Jinxian Sag Research Articles

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.

Petroleum Sulfur Biomarkers Analyzed by Comprehensive Two-Dimensional Gas Chromatography Sulfur-Specific Detection and Mass Spectrometry

An atypical distribution of steroids occurred in oilsand and oil samples in the Jinxian Sag, Bohai Bay Basin, northern China. It is featured by usually abundant C21–C26 short-chain steranes that are derived from the cleavage of the weak C–S bonds at the C-17 and C-20 positions. Sulfur-containing petroleum biomarkers were further analyzed in these sulfur-rich heavy oils by comprehensive two-dimensional gas chromatography (GC × GC) using sulfur-specific detection to locate and confirm sulfur species and GC × GC time-of-flight mass spectrometry (TOF MS) to identify previously unknown sulfur-containing biomarkers. Preliminary GC–MS analysis revealed a series of compounds yielding characteristic fragment ions consistent with thiophenic substructures. For confirmation, GC × GC–sulfur chemiluminescence detection (SCD) was used to confirm the sulfur moiety. The molecular structures of sulfur biomarkers were proposed as unsaturated isoprenoid thiophenes based on fragmentation patterns observed in their 70 eV elect...

Geochemical Explication of Sulfur Organics Characterized by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry on Sulfur-Rich Heavy Oils in Jinxian Sag, Bohai Bay Basin, Northern China

For the sulfur-rich heavy oils in Jinxian Sag, Bohai Bay Basin, northern China, high abundances of organic sulfur compounds (OSCs), including benzothiophenes and dibenzothiophenes, were detected in aromatic hydrocarbon fractions. Short-chain (such as 20-thienylpregnanes and thienylandrostanes) and regular carbon-numbered S-containing steroids are also present in the saturate hydrocarbon fractions. In addition, a series of short-chain steranes (C21–26), unusual short-chain lanostanes (C24–25), 4-methyl steranes (C22–23), 4,4-dimethyl steranes (C22–24), and androstanes (C19–20) were found, accompanied with high-molecular-weight analogues, C27–30 regular steranes, C27–30 4-methyl steranes, and C28–30 4,4-dimethyl steranes. The distinctive distribution characteristics and dominance of the short-chain steroids in these oils are believed to be derived from the cleavage of the weak C–S bond at C-17 and C-20 positions of extensively sulfurized steroids with subsequent microbial attack or bacterial reworking. To o...

Identification and formation of sulfur-containing steroids in sulfur-rich heavy oils in the Jinxian Sag, Bohai Bay Basin, North China

High abundant sulfur-containing steroids were identified and detected in saturate hydrocarbon fractions of heavy oil with a high sulfur content in the Jinxian Sag, Bohai Bay Basin, North China. These sulfur-containing steroids were structurally merged into the D-ring of steroid nucleus with thiophene ring and/or combined into the C-22 in the side-chain. Based on the previous reports of sulfur-containing steroids with methylthio-teroids and intra-molecular form, four formation mechanisms of sulfur-containing steroids and diagenetic pathway of steroids under S-rich conditions were proposed in this paper according to the double bond positions in the sterene compounds. Hydrogenation and sulfurization both occurred in the diagenetic processes of olefinic bond in the side-chain of steroids: abiogenic chemical hydrogenation of H2S and HS− leads to the formation of regular steranes; a successful sulfurization process leads to the formation of the side-chain sulfur-containing steroids whereas unsuccessful cyclization and/or sulfurization result in the generation of short-chain steranes. This kind of mechanism of hydrogenation/sulfurization of side-chain olefinic bond provides a potential genesis clue for the occurrence of high abundance of short-chain steranes (higher than the common regular steroids, phytane and n-alkanes) in S-rich heavy oils and source rocks in the Jinxian Sag, Bohai Bay Basin, North China.

Identification of C 24 and C 25 lanostanes in Tertiary sulfur rich crude oils from the Jinxian Sag, Bohai Bay Basin, Northern China

Unusual short chain lanostanes (C 24 and C 25) and C 30 lanostane were identified in sulfur rich crude oils from the Jinxian Sag, Bohai Bay Basin, northern China. Besides the regular steranes (C 27–30), a series of 4-methyl steranes (C 22−23, C 27−30), 4,4-dimethyl steranes (C 22−24, C 28−30), short chain steranes (C 23−26), abundant pregnanes (C 21−22) and androstanes (C 19−20), together with sulfur containing steroids (20-thienylpregnanes and thienylandrostanes) were detected in the aliphatic and branched-cyclic hydrocarbon fraction of these crude oils. A literature survey of some long chain sterane analogues (e.g., A-nor-steranes, norcholestanes, C 30 steranes, lanostanes) and pregnanes seems to point to a sponge and/or dinoflagellate source. 4-Methyl, 4,4-dimethyl steroids and lanosterols (4,4,14-trimethyl steroids as the basic skeleton of lanostanes) can be derived from methanotrophic bacteria. Thus, a biological origin from a prokaryotic methylotroph can be used to explain the common source of abundant short chain steranes (C 23–26), 4-methyl (C 22–23) and 4,4-dimethyl steranes (C 22–24), as well as lanostanes (C 24–25 and C 30 analogues) in our oil samples. Generally, the steroids appear to have been extensively sulfurized with sulfur substitution at the C-22 position in the side chain during the early stage of diagenesis, which was readily subject to attack by bacterial degradation (enzymatic cleavage) and/or abiotic oxidation. As a consequence, short chain sterane analogues (e.g., abundant pregnanes and androstanes in this study) and short chain lanostanes (C 24−C 25) might later be released through cleavage of weak C–S bonds at the C-22 carbon in the sulfurized steroids and lanostane sulfides. Finally, the formation of the short chain C 24−C 25 lanostanes and distinctive occurrence of short chain steranes in this study can be well explained by microbial biodegradation of sulfurized lanostanoids and steroids in the reservoir.

Distribution and significance of novel low molecular weight sterenes in an immature evaporitic sediment from the Jinxian Sag, North China

An unusual series of C 22–C 27 monounsaturated sterenes and C 24–C 30 tetracyclic terpanes (17,21- secohopanes) were detected in relatively high concentrations in an immature evaporitic marl sediment of the Jinxian Sag, Bohai Bay Basin, North China. The site of unsaturation in these novel sterenes is assigned tentatively to the D ring on the basis of mass spectral interpretation, which also distinguishes them from reported unsaturated sterenes. Other hydrocarbon biomarker or stable isotope characteristics are indicative of microbial (e.g. methyl hopanes), phytoplankton or higher plant (depleted δ 13C values of isoprenoids and hopanes) inputs and an anoxic carbonate depositional environment (hexacyclic hopanes; tetracyclic terpanes). The hydrocarbon composition showed no obvious biodegradation and the relatively high concentration of unsaturated terpenoids (e.g. gammacerene) and low values of other established maturity parameters ( T s/ T m = 0.23; R o = 0.44%; T max = 417 °C), are consistent with sediments of low maturity. The novel, low molecular weight sterenes and the tetracyclic terpanes may be early diagenetic products of microbial sources in a carbonate environment.

Two-dimensional gas chromatograms as fingerprints of sour gas-associated oils

Examining a well-documented case history in Western Canada sedimentary basin using two-dimensional gas chromatography (GC × GC) has produced a useful tool for fingerprinting thermochemical sulfate reduction (TSR)-affected petroleum fluids. Application of the approach and data interpretation strategies to the northeastern Jinxian Sag of Bohai Bay Basin has provided molecular evidence for TSR as a plausible source for the H 2S in the gases associated with the studied oils.

Geochemical characteristics of the Zhaolanzhuang sour gas accumulation and thermochemical sulfate reduction in the Jixian Sag of Bohai Bay Basin by Zhang et al. (Organic Geochemistry 36, 1717–1730)

In this discussion, we compare the interpretation of data on an Eocene sour gas and high-sulfur oil accumulation from the Jinxian Sag, Bohai Bay Basin, China, provided by [Zhang, S.C., Zhu, G.Y., Liang, Y.B., Dai, J.X., Liang, H.B., Li, M.W., 2005. Geochemical characteristics of the Zhaolanzhuang sour gas accumulation and thermochemical sulfate reduction in the Jixian Sag of Bohai Bay Basin. Organic Geochemistry 36, 1717–1730.] with our earlier published interpretation [Cai, C.F., Worden, R.H., Wolff, G.A., Bottrell, S., Wang, D.L., Li, X., 2005. Origin of sulfur rich oils and H 2S in Tertiary lacustrine sections of the Jinxian Sag, Bohai Bay Basin, China. Applied Geochemistry 20, 1427–1444.]. While [Zhang, S.C., Zhu, G.Y., Liang, Y.B., Dai, J.X., Liang, H.B., Li, M.W., 2005. Geochemical characteristics of the Zhaolanzhuang sour gas accumulation and thermochemical sulfate reduction in the Jixian Sag of Bohai Bay Basin. Organic Geochemistry 36, 1717–1730.] considered that TSR was the cause of the elevated H 2S, in contrast we concluded that BSR was the process responsible for the high concentrations of H 2S. This in turn led to the unusually high degree of sulfurization of the oil. We here contend that there is no evidence in support of TSR in the Eocene sediments of the Jinxian Sag, Bohai Bay Basin, China.

06/00021 Origin of sulfur rich oils and H 2S in Tertiarylacustrine sections of the Jinxian Sag, Bohai Bay Basin, China : Cai, C. et al. Applied Geochemistry, 2005, 20, (7), 1427–1444

06/00021 Origin of sulfur rich oils and H 2S in Tertiarylacustrine sections of the Jinxian Sag, Bohai Bay Basin, China : Cai, C. et al. Applied Geochemistry, 2005, 20, (7), 1427–1444

Geochemical characteristics of the Zhaolanzhuang sour gas accumulation and thermochemical sulfate reduction in the Jixian Sag of Bohai Bay Basin

Natural gas from the Zhaolanzhuang field of the Jizhong Superdepression, Bohai Bay Basin contains the highest proportions of H 2S (40–92%) among the sour gases encountered in China. The gas payzones include the Eocene–Oligocene Kongdian Formation (Ek) and the Es4 member of the Shahejie Formation. The sedimentary sequence consists of halite, anhydrite, carbonate, sandstone and shale interbeds deposited in the evaporative brackish water lacustrine – salt lake setting. In the deepest part of the Jinxian sag, the total thickness of evaporites is more than 1000 m, of which halite accounts for over 40%. Various organic-rich mudstones intercalated with the evaporites are currently within the conventional hydrocarbon window (with a depth of 2500–3500 m), and likely the source for the oil and sour gas in the Zhaolanzhuang field. The temperatures of the gas reservoirs range from 75 to 100 °C, too low for significant thermochemical sulfate reduction. The co-occurrence of abundant elemental sulfur with the sour gas and the δ 34S values of the various sulfur-containing compounds indicate that the H 2S gases were most likely derived from much deeper source kitchens where significant thermochemical sulfate reduction has occurred.

Origin of sulfur rich oils and H 2S in Tertiary lacustrine sections of the Jinxian Sag, Bohai Bay Basin, China

Very high S oils (up to 14.7%) with H 2S contents of up to 92% in the associated gas have been found in the Tertiary in the Jinxian Sag, Bohai Bay Basin, PR China. Several oil samples were analyzed for C and S stable isotopes and biomarkers to try to understand the origin of these unusual oil samples. The high S oils occur in relatively shallow reservoirs in the northern part of the Jinxian Sag in anhydrite-rich reservoirs, and are characteristic of oils derived from S-rich source rocks deposited in an enclosed and productive stratified hypersaline water body. In contrast, low S oils (as low as 0.03%) in the southern part of the Jinxian Sag occur in Tertiary lacustrine reservoirs with minimal anhydrite. These southern oils were probably derived from less S-rich source rocks deposited under a relatively open and freshwater to brackish lake environment that had larger amounts of higher plant inputs. The extremely high S oil samples (>10%) underwent biodegradation of normal alkanes resulting in a degree of concentration of S in the residual petroleum, although isoprenoid alkanes remain showing that biodegradation was not extreme. Interestingly, the high S oils occur in H 2S-rich reservoirs (H 2S up to 92% by volume) where the H 2S was derived from bacterial SO 4 reduction, most likely in the source rock prior to migration. Three oils in the Jinxian Sag have δ 34S values from +0.3‰ to +16.2‰ and the oil with the highest S content shows the lightest δ 34S value. This δ 34S value for that oil is close to the δ 34S value for H 2S (∼0‰). It is possible that H 2S was incorporated into functionalized compounds within the residual petroleum during biodegradation at depth in the reservoir thus accounting for the very high concentrations of S in petroleum.

Unprecedented occurrence of novel C26–C28 21-norcholestanes and related triaromatic series in evaporitic lacustrine sediments

A novel series of C26–C28 21-norcholestanes and their triaromatic analogs have been tentatively identified as major components in Eogene evaporitic sediments in Jinxian Sag of the Bohai Bay basin, northern China. These compounds may be viewed as representatives of a new class of molecules whose biological or diagenetic origin remains to be determined. Their unprecedented high abundance in the evaporitic lacustrine sediments, and related oils, show important source and/or depositional environment controls in addition to thermal maturity.