the tremendous rerves in ultra-deep strata,most of the ultra-deep petroleum is unexplored.According to the discoveries,ultra-deep petroleum could be stored in various rervoirs,including carbonates,clastic rocks,and volcanic rocks.Among the,carbonates deposited in marine environments and clastic rocks dimented in terrestrial delta facies are the most important (Table 2).2.1.Carbonate rervoirs
The ultra-deep carbonate rervoirs in China are mainly distributed in the Paleozoic to Proterozoic strata in the Sichuan Basin and the Tarim Basin.In the Sichuan Basin,only natural gas is produced in the ultra-deep buried strata,and the potential rer-voirs are distributed throughout the basin.To date,the Yuanba Gasfield has been discovered in this region in addition to some minor discoveries in
the Longgang area (Fig.2).
The Yuanba Gasfield is located in the northern depression of the Sichuan Basin.The gas is rerved in marine strata of the Upper Permian Changxing to the Lower Triassic Feixianguan Formations.The rervoir,buried at 6500–7110m,is a lithological reef-shoal complex with an average porosity of 5.2%.The total proven gas rerve is 2.19Â1011m 3.The natural gas was compod of 88.35%methane (CH 4),5.22%hydrogen sulfide (H 2S),and 6.8%car-bon dioxide (CO 2)[1].Tens of kilometers to the southeast of the Yuanba Gasfield,the Longgang reef-shoal gas field was discovered,with a shallower burial depth and a proven rerve of about 4.1Â1010m 3in the ultra-deep strata.
In the piedmont zone of the western depression of the Sichuan Basin,high-producing gas flow was also found in the top rervoir of the Leikoupo Formation,with a depth of more than 6000m.The daily gas production of Well Yangshen-1in the fourth member of the Leikoupo Formation reached 0.6Â106m 3,thus demonstrating great exploration potential.
In the Tarim Basin,the ultra-deep large oil/gas fields are mainly distributed in the Tabei and Tazhong areas (Fig.3).The main pro-duction strata are the Middle–Lower Ordovician;the Cambrian also has a certain degree of exploration职业电竞选手
potential.
Fig.1.Bar charts showing geological resources and proven rerves of ultra-deep petroleum in six main onshore basins :tonne of oil equivalent.Table 1
Major progress in science and technology related to ultra-deep petroleum exploration in China since 2010.Year Major progress in ultra-deep petroleum exploration
Awarded organization 2010(Ultra-)deep exploration technology from China ud in the shallow a r
egion promoted the discovery of a giant gas pool in the mature exploration area in the Gulf of Mexico
CNPC 2011The Yuanba exploration project,a sub-project of Marine Exploration in northeastern Sichuan Basin,found the deepest large gas field in China
GSC 2012The hydrocarbon supply theory for deep stratum was widely promoted
CNPC 2012A giant gas field was found in the carbonate strata with a depth greater than 6500m in the Hanilcatam (Halahatang)area,Tarim Basin
GSC 2014A natural gas exploration project was undertaken in Kelasu,Tarim Basin
GSC 2015The tolerance temperature of downhole tools broke the record and reached 200°C
CNPC 2016High and stabilized production technology was developed for big reef gas fields with ultra depth and high sulfur content in the Yuanba Area
GSC 2016
Ultra-deep horizontal well drilling and completion technology were developed
CPS
Table 2
Ultra-deep rervoir rock types and major discoveries.Rervoir types Sedimentary facies
Major discoveries with formations and locations
Carbonates
Marginal platform reef-shoal Upper Permian Changxing Formation and Lower Triassic Feixianguan Formation (Yuanba,Sichuan Basin)Ordovician (Tazhong No.1structure,Tarim Basin)Shoal (grain beach)Carboniferous (Tazhong No.4structure,Tarim Basin)Dolostone
Carboniferous (Sichuan Basin)
Karstic weathering crust Ordovician (Tahe,Hanilcatam,Lunnan and Lungu,Tarim Basin)Fault/fracture Ordovician (Shunbei,Tarim Basin)
Clastic rocks
Delta deposits
Cretaceous (Keshen–Dabei Gasfield,northern Tarim Basin)
X.Guo et al./Engineering 5(2019)458–470459
The largest oil field discovered in the Tabei area is the Lunnan–Tahe karst fracture-cavity field.Its oil-bearing area is 2800km 2,consisting of more than 100karst fracture-cavity oil storage units of various sizes.Even in the buried strata more than 6000m deep,the epigenetic karst fracture-cavities are still well prerved.In 2006,Well Tashen-1,which is 8408m deep,was drilled in the southeastern part of the Akekule Uplift in the northern Tarim Basin,and liquid hydrocarbons were found in Cambrian high-quality dolostone rervoirs.At the end of 2014,the drilling depth of a single well of the Middle–Lower Ordovician in the Well Tashen-3field was 6168.24–6724.00m.A fracture-cavity beneath the weathering crust with a thickness of 160–350m was sub-quently discovered,and high-yield oil flow was obtained.As of the end of 2014,the Tahe Ordovician has yielded 1.377Â109t of proven rerve.
In Tazhong,the ultra-deep oil and gas are mainly distributed along the Tazhong No.1fault zone.In 2013,there were also break-throughs from Well Zhongshen-1in the Middle–Lower
Cambrian
硬座宝Fig.2.Ultra-deep gas fields in the Sichuan
Basin.传染病应急预案
Fig.3.Ultra-deep oil and gas fields in the Tarim Basin.
460X.Guo et al./Engineering 5(2019)458–470
Xiaoerblak Formation at6861–6944m,with158500m3of natural gas produced per day.Subquently,liquid hydrocarbons were found in Well Zhongshen-5at6562–6671m.After acid fracturing, the highest daily oil and gas production were24.17and11804m3, respectively,with a6mm nozzle[2].In the Shuntuoguole Lower Uplift area of the Tazhong No.1fault zone,the rervoir depth is generally6600–8000m,with a formation pressure of82–172MPa and a stratum temperature of184–207°C.Well Shuntuo-1,with a depth of7874m,produced388000m3of natu-ral gas per day in the Ordovician rervoir.In the southern part of Tazhong—that is,the Shunnan area—2.6Â105–1.1Â106m3of natural gas per day was drilled to a well depth of6300–6700m in the Ordovician.In the northern part of Tazhong—that is,the Shunbei area—ultra-deep oil with a burial depth of more than 6300m was also discovered in the Ordovician.Well Shunbei-1CX produced132m3of oil and45000m3of gas per day.
2.2.Clastic rock rervoirs
To reach ultra-deep layered clastic rervoirs,China has drilled multiple ultra-deep wells.The wells include:Well Keshen-1 (8023m),Well Keshen-902(8038m),and Well Bozi-1(7014m) targeting the Cretaceous;Well Adong-1(7680m)targeting the Upper Ordovician in the Tarim Basin;and Well Niudong-1 (6027m)targeting the Paleocene in the Bohai Bay Basin.The Keshen–Dabei Gasfield is currently the only found gas pool in the Kuqa Depression of Tarim.
The Keshen–Dabei Gasfield is located in Kelasu Subsalt in the Kuqa Depression in the Tarim Basin.Its clastic rock rervoirs are rich in natural gas resources.A Cretaceous high-quality delta sand rervoir was found in the ultra-deep buried strata with a burial depth of6000–8000m,stratum pressure of88–136MPa,stratum temperature of120–184°C,and rervoir thickness of200–300m.The rervoir is vertically stacked and is laterally connected with an area of18000km2;it has a porosity of5.7%–7.9%[3].A total industrial oilflow of466000m3ÁdÀ1was obtained in the 6573–6696m Cretaceous interval from Well Keshen-2drilled on August28,2008,which marked the discovery of the Keshen Gas-field.The test of Well Keshen-9in the7445–7552m ction revealed a daily output of1.13Â106m3of high-yield industrial gas.The gas rervoir pressure is127.4MPa with a wellhead oil pressure of100MPa.Thus far,14gas pools have been found by the end of September2018,the includedfive large-scale gas rervoirs(Keshen-8,Keshen-9,Keshen-6,Keshen-13,and Bozi-1)with a total proven gas rerve of8.3Â1011m3and probable and possible rerves of3Â1011m3.
In addition,in the Songliao Basin,ultra-deep clastic rock rer-voirs have been industrially developed.In the Qikou sag area of the Bohai Bay,ultra-deep buried strata have also been identified as a potentially profitable exploration area.
3.Oil/gas sources in ultra-deep rervoirs
Thefirst major issue to be solved in ultra-deep oil/gas explo-ration is the origin of the oil/gas.The classic‘‘kerogen pyrolysis and hydrocarbon generation theory”[4]us a kerogen-generating-hydrocarbon geochemical diagram to establish an oil/-gas genesis model.However,this model is too general[5–7],and it is becoming increasingly difficult to explain complicated ultra-deep oil/gas sources.After long-term geological evolution,the composition of ultra-deep oil/gas is becoming extremely complex and often shows the characteristics of mixed and multistage for-mation from multiple sources.This is becau ultra-deep oil/gas rervoirs often have multiple supplies of hydrocarbons and multi-stage charging mechanisms.3.1.Multiple supplies of hydrocarbons
There are two major sources of ultra-deep gas.One is the crack-ing of crude oil,which includes the direct cracking of crude oil into gas and the cracking of bitumen—the byproduct of crude oil cracking—into gas.The other is the cracking of hydrocarbon source rocks(kerogen)after maturation.
3.1.1.Crude oil-cracked gas
After being deeply buried for a long time,the source rocks became mature and began to generate hydrocarbons.As they were transported,the hydrocarbons accumulated in effective traps and formed ancient rervoirs.As the burial depth further incread,the temperature ro above160°C and the ancient oil rervoir began to crack.An ancient gas rervoir was thus formed. Under the effects of later tectonic adjustment and re-aggregation, the current gas rervoir was then formed.Another product of crude oil cracking was solid bitumen,which could generate hydro-carbons again under the effect of heat.This constituted another important source of natural gas[8,9].
Therefore,paleo-oil or bitumen cracking may be an important source of ultra-deep natural gas.With respect to sapropelic organic matter,most of the natural gas comes from the cracking of crude oil produced by kerogen in the early stage,and only a small part of natural gas originates from kerogen cracking[10].
The marine reef-shoal gasfield of the Changxing Formation in the Yuanba area,Sichuan Basin and the Cambrian subsalt gas rer-voir of Well Zhongshen-1in the Tazhong area of the Tarim Basin are typical rervoirs compod of gas cracked from crude oil [11–13].Analys of ln(C2/C3)–ln(C1/C土星资料
2)and the relationship between(d13C2–d13C3)and ln(C2/C3)of natural gas from the mar-ine reef-shoal rervoir of the Changxing Formation in the Yuanba area show that this natural gas comes from the Upper Permian hydrocarbon source rocks.The ratio of C2/C3obviously increas, and the change in C1/C2is relatively small.This demonstrates that natural gas of the Changxing Formation is mainly sourced from the cracking of crude oil(Fig.4)[1].A large amount of pyrobitumen can be en in the rervoir.Isotopes show that natural gas from Well Zhongshen-1in the Tazhong area is also mainly derived from crude oil cracking[13].
3.1.2.Hydrocarbon source rock(kerogen)-cracked gas
With respect to humic organic matter,most natural gas comes from the cracking of kerogen[10].The Cretaceous sandstone rer-voir of the Keshen–Dabei Gasfield in the Tarim Basin isfilled with kerogen-cracking gas sourced from the Triassic.The current vit-rinite reflectance of the source rock is1.5%Ro(reflectance in oil) or higher,which meets the conditions for the formation of cracked gas[14].
Overall,the propod mechanism of polygenic hydrocarbon generation breaks through the theoretical framework of‘‘kerogen degradation to generate hydrocarbon”[12].By theoretically prov-ing
that ultra-deep buried strata are rich in hydrocarbon sources and have huge exploration potential,this mechanism has impor-tant guiding significance for oil/gas exploration in areas with over-matured source rocks.
3.1.3.Reworking of gas compositions by thermochemical sulfate reduction
H2S content is one of the most distinctive characteristics of the gas composition of different ultra-deep gas rervoirs.The H2S in natural gas rervoirs generally originates from the following sources:①pyrolysis of sulfur compounds in kerogen or crude oil;②bacterial sulfate reduction(BSR)by bacteria or microorgan-isms;and③thermochemical sulfate reduction(TSR).It is gener-
X.Guo et al./Engineering5(2019)458–470461
ally believed that high concentrations of H 2S (>5%)in ultra-deep natural gas indicate a contribution from TSR [15–18].The reaction occurs between sulfate (gypsum/anhydrite)and hydrocarbons:
CaSO 4þhydrocarbon !CaCO 3þH 2S þH 2O ÆS ÆCO 2ð1Þ
The content of H 2S and CO 2in the ultra-deep natural gas of the Changxing Formation in the Yuanb
a area is relatively high.Statis-tics show that the content of H 2S in the Yuanba I block is generally greater than 5.0%with a maximum of 12.0%–15.0%,and that the CO 2content is generally between 5.0%and 20.0%.The H 2S content of the Yuanba II block is generally between 1.0%and 6.0%,and the CO 2content generally ranges from 2.5%to 12.5%.Although the con-tent of non-hydrocarbon gas in wells of blocks I and II has chan-ged significantly,there is a significant positive correlation between the content of H 2S and CO 2in each well [19],indicating that both are products of the TSR,a chemical reworking of the natural gas in this area.
3.1.
4.Mantle-or crust-sourced gas
The injection of mantle-or crust-sourced gas,such as helium (He),nitrogen (N 2),and CO 2,makes the composition of deep gas even more complicated.Tho mantle-or crust-sourced non-hydrocarbon gas may be economically producible,as has been shown globally by many cas.Among them,the rervoirs found in China are mainly distributed in the central and eastern areas near and along the Tanlu Fault Zone,including the Songliao Basin,Liaohe Basin,Huanghua Depression,Jiyang Depression,northern Jiangsu Basin,and Sanshui Basin.Some of the gas wells ha
ve a He gas concentration of more than 0.05%or a CO 2content of 80%or as high as nearly 100%[20].Examples include:①the Permian CO 2gas field in the Huangqiao area,northern Jiangsu Province,with a CO 2purity of 99.9%with proven rerve of 1.42Â1010m 3;②gas from the Wanjinta structure in the Son-gliao Basin and Well Jie-3in the Liaohe Basin,which are rich in He gas;③the Weiyuan Gasfield,western Sichuan Basin,the He content in which ranges from 0.1%to 0.34%;and ④Well Yang-1
and Well Shuang-1on the northern margin of the Nanpanjiang Basin,the He contents in which are as high as 1.28%and 0.1%–1.28%,respectively.
Mantle-or crust-sourced gas generally migrate along deep faults.After entering the gas rervoirs,non-hydrocarbon gas becomes abnormally high in content,which changes the original composition of the gas.Converly,the complication of gas rervoir composition has made it difficult to recognize the source of ultra-deep oil/gas and to understand the gas accumulation mechanism.辛弃疾介绍
3.2.Development of ultra-deep liquid hydrocarbons
Formation temperature increas as the depth of burial increas,and hydrocarbon formation and ev
olution are cloly related to temperature.Not only does the hydrocarbon generation process of source rocks tend to stop above a certain temperature limit,but liquid hydrocarbons are often cracked into gas under high-temperature condition.Therefore,there are ‘‘oil-generation windows”and ‘‘hydrocarbon generation deadlines”for oil/gas exploration [21].Most oil and gas are stored in the zone where the formation temperature is 60–120°C [22].However,in ultra-deep buried strata,the temperature is often higher than 120°C.Therefore,it was believed that the chance of finding liquid hydro-carbons would be very low.
Nevertheless,exploration has confirmed that liquid hydrocar-bons can still be found in ultra-deep buried strata.Liquid crude oil has been found in the Cambrian below 6000m in Well Zhongshen-1and below 8000m in Well Tashen-1in the Tarim Basin,and in the Mesoproterozoic Jixian Group in Well Niudong-1in the Bohai Bay Basin.The examples show that the liquid oil window may change with geological conditions.In particular,the low geothermal gradient and overpressure effect are often the main factors for the development of ultra-deep liquid
hydrocarbons.
Fig.4.Relationship between ln(C 2/C 3)and ln(C 1/C 2),showing sources of natural gas in the Sichuan Basin.Reproduced from Ref.[1]with permission of Marine Origin Petroleum Geology,Ó2014.
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3.2.1.Low geothermal gradient
Ultra-deep strata are commonly characterized by high forma-tion temperature.However,basins with a low geothermal gradient are controlled by low heat-flow values.Their stratum compaction is often relatively weaker than that of basins with a high geother-mal gradient.To a certain extent,the maturing of source rocks postpones and hence the time of hydrocarbon generation lags behind,which is beneficial to the generation of ultra-deep oil/gas in cold basins.Due to the rapid and deep burial at the late stage, the Tarim Basin in western China has a low geothermal gradient (15–25°CÁkmÀ1)and is capable of generating hydrocarbons even in the ultra-deep buried strata.However,in regions in the east of China,such as the Songliao Basin,which has a high geothermal gradient(38–42°CÁkmÀ1),there is basically no hydrocarbon gener-ation capacity[23].Recently,explorationfindings from the Shun-tuoguole Uplift in the northern part of Tazho
ng indicate that the Lower Cambrian source rocks were still in the condensate oil and natural gas generation stage during the Himalayan period due to a long period of low formation temperature[24].Furthermore,as for marine crude oil in the Tarim Basin with a low geothermal gra-dient background and a fast burial history in the late stage,some rearchers believe that the lower limit of the depth of liquid pet-roleum degradation due to cracking is below9000–10000m,and the corresponding rervoir temperature is greater than210°C. Above this depth,liquid petroleum can exist in large quantities [25].
3.2.2.Overpressure effect
Most ultra-deep rervoirs are characterized by overpressure [26].Under overpressure conditions,the thermal maturation of organic matter would be resisted.Therefore,hydrocarbon genera-tion and oil cracking would be postponed[27].Hence,oil could be generated or prerved in ultra-deep strata.
3.3.Multistage charging
In superimpod basins,the superimposition of multistage tec-tonic movements controls the multistage hydrocarbon generation process of multicomponent parent materials.Affected by this, ultr
a-deep hydrocarbon rervoirs often have the characteristics of multipha charging.
Ultra-deep gas rervoirs often originate from the cracking of aggregated crude oil.The pyrolysis and multipha(primary,c-ondary,or even tertiary)hydrocarbon generation of bitumen in dif-ferent evolutionary stages and with different occurrences including in situ rervoir bitumen,offsite rervoir bitumen,and source rock-disperd bitumen residual,often constitute the main accumulation mechanism for deep/ultra-deep natural gas[28].Oil/gas geochem-istry andfluid-inclusion data from the Cretaceous ultra-deep sand-stone rervoir in the Dabei Gasfield of the Kuqa Foreland Basin show that there are two-stage oil and one-stage natural gas charg-ing in the Dabei area,which is an important factor for the high yield and enrichment of the Dabei Gasfield[29,30](Fig.5).
4.Key controlling factors of rervoir formation
脾虚的症状The development of rervoir and its size are two of the key fac-tors that affect the successful exploration of ultra-deep oil/gas. Under deep-burial conditions,the strata are generally under high-temperature and high-pressure conditions,and generally have undergone long-term,multipha tectonic movements and diagenes.The pores of rervoir rocks often disappear due to destructive diagenes such as compaction,pressure dissolution, and cementation[31].
The key factors for the development of ultra-deep rervoirs can be explained from three aspects:the development of primary pores,the formation of condary pores,and the effective prer-vation of rervoir pores.
4.1.Original pores controlled by high-energy dimentary facies
Large-scale high-energy deposits are the basis for the formation of large oil and gas rervoirs.As for carbonates,high-energy deposits are mainly marginal-platform reef-shoal complexes and inner-platform shoals.The are characterized by large deposi-tional areas and abundant original pores.For example,we
found
Fig.5.Three stages of hydrocarbonfilling as indicated byfluorescence obrvation in Well Dabei-1.(a)Single polarized light;(b)fluorescence,where the yellowfluorescent oil inclusions reprent the crude oilfilling with relatively low maturity in thefirst pha;(c)single polarized light,pyrobitumen(cracking product from crude oil);(d)single polarized light;(e)fluorescence,where the blue-whitefluorescent oil inclusions reprent the crude oilfilling with relatively high maturity in the cond pha;and(f)fluid inclusion,where the black gas inclusions reprent natural gasfilling in the late third pha.Reproduced from Ref.[29]with permission of China University of Geosciences,Ó2010,and from Ref.[30]with permission of China University of Mining and Technology,Ó2015.
X.Guo et al./Engineering5(2019)458–470463
