Biomarker characteristics and Origin of Crude Oils from Well Tian 2
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The biomarker assemblage characteristics and origin of crude oils in Well Tian 2 in Tiancao Sag were studied by using quantitative gas chromatography-mass spectrometry analysis.The geochemical characteristics were described as follows: abundance of hopanes from C31 hopane to C35 hopane decline gradually, gammacerane with higher abundance, the absence of rearranged hopane and rearranged sterane, reverses shape of L in the distribution of C27~C29 sterane and higher abundance of C28 sterane.Sterane isomerization parameters and alkyphenanthrene index l indicate that the maturity of crude oil is low.On the basis of comprehensive analysis of the biomarker assemblage characteristics of possible source rocks in this region, and from the characteristics of biomarker fingerprints and their absolute concentrations, it is affirmed that there is a genetic relationship between crude oil of Well Tian 2 and the lower part of source rocks in Bayingobi Formation of lower Cretaceous.It is pointed out that source rocks of lower part of Bayingebi Formation are the main source rocks of Tiancao Sag.Keywords:
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It is the first time that some crude oil has been discovered in the Chagan Sag, the eastern of the Yingen Ejinaqi basin, NW China. The geochemical characteristics of the oils (including oil sands) and source rock have been studied in this paper. The oils, in which saturated hydrocarbon accounts for more than 90%, belongs to maturated oils with a lower density and lower sulfur content at the generaton peak stage (equal to R o=1.0%±). The δ 13 C value of the whole oil is -31.1‰. The features of the biomarkers in the oils are as follows: relatively high abundance of isoprenoidies, no preference between pristane and phytane; the proportion of tricyclic terpanes over pentacyclic terpanes; relatively high amounts of Ts, C 29 Ts, C 30 diahopane and gammacerane; the significant amounts of C 30 hopane and much lower concentration of homohopanes (higher than C 32 ); high amounts of pregnanes; the significant proportion of C 29 sterane, relatively high abundance of C 27 sterane, the proportion of isocholestane over cholestane; low concentration of diacholestane; equivalent amounts between steranes and terpanes. The proportion of methane in the discovered natural gas is up to 80%, reflecting that it belongs to wet gas.The oil source correlation shows that the crude oils in Chagan Sag are generated from Lower Cretaceous Bayingebi Formation, and can be described as typical lacustrine oils. Biomarkers reveal that the oil has a certain migration effect.
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Gas chromatography-mass spectrometry analyses have been carried out to investigate the geochemical characteristics of the Latrobe Group shales and coaly shales from the Gippsland Basin, Australia. The depositional environment, source of organic matter and thermal maturity of hydrocarbon source rocks in the study area were evaluated using molecular biomarker analyses. The distribution of isoprenoid alkanes and pentacyclic triterpanes reveals an oxic environment with fresh water (pristane/phytane > 3.0, gammacerane index < 0.3). The carbon preference indices (CPI) and odd-to-even predominance ratios of the n-alkanes are higher than 1.0, suggesting terrigenous higher plant-derived organic matter in the sediments. The high predominance of C29 sterane over C27 sterane, as well as the occurrence of conifer and angiosperm biomarkers (e.g., labdane, isopimarane, phyllocladane, rimuane, oleanane, retene, anthracene, and cadalene), corroborates input from higher vascular land plants. Biomarker and aromatic thermal maturity indices, such as the methylphenanthrene index, the methylnaphthalene ratio, C31 22S/(22S+22R) hopanes, C30 αβ/(αβ+βα) hopanes and C29 ααα 20S/(20S+20R) steranes, indicate rather thermally immature hydrocarbon source rocks, in agreement with the above CPI data. This maturity trend is also supported by the triaromatic sterane index [TA(I)/TA(I+II)], which is generally lower than 0.2.
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Abstract Three different organic facies for source rocks are recognized in the NW Java Basin based on biomarker composition of oils. The largest group of oils, derived from a deltaic source rock, typically has a high concentration of oleanane, C27/C29 sterane > 1, hopane/sterane > 4, and a predominance of medium weight n-alkanes. A second group of oils, probably of lacustrine origin, has an abundance of botryococcane, hopane/sterane > 8, an unusual sterane distribution (C28 > C29 > C27), and a slight odd-carbon n-alkane predominance in the C21 - C33 range. A third type, represented by two oils, is characterized by a predominance of low molecular weight n-alkanes (n-C11 to n-C17), low diasterane/sterane, hopane/sterane > 2, C23 tricyclic terpane higher than C24 tetracyclic terpane, C29/C30 hopane > 1, a high homohopane index, C35/C34 homohopane > 1, and an intermediate to high sulfur content. These features suggest a marine carbonate depositional setting for the source rock, although high pristane/phytane (4 to 5) and high concentrations of C19 and C20 tricyclic terpanes conflict with this interpretation. Accordingly, these two oils may have been mixed with oil from a noncarbonate source rock. This is the first indication that a carbonate source rock, probably the Baturaja Formation, is present in the NW Java Basin.
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Biomarker studies using oil samples and sediment extracts from wells located in the northern part of the Sabah Basin show that the organic matter source for the basin's hydrocarbon to be dominantly terrigenous, indicated by high triterpane/sterane ratios, the presence of compounds diagnostic of land derived organic matter such as oleanane, and resins W, T and R, and the general predominance of C29 regular steranes over C27 and C28 components. The consistently similar mass fragmentograms of both mJz 191 and mJz 217 suggests that the oils and sediment extracts originated either from the same source rock or, more likely, from different source rocks with similar chemical compositions. The similarity of biomarker distributions in samples from different environments suggests that detrital plant components from similar higher land plant assemblages were redistributed by sedimentary/transport processes to various depositional settings. Petrographic studies of kerogen concentrates confirm the dominance of land-plant contribution and the general absence of marine organic matter even in the marine sediments. For the oils, the 20S/(20S+20R)-sterane biomarker maturity ratios are lower than the equilibrium value of 0.55 and moretane/hopane ratios > 0.10. This supports the apparent retardation of the isomerisation process previously reported by Grantham (1986) to occur in Far Eastern Tertiary oils. The triterpane 22S/(22S+22R) ratio, on the other hand, appears to reflect the expected maturity levels of the oils. The majority of the extracted sediments are shown to be immature by pyrolysis Tmax, vitrinite reflectance and kerogen elemental composition, indicating that the extracted hydrocarbons have migrated into the sediments from elsewhere. Therefore, biomarker maturity ratios from the extracts cannot be used to determine the true maturity levels of the sediments as the in situ biomarkers will have been mixed with those already present in the migrating hydrocarbons. Variations of biomarker maturity ratios with depth resemble those of the Production Index in all the studied wells, thus confirming the influence of non-indigenous residual hydrocarbons on the biomarker maturity ratios.
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Based on gas chromatography mass spectrometry techniques, the biomarker characteristics of the Hongqi Formation source rocks in the Tuquan basin are studied, information and petroleum geological significance on organic matter origin, sedimentary environment and organic matter maturity are revealed. Biomarker assemblage characteristics of the Hongqi Formation source rocks are described as follows: Long-chain tricyclic terpane and gammacerane is abundant, C 27 regular steranes is predominant. The low content of gammacerane in organic matter shows that the source rocks were deposited in the low salinity environment. The mean value of steranes/hopanes is 0.40, showing more contribution from algae than bacteria. High content of C 27 regular steranes indicate that the parent material input is mainly the low aquatic o Subscript text rganism. Biomarker parameters including Ts/(Ts+Tm) ratio, C 29 steranes 20S/(20S+20R) ratio, C 29 sterane αββ/(αββ+ααα) ratio indicate that Hongqi Formation source rock has reached or surpassed the main phase of oil generation.
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Based on the analytical results of basic geochemical features on the source rocks from Oligocene Liushagang Formation from Well Xuwen-1 in MaIchen Sag of Beibuwan Basin,it was found that the abundance and the type of organic matter in the 2nd member of the source rocks in Liushagang Formaton were better than that in the 3rd member,it was implied that the hydrocarbon potential in the 2nd Member was higher than that in the 3rd Member.There was a similarity in the distribution of triterpane series of source rocks in the 2nd Member and the 3 Member,such as with dominant hopane series,moderate oleanane and almost absent gammacerane.It was indicated that those source rocks are deposited in fresh-water environment with abundant terrestrial materials input.In C27-29starane series,the relative content of C27 sterane from algae input in the source rocks of 2nd member is more that in the 3rd Member,and vice versa in C29 sterane from terrestrial matter input.For the crude oil from Well Xuwen-1,high Pr/Ph ratio and almost absent gammacerane are characterized,it shows that it is derived from the source rocks deposited under fresh-water and weakly reduced environment.By comparison,the distributions of triterpanes and steranes in the crude oils are between the source rocks of 2nd and 3rd Members,it is a typically mixed oil.Based on the quantitative analytical results of diasterane C27/C29 ratio,it is found that in this mixed oil,the contribution of the 2nd member and 3rd member 3 is about 77% and 23%,respectively.
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Crude oils representing four different holes from the Tamsagbulag oilfield were analyzed by gas chromatography-mass spectrometry (GC/MS) spectroscopy. Fifty-two biomarker compounds, 34 pentacyclic triterpanes and 18 steranes were identified and semi-quantitatively determined by selective ion monitoring (SIM) chromatography. Depending on the oil source, the variation in the pentacyclic triterpanes distribution is more significant than the variation in the sterane distribution. It is said that pregnane and honopregnane originated during the period of very salty sedimentary accumulation in the condition of diagenesis.
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Biomarker is one of the most effective tools in the study of oil-source correlation gas-oil maturity and gas-oil migration. By using biomarker GC, GC—MS analysis techniques, the geochemical characters of steranes and hopanes from crude oils of Tahe oil field were investigated in this paper. The study showsTahe oilfield is a compound oil reservoir, which is composed of later generated oils with high maturity lighter biodegradation and earliergenerated oils with low maturity stronger biodegradation. The characters of C_ 27 —C_ 29 steranes andhopanes series indicate oils with varied maturities generated from the same sort of mother materials. The three-dimensional diagramformed by Ts/(Tm+Ts),rearranged-sterane/sterane and ββ/(ββ+αα)C_ 29 shows that the migration direction of crude oil is from south to north, which confirms the opinion that oil and gas of Tahe oilfield mainly came from Manjiar Depression——an area to the south of Tahe oilfield.
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