A discussion on the upper limit of maturity for gas generation by marine kerogens and the utmost of gas generative potential： Taking the study on the Tarim Basin as an example

The experimental data yielded by Rock-Eval pyrolysis, kerogen atomic H/C, Py-GC and gold-tube sealing thermal simulation on the marine Cambrian-Ordovician source rock from the Tarim Basin revealed that the upper limit of maturity for natural gas generation or the ＂deadline of gas generation＂ for marine types Ⅰ and II kerogens is equal to 3.0% of vitrinite reflectance （Ro）; while the ＂deadline of gas generation＂ for type Ⅲ kerogens typically like coals is as high as 10% Ro. Thus, different organic matter has obviously different utmost maturities for gas generation. The mass-balance calculation by kerogen elements showed that when Ro〉1.5%, the utmost amount of gas generation for the marine type II kerogen is less than 185 m^3/t TOC, accounting for less than 30% of its total hydrocarbon generative potential; when Ro〉2.0%, it becomes 110 m^3/t TOC, less than 20% of the total hydrocarbon generative potential. The amount of the gas generative potential obtained by Rock-Eval is only around one tenth of the calculated value by the mass balance of kerogen elements at the same thermal evolutionary stage, while those by Py-GC and gold-tube sealing simulation are intervenient between the above two. The utmost of gas generative potential at the over 1.3% Ro stage is around 60--90 m3/t TOC, therefore, the amount of gas generation obtained by Rock-Eval is the minimum of gas generative potential, while that by the mass-balance calculation of kerogen elements is the maximum that the actual amount of gas generation should not exceed.     

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.     

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.     

Mass balance calculation of the pyrolysates generated from marine crude oil： A prediction model of oil cracking gas resources based on solid bitumen in reservoir

Oil cracking gas plays an important role in the resources of natural gas in the basins with high and over mature marine source rocks in China. The prediction of the oil cracking gas resources becomes nec-essary and urgent in the gas exploration in these basins. A marine crude oil sample was pyrolyzed using sealed gold tubes system. The pyrolysates including gas,liquid and solid were quantitatively analyzed. Based on the pyrolysis data and kinetic calculation,the yield correlativity among gas,liquid and solid products was regressed with high correlative coefficients to establish a prediction model suitable for the resource estimation of oil cracking gas. The verification formula for this model was also established on the principle of mass conservation. The affecting factors and the application precondi-tions of this prediction model were discussed. This model would enlighten and provide some new ideas for the resource assessment of natural gas in the high and over mature marine carbonate source rock areas in China. It is expected to be valuable in gas exploration.     

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 dispropor-tionation reactions leading to light hydrocarbons and pol-yaromatic 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.     

Experimental evidence for formation water promoting crude oil cracking to gas

Crude oil cracking to gas is the key to determining the exploration potential and strategy for deep hydrocarbon resources.Identifying the factors that affect the threshold and potential of crude oil cracking to gas as well as other possible influencing factors will provide the scientific basis for deep hydrocarbon exploration.A comparison of pyrolysis simulation experiments of crude oil,hydrous crude oil,and various water media under a constant temperature(350℃) and pressure(50 MPa) shows that water plays a large role in crude oil cracking to gas.(1) When water is added,the gas yields increase significantly,including those of alkane gases and non-hydrocarbon gases:the yield of alkane gases increases 1.8-3 times;the yields of H2 and CO2 also increase significantly.This means that water takes part in the process of crude oil cracking to gas,and supplies hydrogen.Therefore,the presence of water will dramatically enhance the potential of crude oil cracking to gas.(2) Mg2+ ions in the formation water promote the crude oil + water reaction to some extent and increase the total yield of alkane gases and the yields of both H2 and CO2 ;more interestingly,the i-C4/n-C4 and i-C5/n-C5 ratios increase significantly.This indicates that Mg2+ ions in formation water act as a catalyst,and a disproportionation reaction is involved in the crude oil + water reaction.This study helps us to understand the factors influencing crude oil cracked gas and to evaluate the hydrocarbon resources in deep sedimentary basins.     

A breakthrough of researching reservoir adsorption gas and its significance

GC-C-MS on linear isotope analysis equipment makes it possible to measure the hydrocarbon gases at the level of 10 -3-10 -2 μL- By applying this technique the carbon isotopes of C1-C3 of the adsorbed gas from the Triassic oil sand bed of the Aican-l Well in the Turpan-Hami basin were analysed. The δ13C values of C1-C3, are -55.1‰, -38.6‰ and -35.0‰ respectively. In terms of geochemical characteristics of natural gases and crude oils, in combination with basinal geological backgrounds, it is considered that the reservoir adsorbed gas was formed by crude-oil biodegradation, absorbed by reservoir rocks and its oil-gas source is related with the Permain (perhaps including the Carboniferous). The adsorbed gas is obviously different from the Jurassic coal-generated oil and gas.     

Characteristics of hydrocarbon source rocks of No.3 buried-hill region in Nanpu Sag

Based on the data of 44 samples of hydrocarbon source rocks in Nanpu No.3 buffed-hill region, the kerogen type is judged through the pyrolysis and microscopic identification. At the same time, organic matter maturity and hydrocarbon generation threshold are studied by using vitrinite reflectance, pyrolysis yield and hydrocarbon abundance. Meanwhile the hydrocarbon expulsion threshold is calculated. And the characteristics of organic hydrocarbon generation and expulsion are preliminarily revealed and evaluated. The result shows that the No.3 buffed-hill region has abundant hydrocarbon source rocks with high content of organic carbon. And the primary types of kerogen are II, and lI 2. The hydrocarbon source rocks which passed biochemistry, thermolysis and thermal cracking have developed into the mature-postmature phase of different extents. And plenty of oil and gas were expelled out. It is believed the depth of oil-generating window is 3 600 m and the depth of hydro- carbon-expulsion threshold is 4 100 m. The comprehensive analysis indicates that Nanpu No.3 burried-hill region has a certain condition to generate hydrocarbon which is very promising in oil exploration and thus can become an important exploration and development target next.     

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.     

The role of sulfur in the pyrolysis of kerogen

Sulfur plays an important role in the generation and evolution of hydrocarbon from organic matter. Here, a pyrolysis experiment in closed system was performed on Maoming oil shales kerogen (Type Ⅰ), Maoming oil shales kerogen added with sulfur ether and Maoming oil shales kerogen added with sulphur. The results suggest that the existence of sulfur can result in: (i) higher yield of hydrocarbons generated from the kerogen; (ii) decrease of the temperature for the maximum generation of heavy hydrocarbons (the C15+ fraction) by 20℃; (iii) decrease of the temperature for the maximum generation of the aromatics fraction by 40℃, and (iv) acceleration of the aromatization process. The pyrolysates from kerogen added with sulfur are similar to the heating products of the sulfur-rich kerogen as reported in the literatures. It seems that the sulfur catalysis is also an important factor that can make the sulfur-rich kerogen generate low-mature oil at the earlier diagenesis stage, except for the weakness of the C-S and S-S bonds.     

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.     

油气分段捕获揭示震旦——寒武系天然气成藏过程

烃源岩生烃具有时间和空间上的序列性,构造活动和圈闭的形成也具有阶段性,加之富集型古油藏和分散型滞留原油产生的裂解气也会大量运移聚集,因此,油气成藏可能经历多个阶段。基于以上油气分段捕获原理"将今论古"地分析四川盆地南部威远、资阳、高石梯-磨溪3个典型构造的震旦系-寒武系天然气成藏过程与成藏规律,从而预测其他未知区块的成藏潜力。研究认为,高磨地区作为继承性古隆起,震旦系-寒武系圈闭几乎捕获了烃源岩从早期生油到晚期干酪根生气所有阶段的油气;威远、资阳震旦系-寒武系圈闭则仅捕获了部分阶段的油气,对于未知区块-泸州古隆起区来讲,其震旦系-寒武系能捕获生气高峰及其以后阶段所有的天然气,包括分散型滞留原油裂解气和晚期干酪根裂解气,如果落实好储层和圈闭,天然气勘探潜力很大。     

Migration and accumulation of natural gas in Kela-2 gas field

With the guidance of petroleum system theory, the dynamic filling history of natural gas in the Kela-2 gas field is analyzed by using a large suite of oil and gas geo- chemistry evidence in combination with the tectonic evolu- tion history and reservoir evolution history. It concludes that the Kela-2 gas field was formed by capturing the gas gener- ated during the main gas generation period, while the late kerogen cracking gas contributed a little to the gas field. It suggests that the gas generated during the main gas genera- tion accumulated in the early-formed wide-gentle anticline, which is the necessary condition for natural gas to re-migrate and enrich late to form the large-scale gas reservoir. The newest research shows that the filling history of gas in the Dabei-1, Yinan-2, Tuziluoke and Dina-2 gas fields was re- lated with the natural gas accumulation in the early wide- gentle anticline as well as late re-migration and enrichment of natural gas.     

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%.     

用红外光谱表征干酪根和煤的芳碳量

干酪根和煤红外光谱中的芳碳含量常以1600cm~(-1)频带来表征.但该频带的强度受芳碳骨架化学环境和结构的强烈影响.用苯并菲、茬、二羟基蓁、二苯酮和联苄等6种模型化合物进行红外光谱分析发现,1600 cm~(-1)频带因芳环稠化而迅速减弱,因邻接含氧基团而显著增强.对不同类型干酪根的热模拟系列产物进行了红外光谱和固体~(13)C核磁共振波谱的综合研究.结果表明,1600 cm~(-1)的摩尔吸光系数ε_1600有随演化加深而下降的趋势,尤以煤系干酪根为甚,与模型化合物的结果相符.因此,吸收系数K_1600可用以区分干酪根的类型,但不宜用作其成熟度的参数.     

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。     

四川中江气田沙溪庙组天然气成因类型及气源对比

探讨四川盆地中江气田侏罗系沙溪庙组天然气的成因和气源,为找气提供理论依据。利用天然气组分特征、碳同位素特征、轻烃特征等地球化学资料对该区天然气成因类型进行了分析,根据轻烃的气、源配对参数和碳同位素参数对天然气的来源进行了追踪。沙溪庙组天然气以烃类气体为主,含少量的非烃类气体,为腐植型干酪根裂解成因的煤型气;对碳同位素和轻烃的研究均表明,天然气为有机成因的煤型气。气源对比结果表明,须五段烃源岩为沙溪庙组煤型气的主要来源。中江气田侏罗系沙溪庙组气藏为次生气藏。