Ordovician Hydrocarbon Migration along the Tazhong No. 10 Fault

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Ordovician hydrocarbon migration along the Tazhong No.10 fault belt in the Tazhong Uplift, Tarim Basin, northwest China Weibing Shen, Xiongqi Pang, Jianfa Chen, Ke Zhang, Zeya Chen, Zhaofu Gao, Guangping Luo, and Liwen He Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03542 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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Energy & Fuels

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Ordovician hydrocarbon migration along the Tazhong No.10 fault belt in the

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Tazhong Uplift, Tarim Basin, northwest China

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Weibing Shen a*, Xiongqi Pang b, Jianfa Chen b, Ke Zhang c, Zeya Chen b, Zhaofu Gao a, Guangping Luo b, Liwen He b

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a MLR Key Laboratory of Isotope Geology, Institute of Geology, Chinese Academy of Geological

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Sciences, Beijing 100037, China

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b State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum,

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Changping District, Beijing, 102249, China

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c Research Institute of Exploration and Development, Tarim Oilfield Company, PetroChina, Korla,

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Xinjiang, 84100, China

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* Corresponding Author

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E-mail: [email protected]

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Phone number: +8615117973405

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Present address: Chinese Academy of Geological Sciences, No. 26, Baiwanzhuang Road, Xicheng

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district, Beijing, P. R. China, 100037

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Abstract: The Ordovician hydrocarbon migration and accumulation of the Tazhong Uplift in the

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Tarim Basin, northwest China, is investigated from the perspective of geological and geochemical

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analysis. Geochemical parameters successfully analyzed include the oil and gas properties,

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Ts/(Ts+Tm) ratios, and carbon isotope ratios of gas. Results show anomaly parameter values are

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observed in the No.10 fault belt (10FB) and the No.1 fault belt (1FB). As the distance from the 10FB

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and 1FB increases, the parameter value anomalies weak gradually until then become disappeared in

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the north platform belt (NPB). This saddle-like distribution of parameters indicates the hydrocarbon

ACS Paragon Plus Environment

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is introduced into the Ordovician through 10FB and 1FB from the northern Manjiaer Depression and

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the uplift itself. This new conclusion is different from the conventional view to a large extent, which

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indicates that Ordovician hydrocarbon mainly derive from the Manjiaer Depression and the No.1

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fault is the only NW-trending oil source fault. The viewpoint of 10FB as an additional hydrocarbon

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charge place is further supported by the evidence from the hydrocarbon charge intensity, structural

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framework, source rock distribution, and significantly improvement of the reservoir physical

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property (7~8 times at the 10FB). Based on this hydrocarbon charge and migration process and patter,

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the main target for further exploration activities in the Ordovician of the Tazhong Uplift should be

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the SPB (south platform belt) and the south part of the 10FB, especially the south part of the 10FB.

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Abstract: Hydrocarbon migration; hydrocarbon accumulation; carbonate rock; carbon isotope ratio;

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Tazhong No. 10 fault; Tarim Basin

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1 INTRODUCTION

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Faults have always been widely considered as a difficult and disputed topic in petroleum

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geology because they influence the essence features of petroleum systems, especially the petroleum

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migration process. 1-4 The Tazhong Uplift, one of the most prolific petroliferous areas in the Tarim

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Basin, located in the central part of the basin. 5 The uplift is formed with multi-episode structural

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movements, and develops complex faults. 6-7 Faults in the uplift can be divided into two sub-fault

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systems: (1) NW-trending thrust fault system mainly includes Tazhong No. 10 fault, Tazhong No. 1

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fault, and central fault; (2) NE-trending strike-slip fault system mainly includes ZG51, ZG432, ZG43

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and ZG441 faults. These sub-fault systems form X-shape fault combinations together (Fig.1 and Fig.

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2). Recently, amounts of oils and gases with high-economic value have been proved in the

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Ordovician carbonate rock of the uplift. At the end of 2012, the oil and gas reserves are


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approximately equal to 5×108 t. 8 The produced Ordovician hydrocarbon in the uplift includes gases,

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gas condensates, oil condensates, normal oils, and very waxy oils. Most of them found along the

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fault belts (Fig. 1), which indicates it is meaningful to investigate influences of the faults on

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hydrocarbon migration and accumulation.

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In the last decade, significant progress has been made in the study of faults controlling on the

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Ordovician hydrocarbon migration and accumulation in the Tazhong Uplift. Lin and Zhang (1996),

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Zhang et al. (2000), Gu et al. (2003), Sun et al. (2003), and Han et al. (2009) proposed that the

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Manjiaer Depression might be an dominating kitchen for the hydrocarbons in the Tazhong Uplift,

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based on oil-source correlation of biomarkers, and suggested that the hydrocarbons were mainly

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introduced into the uplift along the No.1 fault belt. 9-13 Similarly, Ma et al. (2004), Guo et al. (2008),

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Xiang et al. (2010), Wu et al. (2009) used the grading rules of hydrocarbon production, oil/gas

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properties, inclusion tracer, and diamantane index to predict the Manjiaer source kitchen, and

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deduced the No.1 fault belt was the most important hydrocarbon migration pathway in the uplift, 14-17

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which is supported by Yang et al. (2012) based on the analysis of reservoir physical properties. 18 In

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the past few years, some geologists suspected that the NE-trending strike-slip faults should be gave

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much more attention. Yang et al. (2012), Zhou et al. (2013), Pang et al. (2013a, b), and Lan et al.

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(2014, 2015) indicated that strike-slip faults might be additional pathways for hydrocarbon migration

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in the Tazhong Uplift, based on study of nitrogen-bearing organic compounds, crude oil trace

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elements, and carbon isotope ratios of nature gas, and suggested that the hydrocarbon charged into

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the Ordovician of the uplift though the intersection zones of strike-slip faults and Tazhong No. 1 fault.

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7-8, 18-21

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Despite recent progress in the investigation of strike-slip faults and No.1 fault jointly controlling


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the Ordovician hydrocarbon migration and accumulation in the Tazhong Uplift, the focus study on

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the No.10 fault belt is scarce, and a more thorough control mechanism analysis is required. Only few

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research have inferred that the No. 10 fault belt was also an important channels for vertical

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hydrocarbon migration based on the structural movement research. 22 The underlying reasons for

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preceding deficiency of related study is due to the lack of oil-gas wells drilled along the No.10 fault

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belt for hydrocarbon characterization and migration tracer at that time. With the penetration of

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hydrocarbon exploration and development, lots of exploration or development wells (more than 30)

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have been drilled along the No.10 fault belt. Combined with the geologic and geochemical methods,

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this study investigates the secondary migration of the Ordovician hydrocarbons along the No.10 fault

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belt. The key factors are investigated including regional structural framework, source rock

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distribution, oil and gas properties, Ts/(Ts+Tm), and carbon isotope ratios of gases. A new model for

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further Ordovician hydrocarbon exploration in the Tazhong Uplift has been proposed. The results can

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provide theory and data support for further hydrocarbon accumulation mechanisms study and

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hydrocarbon exploration activities in the Tazhong Uplift of the Tarim Basin.

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2 NOMENCLATURE

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Caledonian period: 570 to 410 Ma;

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Hercynian period: 410 to 250 Ma;

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Indosinian period: 250 to 208 Ma;

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Yanshanian period: 208 to 65 Ma;

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Himalayan period: 65 Ma;

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Gas dry coefficient: C1/Ʃ(C1–C5), the relative abundance of CH4 among CH4, C2H6, C3H6, n-C4H8,

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i-C4H8 and C5H12;


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△R3: |R3-R4|/R4; R3= iC4/nC4; R4 = iC4/C3; C3 is the propane; iC4 is isobutene; nC4 is normal butane.

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23, 24

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Ts/(Ts+Tm): 17α-22,29,30-trisnorhopane /(17α-22,29,30-trisnorhopane +

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18α-22,29,30-trisnorhopane) ratio;

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HI: hydrocarbon index

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3 GEOLOGICAL SETTING

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The Tazhong Uplift, one of three typical uplifts in the Tarim Basin, is surrounded by the

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Tangguzibasi Depression in the southwest, the Manjiaer Depression in the northeast and the Awati

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Depression in the northwest (Fig. 1). The NW-SE-trending Tazhong Uplift converges towards the

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east and open towards the west. It is subdivided into four structural tectonic units: the east burial hill,

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the south slope, the central faulted horst belt, and the north slope.13 The north slope contains: the

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No.1 fault belt (1FB), the north platform belt (NPB), the No.10 fault belt (10FB), and the south

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platform belt (SPB) from north to south (Fig. 1). The 1FB is the boundary between the Manjiaer

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Depression and the Tazhong Uplift, and is about 230 km long and 5-10 km wide with an inverted “S”

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shape. The 10FB is mainly composed of the No.10 fault and accompanied second rifts, with 50 km

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long, west-north-west trending, thrust characteristic, and shows an inverted L-shape. It is adjacent to

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the east burial hill to the southeast, and vanishes along Z1 well area to the northwest (Fig. 1).

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The 10FB and 1FB mainly experienced three discernable evolution stages (Fig. 2): (1) the start

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of fault activity featuring extensional structure during the Cambrian-Early Ordovician. The

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extensional faulting influence during this period set the border between the Tazhong Uplift and the

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Manjiaer Depression. As a result, the 1FB and 10FB developed in the Middle Ordovician, and

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formed the south-north zoning structural framework in the Tazhong Uplift. Influenced by the zoning

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characteristics, the reservoir characteristic and hydrocarbon distribution in the Tazhong Uplift varied


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across the 10FB. 21, 25, 26 The karst reservoir was well developed in the north side because of its flat

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slope and sufficient water leaching. On the contrary, the karstification of the south side was relatively

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poor for the existence of a steep slope. (2) A period of predominantly tectonic reversal occurring and

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transforming into the thrust reversal fault system during the Middle-Later Ordovician. The eastern of

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the Tazhong Uplift experienced intense uplift and formed large erosion unconformities. The 1FB and

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10FB were reactivated, and they came into thrust reversal in this period. The 1FB and 10FB not only

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controlled the structural framework of north-south zoning, but also the distribution of the structural

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traps.

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period, and then a multi-layered and uniform succession of sediments were deposited.6,21,29 As shown

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in representative profiles, the No.1 fault and No.10 fault cut the strata from the oldest to youngest

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(Cambrian, Ordovician, Silurian, and ending in the Silurian) (Fig. 2). The early formed 10FB and

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1FB have been divided into several blocks by the striking faults, i.e., ZG441, ZG46, ZG43, and

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ZG51 from west to east (Fig. 2). The NE strike–slip faults also extended down to the Proterozoic

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basement.7, 21, 27

6, 21,27,28

(3) Final to shape in the late Ordovician. The Tazhong Uplift was completed after this


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Fig. 1. Ordovician reservoired oil-gas distribution in the Tazhong Uplift, Tarim Basin.

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Fig. 2. Representative profiles illustrate the tectonic frameworks of different segments of the No.10

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fault belt.


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Drilled formations in the Tazhong Uplift from the bottom to top include: the Cambrian,

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Ordovician, Silurian, Devonian, Carboniferous, Paleozoic, Triassic, and Cretaceous, with the Jurassic

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missing (Fig. 3). The Ordovician system is characterized by marine carbonate platform facies. There

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are six kinds of micro-facies developing in the study area: mound, reef, slope, restricted platform,

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beach, and interbank sea facies. 19 Excellent reservoir belts with grain beach and organic reef facies

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have been developed in the north slope of the Tazhong Uplift, where is also the best area with the

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advantage for hydrocarbon resources exploration potentiality of the Ordovician carbonate. 7,30-33 Due

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to sea level fluctuation, nine sedimentary cycles have been developed, and the overall thickness

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reached about 100~300 m. 34 The 10FB is located on the carbonate platform inner area, and mainly

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developed

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oolites, intraclast limestones, and most bioclast limestones.8 The scale of a single reef or grain beach

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is small, with a thickness of about tens of centimeters to several meters, and interbedded lime

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mud-mound 25. The single reef or grain beach width varies between 2 km and 5 km. Thus, correlating

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the lithologies, especially of the reef body, from the wells is difficult

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reservoir-caprock assemblages can be identified in the Ordovician (Fig. 3). Among them, the Lower

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Ordovician Yingshan Formation weathered crust is the most important hydrocarbon produced

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reservoir, with Upper Ordovician micritic limestone (Liang 3-5 section) as caprock. The Yingshan

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Formation experienced large-scale weathering and erosion, with large-scale development of karst

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and corrosion holes in structure highs. Furthermore, later fracturing and dolomitisation with

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hydrothermal activity coupled to improve the physical properties of grain bank reservoirs in the

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Yingshan Formation. 35

organic

reef

and

grain

beach

systems

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which

is

composed

by

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. Three sets of

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Fig. 3. Lithology and petroleum geological settings in the Tazhong Uplift.

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The overall distribution of oil and gas fields in the Tazhong Uplift is characterized by laterally

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“E–W blocking, N–S zoning, oil in the west, and gas in the east”, 26 and characterized by the

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superimposition of the multi-layer and multi-type reservoirs in vertical, with Upper Ordovician

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Lianglitage reef reservoirs and Lower Ordovician Yingshan weathering crust reservoirs. 35 It has been


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widely accepted that the Ordovician hydrocarbon in the Tazhong Uplift derived from the

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Middle-Upper Ordovician and the Cambrian-Lower Ordovician source rock. 6,12,36-43 Biomarkers of

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the potential Middle-Upper Ordovician source rocks and related oil are characterized by regular “V”

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type sterane with C27 > C28 < C29. By contrast, biomarkers of the potential Cambrian-Lower

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Ordovician source rocks and oil have the characteristics of low abundance of regular sterane of

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“slanted line type” or “reverse L type” with C27 < C28 < C29, together with high abundance of

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phenylisoprenoids, 4-alkyl-24-ethyl cholane, of triaromatic dinophyceae steroids, and gammacerane.

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A higher abundance of phenylisoprenoids is considered to be related to Cambrian source rocks.12

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4 SAMPLING AND METHODS

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Parts of oil and gas composition testing data obtained from the Tarim Oilfield Company of

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PetroChina. 59 Ordovician oil samples and 49 Ordovician gas samples selected for composition

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analysis were collected from wellhead separators in the Tazhong Uplift.

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4.1 Molecular Composition Measurements of Crude Oil

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The molecular compositions of oil samples were tested using a gas chromatographyrmine

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spectrometry (GC-MS). After removing asphaltenes, oil sample was segregated into saturated and

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aromatic fractions by a silica/alumina chromatographic column. Hexane and benzene were used to

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extract the saturated and aromatic hydrocarbons, respectively. Ether was then used to extract the

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nonhydrocarbon organic compounds. Then, an Agilent 5975i GC-MS was used to analyze

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hydrocarbon content. For operating GC, the oven temperature was set at 100 °C for 1 min and then

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ramped to 140°C at a rate of 10 °C/min, subsequently to 300°C at 2 °C/min, and maintained for 20

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min. The carrier gas was helium, and injector operated at a liner flow of 0.8 cm3/min. The MS was

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handled with an ionization energy of 70 eV.


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4.2 Molecular Composition and Stable Carbon Isotope Measurements of Natural Gases

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The molecular compositions of gas samples (without H2S) were tested using an Agilent 6890N

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gas chromatograph equipped with a flame ionization detector and a thermal conductivity detector.

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Individual hydrocarbon gas components (C1–C4) were separated using a capillary column (PLOT

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Al2O3 50 m × 0.53 mm). Non-hydrocarbon gases were separated using two capillary columns (PLOT

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Molsieve 5 Å 30 m × 0.53 mm, PLOT Q 30 m × 0.53 mm). GC oven temperature was initially set at

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30 °C for 10 min, and then ramped to 180 °C at 10 °C/min. All the gas compositions have made

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oxygen-free correction and the corresponding correction for nitrogen. Stable carbon isotope values

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were measured on a Thermo Delta V Advantage instrument interfaced with a HP 5890II gas

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chromatograph. The gas chromatograph was equipped with a Poraplot Q capillary column (30 m× 0.32

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mm), and carrier gas was helium. Gas components were separated on the gas chromatograph in a

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stream of helium, converted into CO2 in a combustion interface, and then injected into the mass

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spectrometer. Samples were injected at an initial temperature of 50 °C (maintained for 3 min), after

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which the oven was heated to 190 °C at a rate of 15 °C/min, and maintained at that temperature for 15

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min. Gas samples were analyzed in triplicates, and the stable carbon isotope data are expressed in the

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delta notation in permil (‰) relative to VPDB (Vienna Pee Dee Belemnite, δ13CVPDB = 0‰).

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Measurement precision is ±0.5‰ for δ13C. Analytical precision is estimated to be ±0.3‰.

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Different from the other gas compounds which was determined using an Agilent 6890N gas

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chromatograph, the H2S concentrations were measured in oil field locale. The methods for H2S

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measurement were also different from the other gas compounds. In brief, H2S-bearing gas was

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bubbled through 50mL glass jars containing 0.5 × 10-2 zinc acetate to precipitate ZnS. ZnS was

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oxidized by adding 10 mL 0.01mol/L Iodine solution and 10 mL 1mol/L HCl until blended well.


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After 2~3 min reaction, solution was transferred to 250 mL iodine flask and added 2~3 mL starch

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solution. Then, it was added 0.01mol/L sodium thiosulfate until the blue disappeared. The H2S

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concentration of gas was calculated in accordance with the measurement law of mass spectrometry

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(State Standard of China (GB/T 11060).

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5 RESULTS

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As Ordovician oils and gases originated from the Cambrian-Ordovician source rocks, previous

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studies has shown that oils have uniform middle-high maturity, such as TeMNr, TMNr, Ts/(Ts+Tm),

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are approximately > 0.6, > 0.6, > 0.5, respectively. 44-46 In addition, other geochemical parameters,

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such as tricyclic terpane, gamma napalite and methyl sterane, are almost uniform, and indicated that

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the Ordovician hydrocarbon accumulation in the Tazhong Uplift is almost uniform.

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12,40,47-48

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Ordovician hydrocarbon migration of the Tazhong Uplift. 8,15,20-21, 49-50

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5.1 Oil Properties

Subsequently, crude oils and gases properties and compositions could be good tracers for

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The oil compositions and properties vary regularly during the process of migration. 9,51 The

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Ordovician oils are mainly light crude with density of 0.76~0.88 g/cm3, wax content of 1.6%~18.5%,

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sulphur content of 0%~0.67%, and viscosity of 0.75~8.21 mPas. The oil properties have

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characteristics with obvious belts (Fig. 4 and Fig. 5). In generally, oil properties have saddle-like

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distribution: compared to the NPB, there are significant parameter abnormities in the 1FB and 10FB,

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as shown by relatively low densities and high wax contents (Fig. 4 and Fig. 5). For example, the wax

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contents in the 1FB are higher than 7% with highest 16.1% (TZ86), in the NPB are lower than 8%

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with lowest 1.6% (TZ168), and in the 10FB are higher than 7% with highest 18.5% (ZG51). In

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addition, the wax contents are anomaly high along the No.1 fault and No.10 fault and regularly


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decrease with the distance increasing from the two faults. For example, the wax contents decrease

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from 9.9% to 5.8% and then increase to 9.2% (ZG1-ZG503-ZG433C). On the contrary, the densities

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are anomaly low in the two fault belts and regularly increase with the distance increasing from the

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No.1 fault and No.10 fault. For example, the oil densities increase from 0.78 g/cm3 (ZG10) to 0.81

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g/cm3 (ZG106), and then decrease to 0.78 g/cm3 (ZG43).

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Fig. 4. Oil density distribution in the Ordovician reservoirs in the Tazhong Uplift.

234


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Fig. 5. Wax content distribution in the Ordovician reservoirs in the Tazhong Uplift.

237 238

The lateral variations in the oil properties indicate that the 1FB and 10FB are both the places

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where the hydrocarbon charges. After arriving at the Tazhong Uplift, oils continue migrating in two

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directions, namely, from southwest to northeast in the 10FB and from northeast to southwest in the

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1FB. As the oils and gases are mainly from the Cambrian-Ordovician source rocks, the above

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characteristic seems to be caused by the higher maturity gases which generated in later periods

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charging into the crude oils generated in earlier periods; the densities proximal to the No.1 fault and

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No.10 fault reduce, and the wax contents increase. However, on account of the migration obstacles

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and the strong heterogeneity of the Ordovician carbonate rocks, it is rarely observed that oils mixed


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with gases in the areas far away from the No.10 fault and No.1 fault, and the densities are relatively

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larger, and the wax contents lower.

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Analysis of fluid inclusions in reservoirs reveals three stages of oil–gas inclusions in

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gas-condensate reservoirs in the Tazhong Uplift, including the first-stage oil–gas inclusions, the

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second-stage oil–gas inclusions and the third-stage gas inclusions.

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temperatures of the hydrocarbon inclusions correspond to Middle Caledonian (70-90

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Hercynian (100-125 ℃) and Late Himalayan (120-155 ℃) fluids (Fig. 6). 53, 54 This suggests that three

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stages of effective hydrocarbon charge are preserved in the Tazhong Uplift: oil filling occurred in the

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Middle Caledonian and the Late Hercynian, however, gas filling in the Late Himalayan. Analysis of

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oil and gas maturity provide direct evidence that highly overmature gases, which were mainly

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sourced from the Cambrian-Lower Ordovician source rocks during the Himalayan period, flushed

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and invaded into the previous oil reservoirs, which generated in the middle Caledonian and late

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Hercynian periods.

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gas-condensate reservoirs than in oil reservoirs,

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Zhang (2000). 10 The analysis of maturities of oils in the Tazhong Uplift shows that the oils from

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different types of reservoirs have significantly different maturities. Meanwhile, maturity of

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Ordovician gases in the Tazhong Uplift is calculated according to the equation proposed by Dai

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(1992): δ13C1=14.13 lg Ro – 34.39 and oil maturity according to the methylphenanthrene index MPI1

264

proposed by Radke et al. (1982). 56, 57 The results show that gas maturity in gas-condensate reservoirs

265

is substantially different from the oil maturity. However, gas maturity in volatile-oil reservoirs are

266

slightly different from the oil maturity. Oil and gas maturities in normal oil reservoirs are almost the

267

same. These suggest that the oils and gases in gas-condensate reservoirs are not from the same period;

39-40,48

52,53

The peak homogenization ℃),

Late

Zhu et al. (2014) found the δ13Coil is commonly heavier by 1‰ in 55

which is consistent with the opinion expressed in


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 48

268

high maturity gases from a later period may wash and alter an ancient oil reservoir, causing different

269

maturities of oils and gases.

270 271

Fig. 6 Geothermal and burial history of stratums in the Tazhong Uplift

272 273

Charging of the dry gases changed the densities of the oils. Most of the gases dissolved into the

274

oils, and oil densities vary. Along the gas migration direction, dry gas invasion energy decreased.

275

Correspondingly, oil densities increase gradually along the migration direction. Hydrocarbon

276

accumulation near the source faults (No1. fault and No.10 fault) is typically characterized by gas

277

condensate reservoir with lower densities resulting from the charge sequences (Fig. 1 and Fig. 2),

278

and oil densities increase gradually with increasing distance. Meanwhile, the wax contents are also

279

influenced by dry gas invasion. Previous studies have shown that the late charge of a gas into oil

280

reservoirs would result in deasphalting oil and an increase oil wax contents

281

experienced gas invasion commonly show a sharp decrease in the light components of normal


58-62

. Oils that

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282

alkanes but a relatively moderate decrease in the heavy components, which resulted in the forming

283

of waxy oils. 63-64 Proximal to faults, oils with high wax contents have been detected, proving that

284

hydrocarbon was indeed charging along the No.1 fault and No.10 fault.

285

There are two interpretations for the observed oil property distribution. First, depositional facies

286

vary among the Ordovician reservoirs (Fig. 7). Facies in the 1FB and 10FB are mainly reef and

287

beach, and changed to inter-bank sea in the NPB. The facies of inter-bank sea always have poor

288

reservoir physical property, and created migration obstacles. This leads to discontinuity of

289

hydrocarbons (Fig. 7). Second, in the Ordovician carbonate reservoirs, the markedly faulting and

290

karstification lead to the complex pore-fissure-fracture reservoirs space system. It is obvious that the

291

proximal reservoir’s porosity and permeability have been improved significantly by faulting.

292

Pore-fissure-fracture space systems with relatively high porosity and permeability are usually

293

segmented by the surrounding wall-rocks with ultra-low porosity and permeability, which make a

294

direct contribution to the relatively independent fracture-cave units. 65-66 Due to the discontinuity

295

distribution of pore-fissure-fracture reservoir, the hydrocarbon migration with long distance is

296

limited. That is, on account of the migration obstacles caused by the strong heterogeneity of the

297

carbonate rocks, it is rarely observed that oil mixed with gas in the areas far from away from the 1FB

298

and 10FB, and low wax contents and high densities can be observed.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

299 300

Fig. 7. Facies distribution in the Ordovician reservoirs in the Tazhong Uplift.

301 302

5.2 Gas Properties

303

In process of natural gas migration and accumulation, the gas properties and compositions

304

change systematically and regularly and have usually been a good tracer for hydrocarbon migration

305

history. 67-68 Properties of the Ordovician gases sampled from the Ordovician are complex. The CH4

306

contents range from 79.3% to 94.6%. The range of gas/oil ratios are from 0 m3/m3 to 13, 527 m3/m3,

307

and gas dry coefficients are generally higher than 0.90. The H2S contents also change largely,

308

ranging from 0% to 23.10%. Following the same variation rules of the oil properties, the Ordovician

309

gas properties change systematically and regularly. As shown in the plane distribution maps of

310

gas/oil ratio (GOR) (Fig. 8), dryness coefficients (Fig. 9), and H2S contents (Fig. 10), it is clear that

311

natural gas properties can be divided into three belts, which represented the intensity of gas charging

312

and migration are various. The three gas property parameters are characterized by high anomalies in


Page 18 of 48

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313

the 1FB and 10FB. The H2S contents are 1000 times higher and the GOR is 2~3 times higher along

314

the No.10 fault and No.1 fault (Fig. 8 and Fig. 10). The gas dryness coefficients in the 1FB and 10FB

315

are mainly higher than 0.95 and 0.94, respectively, but they are lower than 0.93 with the lowest 0.72

316

(ZG26) in the NPB (Fig. 9). All of them decrease away from the No.10 fault and No.1 fault and can

317

be tracers for the influence of gas migration. The afore-mentioned facts indicate that the late gas

318

charge and migrate along the two faults. As the distance from the No.1 and No.10 faults increase, the

319

gas charge intensity becomes small.

320 321

Fig. 8. GOR distribution in the Ordovician reservoirs in the Tazhong Uplift.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

322 323

Fig. 9. Dry coefficient distribution in Ordovician reservoirs in the Tazhong Uplift.

324

325 326

Fig. 10. H2S content distribution in the Ordovician reservoirs in the Tazhong Uplift.


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327

Energy & Fuels

Fu and Liu (1992) and Miao et al., (2011) summarized the law of △R3.

23-24

Due to the

328

differences of molecular polarity and volume, natural gases produce chromatographic migration

329

fractionation in seepage migration process. With the migration distance increasing, R3 increases and

330

R4 decreases. The |R3-R4| also shows an increasing trend, so △R3 shows a more significant increase

331

as a good reference for natural gas migration. The value of △R3 shows a significant increase in

332

relation to increasing migration distance. The deeper values of △R3 are smaller, but the shallow

333

values are larger in the same wells, for example, in wells ZG44C, ZG43, ZG431 and ZG51 near the

334

No.10 fault (Fig. 11), which indicates an increasing vertical migration distance and the 10FB is a gas

335

charge place.

336

(a)

(b)

337 338 339

(c)

(d)


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

340 341

Fig. 11. Vertical migration characteristic of gas with the parameter of △R3. The △R3 value is smaller

342

in the deeper reservoir. The dashed line is the boundary line of △R3 value with different depth. The

343

wells are seen in Figure 1. (a) Well ZG43; (b) ZG431; (c) ZG44C; (d) ZG51.

344 345

Controlled by the strong heterogeneity of the Ordovician carbonate rocks, gases derived from

346

the Cambrian-Lower Ordovician source rocks in the Himalayan period firstly occupied the nearby

347

pore-fissure-fracture systems after introducing along the No.1 fault and No.10 fault, and then

348

migrated far away. In the presence of Cambrian gypsum, thermochemical sulfate reduction was

349

active, 69 and the overmature gases were mixed with H2S. Therefore, gases with higher H2S contents

350

can be found in the reservoir beds near the No.10 fault and No.1 fault. This resulted in abnormally

351

high GOR, high gas dryness coefficients, high H2S contents, high wax contents and low densities

352

near the No.1 fault and No.10 fault. The afore-mentioned parameter anomalies gradually disappeared

353

as the distances from the two gas source faults increased.

354

5.3 Crude Oil Composition and Maturity Parameter

355

Property and composition discontinuities of crude oils are widely suggested to be the


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Energy & Fuels

356

inheritance which oils firstly charged into the reservoirs. 70-74 For the same source rocks, over time,

357

the maturity of generated hydrocarbon increases constantly. Therefor oil migration pathway and

358

orientation can be speculated based on tendencies of decreasing oil maturity.

359

ratio can be applied to crude oil that experienced the peak period of hydrocarbon generation, to

360

recognize low-mature, mature, and high-mature oil, which is a valuable crude oil maturity index. The

361

smaller the distance from oil to the source rocks is, the higher the Ts/(Ts+Tm) ratio is. To the

362

contrary, with distance becoming larger away from the source, the ratio tends to be smaller. The

363

successful application of this oil maturity index in the Ordovician carbonate reservoir of karst

364

pore-fissure type in the Halahatang Area of the Tarim Basin, 76 clearly shows that the Ts/(Ts+Tm)

365

ratio is effective indicator for tracing oil migration in carbonate rocks.

74-75

The Ts/(Ts+Tm)

366

Synthesized with the Ordovician crude oil data, the Ts/(Ts+Tm) ratios vary as the distance from

367

the faults (Fig. 12), and follow the same variation rules of the wax contents, gas/oil ratios, H2S

368

contents, and gas dryness coefficients. Ts/(Ts+Tm) ratios are high in the 1FB and 10FB and

369

decreases sideways, which indicates that the horizontal migration distance increases and the two

370

faults are the oil source faults (Fig. 12). This viewpoint was further supported by the vertical

371

distribution of hydrocarbon. For example, the TZ11 Oilfield and TZ12 Oilfield, adjacent to No. 10

372

fault, are characterized by compound oilfields, where the counts of vertical hydrocarbon layer are

373

obviously larger than within the oilfields far away from the No. 10 fault, such as the ZG6 Oilfield

374

(Fig. 13). In addition, other geochemical parameters of crude oil sampled from different reservoirs in

375

the TZ11/TZ12 Oilfield vary regularly. Compared to the Silurian oils, the Ordovician oils have larger

376

amount of nitrogen compounds, 4,6-DMDBT/1,4-DMDBT ratio, 2, 4-DMDBT/1,4-DMDBT ratio,

377

and

4-MDBT/

1-MDBT

ratio,

while

have

smaller

1,8-DMCA/2,7-DMCA


ratio

and

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

378

1,8-DMCA/1,7-DMCA ratio (Fig. 13). This comparison of geochemical parameters also indicates

379

that the No. 10 fault is oil migration fault.

380 381

Fig. 12. Ts/(Ts+Tm) ratio distribution in the Ordovician reservoirs in the Tazhong Uplift.

382 383

Fig. 13. Vertical migration characteristic of oil with geochemical parameters in the section.


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Energy & Fuels

384

The afore-mentioned pattern of crude oil molecule parameters occurs because the effects of

385

multiple hydrocarbon charges and the hydrocarbon in short supply during the later accumulation

386

periods. The over-mature hydrocarbon which generated during the later period can hardly migrate

387

through a long distance and charging into reservoirs far away from the source faults. As a

388

consequence,

389

much higher of maturity than that accumulated farther away.

390

5.4 Gas Maturity Parameter

oil

accumulated

near

the

No.1

fault

and

No.10

fault

trend

to

be

391

Geochemical characteristics of natural gas including carbon isotope ratio are inherited from the

392

time when gas charge into the reservoirs. 9, 77 As gas maturity increases, the gas composition would

393

become enriched in the

394

mature they would be, and the more

395

that gas enriched in the 13C usually distributed close to source rocks. The successful application of

396

carbon isotope compositions in the Ordovician carbonate reservoir of karst pore-fissure type in the

397

Lunnan Area of the Tarim Basin indicate carbon isotope ratios of gases can trace the gas migration. 49,

398

80

399

13

C. 70, 78-79 The later the natural gases charged into reservoirs, the more 13

C would become enriched. 9, 70, 79 The above facts indicated

According to the 51 Ordovician gas samples, there are various

13

C values for methane gases

400

(Fig. 14). The gas samples with lower maturity are mainly from the NPB. The more mature gas

401

samples with higher carbon isotope ratios are from the 1FB and 10FB, indicating that they are

402

generated in the relatively late period. Carbon isotope ratios of gases are large along the No.1 fault

403

and No.10 fault, and decrease gradually with increasing distance from the two faults. For example,

404

carbon isotope ratios are -38.0‰ in Well ZG2, decreasing gradually to -47.6‰ and -44.6‰ in Well

405

ZG501 and ZG5, and then increasing to -41.6‰ in Well ZG433C (Fig. 14). This indicates an


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

406

apparent hydrocarbon migration and the No.1 fault and No.10 fault are gas charge pathways. This

407

pattern has the same explanation as the crude oil maturity parameters.

408 409

Fig. 14. Methane carbon isotope composition distribution in the Ordovician reservoirs in the

410

Tazhong Uplift.

411

6 DISCUSSION

412

6.1 Hydrocarbon Charging and Migrating Along the No.10 Fault Belt

413

6.1.1 Theoretical Support

414

Assuming that oils and gases in reservoirs charge from one side, along the oils and gases

415

migration direction, properties and charge intensity of oils and gases may regularly change after a

416

gradient. 9 As discussed previously, traditional viewpoint indicated that the Ordovician oils and gases

417

in the uplift are mainly introduced along the No.1 fault from the northern Manjiaer Depression, and


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Energy & Fuels

418

then migrated from north to south. 7-8,18-21 Based on this model, it can be inferred that the oils and

419

gases charge intensity is anomaly high at the No.1 fault (oil source fault) and tended to decrease as

420

distance increased away from the fault. Consequently, a critical boundary for oils and gases

421

charging and accumulation in the Ordovician of the uplift would exist, such as 35 km away from the

422

No.1 fault. 8,19-20 That is, the area beyond this hydrocarbon charging and accumulation boundary,

423

such as 10FB and SFB, would develop few oil-gas fields. But that is not the case, the 10FB and SFB

424

have lots of oil and gas reservoirs nowadays (Fig. 1). In addition, regional tectonic framework in the

425

hydrocarbon charge period controlled secondary hydrocarbon migration. 81-82 After hydrocarbons

426

charged into a conductive layer, they would migrate from low part to the high part by buoyancy. 83

427

Based on the “volume balance” theory,

428

hydrocarbon charge period, is modeled (Fig. 15). It shows the 10FB is the apparently high part in the

429

north slope of the uplift, which forms a “shelter” and makes the SFB is blocked from the

430

hydrocarbon charge from the Manjiaer Depression. These make the probability of hydrocarbon

431

accumulation in the SFB become much lower. But lots of hydrocarbon wells drilled in the SFB, such

432

as the Z1 well area. Subsequently, it is reasonable to deduce that there is an additional hydrocarbon

433

charge place near to the SFB, and the 10FB is the ideal match. Anomalies of all the parameters in

434

the10FB are the best evidence for the new conclusion.

84

the Ordovician’ top structure form during the major


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

435 436

Fig.15. Structural form of the top of the Ordovician during the major accumulation period in the

437

Tazhong Uplift.

438

6.1.2 Source Condition

439

The requirement for hydrocarbon charging along the 10FB is that the No.10 fault contacts with

440

the source rock. That is, source rocks must develop in the Tazhong Uplift itself. Similar to

441

considerable progress in determination of hydrocarbon source rock intervals in last decade, the

442

researches of source kitchen are also fruitful in the Tazhong petroleum system.

443

For a long time, the Manjiaer Depression was considered to be the primary source kitchen for

444

the Ordovician hydrocarbon in the Tazhong petroleum system by vast majority of geochemists (Fig.

445

16),

446

additional kitchen and be gave much more attention. According to oil-source rock correlation of

8-9,11-12,20

but some geologists who suggested that the Tazhong Uplift itself ought to be an


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Energy & Fuels

447

biomarkers, such as 4-alkyl-24-ethyl cholane, 24-norcholestane, gammacerane, diasteranes, and

448

triaromatic dinosteroids, the geologists deduced Middle-Upper Ordovician source rocks and

449

Lower-Middle Cambrian source rocks deposited in the north slope facies of the Tazhong Uplift itself

450

should be the potential source rocks for the Ordovician oils and gases.

451

Cambrian source rocks deposited in the evaporation lagoon, inner sag, and organic shelf, with the

452

lithology composed mainly of black shale and phosphorite. 50 They have high hydrocarbon potential

453

with thicknesses of 10~300 m, TOC (total organic carbon) of 1.1%~3.5% with an average of 1.4%,

454

and HI (hydrocarbon index) of 2.0 to 230.1 mg/g. The Middle-Upper Ordovician source rocks were

455

of some evaporative lagoonal and organic facies deposited on an inner platform, with the lithology

456

composed mainly of muddy limestone and gray phosphatic silica. The thicknesses of them ranges

457

from 10 m to 150 m, TOC, from 1.1% to 2.5% with an average of 1.0%, HI, from 1.6 mg/g to 156.2

458

mg/g. Most of the rock samples (cores) in the Tarim Basin range in age from Ordovician to

459

Cambrian, too old to contain land-plant-derived vitrinites for the measurement of Ro. The reflectance

460

of solid bitumen in the samples was measured as Rb. The Rb values were transformed into Ro values

461

using the following empirical relationship which was based on measurements of samples from China.

462

85

463

(vitrinite reflectance) ranging from 1.5% to 3.0% and from 0.5% to 1.2%, respectively. 38,40, 86-87 As

464

noted, the source rocks in the Tazhong Uplift itself reached hydrocarbon generation peak and can

465

provide significant support for hydrocarbon charging and migrating along the No.10 fault belt.

37-40

The Lower-Middle

The two set of source rocks in the uplift are both in high mature to over mature stage, with Ro


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

466

(a)

467 468

(b)

469 470

Fig. 16. Source rock distribution in the Tarim Basin (modified from the Zhang et al., 2000a, b, 2012).

471

(a) The Lower-Middle Cambrian source rock. (b) The Middle-Upper Ordovician source rock.


Page 30 of 48

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472

Energy & Fuels

6.1.3 Reservoir Properties Distribution

473

Limited carrier beds with a higher porosity and permeability usually are dominant pathways for

474

oil and gas migration. 88-90 The Ordovician carbonate rocks in the Tazhong Uplift show a low matrix

475

porosity of less than 5% and a low permeability of less than 1 mD. Therefore faults and associated

476

fractures are crucial for reservoir characterization. The faults promote development of a system of

477

micro-cracks and fractures increase reservoir space. These features serve as fluid flow, increasing the

478

reservoir porosity and permeability and enhancing the activity of underground fluids, such as the

479

structurally control hydrothermal reservoir. 91 From the Ordovician carbonate reservoir model of the

480

Tazhong Uplift, it follows that the fractures in different belts are distributed differently. Generally,

481

the 1FB and 10FB develop more open fractures due to multiphase activity of the No.1 fault and

482

No.10 fault. The fracture orientation is parallel to the direction of the trending thrust faults

483

(northwest); in contrast, the fractures in the NPB and SFB grow less intensely and do not show

484

tendency to develop gapping, resulting in relatively poor reservoir conditions in those belts.

485

Meanwhile, the faults break through the land surface and increase the depth of the karstification zone

486

during the karst stage, improving the reservoir bed quality. Karst-weathered crust reservoir beds

487

develop in the Lower Ordovician of the Tazhong Uplift. 92 By integrating the fracture distribution

488

(based on fault density) with weathering crusts (based on sedimentary facies and structural highs),

489

the Ordovician reservoirs’ porosity and permeability have been analyzed. The porosity and

490

permeability of reservoirs in the 1FB and 10FB are 7~8 times higher than that in NPB and SPB (Fig.

491

17). With the distance increasing from the No.1 fault and No.10 fault, the reservoir physical property

492

become worse, and industry hydrocarbon wells decrease (Fig. 17). Based on the analysis of

493

relationship between logged reservoir physical property and reservoir oil-bearing property, the


21

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

494

commercial hydrocarbon yet discovered is all distributed in reservoirs with relatively high porosity

495

and permeability (porosity > 1.8%, permeability > 0.1 MD), and oil saturation increase with the

496

porosity and permeability increasing (Fig. 17). These can deduce that the 1FB and 10FB featuring

497

good reservoir property is prone to be rich in hydrocarbons, which are preferred hydrocarbon

498

migration pathway.

499

(a)

500 501

(b)

502 503

Fig.17. Physical properties distribution of reservoirs in the Ordovician of the Tazhong Uplift. (a)

504

Porosity distribution. (b) Permeability distribution.


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505

Energy & Fuels

6.2 Application for Hydrocarbon Exploration

506

Enormous progress of the Ordovician hydrocarbon exploration activities in the 1FB, NPB and

507

the north part of the 10FB of the Tazhong Uplift have been got, and exploration activities are moving

508

further towards the south part of the 10FB and SFB nowadays. 93-94 Whether the SFB and south part

509

of the 10FB have hydrocarbon exploration potential or not become a large challenge.

510

Hydrocarbon, sourced from the Manjiaer Depression, mainly charges into the uplift along the No.1

511

fault and laterally migrates into the northern part of north slope. However, little hydrocarbon can

512

migrate with a long distance and get accumulation in the south part of the 10FB and SFB based on

513

the afore-mentioned analysis. The lack of sufficient oil sources make that some geochemists deduce

514

the limited exploration potential of south part of the 10FB and SFB. 64 In our work, the Ordovician

515

hydrocarbon discovered in the uplift can derive from both the Manjiaer Depression and the Tazhong

516

Uplift itself, and we confirm that the No. 10 fault is an additional oil-migrated fault (Fig. 18).

517

Hydrocarbon, originated from the Tazhong Uplift itself, mainly charges into the Tazhong Uplift

518

along the 10FB and gets accumulation nearby. In addition, previous studies proved the SFB and

519

south part of the 10FB have the similar reservoir and caprock condition to the 1FB and NPB (Fig.18).

520

18,93

521

and it is wise to choose the south part of the 10FB first.

50,64

Consequently, we can indicate the SPB and south part of the 10FB have exploration potential,


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

522 523

Fig. 18. The Ordovician hydrocarbon migration and accumulation model in Tazhong petroleum

524

system.

525

7 CONCLUSION

526

(1) Hydrocarbons are introduced into the Ordovician of the Tazhong Uplift from the northern

527

Manjiaer Depression and the uplift itself through the 10FB and the 1FB. Hydrocarbon charge and

528

migration patter lead to anomalies of all the parameters in the 10FB and the 1FB.

529

(2) After introducing along the 10FB and 1FB, hydrocarbon migrated into the SPB and NPB

530

along different directions. As the distance from the 10FB and 1FB increases, the anomalies weak

531

gradually until then become disappeared in the NPB.

532

(3) The viewpoint of 10FB as an additional hydrocarbon charge place is further supported by

533

the evidence from the hydrocarbon charge intensity, structural framework, source rock distribution,

534

and significantly improvement of the reservoir physical property (7~8 times at the 10FB).

535

(4) As geological conditions for hydrocarbon accumulation in the SPB and the south part of the

536

10FB are similar to those of 1FB and NPB, SFB and south part of the 10FB have exploration

537

potential, and south part of the 10FB should be emphasized.


Page 34 of 48

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538

Energy & Fuels

ACKNOWLEDGEMENTS

539

The authors thank the Tarim Oilfield Company of PetroChina for supporting the work and

540

providing samples and data. This work was funded by the National 973 Basic Research Program (no.

541

2011CB201100). The authors are particularly grateful to Professor Ryuzo Tanaka and three

542

anonymous reviewers for their constructive review and comments for the manuscript.

543

REFERRNCES

544

(1) Losh, S.; Eglington, L.; Schoell, M.; Wood, J. Vertical and lateral fluid flow related to a large

545

growth fault, south Eugene Island Block 330, offshore Louisiana. AAPG Bulletin 1999, 83,

546

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Fig. 4. Oil density distribution in the Ordovician reservoirs in the Tazhong Uplift.

805

Fig. 5. Wax content distribution in the Ordovician reservoirs in the Tazhong Uplift.

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Fig. 6. Geothermal and burial history of stratums in the Tazhong Uplift

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Fig. 7. Facies distribution in the Ordovician reservoirs in the Tazhong Uplift.

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Fig. 8. GOR distribution in the Ordovician reservoirs in the Tazhong Uplift.

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Fig. 9. Dry coefficient distribution in the Ordovician reservoirs in the Tazhong Uplift.

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Fig. 10. H2S content distribution in the Ordovician reservoirs in the Tazhong Uplift.

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Fig. 11. Vertical migration characteristic of gas with the parameter of △R3. The △R3 value is smaller

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in the deeper reservoir. The dashed line is the boundary line of △R3 value with different depth. The

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wells are seen in Figure 1. (a) Well ZG43; (b) ZG431; (c) ZG44C; (d) ZG51.

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Fig. 12. Ts/(Ts+Tm) ratio distribution in the Ordovician reservoirs in the Tazhong Uplift.

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Fig. 13. Vertical migration characteristic of oil with the geochemical parameters in the section. D =

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Devonian; S = Silurian; O3s = upper Ordovician Sangtamu Formation; O3l = upper Ordovician

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Lianglitage Formation; O1y = lower Ordovician Yingshan Formation; O1p = lower Ordovician

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Penglaiba Formation; ∈= Cambrian.

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Fig. 14. Methane carbon isotope composition distribution in the Ordovician reservoirs in Tazhong

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Uplift.

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Fig.15. Structural form of the top of the Ordovician during the major accumulation period in the

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Tazhong Uplift.

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Fig. 16. Source rock distribution in the Tarim Basin (modified from the Zhang et al., 2000a, b, 2012).


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(a) The lower-Middle Cambrian source rock. (b) The Middle-upper Ordovician source rock.

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Fig.17. Physical properties distribution of reservoirs in the Ordovician of the Tazhong Uplift. (a)

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Porosity distribution. (b) Permeability distribution.

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Fig. 18. Ordovician hydrocarbon migration and accumulation model in the Tazhong petroleum

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system. S = Silurian; O3s = upper Ordovician Sangtamu Formation; O3l = upper Ordovician

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Lianglitage Formation; O1y = lower Ordovician Yingshan Formation; O1p = lower Ordovician

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Penglaiba Formation; ∈= Cambrian.

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Page 48 of 48

Ordovician Hydrocarbon Migration along the Tazhong No. 10 Fault

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