Geochemical characteristics, origin and carbon isotope reversal of the presalt natural gas in the Lower Paleozoic Ordovician carbonates, Ordos Basin, China

Shangru Yang^a,b, Wenzhe Gang^a,b*, Jingjing Cao^a,b*, Jianping Liu^c, Gang Gao^a,b, Wenlong Dang^a,b

^a State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, 102249, China

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

^c Exploration Division of China Petroleum Changqing Oilfield Company PetroChina, Xi’an Shaanxi 710018, China

Abstract: The origin and potential source of the presalt natural gas in the Lower Paleozoic Ordovician carbonates within the Ordos Basin, China, is heavily debated, and the causes of carbon isotope reversal are currently unclear. This study analyzes the source and origin of the presalt natural gas and determines the reasons for carbon isotope reversal. The results show that the nonhydrocarbons contain CO₂, H₂S, and N₂ and some gas samples have high H₂S contents of 9.02%–23.58%. The dryness coefficient (C₁/C₁₊) of 91.06%–99.99% (ave. 98.34%) suggests that the presalt gas is mainly a dry gas with some wet gas and that it originates from a high- to over-mature source. However, the results show that the presalt natural gas is primarily a wet-gas-cracking gas, rather than a kerogen-cracking gas, with a certain amount of coal-type gas mixing and it mainly originated from oil-prone marine carbonate source rocks rather than the Permian–Carboniferous coal measures. The gas has different types of carbon isotope reversal patterns, and the δ¹³C₂ values become negative at Ro_equ > 1.5%. The mixing between the oil-cracking and wet-gas-cracking gas at Ro_equ > 1.5% results in carbon isotope reversal (δ¹³C₁ > δ¹³C₂), rather than Rayleigh and migration fractionation. The occurrence of thermochemical sulfate reduction (TSR) is supported by the high abundance of H₂S in the presalt gas, reservoir bitumen in dissolved pores, secondary calcite with negative δ¹³C_carbonate of −21.1‰ to −9.7‰, and diagenetic cubic pyrite. It is evident that the heavy δ¹³C₂ values (greater than −28‰) and the carbon isotope reversal of δ¹³C₂ > δ¹³C₃ of the presalt natural gas at Ro_equ < 1.5% are related to TSR. This study also establishes a new model of the carbon isotope reversal of the natural gas, which is primarily controlled by its thermal maturity and TSR. Six carbon isotope patterns can be well explained and are supported by real gas samples from the presalt strata, and two types of carbon isotope reversal are predicted. 

Keywords: Carbon isotope reversal; Oil-derived gas; Thermochemical sulfate reduction; Maturity; Presalt natural gas; Ordos Basin

[1] Yang Shangru, Gang Wenzhe, Cao Jingjing, et al. Geochemical characteristics, origin, and carbon isotope reversal of the presalt natural gas in the Lower Paleozoic Ordovician carbonates, Ordos Basin, China[J]. Marine and Petroleum Geology, 2022, 139, 105577. (中科院2区, IF: 4.348)

《鄂尔多斯盆地下古生界奥陶系盐下天然气地球化学特征、成因及碳同位素倒转》 

鄂尔多斯盆地是我国第二大含油气盆地，1989年陕参1井获得28.34×10⁴ m³/d的工业气流，标志着靖边气田的发现。2021年5月18日首次在鄂尔多斯盆地中东部奥陶系盐下地层中发现20.73×10⁴ m³/d的工业气流，证实了奥陶系马家沟组盐下天然气具有巨大的勘探潜力。现有下古生界气藏均接近石炭系-奥陶系不整合面，称为“风化壳气藏”(Cai et al., 2005)，累计探明储量为870 ×10⁸ m³。然而，风化壳气藏的来源仍存在很大争议：(1)主要是源自奥陶系海相烃源岩的油型气(或油裂解气)，存在二叠系-石炭系(P - C)煤系烃源岩生成的少量煤型气；(2)主要来源于P-C煤系烃源岩的煤型气，部分来源于石炭系海陆过渡相灰岩(Dai et al., 2005; Han et al., 2018)或(3)混有奥陶系海相烃源岩生成的油型气。争议的原因如下：(1)多套烃源岩层系。包括P - C煤系烃源岩(Dai et al., 2005; Han et al., 2018)、石炭系海陆过渡相灰岩(Han et al, 2018)以及奥陶系浅海台地相烃源岩(Cai et al., 2005); (2)碳同位素组成复杂，大部分天然气碳同位素倒转特征明显(Han et al., 2018); (3)天然气成因复杂，可能受多因素控制。如混合过程(Dai et al., 2014; Han et al., 2018)、运移、热成熟度(Xia et al., 2013)和热化学硫酸盐还原作用(TSR) (Cai et al., 2005)等。以往的研究主要集中于下古生界风化壳气藏，对奥陶系盐下天然气的研究较少。盐下天然气不同类型的碳同位素倒转样式的成因尚不清楚。本次研究基于鄂尔多斯盆地中东部奥陶系马家沟组盐下地层中127个天然气样品，聚焦盐下天然气的来源、成因及碳同位素倒转的原因。奥陶系盐下天然气主要来源于马家沟组海相碳酸盐岩烃源岩，天然气类型以油型气为主，存在少量煤型气的混入。非烃中含有CO₂、H₂S和N₂，部分天然气样品的H₂S含量较高，介于9.02% ~ 23.58%之间。天然气干燥系数(C₁ / C₁₊)为91.06% ~ 99.99%(平均为98.34%)，表明盐下天然气以干气为主，部分为湿气。盐下天然气主要是湿气裂解气，其次是油裂解气，而不是干酪根裂解气。此外，盐下天然气具有δ¹³C₃ > δ¹³C₁ > δ¹³C₂、δ¹³C₁ > δ¹³C₂ > δ¹³C₃、δ¹³C₂ > δ¹³C₁ > δ¹³C₃等不同类型的碳同位素倒转样式（Fig. 1）。盐下天然气高丰度的H₂S含量、溶蚀孔隙中储层沥青、δ¹³C_carbonate为-21.1‰~ -9.7‰的次生方解石和次生成因的立体黄铁矿等证实了奥陶系马家沟组存在热化学硫酸盐还原作用(TSR)。本次研究首次建立了成熟度和TSR协同控制天然气碳同位素倒转的新模式。研究证实，当Ro_equ > 1.5%时，天然气δ¹³C₂值明显偏负。当成熟度较高时（Ro_equ >1.5%），原油裂解气与湿气裂解气的混合导致碳同位素倒转(δ¹³C₁ > δ¹³C₂)，而不是由于瑞利分馏和运移分馏。当成熟度较低时（Ro_equ<1.5%），盐下天然气δ¹³C₂值偏重(> -28‰)，天然气乙烷和丙烷碳同位素倒转（δ¹³C₂ > δ¹³C₃）主要受TSR控制，也可能受到源自P-C煤系烃源岩的煤型气的混合作用影响。利用新模式较好地解释了盐下天然气6种碳同位素组合样式，均得到天然气实例的证实（Fig. 2），并预测了2种类型的碳同位素反转样式。本文研究结果对鄂尔多斯盆地今后的盐下天然气勘探具有应用价值，对确定海相碳酸盐岩天然气的成因和来源具有非常重要的意义。

 关键词：碳同位素倒转；油型气；TSR；成熟度；盐下天然气；鄂尔多斯盆地

参考文献：

[1] Cai, C.F., Hu, G.Y., He, H., Li, J., Li, J.F., Wu, Y.S., 2005. Geochemical characteristics and origin of natural gas and thermochemical sulphate reduction in Ordovician carbonates in the Ordos Basin, China. Journal of Petroleum Science and Engineering 48: 209-226.

[2] Dai, J.X., Li, J., Luo, X., Zhang, W.Z., Hu, G.Y., Ma, C.H., Guo, J.M., Ge, S.G., 2005. Stable carbon isotope compositions and source rock geochemistry of the giant gas accumulations in the Ordos Basin, China. Organic Geochemistry 36: 1617-1635.

[3] Han, W.X., Ma, W.J., Tao, S.Z., Huang, S.P., Hou, L.H., Yao, J.L., 2018. Carbon isotope reversal and its relationship with natural gas origins in the Jingbian gas field, Ordos Basin, China. International Journal of Coal Geology 196: 260–273.

[4] Xia, X.Y., Chen, J., Braun, R., Tang, Y.C., 2013. Isotope reversals with respect to maturity trends due to mixing of primary and secondary products in source rocks. Chemical Geology 339: 205–212.

鄂尔多斯盆地中东部马家沟组TSR和成熟度控制的天然气碳同位素倒转模式