Ansong Geng

Carbon isotopic composition of individual n-alkanes in asphaltene pyrolysates of biodegraded crude oils from the Liaohe Basin, China

Biodegraded oils are widely distributed in the Liaohe basin, China. In order to develop eAective oil-source correlation tools specifically for the biodegraded oils, carbon isotopic compositions of individual n-alkanes from crude oils and their asphaltene pyrolysates have been determined using the gas chromatography‐isotope ratio mass spectrometry technique. No significant fractionation in the stable carbon isotopic ratios of n-alkanes in the pyrolysates of oil asphaltenes was found for anhydrous pyrolysis carried out at temperatures below 340C. This suggests that the stable carbon isotopic distribution of n-alkanes (particularly in the C16‐C29 range) in the asphaltene pyrolysates can be used as a correlation tool for severely biodegraded oils from the Liaohe Basin. Comparison of the n-alkane isotopic compositions of the oils with those of asphaltene pyrolysates shows that this is a viable method for the diAerentiation of organic facies variation and post-generation alterations. # 2000 Elsevier Science Ltd. All rights reserved.

Characterization of nC(7)-soluble fractions of the products from mild oxidation of asphaltenes

Abstract A significant quantity of hydrocarbons (including alkanes) is occluded in the skeleton of the asphaltene molecule. The hydrocarbons are probably remnants of the “original oil” which had been retained within the asphaltene matrix and protected from the secondary alteration processes that occurred subsequently in the oil reservoirs. In this work we report that oxidation of asphaltenes by stirring with 30%H 2 O 2 –HAc or NaIO 4 –NaH 2 PO 4 can release n C 7 -soluble oxidized products, including the occluded hydrocarbons. Characterization of the n C 7 -soluble fractions of oxidized products can be applied to highlight some geochemical problems, such as in studies of oil–oil correlation, oil–source correlation and secondary alterations of oil reservoirs. It will be especially useful to recover the original geochemical information of some oil reservoirs heavily degraded by post-depositional processes.

Kinetic studies of asphaltene pyrolyses and their geochemical applications

Abstract Pyrolysis kinetics are determined for a series of asphaltene samples associated with different types of kerogens. The activation energy distributions obtained for asphaltenes related to type I kerogens cover a wide range, with significant hydrocarbon generation with activation energies above 350 kJ/mol. The ranges of activation energy distributions are relatively narrow for asphaltenes associated with type II and III kerogens, with little hydrocarbons generated with activation energies above 350 kJ/mol. If the type of associated kerogen can be reasonably constrained, the pyrolysis kinetics of the asphaltenes is potentially a very useful tool for assessing the maturity levels of reservoired oils. The results of pyrolysis kinetics indicate that there is still significant potential for asphaltenes to regenerate hydrocarbons.

Triterpane and sterane biomarkers in the YA13-1 condensates from Qiongdongnan Basin, South China Sea

Triterpanes and steranes in condensates from the YA13-1 gas field, Qiongdongnan Basin, were monitored. The YA13-1 condensates have unusual biomarker distributions dominated by terpanes and steranes derived from higher plants. Anomalously abundant 1 got-oleanane and remarkably abundant bicadinanes are present in the YA13-1 condensates, whereas the 17alpha-hopane contents are extremely low. Taraxastane and significantly abundant 17alpha-diahopanes occur in the condensates. In addition, a number of unknown C-29 and C-30 pentacyclic triterpanes including previously unreported compounds were detected in the condensates, some of which are significantly abundant. The unknown compounds may be terrestrial biomarkers. C-29 homologues are relatively predominant among the regular and rearranged steranes. The diasterane concentrations are markedly higher than those of regular steranes. The maturity of the YA13-1 condensates is relatively high, at the peak to late oil generation stage (corresponding to 0.85-1.10% Ro), based on sterane and terpane and including bicadinane maturity parameters (i.e. T/(T-1 + R) and 2T/R bicadinane ratios). The above maturity assessment result is different from that based on diamondoid maturity parameters (%Ro = 1.60-1.70) [Org. Geochem. 25 (1996) 179], which can be explained by a contribution of hydrocarbons from two sources at different depths. The YA13-1 condensates were probably generated from the Yacheng and Lingshui coal-bearing source rocks buried both in the Qiongdongnan Basin (3400-5000 m) and in the Yinggehai Basin (>5000 m). The possible contribution of lower maturity hydrocarbons from the Yacheng and Lingshui Formations (3400-4100 m) in the Qiongdongnan Basin to the YA13-1 gases and condensates should not be neglected

Geochemical characterization of free versus asphaltene-sorbed hydrocarbons in crude oils: implications for migration-related compositional fractionations

Based on the existing petroleum migration models that focus on the role of organic sorption, a laboratory study has been performed to simulate the migration effects on the various fractions of oil. In this study, one free oil fraction (defined as Bitumen 1, existing outside of or loosely attached to asphaltene), and three to four absorbed oil fractions (defined as Bitumens 2, 3, 4 and 5, more strongly attached to asphaltene macromolecules) that were obtained from three oil samples collected from the Bohai Bay Basin, East China, were analyzed in detail. Based on the laboratory partition of oil components between the solvent phase (oil) and the asphaltene phase, petroleum migrationrelated effect on saturate fraction was re-evaluated. Our observation indicates that the compositional variation from Bitumen 1 to Bitumen 4 or 5 includes the following: (1) The resin content increases while the saturate content and saturates/aromatics ratio decrease; (2) The relative abundance of short chain n-alkanes decreases while that of the long chain n-alkanes increases; (3) Pr/nC17, Ph/nC18 and Pr/Ph ratios stay virtually unchanged while the relative abundances of terpanes to n-alkanes and steranes to n-alkanes decrease; (4) The ratios of C21/(C21 þ C29) steranes and C23/(C23 þ C30) terpanes in the free oil fraction are similar to or even lower than those in the whole absorbed fraction, suggesting relatively stronger affinities of these smaller terpane and sterane molecules to asphaltenes in contrast to the relatively larger ones and (5) The isomerization parameters of C27 and C29 steranes are slightly higher in the free fraction than in the whole absorbed oil fraction. Large variations were observed in the distributions of steranes among the four bitumen fractions of the X52 oil, and were considered to be related to the heterogeneity of the source rocks, in addition to the partition of these compounds between the asphaltene phase and the solvent phase. q 2002 Elsevier Science Ltd. All rights reserved.

Maturity and petrography of bitumens in the Carboniferous of Ireland

Abstract A set of bitumen samples from the Carboniferous of Ireland shows a general trend of increasing maturity southwards across the island. This trend is consistent with a similar southwards increase in maturity exhibited by the host rocks (vitrinite reflectance), reflecting proximity to the Hercynian Orogen to the south of Ireland. Increasing maturity is reflected by decreasing H/C ratio, and increasing reflectance, bireflectance and anisotropy, and is accompanied by heavier carbon isotope composition for the most mature samples. Bitumen reflectance is the most sensitive parameter and ranges up to 3.6%. Most of the bitumens occur in hydrothermal mineral veins and were generated locally by the passage of hot fluids through Carboniferous shales and limestones. Some bitumens contain inclusions of metalliferous and other inorganic minerals. High sedimentation rates during the Carboniferous caused hydrocarbon generation before the Late Carboniferous Hercynian Orogeny, so that the bitumens experienced thermal overprinting by the Orogeny and show the same maturity trend as the country rocks. Anisotropy is exhibited by all bitumen samples with reflectances of 2.0% or over; mesophase textures occur in bitumens with reflectances over 3.0%, including one case where fluid inclusion homogenization temperatures in coeval minerals (calcite and fluorite) are less than 200°C.

Thermal cracking of n-octodecane and its geochemical significance

The mechanism of carbon isotopic fractionation for gaseous hydrocarbons is revealed by investigating the residual liquid hydrocarbons in laboratory pyrolysates of n-octodecane. The results indicate that cracking and polymerization in the relatively low temperatures and disproportionation reactions leading to light hydrocarbons and polyaromatic hydrocarbons at high temperatures are probably causes for the carbon isotope reversal of gaseous hydrocarbons that is commonly observed in pyrolysis experiments. This study provides significant insight for quantitative modeling of natural gas δ13C values and aid in the identification and assessment of natural gases derived from oil cracking.

The origin of crude oils from the Shuguang-Huanxiling Buried Hills in the Liaohe Basin, China: evidence from chemical and isotopic compositions

Abstract Since the Shu103 well was successfully drilled in 1995, the Buried Hill reservoir is receiving a new exploration emphasis in the Liaohe Basin, China. The Buried Hill oils can be divided into 3 main types in the Shuguang-Huangxiling area based on their chemical and isotopic compositions. The first type is collected mostly from the Shuguang area. The similarity to the Es4 oils and the Es4 source rock extracts indicates that they were mainly expelled from the fourth member of the Shahejie formation (Es4) in the Chenjia Sag and/or Panshan Sag, deposited in a stratified paleolake system. The second is charactized by a relative high Pr/Ph ratio, low gammacerane content and depletion of 12 C in individual n -alkanes. These characteristics represent a typical origin from dominantly freshwater paralic lacustrine sediments. Various biomarker indices of the other Buried Hill oils fall between the former two types. The authors infer that these oils may be derived from the adjacent Qingshui Sag and/or Panshan Sag, and were likely generated from transitional sedimentary facies between well-circulated freshwater paralic lacustrine and relatively closed saline lacustrine.

The generation and expulsion of gases in Ya131 gas field, South China Sea: implication of laboratory pyrolysis results

Abstract Laboratory pyrolysis methods were used to determine the source and thermal maturity of the gas accumulation in the Ya131 gas field, South China Sea. The mudstone source rock samples used in the pyrolysis experiments were selected from the Yacheng and Meishan Formations in the Ying-Qiong Basin. The total amount of gases generated and expelled from the Yacheng Formation mudstone is larger than that from the Meishan Formation mudstone, suggesting that the gas generation potential of the Yacheng Formation is higher than that of the Meishan Formation. The similarity between the gases from the Ya131 field and the gases produced at the higher pyrolysis temperatures used in the maturation study, in terms of wetness, the amounts of C 1 , C 2 , C 3 , and the percentages of C 1 , C 2 –C 4 and C 5 –C 7 , implies that the gases of the Ya131 field are high maturity gases. However, there are distinct differences between the normalised molecular compositions of the C 6 and C 7 species of the gases from the Ya131 field and those of the pyrolysis gases produced in the laboratory. Comparison of the stable carbon isotope composition of the n -alkanes, pristane (Pr) and phytane (Ph) in the saturated hydrocarbon fraction of the condensate from the Ya131 field with that of rock extracts from the Yacheng, Meishan, Ying-Huang Formations indicates no definite correlation between the condensate and the extracts. These observations suggest that there may be additional source rocks for the Ya131 field, existing within the deeper part of the basin.

Asphaltenes in oil reservior recovery

Asphaltene is one of the important compositions in oil reservoirs, while it is also a major factor that causes difficulties in oil recovery and oil post-processings. Up to date, study on asphaltenes in oil recovery is still a bottleneck problem. In this paper, the advances of studies on asphaltenes are reviewed, and some directions for further studying are suggested. What is reviewed in the paper includes the precipitation studies of asphaltenes, the degradation studies of asphaltenes and the applications of asphaltene’s studying in oil recovery; furthermore, it is regarded as a promising direction to study the possible applications of asphaltene’s selectively decomposing by chemical reagents in oil recovery.