Ordovician Crude Oils Research Articles

Geochemical characteristics of Ordovician crude oils in the FI17 strike-slip fault zone of the Fuman oilfield, Tarim basin: Implications for ultra-deep hydrocarbon accumulation in the Tarim basin

The Ordovician crude oils in the Fuman Oilfield usually show varying physical properties and phase states along a single strike-slip fault. This study collected nineteen crude oil and gas condensate samples from the fault zones (FI17, FI19, and FI20) located in the eastern zone of the Fuman oilfield. Organic geochemistry methods were used to investigate the oil-source correlation, thermal maturity and secondary alternation processes of the samples, to reveal petroleum charge and accumulation models in ultra-deep reservoirs in the Fuman oilfield. The results showed that biomarkers of most crude oils have been modified by thermal stress, and the primitive geochemical information were distorted. Similar light hydrocarbon chromatogram characteristics and parameter ratios indicated that crude oils in the study area were generated from the same source rock. The carbon isotope of the whole oils (−31 ‰ to −33 ‰) and n-alkanes (−31 ‰ to −37 ‰) was comparable to the kerogen of the Lower Cambrian Yurtus Formation source rock (−30 ‰ to −37 ‰) considering both the carbon isotope fractionation between oils and kerogen and the effect of thermal maturity. The calculated equivalent vitrinite reflectance (%VRE) of oil and gas condensate based on adamantane, aromatic hydrocarbon and light hydrocarbon parameters ranged from 0.77 % to 1.44 %, suggesting a mature to highly mature hydrocarbon-generating stage. The lower dibenzothiophene content, 2-methylhexane/3-methylhexane ratios and relatively stable K1 values from light hydrocarbons revealed that the oil and gas condensates fluids were not subject to significant modification by thermochemical sulfate reduction (TSR). The calculated n-alkanes loss based on the whole oil gas chromatography ranging from 8 % to 26 %, suggested that the oil and gas condensate in the study area underwent relatively weak gas washing. Additionally, the concentrations of 3-methyldiamantane (3-MD) and 4-methyldiamantane (4-MD) in the samples ranged from 40.42 μg/g to 121.43 μg/g, showing realtively low oil cracking degree (<40 %) except for the gas condensates from the FI20 fault zone, whose oil cracking degree can reach 61 %. The comprehensive analyses in this study indicate that the differences in oil and gas physical property and phase state in the FI17 fault zone were primarily controlled by the vertical connectivity of the fault and the differences of crude oil charging periods. The hydrocarbon generation occurred during the early Caledonian period, followed by migration and accumulation in the Hercynian period, and subsequent adjustment and transformation in the Himalayan period. This study could provide significant exploration guidance for further ultra-deep oil and gas exploration in the Fuman oilfield.

Nondestructive investigation on hydrocarbons occluded in asphaltene matrix: The evidence from the dispersive solid-phase extraction

Asphaltenes are the heaviest compound group in crude oil. The complex and porous macromolecular network of asphaltenes both adsorbs and occludes small hydrocarbon molecules, which are considered source-related petroleum biomarkers as they are shielded from secondary alterations. The current method to isolate the occluded content involves mild oxidative degradation of asphaltenes with CH3COOH/H2O2, but it is generally not suitable for quantitative studies, as side reactions that chemically alter the bound hydrocarbons are difficult to mitigate. In the current study, we compared the performance of dispersive solid-phase extraction (DSPE) and mild oxidative degradation in isolating asphaltene-occluded hydrocarbons, using Lower Cambrian solid bitumen from northwestern Sichuan and Ordovician crude oil from the Tazhong area of Xinjiang, China. We demonstrated that DSPE was effective in extracting occluded hydrocarbons from asphaltene aggregates without undesirable chemical alterations, thereby allowing subsequent quantitative analysis. Further investigations revealed that the chemical composition of the occluded n-alkanes provided useful information with regard to the origin of the petroleum. Importantly, the enrichment of aromatic hydrocarbons inside the asphaltene aggregates, particularly the relative abundance of different methylphenanthrene isomers, provided crucial experimental evidence in support of our earlier hypothesis that the occluded hydrocarbons were protected from secondary alterations, and therefore had remained relatively stable over a long geologic period.

Hydrocarbon Accumulation Mechanism of Ordovician in the Halahatang Area, Tarim Basin—Evidence From Organic Geochemistry

As an important oil and gas-bearing area in the Tarim Basin, the Halahatang Area has great potential for resource exploration. However, the research on oil and gas sources and the filling period of the Lower Paleozoic Ordovician is still limited. In this study, the organic geochemical analysis of source rocks, the simulation technology of hydrocarbon generation evolution basin, the distribution characteristics of fluid inclusions, and the measurement of uniform temperature are used and combined with the stable isotope of natural gas and the characteristics of oil-source biomarkers, and the process of oil and gas accumulation is restored. The results show that: ①The Cambrian source rock is the main contributor to Ordovician crude oil in the Halahatang Area, and its crude oil shows the characteristics of lightweight, rich chain hydrocarbons, and poor aromatic hydrocarbons. The composition of C7 compound shows the advantage of n-heptane, which belongs to type I kerogen. The biomarker compounds show the characteristics of high content of long-chain tricyclic terpane and high Ts/Tm values, and the crude oil is considered to be a highly mature sapropelic kerogen source oil. ②The carbon isotopes of methane and ethane of Ordovician natural gas in the Halahatang Area are light (from −35.6 to −29.5‰), showing obvious characteristics of sapropelic gas. ③There are three stages of the oil and gas accumulation process in Ordovician reservoirs in the Halahatang Area. In the middle and late Caledonian periods, the source rock reached the hydrocarbon generation threshold and began to generate oil. The late Hercynian period was the main accumulation period, and the Himalayan period was dominated by dry gas filling. ④The crude oil generated in the Middle-Late Caledonian periods migrated along the faults and conductive layers and finally accumulated in the high parts of the structure in the north. The migration of oil and gas in the late Hercynian period was limited by the bitumen produced by early biodegradation. Since the asphalt plugging condensate gas has almost failed to cause gas invasion from the reservoir, the Ordovician reservoir still maintains the characteristics of crude oil-associated gas, forming an oil and gas reservoir with a gas cap.

GEOCHEMICAL COMPOSITION OF ORDOVICIAN OILS FROM THE TUOPUTAI REGION, NORTHERN TARIM BASIN, NW CHINA: SOURCE ROCK CORRELATION

Molecular and stable carbon isotope compositions of 46 Ordovician crude oil samples from wells in the Tuoputai region of the northern Tarim Basin were investigated using GC–MS, MRM GC–MS and IRMS to determine their genetic relationships and to identify possible source rocks. Thirty‐three source rock samples from outcrops and cores were also investigated. The oil samples varied from light to heavy crudes and showed very narrow δ13C value ranges for the whole oil, saturated and aromatic fractions. The majority of the oils displayed very similar molecular compositions with relatively high concentrations of n‐alkanes and isoprenoids and low concentrations of terpenoids and steroids. Comparison of the compositions of these crude oils strongly suggested their genetic affinity, while maturity parameters indicated maturity variations from the peak to the late oil generation stages. The samples also showed the characteristics of mixtures of biodegraded and fresh oil charges.Bitumen extracts from Cambrian and Ordovician source rocks were studied in detail. The oil compositions suggested a marine marl source deposited in anoxic, hypersaline conditions with significant bacterial and algal organic matter inputs. The distributions of C26–C28 triaromatic steroids, tricyclic terpanes and regular steranes appear to have been greatly influenced by thermal maturation, making them unreliable for correlating the oils and the source rocks. In contrast, dinosteranes and triaromatic dinosteroids seem not to have been affected by maturation and were more useful for correlation studies. They indicated that there was no or little genetic relationship between the Cambrian – Lower Ordovician source rocks and the oils, but in general suggested a possible Middle – Upper Ordovician source for the oil accumulations in the Tuoputai field. However, the occurrence of triaromatic dinosteroids in oil from well TP28XCX may also suggest a minor contribution from Cambrian – Lower Ordovician source rocks.

Geochemical Characteristics and the Origin of Superdeep Condensates in Tarim Basin, China.

A series of trace compounds (diamondoids, ethanodiamondoids, and thiadiamondoids) were detected through two-dimensional gas chromatography/time-of-flight mass spectrometry (GC × GC-TOFMS) analysis of Ordovician condensate samples from the Tazhong area. Gas chromatography-mass spectrometry (GC-MS) analysis showed that the biomarker parameters are less effective for high-maturity oils. Carbon isotope and geochemical features suggested that the gas is a high-temperature cracking gas when its temperature is higher than 170 °C. The H2S content is 8.27%, suggesting that it is affected by thermochemical sulfate reduction (TSR). However, the geological analysis indicated that the Ordovician reservoirs do not satisfy the conditions for TSR. The high-maturity oil in the Ordovician reservoirs may generate diamondoids and ethanodiamondoids when cracking, while TSR and severe cracking occur in deep Cambrian source rocks and produce a large number of diamondoids, ethanodiamondoids, organic sulfur compounds (OSCs), etc. The secondary geochemical products that are carried up by the dry gas and migrate upward through faults and are enriched in Ordovician crude oil reservoirs. On this basis, we proposed that the condensate presented was formed by the mixing of dry gas from Cambrian oil that was altered by cracking and TSR into Ordovician in situ slightly cracked oil, therefore speculating that the favorable reservoir–seal assemblages in this area may contain abundant oil and gas resources. Consequently, improved knowledge of secondary alteration effects on the reservoir and underground fluids is vital for oil and gas prediction and exploration development in the next step.

Comparative analysis of characteristics and genesis of crude oils of the Zs block in the Tazhong area and the Ha 6 block in the Tabei area, Tarim basin

Ha6 block is located in the north of the Tarim Basin. In this study, the characteristics of the Ordovician crude oil in the Ha6 block were analyzed by the gas chromatography-mass spectrometry (GC-MS) method on the basis of exploration and development data. The results show that C7 compounds are produced by type-I kerogen. According to the comparison, the medium-molecular-weight hydrocarbon fingerprint of the Cambrian crude oil in the Zs block is consistent with that of the Ordovician crude oil in the Ha6 block, which means that their sources are the same. In the two blocks, biological marker compounds of crude oil are also very similar: C27R, C28R, and C29R steranes are in a V-shaped pattern; the Ts/Tm ratio and the content of tricyclic terpanes are both high; and the presence of a large amount of aromatized isoprenoid indicates the crude oil was generated in a strong-reducing environment. In addition, the crude oil single molecule carbon isotope curve is similar with that of Wells Zs1 and Zs5. Therefore, it is comprehensively concluded that the crude oil in the Ordovician reservoirs in the Repu area is mainly supplied by the Cambrian source rock.

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.

Characteristics and genesis of the Ordovician crude oil in the RePu area, Halahatang sag, Tarim Basin, China

The RePu area is located in the Tarim Basin, China. Based on the exploration and development, this paper analyzed the characteristics of the Ordovician crude oil and the source rocks by gas chromatography-mass spectrometry. The Ordovician carbonatite reservoir oil in this area exhibited light component, rich chain hydrocarbon and poor aromatic hydrocarbon. The composition of C7 compound showed the advantage of n-heptane, which belonged to type I generative kerogen, and it was deep buried highly mature crude oil. The crude oil biomarker compounds all exhibited high content of long-chain tricyclic terpanes and high Ts/Tm values. The large presence of alkane aromatization compounds indicated strong reducing environmental conditions during crude oil formation. By comprehensive judgment that the main oil source of the Ordovician reservoirs in the RePu area was the Cambrian source rock.

Biodegradation and possible source of Silurian and Carboniferous reservoir bitumens from the Halahatang sub-depression, Tarim Basin, NW China

Five core samples comprising three Silurian and two Carboniferous bituminous sandstones from the Halahatang sub-depression (Tarim Basin, NW China) were studied to determine the nature and extent of microbial alteration and possible source(s) of the reservoir bitumens. A total of twelve Ordovician crude oil samples from this sub-depression were also examined for comparative purposes. Co-occurrence of a series of 25-norhopanes (NHs) (C28–C34), variable amounts of chromatographically unresolved complex mixture (UCM), and diminished concentrations of n-alkanes below ca. C20 in saturated hydrocarbon fractions of the Silurian and Carboniferous bitumens indicates that they are mixtures of heavily biodegraded oil from a former charge and slightly biodegraded oil from a later charge. The presence of C28–C34 25-NHs suggests a biodegradation level of at least PM 6 for these bitumen samples. Nevertheless, minor differences in terpane and sterane distributions between the Silurian and Carboniferous bitumen samples were observed which can be attributed to small variations in biodegradation levels. Values for the 25-NH ratio (Peters et al., 1996), gammacerane/(gammacerane + C30 hopane), C28 25-NH/C29 hopane and C29 25-NH/C30 hopane ratios indicate slightly higher levels of microbial alteration for the Silurian bitumens compared to the Carboniferous bitumens, although caution must be exercised when comparing the biodegradation intensities using molecular parameters. The Ordovician crude oil samples appear to be more severely biodegraded than the Silurian and Carboniferous bitumens, as evidenced by higher concentrations of tricyclic terpanes relative to hopanes, and pregnanes and diasteranes relative to the regular steranes, although thermal maturity effects cannot be ruled out. Geochemical correlation studies were successfully applied based on carbon isotopes and biomarkers including tricyclic terpanes and triaromatic steroids which are highly resistant to severe biodegradation (PM 6). Bulk isotopic compositions, distribution patterns and certain ratios of these biomarkers indicate that the Ordovician crude oils and the Silurian and Carboniferous bitumens originated from a single source. They were probably derived from Middle–Upper Ordovician source rocks.

Distribution of C26 norcholestanes in Ordovician crude oils from Tahe Oilfield and its geological significance

Abstract A series of C26 norcholestanes was detected in the Ordovician crude oils from the Tahe Oilfield, the Tarim Basin, northwest China. The abundance of 24-norcholestanes in most of the Ordovician crude oils was lower than that of 27-norcholestanes. The oils from well T904 in the eastern Tahe Oilfield and well TD2 in the eastern Tarim Basin, however, were characterized by relatively higher 24-norcholestane abundance. The distribution of C26 norcholestane of the Upper Ordovician marls from the wells LN46 and BD2 resembled that of the most Ordovician crude oils from the Tahe Oilfield. The distribution of C26 norcholestanes in typical Cambrian black shales and gray limestones from the wells TD2, He4 and outcrops from Keping profile resembled that of the crude oils from the wells T904 and TD2. Criteria are proposed relative to C26 norcholestanes that could be used to classify oil families within the Ordovician reservoirs. Oils with NCR (defined as 24/ (24+27)-norcholestanes) &gt;0.50 and NDR (defined as 24/ (24+27)-nordia-cholestanes) &gt;0.35 were thought to be derived from the Cambrian source rocks, and oils with NCR&lt;0.40 and NDR&lt;0.35 were thought to be derived from the Upper Ordovician source rocks. Therefore, C26 norcholestanes were effective molecular markers to classify oil genetic families in the Tahe Oilfield. They could also provide molecular evidence for the origin and evolution of diatoms and dinoflagellates and the plate drift of the Tarim Basin.

Erratum to: Geochemical characteristics of Ordovician crude oils in the northwest of the Tahe oil field, Tarim basin

Forty-six crude oil samples were selected from the Ordovician in the northwestern part of the Tahe oilfield for detailed molecular geochemical and isotopic analysis, including group compositions, carbon-hydrogen isotopes and gas chromatograms of saturated hydrocarbons, as well as the characteristics of terpane, sterane and other biomarkers, indicating that crude oils are of the same origin from different districts in the Tahe oilfield and were derived from the same source kitchen (or oil source formation), i.e., mainly stemming from marine hydrocarbons. Detailed studies of oil physical properties of 25-honpane revealed that such oils have heavy or thick oil qualities due to biodegradation. Comprehensive assessment in terms of five maturity parameters shows that the oils from the Ordovician with R ∘ values varying from 0.80% to 1.59% are widely distributed in the northwest of the Tahe oilfield.

Geochemical characteristics of Ordovician crude oils in the northwest of the Tahe oil field, Tarim basin

Forty-six crude oil samples were selected from the Ordovician in the northwestern part of the Tahe oilfield for detailed molecular geochemical and isotopic analysis, including group compositions, carbon-hydrogen isotopes and gas chromatograms of saturated hydrocarbons, as well as the characteristics of terpane, sterane and other biomarkers, indicating that crude oils are of the same origin from different districts in the Tahe oilfield and were derived from the same source kitchen (or oil source formation), i.e., mainly stemming from marine hydrocarbons. Detailed studies of oil physical properties of 25-honpane revealed that such oils have heavy or thick oil qualities due to biodegradation. Comprehensive assessment in terms of five maturity parameters shows that the oils from the Ordovician with R ∘ values varying from 0.80% to 1.59% are widely distributed in the northwest of the Tahe oilfield.

Main progress and problems in research on Ordovician hydrocarbon accumulation in the Tarim Basin

The Tarim Basin is the largest petroliferous basin in the northwest of China, and is composed of a Paleozoic marine craton basin and a Meso-Cenozoic continental foreland basin. It is of great significance in exploration of Ordovician. In over 50 years of exploration, oil and gas totaling over 1.6 billion tonnes oil-equivalent has been discovered in the Ordovician carbonate formation. The accumulation mechanisms and distribution rules are quite complicated because of the burial depth more than 3,500 m, multi-source, and multi-stage accumulation, adjustment, reconstruction and re-enrichment in Ordovician. In this paper, we summarized four major advances in the hydrocarbon accumulation mechanisms of Ordovician carbonate reservoirs. First, oil came from Cambrian and Ordovician source rocks separately and as a mixture, while natural gas was mainly cracked gas generated from the Cambrian-Lower Ordovician crude oil. Second, most hydrocarbon migrated along unconformities and faults, with different directions in different regions. Third, hydrocarbon migration and accumulation had four periods: Caledonian, early Hercynian, late Hercynian and Himalayan, and the latter two were the most important for oil and gas exploration. Fourth, hydrocarbon accumulation and evolution can be generally divided into four stages: Caledonian (the period of hydrocarbon accumulation), early Hercynian (the period of destruction), late Hercynian (the period of hydrocarbon reconstruction and re-accumulation), and Himalayan (the period of hydrocarbon adjustment and re-accumulation). Source rocks (S), combinations of reservoir-seal (C), paleo-uplifts (M), structure balance belt (B) matched in the same time (T) control the hydrocarbon accumulation and distribution in the Ordovician formations. Reservoir adjustment and reconstruction can be classified into two modes of physical adjustment and variation of chemical compositions and five mechanisms. These mechanisms are occurrence displacement, biodegradation, multi-source mixing, high-temperature cracking and late gas invasion. Late hydrocarbon accumulation effects controlled the distribution of current hydrocarbon. The T-BCMS model is a basic geological model to help understanding the control of reservoirs. At present, the main problems of hydrocarbon accumulation focus on two aspects, dynamic mechanisms of hydrocarbon accumulation and the quantitative models of oil-bearing in traps, which need further systemic research.

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.

Reservoir geochemistry of the Tazhong Oilfield in the Tarim Basin, China, Part II. Migration, accumulation and mixing of crude oils

Measurements of the absolute and relative concentrations of nitrogen-containing compounds in crude oils from different reservoir strata (Ordovician, Silurian and Carboniferous) in the Tazhong region of the Tarim Basin, Xinjiang, China, showed that even though there are quite a number of factors affecting the distributional and compositional characteristics of neutral nitrogen-containing compounds in crude oils, the distributional and compositional characteristics of crude oils whose source conditions are approximate to one another are influenced mainly by the migration and fractionation effects in the process of formation of oil reservoirs. In addition, crude oils in the Tazhong region show obvious migration-fractionation effects in the vertical direction. Carboniferous crude oils are characterized by high migration parameters and low compound concentrations, just in contrast to Ordovician crude oils. This indicates that crude oils from shallow-level oil reservoirs were derived from those of deep-level oil reservoirs via faults, unconformable contact or carrier beds. Crude oils from the Tazhong region show some migration-fractionation effects in the lateral direction, but mixing of crude oils derived from different hydrocarbon source rocks in the process of formation of oil reservoirs made it more complicated the migration and accumulation of crude oils, as well as the formation of oil reservoirs.