Immature Crude Oil Research Articles

Origin of polar organic sulfur compounds in immature crude oils revealed by ESI FT-ICR MS

Organic sulfur compounds (OSCs) are abundant in immature crude oils, including “polar” OSCs that are defined here as compounds containing one or more heteroatom(s) in addition to sulfur atoms having sufficient polarity to be analyzable by electrospray ionization (ESI) without derivatization. An understanding of the origins of polar OSCs in crude oils has been hampered by limitations in their analytical characterization. In this paper, we employed a high field (9.4 T) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) coupled with ESI to study the sulfur-rich immature crude oils from the Jianghan Basin in China. The results show that the polar OSCs usually have non-sulfur counterparts, i.e., heteroatom-containing compounds without sulfur, with similar carbon number ranges, but slightly lower DBE (double bond equivalence) distributions. The similar or even identical carbon number distributions of the polar OSCs and their non-sulfur counterparts indicate their inheritance from the same precursors. The sulfur rings in polar OSCs are formed by intramolecular sulfurization that leads to the increases in DBEs compared to their non-sulfur counterparts. The results also indicate that the number of sulfur rings that can be formed in polar OSCs is largely controlled by the number of available reactive functional groups in their precursors. This paper extends our knowledge of intramolecular sulfurization during early diagenesis, especially for polar heteroatom-containing compounds.

Molecular characterization of sulfur compounds in some specieal sulfur-rich Chinese crude oils by FT-ICR MS

Routine GC/MS analysis may apply to the volatilized Low-Molecular-Weight compounds in saturate and aromatic hydrocarbon fractions; thus, relative studies using this technique inevitably bring about some limitations on distribution of miscellaneous sulfur atom. In this article, Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) with high resolution is employed to investigate the distribution of organic sulfur compounds (OSCs) in the crude oil typically derived from the Eogene carbonate-evaporite sediments with further chemical compositional characterization in molecular level by miscellaneous atomic type, carbon number, and double bond equivalent (DBE). A variety of miscellaneous atomic types with S1, S2, S3, OS, OS2, O2S, O2S2, NS, and NOS etc. (S1 means those OSCs with one sulfur atom in a molecule) were identified in OSCs in these oil samples. High levels of alkyl thioether series compounds with one ring structure were presented mainly in the crude oil in the Jianghan Basin whereas high amounts of benzothiophene, dibenzothiophene etc. compounds with higher values in DBE and carbon number range occurred in the sulfur-rich heavy oil in the Jinxian Sag. Although carbonate-evaporite sediments deposited in the saline lacustrine facies in the Eogene basin both occurred in the Jinxian Sag and Jianghan Basin, obviously, they possess different chemical diagenetic pathway of sulfur under various microbial reactions, leading to diverse distributional characteristics on biomarkers, OSCs, and even different hydrocarbon generation mechanism of immature crude oil.

Immature Oils in China and Their Genetic Mechanisms

Abstract Immature crude oils are a kind of unconventional petroleum resources. They are generated through early low‐temperature biochemical/chemical reactions of some specific organic matter. Their geological reserves explored are as high as several hundred million tons in China. Based on a detailed organic geochemical study, five genetic mechanisms of immature oils have been proposed in this paper for early hydrocarbon generation from suberinite, resinite, bacteria‐reworked terrestrial organic matter, biolipids and sulphur‐rich macromolecules respectively.

OCCURRENCE AND DISTRIBUTION OF n-ALKANESAND n-FATTY ACIDS IN SAUDI ARABIAN CRUDE OILS

ABSTRACT Ten crude oil samples, covering wide range of maturity (API gravity = 18·5–36·1), were assembled from Safaniya, Abqiq, Ain Dar, Wafra. Marjan and Zuluf oil fields in the area of Arabian Gulf. n-Alkanes of were separated from the petroleum distillate ((150°C–450°C) of the crude oils by urea adduction. n-Fatty acids were separated from the fraction of n-alkanes by treatment with aqueous solution of KOH. Distribution of n-alkanes and n-fatty acids has been investigated by means of gas chromatography. The studied crude oils showed symmetrical distribution curves of n-paraffins and fatty acids of low molecular weight were abundant as compared with n-paraffins. The n-paraffins distribution curve of Wafra/Iucene biodegraded immature crude oil showed three maxima at C17, C19, and C31 whereas the maxima of n-fatty acids are located at C14,C22and C24. The results were interpreted in terms of origin, maturation and depositional environments of the crude oils.

Immature crude oils in the salt lake depositional environment are related to organic matter precipitated at stage of carbonate in salt lake sedimentation sequences

The present paper studies the mineralogy and geochemistry of a geological section in Jianghan basin. Widely distributed immature oils in this basin have been shown to relate to the carbonate depositional stage of salt lake sedimentation cycle. It is concluded that only when the content of organic carbon in the rocks is taken into account, is it meaningful to calculate the immature oil in the source rocks.

Lanostane sulfides in an immature crude oil

Tetra- n-butylammonium periodate oxidation/LiAlH 4 reduction, thiourea adduction and molecular distillation were employed for the isolation of pentacyclic lanostane sulfides in an immature Chinese crude oil. The position of the sulfide bridges in the molecules was assigned by the deuterium exchange and mass spectra of the corresponding sulfones. Nickel boride desulfurization of the lanostane sulfides yielded C 30 and C 31 lanostanes, the structures of which were established by GC-MS with co-injection of synthetic standards. None of these lanostanes were found to be present in the oil.

A biomarker study of immature crude oils from the Shengli oilfield, People's Republic of China

A suite of crude oils from the Shengli oilfield in northwestern China was subjected to detailed chemical analysis. Crude oils in this suite, believed to be derived from source rocks deposited under fresh, brackish and saline environments have chemical compositions and biological marker compounds indicative of immature source rocks. In this respect the crude oils are unusual, apparently having been formed and expelled from source rocks that have been subjected only to mild thermal stress prior to the main zone of petroleum generation.

Geochemical characteristics of immature hydrocarbons of the Anjihai Depression in the Junggar Basin

On the basis of organic studies, this paper presents the geochemical characteristics of immature crude oil and source rock from the Anjihai Depression, Junggar Basin. The vitrinite reflectivities ( R °) of Tertiary rock samples are less than 0.6% at depths of 3000–4000 m. Sterane αααC 29 S/( R + S) values range between 0.049 and 0.23, and C 29ββ/(ββ + αα) values range from 0.30 to 0.48 in the Tertiary rock samples. The above two ratios of the crude oils are 0.44 and 0.47 respectively. Ratios of Σ rearranged steranes and Σ total steranes are in the range of 0.02–0.15 for both crude oil and Tertiary rock samples. These results show that the Tertiary rock samples are in a lower evolutionary stage, and crude oils originating from the Tertiary system are immature.

Characterisation of highly branched isoprenoid thiophenes occurring in sediments and immature crude oils

A number of C 20 and C 25 highly branched isoprenoid thiophenes (HBIT) (i.e. thiophenes with 2,6,10-trimethyl-7-(3′-methylbutyl)dodecane and 2,6,10,14-tetramethyl-7-(3′-methylpentyl)pentadecane carbon skeletons) have been identified in sediments and immature oils from different geographical locations. Eight C 20 and two C 25 HBIT structural isomers were assigned on the basis of mass spectral characterisation, desulphurisation of isolated or mixtures of C 20 and C 25 HBIT and in some cases by comparison of mass spectral and relative retention time data with authentic compounds. Only a limited number of all theoretically possible isomers were present, indicating that these HBIT are probably formed by selective sulphur incorporation reactions into the widely occurring highly branched isoprenoid alkenes.

Origin of organic sulphur compounds and sulphur-containing high molecular weight substances in sediments and immature crude oils

The distribution patterns of Organic Sulphur Compounds (OSC), occurring in certain sediments and immature crude oils, were compared with those of the corresponding hydrocarbons. Because of the complexity of the OSC mixtures, they were desulphurized to hydrocarbons ( n-alkanes, isoprenoid alkanes, steranes, triterpanes and branched alkanes). The hydrocarbons produced by desulphurization of the OSC exhibited distribution patterns different from those of the hydrocarbons originally present. Therefore reaction of elemental sulphur with these hydrocarbons at elevated temperatures must be considered as an unlikely origin for these OSC. Sulphur incorporation reactions on an intramolecular basis with suitable functionalized precursors at the early stages of diagenesis are probably the major origin for these OSc. Desulphurization of high molecular weight fractions also produced hydrocarbons, dominated by n-alkanes up to C 40. Therefore it is assumed that these substances contain n-alkanes, 2,5-dialkyl-thiophenes and -thiolanes linked to each other by sulphur briddges. These findings stronly suggest that sulphur-containing high molecular weight substances are formed by the same sulphur incorporation reactions as OSC, but in an intermolecular fashion.

Occurrence and geochemical significance of long-chain dialkylthiacyclopentanes

Small sulphur-containing molecules were first detected in petroleums and sediments many years ago1–3, but only recently have complex sulphur components related to biological markers been identified4–14. These discoveries have provided interesting information on the distribution of sulphur-containing compounds in geological samples, but the formation mechanisms of such molecules remain to be clarified. Recently, Connan et al.15 recognized the occurrence of series of long-chain sulphur-containing components in heavy sulphur-rich crude oils from carbonate basins and tentatively identified them as alkylthiacyclopentanes and alkylthiacyclohexanes on the basis of selective detection and gas chromatography/mass spectrometry studies. Here we report the conclusive identification and geochemical significance of long-chain 2,5-dialkylthiacyclopentanes of type 1 (see Fig. 4), occurring as major components in some of these petroleums, as shown for an immature carbonate crude oil from Maruejols (Oligocene Ales basin, France), containing 6.5% sulphur16. In addition to providing new information about the 'aromatic' fractions of these crude oils (which are in fact largely non-aromatic), these results give useful indications of some of the mechanisms of incorporation of sulphur into organic matter in the sub-surface environment. Furthermore, as confirmed by thermal simulation experiments, these dialkylthiacyclopentanes are thermally labile, which seems to restrict their occurrence to immature crude oils. They could therefore serve as useful maturation parameters for the early stages of petroleum formation in carbonate basins.

Sulphur-containing compounds in sulphur-rich crude oils from hypersaline lake sediments and their geochemical implications

Three sulphur-rich commercial crude oils have been studied, which contain sulphur as high as up to 4–12%. These samples were collected from Tertiary hypersaline lake sediments of the Jianghan Basin, Hubei Province at different depths, but above the oil generation threshold (2200m). FPD-GC and GC-MS data show that aromatic fractions of the crude oils are composed of different homologues of sulphur-containing compounds, including long-chain normal alkyl-thiophenes and-thiolanes, long-chain isoprenoid-thiophenes and -thiolanes, and benzothiophenes. It is worth noting that the distribution patterns of long-chain alkyl-thiophenes and -thiolanes from two shallow-seated crude oils are quite similar to those of normal alkanes showing marked even-odd predominance. It seems that the even-odd predominance of sulphur-containing compounds decreases with increasing burial depth of the crude oils. The major component of aliphatic fraction is phytane, and similarly the major peaks of aromatic fractions also represent C20 isoprenoid thiophenes. Some preliminary conclusions have been drawn from the above discussion: (1) Abundant sulphur-containing compounds may be used as an indicator of low mature or immature crude oils produced from hypersaline lake sediments; (2) Sulphur-containing compounds are considered to be early diagenetic products of reactions between elemental sulphur or sulfides and alkanes or their precursors (phytols, fatty acids, alcohols, etc.), or of bacterial activities, but not direct inputs of organisms.

Factors influencing the evolution of petroleum aromatics from an immature crude oil during pyrolysis on a shale matrix

Samples of an immature crude oil have been pyrolyzed at 300°C on a range of crushed sediment matrices. The maturation of the aromatic fraction of the petroleum was monitored using proton magnetic resonance spectroscopy. Factors varied in the matrix included the cation exchange capacity, the nature of the exchange ions, clay type, level of elemental sulphur, and surface area. The variables found to have the greatest effect on maturation rate were the proton exchange capacity and the concentration of elemental sulphur. In addition, removal of the heterocyclic components from the crude oil resulted in an enhanced rate of maturation. The implications of these results for the processes involved in petroleum formation and maturation under geological conditions are discussed.