Cambrian Lower Ordovician Source Rocks Research Articles

Influence of dissolved organic carbon on the Cambrian Ocean: Evidence from the carbon isotopes of the sedimentary rocks of the Eastern Tarim Basin, North‐west China

In recent years, the typical crude oil from the Cambrian Lower Ordovician source rocks has been found in the Tarim Basin, and it is characterized by 13C enrichment. The stable carbon isotope of the oil is concentrated at approximately −28‰, which is 3‰–6‰ heavier than that of the crude oil from the Middle and Upper Ordovician source rocks. The stable carbon isotope between crude oil and its kerogen is characterized by inversion, with an amplitude of approximately 3‰–4‰. However, the origin of this phenomenon remains controversial, which restricts the next exploration of crude oil in the Cambrian–Lower Ordovician in the Tarim Basin. Samples were collected from seven outcrop profiles in the Kuruketage region. The basic geochemical features of the Cambrian strata, including total organic carbon abundance, δ13Corg, δ13Ccarb, and δ18Ocarb, were studied and compared with those of well TD2. The results show that the lithologies, isotope stratigraphic evolutional curves, and correlations between the organic carbon and inorganic carbon of the profiles from the northern Kuruketage region are successively comparable with those of the profiles from the southern Kuruketage region and the well TD2. In this study, the carbon isotope records are supposed to indicate a vertical gradient Cambrian oceanic mode, including the shallow decoupled isotope zone, middle gradient‐coupled isotope zone, and deep isotope‐decoupled zone. The discovery of this isotope record reflects the formation of stable carbon and oxygen isotope stratified oceans during the Cambrian period. Additionally, it provides important insights into the origin of the 13C enrichment observed in some Lower Cambrian crude oils in the Tarim Basin.

Quantitatively unmixing method for complex mixed oil based on its fractions carbon isotopes: A case from the Tarim Basin, NW China

Deep mixed oils with secondary alterations have been widely discovered in the Tarim Basin, but current methods based on biomarkers and isotopes to de-convolute mixed oil cannot calculate the exact mixing proportion of different end-member oils, which has seriously hindered further exploration of deep hydrocarbons in the study area. To solve this problem, we constructed a novel method based on the carbon isotope ( δ 13 C) of the group components to de-convolute mixed liquid hydrocarbons under the material balance principle. The results showed that the mixed oil in the Tazhong Uplift was dominantly contributed at an average proportion of 68% by an oil end-member with heavier δ 13 C that was believed to be generated from the Cambrian-Lower Ordovician source rocks, whereas the mixed oil in the Tabei Uplift was predominantly contributed at an average proportion of 61% by an oil end-member with lighter δ 13 C that was believed to be generated from the Middle-Upper Ordovician source rocks. This indicates that, on the basis of the detailed description of the distribution of effective source rocks, the proposed method will be helpful in realizing differential exploration and further improving the efficiency of deep liquid hydrocarbon exploration in the Tarim Basin. In addition, compared to traditional δ 13 C methods for whole oil and individual n-alkanes in de-convoluted mixed oil, the proposed method has a wider range of applications, including for mixed oils with variations in color and density, indicating potential for promoting the exploration of deep complex mixed oils in the Tarim Basin and even around the world.

Cambrian crude oil in ZS well field, Tazhong area: characteristics, genesis, and comparison with ordovician crude oil

In this paper, the Cambrian crude oil and source-rock geochemistry in the ZS well field of Tazhong area, Tarim Basin, was analyzed using the gas chromatography-mass spectrometry (GC-MC). Moreover, the hydrocarbon source in the Tazhong was identified through oil–source correlation. The analysis of light hydrocarbons and biomarkers indicates that the Cambrian and Ordovician protokerogens are of marine sapropelic type formed in a strongly reducing environment. The result of oil–source correlation shows that both Cambrian crude oil in the ZS well field and Ordovician crude oil in the Tazhong area originated in the Cambrian–Lower Ordovician source rocks.

Typical end-member oil derived from Cambrian-Lower Ordovician source rocks in the Tarim Basin, NW China

Abstract Based on GC-MS data of saturate and aromatic hydrocarbon fractions in crude oils from Well TD 2 and the Yingmaili and Tazhong area in the Tarim Basin, the end-member oil derived from Cambrian-Lower Ordovician source rocks was determined. The analytical results show that the crude oil from Well TD2 is rich in 3-6 ring polycyclic aromatic hydrocarbons without alkyl group, suggesting that this oil was altered by an abnormally thermal event. It contains a complete series of normal alkanes and 25-norhopanes, indicating that it is mixed oil of normal oil and biodegraded oil, so it can't be taken as end-member oil derived from Cambrian source rocks. The geochemical characteristics of crude oils from Well TZ11 and TZ30 are completely different from the crude oils from Well YM101 and YM201 generated by Middle–Upper Ordovician source rocks, so it is inferred that they are derived from Cambrian-Lower Ordovician source rocks. In addition, these crude oils don't contain 25-norhopanne series, indicating that they haven't suffered biodegradation and satisfy with geochemical conditions of end-member oil. It is noteworthy that the two kinds of marine oils have similar triaromatic steroids and methyl triaromatic steroids, but very different C27-29 steranes and C28-30 methyl steranes, showing that they are not a good indicator of organic matter input. Triaromatic dinosteroids are a kind of biomarkers most usually used in marine oil-source correlation in the Tarim Basin, but the big differences in their relative abundances between crude oil from Well TD 2 and two kinds of end-member marine oils suggest that they are easily affected by thermal maturity and should be cautiously used in marine oil-source correlation.

Biomarker geochemistry of crude oils and Lower Paleozoic source rocks in the Tarim Basin, western China: An oil-source rock correlation study

Abstract In order to select biomarker parameters that have potential to quantitatively distinguish between the Cambrian–Lower Ordovician and Upper Ordovician source rocks in the Tarim Basin with a total organic carbon content >0.5%, 42 Lower Paleozoic shale samples, were collected from a number of wells and outcrops in the Tarim Basin, northwestern China. The biomarkers in the extracts of the source rocks were analyzed by gas chromatography–mass spectrometry. The results show that C23 and C30 were dominant in the tricyclic terpanes and pentacyclic triterpanes, respectively, and C27 and C29 were both dominant in C27–29 steranes of the source rock samples. The distribution of n-alkanes, isoprenoids and terpanes indicate that the source rock samples have been slightly to moderately biodegraded, and they originated from a reducing depositional environment with considerable marine algal input in. Comparatively, Upper Ordovician source rock samples presented lower values of C28/C29 regular steranes and higher values of C29 ααα20S/(20S + 20R) and C29αββ/(αββ+ααα). Abnormally lower values of C29 ααα20S/(20S + 20R) and C29αββ/(αββ+ααα) were observed in the Cambrian–Lower Ordovician strata. The set of data in this study show that homohopane index including C31homohopane/C30hopane, C31-35homohopanes/C30hopane, and the ratios related to the triaromatic steroids including C26−20S/C28−20S TAS, C27−20R/C28−20R, (C26−20R + C27−20S)/C28−20S and C28/(C26+C27) TAS, could serve as potential biomarker parameters to distinguish between the two groups of source rocks in the Tarim Basin. The values of those parameters (in above order) in Cambrian–Lower Ordovician source rocks were 0.3, >0.5, >0.85, and 0.8, >1.9, 1.2, respectively. The correlation between the source rock samples and 19 oils obtained from the Tadong, Tabei and Tazhong uplifts in the Tarim Basin show that most of the oils in Tabei uplift are probably from Upper Ordovician source rocks, the oils in Tadong uplift are more likely from Cambrian- Lower Ordovician source rocks, and the Tazhong uplift is more like a mixed oil region with oils from both of the two groups of source rocks.

The application of diamondoid indices in the Tarim oils

This study presents new data for the identification of the source and assessment of the thermal maturity of oils based on the diamondoid indices of oils from the Tazhong and Luntai uplifts in the Tarim Basin in northwest China. The oil samples were divided into three groups according to biomarker characteristics and the abundance and distribution of diamondoids. Group I oils are located along the Tazhong No. 1 fault zone, which contained abundant diamondoids and are of high thermal maturity, suggesting they are late-charged hydrocarbons that were derived from a Middle–Upper Ordovician source. Group II oils are mainly located in blocks close to the Tazhong No. 1 fault zone and are dominated by early formed hydrocarbons from Cambrian–Lower Ordovician source rocks. Group III lacustrine oils comprise relatively low concentrations of diamondoids compared with group I and group II oils. Group III oils were sourced from Jurassic or possibly Triassic units and are hosted by the low-relief Yingmaili section of the Luntai uplift. The thermal maturity of the oils in each group was evaluated using diamondoid parameters; a few group I oils exhibit intense thermal cracking. To estimate the extent of oil cracking using the concentrations of diamondoids in oils, this study proposes a practical approach that facilitates the determination of baseline 4- and 3-methyldiamantane concentrations. Application of this method indicates that the Tarim Basin marine oils contain a baseline concentration of approximately 69 ppm.

Origin, charging, and mixing of crude oils in the Tahe oilfield, Tarim Basin, China

Forty-eight crude oil samples collected from different reservoirs in the Tahe oilfield of the Tarim Basin were investigated. Based on geochemical characteristics, it is concluded that the oil samples originated from multiple marine source rocks deposited in various sedimentary environments and subsequently altered by different levels of thermal maturation. These mixtures were de-convoluted to three endmember oils (EM1, EM2 and EM3) by alternating least squares regression using 38 concentration parameters. EM1 is the minimum contributor with an average of 13%, while EM2 and EM3 are the main contributors to the mixtures with averages of 52% and 35%, respectively. EM1 oil originated from Cambrian–Lower Ordovician source rocks in the early to peak oil window and subsequently experienced two phases of mixing and biodegradation. EM2 and EM3 oils originated from Middle–Upper Ordovician source rocks, but EM2 was generated at lower thermal maturity than EM3. The EM2 oil underwent two phases of mixing and one stage biodegradation, while the EM3 oil mixed with previous existing mixtures in the reservoirs. The final mixtures that might be affected by secondary processes, such as evaporative fractionation, are currently produced from the Tahe oilfield. The general orientation of oil filling was from south to north and east to west based on variations in the relative contributions of EMs, and the total concentrations of dibenzothiophenes and dibenzofurans in the oils. Considering the histories of sedimentary tectonic evolution, hydrocarbon generation and expulsion, and the de-convolution results, a model of three stages of oil charge and two phases of mixing and biodegradation was established for the Tahe oilfield oils.

Evaluation method and application of the relative contribution of marine hydrocarbon source rocks in the Tarim basin: A case study from the Tazhong area

In total, 2.37 million tons of marine crude oil originating from mixed source rocks has been discovered in the Tarim basin. Geological and geochemical analyses have confirmed that these mixed hydrocarbons are mainly from two sets of source rocks, including the Cambrian – Lower Ordovician and Middle-Upper Ordovician hydrocarbon source rocks. In this study, we determined the set of source rocks primarily responsible for the mixed hydrocarbons and the next location to be explored. Differences in n-alkane carbon isotopes in end-member oils from Cambrian–Lower Ordovician and Middle-Upper Ordovician source rocks were examined. A material balance model and simulation methods were used to evaluate the relative amounts contributed by each source. The results from known reserves in the Tazhong area show that the mixing ratio or contribution is up to 65% from Cambrian–Lower Ordovician source rocks and is generally higher than that from Middle-Upper Ordovician source rocks. The discovery of deep hydrocarbons has caused the total oil contribution from the Cambrian–Lower Ordovician to increase. The mixing ratio of Cambrian–Lower Ordovician oil varies depending on the well, formation, and block. It increases from west to east horizontally and from top to bottom vertically. Hydrocarbons from Cambrian–Lower Ordovician source rocks migrate upward along faults, and the mixing ratio decreases as the distance from the oil source fault increases. Favorable areas for Cambrian–Lower Ordovician hydrocarbon exploration are deep layers and areas near the fault zone that are connected to deep layers. The material balance model for carbon isotopes and evaluation methods for relative contributions considered differences in relative concentration and carbon isotope structure of n-alkanes. Herein, new methods for the identification and evaluation of hydrocarbons in the petroleum system of this superimposed basin are presented.

De-convoluting crude oil mixtures from Palaeozoic reservoirs in the Tabei Uplift, Tarim Basin, China

Geochemical characteristics of 61 crude oil samples collected from Palaeozoic reservoirs in the Tarim Basin (60 from the Tabei Uplift and one from the Tazhong Uplift) were analyzed. The samples proved to be mixed oils of different maturity from diverse source rocks. Concentrations of 40 biomarkers and carbon isotopic compositions for the whole oils were analyzed by alternating least squares (ALS) regression to de-convolute the mixtures. Three endmember (EM) oils were identified. EM1 is the minimum contributor to the mixed oils, accounting for less than 10% of most oils. EM1 originated from Cambrian–Lower Ordovician source rocks in the early to peak oil window stage and experienced two phases of mixing and biodegradation. EM2 is the secondary contributor with proportions ranging from 10% to 40% in most oil samples. EM2 originated from Middle–Upper Ordovician source rocks at the early oil generation stage and underwent two phases of mixing and one stage biodegradation in the reservoirs. EM3 is the major contributor to most samples with proportions ranging from 13% to 95%. EM3 was generated from Middle–Upper Ordovician source rocks at the late oil generation stage and mixed with earlier emplaced mixtures in the reservoirs. The final mixtures that were not biodegraded are currently exploited from Palaeozoic reservoirs in the Tabei Uplift. Biomarkers in the crude oils reveal mixed characteristics, including evidence for two phases of oil charge and severe biodegradation.

Comprehensive diagnostic review of the 13C-enriched crude oils exemplified by TD2Є and TZ62S in Tarim Basin, NW China

The crude oils typically from the Cambrian-Lower Ordovician source rocks of Tarim Basin, NW China, such as TD2Є and TZ62S, are 13C-enriched with the stable carbon isotopic ratios (VPDB) approaching −28 ‰. In this paper, the main research viewpoints on this issue are summarized, and combined with results from organic and inorganic carbon isotope stratum curves of the outcrop at the Ya’erdang Mountain in Tarim Basin. In addition, more alternative interpretations are discussed. On one hand, the inverse fractionation features of stable carbon and hydrogen isotopes of these crude oils may imply their protogenous nature. On the other hand, the anisotropy of source rocks and contribution from older stratum source rocks need verifying as well. For the sake of the final resolution of this issue, some further study topics are recommended.

Geochemical quantification of mixed marine oils from Tazhong area of Tarim Basin, NW China

Located in the center of the Tarim Basin, NW China, Tazhong area was found to be enriched in oil and gas resources, and the resources are mainly originated from two sets of marine source rocks, namely, the Cambrian-Lower Ordovician (ϵ-O1) source rocks and the Middle-Upper Ordovician (O2+3) source rocks. Due to complex post-depositional alterations such as multiple episodes of hydrocarbon generation, migration and accumulation, most oil reservoirs in this area occur as mixed oils.Results of geochemical studies of crude oils, combined with bulk stable carbon isotope data of 108 oil samples, indicate that most marine oils from Tazhong area are mixtures sourced from both the ϵ-O1 and O2+3 strata. Based on bulk stable carbon isotope data of the oils, quantification studies were made to evaluate the relative contributions of different source rocks to the mixed oils, and the results show that the Middle-Upper Ordovician source rocks are the main contributor for the oils in the Tazhong area, despite fewer exceptions that some oil reservoirs were mainly originated from the Cambrian-Lower Ordovician source rocks. However, for the oil reservoirs occurring in different subunits in the Tazhong area, the relative contributions from different source rocks do not show regular distribution patterns, implying that Tazhong area was subject to multiple episodes of hydrocarbon generation, migration and accumulation in geological history.

Multiple-sourced features of marine oils in the Tarim Basin, NW China – Geochemical evidence from occluded hydrocarbons inside asphaltenes

The Tarim Basin, NW China, is a large composite basin with multiple sets of petroleum source rocks. The basin has undergone numerous episodes of hydrocarbon generation, migration and accumulation, making it difficult to assess the source rocks and oil–source correlation for the widespread marine oils in this area.Protected by the molecular structure of asphaltenes, occluded hydrocarbons can provide information about the early source rocks. In this work, the occluded hydrocarbons released from the asphaltenes by a mild chemical degradation method were compared with the crude oil maltenes and the adsorbed compounds from asphaltenes. Analysis of biomarker distribution and the carbon isotope composition of individual n-alkanes suggests that the widespread marine oils in the Tazhong Uplift, Tabei Lunnan Uplift and Halahatang Depression were contributed by Cambrian–Lower Ordovician source rocks at an early stage, and later mixed with hydrocarbons derived from Middle-Upper Ordovician source rocks. The marine oils in the Tarim Basin demonstrate extensive characteristics of having been derived from multiple source rocks.

Molecular correlation of crude oils and oil components from reservoir rocks in the Tazhong and Tabei uplifts of the Tarim Basin, China

Molecular data from a large set of source rock, crude oil and oil-containing reservoir rock samples from the Tarim Basin demonstrate multiple sources for the marine oils in the studied areas of this basin. Based on gammacerane/C 31 hopane and C 28/(C 27 + C 28 + C 29) sterane ratios, three of the fifteen crude oils from the Tazhong Uplift correlate with Cambrian–Lower Ordovician source rocks, while the other crude oils from the Tazhong Uplift and all 39 crude oils from the Tahe oilfield in the Tabei Uplift correlate with Middle–Upper Ordovician source rocks. These two ratios further demonstrate that most of the free oils and nearly all of the adsorbed and inclusion oils in oil-containing reservoir rocks from the Tazhong Uplift correlate with Cambrian–Lower Ordovician source rocks, while the free and inclusion oils in oil-containing carbonates from the Tahe oilfield correlate mainly with Middle–Upper Ordovician source rocks. This result suggests that crude oils in the Tazhong Uplift are partly derived from the Cambrian–Lower Ordovician source rocks while those in the Ordovician carbonate reservoirs of Tahe oilfield are overwhelmingly derived from the Middle–Upper Ordovician source rocks. The scatter of C 23 tricyclic terpane/(C 23 tricyclic terpane + C 30 17α,21β(H)-hopane) and C 21/(C 21 + ΣC 29) sterane ratios for the free and inclusion oils from oil-containing carbonates in the Tahe oilfield possibly reflects the subtle organofacies variations in the source rocks, implying that the Ordovician reservoirs in this oilfield are near the major source kitchen. In contrast, the close and positive relationship between these two ratios for oil components in the oil-containing reservoir rocks from the Tazhong Uplift implies that they are far from the major source kitchen.

Origin of the Silurian Crude Oils and Reservoir Formation Characteristics in the Tazhong Uplift

Abstract:The Silurian stratum in the Tazhong uplift is an important horizon for exploration because it preserves some features of the hydrocarbons produced from multi‐stage tectonic evolution. For this reason, the study of the origin of the Silurian oils and their formation characteristics constitutes a major part in revealing the mechanisms for the composite hydrocarbon accumulation zone in the Tazhong area. Geochemical investigations indicate that the physical properties of the Silurian oils in Tazhong vary with belts and blocks, i.e., heavy oils are distributed in the TZ47–15 well‐block in the North Slope while normal and light oils in the No. I fault belt and the TZ16 well‐block, which means that the oil properties are controlled by structural patterns. Most biomarkers in the Silurian oils are similar to that of the Mid‐Upper Ordovician source rocks, suggesting a good genetic relationship. However, the compound specific isotope of n‐alkanes in the oils and the chemical components of the hydrocarbons in fluid inclusions indicate that these oils are mixed oils derived from both the Mid‐Upper Ordovician and the Cambrian–Lower Ordovician source rocks. Most Silurian oils have a record of secondary alterations like earlier biodegradation, including the occurrence of “UCM” humps in the total ion current (TIC) chromatogram of saturated and aromatic hydrocarbons and 25‐norhopane in saturated hydrocarbons of the crude oils, and regular changes in the abundances of light and heavy components from the structural low to the structural high. The fact that the Silurian oils are enriched in chain alkanes, e.g., n‐alkanes and 25‐norhopane, suggests that they were mixed oils of the earlier degraded oils with the later normal oils. It is suggested that the Silurian oils experienced at least three episodes of petroleum charging according to the composition and distribution as well as the maturity of reservoir crude oils and the oils in fluid inclusions. The migration and accumulation models of these oils in the TZ47–15 well‐blocks, the No. I fault belt and the TZ16 well‐block are different from but related to each other. The investigation of the origin of the mixed oils and the hydrocarbon migration and accumulation mechanisms in different charging periods is of great significance to petroleum exploration in this area.

Geochemical constraints on mixed source and hydrocarbon filling process in the Yingjisu Sag, Tarim Basin, Xinjiang

The Yingjisu Sag was petroliferous for normal oil, condensate oil, reservoir bitumen and natural gases. Geochemical studies showed that natural gases in the Yingjisu Sag were a gas mixture consisting mainly of Cambrian pyrolysis gas, Jurassic condensate oil in well Yingnan 2 and normal oil in well Tadong 2, reflecting the characteristics of marine-phase gases and oils, while crude oils in well Longkou 1 demonstrated the characteristics of both marine and terrestrial oils, which were derived from lower algae and higher plants. Jurassic oils from wells Longkou 1 and Huayingcan 1 and Cambrian crude oils from well Tadong 2 were derived mainly from Cambrian-Lower Ordovician source rocks. Jurassic and Silurian reservoir bitumens from well Yingnan 2 were biodegradated, suggesting they are of marine and terrestrial origins. The bitumens have similar geochemical characteristics, which are correlated well with Ordovician crude oils from well Tadong 2 and Jurassic condensate oil from well Yingnan 2. Based on the characteristics of tectonic evolution in this area and the analysis of hydrocarbon accumulation, the constraints on the mixed source and hydrocarbon filling process in the Yingjisu Sag were brought forward.

Molecular correlation of free oil, adsorbed oil and inclusion oil of reservoir rocks in the Tazhong Uplift of the Tarim Basin, China

The free, adsorbed and inclusion oils were recovered by sequential extraction from eleven oil and tar containing reservoir rocks in the Tazhong Uplift of Tarim Basin. The results of gas chromatography (GC) and GC–mass spectrometry analyses of these oil components and seven crude oils collected from this region reveal multiple oil charges derived from different source rocks for these oil reservoirs. The initially charged oils show strong predominance of even over odd n-alkanes in the range n-C 12 to n-C 20 and have ordinary maturities, while the later charged oils do not exhibit any predominance of n-alkanes and have high maturities. The adsorbed and inclusion oils of the reservoir rocks generally have high relative concentrations of gammacerane and C 28 steranes, similar to the Cambrian-Lower Ordovician source rocks. In contrast, the free oils of these reservoir rocks generally have low relative concentrations of gammacerane and C 28 steranes, similar to the Middle-Upper Ordovician source rocks. There are two interpretations of this result: (1) the initially charged oils are derived from the Cambrian-Lower Ordovician source rocks while the later charged oils are derived from the Middle-Upper Ordovician source rocks; and (2) both the initially and later charged oils are mainly derived from the Cambrian-Lower Ordovician source rocks but the later charged oils are contaminated by the oil components from the Silurian tar sandstones and the Middle-Upper Ordovician source rocks.

Petroleum charge history in the Lunnan Low Uplift, Tarim Basin, China – Evidence from oil-bearing fluid inclusions

The petroleum charge history in the Lunnan Low Uplift (Tarim Basin, China) was investigated by comparing the geochemical characteristics of fluid inclusion (FI) oils from Ordovician and Triassic reservoirs with crude oils from stratigraphically different reservoirs. Quantitative Grain Fluorescence (QGF) and Total Scanning Fluorescence (TSF) were used to screen the reservoir samples so as to identify suitable samples for Molecular Composition of Inclusion (MCI) analysis. All of the samples contain C 30 n-propylcholestanes, diagnostic of marine source rocks. The Ordovician-hosted FI oils differ from the Triassic-hosted FI oils, having higher relative abundances of C 19 tricyclic terpane, C 24 tetracyclic terpane and rearranged hopanes (Ts and C 29 Ts), and higher Pr/Ph ratios, while the Triassic-hosted FI oils have lower Pr/Ph ratios and higher relative abundances of C 23 tricyclic terpane, arylisoprenoids, 25-norhopanes and C 35 homohopane. Several of the crude oils are similar to the Triassic-hosted FI oils. Based on the geochemical characteristics of the FI and crude oils combined with information on the burial history of the area, at least two petroleum charge episodes can be distinguished in the Lunnan Low Uplift. An early charge represented by the Ordovician-hosted FI oils was biodegraded, as evidenced by the presence of 25-norhopanes in the Triassic-hosted FI oils and crude oils but not in the Ordovician-hosted FI oils. Ordovician-hosted FI oils provide the only evidence for oil generated from Cambrian-Lower Ordovician source rocks. Triassic-hosted FI oils and crude oils in the Ordovician, Carboniferous and Triassic reservoirs are mixtures of biodegraded oil from an early charge episode, and more mature oil from later charge.

Gas sources of the YN2 gas pool in the Tarim Basin—Evidence from gas generation and methane carbon isotope fractionation kinetics of source rocks and crude oils

The YN2 gas pool in the eastern Tarim basin has been believed to be an accumulation of gases resulting from oil-cracking. However, this gas source model cannot explain such geochemical characteristics of the natural gases as heavier carbon isotope values ( δ 13C 1=−37.5‰ to −36.2‰; δ 13C 2=−34.7‰ to −30.9‰) and high content of nitrogen gas (15.5% on average). In this study, we performed pyrolysis experiments on typical source rock and crude oil samples selected from the basin, evaluated their kinetic parameters of gas generation and methane carbon isotopic fractionation, and extrapolated them to geological conditions. The results show that neither kerogen-cracking gas nor oil-cracking gas can act as the single gas source of the YN2 gas pool. By contrast, a mixing model of 55% kerogen-cracking gas and 45% oil-cracking gas results in an excellent match with the geochemical characteristics of the gas pool. The gas condensate from the YN2 gas pool originated from the cracking of oils in mid-upper Ordovician strata, and the nitrogen gas was mainly derived from the cracking of kerogen in Cambrian–lower Ordovician mudstones in the Yingjisu depression center during the very high level of maturation. The YN2 gas pool went through three stages of formation: (1) oils from Cambrian–lower Ordovician source rocks accumulated in Cambrian and Ordovician reservoirs during the middle Ordovician, and gases were generated from both kerogen and paleo-oil pools in the Cambrian–lower Ordovician strata during late Ordovician to early Devonian; (2) gases leaked during either late Devonian to Triassic due to uplifting, or late Jurassic due to intense faulting activity in this area; (3) gases were regenerated from the late cracking of kerogen in the Cambrian–lower Ordovician strata as well as from oil-cracking in mid-upper Ordovician reservoirs with rapid subsidence after the Cretaceous. The two types of gas mixed and accumulated in the Jurassic traps to form the present wet gas pool with high nitrogen content and a heavier methane carbon isotopic value.

Gas accumulation from oil cracking in the eastern Tarim Basin: A case study of the YN2 gas field

Oil and gas exploration in eastern Tarim Basin, NW China has been successful in recent years, with several commercial gas accumulations being discovered in a thermally mature to over-mature region. The Yingnan2 (YN2) gas field, situated in the Yingnan structure of the Yingjisu Depression, produces gases that are relatively enriched in nitrogen and C 2+ alkanes. The δ 13C 1 (−38.6‰ to −36.2‰) and δ 13C 2 values (−30.9‰ to −34.7‰) of these gases are characteristic of marine sourced gases with relatively high maturity levels. The distributions of biomarkers in the associated condensates suggest close affinities with the Cambrian–Lower Ordovician source rocks which, in the Yingjisu Sag, are currently over-mature (with 3–4%Ro). Burial and thermal maturity modeling results indicate that paleo-temperatures of the Cambrian–Lower Ordovician source rocks had increased from 90 to 210 °C during the late Caledonian orogeny (458–438 Ma), due to rapid subsidence and sediment loading. By the end of Ordovician, hydrocarbon potential in these source rocks had been largely exhausted. The homogenization temperatures of hydrocarbon fluid inclusions identified from the Jurassic reservoirs of the YN2 gas field suggest a hydrocarbon emplacement time as recent as about 10 Ma, when the maturity levels of Middle–Lower Jurassic source rocks in the study area were too low (<0.7%Ro) to form a large quantity of oil and gas. The presence of abundant diamondoid hydrocarbons in the associated condensates and the relatively heavy isotopic values of the oils indicate that the gases were derived from thermal cracking of early-formed oils. Estimation from the stable carbon isotope ratios of gaseous alkanes suggests that the gases may have been formed at temperatures well above 190 °C. Thus, the oil and gas accumulation history in the study area can be reconstructed as follows: (1) during the late Caledonian orogeny, the Cambrian–Lower Ordovician marine source rocks had gone through the peak oil, wet gas and dry gas generation stages, with the generated oil and gas migrating upwards along faults and fractures to form early oil and gas accumulations in the Middle–Upper Ordovician and Silurian sandstone reservoirs; (2) since the late Yanshanian orogeny, the early oil accumulations have been buried deeper and oil has undergone thermal cracking to form gas; (3) during the late Himalayan orogeny, the seals for the deep reservoirs were breached; and the gas and condensates migrated upward and eventually accumulating in the relatively shallow Jurassic reservoirs.

Geochemistry of Palaeozoic marine petroleum from the Tarim Basin, NW China: Part 1. Oil family classification

A series of Palaeozoic marine oils ranging from condensates to very heavy oils from the cratonic region of the Tarim Basin, NW China have been examined to delineate oil families. Crude oils from the Mesozoic foreland Kuqa Depression were also analyzed for comparison. The genetic family was assessed using bulk hydrocarbon isotopic composition, alkylation pattern of polycyclic aromatic hydrocarbons (PAHs) and biomarker distributions. Detailed oil–oil correlation suggests two distinct families occur in the cratonic region. Despite the diversity of the reservoir rocks and some variability in oil physical properties, the Tabei and Tazhong oils are classified, in general, into a single family, with a possible indication for secondary subdivision. These oils are characterized by low pristane/phytane (Pr/Ph) ratio, high dibenzothiophene/phenanthrene (DBT/P) ratio, light bulk isotopic composition, high degree of PAH alkylation, high proportion of C 23 and C 24 tricyclic terpanes and low abundance of C 24 tetracyclic terpane and triaromatic dinosteroids. This family is inferred to have a Middle–Upper Ordovician source rock. The Tadong family, distinctly different from the Tabei and Tazhong families, is characterized by relatively high Pr/Ph ratio, low DBT/P ratio, slightly heavier bulk isotopic compositions and low degree of PAH alkylation. It also has a unique biomarker signature, with a high proportion of triaromatic dinosteroids, possibly indicating Cambrian–Lower Ordovician source rocks. The oils from the Kuqa Depression are easily distinguished from the other two families due to their terrigenous origin. The major characteristics of the Kuqa oils are high Pr/Ph ratio, very low DBT/P ratio, heavy bulk isotopic composition, low PAH alkylation degree, high proportion of C 20 and C 21 tricyclic terpanes and C 24 tetracylic terpane, and abundance of triaromatic dinosteranes and 4-methyl-24-ethyltriaromatic cholestanes. These characteristics are consistent with terrestrial organic source rocks deposited under sub-oxic conditions.