OriginandaccumulationofcarbondioxideintheHuanghuadepressionBohaiBayBasinChina
AUTHO RS
Tongwei Zhang $Power,Environmental and Energy Research Center,California Institute of Technology,738Arrow Grand Circle,Covina,California 91722;zhangtw@https://www.360docs.net/doc/fa11294981.html,
T.Zhang is a research geochemist and labo-ratory manager at the California Institute of Technology (Caltech).He was a postdoctoral scholar in chemistry at the Caltech.He holds a B.S.degree in geology and a Ph.D.in isotope geochemistry.His research focuses on petro-leum and natural gas geology and geochemistry and CO 2sequestration.
Mingjie Zhang $Department of Geology,School of Earth and Environmental Sciences,Lanzhou University,Lanzhou 730000,People’s Republic of China;mjzhang@https://www.360docs.net/doc/fa11294981.html, M.Zhang is currently a professor and director of the Institute of Geological Science at Lanzhou University.He holds a B.S.degree in geology,an M.S.degree in mineralogy and petrology,and a Ph.D.in geochemistry.His research interest mainly focuses on stable and noble-gas isotope geochemistry in natural gas and the Earth’s mantle.
Baojun Bai $Geological Sciences and En-gineering Department,University of Missouri–Rolla,1870Miner Circle,Rolla,Missouri 65409;baib@https://www.360docs.net/doc/fa11294981.html,
B.Bai is an assistant professor of petroleum engineering at the University of Missouri–Rolla.He was a postdoctoral scholar at the California Institute of Technology.He holds a B.S.degree in reservoir engineering and Ph.D.s in both petroleum engineering and geology.His re-search focuses on enhanced oil recovery using chemicals and CO 2/CO 2foam.
Xianbin Wang $Institute of Geology and Geophysics,Chinese Academy of Sciences,Lanzhou 730000,People’s Republic of China;xbwang@https://www.360docs.net/doc/fa11294981.html,
X.Wang is currently a professor of gas geo-chemistry at the Institute of Geology and Geo-physics,Chinese Academy of Sciences.He holds a B.S.degree in geochemistry from the Chinese University of Sciences and Technology.
Origin and accumulation of carbon dioxide in the Huanghua depression,Bohai Bay Basin,China
Tongwei Zhang,Mingjie Zhang,Baojun Bai,Xianbin Wang,and Liwu Li
ABSTRACT
The CO 2content in natural gas in the Huanghua depression,Bohai Bay Basin,China,is highly variable,ranging from 0.003to 99.6%.Understanding the origin and distribution of the CO 2is important to assess risk prior to drilling.This study uses gas geochemistry to identify the origins of CO 2in the sedimentary basin and places these findings within a geologic context.
Chemical compositions,d 13C CO 2,3He/4He,and 40Ar/36Ar were measured for 50gas samples collected from gas-and oil-producing wells located in different tectonic regions in the depression.From these analyses,we determined that the CO 2in the Huanghua de-pression originated from three sources:thermal decomposition of organic matter,thermal decomposition of carbonate minerals,and mantle degassing.Gases with low amounts (<3%)of CO 2tend to be organogenic.This organogenic CO 2occurs in hydrocarbon accu-mulations and is characterized by d 13C CO 2values ranging from à20to à10x and low 3He/4He (R /R a <1,herein R and R a represent the 3
He/4He ratio of sample and air,respectively).Carbon dioxide origi-nating from thermal carbonate decomposition occurs as a minor com-ponent (<10%)in hydrocarbon gas accumulations and is charac-terized by a narrow range of d 13C CO 2(à2to +2x )and R /R a <1.Huanghua depression natural gases with CO 2content in excess of 15%resulted from mantle degassing and mainly occur at the inter-section of faults.These gases have 3He/4He ratios in excess of at-mospheric value (R /R a >1)and d 13C CO 2ranging from à5to à3x .Volatiles from mantle degassing during the postmagmatic stage are the most likely major source for CO 2in these high-CO 2-content reservoirs.Basement faults likely provide pathways for the up-ward migration of CO 2-rich mantle fluids.Consequently,CO 2-rich
AAPG Bulletin,v.92,no.3(March 2008),pp.341–358341
Copyright #2008.The American Association of Petroleum Geologists.All rights reserved.
Manuscript received November 27,2006;provisional acceptance April 17,2007;revised manuscript received September 4,2007;final acceptance October 23,2007.DOI:10.1306/10230706141
gas pools are locally concentrated in the Gangxi and Dazhongwang fault zones within the depression.
INTRODUCTION
Carbon dioxide–rich natural gas(more than15%v/v)occurs in sev-eral sedimentary basins in various geological settings(Wycherley et al.,1999).Understanding CO2formation and accumulation in sedimentary basins is vital for accurate risk assessment in natural gas exploration because(1)CO2lowers the economic value of natural gas by diluting hydrocarbon concentrations and(2)CO2increases the production costs for gas treatment and separation(Ballentine et al.,2001;Marty,2001).The characterization of naturally formed CO2pools in reservoir rock also may provide useful information to guide subsurface sequestration and storage of CO2to reduce green-house gas emissions(White et al.,2003).
Four well-documented processes have been identified as major sources for CO2-rich natural gas accumulations in sedimentary basins:(1)low-temperature decomposition of organic matter during early catagenesis and when d13C CO
2
values are normally lower than à10x(Hoefs,1987);(2)hydrocarbon oxidation by thermochem-ical or microbial sulfate reduction;(3)decomposition of carbonate minerals;and(4)mantle degassing.Determining the source of CO2 in natural gases from its abundance or stable carbon isotope ratio is difficult because CO2readily exchanges with inorganic min-erals under reservoir conditions(Smith and Ehrenberg,1989).Hence, the source for CO2-rich natural gas is inferred using the abundance and isotopic composition of associated hydrocarbons and/or inert gases.For example,d13C values of CO2in the Devonian Nisku Formation of Alberta,Canada(Hutcheon et al.,1995;Hutcheon, 1999),and the Barremain–Jurassic carbonates of the Aquitaine Ba-sin(Connan et al.,1996)are nearly identical with their respective carbonate reservoir rocks,but associated hydrocarbons are isotopi-cally heavy(enriched in13C)relative to those in unaltered reservoirs. High concentrations of H2S in the gases indicate that the CO2orig-inated via thermochemical sulfate reduction of hydrocarbons.Huang et al.(2004)reported that the high CO2content in natural gas res-ervoirs of the Yinggehai Basin,offshore China Sea,originated by thermal decomposition of the lower Miocene calcareous shale. Here,the gases contained15–85%CO2,with the d13C CO
2
ranging fromà0.56toà8.16x and the3He/4He ratio ranging from0.20to 6.79?10à7.In contrast,CO2-rich gases in the Pannonian Basin in Hungary(Sherwood Lollar et al.,1997),Harding County of New Mexico(Staudacher,1987),west Texas Permian Basin(Ballentine et al.,2000,2001),several sedimentary basins along the Tanlu fault zone of China(Xu et al.,1995;Dai et al.,1996,2005;Tao et al., 1997;Xu et al.,1997a,b),and the Taranaki Basin of New Zealand (Hulston et al.,2001)have high3He/4He ratios(or R/R a>1),where R and R a are the3He/4He ratios of the sample and air,respectively,
and d13C CO
2ranging fromà3toà8x,which is consistent with
He has worked on petroleum and natural gas
geochemistry,noble-gas isotope geochemistry,
and deep earth interiors for more than40years.
Liwu Li$Institute of Geology and Geophys-
ics,Chinese Academy of Sciences,Lanzhou,
730000,People’s Republic of China;
llwu@https://www.360docs.net/doc/fa11294981.html,
L.Li is currently a professor of geochemistry
at the Institute of Geology and Geophysics,
Chinese Academy of Sciences.He has worked
on gas composition and stable isotope analysis
since1992.He holds an M.S.degree and a
Ph.D.in nuclear physics from the Institute of
Modern Physics,Chinese Academy of Sciences.
ACKNOWLEDGEMENTS
This research was supported by the National
Natural Science Foundation of China(Grant
40072048).We thank Barry Katz,Kenneth
Peters,and P.Mukhopadhyay for critical and
constructive reviews of our article.We thank
Clifford C.Walters for his constructive com-
ments to improve the manuscript and his great
effort to correct the English writing.This
work benefited from collaborations and dis-
cussions with many colleagues.We espe-
cially thank Xixing Gao and Jianfu Wang from
Dagang Petroleum Co.for their help in gas
sampling and providing valuable local geolog-
ical information.
342CO2Origin and Accumulation,Bohai Bay Basin,China
gases from mid–ocean ridge basalts.These CO2-rich gas reservoirs are thought to arise from mantle degassing and are associated closely with deep faults and in basins that have experienced strong tectonic activities.
The CO2content in natural gas in the Huanghua depression,Bohai Bay Basin,China,one of the basins along the Tan-Lu fault zone,ranges from0.003to 99.6%.The origin of the high CO2content in natural gases has not been well established,leading to difficul-ties in accurately predicting the CO2risk prior to dril-ling.The objectives of this work are to
1.Identify geochemical criteria to differentiate CO2
of different origins in natural gases in the Huanghua depression
2.Characterize high-content CO2gas reservoirs for-
mation for risk assessment prior to drilling
To address the above issues,50natural gas samples were collected from both oil-and gas-producing wells widely distributed across the basin.The chemical com-position of gases,as well as the carbon isotope ratio of CO2,3He/4He,and40Ar/36Ar isotope ratios,was mea-sured.From these measurements,an integrated geo-chemical model to identify the origins of CO2in natural gas was established that considers the tectonic,mag-matic,and geologic evolution of the basin. GEOLOGIC SETTING
The Huanghua depression,located in the Bohai Bay Basin of eastern China,is bounded by the Cangxian uplift to the west and the Chenning uplift to the east (Figure1).The depression extends approximately south-west to northeast and covers a total area of approxi-mately18,529km2(7154mi2).Bohai Bay Basin,one of the major petroleum-and gas-producing areas in east-ern China,is an intracontinental extensional basin that formed on the base of a Proterozoic craton with Paleo-zoic cover rocks,resulting from the interaction between the Eurasian,Kula-Pacific,and Indian plates since the Triassic(Li,1982).The depth of the Moho disconti-nuity is less than28km(17mi)in the central part of basin and gradually increases laterally to more than 36km(22mi)(Ma,1987).As a result,the basin ex-perienced high geothermal heat flow,averaging1.95 heat-flow units(HFU),resulting in a high average geo-thermal gradient of4.3j C/100m(34.35j F/100ft) (Z.Wang,1990,personal communication).
The formation of the Huanghua depression can be divided into three main evolutionary stages:regional compression(Triassic–Early Cretaceous),fault de-pression(Paleogene),and whole depression(Neogene–Quaternary)(Dai et al.,1996).In the Tertiary,because of the effects of the Himalayan orogenesis,tectonic activity increased,and violent faulting caused the plat-form to disintegrate and step into a developing stage of fault depression.The tectonic movement in this period can be divided into two cycles:a faulting phase in the early Tertiary and a thermal subsidence phase in the late Tertiary.The Huanghua depression has experienced ex-tension and sedimentation for about50m.y.since the Eocene,involving about20–25m.y.for a faulting phase and25m.y.for a thermal subsidence phase,respectively (Dai et al.,1996).
Different types of sediments developed within the various stages of the faulting phase,with potential gas-oil source rocks developing in three cycles of sedimen-tation(Figure2):the second member of Kongdian For-mation during the Paleocene(Ek2),the first member of Shahejie Formation during the Oligocene(Es1),and the third member of Shahejie Formation during the Oligocene(Es3).The Huanghua depression is com-posed of5sags and3buried-hill fault zones(Figure1). Of these,source rocks in the Qikou sag(including the southern and northern Qikou sag),Banqiao sag,and Cangdong-Nanpi sag are sufficiently mature for petro-leum generation,whereas those in the Yanshang sag are thermally immature.
Cenozoic alkali basalt and diabase are widespread along northeast-striking faults,especially where they intersect northwest-striking faults.These magmatic fea-tures were regionally distributed during different pe-riods of the depression’s evolution.For instance,the Eo-cene diabase is distributed along the northeast-striking Kongdian fault zone,whereas the Oligocene basalt and diabase are widely distributed along the northeast-extending fault zone in the Qikou sag.In the late Ter-tiary,after the depression entered a thermal subsidence phase from a faulting phase,a series of volcanoes devel-oped in the middle and northern part of the depression. Multiple layers of Miocene basalt cover an area of about 1000km2(386mi2)and have a maximum thickness of 177m(580ft).
Natural gas reservoirs also are distributed across the entire Huanghua depression,but are concentrated mostly in the middle.Nine natural gas formations, including commercial-scale gas reservoirs with burial depths from700to4800m(2300to15,748ft),i.e., Ordovician,Carboniferous,Permian,Ek2,Es3,Es1,Ed,
Zhang et al.343
Ng,and Nm (Figure 2)have been confirmed.In addition to hydrocarbon accumulations,reservoirs containing nearly pure carbon dioxide were encountered in the depression.
SAMPLES AND EXPERIMENTAL METHODS The natural gas samples were collected in 1.5-L stain-less steel gas cylinders with high-pressure valves at each end.Each cylinder was connected directly to a gas well-head by copper pipes.The copper pipes and gas cylin-ders were flushed with the natural gas for 15min to remove air contamination,and then the natural gas was collected into the gas cylinder by closing the end valves.Sampling locations of natural gases are shown in Figure 1and are listed by their identification numbers in Tables 1and 2.
Compositions of the natural gas samples were ana-lyzed using a MAT 271mass spectrometer,which can accurately quantify the contents of inorganic compo-nents,such as CO 2,N 2,H 2,helium,argon,and hy-drocarbon components with carbon numbers smaller than 4.For calibration,reference gases with known com-positions were injected into the mass spectrometer to measure the sensitivity of each component in units of voltage per mole.Gas samples were injected into the MAT 271mass spectrometer using the same procedure as the reference gas,and the voltage of each component was measured.The molar percentage for each gas com-ponent was calculated using the calibration sensitivity
values.
Figure 1.Sampling locations of natural gas and subtectonic divisions in Huan-ghua depression,Bohai Bay Basin,China.The numbers adjacent to gas sampling points in the map are identical with the sample identification in Tables 1–3.I =Banqiao sag;II =North Dagang buried-hill fault belt;III =northern Qikou sag;IV =southern Qikou sag;V =South Dagang buried-hill fault belt;VI =Cangdong-Nanpi sag;VII =Kongdian buried-hill fault belt;and VIII =Yanshang sag.Symbol of leg-ends:1=sea line;2,3=boundary fault of the Huanghua depression;4=gas sam-pling well location;5=high-concentration CO 2gas-producing well (CO 2>10%);6=contour of thermal fluid (j C/100m);and 7=basalts.
344
CO 2Origin and Accumulation,Bohai Bay Basin,China
The 40Ar/36Ar ratio was measured using a MAT 271mass spectrometer.A gas sample was passed through two liquid nitrogen cold traps.Ethane and higher hydrocarbons
were trapped along with most of the methane,where-as the remaining gases,including argon,were trans-ferred to the mass spectrometer.The 40Ar/36Ar
ratio
Figure 2.A stratigraphic column of the Huanghua depression,Bohai Bay Basin,China.Symbol of legends:1=sandstone;2=mudstone;3=sand-stone and intercalated mudstone;4=oil shale;5=biological limestone;6=gypsum and conglom-erate;7=volcanic rocks;8=clastic rocks;9=coal layer;10=limestone;11=gas-producing layer;and 12=unconformity.
Zhang et al.
345
Table1.Chemical Compositions of Natural Gases in Huanghua Depression,China
Sample
Identification Well Formation Depth(m)
Gas Chemical Composition(%)
He(?10à4)Ar(?10à4)N2CO2C1C2+C3
Banqiao
1Bansheng20Es23671.9–3703.227250.2 2.677.819.5 2Bansheng17-5Es23708.6–3720.425730.4 1.873.824.0 3Bansheng16-22Es23879.1–3915.5102530.5 1.385.612.6 4Bansheng17Es12873.4–2878581400.50.388.18.9 5Ban821Es12699.8–2708.443640.50.594.3 4.2 6Ban837Es12928–293451450.7 1.479.216.1 7Ban19-4Es12947.93–2968.7353510.7 1.987.68.8 8Ban G1Es23079.1–3115.542250.2n.d.*84.513.8 Baishuitou
9Ban G5Es23229.9–32342480.1 1.568.130.3 10Gan95-2Es23399.3–3422.125200.20.090.98.9 11Bai21-3Es23176.8–3201.944n.d.0.3 1.884.013.9 12Bai14-2Ed32445.1–2449.753220.60.584.810.2 13Gansheng3Es1-23501.8–3636.815840.2 2.679.917.3 14Gansheng5Es32990–308016260.011.85.613.4 North Dagang
15Gang28Es12651.6–272338390.3 1.180.318.4 16Gangzhong8-33Es12026–2122.819620.20.685.313.4 17Gang2025Es33581.6–3617.273470.3 3.285.211.2 18Hong7-1Es32368.5–2384.8110260 1.50.096.1 1.8 19Gang139Ng1062–1173.481240 1.20.297.90.1 20Gangxi38-1Ng1613.1–1666.891700.70.492.3 5.2 21Gangxi14-258976 3.00.196.20.9 22Gangxi6-5-254890.40.796.80.1 23Gangxi20-19Nm21093–1168331400.50.299.50.0 24Gang137Nm1083–10921001600.637.052.99.5 25Gangxi49-52Nm2942.5–1054.48300.213.181.0 3.1 26Gangxi15-7-2Nm21014–1186791400.6 6.288.3 2.9 27Gang151Es11632.4–1639.420n.d.n.d.99.60.4n.d. 28Fan18-1Es12574.8–2577.757760.30.985.49.7 29Qi452Es12387.4–2438.91001200.80.888.310.2 30Qi450Es12599.6–261670780.3 1.286.4 5.8 31Qi416Es12968.3–3019.646780.4 2.073.424.1 South Dagang
32Qi15Es1+22773–2799210500 2.5 2.283.911.4 33Qi123-5Es12904.6–2939.4662300.9 3.479.815.5 34Qi50-6Es12550.4–2575141500.90.084.713.4 35Yangcun26Ng1530–1748220220 1.40.198.20.3 Kouchun
36Kou11P1552.4–1568.8290370 2.025.466.8 5.8 37Kou38-15Ng1452.3–1455.3240320 2.10.396.80.9 38Kou29Ng1408.4–14261501600.90.298.60.5 39Kong1095Ng1408.4–1418.3270370 2.30.396.0 1.3 346CO2Origin and Accumulation,Bohai Bay Basin,China
was measured from intensities of the m /z =36and m /z =40signal.Air was used as reference for the 40Ar/36Ar ratio measurement (measured data of 40Ar/36Ar in Lanzhou city’s air is 293±5,the standard deviations are expressed as 2s ).
Stable isotope analysis on CO 2was conducted using a SP3400gas chromatograph–combustion–isotope ra-tio MAT 252mass spectrometer (GC-C-IRMS).All steps,including separation,purification,combustion to CO 2,and analysis were conducted on this online sys-tem.A SP3400gas chromatograph was fitted with a packed column GDX5(length =4m [13ft],outer diameter =3mm [0.11in.])and operated under a constant pressure (15psi;103kPa).Excellent chro-matographic separation of the CO 2was achieved at 25j C,and reproducibility of the d 13C CO 2(<0.3x )was achieved routinely in gases where the CO 2concentra-tions varied from 100%to hundreds of ppm.All re-sults are accurate within 0.3x (1s )with respect to the Peedee belemnite standard.
3
He/4He and 4He/20Ne ratios were determined using a VG5400mass spectrometer equipped with a metal gas extraction and purification line.Observed 3
He/4He ratios were calibrated against atmospheric standard gas.Errors are quoted at the 1s level confi-dence,which include statistical analytical error,re-producibility of air standard used to correct mass discrimination,abundance determination,expansion volume uncertainty,and mass spectrometer sensitiv-ity stability.
RESULTS AND DISCUSSION Gas Chemical Compositions
Chemical compositions of natural gases from the Huang-hua depression are listed in Table 1.For most of the samples,methane is the dominant component,with con-centrations above 70%.The concentration of heavier gaseous hydrocarbons ranges from 0.03to 30%.The concentration of carbon dioxide in the natural gases spans a wide range,from 0.003to 99.6%,and is generally lower than 10%(Figure 3a).The natural gases in Huang-hua depression can be divided into two groups,methane-rich gases (CH 4>70%)and CO 2-rich gases (CO 2>15%),which occur mainly in the Gangxi and Dazhong-wang fault zones and the Kouchun transfer fault zone.Helium and argon in the natural gases have concentra-tions of up to 500ppm.A linear relationship between helium and argon contents (Table 1)suggests a common source for their origin.Molecular nitrogen is a trace component in the natural gas samples,with concentra-tion ranging widely from 0.04to 4.6%.CO 2Content versus d 13C CO 2Value and Its Origins The initial carbon isotopic composition of nonbiolog-ical CO 2in natural gases strongly depends on its origins (Pineau et al.,1976;Javoy et al.,1978;Pineau and Javoy,1983).The organogenic CO 2derived from thermal de-composition of organic matter is enriched in the lighter
Wangguantun 40Zhuang 1612-1Es32567.4–29831301400.9 5.079.711.541Zhunag 8-15-2Ng 1768–1946861300.40.394.0 4.742Zhuang 7-17Nm21372.8–146128500.30.096.0 3.143Zao 1539Mz 2783–2803400580 4.60.183.611.444Feng 37-21Ek22498.1–2528.6120210 1.00.881.411.945Cang 1Es11810.4–1821.673860.70.398.60.446Guan 144-2Mz 2490.6–2541160290 3.10.987.48.647Guan 20-33120230 2.10.480.79.348Guan 969Es11904.4–1959.8150220 2.00.793.7 4.149Wang 14Es11880.9–1903.51001100.9 1.994.0 2.450
Ni 44-14
Es1
1407–1430
59
26
0.3
0.2
97.9
0.4
All wells are owned by Dagang Petroleum Co.*n.d.=not determined.
Table 1.Continued Sample
Identification Well Formation Depth (m)Gas Chemical Composition (%)
He (?10à4)
Ar (?10à4)
N 2CO 2C 1C 2+C 3Zhang et al.
347
Table2.Isotopic Compositions of Natural Gases in Huanghua Depression,China
Sample
Identification Well Formation Depth(m)
Isotope Compositions
d13C1x d13C CO
2
x40Ar/36Ar
3He/4He
(?10à7)R/R a40Ar/20Ne
Banqiao
1Bansheng20Es23671.9–3703.2à42.9à2.51680 1.25±0.040.09374 2Bansheng17-5Es23708.6–3720.4à44.5à4.018900.83±0.040.06n.d.* 3Bansheng16-22Es23879.1–3915.5à43.5 3.1n.d. 1.08±0.030.08n.d. 4Bansheng17Es12873.4–2878à47.5à15.9727n.d.n.d.124 5Ban821Es12699.8–2708.4à46.2n.d.463 2.28±0.070.16n.d. 6Ban837Es12928–2934à43.2à12.6810n.d.n.d.n.d. 7Ban19-4Es12947.93–2968.73à43.5à1.1711 3.65±0.10.26n.d. 8Ban G1Es23079.1–3115.5n.d.n.d.n.d.n.d.n.d.n.d. Baishuitou
9Ban G5Es23229.9–3234à37.6à1.0532 4.45±0.120.3239 10Gan95-2Es23399.3–3422.1à37.3n.d.445 4.18±0.120.3303 11Bai21-3Es23176.8–3201.9à37.7à3.17808.74±0.230.6239 12Bai14-2Ed32445.1–2449.7à39.9à5.3648 4.81±0.160.34n.d. 13Gansheng3Es1-23501.8–3636.8à36.70.3665 3.24±0.10.23n.d. 14Gansheng5Es32990–3080à38.4à2.612009.53±0.280.68n.d. North Dagang
15Gang28Es12651.6–2723à44.4à4.115869.48±0.280.68280 16Gangzhong8-33Es12026–2122.8à42.2à3.8n.d.n.d.n.d.n.d. 17Gang2025Es33581.6–3617.2à47.8 1.49429.13±0.250.6554 18Hong7-1Es32368.5–2384.8à45.3n.d.20008.59±0.240.61n.d. 19Gang139Ng1062–1173.4à58.2à11.2n.d.10.5±0.30.75n.d. 20Gangxi38-1Ng1613.1–1666.8à47n.d.39512.5±0.30.89n.d. 21Gangxi14-25n.d.n.d.à59.3n.d.3828.48±0.230.6n.d. 22Gangxi6-5-2n.d.n.d.à45.8n.d.63423.1±0.6 1.65n.d. 23Gangxi20-19Nm21093–1168à52.5à7.9405n.d.n.d.n.d. 24Gang137Nm1083–1092à43.9à4.6184030.3±0.8 2.161695 25Gangxi49-52Nm2942.5–1054.4à44à3.991513.8±0.40.99n.d. 26Gangxi15-7-2Nm21014–1186à49.5à1.397441.9±1.1 2.99n.d. 27Gang151Es11632.4–1639.4à28.3à3.5704552.4±1.4 3.74299 28Fan18-1Es12574.8–2577.7à45.4à7.1164319.1±0.5 1.36n.d. 29Qi452Es12387.4–2438.9à47.5à3.4133219.0±0.5 1.36331 30Qi450Es12599.6–2616à46.3à5.81372n.d.n.d.n.d. 31Qi416Es12968.3–3019.6à42.2à0.912527.39±0.200.5395 South Dagang
32Qi15Es1+22773–2799à43.4à0.7294010.8±0.30.772022 33Qi123-5Es12904.6–2939.4à42.8à2.523489.13±0.280.65384 34Qi50-6Es12550.4–2575à41.3n.d.n.d. 3.94±0.120.28126 35Yangcong26Ng1530–1748à42.7à12.74917.39±0.190.53684 Kouchun
36Kou11P1552.4–1568.8à42.6à5.6181025.3±0.6 1.81995 37Kou38-15Ng1452.3–1455.3à43.3 2.34668.71±0.240.62105 38Kou29Ng1408.4–1426à42n.d.94012.1±0.30.86n.d. 39Kong1095Ng1408.4–1418.3à44.6n.d.50117.3±0.4 1.241389 348CO2Origin and Accumulation,Bohai Bay Basin,China
carbon isotope 12C (Hoefs,1987),and d 13C CO 2values are normally lower than à10x .In contrast,the CO 2derived from abiogenic sources,i.e.,decarbonization (Mattey et al.,1990;Huang et al.,2004)or mantle degassing (Pineau et al.,1976;Pineau and Javoy,1983;Javoy et al.,1984,1986;Marty et al.,1989;Marty and Tolstikhin,1998)is isotopically heavier.Carbon diox-ide arising from carbonates tends to reflect their iso-topic composition.For example,the d 13C values of CO 2originating from the decarbonization of sedimentary carbonates in Baihai Bay Basin,China,are in the range from à2.4to +2.4x (Zheng et al.,2001),and CO 2formed by the thermal metamorphism of carbonate rocks is from à3.5to +3.5x .The carbon isotope com-position of CO 2from mantle degassing or magmatic origins is typically à8to à4x .The origin of CO 2in basinal fluids cannot be resolved by their d 13C values alone because the ranges for magmatic and crustal gases are not unique.These values may be further perturbed through interaction with host rocks and connate waters.
The d 13C CO 2values for natural gas in Huanghua depression have a wide range from à20to +5x ,with two main groupings:à15to à10x ,indicating an or-ganic origin;and à5to +5x ,suggesting an inorganic origin (Figure 3b).The Huanghua depression gases can be further divided into four groups designated A–D by comparing the d 13C CO 2versus CO 2abundance (Figure 4).Group A gases contain less than 2%CO 2and have d 13C CO 2values lower than à10x ,which is
Table 2.Continued
Sample
Identification Well Formation Depth (m)Isotope Compositions
d 13C 1x d 13C CO 2x 40
Ar/36Ar 3
He/4He (?10à7)R /R a 40
Ar/20Ne Wangguantun 40Zhuang 1612-1Es32567.4–2983à46.5à3.71345 5.96±0.150.43n.d.41Zhuang 8-15-2Ng 1768–1946à42.1n.d.465 5.98±0.160.436n.d.42Zhuang 7-17Nm21372.8–1461à43.3n.d.497 4.87±0.130.35n.d.43Zao 1539Mz 2783–2803à35.6n.d.371 6.38±0.170.4614744Feng 37-21Ek22498.1–2528.6à44.7à4.31519 6.25±0.180.45n.d.45Cang 1Es11810.4–1821.6à59.5à10.3358 5.30±0.140.3829646Guan 144-2Mz 2490.6–2541
à32.8à11.91095 4.22±0.110.3162547Guan 20-33n.d.n.d.
à32.4à14.3n.d.n.d.n.d.n.d.48Guan 969Es11904.4–1959.8à40.7à13.7950n.d.n.d.n.d.49Wang 14Es11880.9–1903.5à49.7à0.2n.d. 4.43±0.110.3235850
Ni 44-14
Es1
1407–1430
à48.1
à13.2
n.d.
7.14±0.19
0.51
n.d.
All wells are owned by Dagang Petroleum Co.*n.d.=not
measured.
Figure 3.Frequency distribution of CO 2content (a)and its carbon isotopic composition (b)in natural gases from the Huanghua depression.Stipple peaks (in b)have d 13C values supporting an origin by thermal decomposition of organic mat-ter,whereas the other peaks likely indicate inorganic CO 2.
Zhang et al.
349
the typical range for CO 2generated by thermal alter-ation of organic matter.Group B gases contain less than 10%CO 2and have d 13C CO 2values between à2and +2x ,which are consistent with CO 2originating from the decarbonization of sedimentary carbonate rocks.Group C gases have intermediate values between groups A and B,which is consistent with these gases being mixtures of organogenic and carbonate CO 2.In contrast,group D gases have very high concentrations of carbon dioxide (CO 2>15%),indicating that these gases cannot be a simple mixture from groups A and B and suggesting an alternative origin as discussed below.
3
He/4He and
40
Ar/36Ar Isotope Ratios in Natural Gases
Helium and argon in natural gases are derived from either radiogenic decay or mantle degassing.For radiogenic gas,3
He/4He ratios decrease with the increase of 40Ar/36Ar,reflecting the radiogenic decay contributions of 40Ar from 40
K and 4He from uranium and thorium (Figure 5a).He-lium in the mantle is primarily primordial,and 3He/4He ratios increase with the increase of 40Ar/36Ar ratios in gases derived from mantle degassing (Figure 5b).
The 3He/4He ratios,R ,in natural gas from the Huanghua depression ranged from 0.06to 3.74R a and can be divided into three groups:R <0.5R a ,0.5–1.0R a ,and >1.0R a (Table 2).The 4He/20Ne ratios range from 39up to 2022,indicating that the gas sam-pling and analytical procedures did not introduce air as a contaminant because the measured ratios are much larger than the value of air.Therefore,the wide var-iation in helium isotopic compositions of the natural gases should be attributed mainly to the mixing of crust-derived noble gases enriched in 4He and the mantle-derived noble gases enriched in 3He.
The spatial distribution and relationship of 3He/4He and 40Ar/36Ar of the Huanghua depression natural gases indicate that the sources of the noble gases are con-trolled by their associated faults.Natural gases with R <0.5R a occur mainly in the Banqiao,Baishuitou,and Wangguantun area,and those with 0.5R a
The Banqiao fault zone is a gravitational gliding tectonic zone where basement faults are absent,and the observed faults are limited to the development in the overlying Tertiary formations.Here,the character-istics of noble gases mainly reflect a radiogenic
origin.
Figure 4.Plot of CO 2content versus d 13C CO 2.The various symbols in the figure represent the samples collected from different areas in the depression.
350
CO 2Origin and Accumulation,Bohai Bay Basin,China
In contrast,the Gangxi fault zone,part of North Dagang fault zone,is a basement fault.This area experienced folding,faulting,volcanic activity,tilting of blocks,and faulting of cap formations.The Gangxi fault zone expe-rienced the same tectonic activities and controls other fault development in the area.As a result,the Gangxi fault zone provides a pathway and reservoir for mantle gas migration and accumulation.
The faulting may be associated with the upward migration of hot brines because a relative higher ther-mal fluid occurs in fault zones than sags as shown in Figure 1.The hot brines could be derived from deeper sedimentary sequences.The deeper the faults penetrate,the hotter the brines that migrate upward.Consequent-ly,fault zones are associated with high geothermal fluids.Some correlation exists between high heat fluid and the enrichment of CO 2in natural gas.For example,as shown in Figure 1,the highest thermal fluid contour is 3.7j C/
100m (34.0j F/100ft),CO 2-rich natural gases are mostly distributed there,with an exception of Kou 11.How-ever,not all higher heat fluid areas produce high CO 2.Natural gases with high concentration of CO 2occur mainly along Gangxi and Dazhongwang basement faults,which may provide a pathway for the upward migra-tion of mantle-derived volatiles.The association be-tween basement faults and trap formation can be used to better risk the distribution of high-CO 2reservoirs in future exploration.
Relationship of 3He/4He and d 13C CO 2in Natural Gases Comparisons of the 3He/4He (R /R a )to the d 13C CO 2and CO 2content reveal the source of the CO 2-rich gases (Figure 7a,b).The natural gases with CO 2derived from the decomposition of organic matter and carbonate min-erals are associated with a 3He/4He ratio less than 1.0R
a
Figure 5.Relationship of 3He/4He and 40Ar/36Ar in the natural gases sug-gests two noble gas sources,which is possibly related to the difference in the faulting properties within the Huanghua depression.
Zhang et al.
351
(Figure 7a),suggesting that crustal helium of radiogenic origin is dominant.However,in group C gases,CO 2with d 13C CO 2values ranging from à3to à7x ,which overlaps the range of CO 2from mantle degassing and magma (Mattey et al.,1984;Exley et al.,1986),is as-sociated with 3He/4He ratios larger than 1.0R a ,in-dicating that mantle-derived helium enriched in 3He made a significant contribution.The plot of R /R a versus CO 2content clearly shows that the Huanghua depres-sion natural gases with CO 2content more than 10%are all associated with a high 3He/4He ratio in excess of atmospheric value (>1.0R a ),and that the 3He/4He ra-tio increases with the increasing CO 2(Figure 7b).
To summarize,natural gases with more than 10%carbon dioxide have 3He/4He ratios in excess of the at-mospheric value (R /R a >1),which indicates a significant contribution of mantle-derived helium.For these gases,the CO 2content increases with increasing 3He/4He val-ues,and the d 13C CO 2values range from à5to à3x ,which is consistent with the carbon isotope composi-tion of mantle degassing CO 2.These observations im-ply that the CO 2in natural gases in these gases that have a high CO 2content in the Huanghua depression most likely originates from mantle degassing.Reservoir Characterization of Mantle-Derived CO 2Accumulation
General Distribution of CO 2-Rich Gas Fields in the Huanghua Depression
CO 2-rich gas fields in the Huanghua depression occur mainly along the Dazhongwang and Gangxi fault
zones
Figure 6.Spatial distribution of the 3
He/4He ratio in the natural gases col-lected from the Huanghua depression,Bohai Bay Basin,China.Symbol of leg-ends:1,2=faults;3=boundary fault of the Huanghua depression;4=gas sam-pling well location;5=high-concentration CO 2gas-producing well (CO 2>15%);6=sea line;7=basalts;and 8=R /R a ratio.I =Banqiao sag;II =North Dagang buried-hill fault belt;III =northern Qikou sag;IV =southern Qikou sag;V =South Dagang buried-hill fault belt;VI =Cangdong-Nanpi sag;VII =Kongdian buried-hill fault belt;and VIII =Yanshang sag.All wells are owned by Dagang Petroleum Co.
352
CO 2Origin and Accumulation,Bohai Bay Basin,China
(Figure 6).The gas samples along the Dazhongwang fault zone were collected before Dagang Petroleum Co.’s gas-producing wells,Wang 21,Wanggu 21,and Qigu 1(Table 3;Figure 1),were shut in after drilling was com-pleted in 1977.C.Yang (1990,personal communica-tion)summarized the chemical compositions and gas-producing yields of the CO 2reservoirs in the Huanghua depression based on unpublished reports (Table 3).The content of CO 2in the natural gases in the Dazhongwang fault zone range from 79to 95%.Although d 13C CO 2and 3He/4He isotopic data are not available for gases in the Dazhongwang fault zone to directly identify the ori-gin of the CO 2,the similarity in fault development his-tory between the Dazhongwang and the Gangxi fault zones suggests that they share a common origin for CO 2-rich gas accumulations.As discussed above,the Gangxi fault zone is tectonically favorable to accumulate mantle-derived gases,and the high concentration of CO 2(>15%)in the natural gas most likely originates from mantle degassing.Furthermore,a geochemical profile along the North Dagang fault zone from west to east clearly shows that the CO 2content rapidly decreases
with
Figure 7.Plot of d 13C CO 2
and CO 2content versus R /R a in the natural gases from Huan-ghua depression.The symbols are the same as in Figure 4.
Zhang et al.
353
corresponding decreases in the3He/4He and40Ar/36Ar ratios,whereas the carbon isotopic composition of CO2 is relatively stable(Figure8).The CO2contents,3He/4He and40Ar/36Ar in the Dagang Petroleum Co.Gang151 well,located at the western part of the Gangxi fault zone, were measured to be99.6%,3.74R a and7037,respec-tively.These observations imply that the western part of the Gangxi fault zone is more favorable for accumulat-ing mantle-derived CO2.It is reasonable to assume that CO2-rich gases in the Dazhongwang fault zone originate from the mantle degassing as well because the Dazhong-wang fault zone is a basement fault located about10km (6mi)away to the northwest of the Gangxi fault zone. Main Controls for CO2-Rich Gas Reservoirs in the Huanghua Depression
Volatiles from mantle degassing are most likely the main source of high CO2gases in the depression.Both 3He/4He ratios and current geothermal gradients de-crease from the fault zones to depositional sags,reveal-ing a communication of basement faults with the deep Earth.In the Gangxi basement fault zone and in the Kouchun fault,3He/4He ratios indicate a significant mantle helium contribution(Figure5),especially at the conjunction of northwest-striking faults and northeast-striking faults,where a high geothermal anomaly is de-veloped.At the margin of the Bohai Bay Basin,an anomaly zone of large-scale gold mineralization and large gold deposits have been found in the volcanic rocks.Isotope studies demonstrated that the CO2in these volcanic rocks originated from the mantle(Zhang and Hu,2002).This observation is further supported by a deep geophysical survey that found an uplift of the asthenosphere mantle and the high-conductivity layer in the lithosphere in the Bohai Bay and adjacent areas (Liu,1992;Deng et al.,1998).
Another supporting observation is the temporal and spatial distribution of basalt in the depression.The center of volcanic activity shifted from Eocene to Mio-cene,moving to the north and middle of the depression. Eocene Kongdian basalt occurs only in the southern part of the depression,whereas Oligocene Shahejie ba-salt occurs mainly in the northern and middle part of the depression.During the Miocene Dongying period, volcanism tended to cease in the southern part and became less intensive in the northern and middle of the depression.Carbon dioxide–rich natural gases are main-ly distributed in the middle of the depression,and no high CO2gas reservoirs exist in the southern part of the depression.Based on these observations,we infer that the CO2in the high CO2accumulations was not a product from magma eruption or intrusion.If this were the case,high CO2gases should exist in the southern part of the depression as well.
The basement faults may provide a pathway for the upward movement of mantle-derived volatiles. Because CO2is likely to be the major volatile carrier phase for mantle-derived rare gases in the crust(Zhang et al.,2006),the greatest potential for unambiguous identification of mantle-derived carbon would be in the most CO2-rich gas reservoirs(Staudacher,1987).Un-like the inert rare gas tracers,CO2is a reactive species and is subject to loss to a wide variety of potential crustal sinks and to mixing and dilution by many sources of crustal-derived carbon(Sherwood Lollar et al.,1997). Because mantle-derived helium is enriched in3He by several orders of magnitude compared to crustal-derived4He-enriched radiogenic helium,this rare gas tracer has been widely used to quantify the flux of
Table3.Chemical Compositions of CO2Gas Pools in Huanghua Depression,China*
CO2Gas Pool
Sample
Identification Well Formation and Depth(m)
Gas Composition(%)
Gas Production(m3/day)
CO2N2C1C2+
Zaizhuangzi27Gang151Es11632–163998.60.2n.d.**n.d.CO2gas30,297 51Tai152042–205697.90.2n.d.n.d.26,261 Youaichun52Gang87O2044–210988.10.311.00.4CO2-H2O9249 53Gang59O2084–211074.90.119.7 5.4n.d. Dazhongwang54Wang21Es11734–177579.1 3.717.10.2CO2gas25,500 55Wanggu1O2442–253595.1 3.0 1.9n.d.11,308 Qijiawu56Qigu1O n.d.67.3n.d.15.317.5CO2-H2O n.d. All wells are owned by Dagang Petroleum Co.
*According to C.Yang(1990,personal communication).
**n.d.=not determined.
354CO2Origin and Accumulation,Bohai Bay Basin,China
mantle-derived volatiles in continental regions (Sano,1986;Martel et al.,1989;Stute et al.,1992).Here,we use 3He/4He as an indicator of the extent of commu-nication of basement faults to the deep Earth.Low 3
He/4He with minor mantle helium contribution in the southern and northern parts of the depression im-plies a lack of communication of the basement faults to the deep Earth after the Miocene,whereas high 3
He/4He with major mantle helium contribution in-
dicates a good communication of the Gangxi basement faults and the Kouchun fault to the deep Earth.Reservoir Characterization of CO 2Gas Fields in the Huanghua Depression
Five stratigraphic layers with various lithological char-acteristics have thus far been identified as CO 2-rich gas reservoirs in the Huanghua depression.They are (1)Ordovician carbonate reservoirs with a
paleocrust
Figure 8.Geochemical profile along the Gangxi fault belt from west to east.The Ganxi fault and the location of the five oil-and gas-producing wells are shown in Figure 6.All wells are owned by Dagang Petroleum Co.
Zhang et al.
355
caused by weathering;(2)Permian sandstone reser-voirs;(3)Oligocene organic-rich limestone and dolo-mite reservoirs in the first member of Shahejie Forma-tion (Es1);(4)Miocene fine silt and sandstone reservoirs in the Dongying Formation (Ng);and finally,(5)Plio-cene sandstone reservoirs in the Minghuazheng Forma-tion (Nm).
The Zezhuangzhi gas pool,one of the CO 2-rich gas fields in the Gangxi fault zone,is used as an example to discuss how mantle-derived CO 2gas from mantle degassing accumulates in shallow traps (Figure 9).This gas pool with CO 2content above 98%is located at the southeastern slope of Gangxi buried hill where accu-mulation is mainly controlled by the Gangxi basement fault.Two additional sets of faults,a northeast-striking extension fault south of Zaizhuangzhi and a northwest-striking extension fault north of Zaizhuangzhi,divide the trap into two parts and determine the scale of the gas pool (Figure 9).Oligocene organic-rich limestone and dolomite in the first member of Shahejie Forma-tion (Es1)with porosity from 17.4to 21%constitute the gas reservoir.A 200–300-m (660–1000-ft)-thick black organic-rich mudstone deposited at the end of Es1has a good sealing efficiency and is thought to be an effective cap rock for the reservoir.The east-west Gangxi basement fault controlled Tertiary sediment deposition.Two units,Sha-3(Es3)and Sha-2(Es2),were deposited on the Mesozoic unconformity inter-face during the Eocene.At the end stage of Es2group deposition,the trap started to form.The nose-shaped fracture trap was not completed until the early Miocene after the Oligocene Es1and Miocene Ed sediments were deposited.In other words,mantle-derived CO 2started to fill in the trap after the early Miocene.
CONCLUSIONS
This study used gas geochemistry to differentiate the origins of CO 2in the Huanghua depression.An inte-grated geochemical model,based on gas chemical com-positions,d 13C CO 2,3He/4He,and 40Ar/36Ar,shows that CO 2in Huanghua depression natural gases arise from three sources:thermal cracking of organic
matter,
Figure 9.Geological cross section of Zaizhuangzi mantle-derived CO 2pool.The sampling identification of Tai 15well is 51in Figure 1,and it is located adjacent to Gang151.Symbol of legends:1=mantle-derived fluids;2=fluids from Qikou sag;3=gas;4=oil;and 5=normal fault.All wells are owned by Dagang Petroleum Co.
356
CO 2Origin and Accumulation,Bohai Bay Basin,China
thermal decomposition of carbonate minerals,and man-
tle degassing.Based on the natural gas geochemistry inte-
grated with the tectonic evolution,fault development,
and magmatic activities in the Huanghua depression,
we conclude the following:
https://www.360docs.net/doc/fa11294981.html,anogenic CO2with d13C CO
2
values less than
à10x and the decarbonizated CO2with d13C CO
2 values ranging fromà2to2x occur in low con-
centrations in produced gases,generally less than
10%.Carbon dioxide derived from these sources is
associated with low3He/4He ratios(R/R a<1).
2.Gases in the Huanghua depression with high CO2
concentrations(>15%in natural gas)likely originate
from mantle degassing.These gases are associated
with helium having high3He/4He ratios in excess of
atmospheric value(R/R a>1),indicating a signifi-
cant mantle-derived helium contribution.Most of
the associated CO2,with values ranging fromà5
toà7x,is inferred to have a similar origin.
3.Natural gases with high concentrations of CO2oc-
cur mainly along the Gangxi and the Dazhongwang
basement faults,which may provide a pathway for
the upward migration of mantle-derived volatiles.
The lack of a correlation between the occurrence of
basalt and high-CO2accumulations suggests that
this gas is not magmatic.
4.The association between basement faults and trap
formation can be used to better assess the distribu-
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