Yongge Sun

Compound-specific stable carbon isotope analysis as a tool for correlating coal-sourced oils and interbedded shale-sourced oils in coal measures: an example from Turpan basin, north-western China

Molecular geochemical methods have shown that it is often diAcult to diAerentiate between coal- and interbedded shale-sourced oils, even though coals and interbedded shales may exhibit considerable organic influx variation (e.g. land plant vs algal organic matter) due to the changes of depositional setting. However, compound-specific stable carbon isotopic compositions are sensitive to the source input variations. Typically, specific molecules are more depleted in 13 C with increasing content of aqueous biota. This hypothesis is examined and exemplified by comparing the stable carbon isotopic ratios of n-alkanes from source rock extracts and related oils of the Turpan basin, northwestern China. Stable carbon isotopic values of n-alkanes extracted from coals and interbedded shales show that d 13 C values of n-alkanes with less than 20 carbon atoms vary only slightly. However, there are dramatic changes in the isotopic compositions of higher molecular weight n-alkanes. Furthermore, n-alkanes from coal extracts are enriched in 13 C relative to that of interbedded shales with excursions up to 2‐3%. This comparison enables the diAerentiation of coal- and interbedded shale-sourced oils, and provides information useful in assessing the hydrocarbon system of a basin. # 2000 Elsevier Science Ltd. All rights reserved.

Molecular and stable carbon isotopic composition of monomethylalkanes from one oil sand sample: source implications

Abstract Unusually abundant long-chain monomethylalkanes (MMAs) (C 25 –C 36 ) and their short-chain homologues (C 14 –C 22 ) have been tentatively identified in a Late Triassic oil sand sample from the Lunnan oilfield, Tarim Basin, NW China. Molecular distributions and stable carbon isotopic compositions of these compounds have been determined to investigate their bio- and/or geosynthetic sources. The results show that the most abundant isomers of short-chain MMAs were 4-methyl ( m/z 70), 5-methyl ( m/z 84), 6-methyl ( m/z 98) and 7-methyl ( m/z 112) alkanes, whereas in long-chain MMAs, there is a clear predominance of the 9-methyl and 10-methyl isomers. Combined with the compound-specific stable carbon isotopic analysis, it is concluded that short-chain MMAs and long-chain MMAs must have different sources. The similar isotopic compositions of MMAs and n -alkanes in this sample suggest that they may share the same sources. The short-chain MMAs, as previously suggested, are probably related to a cyanobacterial origin, while the long-chain MMAs may be mainly associated with microorganisms or heterotrophic bacteria, rather than cyanobacteria.

Experimental study and evaluation on hydrocarbon generation of macerals

Some typical coal and maceral samples are selected for oil and gas-generating systematic thermal simulation experiments, Rock-Eval, GC and GC-MS analyses. Results cause productivity curves of extracts and gaseous, light, liquid as well as total hydrocarbon. Effects of macerals and maturation on hydrocarbon productivities and compositions are synthetically discussed. Evaluation indexes and plan on coal-generated oil and gas in bituminous coal rank are suggested according to the data from experiments and analyses.

Constraints on the diversity of crude oil types in the Lunnan Oilfield, Tarim Basin, NW China

Hydrocarbons, include heavy oils, normal oils, waxy oils and light oils, condensate oils and gases were all found in Lunnan Oilfield, the occurrences of hydrocarbons with complicated physical properties indicates a complicated distribution of reservoirs. By the drilling data, the distribution pattern had been found that, in plan view, the density of crude oils in the western part is heavier than that of oils in the eastern part in Lunnan region, namely the crude oils were mainly found in the western part while the natural gases were mainly found in the eastern, which shows that an obvious differentiation in the east and west part was presented in the Lunnan region. Furthermore, in vertical view, the light hydrocarbons were mainly found in the Carboniferous reservoir while the heavier hydrocarbons were mainly found in the Ordovician and Triassic reservoirs. By analysis of the semi-logarithm distribution ofn-alkanes, biomarker ratios and stable carbon isotopic compositions of crude oils, we have found that two oil families and discontinuous distribution were presented in hydrocarbons in Lunnan Oilfield. This kind of discontinuous distribution of geochemical parameters was not from continuous fractionation in migration process. The biodegraded heavy oil reservoirs in western part belong to the lower matured oil family, while the light or condensate oils reservoirs belong to higher matured oil family. Thus, a mixed filling of hydrocarbons was occurred in the middle part in Lunnan Oilfield and resulted in the occurrence of waxy oil. The mixed filling model may help to explain the distribution pattern of hydrocarbon reservoirs in Lunnan Oilfield, especially can lead to a relatively consistent explanation for all kinds of geochemical data, such as physical properties, pyrrolic nitrogen compounds, biomarker parameters and stable carbon isotopic compositions.

Application of pyrolysis gas chromatography technique to evaluation of coal-generated hydrocarbon

Based on kerogen-generated hydrocarbon model, a new method to calculate hydrocarbon yields for coals and coaly samples was put forward by means of pyrolysis technique. At the same time, the empirical criteria suggested by Powell were revised. The threshold value was preliminarily defined as HC yields >30 mg HC per gram TOC for effective gas source rocks and >60 mg HC per gram TOC for effective oil source rocks. Additionally, it was also confirmed that the relative compositions of the three ranges of C1-C5 total hydrocarbons, C6-C14n-alkanes plusn-alkenes and C15+n-alkanes plusn-alkenes from pyrolysates can be effectively used to distinguish the coal-generated hydrocarbon types.

Factors controlling the enrichment of natural gas in Kuche depression, Tarim Basin, NW China: Molecular geochemical evidence from sedimentary organic matter

Using molecular geochemical data from infrared spectrometer and pyrolysis gas chromatography-mass spectrometry, this paper investigates the petroleum generation characteristics of Jurassic coal measures from Kuche depression, Tarim Basin, NW China. The results showed that the Jurassic coaly rocks with medium maturity (R0%: 0.8–1.1) were enriched in gas-prone functionalities (-CH3) and low molecular weight pyrolysates (< nC21), indicating that the coaly rocks from Kuche depression were gas/condensate prone at the stages of middle to high maturation, and it was further supported by the oil/source correlation from well Yinan 2 in this region.