The evolution characteristics and fractionation mechanism of carbon isotopes in the process of "multi-stage hydrocarbon generation"

The Jiyang Sag and the Liaohe Basin are the two important areas where immature oil resources are distributed in China. From these two areas immature-low mature to mature oil samples were collected for carbon isotopic analysis. The extracts of source rocks are dominant in the Jiyang Sag while crude oils are dominant in the Liaohe Basin. The maturity index, R., for source rocks varies from 0.25%(immature) to 0.65% (mature). Studies have shown that within this range of R. values the extracts of source rocks and crude oils, as well as their fraction components, have experienced observable carbon isotope fractionation. The carbon isotopic values tend to increase with burial depth, the oils become from immature-low mature to mature, and the rules of evolution of oils show a three-stage evolution pattern, i. e. ,light→heavy→light→heavy oils. Such variation trend seems to be related to the occurrence of two hydrocarbon-generating processes and the main hydrocarbon-forming materials being correspondingly non-hydrocarbons and possessing MAB characteristics, lower thermodynamic effects and other factors. In the process towards the mature stage, with increasing thermodynamic effects, the thermal degradation of kerogens into oil has become the leading factor, and correspondingly the bond-breaking ratio of 12 C-13C also increases,making the relatively 12C-rich materials at the low mature stage evolve again towards 13C enrichment.     

Geochemistry of source rocks in the Es<Subscript>3</Subscript> and Es<Subscript>4</Subscript> members in the Shahejie Formation of the Western Depression,Liaohe Oilfield,China

The Western Depression of the Liaohe Basin is the major exploration area of the Liaohe Oilfield, and its main source rocks consist of the third and fourth members of the Shahejie Formation (Es3 and Es4). These source rocks are widely distributed in the depression, with semi-deep lake and fan delta as the main sedimentary facies, brown oil shale and black gray-dark gray mudstone as the main rocks, and a total thickness of 270-1450 m. The kerogens are mainly of the types I and IIA, and partly of the type IIB and least of the type III. The Ro values range from 0.4%-0.8%, indicating an evolution stage from immature to mature. The maturity of Es4 source rocks is rela-tively high, reaching the early mature stage, but their distribution and thickness are lower than those of Es3. Besides, according to biomarker analysis, it is thought that the source rocks of Es3 and Es4 are characterized by mixed input, and most of the source rocks were formed in the brackish water-saline and strongly oxygen-free environment. Fur-thermore, the Qingshui, Niuxintuo and Chenjia sags are believed to possess greater potential for hydrocarbon gen-eration and expulsion, for they are source rocks with a larger thickness, have higher organic carbon contents, belong to better organic matter types and possess higher maturities.     

Geochemical characteristics of crude oils from Well Zheng-1 in the Junggar Basin, Xinjiang, China

Well Zheng-1 is located in the combined area of the central uplift and the north Tianshan piedmont depression in the Junggar Basin. Two oil-bearing beds are recognized at 4788–4797 m of the Lower Cretaceous Tugulu Formation (K1tg) and 4808.5–4812.5 m of the Lower Jurassic Sangonghe Formation (J1s). The geochemical characteristics of family composition, carbon isotopic composition, saturated hydrocarbons, sterane and terpane biomarkers and carotane of two crude oils are described in this paper. The results show that the geochemical characteristics of the two crude oils are basically similar to each other, indicating they were all derived mainly from the high mature, brine, algae-rich lake facies sediments. Oil-source correlation revealed that crude oils of the two beds were derived mainly from the source rocks of Permian and mixed by the oil derived from the source rocks of Jurassic and Triassic. This is consistent with the geological background with several sets of source rocks in the area studied.     

辽河盆地西部凹陷稠油成因类型及其油源分析

根据原油的成熟度、生物降解程度及生物降解后油气的注入情况等多元因素,将辽河盆地西部凹陷的稠油划分为原生稠油、降解型稠油和降解—混合型稠油,在此基础上对该区稠油的油源进行了分析;研究表明,富伽马蜡烷的未熟原生稠油和未熟—降解型及低熟—降解型稠油可能主要来源于沙四段源岩,富4-甲基甾烷型的原油可能与沙三段具有亲缘关系。     

Characteristics and Evaluation of the High-Quality Source Rocks of Cretaceous Continental Shale Oil in Tonghua Basin,China

The presence of shale oil in the Cretaceous Hengtongshan Formation in the Tonghua Basin, drilled by the well TD-01, has been discussed in this geological investigation for the first time. To evaluate the high-quality source rocks of Cretaceous continental shale oil, the distribution characteristics and the evolution of the ancient environment, samples of shale were systematically analyzed in terms of sedimentary facies, organic geochemistry, and organic carbon isotopic composition. The results demonstrate that a TOC value of 1.5% represents the lower-limit TOC value of the high-quality source rocks. Source rocks have an aggregate thickness of 211 m and contain abundant organic matter, with TOC values of 2.69% on average and a maximum value over 5.44%. The original hydrocarbon-generative potential value(S_1+S_2) is between 0.18 mg/g and 6.13 mg/g, and the Ro is between 0.97% and 1.40%. The thermal maturation of the source rocks is relatively mature to highly mature. The δ¹³C value range is between -34.75‰ and -26.53‰. The ratio of saturated hydrocarbons to aromatic hydrocarbons is 1.55 to 5.24, with an average of 2.85, which is greater than 1.6. The organic types are mainly type Ⅱ_1, followed by type Ⅰ. The organic carbon source was C_3 plants and hydrophytes. The paleoclimate of the Hengtongshan Formation can be characterized as hot and dry to humid, and these conditions were conducive to the development of high-quality source rocks. A favorable paleoenvironment and abundant organic carbon sources provide a solid hydrocarbon generation base for the formation and accumulation of oil and gas in the shale of the Tonghua Basin.     

Origin of the Neogene shallow gas accumulations in the Jiyang Superdepression, Bohai Bay Basin

Natural gas resources occur extensively along the east coast of China, with a number of large and medium-sized gas fields being discovered in recent years. Gas reservoirs include Neogene, Paleogene and the underlying Mesozoic and Paleozoic basement. Of the total proven natural gas reserves in the Jiyang Superdepression, Bohai Bay Basin, almost 89.7% is present in the shallow Neogene gas pools, in traps formed on top of the paleotopographic highs and along the margin of the secondary depressions. These gases are closely associated with heavy oils, occurring as gas caps or associated gases within the heavy oil pools, or in separate gas pools above, or updip from, the heavy oil pools. The gases contain over 95% methane and small quantities of alkanes, nitrogen and carbon dioxide. The stable carbon isotopes of methane in these gases are up to 10‰ more positive than those of the thermogenic gases in the deep Paleogene reservoirs, with propane more enriched in ¹³C than butane. This study demonstrated that the majority of the petroleum source rocks in the Jiyang Superdepression tend to be oil-prone, and are currently within or shallower than the conventional oil window (0.45–1.0% Ro). The chemical and carbon isotopic compositions of the gases, together with the moderate to severe biodegradation of the associated heavy oils in the shallow Neogene strata, clearly suggest that the formation of the shallow natural gases in the Jiyang Superdepression result from the anaerobic degradation of accumulated oils in reservoir.     

Origin of sulfur rich oils and H2S in Tertiary lacustrine sections of the Jinxian Sag,Bohai Bay Basin,China

Very high S oils (up to 14.7%) with H₂S 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 H₂S-rich reservoirs (H₂S up to 92% by volume) where the H₂S was derived from bacterial SO₄ reduction, most likely in the source rock prior to migration. Three oils in the Jinxian Sag have δ³⁴S values from +0.3‰ to +16.2‰ and the oil with the highest S content shows the lightest δ³⁴S value. This δ³⁴S value for that oil is close to the δ³⁴S value for H₂S (∼0‰). It is possible that H₂S 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.     

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 effective 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 340°C. 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 differentiation of organic facies variation and post-generation alterations.     

塔里木盆地喀什凹陷克拉托天然气来源分析及聚气特征

塔里木西南喀什凹陷的克拉托天然气主要表现为原油的溶解气或者湿气,甲烷含量为74.59%~85.58%,克4井和克30井天然气则为较干的湿气。克拉托天然气的δ¹³C₁值为-41.2‰~-40.6‰,δ¹³C₂值为-30.0‰~-27.4‰。气源对比表明克拉托天然气主要源自具有混源母质特征的中侏罗统湖相烃源岩,不同于源自石炭系烃源岩的阿克莫木天然气。喀什凹陷的中-下侏罗统烃源岩主要是由于新近系的巨厚沉积才从未成熟—低成熟阶段进入成熟—高成熟阶段,生成的油气在克拉托背斜圈闭中聚集,虽也属晚期成藏,却具有连续聚气的特征。上新世末期,喀什凹陷的周缘开始抬升,早期油气藏受到破坏,形成了现今的地表油气苗或油砂。     

Stable carbon isotopes of alkane gases from the Xujiahe coal measures and implication for gas-source correlation in the Sichuan Basin,SW China

The Upper Triassic Xujiahe Formation in the Sichuan Basin, SW China consists of a series of coal measures. The first, third and fifth members of this formation are dominated by gas prone dark mudstones and coals. The mudstones contain Type II and III kerogens with average organic carbon contents around 1.96%. These source rocks are mature in the central Sichuan and highly mature in the western Sichuan Basin, characterized by gas generation with subordinate amounts of light oil or condensate oils. The source rocks are intercalated with the sandstone dominated second, fourth and sixth members of the Xujiahe Formation, thus leading to three separate self contained petroleum systems in the region. The proven gas reserves in the Xujiahe Formation are only less than that of the Triassic Feixianguan Formation and the Xujiahe Formation has the second largest gas field (Guang’an gas field) in the basin. Gases derived from the Xujiahe Formation coals generally show a normal stable carbon isotopic trend for C₁–C₄ n-alkanes, with the highest δ¹³C₂ values among the nine gas pay zones in the basin (?20.7‰ to ?28.3‰), and δ¹³C₁ values as low as ?43.0‰ in the central Sichuan. Gas accumulations with an oil leg have also been found in the eastern and southern Sichuan where the thickness of the Xujiahe Formation is significantly reduced. Gases in these accumulations tend to show low δ¹³C₂ values (?30.0‰ to ?36.3‰), characteristic of oil prone source rocks.