Article Cite This: Energy Fuels 2019, 33, 4849−4856
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Geochemical and Isotopic Evidence 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† †
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
‡
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ABSTRACT: The exploration activities in the eastern Tarim Basin have been thwarted over the last decade after the discovery of several Jurassic gas condensate fields. In this study, the two-dimensional gas chromatography/time-of-flight mass spectrometry (GC × GC-TOFMS) and compound-specific carbon isotope analysis were performed on a Jurassic condensate (Yingnan2) and the associated gas to determine its genesis and the accumulation process in this area. The geochemical and isotopic features suggested that the condensate analyzed was a mixture of the Ordovician cracked paleo-oil with the Jurassic intact oil as evidenced by the concentrated diamondoids and ethanoadamantanes, the high gas generation temperature (∼195 °C), the heavy whole oil δ13C (−28.6‰), and the significant variation in the isotopic profile of n-alkanes. As the gas amount was constantly elevated due to both oil cracking and the mixing of the kerogen-cracking gas from the Cambrian source rock, phase transition occurred and thus formed the Yingnan2 secondary condensate. The constant mixing of nitrogen-rich kerogencracking gas complementarily caused an increase in the nitrogen gas content. The accumulation model of the condensate in the eastern Tarim Basin was proposed with consideration of the post-accumulation alterations including thermal cracking and mixing, which complicated the quality and distribution of subsurface petroleums. It was further speculated that abundant gas and condensate resources may be preserved in favorable reservoir-seal assemblages in this field. subsurface fluid prediction and next-field exploration deployment. In this study, we focus on the geochemical and isotopic features of the Yingnan2 condensate and the associated gas. The results from gas chromatography/time-of-flight mass spectrometry (GC × GC-TOFMS) and compound-specific carbon isotope analysis (CSCIA) presented here, in combination with geological analysis, demonstrate improved understanding of the postaccumulation processes in the subsurface. The genesis, accumulation process, and possible alterations of the Yingnan2 condensate were determined, and future exploration potential in the eastern Tarim Basin was implied.
1. INTRODUCTION Postaccumulation physiochemical alterations may exert significant impact on primary petroleum accumulations,1,2 leading to the adjustment, redistribution, and even complete destruction of primary petroleums.3−5 In recent years, exploration interests have shifted toward greater depths where complex secondary alterations under high-temperature and high-pressure conditions are involved.6−9 Thermal cracking, gas invasion, thermochemical sulfate reduction, water washing, and hydrocarbon diffusion may solely or jointly impact the chemical features and distribution of accumulated petroleums, and thus to determine which of the processes are operative is significant for the improved understanding of the petroleum accumulation and exploration potential in the field. Tarim Basin is the largest petroliferous districts in China and tremendous petroleum resources have been discovered in the central and northern parts.10 However, only a few gas condensate accumulations were obtained in the eastern Tarim Basin despite its coverage of over 100 000 km2.11 As to the Yingnan2 condensate, the unique industrial condensate accumulation in the eastern Tarim Basin, previous studies focused more on the geological analyses regarding the source− reservoir−seal assemblages,12,13 migration pathways,14,15 and the hydrocarbon generation and expelling history,16,17 and thus the condensate was believed to have been originated from cracked oils in Ordovician paleo-oil pools. Nevertheless, controversies remain about the genesis and accumulation process of the Yingnan2 condensate and thus hinders the © 2019 American Chemical Society
2. MATERIALS AND METHODS 2.1. Geological Settings and Samples. The Yingnan2 well is located in the Yingjisu depression in the eastern Tarim Basin; this area has experienced rapid subsidence during the Cambrian−Silurian, followed by uplift and erosion from the late Devonian to Triassic and constant subsidence after Triassic.18 The strata drilled by the Yingnan2 well include, from old to new, the upper Ordovician, Silurian, Jurassic, the lower Cretaceous, and the whole Cenozoic. According to the seismic interpretation and regional field studies, the lower Ordovician consists of huge clastic sedimentary rocks (over 5000 m) and the Cambrian high-quality source rock is developed at depths of over 10 000 m.19 Received: February 17, 2019 Revised: May 5, 2019 Published: May 28, 2019 4849
DOI: 10.1021/acs.energyfuels.9b00484 Energy Fuels 2019, 33, 4849−4856
Article
Energy & Fuels
Figure 1. GC × GC-TOFMS color contour chromatograms of the Yingnan2 condensate. (a) Total ion current chromatogram highlighting distinctive groups of aliphatics, aromatics, diamondoids, and ethanoadamantanes, each compound series is marked with circles or boxes. (b) m/z 71 + 85 + 99 highlighting n-alkanes. Two sets of source rocks were developed in this region including the Cambrian marine shales and the Jurassic coal measures. The Cambrian marine shales have been confirmed as the major petroleum source of the Ordovician reservoirs with a wide distribution in the Tarim Basin,19,20 while the Jurassic coal measures are relatively locally formed in the eastern Tarim Basin. Based on the geothermal and tectonic evolution history, the Cambrian source rock has reached overmature stage, while the Jurassic coal measures are overall immature (% Ro < 0.7). Thus, the oil and gas obtained in this area were majorly from the Cambrian sources. Several sets of pay zones were penetrated in the Yingnan2 well and the condensates were mainly obtained in the lower Jurassic sandstones and conglomerates (3500−3800 m). Downhole pressure and temperature measurements showed that the condensate reservoir was in a normal pressure condition, with temperatures lower than 105 °C. The caprocks in this field include the mudstone and tight sandstones and siltstones in the upper Jurassic. The Yingnan2 well had the daily production of ∼4.5 m3 oil and ∼66 040 m3 gas with the GOR (gas-to-oil ratio) of 14 675 m3/m3. The condensate and associated gas samples from the Yingnan2 well were collected at the interval of 3648−3680 m at the well head after the separator. Sample vials and high-pressure steel vessels were used for the collection of condensate oil and gas, respectively. The condensate oil obtained was light brown in color with a low density of 0.772 g/cm3. Four Cambrian-sourced oil samples including two typical Ordovician condensates (Tazhong83 and Zhonggu21), an Ordovician oil (Zhonggu163) and a severely cracked Cambrian oil (Zhongshen1C) were geochemically and isotopically compared with the Yingnan2 condensate. 2.2. Methods. GC × GC-TOFMS was conducted on a Leco Corporation instrument equipped with two Agilent GCs interfaced to a Pegasus 4D TOF mass spectrometer. A PONA column (50 m × 0.2 mm × 0.5 μm) and a Rxi-17 column (2 m × 0.1 mm × 0.1 μm) were used for the one- and two-dimensional GC, respectively. The concentrations of diamondoids, ethanodiamondoids, and other products were quantified based on the peak areas relative to an internal standard (D16-adamantane). Further details of the sample pretreatment procedures and operating conditions can be found elsewhere.21 Gas chromatography−mass spectrometry (GC−MS) analysis was conducted with a TRACE GC ULTRA/DSQII instrument equipped with an HP-5MS silica column (60 m, 0.25 mm i.d., film thickness = 0.25 μm). The initial GC oven temperature is 100 °C, held for 5 min, and then programmed to 220 °C at the rate of 4 °C/min; finally, it was programmed to 320 °C at 2 °C/min and held isothermally for 20 min. CSCIA of oil was carried out on a Micromass IsoPrime mass spectrometer attached to a HP 6890 GC. A 60 m × 0.25 mm capillary column coated with 0.25 μm 5% phenyl-methyl-silicone stationary phase was fitted to the GC. The GC oven was programmed from 50 to 310 °C at 3 °C/min with the initial and final holding times of 1 and
30 min, respectively. Helium was used as a carrier gas at a constant flow rate of 1 mL/min. The carbon isotopic compositions (δ13C) were calculated by the CO2 peaks produced by the combustion of hydrocarbons separated by GC. A CO2 reference gas with a known δ13C value (relative to the Vienna Peedee Belemnite (VPDB)) was pulsed into the mass spectrometer and the isotopic composition of the samples was reported in the δ notation relative to the reference gas. Stable carbon isotope ratios of gases were analyzed by the Thermo Delta V Advantage instrument. A Trace GC Ultra was used to fractionate the components, and the temperature was initially held at 33 °C, programmed to 80 °C at the rate of 8 °C/min and finally programmed to 250 °C at 5 °C/min and held isothermally for 20 min. GC Combustion III was the transfer interface, and the temperature of the oxidation oven was kept at 980 °C and that of the reducing oven was 640 °C. A Delta V Advantage Isotope-Ratio Mass Spectrometry was used to acquire mass spectral data from the GC by using 3.07 kV electron impact ionization. Stable carbon isotopic values were reported in parts per thousand relative to the VPDB.
3. RESULTS 3.1. Geochemical Features of Oil and Gas. Several distinctive compound groups were identified in the Yingnan2 condensate with a signal-to-noise >100 through the GC × GCTOFMS analysis, including aliphatics (n-alkanes and cycloalkanes), aromatics (benzenes, naphthalenes, and phenanthrenes), diamondoids (adamantanes and diamantanes), and ethanoadamantanes (Figure 1a). As illustrated in Figure 1b using EICs (extracted ion chromatograms) of m/z = 71, 85, and 99, respectively, the condensate covers a wide range of nalkanes series (n-C6−n-C29), in which lower-MW (molecular weight) compounds are more enriched and >C20 compounds are of relatively low concentrations. Unlike the primary thermal condensates generated at high-maturity stages that are dominated by lower compounds before n-C10,22 the Yingnan2 condensate is more likely of secondary genesis. A total of 79 isomers and homologues of alkylated adamantanes were detected in the Yingnan2 condensate (Figure S1 and Table 1), and the exact isomeric configuration of a large proportion of the alkylated adamantanes detected could not be unequivocally established except several specific isomeric assignments according to previous studies.23,24 Compared with the concentration of alkylated diamondoids in typical condensates, volatile oils and oils in the Tarim Basin (Table 2), the Yingnan2 condensate has an extensive series of alkylated adamantanes, extending to C5, with the total concentration of 21.09 mg/g. Alkylated diamantanes showed fewer isomers and homologues with the lower summed 4850
DOI: 10.1021/acs.energyfuels.9b00484 Energy Fuels 2019, 33, 4849−4856
Article
Energy & Fuels
densate is more related to the marine oils generated from the Cambrian source rocks rather than the Jurassic coal measures. Hydrocarbon gases account for 84.0% of the associated condensate gas, and it was typical of the wet gas according to the gas dryness (defined as C1/C1−4 by volume) of 0.88. Nonhydrocarbon gases consisted of major nitrogen gas (15.1%) and minor carbon dioxide (0.1%), and no hydrogen sulfide was detected. 3.2. Isotopic Features of Oil and Gas. In Figure 4, the carbon isotopic profiles of n-alkanes from the five oil samples are plotted against the carbon chain length, among which Zhongshen1C is a severely cracked Cambrian oil, Tazhong83 and Zhonggu21 are typical condensates, and Zhonggu163 is a typical oil. The isotopic profiles except that of Zhongshen1C show a relative consistency in the range of approx. n-C21 to nC30; however, significant differences can be seen in the lower MW range (below n-C20). The Zhonggu21 sample shows an overall flat isotopic profile at around δ13C = −32.0‰, and the Tazhong83 sample becomes progressively lighter isotopically with longer carbon chains, with the result that the n-C26 to nC30 are approx. 1‰ depleted in 13C relative to the n-C13 to nC16. Significant isotopic variation is observed in the Yingnan2 sample, which showed the most enriched 13C similar to that in Zhongshen1C in their front end and an extremely steep slope (the carbon isotopic range from −28.3 to −31.5‰) in the range of n-C13 to n-C20. As to the Zhongshen1C sample, the profile is overall isotopically heavier than the rest of the samples, especially in the range of >n-C20. The carbon isotopic values of methane, ethane, propane, and normal butane of the Yingnan2 condensate gas is −37.1, −33.1, −29.8, and −30.1‰, respectively, showing an overall positive isotopic profile with a more enriched 13C with the increasing carbon number, although propane is slightly heavier isotopically than normal butane.
Table 1. Number of Alkylated Diamondoids and Ethanoadamantanes Detected in the Yingnan2 Condensate compounds
MW
adamantane C1-adamantane C2-adamantane C3-adamantane C4-adamantane C5-adamantane diamantane C1-diamantane C2-diamantane C3-diamantane ethanoadamantane C1-ethanoadamantane C2-ethanoadamantane
136 150 164 178 192 206 188 202 216 230 162 176 190
isomer numbera 2 (2) 8 (7) 22 (6) 32 (5) 15 (2) 3 (3) 6 (6) 2 (2) 3 4
a
Total isomer numbers detected in this study; numbers in parentheses represent the isomer numbers from previous GC × GC−MS studies.23−26
concentrations of 0.17 mg/g (Figure S2 and Tables 1, 2). Notably, a series of ethanoadamantanes with the total concentration of 0.45 mg/g were identified in the condensate (Tables 1 and 2), and their distributions are marked in Figure 2, and the corresponding mass spectra are shown in supplementary Figure S3. The Yingnan2 condensate was geochemically compared with a typical Cambrian-sourced marine oil from the Zhonggu163 well and the Jurassic coal sample from the Huayingcan1 well through GC−MS analyses (Figure 3). Results show that both Yingnan2 condensate and Zhonggu163 oil have abundant terpenoid products peaked at C23TT (tricyclic terpane), the higher MW hopanes (e.g., C29 diahopane and 17α(H)diahopane) are also abundant and the homohopane series (C30−35HH) show a gradual decreasing trend in abundance. Contrarily, the terpanes of the Huayingcan1 coal sample has lower abundance and the homohopanes distributions are different from those of the two oil samples. As to the sterane series, the C27−29 20 R steranes of the Huayingcan1 coal sample shows quite a different distribution compared with the typical “V”-shape distribution in the oil samples. Based on the biomarker characters, the presently analyzed Jurassic con-
4. DISCUSSION 4.1. Severe Cracking of Paleo-Oil. Diamondoids are thermally stable compounds with a structure similar to that of diamond and are typical products indicative of oil cracking.24 Diamondoids maybe in-reservoir generated and concentrated during maturation27 or migrated from secondary sources during gas invasion and mixing,21,28 and the high abundance of diamondoids in oils generally represents extensive oil
Table 2. Concentration of Alkylated Diamondoids and Ethanoadamantanes Detected in the Yingnan2 Condensate, and Typical Condensates, Volatile Oils, and Oils in the Tarim Basin concentration (mg/g) compound
Yingnan2
condensates
volatile oils
oils
adamantane C1-adamantane C2-adamantane C3-adamantane C4-adamantane C5-adamantane diamantane C1-diamantane C2-diamantane C3-diamantane ethanoadamantane C1-ethanoadamantane C2-ethanoadamantane
0.64 2.96 7.03 6.36 3.44 0.66 0.01 0.07 0.07 0.02 0.02 0.15 0.27
0.15−0.65 1.58−4.2 1.78−7.33 0.55−6.46 0.35−3.52 0.01−0.75 0.02−0.23 0.11−0.76 0.07−0.68 0.01−0.19 0.01−0.03 0.01−0.43
