Groundbreaking gas source rock correlation research based on the application of a new experimental approach for adsorbed gas

Many years of experimentation have led to the development and improvement of equipment and methods used to make gas source rock correlations.By crushing samples using a ball mill and directly collecting adsorbed gases in the absence of aqueous media under high vacuum conditions,most possible interference factors,such as atmospheric pollution,crushing-induced pyrolysis,and gas collection by aqueous media are eliminated.This greatly enhances the volume percentage of hydrocarbon gas in the acquired adsorbed gases,with maxima up to more than 80%.The actual measurement of carbon isotopic series can be carried out to such an extent as to be δ13C1-δ13C5.A preliminary study using newly established equipment and methods has indicated the following.(1) The carbon isotopic composition of ethane in adsorbed gases on hydrocarbon source rocks can be used to distinguish the types of source rocks.This is consistent with results obtained by using organic geochemical parameters for source rocks,and illustrates that it is highly feasible to use the carbon isotopic composition of ethane in natural gases as a parameter for distinguishing the types of source rocks.(2) The thermal evolution degree(Ro) of hydrocarbon-source rocks calculated in terms of the carbon isotopic composition of methane in adsorbed gases on hydrocarbon source rocks agrees well with the vitrinite reflectance actually measured in the source rocks.This confirms the reliability of the relationship between the carbon isotopic composition of methane in natural gases and the thermal evolution degree determined using statistics.(3) Finally,a direct gas source correlation method for natural gases has been established,and the expression of log Ro vs.δ13C1 established in terms of actually measured δ13C1 values of methane in absorbed gases.Ro values of hydrocarbon source rocks have been established as well,thus creating favorable conditions for precise oil-and-gas source correlations in exploration areas.     

Natural gas geochemistry of low evolutionary stage of medium- and small-sized basins in Yunnan Province, southwestern China

The biogenic gas and premature-low mature associated gases in some medium and small-sized basins of Yannan Province, such as Luliang, Yanglin, Baoshan and Jinggu basins, have beed researched. The results show that the biogenic-gas consists mainly of methane which is more than 99% in gasous hydrocarbons and of lighter carbon isotopic composition with δ13C1 values from -60.0‰ to - 75.4‰. The methane carbon isotopic compositions in the Baoshan Basin is relatively heavy (δ13C1 - 60‰- - 65‰), but those in Luliang and Yanglin basins are lighter (δ13C1, less than -70‰), which implies that the gas field of the Baoshan Basin formed earlier than the others. In the Jinggu Basin, where crude oil is premature-lower mature, the natural gas of associated oil relatively wet and the relative content of methane about 58%-95% in gasous hydrocarbons. Constituently the gas composition is much similar to associated one, but the methane carbon isotopic compositions from - 53.8‰ to - 57.8‰ are obviously richer in 12C than those of general oil fields and similarly characterize the biothermo-catalytic transitional zone gas. Their ethane carbon isotopic compositions from - 34.6‰ to -29.0‰ show that they may be derived from type Ⅰ or Ⅱ source rock. But for the associated gas from lower evolutionary stage, the heavier ethane carbon isotopic composition as well as the reversed order among the carbon isotopic composition of ethane, propylane and butane also implies that some gases from the type Ⅲ organic matter are mixed. The δ13CCo2 of the samples essentially less than - 10‰ may be generated from organic matter.     

Origin and migration of natural gases in central area of Western Sichuan Depression

The chemical and isotopic compositions of nine natural gas samples from Xiaoquan, Xinchang and Hexinchang fields in the central part of Western Sichuan Depression were measured. All of the samples consist mainly of CH, and the contents of hydrocarbons are > 99 % in volume with the gas wetness > 0.5 % . The methane 8" C, and ethane S^C; values of these gases are > - 37%c and > - 27%e respectively. The results indicate that hydrocarbons were formed via thermogenesis. The hydrocarbons in Xiaoquan field were generated from source rocks at early mature and mature stages, while the hydrocarbons in Xinchang and Hexinchang fields came from source rocks at mature and late mature stages respectively. In addition, the isotope ratios and the chemical compositions of the gases from Xinchang field varied with depth, indicating that the hydrocarbons were derived from the deeply buried and the mature source rocks of Upper Triassic age and had experienced a long distance migration.     

Evolution of the CBM reservoir-forming dynamic system with mixed secondary biogenic and thermogenic gases in the Huainan Coalfield,China

The secondary biogenic gas is an important original type of the coalbed methane (CBM) in China. Based on the analyses of sedimentary and burial history of the Permian coal-bearing strata, combined with thermal history and gas generation process of coals, the CBM reservoir-forming dynamic system with mixed secondary biogenic and thermogenic gases in the Huainan Coalfield is subdivided into four evolutionary stages as follows: (i) shallowly-buried peat and early biogenic gas stage; (ii) deeply buried coal seams and thermogenic gas stage; (iii) exhumation of coal-bearing strata and adsorbed gas lost stage; and (iv) re-buried coal-bearing strata and secondary biogenic gas supplement stage. The Huainan CBM reservoir-forming model has the features of the basin-centered gas accumulation. The evolution of the reservoir-forming dynamic system proves that the thermogenic gas is not the main gas source for the Huainan CBM reservoir. Only the secondary biogenic gases as an additional source replenish into the coal bed after basin-uplift, erosional unroofing and subsequent scattering of thermogenic gases. Then this kind of mixed CBM reservoirs can be formed under suitable conditions.     

Kinetics of early methane generation from Green River shale

During oil/gas exploration in an area, there is a question as to whether methane in the strata was generatedin situ mainly by thermal degradation of local organic matter. The key to the question is whether the kerogen could produce methane of more than 80 mL/g TOC, when its R_o increased from initial (0.36%) to current value of 0.70% during thermal evolution. The authors have made efforts to answer this question by the kinetics method. First, a series of pyrolysis experiments were carried out to obtain data of methane yield and R_o evolution, from which kinetic parameters of methane and R_o were derived; then these parameters were extrapolated to the geological condition and finally, a methane yield, 96 mL/g TOC, was predicted. The result suggests that from the view of hydrocarbon generation, methane in the seam should be from thermal degradation of local kerogen.     

成煤作用与煤岩生气特点分析

煤层气研究的基础之一是对煤的质量的研究,即生气物质质料的研究,包括煤的岩石学特征、煤质特征、煤的地球化学特征。就煤田地质研究而言,煤岩学和煤质学研究是传统研究范畴,在国内外是非常成熟的,但与煤的生、产气结合起来进行煤层气聚集规律研究则形成了新的研究领域,其中煤作用研究出现了新的理论和成因学说。在不同的成煤环境下形成的煤,其岩石学特点不同,煤的微观组织也不同。不同的煤岩组分其生、产气的能力是不同的,在盆地演化的不同时期具有不同的生、产气高峰。结合煤的成煤作用研究与沁水盆地富煤单元的形成与分布分析,提出成煤作用是奠定煤层气原始质料的前提、成煤环境演化是煤层气形成的控制因素、富没煤带和富煤单元与煤层气聚集单元具有成因联系等结论。     

Pyrolysis of oil at high temperatures: Gas potentials, chemical and carbon isotopic signatures

Although the gas cracked from oil has been believed to be one of the important sources in highly ma-tured marine basins, there are still some debates on its resource potentials and chemical and isotopic compositions. In this study a Cambrian-sourced marine oil sample from the Silurian reservoir of well TZ62 in the central Tarim basin was pyrolyzed using sealed gold tubes with two different pyrolysis schemes: continuous pyrolysis in a closed system and stepwise semi-open pyrolysis. The results show that the maximum weight yield of C_1–5 gases occurs at EasyRo=2.3% and the residual gas poten-tial after this maturity is only 43.4 mL/g, about 12% of the yield of 361 mL/g at EasyRo=2.3%. Combined with the results of kinetic modeling, the main stage of gas generation from oil cracking is believed within the EasyRo=1.6%―2.3%. The increase in the volume yield of C_1–5 gases at EasyRo>2.3% in a closed system is mainly related to the re-cracking of previously formed C_2–5 wet gases, not the direct cracking of oil. The stepwise pyrolysis experiments show that the gas from the cracking of residual oil at EasyRo>2.3% is characterized by very high dryness index (higher than 92%) and heavy methane carbon isotopes ranging from -28.7‰ to -26.7‰, which is quite different from the gases from the con-tinuous pyrolysis in a closed system. The kinetic modeling of methane carbon isotope fractionation shows that the carbon isotopes of methane within the main stage of gas generation (EasyRo<2.3%) are far lighter than the carbon isotopes of the precursor oils under a geological heating rate of 2 ℃/Ma. The above observations and results provide some new clues to the accurate recognition and objective re-source evaluation of oil cracking gas in highly mature marine basins.     

Thermal simulation of the formation and evolution of coalbed gas

Thermal simulation experiment of gas generation from the peat and the coals were performed using the high temperature and pressure apparatus, at temperature ranging from 336.8-600℃, a pressure of 50MPa and two heating rates of 20℃/h and 2℃/h, and the evolution and formation of coalbed gas components were studied. Results show that for the coals, the gaseous products are mainly composed of hydrocarbon gases. However, for the peat the content of hydrocarbon gases in gaseous products is lower than that of non-hydrocarbon components. In the generated hydrocarbon gases methane is predominant and heavy hydrocarbon gases (C2-5) are present in small amount. Meanwhile, carbon dioxide (CO2) predominates the generated non-hydrocarbon gases, and hydrogen (H2) and sul-furated hydrogen (H2S) are existent in trace amount. It is also observed that temperature is the main factor controlling the evolution of coalbed gas generation. With increasing vitrinite reflectance, methane rapidly increases, CO2 sightly increases, and C2-5 hydrocarbons first increase and then decrease. The peat and Shanxi formation coal have a higher generative potential of coalbed gases than coals and Taiyuan formation coal, respectively, reflecting the effect of the property of organic matter on the characteristics of coalbed gas component generation. In this study, it is found that low heating rate is favorable for the generation of methane, H2 and CO2, and the decomposition of C2-5 hydrocarbons. This shows that heating time plays an important controlling role in the generation and evolution of coalbed gases. The results obtained from the simulation experiment in the study of coalbed gases in natural system are also discussed.     

Discussion of gas enrichment mechanism and natural gas origin in marine sedimentary basin,China

There are abundant natural gas resources in Chinese marine sedimentary basin. The exploration hot shots of natural gas are the Palaeozoic marine strata here in recent years, and several large scale gas fields have been discovered. Chinese Palaeozoic high-post matured and coal measure hydrocarbon source rocks are mainly prone to gas generation in the present. This research considered that gas source rocks and TSR are the key cause of gas enrichment of marine strata. High-quality argillaceous and coal measure hydrocarbon rocks are distributed widely in the Palaeozoic marine strata, which have been in highly matured phase in the present. The argillaceous source rock generally contains various sulfates that could accelerate crude oil cracking to gas for TSR occurrence, and coal measure source rock mainly generates gas, so Chinese marine basin gives priority to accumulating gas. Marine strata have not founded oil reservoirs in the Sichuan Basin and Ordos Basin, and they consist mainly of dry gas. Marine natural gases are the mixed gases of oil cracking gas and coal-formed gas in a general way, oil cracking gases contain usually some H2S and CO2. Hydrocarbon carbon isotopes are very complicated, and methane and ethane isotopic values bear apparent reversal caused by thermal evolution and mixing among different genetic types of natural gas. Coal-formed gases are the main component of Chinese marine natural gas. The Upper Permian of the Sichuan Basin and the Carboniferous-Permian of the Ordos Basin coal measure hydrocarbon source rock present large hydrocarbon generation potential, which are the prospecting highlight of marine natural gas hereafter. Oil cracking gas exploration will be paid much attention to in the Tarim Basin because of the lack of coal measure hydrocarbon source rock.     

Discussion of gas enrichment mechanism and natural gas origin in marine sedimentary basin, China

There are abundant natural gas resources in Chinese marine sedimentary basin. The exploration hot shots of natural gas are the Palaeozoic marine strata here in recent years, and several large scale gas fields have been discovered. Chinese Palaeozoic high-post matured and coal measure hydrocarbon source rocks are mainly prone to gas generation in the present. This research considered that gas source rocks and TSR are the key cause of gas enrichment of marine strata. High-quality argillaceous and coal measure hydrocarbon rocks are distributed widely in the Palaeozoic marine strata, which have been in highly matured phase in the present. The argillaceous source rock generally contains various sulfates that could accelerate crude oil cracking to gas for TSR occurrence, and coal measure source rock mainly generates gas, so Chinese marine basin gives priority to accumulating gas. Marine strata have not founded oil reservoirs in the Sichuan Basin and Ordos Basin, and they consist mainly of dry gas. Marine natural gases are the mixed gases of oil cracking gas and coal-formed gas in a general way, oil cracking gases contain usually some H2S and CO2. Hydrocarbon carbon isotopes are very complicated, and methane and ethane isotopic values bear apparent reversal caused by thermal evolution and mixing among different genetic types of natural gas. Coal-formed gases are the main component of Chinese marine natural gas. The Upper Permian of the Sichuan Basin and the Carboniferous-Permian of the Ordos Basin coal measure hydrocarbon source rock present large hydrocarbon generation po- tential, which are the prospecting highlight of marine natural gas hereafter. Oil cracking gas exploration will be paid much attention to in the Tarim Basin because of the lack of coal measure hydrocarbon source rock.     

Kinetic study of hydrocarbon generation of oil asphaltene from Lunnan area, Tabei uplift

The molecular compositions and molecular carbon isotopic compositions of gas hydrocarbons produced in the hydrocarbon generation of oil asphaltene from Lunnnan area were investigated by pyrolysis of asphaltene sealed in gold tube in a limited system. The experimental results indicated that oil asphaltene from Lunnan area had relatively high generation potential of methane. However, the molecular compositions and molecular carbon isotopic compositions of gas hydrocarbons in the hydrocarbon generation of oil asphaltene exhibited different characteristics from that of gas hydrocarbons by primary cracking of kerogen and secondary cracking of oil. Based on kinetic simulation with paleo-geothermal data of oil reservoir, the methane produced by cracking of oil asphatene was characterized by relatively light carbon isotopic compositions. This result could not explain relatively heavy carbon isotopic compositions of natural gas from Lunnan area. Pyrolysis of kerogen from source rocks under very high temperature probably made remarkable contributions to natural gas from Lunnan area.     

Geochemical characteristics of light hydrocarbons in natural gases from the Turpan-Hami Basin and identification of low-mature gas

Theoretical and practical knowledge regarding low-mature gasses is of significant importance to identifying potential natural gas resources. Light hydrocarbon parameters and C and H isotopes are useful tools to identify low-mature gas. Twenty gas samples were collected from the Turpan-Hami Basin for light hydrocarbon analyses. The results showed that the light hydrocarbon components of natural gases contain high methylcycloxane, high isoparaffin and low benzene. This implies that the gas-generating parent materials are of typical humus type and the paleoenvironment is a fresh water sedimentary environment. These features are consistent with the geological setting of the basin. Comparative studies of isoheptane, heptane, and the carbon isotopic compositions of methane in natural gases, and other maturity indices indicated that natural gases in the Turpan-Hami Basin are dominated by low-mature gas formed during the low evolution stage of Jurassic coal seams. The parent materials are of type III, and the maturation degree was in the low evolution stage. These are the fundamental characteristics of low-mature gas. Results of light hydrocarbon research provided further evidence to suggest that the Turpan-Hami Basin is a large-scale gas producer of low-mature gas in China. It is likely that this resource will play an important role in future exploration and development of low-mature gas in China.     

Oil cracking to gases： Kinetic modeling and geological significance

A Triassic oil sample from LN14 of Tarim Basin was pyrolyzed using the sealed gold tubes at 200-620℃under a constant pressure of 50 MPa. The gaseous and residual soluble hydrocarbons were analyzed. The results show that the cracking of oil to gas can be divided into two distinct stages: the primary generation of total C1-5 gases from liquid oil characterized by the dominance of C2-5 hydrocarbons and the secondary or further cracking of C2-5 gases to methane and carbon-rich matters leading to the progressive dryness of gases. Based on the experimental data, the kinetic parameters were determined for the primary generation and secondary cracking of oil cracking gases and extrapolated to geological conditions to predict the thermal stability and cracking extent of crude oil. Finally, an evolution model for the thermal destruction of crude oil was proposed and its implications to the migration and accumulation of oil cracking gases were discussed.     

长岭断陷南部地区断陷层油气成藏机制及勘探潜力

利用天然气组成、轻烃指纹、碳同位素和生物标志化合物以及储层流体包裹体等地球化学特征,结合地质条件以及生烃史-热史模拟研究长岭断陷南部的龙凤山地区油气成因及成藏过程,揭示其油气成藏机制。结果表明:龙凤山地区断陷层天然气属于腐殖型和腐泥型的混合气,且为裂解气和干酪根热降解气组成的混合气,油气源主要为本地的沙河子组烃源岩,原油成熟度低于天然气,为同一油源不同热演化阶段的产物,属于次生凝析气藏,成藏表现为"近源多向供烃,复合输导,早期干酪根热降解成气与晚期原油裂解成气"的多期成藏模式;长岭南部地区发育优质烃源岩、营城组末期形成的反转构造提供了圈闭条件,具备较好油气输导条件、存在多期油气充注,油气勘探潜力大。     

Geochemical characteristics and source rock of Ordovician gas reservoir, Changqing Gasfield

Carbon isotopic composition of methane in Ordovician gas reservoir in the Changqing Gasfield can reflect the dominance of Upper Paleozoic coaltype gas despite its high mature degree. The¹²C-rich ethane in the Ordovician reservoir (δ¹³C₂ < −29%), known as a marker as oiltype gas, does not indicate the Ordovician marine carbonate as main source rock, it is because of the relative less ethane content in coaltype gas than in oiltyped gas due to generation and migration. The way of Upper Paleozoic coaltype gas migrating into the Ordovician reservoir is widely through the unconformlty instead of only through the erosion groove in the weathering crust.     

塔里木盆地塔东英南2气藏气源分析

英南2凝析气藏天然气属富氮湿气,烃类气体体积分数一般为78.86%~87.67%;甲烷68.92%~76.67%,重烃气体为9.17%~14.01%,非烃气体氮气为13.89%~21.07%,甲烷和乙烷的碳同位素分别为-38.6‰~-36.2‰和-30.9‰~-34.7‰.根据天然气组分、天然气碳同位素的特征,认为英南2凝析气藏的天然气来源于高过成熟的下古生界海相寒武系—下奥陶统腐泥型干酪根形成的原油裂解气,同时应用ln(C2/C3)-ln(C1/C2)及δ13C2-δ13C3与ln(C2/C3)判识模式也印证了英南2气藏气源为原油裂解气,气藏凝析油中存在高含量的金刚烷,指示原油经历了裂解,裂解程度为80%~90%.     

论冀中地区O、C—P凝析气藏的形成条件

据凝析油轻烃组成、甲烷及其同系物碳同位素组成、天然气与源岩吸附气指纹时比、双环倍半萜组合特征及地质结构的综合分析,查明O、C—P大多数提析气藏的油气主要来源于石炭—二叠系煤系腐植型有机质,混入了部分下第三系的油气。对石炭—二叠系成烃特点的分析表明,R。=0.6%～1.3%,以成气为主,生油为辅,原始油气比一般大于1OOCm3/t。成熟期的石炭—二叠系有机质的成烃特点和具备油气富集的良好地质条件是O、C—P凝析气藏形成的基本条件。     

湖相碳酸盐岩有机质热演化产物及其碳同位素组成特征

通过泌阳凹陷第三系核桃园组核三段湖相碳酸盐岩的热模拟实验,对有机质演化特征、释放气体组分及含碳气体碳同位素组成特征获得了以下认识：（1）释放气体主要以二氧化碳和烷烃类气为主,含有少量烯烃气体,在300～400℃时达到气体释放峰,随温度的升高,C4～C5较重烷烃气含量增加。二氧化碳随温度的升高释放量增大,主要为碳酸盐分解贡献;（2）含碳气体碳同位素随温度的升高增重,在气体释放峰温度段350℃左右时发生明显“转折”,碳同位素组成明显变重,结果造成在不同的热演化阶段碳同位素值有较大范围的变化。     

城市生活排水系统废气产排量测算模拟研究——以餐饮污水为例

通过实验室静态模拟试验,研究了城市餐饮污水水质的变化规律,特征气体污染物产生规律,并分析了CH4、CO2、H2S产生量与水质变化规律之间的关联性,计算了各种有害气体的产气系数.结果表明,CH4、CO2、H2S的产排量随着污水停留时间的延长呈上升趋势;CODcr是影响甲烷和二氧化碳产生的主要指标,硫酸根是影响硫化氢产生的主要指标.三者的产气系数分别为:3.71～17.9g/kgCODcr去除量、24.5～40.4g/kgCODcr去除量、1.50～7.10g/kg硫酸根去除量.     

Research progress in studies on the coalbed gas geochemistry

The current situation of geochemical studies on coalbed gas is reviewed in this paper. Generally, coalbed gas is compositionally dominated by methane with δ¹³C₁ values ranging approximately from -80‰ to -10‰. However, few isotopic studies have been carried out on other components of coalbed gas except for hydrogen and carbon dioxide, whose δD_CH4 values available for utilization vary from -333‰ to -117‰, and δ¹³C_CO2 values from -29.4‰ to +18.6‰. Two major types of coalbed gas, thermogenic gas and secondary biogenic gas, have been identified, and there are also some other classification criteria. Compared with conventional natural gases, coalbed gas has a wide distribution range of δ¹³C₁ and δ¹³C_CO2 values, especially possessing some extremely heavy values. Current problems that remain unsolved in the coalbed gas geochemistry include the variation mechanism, controlling factors and application of carbon and hydrogen isotopes of methane, the relation between the values of δ¹³C₁ and Ro, the systematic classification scheme and criterion of genetic types, and the application of the coalbed gas geochemistry in evaluating target districts of the coalbed gas exploration.