Natural Gas Geoscience ›› 2020, Vol. 31 ›› Issue (7): 980-992.doi: 10.11764/j.issn.1672-1926.2020.01.008

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Oil-gas origin and accumulation characteristics of Wenchang Depression, western Pearl River Mouth Basin

Bi-wei WANG(),Xin-de XU,Yang-yu WU,Jun-jun YOU,Ming-zhu LEI   

  1. Zhanjiang Branch of China National Offshore Oil Corporation (CNOOC) Limited, Zhanjiang 524057, China
  • Received:2019-12-24 Revised:2020-01-27 Online:2020-07-10 Published:2020-07-02
  • Supported by:
    Science and Technology Project of China National Offshore Oil Corporation Limited(YXKY-2018-KT-01)

Abstract:

Wenchang Depression, which is a Cenozoic dustpan fault depression stretching towards NE-SW trend and faulted in the south part and overlapped in the north part, is located in Zhusan Depression of Pearl River Mouth Basin in the north of South China Sea. Based on the organic geochemistry and fluid inclusion and basin simulation analysis, the source of oil and gas and the characteristics of hydrocarbon accumulation in Wenchang Depression are studied. The results show that the oil and gas from the middle deep lacustrine facies sapropelic source rocks of Wenchang Formation in the deep part of Wenchang B Depression migrate vertically along the South fault to the reservoir of Zhuhai Formation for accumulation. The oils from Wells W1, W2, W3 in the South and Well W4 in the north of Wenchang 19-1 structure are typical middle deep lacustrine oil. The oils from Wells W5 and W6 in the south of Wenchang 19-1 structure are mixtures from the middle deep lacustrine facies and shore shallow lacustrine facies source rocks of Wenchang Formation. The crude oil in Enping Formation of Well W14 in the north of Wenchang 19-1 structure, which came from the humic mixed source rock of shore shallow lacustrine facies of Wenchang Formation, was charged during 8.7-1 Ma. The crude oil in the third member of Zhuhai Formation of Well W15 in the central area of Wenchang A Depression was mainly from the source rock of Enping Formation, and the oil and gas filling time was about 4.1-0.2 Ma.

Key words: Western Pearl River Mouth Basin, Wenchang Depression, Geochemical technique, Fluid inclusions, Oil-gas origin, Accumulation characteristics

CLC Number: 

  • TE122.1

Fig.1

Tectonic units division of western Pearl River Mouth Basin"

Table 1

Geochemical features in source rocks of Wenchang Sag"

层位相带TOC/%IH/(mg/gTOC)δ13C/‰
文昌组中深湖1.31~5.22/2.84(61)298.96~962.69/545.39(56)-22.13~-25.82/-24.49(9)
滨浅湖0.4~9.45/2.15(68)13.56~935.48/375.26(65)-25.8~-27.8/-26.42(5)
恩平组滨浅湖0.41~13.79/1.37(324)33.96~548.15/139.42(314)-26~-28/-27.47(13)
河沼相0.42~9.88/2.73(61)40.68~246.3/124.7(52)-26.67~-30.02/-28.43(11)

Fig.2

Distribution of element types of kerogen from source rocks in Wenchang Sag"

Fig.3

Mass chromatograms of saturated hydrocarbon of source rocks in Wenchang Sag"

Fig.4

Distribution of maturity of top surface from source rocks in Wenchang Sag"

Table 2

Physical properties of oils in Wenchang Sag"

凹陷井号深度/m层位δ13C/‰主要物理性质
密度/(g/cm3)含蜡量/%含硫量/%
文昌A凹陷W113 490~3 525E3zh2-290.783 75.440.06
W113 390~3 415E3zh2-29.40.765 14.480.04
W113 230~3 250E3zh2-29.40.762 72.510.04
W123 661~3 699E3zh2-29.30.8048.750.07
W132 223~2 240N1zj2-29.40.775 94.990.09
文昌B凹陷W11 759~1 771E3zh2-24.30.853 122.730.07
W42 019~2 045E3zh2-23.70.8673.20.1
W31 381.2N1zj2-23.90.875 621.630.11
W51 260~1 276N1zj2-25.10.910 69.730.16
W61 277~1 281N1zj2-26.60.9213.210.14
W142 919.81E3ep1-30.20.811 417.40.05

Fig.5

Mass chromatograms of saturated hydrocarbon of oils in Wenchang Sag"

Table 3

Compositions and isotopes of natural gas in Wenchang Sag"

凹陷井号井深/m层位组分/%C1/∑Cnδ13C/‰
C1N2CO2C1CO2
文昌A凹陷W132 285~2 308E3zh143.621.3138.870.73-41.88-4.53
2 343~2 365E3zh237.990.0447.990.73-41.66-4.65
2 378~2 417E3zh241.581.7942.690.75-41.94-4.42
W93 799.9E3ep112.982.0580.960.77-43.73-4.62
3 860.55E3ep123.540.2369.040.77-41.91-4.90
3 915.5E3ep120.260.6174.450.81-39.44-4.73
3 954.5E3ep112.810.5983.250.80-40.01-4.75
3 981E3ep17.760.3789.260.76-41.13-4.57
W113 230~3 250E3zh277.860.055.870.83-39.77-10.15
3 390~3 415E3zh268.700.416.520.74-43.44-8.54
W123 153.7E3zh177.91.494.630.83-38.25-11.34
3 661~3 699E3zh276.893.394.780.84-39.69-7.88
文昌B凹陷W142 877.34E3ep154.664.152.780.59-37.52-17.38
W11 700~1 712E3zh270.251.8012.190.82-40.00-8.40
1 759~1 771.5E3zh260.311.397.480.66-39.90-12.80
W51 260~1 280N1zj264.485.1322.430.89-50.40-5.90
W72 546.20E3zh234.383.306.120.38-49.49-16.22

Fig.7

Distribution of δ13C and ∑C30-4-MST/∑C29ST from crude oil and typical source rocks in Wenchang B Sag"

Fig.8

Hrdrocarbon inclusions from Well W15 in Wenchang Sag"

Fig.9

Distribution of homogenization temperatures of fluid inclusions of Well W15(a) and Well W14(b) in Wenchang Sag"

Fig.10

Burial history of Well W15(a) and Well W14(b) in Wenchang Sag"

Fig.11

Hrdrocarbon inclusions from Well W14 in Wenchang Sag"

Fig.12

Oil-gas accumulation pattern of Wenchang19-9 structure(see Fig.1 for section locution)"

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