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Geochemical and Isotopic Evidences of the Genesis of a Condensate in the Eastern Tarim Basin, China: Implications for Petroleum Exploration Zhiyao Zhang, Yijie Zhang, Guangyou Zhu, Jianfa Han, and Linxian Chi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00484 • Publication Date (Web): 28 May 2019 Downloaded from http://pubs.acs.org on May 31, 2019
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Energy & Fuels
Geochemical and Isotopic Evidences of the Genesis of a Condensate in the Eastern Tarim Basin, China: Implications for Petroleum Exploration
Zhiyao Zhang†
†
Yijie Zhang†
Guangyou Zhu*,†
Jianfa Han‡
Linxian Chi†
Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083,
China ‡
Research Institute of Petroleum Exploration and Development, Tarim Oilfield Company,
PetroChina, Korla 841000, China
* Corresponding author. Tel.: +86 10 8359 2318;
+86 18601309981
E-mail address:
[email protected] (G. Zhu)
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Abstract:
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The exploration activities in the eastern Tarim Basin have been thwarted over the last decade
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after the discovery of several Jurassic gas condensate fields. In this study, the 2D gas
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chromatography/time-of-flight mass spectrometry (GC×GC-TOFMS) and compound specific
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carbon isotope analysis (CSCIA) were performed on a Jurassic condensate (Yingnan2) and
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the associated gas to determine its genesis and the accumulation process in this area. The
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geochemical and isotopic features suggested that the condensate analyzed was a mixture of
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the Ordovician cracked paleo-oil with the Jurassic intact oil as evidenced by the concentrated
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diamondoids and ethanoadamantanes, the high gas generation temperature (~195 oC), the
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heavy whole oil δ13C (-28.6‰), and the significant variation in the isotopic profile of the
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n-alkanes. As the gas amount was constantly elevated due to both oil cracking and the mixing
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of the kerogen-cracking gas from the Cambrian source rock, phase transition occurred and
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thus formed the Yingnan2 secondary condensate. The constant mixing of nitrogen-rich
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kerogen-cracking gas complementarily caused the increased nitrogen gas content. The
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accumulation model of the condensate in the eastern Tarim Basin was proposed with
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consideration of the post-accumulation alterations including thermal cracking and mixing
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which complicated the quality and distribution of subsurface petroleums. It was further
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speculated that abundant gas and condensate resources may be preserved in favorable
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reservoir-seal assemblages in this field.
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Key words:
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Gas condensate; Oil cracking; Gas mixing; Hydrocarbon accumulation; Jurassic; eastern
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Tarim Basin 2
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1. Introduction Post-accumulation physiochemical alterations may exert significant impacts on primary 1,2,
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petroleum accumulations
leading to the adjustment, redistribution and even complete
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destruction of primary petroleums
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towards greater depths where complex secondary alterations under high temperature and high
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pressure conditions are involved
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reduction, water washing and hydrocarbon diffusion may solely or jointly impact the
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chemical features and distribution of accumulated petroleums, and thus to determine which of
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the processes are operative is significant for the improved understanding of the petroleum
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accumulation and exploration potential in the field.
6–9.
3–5.
In recent years, exploration interests have shifted
Thermal cracking, gas invasion, thermochemical sulfate
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Tarim Basin is the largest petroliferous districts in China and tremendous petroleum
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resources have been discovered in the central and northern parts 10. However, only a few gas
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condensate accumulations were obtained in the eastern Tarim Basin despite its coverage of
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over 100000 km2
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accumulation in the eastern Tarim Basin, previous studies focused more on the geological
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analyses regarding the source-reservoir-seal assemblages
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the hydrocarbon generation and expelling history 16,17 and thus the condensate was believed to
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have been originated from cracked oils in Ordovician paleo-oil pools. Nevertheless,
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controversies remain about the genesis and accumulation process of the Yingnan2 condensate
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and thus hindered subsurface fluid prediction and next-field exploration deployment.
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11.
As to the Yingnan2 condensate, the unique industrial condensate
12,13,
migration pathways
14,15,
and
In this study, we focus on the geochemical and isotopic features of the Yingnan2 3
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condensate and the associated gas. The results from GC×GC-TOFMS and CSCIA presented
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here, in combination with geological analysis, demonstrate improved understanding of the
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post-accumulation processes in the subsurface. The genesis, accumulation process and
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possible alterations of the Yingnan2 condensate were determined and future exploration
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potential in the eastern Tarim Basin was implied.
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2. Materials and methods
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2.1 Geological settings and samples
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The Yingnan2 well is located in the Yingjisu depression in the eastern Tarim Basin, this
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area has experienced rapid subsidence during the Cambrian-Silurian, followed by uplift and
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erosion from the late Devonian to Triassic and constant subsidence after Triassic 18. The strata
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drilled by the Yingnan2 well include, from old to new, the upper Ordovician, Silurian,
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Jurassic, the lower Cretaceous and the whole Cenozoic. According to the seismic
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interpretation and regional field studies, the lower Ordovician consists of huge clastic
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sedimentary rocks (over 5000m) and the Cambrian high-quality source rock is developed at
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depths over 10000m 19.
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Two sets of source rocks were developed in this region including the Cambrian marine
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shales and the Jurassic coal measures. The Cambrian marine shales have been confirmed as
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the major petroleum source of the Ordovician reservoirs with wide distribution in the Tarim
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Basin 19,20, while the Jurassic coal measures are relatively locally formed in the eastern Tarim
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Basin. Based on the geothermal and tectonic evolution history, the Cambrian source rock has
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reached over-mature stage while the Jurassic coal measures are overall immature (%Ro100 through the GC×GC-TOFMS analysis, including aliphatics (n-alkanes
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and cycloalkanes), aromatics (benzenes, naphthalenes and phenanthrenes), diamondoids
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(adamantanes and diamantanes) and ethanoadamantanes (Figure 1a). As illustrated in Figure
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1b using EICs (extracted ion chromatograms) of m/z = 71, 85, 99, respectively, the
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condensate covers a wide range of n-alkanes series (n-C6 – n-C29), in which lower MW
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(molecular weight) compounds are more enriched and >C20 compounds are of relatively low
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concentrations. Unlike the primary thermal condensates generated at high-maturity stages that
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are dominated by lower compounds before n-C10 22, the Yingnan2 condensate is more likely
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of secondary genesis.
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Figure 1. GC×GC-TOFMS color contour chromatograms of the Yingnan2 condensate. (a) 7
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Total ion current chromatogram highlighting distinctive groups of aliphatics, aromatics,
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diamondoids and ethanoadamantanes, each compound series is marked with circles or boxes;
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(b) m/z 71 + 85 + 99 highlighting n-alkanes.
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A total of 79 isomers and homologues of alkylated adamantanes were detected in the
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Yingnan2 condensate (Figure S1, Table 1), and the exact isomeric configuration of large
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proportion of the alkylated adamantanes detected could not be unequivocally established
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except several specific isomeric assignments according to previous studies
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with the concentration of alkylated diamondoids in typical condensates, volatile oils and oils
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in the Tarim Basin (Table 2), the Yingnan2 condensate has an extensive series of alkylated
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adamantanes, extending to C5, with the total concentration of 21.09 mg/g. Alkylated
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diamantanes showed less isomers and homologues with the lower summed concentrations of
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0.17 mg/g (Figure S2, Table 1, 2). Notably, a series of ethanoadamantanes with the total
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concentration of 0.45 mg/g were identified in the condensate (Table 1, 2) and their
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distributions were marked in Figure 2 and the corresponding mass spectra were shown in
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supplementary Figure S3.
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23,24.
Compared
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Energy & Fuels
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Figure 2. GC×GC-TOFMS color contour chromatogram of ethanoadamantanes in the
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Yingnan2 condensate using EICs of m/z 162 + 161 + 176 + 175 + 189. EA =
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ethanoadamantane.
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Table 1. The numbers of alkylated diamondoids and ethanoadamantanes detected in the
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Yingnan2 condensate. A Total isomer numbers detected in this study; numbers in the brackets
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represent the isomer numbers from previous GC×GC–MS studies. 23–26 Isomer Number A
Compounds
MW
Adamantane
136
C1-adamantane
150
C2-adamantane
164
8 (7)
C3-adamantane
178
22 (6)
C4-adamantane
192
32 (5)
C5-adamantane
206
15 (2)
2 (2)
Diamantane
188
C1-diamantane
202
3 (3)
C2-diamantane
216
6 (6)
C3-diamantane
230
2 (2)
Ethanoadamantane
162
C1-ethanoadamantane
176
3
C2-ethanoadamantane
190
4
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Table 2. The concentration of alkylated diamondoids and ethanoadamantanes detected in the
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Yingnan2 condensate, and typical condensates, volatile oils and oils in the Tarim Basin. Compound
Concentration (mg/g) Yingnan2
Condensates
Volatile Oils
Oils
Adamantane
0.64
0.15-0.65
0.01-0.11
