In recent years, breakthroughs have been made in the Chang-8 member of the western part of the Tianhuan Depression in the Ordos Basin. The unclear understanding of the oil source of Chang8 crude oil in the Huanxi region has constrained subsequent oil exploration. To solve this problem, the characteristics and sources of crude oil were studied by physical property analysis, crude oil chromatography and crude oil chromatography-mass spectrometry. The analysis results show that the crude oil is light or medium quality. The main peak carbon of the total hydrocarbon chromatogram of crude oil is C14 to C15 with odd-even dominance value (OEP) ranging from 0.94 to 1.09, pristine/phytane (Pr/Ph) ranging from 1.16 to 1.14, and C21−/C21+ranging from 2.00 to 3.73. The distribution and main parameters of steroterpane biomarkers are characterized by low relative abundance of rearranged hopanes, low gammacerane, low C30rearranged hopanes/C30 hopanes (C30*/C30) ratio (0.07-0.33), ααα- 20R configuration steranes are distributed in a "V" shape. The crude oil parent material comes from aquatic organisms and is formed in a fresh water, weak reduction and weak oxidation sedimentary environment. The oil source comparison shows that the Chang 8 crude oil in the Huanxi area comes from the Chang 7 black shale inside the Basin. The Huanxi area is adjacent to the high-quality source rock in the eastern lake basin. It is speculated that the oil generated from the Chang 7 black shale in the accumulation period is driven by the hydrocarbon generation pressurization, and is laterally channeled along the fault, unconformity surface and sand body, and accumulates in favorable positions to form reservoirs. The Chang 8 oil-bearing formation in the Huanxi area shows good potential for reservoir formation and exploration.

The Jurassic Yan'an Formation (J1y) crude oil-associated gas in the Pengyang area of the Ordos Basin contains a certain amount of H2S. The study of its genesis is important for us to predict the distribution of H2S in Mesozoic in the basin. The Chang7 source rock of the Yanchang Formation (T3y) is the principal source of crude oil in the Jurassic Yan'an Formation area. However, H2S is not detected in the crude oil-associated gas of Chang10 to Chang3 members of the Yanchang Formation. As a result, the formation of H2S in the crude oil-associated gas of Yan'an Formation may be related to reservoir and accumulation factors. The study shows that Jurassic formation water has a high salinity and is rich in divalent sulfur, which inhibits the development of sulfate-reducing bacteria (SRB). It is less likely to biologically reduce sulfate to produce H2S. The δ34S values of H2S in crude oil-associated gas of Yan'an Formation are greater than 25‰, which are obviously different from those of volcanic hydrothermal fluid, organic matter, and microbial reduction products, but similar to divalent sulfur isotope values in formation water and about 10‰ negative to sulfate ion in formation water. TSR has generated sulfur isotope distribution and fractionation characteristics in H2S. The temperature measurement of reservoir inclusions in the Jurassic Yan'an Formation shows that the initial filling temperature of oil and gas is higher than 100 °C, and the maximum accumulation forming temperature is 150 °C. The formation water of Yan'an Formation contains a large number of sulfate ions from anhydrite or buried stage. Those rich sulfate ions meet the conditions for a sulfate thermochemical reduction reaction. At the same time, the rich magnesium ions in formation water catalyzed the reaction. Therefore, H2S in Jurassic crude oil-associated gas in the Pengyang area of the Ordos Basin has thermochemical sulfate reduction (TSR) genesis.

鄂尔多斯盆地彭阳地区侏罗系延安组原油伴生气含有一定量的H2S,其成因研究对预测盆地中生界H2S的分布具重要意义.研究区侏罗系延安组原油主要来自于延长组7段(长7段)主力烃源岩,而具有相同油源的长10段—长3段原油伴生气中均未检测到H2S.因此,延安组原油伴生气中H2S的形成可能与储集和成藏因素有关.分析表明,侏罗系地层水具高矿化度且富含二价硫,抑制了硫酸盐还原菌的发育,因此生物还原硫酸盐生成H2S的可能性较小.延安组原油伴生气中H2S的δ34S值均大于25‰,明显不同于火山热液、有机质及微生物还原生成物的δ34S分布特征,而与地层水中二价硫同位素值相近,比地层水中硫酸根离子的δ34S值偏低约为10‰,具有TSR生成H2S的硫同位素分布和分馏特征.研究区侏罗系延安组储层岩石包裹体测温显示,油气初次充注温度高于100℃,最大成藏温度高达150℃,延安组地层水中含有大量的来自于硬石膏或埋藏时赋存的硫酸根离子,满足了发生硫酸盐热化学还原反应的条件,同时地层水中丰富的镁离子催化了反应的发生.因此,鄂尔多斯盆地彭阳地区侏罗系原油伴生气中H2S为TSR成因.

To reveal the effects of lacustrine hydrothermal activities on the organic matter input of source rocks and the formation of excellent source rocks in the Ordos Basin during the Middle and Late Triassic Yanchang period, samples of the Yanchang Formation in the YK1 well in the southern Ordos Basin were selected; source rock evaluations, element geochemical analysis, and palynofacies studies were systematically conducted. The results show that excellent source rocks are developed in the Chang 7-3 interval. The intensity indicators of hydrothermal activity (IIHAs), namely Al/(Al + Fe + Mn) (IIHA1) and (Fe + Mn)/Ti (IIHA2) show that the lake underwent three episodes of obvious thermal fluid activity during the depositional period of the Chang 7-Chang 6 members of which the Chang 7-3 period was the peak of hydrothermal activity. The main hydrocarbon-generating components in the source rocks of the Yanchang Formation were granular amorphous organic matter (AOM) and algae and gelified particles were the secondary hydrocarbon-generating components. Three palynofacies assemblages have been identified in the source rocks. Assemblage A is distributed in the Chang 8-Chang 9 members and Chang 6-2 + 3 interval. The organic matter present in this assemblage mainly consists of type III and translucent lignocellulosic fragments and gelified particles comprise the dominant organic debris. Assemblage B is distributed in the Chang 7-3 interval which consists mainly of Type I and Type II organic matter and is characterized by high abundances of algae and AOM. Assemblage C is distributed in the Chang 7-1 + 2 interval and consists mainly of Type II organic matter which is characterized by the simultaneous development of AOM and phytoclasts. The correlations between the contents of AOM and algae in the source rocks and the IIHAs indicate that hydrothermal activity may have led to the development of algae and source rock formation. The distribution of AOM and algae show consistently high contents over the entire Chang 7-3 interval which are not only coupled with the peak of total organic carbon content, liquid hydrocarbon potential, hydrogen index, and peak of hydrothermal activity but also shows different degrees of correlation with the IIHAs. Their relationships indicate that the peak of the lake's hydrothermal activity may have induced algae and plankton blooms, which may have been one of the important controlling factors for the formation of excellent source rocks. The close relationship between the IIHA and sulfur content indicates that hydrothermal fluids may have further deprived the oxygen of lake bottom and formed a reducing environment which was helpful for the preservation of organic matter and deposition of excellent source rocks.

探讨鄂尔多斯盆地奥陶系盐下天然气的成因类型及来源,为勘探部署提供理论依据.以天然气地球化学特征分析为基础,结合成藏组合特点剖析,通过烷烃气及轻烃组分碳同位素组成比对、δ13 C1—RO相关性分析等综合判识天然气成因类型与来源.结果显示,鄂尔多斯盆地奥陶系盐下天然气组分以烃类气体为主,烃类组分含量平均为94.1％.烃类气体中甲烷占优势,甲烷化系数[C1/∑(C1—Cn)]随区域热演化程度的不同而变化明显,从靖边—乌审旗—神木地区,甲烷化系数依次由0.99→0.95→0.85逐渐减小.除高含硫天然气外,奥陶系盐下天然气的δ13 C1值、δ13 C2值整体偏低,δ13 C1值分布于-45.90‰～-37.29‰之间,平均为-39.58‰,δ13 C2值分布于-35.58‰～-25.77‰之间,平均为-29.9‰.奥陶系膏盐岩下高含硫天然气的甲烷、乙烷碳同位素值显著偏高,是硫酸盐热化学还原反应(TSR)的结果.综合各项地球化学指标气气、气源比对,结果显示鄂尔多斯盆地奥陶系盐下天然气属于自生自储油型气,奥陶系海相气源岩是其主力供烃源岩.

The hydrocarbon source rock development of the Yanchang formation in the Ordos Basin is closely related to the hydrothermal fluid activities.A total of 87 core samples were collected continuously for the high resolution element geochemical analysis.The results show that the high-quality hydrocarbon source rocks generally belong to atypical hydrothermal sedimentation with more terrigenous input,but the intensity of hydrothermal input became stronger during the Chang 7-3 period when the high-quality source rocks deposited.The correlation coefficients between the hydrothermal index Al/(Al + Fe+ Mn) or (Fe + Mn)/Ti ratio and TOC abundance are as high as-0.89 and 0.89,respectively,indicating that the hydrothermal sedimentation is closely related to the source rocks,i.e.increasing hydrothermal input intensity increases organic matter abundance in the hydrocarbon source rocks.The correlation coefficients between Al/(Al+Fe+ Mn) ratio and P/Al,P/Ti,TS content are-0.81,-0.80 and-0.89,respectively,suggesting the correlation of the hydrothermal sedimentation and hydrocarbon source rock may be due to hydrothermal activities.Such activities can promote surface productivity and anoxic condition in lake bottom,providing sufficient organic matter source and good preservation conditions in the process of hydrocarbon source rock development.Studies show that hydrothermal sedimentation are potential organic-rich hydrocarbon source rocks,and correlation analysis of quantitative elemental indices is an effective way to uncover the relationship between hydrothermal sedimentation and source rocks.

随着靖西地区奥陶系中组合天然气勘探的突破,判识天然气成因类型、明确其主要来源成为有效指引勘探方向的关键问题.以天然气地球化学特征分析为基础,通过气—气、气—源对比、源—储—盖空间组合分析等方法,在深化高—过成熟天然气成因判别认知的同时,探讨靖西地区奥陶系天然气的主要来源.研究表明,靖西地区下古生界奥陶系天然气以煤成气为主,局部存在自生自储的油型气;对于高演化干气的成因判识,适宜选取甲烷碳同位素为主要依据.此外,本文对天然气组分碳同位素序列倒转现象提出了新的解释思路.

The Paleozoic condensates in the Ordos basin are characterized as low density,low freezing point,and low dropping point,as well as pristine predominance and high Pr/Ph ratios（Pr/Ph2.0）.The relative content of methyl cyclohexane in the light hydrocarbon is more than 60%.Carbon isotope values（δ13C） of n-alkane monomers are generally ranged from-20‰ to-29‰ and δ13C value decreases with carbon number increase,which shows the sectionalized distribution.Comparing geochemical characteristic of condensates with that of dark-colored mudstones and coal in upper Paleozoic and carbonates in lower Paleozoic,the results show that the geochemical characteristic of Paleozoic condensates in the Ordos basin is similar to that of high mature coal.The determination of Paleozoic condensate supports the Paleozoic natural gas sourced from coal.The carbon isotope value（δ13C） of hydrocarbon gases in the condensates is heavier,indicating they are sourced from coal,in contrast that natural gases with lighter carbon stable isotopes are related to mudstone thickness and distribution in upper Paleozoic,inferring that they would stem from the upper Paleozoic mudstones.

The geochemical parameters of natural gas in the northern Ordos basin show the characteristics of coal-derived gas which has high or much higher methanation parameter, low or lower C2+ component concentration, low condensate oil concentration and the isotope of 13C is abundant. Besides, the content of high cycloparafinic hydrocarbon is equivalent in light hydrocarbon, indicating that the source rock is the gas-bearing coal seams of the cross-facies between marine and continental environment. The changing trend of methanation modelus on the surface or vertical direction, connected with the thermal evolution of gas lava, shows that the gas migration and accumulation is persistent and throughout in the low penetrative deposited lava. The gas accumulation period is in line with the hydrocarbon migration period (J2 - K1). But in the local area, the Shiqianfeng group accumulates gas after K1. The heterogeneity and maturity difference of parent substance, and the migrating and accumulating characteristic, these factors superimposed each other causing the variation of whole geochemical features, and it is complex in local area.

Kinetic parameters were characterized for the formation of the C1–C5 hydrocarbons generating from Taiyuan and Shanxi coal in the Upper Paleozoic of the Ordos Basin, through pressurized simulation experiment designed for the kinetic study of hydrocarbon generation. Based on the palaeogeothermal evolution of the Upper Paleozoic strata in the Ordos Basin, the formation and evolution of gas from Taiyuan and Shanxi coal in the eastern and central regions of the Ordos Basin were reproduced, and it is suggested that large scale secondary hydrocarbon generation does not exist during the uplifting and cooling process of the basin at the end of Early Cretaceous. Some of the factors affecting the behavior of the coal-generated gas were also discussed, and it is emphasized that the most fundamental element determining the hydrocarbon generation behavior is the characteristic of the precursor hydrocarbon-source rock.

通过有压力的生烃动力学模拟实验, 获得了鄂尔多斯盆地上古生界太原组和山西组煤成C1-C5烃类组分的生成动力学参数. 结合鄂尔多斯盆地上古生界的古地温演化特征, 对鄂尔多斯盆地东部和中部地区的上古生界太原组和山西组煤成气的生成演化历史进行了恢复, 认为在早白垩世末盆地抬升降温过程中不存在大规模的二次生烃. 对影响煤成气行为的一些因素进行了讨论, 强调烃源岩母质的性质是决定其生烃行为的最本质原因.

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.