Analysis of secondary migration of hydrocarbons in the Ordovician carbonate reservoirs
Analysis of secondary migration of hydrocarbons in the Ordovician carbonate reservoirs in the Tazhong uplift, Tarim Basin,China
Hong Pang,Junqing Chen,Xiongqi Pang,Keyu Liu, Luofu Liu,Caifu Xiang,and Sumei Li
ABSTRACT
The process and mechanisms of secondary hydrocarbon mi-gration in the Tazhong uplift,Tarim Basin,were investigated based on the analysis of the regional structure and by inte-grating geologic,hydrodynamic,and geochemical parameters. Parameters successfully analyzed included the fluid potential, fluid properties,production outputs,and diamantane index. The results indicated that hydrocarbons migrated into the Tazhong uplift from the northern part of the Manjiaer de-pression through a series of injection points(IPs)during four orogenies,that is,the early Caledonian(510Ma),the late Cal-edonian(439Ma),the late Hercynian–Indosinian(290Ma), and the Yanshanian–Himalayan(208Ma).A total of six IPs were identified at the intersections of the northeast-trending faults and the northwest-trending flower strike faults.The hydrocarbons migrated from the IPs into traps along regional trends from northwest to southeast and from northeast to southwest.The hydrocarbon migration process and patterns determined the distribution of hydrocarbon properties and production rates in the Tazhong uplift.With increasing dis-tance from the IPs,daily hydrocarbon production decreases, and the hydrocarbons become progressively heavier and dis-play lower gas:oil ratios.AUTHORS
Hong Pang State Key Laboratory of Petro-leum Resources and Prospecting,China Univer-sity of Petroleum,Beijing,People’s Republic of China;panghong19820107@https://www.360docs.net/doc/7a11780616.html,
Hong Pang graduated from Wuhan University of Science and Technology in2004with a B.S.degree in engineering management.He received his M.S. degree(2007)and his Ph.D.(2011)in geologic sciences from China University of Petroleum.He currently is engaged in research on the petroleum migration and petroleum resources assessment. Junqing Chen State Key Laboratory of Petroleum Resources and Prospecting,China University of Petroleum,Beijing,People’s Re-public of China;cjq7745@https://www.360docs.net/doc/7a11780616.html,
Junqing Chen graduated from China University of Petroleum in2009with a B.S.degree in geologic sciences.In2012,she graduated from China Uni-versity of Petroleum with an M.S.degree.She currently is engaged in research on the basin analysis and petroleum resource assessment. Xiongqi Pang State Key Laboratory of Pe-troleum Resources and Prospecting,China Uni-versity of Petroleum,Beijing,People’s Republic of China;pangxq@https://www.360docs.net/doc/7a11780616.html,
Xiongqi Pang graduated from China University of Geosciences in1991with a Ph.D.in geologic sciences.He is currently a professor and vice-principal of China University of Petroleum.His interests include basin analysis and petroleum resources assessment.
Keyu Liu PetroChina Research Institute of Petroleum Exploration and Development,Beijing, People’s Republic of China;present address: Commonwealth Scientific and Industrial Research Organization Earth Science and Resource Engi-neering,Bentley,Western Australia;
Keyu.Liu@csiro.au
Keyu Liu is a principal research scientist at the Commonwealth Scientific and Industrial Research Organization(CSIRO)Earth Science and Resource Engineering and is a research professor at the Research Institute of Petroleum Exploration and Development,PetroChina.He received his M.S. degree from Sydney University and his Ph.D.from the Australian National University.Before his present employment,he held various research positions in Australia at CSIRO Petroleum,Adelaide University,and James Cook University.His current
Copyright?2013.The American Association of Petroleum Geologists.All rights reserved. Manuscript received June30,2012;provisional acceptance August28,2012;revised manuscript received October13,2012;revised manuscript provisional acceptance January21,2013;2nd revised manuscript received March9,2013;final acceptance April23,2013.
DOI:10.1306/04231312099
AAPG Bulletin,v.97,no.10(October2013),pp.1765–17831765
INTRODUCTION
Hydrocarbon migration is by far one of the most important and yet least understood topics in petroleum geology (e.g.,Losh et al.,1999;Aydin,2000;Fall et al.,2012).It is also a major challenge for petroleum research because limited direct phys-ical evidence can be found in the geologic record,and physical simulation under realistic geologic conditions in the laboratory is difficult to achieve.Various tools have been applied to study secondary hydrocarbon migration in the Tazhong area,includ-ing the analysis of geochemical parameters,such as hydrocar-bon fluids (Lin and Zhang,1996);biomarkers (Gu et al.,2003);nitrogen-bearing organic compounds (Larter et al.,1996;Liu and Xu,1996);and fluid inclusions (Ma et al.,2004),as well as geologic methods such as fluid potential (Cai,1995).However,the mechanisms behind the secondary migration of hydro-carbons in the Ordovician carbonate reservoirs in the Tazhong uplift are poorly understood despite intensive research for sev-eral decades.
The Tazhong uplift,located in the central uplift of the platform in the Tarim Basin (Figure 1),is one of the most hydrocarbon-rich areas in the basin.The Ordovician hydro-carbon reserves in this area are estimated to be 340billion bbl (Yang et al.,2007).However,the area has undergone multiple tectonic movements,resulting in complicated hydrocarbon traps (Yang et al.,2007),which makes the analysis of hydrocarbon migration history extremely challenging.At present,insufficient information exists on the hydrocarbon charge process in the Tazhong area.Previously,research had been focused on the ap-plication of the pyrrolic nitrogen-bearing compounds to identify the direction and distance of hydrocarbon migration (Guo et al.,2008).However,this method requires that the oil samples be from the same source.An additional complication is that the pa-rameter is also influenced by thermal maturity.This study inte-grates geologic and geochemical methods to analyze the secondary migration of hydrocarbons in the Ordovician carbonate reservoirs in the Tazhong area.The key factors analyzed include regional structural configurations,fluid potentials,fluid properties,well production data,and the diamantane index.A new model for future hydrocarbon exploration in the Tazhong area is proposed.
NOMENCLATURE
Caledonian orogeny:570to 410Ma Hercynian orogeny:410to 250Ma Indosinian orogeny:250to 208Ma
research interest includes clastic sedimentology,petroleum system analysis,and laboratory inves-tigation of oil migration and enhanced oil recovery.Luofu Liu State Key Laboratory of Petro-leum Resources and Prospecting,China Univer-sity of Petroleum,Beijing,People ’s Republic of China;liulf@https://www.360docs.net/doc/7a11780616.html,
Luofu Liu graduated from the University of Bristol in 1992with a Ph.D.He is currently a professor of China University of Petroleum.His research interests focus on petroleum geo-chemistry and petroleum geology.
Caifu Xiang State Key Laboratory of Petro-leum Resources and Prospecting,China University of Petroleum,Beijing,People ’s Republic of China;495821445@https://www.360docs.net/doc/7a11780616.html,
Caifu Xiang received his Ph.D.in geologic sci-ences in 2003from China University of Geosciences.He currently is an associate professor of China University of Petroleum.His research interests focus on petroleum geology.
Sumei Li State Key Laboratory of Petroleum Resources and Prospecting,China University of Petroleum,Beijing,People ’s Republic of China;smli8888@https://www.360docs.net/doc/7a11780616.html,
Sumei Li received his Ph.D.in geologic sciences in 1999from China University of Geosciences.She is currently a professor at the China Uni-versity of Petroleum.Her research interests focus on petroleum geochemistry.ACKNOWLEDGEMENTS
This work was supported by the National Basic Research Program of China (grant number 2011CB201100).We thank the Exploration and Development Research Institute of the Tarim Oil-field Company,PetroChina,for providing back-ground geologic data and permission to publish the results.We also thank the reviewers for their con-structive comments and suggestions,which im-proved the manuscript.
The AAPG Editor thanks the following reviewers for their work on this paper:Barry J.Katz,Fang Lin,and an anonymous reviewer.
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Secondary Hydrocarbon Migration in Ordovician Carbonates
Yanshanian orogeny:208to 65Ma Himalayan orogeny:65Ma
GEOLOGICAL BACKGROUND
The Tazhong area,located in the central Tarim Basin,is an inherited structural uplift that is rich in hydrocarbons and has an exploration area of 22,000km 2(8494.2mi 2).It is located south of the
Manjiaer depression,north of the Tangguzibasi depression,west of the Tadong uplift,and east of the Bachu uplift (Figure 1;Liu et al.,2010).The Tazhong uplift can be further subdivided into sev-eral structural belts from north to south as the Tazhong 1slope belt,the Tazhong 10structural belt,the central horst belt,and the Tazhong 1–8buried hill belt (Figure 1).Their structural positions are relatively higher in elevation in the north than in the south (Yang et al.,2007).Recent
exploration
Figure 1.Geologic structures and distribution of hydrocarbon reservoirs in the Ordovician strata in the Tazhong area,Tarim Basin,western China.
Pang et al.
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suggests that the potential oil reserves in the Or-dovician carbonate reservoirs of this area amount to 22billion bbl,and the gas reserves are more than 1.78tcf (Yang et al.,2007).This further at-tests to the hypothesis that the Tazhong uplift is one of the major hydrocarbon-rich areas in the Tarim Basin.
The Ordovician stratigraphic sequence in the Tazhong uplift from youngest to oldest includes the Upper Ordovician Sangtamu Formation,the Upper Ordovician Lianglitage Formation,the Lower Ordovician Yingshan Formation,and the Lower Ordovician Penglaiba Formation (Figure 2).The Sangtamu Formation is composed primarily of
siliciclastic rocks with a thickness as much as 1000m (3281ft),whereas the other units in the area consist mainly of carbonate rocks.The Tazhong area has three reservoir-caprock assemblages (Figure 2):(1)the Upper Ordovician Lianglitage Formation reef as reservoir and the Upper Ordovician Sangtamu Formation claystone as caprock (e.g.,the Tazhong-1condensate field);(2)the Lower Ordovician Ying-shan Formation weathered crust as reservoir and the Upper Ordovician micritic limestone as caprock (e.g.,the Tazhong-83condensate gas reservoir);and (3)the Lower Ordovician Penglaiba Formation dolomite as both reservoir and caprock (e.g.,the Tazhong-162condensate gas
reservoir).
Figure 2.Generalized strati-graphic column of the Ordovi-cian strata in the Tazhong area.
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Secondary Hydrocarbon Migration in Ordovician Carbonates
The hydrocarbons in the Ordovician carbonate rocks of the Tazhong area were primarily derived from the Lower–Middle Cambrian and Middle–Upper Ordovician source rocks,but which one is the principal hydrocarbon supplier remains de-batable(Chen et al.,2000;Hanson et al.,2000; Zhang et al.,2000;Li et al.,2008;Cai et al.,2009; Pan and Liu,2009).The Lower–Middle Cambrian source rocks show typical characteristics of source rocks deposited in an oxygen deficit sedimentary environment,with the lithology composed mainly of black phosphatic silica,phosphorite,and black shale.The source rocks are dominated by plank-tonic algal organic facies deposited in a marginal depression of a starved basin and some evaporative lagoonal saline algal organic facies deposited on an inner platform.The thickness of the source rocks ranges from300to450m(984–1476ft);the total organic carbon(TOC),from1.20%to3.30%;and the hydrocarbon index,from2.10to250.37mg/g ([pyrolytic hydrocarbon(S2)/TOC]×100).The source rocks reached high to over mature stages and have vitrinite reflectance(R o)values ranging be-tween2.0%and4.0%(the R o value was trans-formed from solid bitumen reflectance)(Wang et al.,2003;Yang et al.,2004).The Middle–Upper Ordovician source rocks were of planktonic algal organic facies deposited in an underfilled marginal basin and planktonic algal organic facies of a limy mud mound deposited on the margin of a plat-form.The thickness of the source rock ranges from 100to200m(328–656ft);TOC,from1.20%to 2.56%;and the hydrocarbon index,from6.10to 531.06mg/g([S2/TOC]×100).The thermal ma-turity reached mature to high mature stages,with R o values ranging between0.5%and1.2%(the R o value was transformed from solid bitumen reflec-tance)(Hanson et al.,2000;Zhang et al.,2000).
There were four periods of hydrocarbon accu-mulation in the area:the early Caledonian(510Ma), the late Caledonian(439Ma),the late Hercynian–Indosinian(290Ma),and the Yanshanian–Himalayan (208Ma).The last two periods marked the most important events for resource development in the Tarim Basin(Pang et al.,2006).The first three periods were dominated by liquid hydrocarbons. During the last period,the Lower–Middle Cambrian source rocks became overmature and generated dry gas,much of which migrated into the Tazhong uplift.This charge episode is evidenced from the presence of extensive condensate gas reservoirs, believed to have originated by dry gas flushing of the early oil,in the area dating to the Ordovician Period.
The Tazhong uplift underwent several tectonic events from the Caledonian to the Himalayan orog-enies,and its structural pattern is controlled by two sets of northwest-trending and northeast-trending strike faults(Xiang et al.,2010).The uplift area was created through latitudinal zoning and lon-gitudinal blocking.Zoning controls the order in which the structural belts were formed,decreasing in age from north to south.The Tazhong-1slope belt was formed in the middle Caledonian orog-eny and was developed after the Sangtamu For-mation was exposed in the late Caledonian orog-eny,after which the area has remained stable. The Tazhong-10structural belt was formed in the Hercynian orogeny,after which subsequent tec-tonic movements were weak.The central horst and the Tazhong-1–8buried hill belts were formed during the Indosinian–Yanshanian orogeny.Strike-slip movements continued to occur into the Hima-layan orogeny.Blocking defines the drift and distri-bution of hydrocarbon reservoir types from east to west.Interestingly,the points separating the blocks generally coincided with the intersections of the northeast-and northwest-trending faults(Figure1). Injection point(IP)refers to the intersection of northeast-and northwest-trending faults,where charged hydrocarbon first entered into the Tazhong area.The Tazhong uplift area has six potential IPs:ZG17,TZ85,ZG3,TZ82,TZ622,and TZ242, dividing the entire Tazhong area into six blocks.In each block,major gas reservoirs have been found close to the IPs(Figure1),representing the domination of the last hydrocarbon(gas)charge and migration. METHODS AND WORKFLOW
This study applied both geologic and geochemical methods to analyze secondary migration of hydro-carbons in the Ordovician carbonate rocks in the
Pang et al.1769
Tazhong area.Fluid properties and production data were collected from the Tarim Oilfield Company, PetroChina.The21Ordovician oil samples selected for the diamantane analysis were evenly distributed across the Tazhong area.Gas chromatography–mass spectrometry(GC-MS)is the main method in the analysis of diamantane of oil samples.After the samples were collected,they were analyzed for hy-drocarbon and water contents,and were then stored at low temperatures for further laboratory analyses. In the laboratory,after the removal of asphaltenes, an alumina or silica chromatographic column was used to separate the crude oil into fractions.Hexane and benzene were used to extract the saturated and aromatic hydrocarbons,respectively.Ether was then used to extract the nonhydrocarbon organic compounds.Diamantane compounds exist in satu-rated hydrocarbon fractions because of the low content,small molecular weight,and low boiling point of the diamantane compounds.To reduce the loss of diamantane compounds,we used a vacuum rotary evaporation solvent to concentrate saturated hydrocarbons and preserve them in low tempera-ture.Then,we analyzed the content of saturated hydrocarbons with GC-MS.The instrument used was a SSQ710Model GC-MS(Finnigan).Saturated hydrocarbons were analyzed using a DB-5quartz capillary column(30m×0.53mm).The GC heating program was set as follows:first,a constant temper-ature of100°C was set for1min;it was then heated to200°C at a rate of4°C/min,followed by heating from200°C to300°C at a rate of2°C/min and maintained for20min.Helium was used as carrier gas,and the vaporizing chamber temperature was set to300°C.Finally,we used an ion scan to improve the sensitivity of the diamantane compound analysis.
The workflow of secondary hydrocarbon migra-tion analysis used in this article includes identifying the regional structural configurations and trends; reconstructing the migration paths using the fluid potential concept;integrating the hydrocarbon prop-erties and geochemical characteristics with well production and methyl diamantane property to in-vestigate the process and mechanisms of secondary hydrocarbon migration in the Tazhong area;and, finally,predicting hydrocarbon distribution in un-drilled intervals.RESULTS
Fluid Potential
The concept of fluid potential,first proposed by Hubbert(1953),refers to the mechanical energy of a unit mass of fluid.The expression for cal-culation is
Φ?gzt∫P0dP rtq22e1TwhereΦ=fluid potential(J/kg);g=acceleration caused by gravity(9.81m/s2[32.19ft/s2]);z=alti-tude(m);p=reservoir pressure(Pa);r=fluid density (kg/m3);q=fluid velocity(m/s).Considering that the natural motion of fluid in the subsurface envi-ronment was so slow,the flow velocity can be as-sumed to be zero.
Fluid movement within a reservoir,controlled by fluid potential,always migrates from areas with high potential toward those with low potential.All mobile fluids are controlled by the potential field and are distributed according to fluid potential gradient (Hubbert,1953;England et al.,1987).The mor-phologic characteristics of source rocks in a basin mostly determine the spatial distribution of the fluid potential field,which makes it possible to determine the primary migration direction of hydrocarbons. Because the hydrocarbons in the Tazhong uplift originated from the Cambrian–Ordovician source rocks(Figure3),any identification of migration di-rections using the fluid potential gradient requires the delineation of compartmental troughs in the potential fields within the area of interest.These troughs represent the boundaries determined by the potential contour patterns in the carrier beds,which cause the hydrocarbons to migrate and accumulate (Liu et al.,2002).It can be divided into different hydrocarbon accumulation units.Hydrocarbons on opposite sides of a compartmental trough moved toward two different directions or areas.For exam-ple,if the potential contour is represented as a to-pographic contour,the ridge would equate to the compartmental troughs for hydrocarbon migration, the valley would equate to the principal migration direction,and the drainage area surrounded by
1770Secondary Hydrocarbon Migration in Ordovician Carbonates
Figure3.Hydrocarbon migration analysis by fluid potential distribution features in the Ordovician strata in the Tazhong area,Tarim Basin.
(A)Structural contour map on the top of the Ordovician,showing the accumulation unit division in the platform of Tarim Basin.Accu-mulation unit:I=northern Manjiaer area;II=Tabei area;III=Tadong area;IV=Tazhong area.(B)Fluid potential distribution features in the Ordovician strata in the Tazhong area.
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multiple ridges would equate to a relatively inde-pendent hydrocarbon accumulation unit(Hubbert, 1953;Toth,1980;England et al.,1987).Figure3 shows the compartmental troughs of hydrocarbon migration and the associated hydrocarbon units of the Tarim Basin.The Tazhong uplift is located in the hydrocarbon accumulation unit IV.Hydrocarbons migrated into the Tazhong area along an axis from northeast to southwest.After arriving at the Tazhong area,hydrocarbons continued migrating in two di-rections,namely,from northwest to southeast and from northeast to southwest.
Oil Properties
Oil properties represent the result of a series of geo-logic processes that enable oil generation,migra-tion,accumulation,and alteration.As the maturity of source rocks increases,the composition of the migrating oil changes.Because most of the hydro-carbons in the reservoirs were injected on one side, along the principal direction of migration,it can be inferred that the crude oil compositions may change after a gradient.Assuming that the hydrocarbons were charged from the same source,the later the oil arrived at an accumulation,the more mature it would be(Hwang et al.,1994;Lin and Zhang,1996).
The crude oil in the Ordovician reservoirs in the Tazhong area is primarily light crude,with a density and viscosity range of0.77to0.91g/cm3 and1.02to1347mPa·s(147.94–195,366psi·s), respectively(Figure4)(Table1).Oil with lower densities and viscosities was distributed near the intersections of the northeastern and northwestern faults(Figure4)(Table1).Along the axis from northwest to southeast,the oil densities and viscos-ities increased away from the structural intersec-tions.Take the IP TZ622for example.The
crude Figure4.Oil density contour map in the Ordovician reservoirs in the Tazhong area.
1772Secondary Hydrocarbon Migration in Ordovician Carbonates
oil density increased from0.82to0.91g/cm3,and the viscosity increased from4.3to31mPa·s(623.7–4496psi·s)away from the IP(TZ621-TZ161-TZ75). As the distance from the IP ZG3increases(TZ54-TZ824-TZ122),the density and viscosity change in a manner similar to that of TZ622,with a density from0.82to0.87g/cm3and a viscosity from3.27 to1347mPa·s(474.27–195,366psi·s;Table1). This seems to be caused by the mix of the crude oil and gas generated in later periods with higher
Table1.Oil Properties of the Ordovician Reservoirs in the Tazhong Area
Hydrocarbon
Injection Point Well Depth(m)Density
(g/cm3)
Kinematic Viscosity,
50°C(mPa·s)
Wax Content
(%)
ZG17TZ8662800.80 2.199.25 TZ85TZ8563500.84 4.61 6.70 ZG3TZ5458320.82 3.277.75 TZ82456130.83 3.847.10
TZ12247100.871347.00 5.10 TZ82TZ82353700.81 2.8510.20 TZ82152130.81 2.4110.40 TZ622TZ62148550.82 4.308.20 TZ16142900.85 4.60 6.10
TZ7539200.9131.00 4.72 TZ242TZ24343880.77 1.028.40 TZ24146190.80 2.427.90
TZ26343100.80 2.40
3.70 Figure5.Stratigraphic-structural cross section(AA′)of geologic features and oil properties in the Tazhong area showing reservoir distribution and types.See Figure4for the location of the cross section.C=Carboniferous;D=Devonian;S=Silurian;O3S=Upper
Ordovician Sangtamu Formation;O3l1–O3l2=member2to member1of the Upper Ordovician Lianglitage Formation;O3l3–O3l5:
member5to member3of the Upper Ordovician Lianglitage Formation;O1=Lower Ordovician.
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maturity;the density and viscosity of crude oil proximal to the IPs are reduced.Nevertheless,be-cause of the heterogeneity of the carbonate rocks and the existence of the migration barriers,in the areas away from the IPs,little oil mixed with gas was observed,and the density and viscosity were rel-atively larger.Previous research indicated that the oil in the Tazhong area accumulated primarily in the Ordovician Period and was later biodegraded during the late Hercynian orogeny associated with strong tectonic movements(Li et al.,2008).Dur-ing the Himalayan orogeny,however,tectonic movements weakened,and seal integrity remained excellent.Thus,the last hydrocarbon charge dur-ing the Himalayan orogeny involved the migration of these hydrocarbons from northwest to southeast and from northeast to southwest after the fluid potential gradient.Based on the distribution of oil densities in the cross section(Figure5),the crude oil proximal to the intersections of the faults had a light color with mainly gas condensate accu-mulations.However,away from the intersections, oils have darker colors,and the accumulations had gradually transformed to normal oil reservoirs in the distal parts.Analysis results showed that the carbon isotopes of Ordovician gas in the Tazhong area were relatively lighter than those of the other areas of the Tarim Basin.The carbon isotope of methane ranges from–39.6‰to–37.7‰(which was generally larger than–36‰in other areas of the Tarim Basin),and that of ethane,from–37.2‰to –33.1‰,indicating that the main gas source rock was sapropelic.In addition,according to the R o equivalence calculated from methane carbon iso-tope,gas maturity ranges from1.46%to1.93%,proving that gas is highly overmature and gener-ated from the Lower–Middle Cambrian source rocks that reached peak generation in the Hima-layan period(Table2).(Han et al.,2009).The Himalayan orogeny was characterized by dry gas invasion and flushing of previous oil accumula-tions at the injection intersection points.This yiel-ded an unusual distribution pattern,with the higher density fluids occurring stratigraphically above the lower density fluids.There were two main reasons for the observed distribution.First,depo-sitional facies changed in the reservoirs(Figure6). Take the ZG17for example.The facies of well ZG17and well TZ86are rich in condensate near to the IP and were reefal and flat.The facies changed to grainstone and flat away from the IP and created migration barriers between the ZG17condensate and TZ88oil reservoirs,so that the oil and gas res-ervoir distribution was not continuous.It was the same reason for the discontinuity in hydrocarbon distribution between the ZG2and ZG3(Figure6). Second,porosity and permeability changed in the reservoirs.Take the ZG17for example.The po-rosity and permeability of well ZG17and well TZ86near to the IP were relatively higher with an average porosity of12.3%and5.8%,respectively, and an average permeability of12.17×10–3and 0.37×10–3m m2,respectively.It appears that the faulting improved the proximal reservoir’s porosity and permeability.The porosity and permeability of TZ88away from the IP were relatively poor, with an average porosity of1.8%and an average permeability of0.0257×10–3m m2.It was the same reason for the discontinuity of hydrocarbons between ZG2and ZG3,the average porosity of
Table2.Gas Isotope Data of the Ordovician Reservoir Formations in the Tazhong Area(from Han et al.,2009)
Well Depth(m)d C1(‰)d C2(‰)d C3(‰)d C4(‰)R o(%) TZ825430–39.6–33.8–30.6–29.0 1.46 TZ8215215–38.3–33.1–30.2–29.4 1.77 TZ62-35075–38.7–33.5–30.1–29.9 1.67 TZ6224920–38.9–33.8–31.1–30.3 1.62 TZ62-14895–38.0–33.6–30.2–29.3 1.84 TZ2414650–38.4–37.2–33.5–31.1 1.74 TZ264300–37.7–36.8–32.6–29.8 1.93
1774Secondary Hydrocarbon Migration in Ordovician Carbonates
which changed from12.74%to3.5%,and the average permeability,from3.09×10–3to0.038×10–3m m2.The dry gas injections changed the den-sities of the oil and lightened their colors.Hydro-carbon accumulations proximal to source rocks were normally dominated by gas condensate reservoirs and light crude oil resulting from the relatively higher porosity and permeability of the reservoirs and charge sequences.Proximal to faults,hydro-carbon shows have been detected,indicating the presence of hydrocarbon migration along the faults (Figure5)and confirming that hydrocarbons were indeed injected at the intersections of the northeast and northwest faults.Hydrocarbons migrated up-ward subsequently and dispersed along the flower faults vertically and migrated from northwest toward southeast(Figure5).
Gas Properties
The compositions of the gas in the Ordovician reservoirs in the Tazhong area vary considerably with methane,generally constituting80.6%to 92.5%,and CO2and N2constituting0.14%to 3.48%and3.29%to9.12%,respectively.The gas densities range from0.73to0.81g/cm3.The dry-ness coefficient(C1/∑C2+)is generally more than 0.95,typical of dry gas.Hydrogen sulfide is also present and varies among wells.The properties of the gas change as the distance from the
source Figure6.Facies distribution in the Ordovician strata in the Tazhong area,Tarim Basin,western China.
Pang et al.1775
points increases in a manner similar to that for the crude oil.For example,the gas:oil ratio,dryness co-efficient,and hydrogen sulfide content are all ab-normally high at the intersections of the northeastern and northwestern faults and gradually decrease from northwest to southeast away from the intersections (Figures7–9).Gases discovered in the Tazhong uplift were derived from overmature source rocks during the late Himalayan orogeny(Hanson et al., 2000;Xiao et al.,2000;Zhang and Huang,2005). The more proximal the reservoir is to the source points,the greater the gas accumulation becomes. This explains the higher gas:oil ratio,dryness co-efficient,and hydrogen sulfide levels in the proxi-mal areas.Therefore,it can be reasonably main-tained that,during the last accumulation period,gas was injected into the Tazhong area at the intersec-tions of faults and migrated toward the southeast from the northwest.Well Production Data
Demaison(1984)studied the distribution features of12generative basins in the world and concluded that the exploration risk increased with the dis-tance away from the effective source.Pang(2003) investigated the relationships among73large-and medium-size oil-gas fields and their respective distances of hydrocarbon migration and concluded that the number of oil-gas fields diminished as the distance of hydrocarbon migration increased. Hu(2005)seconded these conclusions on the basis of examining more than200sedimentary basins around the world and further concluded that a higher probability to discover large-and medium-size oil-gas fields in areas proximal to the hydro-carbon source exists,as opposed to the regions farther away from the source.Therefore,the dis-tance from mature source rocks seems to
determine Figure7.Gas:oil ratio(GOR)contour map of the Ordovician reservoirs in the Tazhong area.
1776Secondary Hydrocarbon Migration in Ordovician Carbonates
the size,distribution,and resource potential of hydrocarbon reservoirs.Most of the hydrocarbon reservoirs derived from the Ordovician source rock are in stratigraphic traps,indicating that migrated hydrocarbons accumulated in the nearest trap. Previous research showed that hydrocarbons in the Tazhong area originated and migrated from the distant Manjiaer depression to the north(e.g., Hanson et al.,2000).By analyzing the well pro-duction data around the Tazhong area,it can be observed that the daily production was abnormally high at the IPs(intersection points of the north-eastern and northwestern faults)(Figure10)and tends to diminish with increasing distance away from the IPs along the northwestern to southeast-ern and northeastern to southwestern axes because of the higher permeability and porosity of the res-ervoirs near the intersections of the faults.Mean-while,with increasing migration distance,the well test results changed gradually from commercial production to hydrocarbon shows or even dry wells (Figure10).
Methyl Diamantane Property
Methyl diamantane is a type of rigid polymeric hy-drocarbon with a structure similar to diamond;it is a product of polycyclic hydrocarbons that under-went a catalyzed polymerization reaction in a high-temperature environment rich in Lewis acids(Chen et al.,1996).Methyl diamantane is stable,and once formed it is hard to destroy by thermal processes or by biodegradation.Because of this property,it can be used as a maturity index of crude oil and hy-drocarbon source rocks(Dahl et al.,1999;Wei et al.,2007).However,the formation of diaman-tanes is typically influenced by multiple
factors, Figure8.Gas dryness coefficient contour map of the Ordovician reservoirs in the Tazhong area.
Pang et al.1777
in addition to thermal stress,which still require verification.These factors include the type of source rocks and the sedimentary environment (Schulz et al.,2001).Methyl diamantane index (MDI)refers to the peak area ratio of4-MD to 1-MD+3-MD+4-MD(%).Previous research has indicated that the MDI can reflect the direction of hydrocarbon migration,which is supported by other geologic and geochemical data(Shao,2005; Duan et al.,2007;Gu et al.,2007).For example, in the Ordovician carbonate oil reservoirs in the Tahe area of the Tarim Basin,all geologic and geo-chemical researches showed that the oil migrated from south to north and from east to west,including fluid potential;18a-22,29,30-trisnorneohopane (Ts)/18a-22,29,30-trisnorneohopane(Ts)+17a-22,29,30-trisnorhopane(Tm)and diasterane/ regular steranes index of saturated hydrocarbon biomarkers;and nitrogenous compounds in crude oil,such as carbazole and methyl carbazole indicators (Shao,2005;Gu et al.,2007).At the same time,the MDI of crude oil also indicated the same migration trend,gradually decreasing from south to north, and from east to west(Duan et al.,2007).Hence, the MDI can be used to effectively delineate the direction of hydrocarbon migration in the Tazhong area.The more proximal the oil is to the source rocks,the greater the MDI is.Conversely,the index tends to be smaller with distance from the source.
Among the21samples from the Tazhong area, the MDI indices are distributed between0.32and 0.62.The samples from the TZ-242wells are the most mature ones,indicating that they were gen-erated relatively late(Table3).In addition,the index values are relatively higher at the hydro-carbon IPs(Figure11)and tend to decrease with distance away from these points.An explanation for this pattern is that hydrocarbons were
first Figure9.Hydrogen sulfide content contour map of the Ordovician reservoirs in the Tazhong area.
1778Secondary Hydrocarbon Migration in Ordovician Carbonates
injected at those IPs;then,they migrated along the axes from northwest to southeast and north-east to southwest.In the Tazhong uplift,as dis-cussed previously,four known periods of hydro-carbon charge exist:(1)in the early Caledonian orogeny,when the R o values of the source rocks reached thermal maturity levels equivalent to 0.5%to 1.3%,and crude oil and condensate were gener-ated;(2)in the late Caledonian orogeny,when the equivalent R o values of the source rocks reached 1.3%to 1.5%,and only condensate was generated;(3)during the late Hercynian and Indosinian orog-enies,when R o equivalent values of the source rocks reached 1.5%to 2.0%,and condensate and dry gas were generated;and (4)in the Yanshanian and Himalayan orogenies,when R o equivalent values of the source rocks reached more than 2.0%,and dry gas was generated (Wang et al.,2003).Because of the effects of multiple charges and the lack of sufficient hydrocarbon supply in later periods,only areas proximal to the hydrocarbon IPs wit-nessed generation and accumulation of highly ma-ture hydrocarbons during the later periods.In other words,hydrocarbons that accumulated near these points were more mature than those that accu-mulated farther away.The migration direction of hydrocarbons in the Ordovician reservoirs as pre-dicted by the MDI is consistent with the data obtained from other geologic and geochemical parameters.
DISCUSSION
Some works show that secondary hydrocarbon migration is primarily controlled by the carrier beds (Hunt,1994),and migration commonly occurs in dominant conduits with superior petrophysi-cal properties (England et al.,1987).Many others believe that the hydrocarbon migration pathways are controlled mostly by structural morphology (Pratsch,1986;Hindle,1997).Hao et al.(2000)discovered that widespread planar migrating hy-drocarbons commonly transform into focused linear flows as they migrate from the center of the basin to the margin.Based on the above theories and em-pirical observation,and considering the actual geo-logic characteristics of the Ordovician source rock in the study area,we propose that the hydrocarbon IPs and structure morphology together determined the hydrocarbon migration in the Tazhong uplift.
Previous research has shown that most of hy-drocarbons in the Tazhong area were originated in the Manjiaer depression to the north (Figure 1),which,at the time of charging,was approximately 40km (25mi)away from the Tazhong uplift (Li et al.,2008).After their generation in the depres-sion,hydrocarbon migrated into the Tazhong area along an axis from northeast to southwest (Figure 3)and transforming from planar migration paths grad-ually into focused linear pathways.Because the mi-gration paths of hydrocarbons were determined
by
Figure 10.Relationship between distance to hydrocarbon injection points and hydrocarbon production of the Ordovician reservoirs in the Tazhong area.
Pang et al.
1779
the topographic geometry of the carrier beds,prox-imal to the hydrocarbon generation area,the Man-jiaer depression and its adjacent oil and gas migration paths formed a dense network as planar distribution, and far from the hydrocarbon generation area,mi-gration paths gradually converged as linear distribu-tion.Eventually,these hydrocarbons reached injected points that led to the hydrocarbon accumulations in the Tazhong area.The northwest and northeast strike faults in the Tazhong area became tectonically active in the early Caledonian orogeny and formed the current structural configuration in the late Caledonian orogeny,providing good geologic conditions for sub-sequent hydrocarbon injection.In addition,the in-tersection points of these faults improved the porosity and permeability of these carbonate rocks because of the tectonic stresses.Hydrocarbons tend to migrate along carrier beds with relatively higher porosity and permeability.The porosity and permeability of res-ervoir rocks in areas proximal to the intersection points were higher than those in the distal areas (Xiang et al.,2010).After being injected into the Tazhong area,the hydrocarbons migrated upward from the roots of the faults and then were dispersed along the flower structures(Figure5).Laterally,hy-drocarbons migrated along two directions,namely, from northwest toward southeast and from northeast toward southwest.The hydrocarbon migration pat-terns in the area conform to the regional structural configuration,fluid potential gradient,well produc-tion output,and geochemical characteristics,which show a consistent first-order trend.
We classified the features of secondary migration in the Tazhong area according to the geologic and geochemical characteristics of hydrocarbons in the late periods of the Paleozoic era.Hydrocarbons generated in the late periods were more likely to migrate along the routes of the previous hydro-carbon migration pathways(Thompson,1991). Assuming that late tectonic movements were not
Table3.Methyl Diamantane Index of Oils from the Ordovician Reservoir Formations in the Tazhong Area*
Hydrocarbon
Peak Area
Injection Point Well Depth(m)1-MD3-MD4-MD4-MD/(1-MD+3-MD+4-MD) ZG17TZ86628038,89634,26662,3240.46
TZ451625339,57334,94458,5490.44
TZ45602032,31527,68643,4490.42
TZ88656044,46739,83839,6730.32
TZ85TZ85635044,37839,74866,0990.44
ZG3TZ54583245,54340,91473,6490.46
TZ824561333,61428,98547,2240.43
TZ12469530,86026,23039,6730.41
TZ82TZ823537048,09743,46881,2000.47
TZ821521343,89739,26865,3440.44
TZ622470343,65739,02864,9670.44
TZ721503024,48419,85528,3480.39
TZ622TZ44482252,43247,80392,5250.48
TZ623480939,24834,61860,4360.45
TZ161429038,85234,22357,4160.44
TZ75392025,96121,33130,2360.39
TZ242TZ261435756,12051,491175,5770.62
TZ24446252,40847,778156,7020.61
TZ243438860,91056,280149,1510.56
TZ241461969,34064,711134,0510.5
TZ263431068,96364,333133,2960.5
*1-MD=1-Methyl diamantane;3-MD=3-Methyl diamantane;4-MD=4-Methyl diamantane.
1780Secondary Hydrocarbon Migration in Ordovician Carbonates
particularly strong,hydrocarbon migration in the late periods basically reflects the early migration routes and processes.After the Hercynian orogeny, tectonic movements became weak in the Tazhong uplift(Lu et al.,2004).Although there were four distinct periods of hydrocarbon accumulations in the area,the Hercynian and Himalayan orogenies experienced the most significant reservoir filling (Pang et al.,2006).In other words,the last two episodes of hydrocarbon accumulations were not followed by any strong tectonic movements,sug-gesting that this study effectively reflects the mi-gration of the bulk hydrocarbons accumulated in the uplift.The Cambrian dolomites underneath the Ordovician strata were inferred to contain hy-drocarbon accumulations with structural patterns similar to those of the Upper Ordovician intervals. Hence,the hydrocarbon migration patterns of the Ordovician reservoirs may be an effective analog for understanding the migration characteristics of hy-drocarbons within the Cambrian dolomite.The results from this study thus provide an important foundation for future petroleum exploration in the deeper part of the Tazhong area. CONCLUSIONS
On the basis of the analysis of the regional structures and by integrating geologic,hydrodynamic,and geo-chemical parameters,the following conclusions on hydrocarbon charge and migration in the Tazhong area,Tarim Basin,western China,can be obtained: 1.Hydrocarbons that migrated during the Ordovi-
cian Period were injected into the Tazhong uplift at six IPs,coinciding with the intersections of the northeast-and northwest-trending
faults. Figure11.Distribution of methyl diamantane index of hydrocarbons of the Ordovician reservoirs in the Tazhong area.
Pang et al.1781
2.Hydrocarbon injected into the Tazhong area via
discrete point sources migrated along two direc-tions,from northwest toward southeast,and from northeast toward southwest.
3.The findings of this research can be potentially
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