Available online at www.sciencedirect.com

ScienceDirect

Journal of Natural Gas Geoscience 1 (2016) 309e318http://www.keaipublishing.com/jnggs

Original research paper

Accumulation mechanism and controlling factors of the continuoushydrocarbon plays in the Lower Triassic Baikouquan Formation of the Mahu

Sag, Junggar Basin, China*

Ablimit Imin a,*, Yong Tang a, Jian Cao b, Gangqiang Chen a, Jing Chen a, Keyu Tao b

a Research Institute of Exploration and Development, PetroChina Xinjiang Oilfield Company, Karamay, 834000, Chinab School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu, 210023, China

Received 24 July 2016; revised 22 August 2016

Available online 19 October 2016

Abstract

Hydrocarbon exploration in the Mahu Sag of the northwestern Junggar Basin has recently resulted in a significant discovery that defined thenew “continuous oil plays” in the Lower Triassic Baikouquan Formation. This formation hosts glutenite reservoirs with oil reserves of more thanhundreds of millions of tons. This resource is an example of an extensive hydrocarbon accumulation that's far from source rocks. Three con-ditions are necessary for this type of hydrocarbon accumulation: large hydrocarbon source, good reservoirs and effective seals, and favorabletiming of hydrocarbon charge and migration. In the Mahu Sag, high-quality lacustrine source rocks are prolific hydrocarbon source, large-scalestrike-slip faults enabled and facilitated long-distance hydrocarbon migration, and fan-delta glutenites are excellent reservoirs for hydrocarbonaccumulation. The hydrocarbon accumulation is controlled by three factors: pro-delta sedimentary facies, structural highs, and faults. Abnormalreservoir over pressuring and fracturing has had an important impact on enhancing hydrocarbon production. The consideration of all thesefactors should govern the selection of future exploration targets.Copyright © 2016, Lanzhou Literature and Information Center, Chinese Academy of Sciences AND Langfang Branch of Research Institute ofPetroleum Exploration and Development, PetroChina. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This isan open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Continuous oil and gas plays; Fan-delta; Lacustrine high-quality source rocks; Highly mature light oil and gas; Distal hydrocarbon accumulation;

Baikouquan Formation; Mahu Sag

1. Introduction

The Junggar Basin is a typical large petroliferous basin inthe western China. Recent exploration was conducted in thisbasin by PetroChina [1,2], they discovered new and unusualcontinuous light oil (and gas) plays that are extremely distal

* This is English translational work of an article originally published in

Natural Gas Geoscience (in Chinese). The original article can be found at: 10.

11764/j.issn.1672-1926.2016.02.0241.

* Corresponding author.

E-mail address: ablmt@petrochina.com.cn (A. Imin).

Peer review under responsibility of Editorial Office of Journal of Natural Gas

Geoscience.

http://dx.doi.org/10.1016/j.jnggs.2016.10.003

2468-256X/Copyright © 2016, Lanzhou Literature and Information Center, Chinese Academy of Science

China. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open

from source sequences in the Lower Triassic BaikouquanFormation. This formation contains glutenite reservoir rockswith proven oil reserves of hundreds of millions of tons.

The Mahu Sag lies on the downthrown block of the “hun-dreds of miles oil region” fault zone. This is the first discoverywithin the Junggar Basin possessing an “old source and newreservoir” type of accumulation, which covers a large areawithin a sag structure. Compared to local and internationalexamples of hydrocarbon reservoirs, this “old source and newreservoir” type that's associated with glutenites is unusual[3e6]. In order to better understand this hydrocarbon reser-voir, this paper investigates the accumulation mechanism andcontrolling factors of this hydrocarbon resource.

s AND Langfang Branch of Research Institute of Petroleum Exploration and Development, Petro-

access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

310 A. Imin et al. / Journal of Natural Gas Geoscience 1 (2016) 309e318

2. Basic characteristics of the hydrocarbon plays

Oil and gas exploration has proven to be successful in theBaikouquan Formation in the Mahu Sag of the Junggar Basin(Fig. 1). Exploration wells, namely, Ma 131, Ma 15, and Xia72 in the Xiazijie fan group of the western part of the basinhave reserves of hundreds of millions of tons. The Well Mahu1in the Karamay fan group and Well Ma 18 in the Huan-gyangquan fan group could produce oil as much as 60 t/d. Inthe Fengnan fan in the western part of the basin, large reserveshave also been discovered. Moreover, there are some otherexploration wells, such as Well Yanbei 1 and Well Yanbei 2 inthe Madong-Xiayan fan group, which also have oil flows.These wells show that the hydrocarbon-producing zone isextensive and highly productive and such wells will be animportant area for continued exploration of the Xinjiang OilField Company, or even PetroChina.

Through Well Ma 131, the Baikouquan hydrocarbon re-sources were first discovered, and some insights about thecharacteristics of the Baikouquan Formation provided weredetermined. According to lithological, electrical resistivity, oil,and sedimentary facies' data, the target layer (Triassic Bai-kouquan Formation) can be divided into three sections: T1b1,

Fig. 1. Geological sketch map showing hydrocarbon exploration results and predicte

Mahu Sag.

T1b2, and T1b3. T1b2.; the said layers can be subdivided intoT1b2

1 and T1b22, with T1b2

1 being the main oil reservoir.T1b2

1 rocks are mostly comprised of gray and brownegrayconglomerate, along with pebbly argillaceous siltstone andargillaceous siltstone (Fig. 2).

The distribution of oil and gas in these rocks is related tothe location of the faults, for example, in Well Ma 131, oil issealed by a fault from the Well Ma 13. The reservoir pressuredata shows that a single fault block is cut into three sub-faultblocks by the east fault of the Well Ma 131 and the east faultof the Well Ma 15.

Hydrocarbons from the Baikouquan Formation of the MahuSag are mainly crude oils with gas. Oil densities range0.78e0.86 g/cm3, and are predominantly 0.82e0.86 g/cm3.The methane content is 59.7%e92.8% (average 76.8%),whereas the ethane content is estimated to be 2.5%e11.9%(average 7.4%). These gas ratios are indicative of wet gas, butdeep exploration has recently discovered dry gas. Therefore,according to these basic oil and gas characteristics, this hy-drocarbon resource is the product of mature to highly maturesource rocks.

In general, such oil and gas systems have characteristicsthat include oil (and gas) distributed throughout the whole

d hydrocarbon occurrences in the Lower Triassic Baikouquan Formation of the

Fig. 2. Cross-section of the Lower Triassic Baikouquan Formation in the Mahu Sag showing oil accumulation zones.

311A. Imin et al. / Journal of Natural Gas Geoscience 1 (2016) 309e318

reservoir belts, localized areas of enrichment, and high pro-ductivity. This corresponds to typical “continuous oil (and gas)play” characteristics [3,7,8]. Hydrocarbon-source correlationstudies have shown that the hydrocarbons are mainly derivedfrom deep lower Permian Fengcheng lacustrine source rocks[9,10] (see Section 3.3), which is different from the typical“continuous oil (and gas) plays” that are generally source-reservoir integrated [8,11,12]. Therefore, this petroleumresource is termed an extensive “continuous hydrocarbonplay” that is distal from its source rocks and represents aglobal model for the study of the formation and nature of suchoil (and gas) reservoirs.

3. Hydrocarbon accumulation mechanism

The average depth of the Baikouquan Formation in theMahu Sag is 3.5 km. The Baikouquan reservoir is separatedfrom the underlying Permian source rocks by 1e2 km. Thevarious lines of evidence suggest that three main factors led tothe formation of the large continuous hydrocarbon plays,which are discussed as follows.

3.1. Hydrocarbon source rocks

The deep CarboniferousePermian rocks in the Mahu Saginclude four types of high-quality source rocks (Fig. 3).Mudstone samples with a mediumehigh abundance of organicmatter are found in all the four types of source rocks. TheLower Permian Fengcheng lacustrine source rocks are of thehighest quality, although they do not have the highest organiccarbon content, these rocks have the greatest hydrocarbongeneration potential. This reflects the fact that the organic

matter type in the Fengcheng lacustrine source rocks isoptimal for hydrocarbon generation as proved by the carbonisotope (relatively light) and hydrogen index (relatively high)data.

The Lower Permian Fengcheng lacustrine source rockswere deposited in a saline, alkaline, and reducing environment[9,13]. Previous studies have shown that alkaline lacustrinesediments with abundant microbial organic matter have ahydrocarbon generation capacity that is often several timesthan that of non-alkaline lacustrine source rocks. Thus, thegenerated crude oil has a lower density and often times readilymigrates [14,15].

Faults cutting through the source rock sequence (Fig. 4)constitute favorable conditions for hydrocarbon migration[16]. During the early Carboniferous to Permian times (fore-land uplift period) the Junggar Basin was affected bycompressive stress, as a result, it formed a series of large-scalereverse faults that amended the source rock sequence. Reversefaults continued to develop until the EarlyeMiddle Triassicperiods. These faults cut through the Baikouquan Formation,causing the Fengcheng source rocks to link with the Baikou-quan reservoir, forming an effective conduit for hydrocarbonmigration (Fig. 4) [17,18]. In the Zhongguai-Wuba and Mananslope areas, large strike-slip fault systems have developed,where the reservoir is directly linked to the source rocks. TheBaikouquan Formation forms a topographic and lithologic oilreservoir. In the Mabei slope area, the Xiazijie structural belthas formed a nose structure that abuts against a reverse fault.The fault breaks off to the bottom of T2k, thereby linking thelower hydrocarbon generation layers with the reservoir. In theMadong slope area, a series of reverse faults have formedalong the nose structure due to the frontal uplift belts. These

Fig. 3. Geochemical profiles of CarboniferousePermian hydrocarbon source rocks in the Mahu Sag.

312 A. Imin et al. / Journal of Natural Gas Geoscience 1 (2016) 309e318

faults break off into T2k, yet again linking the hydrocarbongeneration layers with the reservoir.

The study area is located in the central part of ahydrocarbon-rich depression with faults connecting sourcerocks with reservoirs. Thus, high maturity source rocks for oiland gas are abundant, and it forms the basis for extensivehydrocarbon accumulation distal from these source sequences,which ultimately led to a high production area.

3.2. Reservoir nature and conditions

The Baikouquan Formation in the Mahu Sag has thereservoir qualities to form a “continuous hydrocarbon play”. Itspecifically includes widespread conglomerate reservoirs that

have excellent permeability and porosity that aid in theaccumulation of large amounts of oil and gas. Previous studieshave shown that the conglomerate was deposited in a fan-deltasedimentary system [19e21].

There are four distinct genesis regions to the depositionalsystems around the Mahu Sag and the six fan bodies that areassociated. The six fan bodies are Xiazijie, Huangyangquan,Karamay, Zhongguai, Yanbei, and Xiayan. The six fan bodiesprovided abundant terrigenous detrital material during thedeposition of the Baikouquan Formation. The slope of thesedimentary system was gentle during deposition, thus,resulting in a well-developed pro-delta sedimentary facieswith sand bodies extending into the center of the basin (i.e.,T1b1 and T1b2). The thick layers of glutenite are distributed

Fig. 4. Seismic section showing the structural highs in the Baikouquan Formation, Mahu Sag.

313A. Imin et al. / Journal of Natural Gas Geoscience 1 (2016) 309e318

widely throughout the basin and have good physical propertiesfor a hydrocarbon reservoir. The frontal facies of each fan aredistributed over a large area, with each facies covering severalhundreds of square kilometers. The fan bodies developed inresponse to the paleo-geomorphological controls, withmountain passes and valleys defining the main groove andplains facies. Large strike-slip faults extended the fan deposits.In addition, mudstone units were formed adjacent to the paleo-mountains (Fig. 5).

Oil and gas migrated along the faults of the deep Carbon-iferousePermian source rocks to the Triassic pro-delta reser-voirs, which are sealed at the top, bottom, and sides. Ever sincethe LateCretaceous the regional tectonic activity has beenminor,and the faults have become less active and sealed, thereby pre-venting re-migration or escape of oil and gas. All of these con-ditions are favorable for oil and gas accumulation, it alsoexplains the wide distribution of oil and gas [22]. The MiddleTriassic Karamaymudstone overlies the Baikouquan Formation.At the bottom of the Baikouquan Formation, dense glutenitesoverly themiddle Permian lowerWuerhemudstone (Fig. 6). Thesides of the reservoir are the near-provenance (structural highs),

Fig. 5. Fan-delta sedimentary system model, where the Lower Tri

these are comprised of fan-delta-plain facies overlying the denseglutenites; these and the mudstones above and below the Bai-kouquan Formation provide effective reservoir seals.

The fan-delta-plain facies sandstone and conglomerate areclay-rich but poorly sorted. They have poor reservoir physicalproperties with porosities being >5% and permeability mostly<0.1 mD, for example, such rocks in the Well Xia 72 have aporosity that is 2.8%, whereas in the Well Xia 9, the porosity is3.2%. On the contrary, the main Baikouquan sealed reservoiris predominantly fan and pro-delta front facies gray sandstoneand conglomerate. These reservoir rocks are well sorted butpoor in clay. They have porosities ranging 6%e14% thataveraged ~10%, and permeability that is generally >1 mD. Anexample is the rocks in the Well Ma 18 that possesses anaverage porosity and permeability of 10.5% and 5.66 mD,respectively, whereas, in the Well Ma 2, the average porosityand permeability are 9.4% and 1.41 mD, respectively. There-fore, these reservoir properties such as overlying and under-lying seals, and absence of bottom water all contribute tomaking the formation favorable for hydrocarbon charging andaccumulation [22,23].

assic Baikouquan Formation of the Mahu Sag was deposited.

Fig. 6. Lithological correlation and conditions of overlying, underlying, and side preservation of the Lower Triassic Baikouquan Formation in the Mahu Sag.

314 A. Imin et al. / Journal of Natural Gas Geoscience 1 (2016) 309e318

3.3. Hydrocarbon charging and evolution

There are several types of high-quality source rocks in theBaikouquan Formation (Fig. 2) with the best being theFengcheng rocks (Fig. 3). The Fengcheng source rocks aremost likely the ideal source rock for hydrocarbon generationin the study area as validated by the hydrocarbon geochemicalcharacteristics. Carbon isotope data for all crude oils in thestudy area have ad13C less than �28‰, indicating a typicaloilekerogen crude oil. Pr/Ph values are less than 1.3, thisreflects the reducing sedimentary in the environment in whichthe source rocks were deposited (Fig. 7a). In addition, thegammacerane indices of all the crude oils are greater than 0.2,indicating that the oil is from saline (alkaline) lacustrinesource rocks. Tricyclic terpanes C20, C21, and C23 are mainlyin increasing proportions. All these characteristics are indic-ative of the Fengcheng source rocks in the study area [9,10].Isotopic analyses show that the natural gasses are mainly oil-type gas (Fig. 7b), corresponding to Fengcheng source rocks[9,10,24]. Therefore, crude oil and natural gas of the Bai-kouquan reservoir in the Mahu Sag are mainly derived fromthe Fengcheng source rocks.

The Fengcheng source rocks had two hydrocarbon gener-ation peaks: Late Triassic and Early Cretaceous (Fig. 8). The

Fig. 7. Hydrocarbon geochemistry of the Lower

two-stage oil and gas charging process affected the Baikou-quan frontal facies. This hydrocarbon charging and evolution,as well as three-dimensional sealing brought by the denseglutenites and mudstones, good overlying and underlyingpreservation, and ideal storage conditions (Fig. 6) all led to theenrichment and accumulation of hydrocarbons. As shown inFig. 8, the two hydrocarbon expulsion stages were in theTriassic to the Early Jurassic and the Early Cretaceous [9,10].The first stage was mainly continuous mature oil charging,with bitumen being involved leaving yellow fluorescent-freehydrocarbon inclusions in the reservoirs. In contrast, the sec-ond stage was mainly charging of highly mature oil, repre-sented by blue and white fluorescent hydrocarbon inclusions.Combined with studies of the reservoir evolution, it can beinferred that the Baikouquan Formation was buried in ashallow area (0.5e1.0 km) during the first oil charging stage(Triassic to Early Jurassic). This resulted in a weak compac-tion and preservation of good reservoir properties (porosity of20%). The Baikouquan reservoir was then buried deeply(3e4 km) during the second oil charging stage (Early Creta-ceous). The compaction then became more striking with aresultant deterioration of reservoir properties (porosity of10%), therefore, the oil and gas accumulated in the favorablefrontal facies reservoir. These essential reservoir physical

Triassic Baikouquan Formation, Mahu Sag.

Fig. 8. Two-stage hydrocarbon generation and charging in the Lower Triassic Baikouquan Formation, Mahu Sag.

315A. Imin et al. / Journal of Natural Gas Geoscience 1 (2016) 309e318

properties resulted in highly mature oil accumulating in a largearea of the slope belt. The threshold porosity for charging ofintermediate and light oils into the Baikouquan Formation is10.7% and 7.7%, respectively. Therefore, the oil and gas(mature and highly mature) accumulated in two stages whilebeing in favorable reservoirs.

In summary, the Baikouquan “continuous hydrocarbonplays” in the Mahu Sag was formed due to three factors: high-quality and abundant source rocks, good reservoir quality andseals, and well-matched hydrocarbon charging and evolution.

4. Regularities of hydrocarbon accumulation and factorsinfluencing high hydrocarbon production

4.1. Regularities of hydrocarbon accumulation

Through comparison with similar hydrocarbon plays inChina and internationally, there are three favorable charac-teristics that can be identified for the development of“continuous hydrocarbon plays” distal from source rocks inthe Mahu Sag of the Junggar Basin. Firstly, high-qualitylacustrine source rocks provide a good hydrocarbon source.Secondly, large-scale strike-slip faults connect the source andreservoir rocks allowing long-distance hydrocarbon migration.Thirdly, the fan-delta sedimentary system provides goodconditions for reservoir formation and continuous hydrocar-bon accumulation. The Mahu Sag has the greatest hydrocar-bon generation potential in the Junggar Basin (Fig. 9). Itshigh-quality source rocks are mainly Permian (as well asCarboniferous) in age. Permian sedimentary rocks havecharacteristics typical of deposition in a foreland basin,including the Lower Permian Jiamuhe and Fengcheng For-mations and middle Permian Wuerhe Formation. These threesets of Permian source rocks, along with the other Carbonif-erous age, are thick and widely distributed. They also gener-ated high amounts of oil and gas, providing the basis forhydrocarbon accumulation. The Fengcheng lacustrine sourcerocks are the most significant of these four sets of source rocks

(Fig. 3). After the oil and gas expulsion from the source rocks,they migrate upwards along the NW to NWW-trending strike-slip faults. These faults are steeply dipping and typicallyunconformable, they're overlain by Triassic rocks or have onlyexperienced small amounts of later activity (Fig. 4) [17,25].The faults extend down to the high-quality source rocks(Fengcheng) and extend upwards to the Triassic unconformity.The faults are sealed by thick Triassic mudstones, which forma three-dimensional oil and gas migration and trapping system(Fig. 4). Given that these faults intersect, the relatively high-quality Baikouquan fan-delta reservoir rocks have beenfavorable sites for hydrocarbon accumulation.

The Baikouquan glutenite reservoir has a thickness ranging50 and 100 m, its porosity is between 5% and 12%, and the oilzone thickness is between 10 and 50 m. The main types ofreservoir space are primary grain and secondary solutionporosity (feldspar), with minor clay shrinkage porosity andmicro-cracks. Organic-rich acidic waters produced in the un-derlying Permian hydrocarbon source rocks resulted in upwarddissolution along the cracks and unconformities that was themain factor in generating secondary porosity. The secondaryporosity constitutes a complex reservoir space in addition tothe primary porosity and cracks. The primary porosity featuresof the reservoir are controlled by the six fan bodies around thesag during its formation, which supplied abundant terrigenousdetrital material along a gentle sediment slope allowing a sandbody to extend into the lake center. These provided favorableconditions for the formation of this oil and gas “continuous”plays.

4.2. Factors influencing hydrocarbon accumulation andhigh production

Hydrocarbon accumulation is controlled by three factors: afavorable sedimentary facies, structural highs, and faults.Abnormal reservoir over pressuring and fracturing has had animportant influence on hydrocarbon production. These threefactors are discussed in detail below:

Fig. 9. Geological cross-section showing petroleum migration and accumulation in the Lower Triassic Baikouquan Formation, Mahu Sag.

316 A. Imin et al. / Journal of Natural Gas Geoscience 1 (2016) 309e318

(1) Sedimentary facies: In the exploration area, the mostproductive reservoir area is the Xiazijie fan group, whichcan be further subdivided into two fan bodies (Xiazijie andFengnan fan bodies). Oil and gas are also found in otherfan bodies, including the Huangyangquan, Karamay, andMadongeXiayan fan groups. The frontal sub-facies ofeach fan body further control the degree of oil and gasenrichment. The Baikouquan reservoirs were formed inthe pro-delta and plain sedimentary facies environments.The fan delta and plain facies rocks are brown gluteniteand conglomerate, and for the pro-delta underwaterchannels are gray glutenite and conglomerate. The fandelta and plain facies rocks are poorly sorted with mixedaccumulations of sand, mud, and gravel including anargillaceous matrix, which is responsible for its relativelypoor reservoir properties. In contrast, the pro-delta un-derwater channel rocks formed in a more active hydro-dynamic setting, thus, these rocks have low clay contents;they are also well sorted and are relatively good reservoirmaterial. Therefore, the best reservoirs are the gray glu-tenites and conglomerates, an example would be the sub-facies of the deposition which controlled the quality ofoil and gas accumulation.

(2) The accumulation of oil and gas was also controlled by thedistribution of structural highs and faults. In general, oiland gas are located in the noses of the structures in the

study area. These noses are connected by faults that pro-vide favorable locations for the accumulation of migratedhydrocarbons [26]. Typically, structural highs are associ-ated with faults, which are important for hydrocarbonaccumulation and enrichment, especially considering thelong-distance migration that has taken place in this region.The faults are of great importance as they facilitate anupward migration of hydrocarbon generated by deepsource rocks to the reservoirs (Fig. 4). The northeasternmain faults controlled the distribution of the fan body anddense series of channels, and the east-west-striking sub-fault on the flank controlled the distribution of slopebreaks laterally. Thus, these faults controlled the distri-bution of pro-delta sand bodies and the subsequentdevelopment oil-bearing layers (Fig. 9).

(3) Reservoir over pressuring and fracturing also affect theproduction of oil and gas. In general, it is considered thatthese processes “open up” the reservoir, thus, allowinghigh production which is important for the tight contin-uous reservoirs [27e30]. The Baikouquan Formation inthe Mahu Sag is over pressured (pressure coefficient >1.3)and is favorable for high oil and gas flow rates. Core ob-servations show that the reservoir is highly fractured,particularly in brittle lithologies. These fractures alsoimprove the reservoir properties and enhance productionrates.

317A. Imin et al. / Journal of Natural Gas Geoscience 1 (2016) 309e318

In summary, hydrocarbon accumulation and enrichment inthe Baikouquan Formation of the Mahu Sag is controlled byfavorable sedimentary facies (pro-delta), structural highs, andfaults. High hydrocarbon production is further enhanced byreservoir over pressuring and fracturing. The heavy consider-ation of these factors should be used to guide future explora-tion of oil and gas in this region (Fig. 1). The favorable areasfor exploration were predicted (Fig. 1).

5. Conclusions

(1) Hydrocarbon exploration in the Mahu Sag of the north-western Junggar Basin, China for the first time identified a“continuous hydrocarbon play” distant from source rocks.The plays are extensive in area and they have a largelyproven capability to produce hydrocarbons.

(2) There are three main conditions that have contributed tothis hydrocarbon accumulation, namely, large hydrocarbonsource, good reservoir and seals, and balanced hydrocar-bon charging and evolution. High-quality lacustrine sourcerocks provide an abundant source of hydrocarbons. Large-scale strike-slip faults link the source and reservoir rocks,thus, facilitating long-distance hydrocarbon migration.The fanedelta sedimentary system deposited sedimentsare excellent reservoirs for hydrocarbon accumulation.

(3) Hydrocarbon accumulation is controlled by three factors:pro-delta sedimentary facies, structural highs, and faults.These factors should all be consideredwhen selecting futureexploration targets. In addition, reservoir over pressuringand fracturing enhance hydrocarbon production.

Foundation item

Supported by Projects of National Science and TechnologyMajor Project of China (2016ZX05001-005) and PetroChinaScience and Technology Major Project “Xinjiang's Daqing”(2012E-34-01).

Conflict of interest

The authors declare no conflict of interest.

Acknowledgments

We thank editors and anonymous reviewers for their valu-able reviews and comments, which improved the article.

References

[1] L.C. Kuang, Y. Tang, D.W. Lei, T. Wu, J.H. Qu, Exploration of fan-

controlled large-area lithologic oil reservoirs of Triassic Baikouquan

Formation in slope zone of Mahu depression in Junggar Basin, China Pet.

Explor. 19 (6) (2014) 14e23.

[2] D.W. Lei, I. Abulimit, Y. Tang, J. Chen, J. Cao, Controlling factors and

occurrence prediction of high oil-gas production zones in Lower Triassic

Baikouquan Formation of Mahu Sag in Juggar Basin, Xinjiang Pet. Geol.

35 (5) (2014) 495e499.

[3] J.W. Schmoker, Resource-assessment perspectives for unconventional

gas systems, AAPG Bull. 86 (11) (2002) 1993e1999.

[4] J.Z. Li, B.C. Guo, M. Zheng, T. Yang, Main types, geological features

and resource potential of tight sandstone gas in China, Nat. Gas. Geosci.

23 (4) (2012) 607e615.[5] C.Z. Jia, M. Zheng, Y.F. Zhang, Unconventional hydrocarbon resources

in China and the prospect of exploration and development, Pet. Explor.

Dev. 39 (2) (2012) 139e146.[6] C.N. Zou, S.Z. Tao, X.J. Yuan, R.K. Zhu, D.Z. Dong, W. Li, L. Wang,

X.H. Gao, Y.J. Gong, J.H. Jia, L.H. Hou, G.Y. Zhang, J.Z. Li, C.C. Xu,

H. Yang, Global importance of “continuous” petroleum reservoirs:

accumulation, distribution and evaluation, Pet. Explor. Dev. 36 (6) (2009)

669e682.

[7] C.N. Zou, Z. Yang, S.Z. Tao, X.J. Yuan, R.K. Zhu, L.H. Hou, S.T. Wu,

L. Sun, G.S. Zhang, B. Bai, L. Wang, X.H. Gao, Z.L. Pang, Continuous

hydrocarbon accumulation over a large area as a distinguishing charac-

teristic of unconventional petroleum: the Ordos Basin, North-Central

China, Earth Sci. Rev. 126 (2013) 358e369.

[8] S.A. Sonnerberg, A. Pramudito, Petroleum geology of the giant Elm

Coulee field, Williston Basin, AAPG Bull. 93 (9) (2009) 1127e1153.

[9] X.L. Wang, S.F. Kang, Analysis of crude origin in hinterland and slope of

northwest margin, Junggar Basin, Xinjiang Pet. Geol. 20 (2) (1999)

108e112.[10] J. Cao, Y.J. Zhang, W.X. Hu, S.P. Yao, X.L. Wang, Y.Q. Zhang, Y. Tang,

The Permian hybrid petroleum system in the northwest margin of the

Junggar Basin, northwest China, Mar. Pet. Geol. 22 (3) (2005) 331e349.

[11] B.E. Law, Basin-centered gas systems, AAPG Bull. 86 (11) (2002)

1891e1919.

[12] K.A. Bowker, Barnett Shale gas production, Fort Worth Basin: issues and

discussion, AAPG Bull. 91 (4) (2007) 523e533.[13] L.C. Kuang, Y. Tang, D.W. Lei, Q.S. Chang, M. Ouyang, L.H. Hou,

D.G. Liu, Formation conditions and exploration potential of tight oil in

the Permian saline lacustrine dolomitic rock, Junggar Basin, NW China,

Pet. Explor. Dev. 39 (6) (2012) 700e711.[14] M.R. Mello, J.R. Maxwell, Organic geochemical and biological marker

characterization of source rocks and oils derived from lacustrine envi-

ronments in the Brazilian continental margin, in: B.J. Katz (Ed.),

Lacustrine Basin Exploration: Case Studies and Modern Analogs,

AAPG, Tulsa, 1990, pp. 77e97.

[15] B. Horsfield, D.J. Curry, K. Bohacs, R. Littke, J. Rullk€otter, H.H. Schenk,

M. Radke, R.G. Schaefer, A.R. Carroll, G. Isaksen, E.G. Witte, Organic

geochemistry of freshwater and alkaline lacustrine sediments in the

Green River Formation of the Washakie Basin, Wyoming, USA, Org.

Geochem. 22 (3e5) (1994) 415e440.

[16] K.Y. Wu, J.H. Qu, H.H. Wang, Trike-slip characteristics, forming

mechanisms and controlling reservoirs of Dazhuluogou fault in Junggar

Basin, J. China Univ. Pet. Ed. Nat. Sci. 38 (5) (2014) 41e47.

[17] Y. Shao, R.F. Wang, Y.Q. Zhang, X. Wang, Z.H. Li, H. Liang, Strike-slip

structures and oil-gas exploration in the NW margin of the Junggar

Basin, China, Acta Pet. Sin. 32 (6) (2011) 976e984.

[18] G. Yang, X.B. Wang, B.L. Li, X. Shi, X. Li, S.W. Guan, Oblique

compressional structures and hydrocarbon distribution patterns in the

northwestern margin of the Junggar Basin, Oil Gas Geol. 30 (1) (2009)

26e32.

[19] Z.Y. Lei, D.Z. Bian, S.K. Du, Y.J. Wei, H.S. Ma, Characteristics of fan

forming and oil-gas distribution in west-north margin of Junggar Basin,

Acta Pet. Sin. 26 (1) (2005) 8e12.

[20] Y.J. Wei, D.S. Li, S.Y. Hu, Z.Y. Lei, D.F. He, Fans sedimentation and

exploration direction of fan hydrocarbon reservoirs in foreland thrust belt

of the northwestern Junggar Basin, Acta Geosci. Sin. 28 (1) (2007) 62e71.[21] Y. Tang, Y. Xu, J.H. Qu, X.C. Meng, Z.W. Zou, Fan-delta group char-

acteristics and its distribution of the Triassic Baikouquan reservoirs in

Mahu Sag of Junggar Basin, Xinjiang Pet. Geol. 35 (6) (2014) 628e635.

[22] Z. Cao, G.D. Liu, Z.X.X. Liu, Y.F. Yuan, P. Wang, Z.C. Niu, J.Y. Zhang,

Research status on tight oils and its prospects, Nat. Gas Geosci. 25 (10)

(2014) 1499e1508.

[23] W.W. Yang, G.D. Liu, X.Y. Liu, Y. Feng, Y.G. Du, D.X. Cheng, The

accumulation mechanism and accumulation models of oil in low

318 A. Imin et al. / Journal of Natural Gas Geoscience 1 (2016) 309e318

permeability reservoirs in Yangchang Formation in Longdong area,

Ordos Basin, Earth Sci. Front. 20 (2) (2013) 132e139.

[24] J.X. Dai, S.F. Qin, S.Z. Tao, G.Y. Zhu, J.K. Mi, Developing trends of

natural gas industry and the significant progress on natural gas geological

theories in China, Nat. Gas Geosci. 16 (2) (2005) 127e142.[25] Z.X. Cai, F.J. Chen, Types and tectonics evolution of Junggar Basin,

Earth Sci. Front. 7 (4) (2000) 431e440.

[26] Y. Xin, W.F. Wang, K.Y. Wu, Oil-gas plays characteristics analysis of

Wu-Xia fault belt, the northwestern margin in Junggar Basin, J. China

Univ. Pet. Ed. Nat. Sci. 35 (2) (2011) 32e38.

[27] C.Z. Jia, C.N. Zou, J.Z. Li, D.H. Li, M. Zheng, Assessment criteria, main

types, basic features and resource prospect of the tight oil in China, Acta

Pet. Sin. 33 (3) (2012) 343e350.

[28] F. Hao, S.T. Li, Y.C. Sun, Q.M. Zhang, Characteristics and origin of the

gas and condensate in the Yinggehai Basin, offshore South China Sea:

evidence for effects of overpressure on petroleum generation and

migration, Org. Geochem. 24 (3) (1996) 363e375.

[29] Z. Yang, J.H. Wang, S.H. Lin, S.T. Wu, Y. Liu, Q.Y. Li, P.Z. Zhang,

Hydrocarbon accumulation mechanism near top overpressured surface in

central Junggar Basin, J. China Univ. Pet. Ed. Nat. Sci. 35 (3) (2011)

19e25.[30] G. Fu, Y.C. Xue, Role of abnormal high pressure in the formation and

preservation of oil(gas) reservoirs, Xinjiang Pet. Geol. 20 (5) (1999)

379e382.