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Journal of Natural Gas Science and Engineering 29 (2016) 101e109

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Journal of Natural Gas Science and Engineering

journal homepage: www.elsevier .com/locate/ jngse

Formation and accumulation of lower Jurassic tight gas sands field inKekeya area of Tuha Basin, northwestern China

Shuangbiao Han a, b, c, *, 1, Jinchuan Zhang a, b, Yuqi Zhou d, Songtao Bai e,Longxing Huang e, Chengshan Wang a, Weidong Huang f

a China University of Geosciences, Beijing 100083, Chinab Key Laboratory of Shale Gas Exploration and Evaluation, Ministry of Land and Resources, Beijing 100083, Chinac Key Laboratory of Tectonics and Petroleum Resources (China University of Geosciences), Ministry of Education, Wuhan 430074, Chinad ConocoPhillips School of Geology and Geophysics, University of Oklahoma, Norman, OK 77069, USAe China Petroleum Logging CO. LTD., Xi'an 710077, Chinaf Research Institute of Exploration and Development, Tuha Oilfield Company, CNPC, Hami 839009, China

a r t i c l e i n f o

Article history:Received 15 September 2015Received in revised form16 November 2015Accepted 28 December 2015Available online 31 December 2015

Keywords:Tuha foreland basinKekeya thrust fold beltLower jurassicTight gas sandAccumulationCoal-bearing strata

* Corresponding author. China University of GeosciE-mail address: bjcuphan@163.com (S. Han).

1 Shuangbiao Han obtained a Ph.D. in petroleum gof Geosciences Beijing in 2014. Now he is a Postdoctgas sand.

http://dx.doi.org/10.1016/j.jngse.2015.12.0461875-5100/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

The lower Jurassic in Kekeya area of Tuha Basin contains great tight gas sand resources. This study revealsthe geologic controls on the formation and accumulation of tight gas sand reservoirs in Kekeya areacomprehensively and systematically, based on tectonic evolution, depositional setting, source rockcharacteristics, reservoir properties, charging-accumulation history and mechanisms. Our study revealsthat tectonics and depositional systems play important roles in the tight gas sand petroleum system.Located at the thrust fold belt of Tuha Foreland Basin with continuous stratigraphic succession of braidedfluvial e deltaic e lacustrine depositional systems, the lower Jurassic in Kekeya area contains favorablepetroleum geologic conditions for the tight gas sands field. The source rocks contain abundant organicmatters dominated by gas-prone type III kerogens and started generating primarily gas in early burialstage. The tight sand reservoirs are mainly composed of medium to coarse grained feldspathic lithar-enites with primary reservoir space as remnant interparticle pores, solution enlarged intraparticle poresas well as micro-fractures. The more active burial compactions and chemical diagenesis in coal-bearingstratigraphy along with the fast subsidence rate in Tuha Basin are responsible for the tightness of thesand reservoir. The favorable source e reservoir e seal arrangement along with the right timing of hy-drocarbon generation, the developing of overpressure in source rocks as well as faulting and fracturingcontribute to the widespread accumulation of tight gas sands in lower Jurassic in Kekeya area. Thefindings in this study would provide reference and guide for future exploration and development of tightgas sand fields in this area and in other basins.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Tight gas sands refer to lowporosity and permeability sandstonereservoirs that primarily produce dry natural gas. It is treated asreservoir in the sense of poor quality by conventional natural gasstandards. The tight gas sand play was originally named as deepbasin gas accumulation (Masters, 1979) and was later named as

ences, Beijing 100083, China.

eology from China Universityoral Fellow focusing on tight

basin-center gas (Law, 2002), source-contacting gas reservoir(Zhang et al., 2004a) and continuous gas reservoir (Zou et al., 2009)etc., as the understanding on the geological aspects of such gasreservoir has advanced with time. The formation of tight gas sandreservoirs requires relatively quiescent tectonics and the depositionof regionally continuous tight sandstones. Later tectonic alterations,such as faulting and fracturing would enhance reservoir quality andprovide pathways for gas tomigrate and accumulate at sweet zonesin tight sands. Good sealing capacity of the seal rocks is alsoessential respecting the preservation of the tight gas sand reservoir.

Since 1960, the proven recoverable reserves of tight gas sandshave reached to the point that is quite close to the global under-developed recoverable reserves of conventional natural gas(Kazemi, 1982; Law, 2002; Schmoker, 2002; Zhang, 2006).

S. Han et al. / Journal of Natural Gas Science and Engineering 29 (2016) 101e109102

Considering the continuously declining conventional natural gasreserves, exploration and development of tight gas sands is crucialto ease energy shortage (Wang, 1993; Zhang et al., 2004b). Ac-cording to statistics, tight gas sands develop in more than 70 basinsmainly distributed in North America, Europe and Asian-Pacific re-gion (Schmoker, 2002; Rose and Everett, 1984; Spencer, 1985; Zhaoet al., 2014). Among these, large-scale tight gas sand fields aremainly located in the United States and Canada (Masters, 1979; Fallet al., 2012). Substantial amount of tight gas sand fields also exist inChina (Zhang et al., 2000; Zha and He, 2003; Dong et al., 2007),where the majority of them are distributed in Sichuan Basin, OrdosBasin, Tuha Basin, Songliao Basin and Southern Junggar Basin (Tianand Luo, 2004; Yang et al., 2008; Han et al., 2014). According tonational resource evaluation, tight gas sands approximately ac-count for 40% of onshore natural gas resource potential in China(Ning et al., 2009). Recently, tight gas sands have become a majorcontribution to proven natural gas reserves in China.

Most tight gas sand reservoirs occur in relatively deeper parts ofbasins with usually continuous gas accumulation occurringdowndip from water-bearing rocks (Law, 2002). The depth of tightgas sand reservoirs ranges from a few hundred meters to severalkilometers, e.g. 900 me4500 m in Alberta basin (Masters, 1979),2400 me6100 m in Green River basin (Law and Spencer, 1981) and1600 me2100 m in San Juan basin (Spencer, 1985). Due to thecomplex geology of sedimentary basins in China, conditions for theformation of tight gas sands may not be similar as that of NorthAmerica. Thus, the current strategy is exploring and developingtight gas sands, especially the sweet spots, first in coal-bearingbasins with regionally continuous coal-beds and tight sands. InChina, upper Paleozoic tight gas sand reservoirs of Ordos Basinoccur from 2100 m to 3800 m (Zhao et al., 2014), while upperTriassic tight gas sand reservoirs in western Sichuan Basin occurfrom 2000 m to 5000 m (Zhang et al., 2000).

Tuha Basin is a coal-bearing basin, with abundant tight gas sandreserves generated from coal-bearing source rocks. The Kekeya areaof Tuha Basin, with favorable petroleum geologic conditions, ownsa series of tight gas sand characters such as sufficient source of gas,tight sand reservoirs, formation of overpressure, and widespreadgas accumulations. High industrial gas flow had been firstly ach-ieved from lower Jurassic tight sand formation in well Ke19 locatedin Kekeya area, confirming tight gas sand exploration concept inthis region. Nowadays, Kekeya area is one of the exploration hot-spots in Tuha basin (Wang, 2010; Zheng et al., 2010). Currently, themajor producing intervals of lower Jurassic tight gas sands inKekeya area are Badaowan-1 Member (J1b1) and Badaowan-2Member (J1b2). The gas pay zone is more than 60 m with highsingle-well production rate. The produced gas is primarily dry with75%e85% methane.

The gas accumulation mechanism of tight gas sands is differentfrom that of conventional natural gas, which requires more so-phisticated trapping conditions as well as temporal and spatialmatching of the petroleum system elements (Bachu, 1999; Zhangand Zhang, 2001; Jiang et al., 2006; Dai et al., 2012). This paperattempts to evaluate tight gas sand in Kekeya area of Tuha basincomprehensively and systematically based on tectonic evolution,depositional setting, source rock characteristics, reservoir proper-ties, charging-accumulation history and mechanisms, which wouldprovide reference and guide for tight gas sand exploration anddevelopment for Tuha Basin and other basins.

2. Geological background

2.1. Tectonics

Located in eastern Xinjiang Province in China, Tuha basin is an

East-West trending inter-mountain continental sedimentary basin(Zhai, 1986) mainly consisted of Mesozoic and Cenozoic formations,which were deposited on Permian basement. It is also one of themain petroliferous basins in northwestern China with an area of4.8 � 104 km2 (Zhang, 2000). Kekeya area is located at the thrustfold belt in Bogeda Mountain Front of northern Taibei Depression,Tuha basin (Fig. 1). Kekeya area has developed as a thrust faultfoldsince Yanshan Orogeny (Late Triassic to Cretaceous), and thus is agood location for hydrocarbon migration and accumulation (Li,2001).

Kekeya Structural Belt develops on arcuate thrust folds. It is asteep and narrow thrust anticline with a length of 14.5 km and awidth of 1.0 km. The structural relief is 1.0 kmwith approximate dipangle of 40� on two sides. Thrust faults develop following trends ofNW and NE. The largest faults strike NW, which is consistent withthe strike of the axis of the thrust anticline (Fig. 1).

Before middle Jurassic, the Kekeya area of Taibei depressionwasa huge rifting zone. The subsidence center is located at the hangingwall of the regional normal fault and received thick layer of sedi-ments, while relative thinner layer of sediments was deposited onthe foot wall of the fault (Yuan et al., 2001). After that, a series oflower Jurassic strata were deposited in braided fluvial, deltaic andlacustrine environments (Wu et al., 1994). Sincemiddle Jurassic, thesubsidence of Taibei foreland depression started in response to thecontinuing uplifting of Bogeda Mountain north to the basin,spreading the most area of the southern basin. Before the deposi-tion of Qiketai Formation (J2q), Tuha basin was a pseudo-forelandbasin, whose major structure and tectonic evolution werecontrolled by the rising of Bogeda Mountain (Tao, 2010). In thesame time, the Kekeya Structural Belt started to be inverted intothrust fault-fold belt. (Fig. 2). By the end of middle YanshanOrogeny (J2q), the major structures of Bogeda Mountain Front andKekeya Structural Belt had been formed (Fig. 2). During late Yan-shan Orogeny and Himalayan Orogeny (Cretaceous to Eocene), aseries of near NW and NE trending overturned thrust faults weredeveloped in Bogeda Mountain Front with the formation of struc-tural traps such as faulted anticline and fault-block due to thecontinuous rising of Bogeda Mountain (Wu et al., 1996). KekeyaStructural Belt has been greatly inverted into thrust fault-anticline(Fig. 2).

2.2. Stratigraphy and facies

The Jurassic strata in Tuha Basin, with an average thickness ofmore than 2 km, are mainly composed of sandstones, shales andcoals deposited in braided fluvial, deltaic and lacustrine environ-ments (Wu et al., 1994), which contain abundant oil, gas and coalresources (Li, 2001). The tight gas sands in Kekeya area primarilyoccur in middle-lower Jurassic Shuixigou Group (Yuan et al., 2001),which could be divided to Badaowan Formation (J1b), SangongheFormation (J1s) and Xishanyao Formation (J2x) respectively frombottom up. Badaowan Formation (J1b) is comprised of interbeds ofgray sandstones, pebble sandstones and dark-gray shales. San-gonghe Formation (J1s) consists of interbeds of light-gray pebblesandstone, medium-grained sandstone and gray shale. XishanyaoFormation (J2x) is composed of interbeds of light-gray fine-grainedsandstone and shale with multiple interlayers of thin coal seam.This study focuses on lower Jurassic Badaowan Formation (J1b) andSangonghe Formation (J1s), which are comprised of interbeddedsandstones, siltstones, shales and coals (Fig. 3).

The lower Jurassic of Shuixigou Group (J1b e J1s) in Kekeya areais dominated by fining upward cycles based on the observations oncores of tight gas sand reservoir intervals and cuttings from wellsdrilled in this area. The depositional environment is braided riverdelta and inland lake, with major facies as delta plain coals, stacked

Fig. 1. The location and structure map of Kekeya Area in Tuha Basin, China (AeB: Tectonic evolution cross-section).

S. Han et al. / Journal of Natural Gas Science and Engineering 29 (2016) 101e109 103

distributary channel sands and mouth bars, interdistributary bayfine-grained deposits, lacustrine shales. The Badaowan Formation(J1b) is divided into three members as Ba-3 (J1b3), Ba-2 (J1b2) andBa-1 (J1b1) from bottom up, based on the cyclicity of lithofacies andthe producing tight gas sand reservoir intervals. Ba-3 (J1b3) startswith thin to medium bedded delta front sands (pebble channel

Fig. 2. Tectonic evolution in Kekeya area (J: Jurassic; J1: lower Jurassic; J1b: Badaowan ForSanjianfang Formation; J2q: Qiketai Formation; Esh: Eocene Shanshan Group).

sands and mouth bar sands) followed by very thin to mediumbedded lacustrine argillaceous siltstones and pro-delta shales in-terbeds. Ba-2 (J1b2) starts with medium to thick bedded mouth barsands followed by regionally extensive lacustrine shales. Ba-1 (J1b1)starts with thin tomedium bedded distal mouth bar sands followedby regionally extensive coal beds. Sangonghe Formation (J1s) is

mation; J1s: Sangonghe Formation; J2: middle Jurassic; J2x: Xishanyao Formation; J2s:

Fig. 3. General stratigraphy in Kekeya area with showing principal hydrocarbonsources and reservoir horizons.

S. Han et al. / Journal of Natural Gas Science and Engineering 29 (2016) 101e109104

composed of very thin to medium bedded distal mouth bar sands,interdistributary bay siltstones, shales and coals. Overall, similardepositional systems of lower Jurassic in Kekeya area stack verti-cally and extend laterally in a regional extent. The reservoir sand-stones extend in a wide area with great bulk thickness. As theprovenance is to the north, the strata get thinner and pinch outtowards the south (downdip). Meanwhile, sandstones are lessabundant and become thinner and discrete towards the top oflower Jurassic.

3. Source rock characterization

Lower Jurassic coals and shales are major source rocks for tightgas sand reservoirs in Kekeya area. Eighty (80) lower Jurassicsamples from coals and shales of seven wells in Kekeya area werecrushed into powders and went through routine petroleumgeochemical laboratory analyses including extraction of organicmatters, elemental analysis and Rock Eval pyrolysis (Espitalie et al.,1977; Peters, 1986). The laboratory results were then used toevaluate the hydrocarbon potential, i.e. the type, abundance andmaturity of the organic matters within the source rocks.

The hydrogen-carbon ratios (H/C) and oxygen-carbon ratios (O/C) from elemental analysis were cross-plotted into Van Krevelendiagram (Fig. 4), which suggests the majority of the lower Jurassiccoals and shales in Kekeya area are dominated by typeⅢ gas-pronekerogens. The abundance of organic matters in coals is very highwith total organic carbon content (TOC) ranging from 57% to 71%,chloroform bitumen “A” varying from 1% to 3% and total hydro-carbon content ranging from 4785 ppm to 13,171 ppm. The sum offirst two peaks from Rock Eval pyrolysis represents the hydrocar-bon generation potential (S1þS2) and ranges from 128 mg/g to191 mg/g. The organic matter abundance in black shales is highwith TOC ranging from 0.8% to 2.4%, chloroform bitumen “A”varying from 0.2% to 0.3% and total hydrocarbon content rangingfrom823 ppm to 1451 ppm. The sum of (S1þS2) ranges from8.9mg/g to 28.5 mg/g. For both coals and black shales, vitrinite reflectance(Ro) is in the range of 0.8%e1.3% and Tmax from Rock Eval pyrolysisis in the range of 420 �Ce470 �C, suggesting that lower Jurassic

Fig. 4. Van Krevelen diagram of lower Jurassic source rocks (H/C: HydrogeneCarbonRatio; O/C: OxygeneCarbon Ratio).

S. Han et al. / Journal of Natural Gas Science and Engineering 29 (2016) 101e109 105

source rocks are mature to highly mature and have been in the gasgeneration window.

Based on the laboratory results, the lower Jurassic source rocksin Kekeya area contain abundant organic matters with high hy-drocarbon generation potential. The organic matters of the sourcerocks are dominated by gas-prone type III kerogens and arecurrently generating gas with relatively high maturities. Previousstudies on Alberta Basin, San Juan Basin, Ordos Basin and TuhaBasin show tight gas sands usually occur in coal-bearing stratig-raphy because the onset of generation and expulsion of hydrocar-bons from coals requires relatively lower threshold of maturity andtherefore occur relatively earlier in basin burial history. Earlystudies (Cheng et al., 1997; Hu and Wu, 1997) on the maturationhistory of coal-bearing strata in lower Jurassic of Tuha Basin showbimodal distribution of hydrocarbon transformation ratio in its plotagainst vitrinite reflectance (Ro). The bi-peaks are located at thevitrinite reflectance (Ro) of 0.4%e0.6% and 1.0%e1.2%. The LowerJurassic coal-bearing source rocks have already been in the first gasgeneration peak and are transitioning to the second gas generationpeak. In sum, the gas generation potential of lower Jurassic sourcerocks is very high. The sourcing of gas for tight sand reservoirs issufficient.

4. Reservoir properties

A hundred and ninety (190) thin-sections of lower Jurassic tightreservoir sandstones were obtained from cores of seven wells inthis study. The constituent detrital grains were point-counted todetermine the lithology of the samples. Forty (40) core plugs fromfour producing wells were obtained for porosity and permeabilitymeasurements.

Fig. 5. Thin-section photographs of lower Jurassic tight sand reservoirs in Kekeya area (QFracture).

Based on petrographic studies, lower Jurassic tight sand reser-voirs are composed of poorly sorted, medium to coarse grainedfeldspathic litharenites and litharenites. The primary pore space offeldspathic litharenites was destructed by the growth of calcitemicrite, quartz overgrowth and burial compaction. For litharenites,the percentage of rock fragments is 50%. The interstitial spaceconstitutes 25e35% of the rock and is filled with mud matrix andcalcite micrite. The weathering of sediments was moderate,resulting in both low compositional maturity and low texturalmaturity with poor sorting and subangular roundness. Reduction ofprimary interparticle pore space is common due to great burialcompaction, by means of concavo-convex to long grain contact,quartz overgrowth, deformed rock fragments and matrix-supportframework (Zhang et al., 2009; Xu et al., 2015). The primary typesof reservoir space are the remnant interparticle pores, solutionenlarged intraparticle pores and natural micro-fractures. (Fig. 5,Fig. 6).

Measurements of porosity and permeability suggest that lowerJurassic sand reservoirs in Kekeya area are very tight. The porosityis below 9% with an average of 6%. The majority of permeabilityvalues are ranging from 0.1 mD to 0.5 mD with a mean of 0.3 mD(Fig. 7). Overall, the reservoir quality is considered poor by con-ventional standards. The sands are tighter as burial depth increases.Within strata, the distribution of porosity and permeability iscontrolled by regional structures and depositional settings, that is,the porosity and permeability increase updip and decrease down-dip. The succession of lower Jurassic depositional systems is stablewith both vertically and laterally continuous depositional processesand facies, which are favorable for the formation of regional tightsand reservoirs.

The tightness of the reservoir in Kekeya area is largely controlled

e Quartz; F e Feldspar; BP e Interparticle Pore; IP e Intraparticle Pore; Fr e Micro-

Fig. 6. Micro-fractures in lower Jurassic tight sand reservoirs in Kekeya area.

S. Han et al. / Journal of Natural Gas Science and Engineering 29 (2016) 101e109106

by burial history and diagenesis. Lower Jurassic sands containabundant plastic rock fragments such as phyllites and argillites,which were greatly deformed and flew to primary pore spaces inresponse to burial compactions, resulting in the reduction of pri-mary pore space. Moreover, physical-chemical diagenesis in coal-bearing formation is much more active, especially as the mechan-ical crushing and the chemical solution of the grains (Zhang, 2008).In late burial stage, silica and carbonate cements developed be-tween detrital grains, resulting in the great loss of porosity andpermeability of the reservoir (Leder and Park, 1986; Scherer, 1987).

5. Source e reservoir e seal arrangement

The lower Jurassic tight gas sand fields in Kekeya area arelocated on the Bogeda Mountain Front Thrust Fold Belt (Fig. 1).Badaowan Formation (J1b) contains three thick regionally extensivetight sand reservoirs in Ba-3 (J1b3), Ba-2 (J1b2) and Ba-1 (J1b1)respectively. Sangonghe Formation (J1s) contains relatively thin anddiscrete tight sand reservoirs (Fig. 8). The thick coals and shalesbetween J1b3, J1b2, J1b1 and J1s are efficient source rocks providingprimarily gas for tight sand reservoirs. In the meantime, thesesource rocks provide good sealing capacity. The braided fluvial edeltaic e lacustrine stratigraphic succession of lower Jurassic pro-vides favorable source e reservoir e seal arrangement for tight gassand accumulations.

6. Charging and accumulation

The lower Jurassic tight gas sand reservoirs were charged and

Fig. 7. Porosity and permeability distribution of low

accumulated in two stages. With high subsidence rate in Kekeyaarea, by late Jurassic, lower Jurassic strata had been buried at depthsgreater than 3 km (Fig 2). Lower Jurassic sand reservoir had beentightly compacted and cemented, while the solution of the detritalgrains started and secondary intraparticle pores began to develop.During middle Yanshan Orogeny, lower Jurassic source rocks haveentered the hydrocarbon generation window and were generatingprimarily dry gas. The migration pathways, such as faults andfractures related with the thrusting, had not developed at thisperiod. Therefore, the hydrocarbons generated from the sourcerocks could only be expulsed when the expansion force from hy-drocarbon generation was larger enough to overcome the capillarypressure in the pore network. Owing to the frequent interbeddedsource rock and reservoir arrangement (Fig. 8), the hydrocarbonsgenerated at this time were able to be charged into the reservoirsabove or below the source rocks. The hydrocarbons were expulsedand charged to the tight sand reservoirs by pulses in a manner likepiston.

During late Yanshan Orogeny and Himalayan Orogeny, with thecontinuous rising of Bogeda Mountain, the northern part of TaibeiDepression has been inverted by thrust folding and faulting andformed the anticlinal Kekeya Structural Belt (Fig 2). Governed byregional and local structural stress fields, a series of NW and NEtrending faults and fractures were developed on the thrust fold belt(Figs. 1 and 6), which have been migration pathways betweensource rocks and tight sand reservoirs. In the meantime, TaibeiDepression to the south of Kekeya Structural Belt has been sub-siding faster and the Jurassic strata have been continuously buriedto deeper depths. The middle Jurassic source rocks of Shuixigou

er Jurassic tight sand reservoirs in Kekeya area.

Fig. 8. Stratigraphic cross-section showing the lower Jurassic tight gas sand reservoir units in Kekeya area.

S. Han et al. / Journal of Natural Gas Science and Engineering 29 (2016) 101e109 107

Group (J2x) have also entered the hydrocarbon generation window.The source rocks of the entire Shuixigou Group have been gener-ating primarily dry gas consecutively. The extra large amount ofgenerated hydrocarbons increased the expansion force withinsource rocks along with deep burial depth, which has led to theprevailing overpressure in lower Jurassic source rocks (Fig. 9).

Previous studies by Law and Spencer (1981) show overpressure

Fig. 9. Sonic transient time log and depth plot from

in source rocks is common in tight gas sand basins and gas-driving-water is the unique gas accumulation mechanism in tight sands.The gas accumulation would shrink when gas-charging is insuffi-cient or stops. Only with sufficient and continuous gas charging thereservoir, could the tight gas sand accumulations being preserved.The overpressure in lower Jurassic source rocks provides contin-uous piston forces (Law and Spencer, 1981; Spencer and Law, 1981;

shales and coals in Well Ke19, Ke21 and Ke191.

Fig. 10. Structural cross-section of lower Jurassic tight gas sand system in Kekeya area.

S. Han et al. / Journal of Natural Gas Science and Engineering 29 (2016) 101e109108

Zhang, 2006) for hydrocarbons expulsion and migration to sweetspots in the tight sands via faults and fractures. The favorablesource e reservoir e seal arrangement along with the right timingof hydrocarbon generation, the developing of overpressure insource rocks as well as faulting and fracturing contribute to thewidespread accumulation of tight gas sands in lower Jurassic inKekeya area (Fig. 10).

7. Conclusions

(1) Kekeya Structural Belt of the inter-mountain Tuha ForelandBasin is a thrust faulted fold in Bogeda Mountain Front.Lower Jurassic Badaowan (J1b) and Sangonghe (J1s) Forma-tions were deposited in braided fluvial e deltaic - lacustrinesystems with the major provenance to the north.

(2) The lower Jurassic source rocks are consisted of coals andblack shales dominated by gas-prone type Ⅲ kerogens. Thehigh abundance of organic matters along with high matu-rities provide sufficient source of gas to the tight gas sandpetroleum system. The overpressure within source rocksenables piston type charging for tight gas sand.

(3) The tight sand reservoirs are mainly composed of medium tocoarse grained feldspathic litharenites with primary reser-voir space as remnant interparticle pores, solution enlargedintraparticle pores as well as micro-fractures. The reservoir istight with matrix porosity below 9% and average perme-ability as 0.3 mD. The more active burial compactions andchemical diagenesis in coal-bearing stratigraphy along withthe fast subsidence rate in Tuha Basin are responsible for thetightness of sand reservoir.

(4) The favorable source e reservoir e seal arrangement alongwith the right timing of hydrocarbon generation, the devel-oping of overpressure in source rocks as well as faulting andfracturing contribute to the widespread accumulation oftight gas sands in lower Jurassic in Kekeya area.

Acknowledgments

Our research is supported by Key Laboratory of Tectonics andPetroleum Resources, Ministry of Education (Grant No. TPR-2015-02), China Universities Research Foundation (Grant No. 2-9-2013-

144), National Natural Science Foundation of China (Grant Nos.41272167 and 41102088) and Beijing Municipal Science & Tech-nology Commission (Grant No. Z141100003514004). Editor-in-Chief David A. Wood, Associate Editor and anonymous reviewersare gratefully acknowledged.

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