Origin and Source of the Cenozoic Gas in the Beach Area of the Nanpu Sag,Bohai Bay Basin,China

Abstract

Natural gas and condensate oil and gas have been discovered in structures in the beach area of the Nanpu sag, Bohai Bay Basin, eastern China. The gas is dominantly hydrocarbon gas with less non–hydrocarbons, it is wet gas and has a dry coefficient in the range of 0.7–0.9. The gas has a methane carbon isotope in the range of −44‰–36‰ and a ethane carbon isotope in the range of −30‰– −24‰. Analysis of the relationship charts between ¹³C₁–C₁/C₂₊₃ and δ¹³C_CH4–δD_CH4, δ¹³C–R_o equation, Pr/Ph of associated oil or condensate, suggests that the Cenozoic gas in the beach area of the Nanpu sag is mature – high mature oil–type gas. According to analysis of origin and source of the gas and condensate oil, in combination of monomer hydrocarbon carbon isotope and biomarker, the Cenozoic gas in the beach area of the Nanpu sag is sourced from the Tertiary Shahejie Formation Sha 3 Member (Es3) source rocks and Sha 1 Member (Es1) source rocks. Among these gases, the gas from Es3 source rocks is equivalent to condensate gas formed when source rock vitrinite reflectance (R_o) is at or above 1.3%. This type of gas has heavier carbon isotope, ethane carbon isotope generally more than −27‰, associated oil C₂₄–tetracyclic terpane /C₂₆–tricyclic terpane ratio less than 0.7, the distribution of monomer hydrocarbon carbon isotopes being flat. The gas from Es1 source rocks is equivalent to oil–associated gas formed when source rock vitrinite reflectance (R_o) is at or below 1.0%. This type of gas has lighter carbon isotope, ethane carbon isotope generally less than −28‰, associated oil C₂₄–tetracyclic terpane /C₂₆–tricyclic terpane ratio more than 1.0, the distribution of monomer hydrocarbon carbon isotopes having clear inflection points. This study provides reference to study of gas origin in a complicated region, and makes a further step to show exploration potential and fields for gas in the Nanpu sag.

Keywords

Gas Origin Carbon Isotope Condensate Oil Cenozoic Nanpu Sag Bohai Bay Basin

References

Dai

J.X.

Pei

X.G.

and Qi

H.F.

, 1992. Natural Gas Geology in China(Vol. 1). Beijing: Petroleum Industry Press, pp. 298 (in Chinese).

Dai

J.X.

and Song

, 1987. The index to distinguish coal–genetic gas. In: The geological research of coal–genetic gas. Beijing: Petroleum Industry Press, 156–170 (in Chinese).

Dai

J.X.

Zou

C.N.

Zhang

S.C.

Y.Y.

G.Y.

Luo

Tao

S.Z.

Zhu

G.Y.

J.K.

Z.S.

A.P.

Yang

Zhou

Q.H.

and Shuai

Y.H.

, 2008. Discrimination of abiogenic and biogenic alkane gases. Science in China (Series D) 51, 1737–1749.

Dai

J.X.

, 1992. Identification and distinction of various alkane gases. Science in China (Series B) 2, 187–193.

Dai

J.X.

Xia

X.Y.

Qin

S.F.

and Zhao

J.Z.

, 2004. Origins of partially reversed alkane ?13C values for biogenic gases in China. Organic Geochemistry 35, 405–411.

Dai

J.X.

Zou

C.N.

Qin

S.F.

Tao

S.Z.

Ding

W.W.

Liu

Q.Y.

and Hu

A.P.

, 2008. Geology of giant gas fields in China. Marine and Petroleum Geology 25, 320–334.

Faber

and Stahl

, 1987. Gaseous hydrocarbons of unkown origin found while drilling. Organic Geochemistry 13, 875–879.

C.Y.

, 1982. Source bed controls hydrocarbone habitat in continental basins, east China. Acta Petrolei Sinica 3, 9–13.

Kadir

Philp

R.P.

and Chris

Hasan

Muzaffer

, 2005. Geochemical and isotopic approach to maturity–source–mixing estimations for natural gas and associated condensates in the Thrace Basin, NW Turkey. Applied Geochemistry 20(11), 2017–2037.

10.

Liu

W.H.

Chen

M.J.

Guan

Zheng

J.J.

Jin

Wang

W.C.

G.Y.

Xia

Y.Q.

and Zhang

D.W.

, 2007. Ternary geochemical–tracing system in natural gas accumulation. Science in China(series D) 37, 908–915.

11.

Liu

W.H.

Zhang

D.W.

Wang

X.F.

and Nan

Q.Y.

, 2004. Geochemistry study on gas–source corrlation of natural gas. Acta Sedimentologica Sinica 22, 27–32.

12.

Schoell

, 1980. The hydrogen and carbon isotopic composition of methane from natural gases of various origins. Geochim Cosmochim. Acta 44, 649–661.

13.

Song

and Xu

Y.C.

, 2005. Origin and identification of natural gases. Petroleum Exploration and Development 4, 24–29.

14.

Shen

Y.C.

Wang

X.B.

Liu

D.H.

Shen

Q.X.

and Liu

W.H.

, 1991. Studies on geo–chemical characteristics of gas–source rock and natural gas and mechanism of genesis of natural gas. Lanzhou: Gansu. Science and Technology Press, pp. 243 (in Chinese).

15.

Stahl

W.J.

and Carey

B.D.

, 1975. Source–rock identification by isotope analyses of natural gases from fields in the Val Verde and Delaware basins, West Texas. Chemical Geology. 16, 257–267.

16.

Wang

Z.J.

Zhao

Z.X.

Xia

J.S.

Zhang

Y.C.

Liu

Y.C.

and Wang

J.W.

, 2012. Natural gas origin and model of gas accumulation in deep volcanic rocks in Nanpu Sag, Bohai Bay Basin. Acta Petrolei Sinica, 33, 772–780.

17.

Zhang

S. C.

and Zhu

G. Y.

, 2008. Natural gas origins of large and medium–scale gas fields in China sedimentary basins. Science in China(series D), 51(Supp1), 1–13

18.

Zhao

W.Z.

Zou

C.N.

Wang

Z.C.

J.Z.

and Niu

J.Y.

, 2004. The intension and signification of “Sag–wide Oil–Bearing Theory” in the hydrocarbon–rich depression with terrestrial origin. Petroleum Exploration and Developmen 31, 5–13.

19.

Zheng

H.J.

Dong

Y.X.

Zhu

G.Y.

Wang

X.D.

and Xiong

, 2007. High–quality source rocks in Nanpu sag. Petroleum Exploration and Development 34, 385–391 (in Chinese).

20.

Zhou

H.M.

Dong

Y.X.

Xie

Z.N.

Liao

B.F.

and Zhou

F.M.

, 2004. Precise Exploration in faulted basin: Precise exploration practice and knowledge in Nanpu depression, Bohai Bay Basin. Beijing: Petroleum Industry Press, pp. 148 (in Chinese).

21.

Zhu

G.Y.

Jin

Dai

J.X.

Zhang

S.C.

Zhang

L.Y.

and Zhang

Y.L.

, 2005. Composite hydrocarbon–generation system: An important concept for source rock evaluation and hydrocarbon prediction in rift lacustrine basin. Chinese Journal of Geology 40, 133–144.

22.

Zhu

G.Y.

Jin

Zhang

S.C.

Dai

J.X.

Zhang

L.Y.

and Li

, 2005. Character and genetic types of shallow gas pools in Jiyang depression. Organic Geochemistry 36, 1650–1663.

23.

Zhu

G.Y.

Zhang

S.C.

Huang

H.P.

Liang

Y.B.

Meng

S.C.

and Li

Y.G.

, 2011a. Gas genetic type and origin of hydrogen sulfide in the Zhongba gas field of the western Sichuan Basin, China. Applied Geochemistry 26, 1261–1273.

24.

Zhu

G.Y.

Zhang

S.C.

Wang

Y.J.

Wang

Z.J.

Zheng

H.J.

Xiong

Dong

Y.X.

Wang

X.D.

and Zhang

, 2011b. High quality source rocks and hydrocarbon accumulation in Nanpu sag, Bohai Bay Basin, China. Acta geologica sinica 85(1), 97–113.

25.

Zhu

G.Y.

L.J.

Dai

J.X.

Ding

W.L.

Zhang

J.C.

and Song

L.C.

, 2012a. Sedimentary association of alternated mudstones and tight sandstones in China's oil and gas bearing basins and its natural gas accumulation. Journal of Asian Earth Sciences 50, 88–104.

26.

Zhu

G.Y.

Zhang

S.C.

Huang

H.P.

Yang

H.J.

L.J.

Zhang

and Zhu

Y.F.

, 2012b. The occurrence of ultra–deep heavy oils in the Tabei Uplift of the Tarim Basin, NW China. Organic Geochemistry 52, 88–102.

27.

Zhu

G.Y.

Zhang

S.C.

Liu

K.Y.

Yang

H.J.

Zhang

and Zhang

Y.G.

, 2013. A well–preserved 250 million–year–old oil accumulation in the Tarim Basin, western China: Implications for hydrocarbon exploration in old and deep basins, Marine and Petroleum Geology 43, 478–488.