Secondary migration of hydrocarbons in Ordovician carbonate reservoirs in the Lunnan area, Tarim Basin

Journal Pre-proof Secondary migration of hydrocarbons in Ordovician carbonate reservoirs in the Lunnan area, Tarim Basin Junqing Chen, Kuiyou Ma, Xiongqi Pang, Haijun Yang PII:

S0920-4105(20)30059-0

DOI:

https://doi.org/10.1016/j.petrol.2020.106962

Reference:

PETROL 106962

To appear in:

Journal of Petroleum Science and Engineering

Received Date: 26 July 2019 Revised Date:

6 January 2020

Accepted Date: 15 January 2020

Please cite this article as: Chen, J., Ma, K., Pang, X., Yang, H., Secondary migration of hydrocarbons in Ordovician carbonate reservoirs in the Lunnan area, Tarim Basin, Journal of Petroleum Science and Engineering (2020), doi: https://doi.org/10.1016/j.petrol.2020.106962. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Junqing Chen: Methodology, Writing- Reviewing and Editing, Investigation, Funding acquisition Kuiyou Ma: Investigation, Software, Formal analysis, WritingOriginal draft preparation, Xiongqi

Pang:

Conceptualization,

administration, Funding acquisition Haijun Yang: resources, Data Curation

Supervision, Project

1

Secondary migration of hydrocarbons in Ordovician carbonate

2

reservoirs in the Lunnan area, Tarim Basin

3

Junqing Chen1, 2, Kuiyou Ma1, 2, Xiongqi Pang1, 2, Haijun Yang4

4

1

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

5

Beijing 102249, China 2

6 7

4

College of Geosciences, China University of Petroleum, Beijing 102249, China

Research Institute of Petroleum Exploration and Development, PetroChina Tarim Oilfield

8

9

Company, Korla, 841000, China

Abstract

10

The study on the secondary migration of hydrocarbons in Ordovician carbonate

11

reservoirs in the Lunnan area has been a challenge caused by limited geologic records

12

as direct physical evidence, and physical simulations are difficult to conduct since the

13

parameters of realistic geologic conditions are rare. Based on the analysis of the

14

regional structure, and by integrating the study on the fluid properties and

15

benzocarbazole nitrogen compounds parameters, the secondary hydrocarbon

16

migration in Ordovician carbonate reservoirs in the Lunnan area, Tarim Basin, were

17

investigated. The results indicated oil density show gradually increasing trends from

18

Lungudong Fault to the Middle Slope in eastern Lunnan Uplift; while the trend is

19

converse from southwest of Lunxi and Sangtamu Faults to the Middle Slope in

20

western area. The benzocarbazole nitrogen compounds contents of Ordovician

21

reservoir oil exhibit gradually increasing trends from southwest of Lunxi and

22

Sangtamu Faults and southeast of Lungudong and Sangtamu Faults to the Middle 1

23

Slope. The wax contents, gas dryness coefficients and gas/oil ratios show decreasing

24

trends from east to west and away from the Lungudong Fault and Sangtamu Fault.

25

The crude oil generated at the late Hercynian stage (about 290Ma ago) in the Lunnan

26

area migrated along the directions from southwest of Lunxi and Sangtamu Faults and

27

southeast of Lungudong and Sangtamu Faults to the Middle Slope; the gas formed at

28

the late Himalayan stage (about 208 Ma ago) migrated from the east to west laterally,

29

and along the natural gas invasion faults Lungudong Fault and Sangtamu Fault

30

upwards vertically.

31

Keywords: Tarim Basin; Lunnan area; Ordovician carbonate; hydrocarbon secondary

32

migration

33

34

1 Introduction

35

Secondary migration of hydrocarbons runs through the entire process of

36

hydrocarbon expulsion, transportation and accumulation; therefore, the issue is one of

37

the most significant topics in petroleum geology (Fall et al. 2012). There are still

38

many challenges to study secondary migration due to limited geologic records as

39

direct physical evidence, and physical simulations are also difficult to conduct since

40

the parameters of realistic geologic conditions are rare (Pang et al. 2013). In Lunnan

41

area in the Tarim Basin, China, oil and gas reservoirs are discovered in the Ordovician,

42

Carboniferous Triassic and Jurassic Systems, of which the Ordovician is the most

43

important hydrocarbon-bearing system (Li et al. 2010). It was reported that the

44

prognostic reserves of oil and gas equivalent of Ordovician are up to 1.878 billion 2

45

tons, with the proved reserves up to 0.863 billion tons so far in the region (Li et al.

46

2010), both of which show that it is an outsize marine carbonate oil and gas field.

47

However, the Ordovician hydrocarbons in the Lunnan area with multiple stages of

48

hydrocarbon accumulation (Wu et al. 2013) have complex phase, including heavy oil,

49

normal oil, light oil, condensate gas, etc (Chen et al. 2015). Besides, the hydrocarbon

50

reservoirs underwent multiple periods of tectonic evolution after accumulation (Zhu et

51

al. 2013), making the characteristics of the Ordovician hydrocarbons reservoirs very

52

complicated and creating elevated difficulty for the study on the secondary

53

hydrocarbon migration.

54

At present, the study on the secondary migration of Ordovician hydrocarbons in

55

the Lunnan area is still emerging. Wang et al. (2004) analyzed oil migration in the

56

Lunnan area based on the pyrrolic nitrogen compound distribution. Results showed

57

that the Ordovician and Carboniferous oils migrated laterally from west to east, and

58

the main filling points were on both southwest sides of the Lunnan and Sangtarmu

59

Fault-horst Belts (Fig. 1). Li et al. (2010) conducted oil-oil and oil-source rock

60

correlations research in Lunna area by biomarkers correlations and the compound

61

specific stable carbon isotopes of n-alkanes correlations. Their results showed that oil

62

of Ordovician reservoirs in the Lunna area are from an extensively mixed source and

63

they are also comprehensive accumulations from multiple hydrocarbon charge periods

64

with different chemical constitutions and different sources. The other studies by

65

Zhang et al. (2005) and Yu et al. (2012), utilizing a detail biomarker analysis,

66

indicated that crude oil of Ordovician reservoirs in Lunna area are from two families, 3

67

i.e. Upper-Middle Ordovician source rock and Cambrian–Lower Ordovician source

68

rock. In addition, diamondoids and crude oil physical properties are used to analyze

69

detail accumulation periods in Lunna area (Zhang et al, 2011), and results showed that

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there are at least two hydrocarbon charge periods. One is oil charge that came from

71

Upper-Middle Ordovician source rock and the other is dry gas charge that came from

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Cambrian–Lower Ordovician source rock (Zhang et al, 2011). And a further study by

73

Zhang et al. (2014) suggested that the first period of oil charge had been biodegraded

74

before dry gas charged in Lunnan area. Generally, since the number of samples for the

75

biomarker and isotope analysis is limited, previous studies only focused on the oil

76

sources and oil accumulation period. They proposed that oil of Ordovician reservoirs

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is from the mix of Upper-Middle and Cambrian–Lower Ordovician source rock in the

78

Manjiaer Depression to the south of the Lunnan area (Fig. 1), which just indirectly

79

identified a regional migration direction from south to north. Besides, gas reserves

80

have been discovered in the Lunnan area; however, the gas direction remains

81

unrevealed. This is not sufficient for further hydrocarbon exploration in the study area

82

considering the complicated geological conditions and complex oil and gas phase

83

distribution.

4

84 85

Figure 1. Geologic structures of the Lunnan area in the Tabei Uplift, Tarim Basin

86

Based on the previous studies, we studied the secondary migration direction in

87

the Lunnan area mainly based on oil and gas properties, combining the distribution of

88

carbazole nitrogen compounds. We extensively collected 388 data for oil and gas

89

properties and 18 samples for biomarker characteristics analysis and carbazole

90

nitrogen compounds content that cover the whole Lunnan area to reveal more detailed

91

and specific oil and gas migration directions of Ordovician reservoirs hydrocarbons.

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The study results can provide an important basis for the exploration of Ordovician

93

carbonate rocks in the Lunnan area.

94 5

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2 Geological setting

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The Lunnan uplift, is located in the north of the Tarim Basin, China, covers an

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area of 4420 km2, which belongs to the south wing of the Tabei Uplift as a secondary

98

normal structural unit (Figs. 1a and 1b). This paleo-uplift is encompassed by the

99

Caohu sag in the east, bounded by the Manjiaer Depression and the Halahatang sag

100

respectively in the south and west, with particularly superior geologic conditions for

101

hydrocarbons accumulation (Fig. 1b). It is comprised of seven secondary structural

102

units: The East, South, West, North, Middle slope belts and the Lunnan, Sangtamu

103

fault horst belts, respectively (Fig. 1c). The Ordovician stratigraphic sequences in the

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Lunnan area from young to old include the Upper Ordovician Sangtamu Formation,

105

Lianglitage Formation and Tumuxiuke Formation (Fig. 2), the Middle Ordovician

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Yijianfang Formation and the Lower Ordovician Yingshan Formation and Penglaba

107

Formation (Fig. 2). Among which, the Sangtamu Formation are dominated by clastic

108

rocks, while the other formations are mainly developed carbonate rocks (Fig. 2).

6

109 110

Figure 2. Generalized stratigraphic column of the Ordovician strata in the Lunnan

111

The Lunnan area underwent multiple periods of tectonic movements. During the

112

late Caledonian stage, unbalanced tectonic uplift caused a large south slope in Lunnan

113

area; and in the early Hercynian stage, it was subjected to the regional 7

114

northwest–southeast compression, which resulted in regional uplifting. The deposition

115

of the Upper Ordovician, Silurian and Devonian were removed during this long-term

116

uplift and the Ordovician buried-hill were formed (Cai et al. 2016). During the late

117

Hercynian stage, the regional north–south compression led to forming of the Lunnan

118

Fault and Sangtamu Fault (Fig. 1c). With the increase of compressive stress, the fault

119

activity gradually became more and more intense, and the upper plate of fault rose up

120

higher and higher, forming the Lunnan fault horst belt and Sangtamu fault horst belt

121

(Cai et al. 2016). The early Yanshan to early Himalayan stage was the period of stress

122

transformation in the Lunnan area. Due to the subsidence of the Kuqa Depression, a

123

series of northeast trending tensile faults were developed in the Lunnan area (Cai et al.

124

2016); and in the middle–late Himalayan stage, the Lunnan area was relatively stable

125

and activities of faults stopped.

126

Oil and gas were discovered in whole carbonate Ordovician formation indicating

127

all of the interval can be reservoirs, and the overlying Carboniferous formation is the

128

main caprock. There are two sets of source rocks in the Tabei Uplift, i. e. the

129

Middle-Upper Ordovician and Cambrian-Lower Ordovician source rocks (Li et al.

130

2008, Zhang and Huang. 2005, Zhang et al. 2011, Zhang et al. 2014). There are three

131

periods of hydrocarbon accumulations identified in the Lunnan area, including the

132

Caledonian stage (about 510 Ma ago), the late Hercynian stage (about 290Ma ago)

133

and the late Himalayan stage (about 208 Ma ago) (Gong et al. 2007, Zhang et al. 2013,

134

Wu et al. 2013). During the Caledonian stage and the late Hercynian stage,

135

hydrocarbon accumulations are dominated oil charging; and in the late Himalayan 8

136

stage, the Cambrian–Lower Ordovician source rocks became over mature and

137

generated dry gas, much of which were invaded into the Lunnan area. The large

138

amount of the condensate gas reservoirs discovered to date in the eastern slope of the

139

Lunnan area are believed to be the direct evidence of dry gas flushing of the early oil.

140

141

3 Data and experiment

142

We analyze the secondary migration of Ordovician hydrocarbons in different

143

well blocks of Lunn area, based on the variations of oil and gas properties, and the

144

distribution characteristics of the carbazole nitrogen compounds. The 388 data of oil

145

and gas properties are collected from the Tarim Oilfield Company, PetroChina.

146

The 4 oil samples are selected for biomarker characteristics analysis. Briefly, oil

147

samples were de-asphalted before fractionation on a neutral alumina chromatographic

148

column, and detail procedure can follow Nazir et al. (2014). Using sequential elution

149

method by n-hexane, toluene, and chloroform, the oil samples were divided into

150

saturated hydrocarbons, aromatic hydrocarbons, and a polar fraction. Then, these

151

three hydrocarbon components were then examined with gas chromatography–mass

152

spectrometry (GC-MS). The measurements were conducted using an HP 6890 GC

153

coupled to an HP5973 Series Mass Selective Detector (MSD) with the carrier gas of

154

helium. In order to analyze the saturated hydrocarbon fraction, the temperature of GC

155

oven was programmed to rise from 50 °C to 310 °C at a rate of 3 °C/min with 1

156

minute an initial holding at 50 °C for 1 minute and a finial holding at 310 °C for 30

157

minutes. The MS was mostly operated in the mode of selective ion monitoring (SIM) 9

158

and occasionally in the full scan mode. The methods of accurate identification and

159

quantitative calculation of each peak have been introduced detailly by Jiang et al.

160

(2001).

161

The 14 oil samples are selected from the company to conduct the experiments for

162

analysis of the carbazole nitrogen compounds content. Solid phase extraction method

163

was utilized for the separation of pyrrolic nitrogen compounds by a 3-ml C18

164

ISOLUTETM column (Int. Sorbent Tech., UK) (Wang et al. 2004). In more detail, the

165

oil samples were deasphaltened by precipitation in n-hexane and added to the wetted

166

columns,

167

dichloromethane eluant contained the pyrrolic nitrogen compounds, which were

168

concentrated for further analysis. The GC-MS of the pyrrolic compounds were

169

performed on a Finnigan Model SSQ-710 GC-MS system equipped with a HP-5 fused

170

silica capillary column (25 m in length, 0.32mm i.d.). The operating temperatures

171

were programmed to increase from 35 to 120 °C at a rate of 2°C /min, followed by

172

another increase to 310 °C at 3°C /min with an initial hold time of 5 min at 35 °C and

173

a final hold time of 15 min at 310 °C. N2 was used as the carrier gas. A commercial

174

internal standard compound, N-phenylcarbazole, was co-injected as an internal

175

reference for the quantification of the pyrrolic nitrogen compounds (Li et al., 1992).

eluted

successively

with

n-hexane

176

177

4 Results

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4.1 Oil properties

10

and

dichloromethane.

The

179

Oil properties (both physical properties and chemical properties) are determined

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by geologic processes including oil generation, migration, accumulation, preservation

181

and alteration. The Ordovician oils discovered to date include heavy oil, normal oil

182

and condensate oil. The density of oil ranges from 0.78 to 0.95 g/cm3, viscosity from

183

1.82 to 373 mPa·s, and the maximum content of asphaltene can reach 15.68% (Table

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1). The oils with relatively light density are distributed in the East Slope (the

185

LG47-LG45-LG42 well block and surrounding area, with oil density of 0.85 ~ 0.88

186

g/cm3), the West Slope (LN4-LN-25-LN30 well block and surrounding area, oil with

187

density of 0.84 ~ 0.89 g/cm3), and the regions near the Lungudong Fault and

188

Sangtamu Fault with the minimum value in the Lunnan area (oil density of 0.80 ~

189

0.81 g/cm3, Fig. 3).

190

The composition of the migrating oil can be different with the source rocks

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maturity increasing. Because most of oils are charged to reservoirs on one side and

192

migrated along a principal direction, it can be inferred that the crude oil compositions

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may change after a gradient. Assuming that the hydrocarbons which come from same

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source were charged, hydrocarbons would have higher maturity as the time of

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migration longer (Hwang et al. 1994; Pang et al. 2013). For instance, along the

196

southwest to northeast direction from the South Slope towards the Middle Slope, the

197

oils gradually become light from 0.89 to 0.83 g/cm3 (Fig. 3), showing that oils are

198

from the southern Manjiaer Depression. This is consistent with the regional general

199

direction in previous studies (Wang et al. 2004; Li et al. 2010).

11

200 201

Figure 3. Oil density contour map in the Ordovician reservoirs in the Lunan area

202

However, the oils in the other Slopes in the Lunnan area have quite complex

203

distributions. In the West Slope to the Middle Slope (LG15–LG901–LN11 direction),

204

the oil density increases from 0.87 to 0.93 g/cm3, and from the East Slope to the

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Middle Slope (LG34 –LG35–LN631–LN17–LG2–LG4–LG7–LN11 direction) oil

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density increases from 0.81 to 0.93 g/cm3 (Fig. 3 and 4b). The oil density in the

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regions near the Lungudong Fault and Sangtamu Fault is apace increasing from the

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lowest value 0.80 to 0.83 g/cm3 as the distances to the two faults increase (Fig. 3 and

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4a). There are obvious increasing trends of oil density from the West and East Slope

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to the Middle Slope and away from the Lungudong and Sangtamu Fault. The oil has

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relatively high density of larger than 0.90 g/cm3 near the Lunnan Fault and Lunxi

212

Fault, and gradually decreases as away from the faults (Fig. 3). This may be due to the

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combined effects of multiple periods of oil and gas injection and structural alteration

214

and destruction. 12

215 216

Figure 4. Section and geochemical parameters of Ordovician reservoirs in the Lunnan

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area (section location is shown in Fig. 1)

218

The first period of hydrocarbon accumulation in the Lunnan area occurred at the

219

late Caledonian stage (about 510 Ma ago) (Wu et al. 2013). After the accumulation,

220

due to the tectonic uplifting (at the early Hercynian stage) (Cai et al. 2016), the

221

overlain Devonian, Silurian and Middle–Upper Ordovician caprocks were gradually

222

eroded. The oil reservoirs accumulated in the Ordovician were destroyed and heavy

223

oil was formed. Meanwhile, abundant 25-norhopanes indicating biodegradation are

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detected in the oil samples from the LN30, LG39, LG9, LN1 and LG2 wells (Zhu et

225

al. 2013, Nazir et al. 2017, Nazir et al. 2016), which suggests that the Ordovician oil

13

226

reservoirs in the Lunnan area have been destroyed. Meanwhile, heavy oils are

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distributed near the Lunnan Fault and Lunxi Fault in the northwest (Fig. 3). The two

228

sets of faults cut through the Carboniferous caprock (Fig. 4b), indicating that these

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two sets of faults ruptured the reservoir. Although the damage by the tectonic uplifting,

230

the total ion chromatograms (TICs) for saturated hydrocarbons of Ordovician oils in

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the Tabei Uplift still performs relatively complete distribution of n-alkane, and the

232

light n-alkane components occupy obvious advantages (Fig. 5), indicating the

233

presence of multi-period oil charging. Gong et al. (2007) found there are two periods

234

of oil accumulations in the Ordovician reservoirs by analyzing fluid inclusion, and the

235

second period of oil accumulation took place at the late Hercynian stage (about

236

290Ma ago). From then on, the tectonic movements weakened, and seal integrity

237

remained excellent.

14

nC20

O

nC20

48 LN48 5436-5470 O 5436-5479

LN8 5145-5220 O

nC18

8 5145-5220 O

LN14 5274-5363 O

nC2 0

14 5274-5363 O

LN1 5038-5052 O

1 5038-5052 O

238 239

Figure 5. The total ion chromatograms (TICs) of saturated hydrocarbon fractions of

240

marine oil discovered in Ordovician carbonates within Lunnan area (sample locations

241

shown in Fig. 1)

242

As shown in the section (Fig. 4b), the eastern Lunnan area is mainly developed

243

condensate gas reservoirs, while, the western is primarily developed oil reservoirs.

244

Besides, the oil density is relatively light proximal to the Lungudong Fault and

245

Sangtamu Fault (LG 35 and LN14 wells both have oil with density of 0.81 g/cm3).

15

246

This may be due to the charge of gas that was generated in later stages with high

247

maturity into crude oils. This also leads to the decrease of density and viscosity of

248

crude oil near Lungudong Fault and Sangtamu Fault in the eastern Lunnan area.

249

Nevertheless, little oil mixed with gas was observed in the areas away from the

250

injection faults. This is resulted by the heterogeneity of the carbonate rocks and the

251

existence of the migration barriers, leading relatively larger density and viscosity of

252

oils.

253

The Ordovician gas in the area has the calculated Ro of 1.46% ~ 1.93%. The high

254

maturity indicates it was the dry gas generated by the highly–over mature

255

Cambrian–Lower Ordovician source rock at Himalayan stage (Zhu et al. 2013). In the

256

eastern Lunnan area where near the Lungudong Fault and Sangtamu Fault, oil with

257

abnormal high wax content was discovered (Figs. 4a and 6). The wax contents are

258

generally larger than 20% with a maximum of 33.58% and reduces gradually from

259

east to west as far away from the Lungudong Fault and Sangtamu Fault (Fig. 6). From

260

the source of the parent materials, high waxy oil is usually found in oils of higher

261

terrestrial origin (Li et al. 2006); however, oil in the study area is predominantly

262

marine origin. Therefore, gas invasion is reasonably considered to be the main cause

263

of the formation of secondary high waxy crude oil. The gas from invasion dissolved

264

and extracted the light hydrocarbons in the oil reservoir, resulting in the relative

265

increase of the remaining high carbon compounds in the residual crude oil, and

266

leading a corresponding increase in the content of wax (the wax is usually an alkane

267

compound with carbon number greater than C21). Thus, it can be concluded that the 16

268

direction of natural gas invasion in the Himalayan stage (about 208 Ma ago) was from

269

east to west in lateral and along the Lungudong Fault and Sangtamu Fault vertically

270

(Figs. 4b and 6). What is worth to note, in the western Lunnan area, there is also a

271

relative high wax content area where near the LG42-LG15-LG901 well block with

272

wax content approximately 15%. The formation of this high wax content area may be

273

not the result of the dry gas filling from the Cambrian–Lower Ordovician source rock,

274

because it is far from the gas source faults Lungudong Fault and Sangtamu Fault; thus,

275

the formation reasons require further studies. This local high wax as well as relatively

276

low density distribution (Fig. 3) may be due to another potential gas source (Pang et

277

al., 2018).

278 279

Figure 6. Wax content contour for the marine oil discovered in Ordovician carbonates

280

in the Lunnan area.

281 282

4.2 Gas properties

17

283

The methane contents in the natural gas in the Lunnan area are distributed in the

284

range from 65.30% to 95.60%, N2 contents from 0.72% to 29.2%, CO2 contents from

285

0.16% to 12.70%, and the dryness coefficients (C1 /ΣC2+) from 0.77 to 0.99 (Table 2).

286

The variation characteristics of gas properties are similar to those of oil. Take the

287

dryness coefficients and GOR (gas to oil ratios) for examples, they present abnormal

288

high values in the eastern Lunnan with dryness coefficients value larger than 0.97 and

289

GOR not less than 10000, especially near the Lungudong Fault and Sangtamu Fault

290

where the dryness coefficients value nearly reach 0.99 ~ 1 and GOR close to 20000.

291

As away from the eastern area, they exhibit decreasing trend from east to west and

292

both value of them close to 0 in the Middle Slope area (Figs. 4, 7 and 8).

293

The Ordovician dry gas in the eastern Lunnan area was the products of

294

highly–over mature source rocks at the late Himalayan stage (Zhu et al. 2013). The

295

amount of gas accumulation in reservoirs reduces as the distance from reservoir to

296

source rock becoming large. This explains the higher GOR and dryness coefficients

297

levels in the proximal areas (Figs. 7 and 8). And away from the source rocks, both of

298

the parameters exhibit decreasing trends (Figs. 7 and 8). Therefore, it can be

299

reasonably maintained that, during the last accumulation period, a mass of gas

300

emerged in Ordovician reservoirs by vertical migration along the Lungudong Fault

301

and Sangtamu Fault and begun to lateral migrate from east to west by the overlapping

302

cap rock effect.

18

303 304

Figure 7. Gas dryness coefficient contour map of the Ordovician reservoirs in the

305

Lunnan area.

306 307

Figure 8. Gas/oil ratio (GOR) contour map of the Ordovician reservoirs in the

308

Lunnan area.

309 310 19

311

4.3 Benzocarbazole nitrogen compounds

312

The difference in carbon positions between the benzene ring and the

313

benzocarbazole results in different benzocarbazole structure isomers in oil.

314

Commonly in the crude oil are two kinds of structure isomers nearly linear benzo [a]

315

carbazole and hemispherical benzo [c] carbazole (benzo [b] carbazole content is

316

generally lower). The migration of linear benzo [a] carbazole molecular is faster than

317

that of benzo [c] carbazole hemispheres; therefore, with increasing migration distance,

318

the linear molecular isomers are relatively enriched, that is, the ratio of benzo [a]

319

carbazole / (benzo [a] carbazole + benzo [c] carbazole) will increase (Liu et al. 1998).

320

Among the 14 Ordovician oil samples in the Lunnan area, the ratios of benzo [a]

321

carbazole / (benzo [a] carbazole + benzo [c] carbazole) are distributed in the range

322

between 0.41 and 0.64. The ratios are relatively low in southwest of the Lunxi and

323

Sangtamu Faults and southeast of Lungudong and Sangtamu Faults with values

324

mostly no more than 0.58 (Fig. 9). As the distance from these parts increases, the ratio

325

tends to increase gradually with values larger than 0.59 and reach the maximum value

326

of 0.64 in the Middle Slope. This elucidate the migration direction of oil in Lunnan

327

area maybe is from southwest of the Lunxi and Sangtamu Faults and southeast of

328

Lungudong and Sangtamu Faults to the Middle Slope.

20

329 330

Figure 9. Benzocarbazole content for the marine oil discovered in Ordovician

331

carbonates within Lunnan area.

332

4.4 Comprehensive analysis

333

There are two oil charge periods in Lunnan area according to the direct evidence

334

fluid inclusion and the first period of oil accumulation at the late Caledonian stage

335

(about 510 Ma ago) has been destroyed and affected by strong biodegradation (Gong

336

et al. 2007). Most of biomarkers in the oils can be eliminated since strong

337

biodegradation affect (Fazeelat et al. 2011). Thus, the distribution of the ratio of benzo

338

[a] carbazole / (benzo [a] carbazole + benzo [c] carbazole) is used to indicate the oil

339

migration direct of the second oil charging period at the late Hercynian stage (about

340

290 Ma ago). In the Ordovician oil reservoirs, the ratio of benzo [a] carbazole /

341

(benzo [a] carbazole + benzo [c] carbazole) shows that in southwest of the Lunxi and

342

Sangtamu Faults and southeast of Lungudong and Sangtamu Faults in the Lunnan

21

343

area are distributed relatively small; while in the Middle Slope is relatively large (Fig.

344

9). Therefore, directions of the second period of oil charging appeared to be from

345

southwest of the Lunxi and Sangtamu Faults and southeast of Lungudong and

346

Sangtamu Faults to the Middle slope in the Lunnan area. Besides the southwest sides

347

of the Lunxi and Sangtarmu Fault-horst Belts as main filling points to form the

348

migration from west to east according to previous studies (Wang et al. 2004), the

349

results also indicate the southeast Sangtamu Fault as filling point to form another

350

migration direction from east to west (Fig. 9).

351

Subsequently, dry gas charge occurred at the Himalayan stage (about 208 Ma ago)

352

in Lunnan area (Zhang et al. 2011). The GOR and gas dryness coefficient have a

353

consistent change pattern that in eastern area (especially Lungudong Fault and

354

Sangtamu Fault and surroundings area) relatively high and gradually decreases to the

355

west (Figs. 7 and 8). These seem to indicate the direction of gas migration is from east

356

to west. The GOR and gas dryness coefficient exhibit the extremely high value in

357

Lungudong Fault and Sangtamu Fault and surroundings area (Figs. 7 and 8). This may

358

be because the Lungudong Fault and Sangtamu Fault are gas source faults, which

359

connected the reservoirs and source rocks and provided the vertical migration

360

pathways for gas.

361

The preservation condition of the previous oil accumulations has not changed

362

during the last period of gas charge (Gong et al. 2007). Therefore, the gas charge will

363

result in oil property changes that accumulated earlier. Understanding this would help

364

to illustrate the secondary migration direction according to the current oil properties. 22

365

Gas injected into previous oil reservoirs will make the oil density lower due to the

366

dissolution of gas into the oil. The wax contents of oil also will become higher

367

because light contents of oil decrease. In fact, the oil density is extremely low and the

368

wax contents of the oil is extremely high in the source faults of gas area, the

369

Lungudong Fault and Sangtamu Fault and surroundings area (Figs. 3 and 6). The oil

370

density gradually increases, and the wax contents of the oil gradually decrease to

371

western area. This phenomenon is consistent with the above mentioned direction of

372

hydrocarbons migration.

373

374

5 Discussion

375

Previous researches mainly conducted oil-source correlation using biomarker

376

compounds (Zhang et al. 2005; Yu et al. 2012) and the compound specific stable

377

carbon isotopes of n-alkanes (Li et al. 2010), or only based on pyrrolic nitrogen

378

compound distribution (Wang et al. 2004) to study the secondary migration in the

379

study area. Besides the previous study results on oil-source correlations and

380

accumulation periods, this study is mainly based on fluid properties, combining the

381

typical carbazole parameters. Compared with the previous studies, the fluid property

382

data used can be widely collected and can cover the whole study area, making the

383

results more convincible compared to base on several samples. Additionally, the

384

current oil and gas properties are the eventual results after multiple periods of tectonic

385

movements and accumulations in the study area.

386

A comprehensive methodology working flow to study secondary migration of oil 23

387

and gas can be summarized according to the case study in the Lunnan area. First, it is

388

necessary to figure out the source and reservoir relationship to understand geological

389

conditions and obtain a regional migration direction. The geochemical methods for

390

oil-source correlations can be applied. Second, periods of oil and gas accumulations

391

and structural movements are required to clearly clarify. When studying the specific

392

directions, pyrrolic nitrogen compounds can be used as molecular tracers for

393

monitoring petroleum migration when one period of oil charge occurred (Stoddart et

394

al. 1995, Larter et al. 1995, Larter et al 1996). Or multiple periods of charges occurred

395

but only the last one period of accumulation was preserved after tectonic movements.

396

The crude oil composition and properties would show changes after a gradient if oils

397

are charged to reservoirs on one side and migrated along a principal direction due to

398

geochromatographic fractionation effect (Hwang et al. 1994). Thus, oil properties are

399

another indirect way to study the secondary migration directions. What is quite

400

important, when it comes to more complicated geological conditions, many factors

401

would influence the migration direction study. For example, in superimposed basins,

402

there are multiple sets of source rocks with different characters, multiple periods of

403

accumulation and tectonic movements, such as the Lunan area in this study. The

404

different maturity of source rocks, gas flushing effect, and damage by tectonic

405

movements will seriously result in complex oil and gas properties. All the above

406

influencing factors needs to be considered. Heavy oil would be formed by tectonic

407

damage and biodegradation. Oil density would decrease and wax content would

408

increase due to the gas invasion. It is suggested to study the secondary migration 24

409

direction combining nitrogen compounds and fluid properties.

410

6 Conclusions

411

On the basis of the analysis of the oil and gas properties and nitrogen compounds

412

parameters, the following conclusions can be obtained about hydrocarbon charge and

413

migration in the Lunnan area, Tarim Basin:

414

1. There are two periods of hydrocarbon accumulation in the Ordovician

415

reservoirs in the Lunnan area at the late Caledonian stage (about 510 Ma ago) and at

416

the late Hercynian stage (about 290 Ma ago), respectively. The first period of oil

417

accumulation have been destroyed due to tectonic movements. The directions of the

418

second oil charging period are from southwest of the Lunxi and Sangtamu Faults and

419

southeast of Lungudong and Sangtamu Faults to the Middle Slope in the Lunnan area.

420

2. The direction of natural gas formed at the Himalayan stage (about 208 Ma ago)

421

was from east to west in lateral and along the Lungudong Fault and Sangtamu Fault

422

vertically.

423 424

Acknowledgements

425

This work conducted successfully depends on the support from the National

426

Natural Science Foundation of China (Grant number 41402107), the China

427

Postdoctoral Science Foundation (Grant number 2017M611108), and the National

428

Basic Research Program of China (Grant number 2011CB2011-02). We appreciate the

429

Tarim Oilfield Exploration and Development Research Institute for providing

430

background geologic data and permission to publish the results. We also thank 25

431

anonymous reviewers, which improved the manuscript.

26

432

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31

525

Tables

526

Table 1 Physical properties of oil in Ordovician carbonate reservoirs, Lunnan area

527

Table 2 Properties of gas in Ordovician carbonate reservoirs, Lunnan area

32

528

Tables

529

Table 1: Physical properties of oil in Ordovician carbonate reservoirs, Lunnan area

Well LG45 LG15 LG15-21 LG901 LG9 LN11 LG3 LG7 LG4 LN1 LG2-1 LN8 LG1 LG2-2 LG2 LG100 LG17 LN54 JF128 LG11 LG12 LG13 LN48 LG19 LN621 LN634 LG38 LN633 LN635 LN63 LN631 LG391 LG39 LN25 LG352 LG35 LG351 LG353 LG34

depth (m)

density (g/cm3)

viscosity (mPa.s)

wax (%)

asph (%)

sulfur (%)

5763.16

0.85

8.27

10.60

6.48

0.20

5726.73

0.87

45.10

16.00

11.00

1.40

5697.00

0.89

163.50

13.98

15.68

1.51

5572.00

0.87

15.10

16.10

8.86

0.64

6020.00

0.87

16.72

6.20

9.94

5272.00

0.94

373.04

6.42

14.84

2.14

5515.00

0.89

40.87

4.60

5.83

1.42

5165.00

0.91

142.90

7.67

13.13

2.18

5267.00

0.90

2.59

2.99

0.00

5038.00

0.89

9.91

6.14

4.06

0.30

5379.37

0.86

15.70

2.08

0.36

0.16

5179.00

0.84

9.51

4.30

0.30

0.50

5520.00

0.84

10.40

5.06

0.21

0.07

5479.68

0.86

18.18

19.46

1.92

0.08

5345.00

0.84

15.34

9.18

1.94

0.10

5431.70

0.84

10.72

14.30

0.20

0.27

5464.00

0.85

12.64

4.70

0.40

0.43

5440.90

0.83

7.59

10.50

0.30

0.26

5490.83

0.81

1.82

14.10

0.00

0.07

5187.87

0.78

2.15

2.27

0.09

0.03

5407.20

0.83

10.28

17.90

0.20

0.17

5544.00

0.83

12.61

12.20

0.20

0.00

5304.63

0.81

2.30

2.70

0.00

0.06

5591.77

0.83

11.42

15.90

0.30

0.44

5720.80

0.80

3.16

8.96

0.26

0.24

5780.00

0.82

15.73

11.78

0.00

0.24

5619.38

0.80

3.02

5.28

0.00

0.33

5879.00

0.81

3.55

0.00

0.13

5815.00

0.83

9.75

21.46

0.16

0.26

5836.00

0.82

5.53

12.90

0.10

0.21

5902.88

0.83

15.42

33.58

0.11

0.31

5758.00

0.80

2.71

4.14

0.00

0.16

5681.09

0.81

4.20

10.30

0.00

0.20

5381.59

0.95

11.85

1.37

0.00

5872.50

0.85

18.83

30.14

0.24

0.23

6155.00

0.81

11.75

22.46

0.27

0.20

6310.00

0.83

10.73

16.23

0.32

0.32

6411.74

0.83

24.98

21.50

0.08

0.14

6698.00

0.81

5.39

24.75

0.00

0.18

33

LG341 LN632 LD1 LG392 LG32

6490.40

0.81

4.13

20.02

0.17

0.12

6452.00

0.83

9.92

12.90

0.13

0.20

6785.00

0.80

4.83

12.10

0.00

0.22

6330.00

0.83

9.28

6.67

1.18

0.21

6185.48

0.80

2.39

7.77

0.11

0.23

530 531 532

Table 2: Properties of gas in Ordovician carbonate reservoirs, Lunnan area

well LN11 LG7 LG4 LG1 LG100 LN54 JF128 LG11 LN48 LN17 LN14 LN634 LG38 LN631 LG391 LG39 LN301 LN10 LG352 LG353 LG34 LG341

depth

CO2

H 2S

dryness

(%)

(%)

coefficient

C1 (%)

C2 (%)

C3 (%)

C4 (%)

N2 (%)

5157.01

76.60

6.14

4.10

0.87

1.92

6.80

5165.00

72.10

8.56

6.65

1.43

3.92

2.26

5281.00

79.70

3.45

1.21

0.27

1.90

12.70

5520.00

90.10

3.05

0.32

0.08

0.72

2.38

16

5431.17

92.50

2.22

0.91

0.22

1.26

2.10

120

5448.00

93.00

1.94

0.69

0.18

1.05

1.81

4539.50

65.30

1.86

0.30

0.05

29.20

2.93

2

5187.87

95.10

1.14

0.26

0.06

1.38

1.82

390

5304.63

94.20

1.19

0.22

0.05

1.45

2.56

900

5502.16

94.20

1.16

0.35

0.05

2.07

1.87

0

4430.00

81.90

3.00

1.26

0.26

11.70

1.20

0

5780.00

94.80

1.30

0.45

0.12

0.81

1.86

1000

5653.17

94.10

1.24

0.29

0.04

2.86

1.32

5769.00

95.00

0.93

0.32

0.07

1.09

2.17

590

5804.37

94.80

1.10

0.25

0.04

2.33

1.28

180

5690.00

95.60

0.71

0.15

0.02

1.41

1.98

690

5431.50

90.30

1.49

0.49

0.06

6.31

1.03

5331.00

82.20

2.15

1.15

0.22

13.50

0.16

0

5872.50

94.20

1.25

0.29

0.07

1.75

1.96

23

5872.50

94.20

1.25

0.29

0.07

1.75

1.96

23

6698.00

95.40

1.00

0.24

0.05

1.13

1.90

200

6490.40

93.50

0.86

0.23

0.05

3.44

1.71

13

(m)

533

34

0

0.84 0.77 0.93 0.96 0.96 0.96 0.96 0.98 0.98 0.98 0.94 0.97 0.98 0.98 0.98 0.99 0.97 0.95 0.98 0.98 0.98 0.99

1. The Ordovician oil and gas properties were analysed in the Lunnan area. 2. Carbazole nitrogen compound distribution was studied. 3. Secondary migration direction of Ordovician oil and gas was identified.