characteristics of the upper jurassic marine source rocks and prediction of favorable source rock...

Journal of Earth Science, Vol. 24, No. 5, p. 815–829, October 2013 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-013-0375-5

Characteristics of the Upper Jurassic Marine Source Rocks and Prediction of Favorable Source

Rock Kitchens in the Qiangtang Basin, Tibet

Libo Wang* (王丽波) Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China;

Northeast Petroleum University, Daqing 163318, China Yeqian Zhang (张业倩), Junjie Cai (蔡俊杰)

School of Energy Resources, China University of Geosciences, Beijing 100083, China; Key Laboratory for Marine Reservoir Evolution and Hydrocarbon Abundance Mechanism, Ministry of Education, China University of

Geosciences, Beijing 100083, China Guangzhi Han

Baker Hughes, Drilling System, Rocky Mountain Area, Denver CO 80202, USA

ABSTRACT: This study evaluated the hydrocarbon-bearing potential of Upper Jurassic marine source

rocks in the Qiangtang (羌塘) Basin through a comprehensive organic geochemical analysis of the samples

from a large number of outcrops in different structural units to predict the location of favorable hydro-

carbon kitchens, based on the evaluation standards of Mesozoic marine source rocks in the Qiangtang Ba-

sin. Rocks’ depositional environment, thickness and organic geochemistry feature were analyzed in this

study. The principal controlling factors of the occurrences of favorable source rocks were analyzed. Upper

Jurassic Suowa (索瓦) Formation source rocks are mainly platform limestone in the Dongcuo (洞错)-Hulu

(葫芦) Lake deep sag and Tupocuo (吐坡错)-Baitan (白滩) Lake deep sag. Lithologically, the Suowa Fro-

mation is made up of a suite of marls in intra-platform sags, micrites and black shales, which were all de-

posited in the closed, deep and static water depositional environment. Marl could form hydrocarbon-rich

source rocks and its organic matter type is mainly II type in mature to highly-mature stage, the limestone

forms a medium-level source rock. In addition, the favorable source kitchen of limestone is larger than

that of mudstone. This study provides an important reference for the evaluation of Jurassic marine source

rocks and for prediction of petroleum resources in the Qiangtang Basin.

This study was supported by the National Natural Science

Foundation of China (Nos. 41372139, 41072098, 41002027)

and the National Major Projects of China (Nos.

2011ZX05018-001-002, 2011ZX05009-002-205).

*Corresponding author: [email protected]

© China University of Geosciences and Springer-Verlag Berlin

Heidelberg 2013

Manuscript received October 18, 2012.

Manuscript accepted December 25, 2012.

KEY WORDS: Qiangtang Basin, Upper Jurassic,

marine source rock, favorable source kitchen.

INTRODUCTION The Qiangtang Basin is composed of the northern

Tibet Autonomous Region and the southern Qinghai Province, covering the eastern section of the Tethys structural domain, which is rich in oil and gas reserves. Tectonically, the Qiangtang Basin lies in the northern Tibetan Plateau, between the Kekexili-Jinsha River and the Bangong Lake-Nujiang suture zones and cov-

Libo Wang, Yeqian Zhang, Junjie Cai and Guangzhi Han

816

ers an area of approximately 183 000 km2. This large-scale residual petroleum-bearing terrestrial basin has received little exploration or research attention so far. The tectonic framework comprises an uplift be-tween two depressions (Fig. 1). Mesozoic marine strata (Triassic and Jurassic sequences) develop exten-sively in the Qiangtang Basin, which reveal the con-sistence and completeness of the internal Tibetan Plate.

The Triassic lithology consists primarily of clastic rocks intercalated with carbonates and some volcanic rocks. The Jurassic lithology comprises extremely thick interbedded carbonate and clastic rocks (Fig. 2). The strata are more than 10 000 m thick and are mostly of Jurassic age (Wu et al., 2008).

The discovery of several oil and gas indicates that the hydrocarbon generation, migration and accu-

Mayigangri high uplift

Tupocuo-Baitan Lake deep sag

Northern Qiangtang depression

Central uplift zoneBiluocuo-Qixiang uplift

Dirangbicuo-Tumen sag

Southern Qiangtang depression

Northern marginal step-fault zone

Naducuo-Najiangcuo sag

Queerchaka low uplift

Dongcuo-Hulu Lake deep sag

Boundary ofentral pliftc u

0 40 km

2

3

4 5

67 8

9

1011

1213

14

15

16

17

1819

20 21

22

23

24

2526

272829

303132

33

3536

37

38

39 40

41

42

43

44

45

46

47

48 4950

51

5253

54

5556

57

58

59

60

34

1

Basinboundary

Sutureline

Positionname

Unitboundary

N

Kek i haex l si-Jin River

on

f

z

r

e

a tc ur

e

ed sutur

Bango g Lne-ak Nujiang fractured suture zone

Measuredprofile site

Possible oilseepage

Geologicalsurvey route for

petroleum

Sai pori

E11

N

T3x

Bangongcuo-Nujiangfold belt

J

J

South Qiangtangdepression

J

J

Nk

JJ

J

J J J

J J JNk Nk Nk Nk Nkγ52

γ52

T3x T3x

AnDgm

Centraluplift zone

North Qiangtangdepression

Amu hillock Yuejin Kou

Ns Ns Ns Ns T3x

Dogai coring Dogai Coring Qiangcuo

Kekexilifold belt

0

(a)

(b)

40 km

Figure 1. (a) Geotectonic divisions in the Qiangtang Basin, Tibetan Plateau. 1. Laxiongcuo (LP0); 2. Duxiu Mountain (DP05-TP02); 3. Zhaosha Mountain ZP06; 4. North Gemucuo (CP15); 5. Juhua Mountain (JP03-04); 6. Nadigangri (P04); 7. Duxue Mountain (DP05-TP02); 8. Guoganjianiu Mountain (GP13); 9. Gaicui; 10. South Bailongbing River (BP); 11. Tupocuo (TP09-10); 12. Yeniu Channel (YP); 13. Tianshui- bing (TP); 14. Suona Lake (SP); 15. Rejuechaka (DPL-11); 16. Mayigangri (MP); 17. Hanbuchaka (YBP); 18. Xiaxiancuo (XXP); 19. Dongsangbei (DP); 20. P07; 21. P08; 22. Nadigangri (PM); 23. Hongshui channel (HP19-JP10); 24. Jiaomuchaka (PL0); 25. P05; 26. Xiaochaka (P02); 27. South Bandao Lake (BP); 28. Bandao Lake (BNP); 29. Dongba (DP); 30. Niudu Lake (NP); 31. North Jiantou Mountain (JBP); 32. Jian-tou Mountain (JP); 33. Beileicuo (BCP); 34. Biluocuo (POG); 35. Luxiongcuo (LXP); 36. South Luxiongcuo (LNP); 37. Riasa (RAP); 38. East bank of Donggecuoren; 39. Duoersuodongcuo; 40. Chibuzhangcuo; 41. Suobucha; 42. Songker; 43. East Mingjing Lake Mountain (MP); 44. East Wulanwulacuo Mountain (WJP); 45. Zuerkenwula Mountain (ZP); 46. Zhamuna (ZHP); 47. Zhuoqing (ZPP); 48. Tuotuo River (PG); 49. Tuotuo River (PF); 50. Tuotuo River (PH); 51. Tuotuo River (PA); 52. Tuotuo River (PB); 53. Tuotuo River (PN); 54. Tuotuo River (PJ); 55. Duogaerqu (PC); 56. Duogaerqu (PI); 57. Anduo (PA); 58. Dongqu; 59. Yi-cangma; 60. Baikamushouma. (b) Geotectonic profile in the Qiangtang Basin, Tibetan Plateau.

Characteristics of the Upper Jurassic Marine Source Rocks and Prediction of Favorable Source Rock Kitchens

817

Stratigraphy Lithology

MiddleCretaceous

UpperTriassic

GachaeriFormation

XueshanFormation

SuowaFormation

XialiFormation

BuquFormation

QuemocuoFormation

QuseFormation

XiaochakaFormation

800

1 600

2 400

3 200

4 000

4 800

5 600

6 400

Facies Lithology

Relic sea

Alluvial fan

Platform facies

Littoral facies

Depth(m)

LowerJurassic

MiddleJurassic

UpperJurassic

LowerCretaceous

Alluvial fan

Delta facies

Platform facies

Marine-continentaltransitional facies,d felta acies

Marine fan,basin facies

Platform facies

Platform facies

Marine fan,basin facies

Marine carbonate, micrite

Purplish red sandy conglome-rate interbedded withmudstone

P meurplish red sandy conglo -rate interbedded with micrite

Sandstone, siltstoneinterbedded with mudstone

Deep pray mudstone,calcareous mudstone,marlite interbeddedlimestone and sandstone

with

Purplish red, green-gray,yellow-gray arkose, quartzsandstone, lithic quartzsandstone, siltstone andmuddy siltstone

W

shale

ell-developed marinecarbonate rocks, biolithitelimestone and limestonemainly, gray, dark grayinterbedded with micrite

Gray shale and micritebedded with siltstone,interbedded with spotsvolcanic rock

inter-andof

Lithic arkose, fine-sandstone,siltymudstone interbeddedwith a single layer oflimestone

Dark-gray mudstone,siltstone and sandstoneinterbedded with limestone

Light-gray biolithite clasticlimestone, sparry limestone,calcareous mudstoneGreen-gray calcareous shale,marlite interbedded withsandstone

Delta facies

Delta facies

Samplesites

Sourcerocks

Calcareousshale

Sparrylimestone

Biolithite clasticlimestone

MicriteShaleCalcareousmudstone

Muddy siltstoneSiltstoneSandstoneSandy conglomerate

Mudstone

Source rockSample ites

LimestoneMarlite

Figure 2. Profile of the Mesozoic marine source rocks in the Qiangtang Basin, Tibetan Plateau. mulation took place within the Qiangtang Basin. Consequently, the Jurassic–Upper Jurassic strata with no outcrops in the basin are good prospects for oil ex-ploration and are considered to be one of the strategic backup areas in China (Qiu et al., 2007).

A considerable amount of information on the

evaluation and distribution of the source rocks was pro-vided by published data from the organic geochemical analyses of outcrop samples and from the studies of se-dimentary facies and organic petrology (Dai et al., 2011, 2010; Koeverden et al., 2011; Zahie et al., 2010; Al-sharhan and Abdel-Gawad, 2008; Matthias et al., 2006;


818

Xu and Qin, 2004; Sachsenhofer et al., 2003; Henrik and Trond, 2001). Previous studies have also guided the early exploration and resource evaluation in the Qiang-tang Basin (Chen et al., 2007; Qin, 2006). The study on the Mesozoic Jurassic marine source rocks in various structural units was designed to predict the locations of favorable hydrocarbon-generating areas (Wu et al., 2008). That study covered 10 routes that were 3 210.5 km in length and included 29.2 km of measured stratigraphic sections, 68 measured field profiles and observation points, and the collection of numerous samples for laboratory analysis (110 samples for or-ganic carbon analysis, 108 for maceral analysis, 110 for pyrolysis, 90 for vitrinite and bitumen reflectance, and 30 for carbon isotope measurement). And that study al-so incorporated a review of the research data for the Qiangtang Basin that were published in China and elsewhere.

In this study, a series of evaluation standards for the Mesozoic marine source rocks in the Qiangtang Basin were developed based on the following: (1) ini-tial geochemical analysis of the organics found in the samples obtained from several profiles along the geo-logical survey routes and the Mesozoic marine source rocks from the Qiang-2D Well; (2) the analysis of the abundance, type and maturity of the organic content; (3) the depositional environment, thickness and li-thology of the source rocks; and (4) the lower limits of the organic carbon as determined from the studies of the marine carbonate rocks in China and elsewhere. Subsequently, a systematic study of the characteristics of the major Jurassic source rocks was conducted, and the locations for potential hydrocarbon kitchens were demarcated.

This study provides a sound theoretical basis for hydrocarbon generation in the Mesozoic petroliferous marine basins and aims at in the research of petroli-ferous systems in the Tethys structural regime. Fur-thermore, this study is useful for practical resource es-timations and evaluation of hydrocarbon-bearing po-tential and for further exploration planning within the Qiangtang Basin.

GEOLOGICAL SETTING Basin Evolution and Tectonic Framework

The tectonic evolution of the Qiangtang Basin,

which was marked by the Gondwana rifting in the Permian and the final collision between the Indian subcontinent and Eurasia was from the Late Creta-ceous to the Early Tertiary, can be divided into the following three phases (Wang et al., 2011; DeCelles et al., 2007; Kapp et al., 2007, 2005; Taner and Meyer-hoff, 1990a, b): (1) The geosyncline and platforms were formed before the Permian, which can be subdi-vided into the Precambrian platform basement forma-tion, platform accretion in the Early Paleozoic and transition from the Devonian to the Carboniferous. (2) Plate rifting and closure with basin infill and deposi-tion that occurred between the Permian and the Late Cretaceous. (3) Plate deformation and basin recon-struction since the Early Tertiary.

The fault structure belt of the northern and southern boundaries of the Qiangtang Basin developed from the original suture zone and the adjacent areas, and it experienced multiple periods of transformation and fracture dislocation during later, more intense ac-tivities. During the different stages of the basin evolu-tion and transformation, the northern and southern borders of the basin were different in nature and scope. The Paleozoic Qiangtang basin’s formation and evolu-tion came into an end with the closure of the northern margin of the Paleo-Tethys in the Late Permian. The Qiangtang Block resembled the current form after the Hercynian orogeny and formed the central uplift belt that runs across its center, which forms a natural bar-rier that divides the depositional environments into the North and the South Qiangtang depression in Meso-zoic.

The formation of the Triassic Qiangtang Basin is due to the north-south stretch expansion of the Ke-kexili-Jinsha River rift, and it is also related to the ini-tial expansion of the Bangong Lake-Nujiang rift (ocean) to a certain extent. As early as the Early to Middle Triassic, deposition began in the low-lying parts of the Late Permian uplift background and reached the maximum range of transgression in the Late Triassic. The Kekexili-Jinsha River sea canal was closured in the Late Triassic Indosinian orogeny, and simultaneously, the central belt further uplifted and the northward thrust nappe experienced a north-south squeeze in the Qiangtang Block, which resulted in the intensified separation of the north and the south in the


819

basin. The Jurassic Qiangtang Basin has experienced

two episodes of transgression-regression. The Jurassic transgressive temporal range may have been slightly less than the Late Triassic, but the deposition thick-ness was much larger in the former. From the Late Ju-rassic to the Early Cretaceous, due to the impact of the Yanshan orogeny, the Gangdese Block moved rapidly northward and collided with the Qiangtang Block col-lage, which resulted in the closure of the Bangong Lake-Nujiang sea canal in the southern border of the basin and a seawater retreat from the east to the west that led to the exposure of most of the Qiangtang Block. Thus, the accumulation of a wide range of ma-rine deposits in the Qiangtang Basin ended. The Late Yanshan and Himalayan orogeny pushed the Qiang-tang Basin into a full deformation and transformation stage. During the Himalayan orogeny, the basin was strongly uplifted and transformed, which ultimately led to the formation of the modern tectonic frame-work.

Erosion lines form the boundaries of the Qiang-tang Basin in Upper Triassic and Jurassic. The general tectonic framework consists of north-south zoning with an east-west block division. The third-order structural units are classified from north to south based on the evidence provided by basement fluctua-tions and the Mesozoic strata outcrops and their dis-tribution. (1) The North Qiangtang depression is sub-divided into the following: northern marginal step-fault zone; Tupocuo-Baitan Lake deep sag; and Dongcuo-Hulu Lake sag. (2) The central uplift is sub-divided into the following: Mayigangri high uplift; and Querchaka low uplift. (3) The South Qiangtang depression is subdivided into the following: Paducuo- Najiangcuo sag; Biluocuo-Qixiangcuo uplift; and Di-rangbicuo-Tumen sag. Lithology and Depositional Environment of the Source Rocks

In the Qiangtang Basin, marine source rocks are primarily carbonates, such as micrite, organic limes-tone, bioclastic limestone and marl. Some mudstones are also present, including argillaceous rocks and shale. The Upper Jurassic Suowa Formation (J3s) is one of the major Mesozoic source rocks. The lithology

consists of marine limestones and argillaceous rocks (Fig. 2), which display an extensive distribution with the maximum thickness of 3 000 m (Wu et al., 2008).

Depositional environments are the dominant con-trolling factor in the development of useful hydrocar-bon source rocks. The J3s Formation was produced as the basin underwent large-scale marine transgression, during which time large thicknesses of marine carbo-nates were deposited on the platform. The climate was semi-warm and semi-dry. The lithology of the intra- platform sag is primarily marl, micrite and black muddy shale, which generally implies a closed, deep-water depositional environment. The marl formed the hydrocarbon- rich source rocks. EXPERIMENTAL METHODS

Conventional chemical methods were used to ex-tract the organic materials. The procedure for obtain-ing the saturated hydrocarbons, aromatic hydrocar-bons and non-hydrocarbons was as follows: 0.2 mm thick rock samples were placed in a Soxhlet extractor for 78 h, and the bitumen was precipitated with petro-leum ether; silica gel and alumina chromatographic columns were used to separate the group components with hexane, benzene and ethanol washing agents; gas chromatography-mass spectrometry (GC-MS) was used to test and analyze the saturated hydrocarbons. A platform II chromatograph mass spectrometer was used, with an ion source temperature of 180 ℃, elec-tron energy of 70 eV and an HP-5 elastic capillary quartz chromatographic column measuring 50 m×0.32 mm×0.17 mm. The temperature was increased from 60 to 100 ℃ at 8 ℃/min and from 120 to 300 ℃ at 3 ℃ /min. The analyses were carried out at the Guangzhou Institute of Geochemistry, the Chinese Academy of Sciences and the Laboratory Center of the Research Institute of Petroleum Exploration and Development.

Mean vitrinite reflectance (Ro%) measurements and maceral analyses (500 points) were performed on the same polished sections of the coal samples using a Leitz MPV-3 photometer microscope, following con-ventional methods according to China National Stan-dards GB/T 6948-1998 and GB/T 8899-1998, respec-tively.

Kerogen carbon isotope δ13C‰ analyses were


820

performed by the Chinese Academy of Sciences and the Laboratory Center of the Research Institute of Pe-troleum Exploration and Development according to the China National Standard GB/T 18340.2-2001.

RESULTS AND DISCUSSION Evaluation of the Upper Jurassic Suowa Formation (J3s)

Approximately 200 samples were collected in the Suowa Formation, which yielded abundant organic

geochemical data. The samples were taken primarily from the Paducuo-Najiangcuo sag, the Tupocuo- Baitan Lake deep sag, the Dirangbicuo-Shangmen sag, the Dongcuo-Hulu Lake deep sag, the Queerchaka low uplift and the Baikamushouma profile.

Organic Material Abundance

The organic geochemical data of the source rocks are presented in Table 1. The evaluation standards for the abundance of the source rocks are listed in Tables 2 and 3.

Table 1 Organic matter abundance of the Suowa Formation, Upper Jurassic, in different structural

units of the Qiangtang Basin

Area Lithology Organic

carbon

content

(%)

Hydrocarbon-

generating

potential

(mg·g-1)

Chloroform

bitumen A

(ppm)

Total

hydrocarbon

(ppm)

Profile

Paducuo-Najingcuo

sag

Limestone 0.12–0.26 0.16(10)

0.04–0.26 0.12(10)

28–259 125(6)

46–166 103(4)

South Luxiongcuo

Tupocuo-Baitan

Lake deep sag

Limestone 0.08–0.82 0.14(90)

0.01–0.14 0.06(77)

10–77 46(12)

13–49 36(7)

East Lake, Yeniu channel,

Tupocuo, Bailongbin River

Mudstone 0.23–0.55 0.44(3)

0.12–1.67 0.69(3)

94(1) 74(1) Bailongbin River

Dirangbicuo-Tumen

sag

Limestone 0.12–2.15 1.00(39)

0.06–7.53 2.3(39)

533–53941215(18)

297–2487 668(17)

Anduo, Duogaerqu

Mudstone 1.08–1.75 1.42(2)

1.37–1.58 1.48(2)

Anduo

Dongcuo-Hulu Lake

deep sag

Limestone 0.08–0.17 0.11(17)

0.03–0.06 0.04(18)

5(1) Quemocuo, East Wulanwula

Lake Mountain, Zuerkenwula

Mountain

Mudstone 0.15–0.32 0.25(3)

0.05–0.06 0.05(3)

46–330 188(2)

281(1) Zuerkenwula Mountain

Coal 17.57(1) 1.75(1) 78(1) Zuerkenwula Mountain

Mayigangri high

uplift

Limestone 0.08–0.58 0.22(10)

0.08–0.33 0.15(10)

North Beileicuo

Queerchaka

low uplift

Limestone 0.05–0.25 0.14(4)

0.07–0.44 0.19(4)

40(1) 29(1) Yicangma

Mudstone 0.02–0.31 0.18(19)

0.04–0.26 0.12(19)

8–50 20(4)

3–39 21(2)

Yicangma, Dongqu

Baikamushouma

profile

Limestone 0.10–0.98 0.26(9)

0.14–2.74 0.53(9)

23–97 56(5)

4–66 37(5)

Baikamushouma

Mudstone 0.25–1.17 0.77(9)

0.23–1.41 0.90(9)

39–64 52(2)

27–44 36(2)

Baikamushouma

Note. minimum–maximumaverage(no. of samples) .


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Table 2 Division standards for the organic matter abundance in the Mesozoic marine carbonate

source rocks in the Qiangtang Basin

Hydrocarbon-generating rock division Good Medium Poor No

Organic carbon (%) >0.25 0.25–0.15 0.15–0.1 <0.1

Hydrocarbon-generating potential (mg/g) >0.30 0.30–0.10 0.01–0.06 <0.06

Chloroform bitumen A (ppm) >75 75–40 40–25 <25

Total hydrocarbon (ppm) >60 60–35 35–20 <20

Table 3 Division standards for the organic matter abun-

dance in the Mesozoic marine argillaceous source rocks in

the Qiangtang Basin

Grade of source

rocks

Good Medium Poor No

Organic carbon (%) >1.0 1.0–0.6 0.6–0.4 <0.4

Hydrocarbon

generation

capacity (mg/g)

>0.6 0.6–0.30 0.30–0.20 <0.20

Chloroform bitumen

A (ppm)

>120 120–70 75–45 <45

Total hydrocarbon

(ppm)

>100 100–60 60–40 <40

Salient inferences from the organic material test

results are as follows: (1) The limestones in the Paducuo-Najiangcuo sag were classified as interme-diate source rocks. (2) In the Tupocuo-Baitan Lake deep sag, the limestone forms a medium-level source rock. However, due to the small number of mudstone samples available, the limestone was provisionally classified as medium level until more evidence be-comes available. (3) In the Dirangbicuo-Tumen sag, both the limestone and mudstone were classified as high-quality source rocks. In particular, the limestones were of superior quality: an organic carbon content of 0.12%–2.15% (average value of 1.0%), with approx-imately 90% of the material testing above 0.25%; an average hydrocarbon-generating potential of 2.30 mg/g, with approximately 90% of the material testing above 0.3 mg/g (Fig. 3); a very high soluble organic material content, with an average chloroform bitumen A content of 1 215 ppm and an average total hydro-carbon content of 668 ppm.

(4) In the Dongcuo-Hulu Lake deep sag, both the limestone and mudstone are poor source rocks with

very low parameter values. (5) The limestone in the Mayigangri high uplift

comprises medium-level source rocks with the fol-lowing parameters: an organic carbon content of 0.08%–0.58% (average value of 0.22%), with ap-proximately 50% of the material yielding values above 0.15%; an average hydrocarbon-generating po-tential of 0.15 mg/g, with approximately 70% of the material yielding values above 0.1 mg/g.

(6) The Queerchaka low uplift limestone is a medium-level source rock, and the mudstone is not considered as a source rock.

(7) In the Baikamushouma profile at the margin of the Qiangtang Basin, the limestone is a good source rock but the mudstone is a poor hydrocarbon- ge-nerating rock.

Limestone is generally superior to mudstone as a source rock in the Suowa Formation; the best example of this superiority occurs in the Dirangbicuo-Tumen sag. The lateral distribution of the sedimentary facies, source rock thickness and residual organic carbon content (Figs. 4 and 5) indicate that the most favorable source rocks in this formation are present in the Anduo-Tumen area in the south-east segment of the basin, with non-hydrocarbon generating rocks present elsewhere.

Suitable limestone source rocks are located in the central North Qiangtang Basin and the southern South Qiangtang Basin, with the highest quality occurring in Ando toward the southeast of the basin. In this area, the average organic carbon content value was 1.0%, with approximately 65% of the material yielding a value of 1.0%; the average hydrocarbon-generating potential was 2.3 mg/g, with a maximum value of 7.53 mg/g; the average chloroform bitumen A value was 1 215 ppm, with a maximum value of 5 394 ppm; and the total hydrocarbon content value was 668 ppm,


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with a maximum value of 2 487 ppm. Organic materi-al contents are rarely as high as this in a limestone.

Limestone is a medium source rock in the Bai-kamushouma profile, the Paducuo-Najiangcuo sag, the Tupocuo-Baitan Lake deep sag, and the Queerchaka low uplift. The poorest source rocks are the limestone and mudstone in the Dongcuo-Hulu Lake sag and the mudstone in the Baikamushouma profile. The Queerchaka low uplift mudstone is not considered to be a source rock.

Organic Material Types

The outcrops of the J3s Formation rocks are widespread in this area; Table 5 summarizes the prop-

erties and the complex evolution of the organic mate-rials in each profile. Division standards for the organic material types are listed in Table 4.

(1) In the Tupocuo-Baitan Lake deep sag, the or-ganic material in the limestone of the Bailongbin Riv-er and the Tupocuo platform in the north is generally classified as Type IIA, whereas in the Yeniu channel and Suona Lake in the central region of the sag and in East Lake and Bandao Lake, the organic material is classified as Type IIB. In summary, the organic materi-al in the source rocks in this sag is largely classified as Type II but grades to Type IIA toward the north.

N=39 AVE=2.30

S S1 2+ (mg/g)

0 0.03 0.05 0.1 0.5 1 2 4 6 10

0

10

20

30

40(%

)N=39 AVE=1.00

TOC (%)

0 0.01 0.2 0.4 0.8 1 1.5 2 2.5 3

0

10

20

30

40

(%)

50

Figure 3. Abundance frequency of the limestone organic matter in the Upper Jurassic Suowa Formation in the Dirangbicuo-Tumen sag, Qiangtang Basin.

Basinboundary

0 40 km N

Sedimentaryfacies boundary

Positionname

Limestoneisopach

Geologicalsuture line

Upliftboundary

Geologicalprofile

Mudstoneisopach

40030020010050

5010

0

20

0

30

0

200

10050

40

0

100200

400600

100

200

400

600100

Transitional facies

Basin

Platform

Reef in

Marginalshore

Evaporatedlatformp

Marginplatform

Tuotuo River

Buqu

Esima

Nierong

LunpolaNima

WenbuLangtaideng

AnduoSuo County

114 railwaymaintenance squad

Suowa

Rongma Xiaochaka

Chalukou

Changdong

Juhua MountainRejuechaka

YarongDongbeisa

ng

Riasa

Biluocuo

Duxue M

ountain

Shuanghu

Bailongbing RiverDuogecuoren

East Lake

Nadigangri

Yeniu channel

QuemocuoYanshipinChibuzhangcuo

Tumencoal mine

Tanggula Army

Service Station

Kekexili-Jinsha River fractured suture zone

fractured suture zone

Bangong Lake-Nujiang

Figure 4. The depositional facies and source rock distribution in the Upper Jurassic Suowa Formation of the Qiangtang Basin.


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Table 4 Division standards for the organic matter types in

the Qiangtang Basin

Index and Type I IIA IIB III

Carbon isotope in

kerogen δ13C (‰)

<-28 -28 to

-26

-26 to

-24

>-24

Testing type index of

kerogen in the

microscope

>80 80–40 40–0 <0

Initial hydrogen index

after the recovery IHo

(mg hydrocarbon/g

organic carbon)

>750 750–500 500–250 <250

(2) In the Dongcuo-Hulu Lake deep sag, the core samples were obtained from the Zuerkenwula Moun-tain profile. Here, the source rocks were found to be associated with small amounts of coal, both of which contain Type III organic material, as is evident from the δ13C analysis. A limestone sample from the East Wulanwula Lake Mountain profile in the northern marginal step-fault zone also contains Type III organic material, with some Type IIB materials as well. In summary, the organic content of the mudstone, coal and limestone is poor, and it belongs primarily to the Type III category.

(3) In the Paducuo-Najiangcuo sag, mostly li-mestone samples were collected along the profiles in Luxiongcuo (Type IIA organic material), Dongbeisang (types IIB–III) and Beileicuo (Type IIA). Overall, Type II organic material predominates in the limestone, and the quality worsens from east to west.

(4) In the Dirangbicuo-Tumen sag, primarily li-mestone and argillaceous rocks were collected from the Anduo profile. Both the limestone and mudstone are of Type IIB in the eastern Anduo area.

(5) In the Queerchaka low uplift, mostly mud-stone and limestone samples were collected from the Yicangma and Dongqu profiles. The samples from both of the profiles are of Type III.

In summary, the organic content in the Suowa Formation was found to vary but consisted mainly of types II and III. To underscore the type regularity across the formation, Fig. 5 shows the distribution of the organic material as contours of the kerogen δ13C content. The organic contents of both the limestone and mudstone are generally poor in the east and im-

prove towards the west, despite no apparent difference between the rock types. This improvement may be associated with the major supply of muddy deposi-tional material and the presence of terrestrial organic material in the east. The source rock with the highest classification is Type IIA in the limestone, which was primarily found in Tupoco in the north-western basin and in Luxiongcuo in the southern basin. Organic Material Maturity

Outcrops of the Suowa Formation rocks are widely distributed and have provided a large quantity of thermal maturity data (Table 7). Evaluation stan-dards for the organic material maturity are listed in Table 6.

Figure 5 shows the lateral maturity distribution of the vitrinite reflectance (Ro), from which the follow-ing inferences were drawn.

(1) It is presently in the mature-to post-mature phase, with Ro ranging between 1.0% and 2.5%.

(2) The maturity is high in the western part of the basin, generally being in the high-mature to post- mature phase with large production of condensate and natural gas.

(3) The middle and eastern areas of the basin— Duogecuoren-Quemocuo to the south, Nadigangri- Biluocuo to the east, Anduo to the north and Yicang-ma to the west are low-maturity areas at late stages of peak hydrocarbon generation, wherein oil with Ro between 1.0% and 1.3% is primarily generated.

(4) The maturity level is higher in the central area and lower toward the margins and is significantly controlled by the thermal evolution of the basin, which is most active at the basin margins close to the plate suture zone and least active in the central uplift zone.

Controlling Factors of High Quality Source Rocks and Predictions of Favorable Source Rock Kitchens

The total organic carbon content of marine car-bonate source rocks is mainly controlled by the clay content, they show a positive correlation. The clay content often associated with low energy of carbonate depositional environment. Low energy depositional

Tabl

e 5

Com

preh

ensi

ve e

valu

atio

n of

org

anic

mat

ter

type

s in

prof

iles o

f the

Upp

er J

uras

sic

Suow

a Fo

rmat

ion,

Qia

ngta

ng B

asin

Prof

ile n

ame

Lith

olog

y K

erog

en (δ

13C

)

(‰)

Ker

ogen

type

inde

x

Sapr

opel

ic

(%)

Exin

ite

(%)

Vitr

inite

(%)

Iner

tinite

(%)

IHo

(mg·

g-1)

IH

(mg·

g-1)

Org

anic

mat

ter

type

Bai

long

Riv

er-T

upoc

uo

Lim

esto

ne

-26.

88

34

63

1 28

8

665

29

IIA

Mud

dy sh

ale

-24.

06

II

B–I

II

Yeni

u ch

anne

l-Suo

na L

ake

Lim

esto

ne

-25.

56

34

5 24

II

B

East

Lak

e-B

anda

o La

ke

Lim

esto

ne

-25.

56

II

B

East

Wul

anw

ula

Lake

Mou

ntai

n Li

mes

tone

-2

3.69

5

70

30

IIB–

III

Zuer

kenw

ula

Mou

ntai

n M

uddy

shal

e-2

2.52

66

83

5 13

43

6 13

II

I

Zuer

kenw

ula

Mou

ntai

n C

oal

-20.

53

-73

2

91

7 40

3 10

II

I

Luxi

ongc

uo

Lim

esto

ne

-27.

49

59

72

2 13

9

IIA

Don

gbei

sang

Li

mes

tone

-2

3.13

43

70

10

20

IIB

–III

Bei

leic

uo

Lim

esto

ne

-27.

83

71

83

2 11

5

IIA

And

uo

Lim

esto

ne

-24.

71

14

72

23

5

344

176

IIB

Mud

ston

e -2

4.10

19

76

18

6 20

8 10

4 II

B

Yic

angm

a-D

ongq

u Li

mes

tone

-61

7 4

76

13

379

252

III

Mud

dy sh

ale

-22.

41

-45

14

9 61

17

29

8 12

6 II

I

Ta

ble

6 D

ivis

ion

of th

erm

al e

volu

tion

phas

es o

f the

org

anic

mat

eria

ls in

the

Qia

ngta

ng B

asin

Evol

utio

n ph

ase

Prod

ucts

in d

iffer

ent e

volu

tion

phas

es

Vitr

inite

refle

ctan

ce

(Ro)

(%)

Polle

n co

lor

Polle

n co

lor

inde

x (S

CI)

The

max

imum

pyro

lysi

s pea

k (T

max

)

Imm

atur

e B

ioge

netic

gas

, im

mat

ure

heav

y oi

l <0

.5

Ligh

t yel

low

<2

.5

<430

Low

mat

ure

Low

-mat

ure

oil

0.5–

0.7

Yello

w

2.5–

3.5

430–

440

Mat

ure

Nor

mal

cru

de o

il B

efor

e pe

ak

0.7–

1.0

Dar

k ye

llow

3.

0–3.

5 44

0–45

0

Afte

r pea

k 1.

0–1.

3 B

row

n 3.

5–4.

0 45

0–48

0

Hig

hly

mat

ure

Con

dens

ate-

wet

gas

1.

3–2.

0 B

row

n 4.

0–4.

5 >4

80

Post

mat

ure

Met

hane

>2

.0

Bla

ck

>4.5

>4

80


825

environment is often conducive to the development and preservation of the organic matter, the carbonate rocks with high clay content may be the better source rocks (Zhu et al., 2007; Liu et al., 2006). Depositional environment is an important factor in controlling the development and distribution of high quality source rocks (Chen, 2005). The distribution and hydrocarbon- generating of marine carbonate source rocks are closely related to the depositional environment. The depositional environment not only controls the lithol-ogy, the abundance of organic matter, but also affects the element composition of kerogen and the hydro-carbon potential (Wang and Zhao, 2004). Marine hydrocarbon-rich source rocks (effective source rocks) mainly formed in underfilled deepwater basin facies, deepwater slope facies, deep margin continental shelf facies, platform foreslope facies, An enclosed to semi-enclosed environment of lagoon facies, restricted gulf facies and partial marine platform facies (mud-stone source rocks), coastal lagoon facies and marine delta facies.

Compared to the sedimentary environment of marine carbonate high quality source rocks mentioned above, during the deposition time of the Qiangtang Basin Upper Jurassic Suowa Formation (J3s). The ba-sin underwent a large scale of transgression, the cli-mate of the basin was semi-warm and semi-dry. The basin deposited a large area of marine carbonate plat-form during that time. The lithology of the intra-

platform sag is primarily marl, micrite and deep grey muddy shale, which generally implies a closed, deep-water depositional environment. The marl formed the hydrocarbon-rich source rocks. This is consistent with the hydrocarbon source rocks development characte-ristics of the carbonate reservoirs for oil and gas fields in the world. Many of the world’s organic rich source rocks associated with carbonate facies distributed in the vast continental shelf basin sag or platform sag. The main lithology of the corresponding sedimentary facies of source rocks in Qiangtang Basin is black muddy shale of deep sea facies to bathyal facies, marl and shale of intra-platform sag of carbonate platform facies, oil shale and mudstone of the lagoon subfacies or prodelta subfacies of the evaporation platform fa-cies. The source rocks relates to the evaporation plat-form facies deposited in the mesohaline environment, which represents restricted environment to evapora-tion environment. Among these sedimentary environ-ments, the most common type of the source rocks is Type II. The clastic rocks in deed provides carbonate oil-gas field with source rocks. Besides the deposi-tional environment, the maturity of the organic and the preserve conditions are the main controlling factors in the development of useful hydrocarbon source rocks. For example, the depth of the burial, the intense tec-tonic activities like uplift and erosion, the volcanic ac-tivity and the deep heat flow activity (Gao et al., 2009; Guo et al., 2008; Wang et al., 1999).

Table 7 Organic matter maturity in the Upper Jurassic Suowa Formation in

different structural units of the Qiangtang Basin

Area Vitrinite reflectance Ro% Profile name

Birangdicuo-Tumen sag 1.07–1.27 1.17(21)

Anduo-114 Railway

Maintenance Squad

Tupocuo-Baitan Lake deep sag 1.09–2.94 2.11(25)

Bailongbin River, Tupocuo,

Yeniu channel, Duogecoren

Paducuo-Najiangcuo sag 2.12(1) South Luxiongcuo

Dongcuo-Hulu Lake

deep sag 1.02–1.66 1.21(10)

East Lake, Duoersuodongcuo,

Zuerkenwula Mountain

Queerchaka

low uplift 1.26–1.61

1.32(9) Baikamushouma,

Yicangma

Note.

minimum–maximumaverage(no. of samples) .


826

Predictions of Favorable Source Rock Kitchens Analyses and evaluations of the marine source

rock properties in the different structural units of the J3s Formation has provided information that allows for the determination of the controlling factors for favora-ble and available kitchens; these factors include, for example, lithology, depositional facies, source rocks thickness, organic material abundance, type and ma-turity, etc..

The source rocks are mainly limestone and mud-stone in the J3s Formation. Bands of oil-bearing li-mestone occupy large areas parallel to the southern margin of the South Qiangtang depression and the central North Qiangtang depression and are sur-rounded by zones of available limestone. The oil-bearing areas for mudstone are more limited, com-prising a small exposure to the north of the No. 114 Railway Maintenance Squad in the south-eastern Di-rangbicuo-Tumen sag. The areas with potential utility are limited to the deep sag in the north-eastern corner of the Dongcuo-Hulu Lake in the North Qiangtang depression and a semicircular zone that includes the Tumen coal mine and the Tanggula Army Service Sta-

tion in the south-eastern Dirangbicuo-Tumen sag of the South Qiangtang depression (Fig. 6). The proper-ties of the favorable hydrocarbon-generating areas in the J3s Formation are listed in Table 8.

CONCLUSIONS

Marine source rocks in the Upper Jurassic Suowa Formation are mainly open platform limestones that occur in Dongcuo-Hulu Lake deep sag and Tupocuo- Baitan Lake deep sag of Qiangtang Basin. They are intermediate and the favorable source rock kitchen of limestone is larger than that of mudstone.

The organic matter type of the marine source rocks in the Upper Jurassic Suowa Formation is mainly Type II; the maturity is generally mature- highly-mature in a wide range from mature to post-mature stage and varies significantly but has spe-cific regularities in different areas. Low-maturity areas are mainly distributed in uplifts and the ones relevant with the central uplift in eastern basin.

Depositional environments are the dominant con-trolling factor in the development of useful hydrocar-bon source rocks. The J3s Formation was produced as

1.3

1.5

2.5

2.0

1.3

1.1

1.51.31.5

2.5

2.01.31.52.0

1.10.2

0.10.1

0.2

1.0

0.2

0.60.1

0.1

0.2

0.61.0

0.2

-24.0

-26.0

-26.0

-24.0

-26.0

-22

.0

0 40 km N

Tuotuo River

Buqu

Esima

NierongLunpolaNima

WenbuLangtaideng

AnduoSuo County

114 RailwayMaintenance SquadSuowa

RongmaXiaochaka

Chalukou

Changdong

Juhua

Mountain

Rejuechaka

Yarong

Dongbeisang

Riasa

Biluocuo

Duxue

Mountain

Shuanghu

Bailongbin RiverDuogecuoren

East Lake

Nadigangri

Yeniuchannel

Quemocuo

YanshipinChibuzhangcuo

Tum

en c

oalm

ine

Tanggula ArmyService Station

Mayigangri uplift


Bangong Lake-Nujiang fractured suture zone

Eastern WulanwulaLake Mountain

ZuerkenwulaMountain

Organic carbonabundance contour of

mudstone

Organic carbonabundance contour of

limestone

�13

KC contour ofmudstone

�13

KC contour oflimestone


Basinboundary

Upliftboundary

Geologicalprofile

Positionname

Vitrinitereflectance

--IIB

-III

-III

--IIA

--IIB

--IIA

-II

Figure 5. The distribution of the residual organic carbon content, Ro, the types of source rocks in the Upper Jurassic Suowa Formation of the Qiangtang Basin.


827

Table 8 Characteristic parameters of the favorable hydrocarbon-generating areas in the

Upper Jurassic Suowa Formation, Qiangtang Basin

Parameter and grade Lithology Depositional facies Source rocks

thickness (m)

Organic carbon

content (%)

Kerogen

type

Vitrinite reflec-

tance Ro (%)

Favorable hydrocar-

bon generation area

Limestone Platform 400–600 >0.25 II 1.1–2.5

Mudstone Platform 100–300 >1.0 IIA 1.3–2.0

Available hydrocar-

bon generation area

Limestone Platform, basin and

evaporated platform

200–600 0.15–0.25 II, III 1.1–2.5

Mudstone Platform and

transitional facies

50–400 0.6–1.0 IIA 1.1–2.0

Tuotuo River

Buqu

Esima

Nierong

LunpolaNima

WenbuLangtaideng

AnduoSuo County

114 RailwayMaintenance Squad

Suowa

RongmaXiaochaka

Chalukou

Changdong

Juhua

MountainRejuechaka

Yarong

Dongbeisang

Riasa

Biluocuo

Duxue

Mountain

Shuanghu

Bailongbin River Duogecuoren

East LakeNadigangri

Yeniu channelQuemocuo

YanshipinChibuzhangcuo

Tumen

coal

min

e

Tanggula ArmyService Station

Mayigangri uplift


Eastern Wulanwula Lake MountainZuerkenw

ulaM

ountain

0 40 km N

fractured suture zone

Bangong Lake-Nujiang

GeologicalProfile

II I III

III

II


I

I

II

II

Upliftboundary

Basinboundary

Positionname

Favorable hydrocarbongeneration area of limestone

Available hydrocarbongeneration area of limestone

Favorable hydrocarbongeneration area of mudstone

Available hydrocarbongeneration area of mudstone

Figure 6. Evaluation and prediction of the areas with hydrocarbon-generating potential in the Upper Ju-rassic Suowa Formation, Qiangtang Basin. the basin underwent large-scale marine transgression, when thick marine carbonates were deposited on the platform. The climate was semi-warm and semi-dry. The lithology of the intra-platform sag is primarily marl, micrite and black muddy shale, which generally implies a closed, deepwater depositional environment. The marl formed the hydrocarbon-rich source rocks. ACKNOWLEDGMENTS

The authors would like to thank the staff of the Guangzhou Institute of Geochemistry, the Chinese

Academy of Sciences and the Laboratory Center of Research Institute of Petroleum Exploration and De-velopment for performing the organic geochemical tests and analyses and are grateful to the anonymous reviewers whose comments improved the quality of this manuscript.

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characteristics of the upper jurassic marine source rocks and prediction of favorable source rock...

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