四川盆地黄龙组烷烃气碳同位素倒转成因的探讨

20世纪80年代后期至21世纪初,黄龙组成为四川盆地勘探开发热点,也是其主力储量层和产层,2008年天然气产量为68.1×10⁸m³,占四川盆地年总产量的45.8%。黄龙组天然气的地球化学特征主要表现为氦同位素是典型的壳源型,R/Ra多数为0.01;硫化氢含量低,平均值为0.27%;属于干气,甲烷平均含量为96.71%,重烃气含量低,平均为0.71%。总的看来,黄龙组天然气是低碳优质能源。该烷烃气碳同位素发生倒转,即δ¹³C₁>δ¹³C₂<δ¹³C₃。通过综合分析有机成因烷烃气碳同位素倒转的4种成因,确定黄龙组烷烃气碳同位素倒转的原因是其志留系烃源岩先期形成的具有轻的δ¹³C₂的伴生气和后期形成的具有重的δ¹³C₂的裂解气混合。     

中国有机烷烃气碳同位素系列倒转的成因

所谓烷烃气碳同位素系列是指依烷烃气分子中碳数顺序递增,δ¹³C值依次递增或递减。递增者称正碳同位素系列,递减者叫负碳同位素系列。当烷烃气的δ¹³C值不按正、负碳同位素系列规律,排列出现混乱时,谓之碳同位素系列倒转。致使倒转的原因有有机烷烃气和无机烷烃气的混合;煤成气和油型气的混合;同型不同源气或同源不同期气的混合和烷烃气中某一或某些组分被细菌氧化。倒转可由其中一种原因,也可由两种或更多种原因所致。     

天然气组分碳同位素倒转成因分析及地质应用

为促进稳定碳同位素倒转现象在天然气地质勘探中的应用,通过调研大量国内外相关文献,系统地梳理和归纳了天然气烷烃组分稳定碳同位素序列倒转的成因及原理,包括有机成因气与无机成因气混合、细菌氧化降解作用、不同类型天然气（油型气和煤成气）混合、不同源或不同期天然气（如原生气与次生气）混合、高温及高压作用（气层气和水层气混合、硫酸盐热氧化还原反应、瑞利分馏作用）以及天然气运移扩散效应等。分析认为,碳同位素倒转现象在天然气地质勘探中具有广阔的应用前景,包括判识天然气的成因及来源,研究母质成熟度及天然气次生变化,反映气藏的地质特征（如成藏期次和沉积环境）,以及判断天然气远景区等。     

塔里木盆地和准噶尔盆地烷烃气碳同位素类型及其意义

将烷烃气碳同位素划分为正碳同位素系列(是有机成因烷烃气的表征)、负碳同位素系列(是无机成因烷烃气的表征)和烷烃气碳同位素倒转(是次生改造气的特征)等3种类型.塔里木盆地和准噶尔盆地发育着大量正碳同位素系列的原生型煤成气和油型气,也有大量碳同位素倒转的次生型的有机成因烷烃气,但无原生型负碳同位素系列无机成因烷烃气,仅有个别次生改造型负碳同位素系列,它们是由有机成因烷烃气改造来的.认为烷烃气碳同位素倒转的成因有4种:①同源不同期气或同型不同源气的混合;②煤成气和油型气的混合;③烷烃气中某一或某些组分被细菌氧化;④有机烷烃气和无机烷烃气的混合.     

塔里木盆地海相天然气乙烷碳同位素分类与变化的成因探讨

分析塔里木盆地海相天然气碳同位素组成 ,发现天然气的乙烷碳同位素值域分布范围较大 ( - 43‰～- 2 9‰ ) ,并且具有很强的母质继承性。据此并结合天然气甲烷碳同位素组成变化特征 ,将该盆地海相天然气分成 4类。第一类 :δ13 C1值小于 - 40‰ ,δ13 C2 值小于 - 3 7‰ ,主要来源于寒武系—下奥陶统腐泥型母质 ,分布于塔中主垒带、英买力和东河塘地区 ;第二类 :δ13C1值小于 - 40‰ ,δ13 C2 值大于 - 3 4‰ ,主要来源于中、上奥陶统偏腐殖型母质 ,分布于塔中北斜坡、巴楚、东河塘及雅克拉等地区 ;第三类 :δ13 C1值大于 - 40‰ ,δ13 C2 值为 - 3 9‰～ - 3 5‰ ,主要来源于寒武系—下奥陶统腐泥型母质 ,并受热成熟作用影响 ,主要分布在轮南—吉拉克地区 ;第四类 :δ13 C1值大于 - 40‰ ,δ13 C2值大于 - 3 5‰ ,主要来源于寒武系—下奥陶统腐泥型母质 ,同时混入有中、上奥陶统或石炭系偏腐殖型母质生成的气 ,主要分布于桑塔木断垒带及解放渠东—吉拉克地区。图 1表 3参 4(梁大新摘 )     

渤中凹陷天然气烷烃碳同位素异常的成因探讨

渤中凹陷天然气为有机成因气，但部分气藏烷烃碳同位素出现异常。通过对碳同位素异常的分析认为：QHD30—1N-1井馆陶组天然气甲烷碳同位素偏负的原因可能是深部高成熟气运移至浅层与低熟气混合和／或天然气的运移、扩散效应造成；而CFD18—1—1井、BZ26—2—1井及BZ22—2—1ST井天然气烷烃系列碳同位素局部倒转的原因则分别是由“既不同型又不同源”气、“同源不同型”气和“同型不同源”气的混合所造成。     

准噶尔盆地烷烃气碳同位素组成及来源

通过对全盆地烷烃气碳同位素组成进行分析,明确了准噶尔盆地烷烃气成因类型、来源及分布。准噶尔盆地天然气主要包括成熟—过成熟油型气、成熟—过成熟煤型气、过渡气和生物气等。准噶尔盆地烷烃气碳同位素倒转普遍,碳同位素倒转原因包括细菌氧化作用、油型气和煤型气混合、不同源煤型气混合以及同源不同阶煤型气混合。准噶尔盆地煤型气包括3类：①煤型气重烃气碳同位素组成较重(δ¹³C₂＞-26.0‰),主要为成熟—高成熟,来源于侏罗系烃源岩;②煤型气重烃气碳同位素组成相对较轻(δ¹³C₂＜-26.0‰),成熟度范围广,来源于侏罗系、二叠系乌尔禾组和佳木河组烃源岩中的一套或多套;③煤型气重烃气碳同位素组成分布范围广,主要为高—过成熟,来源于石炭系烃源岩。     

利用天然气的碳同位素比值建立热成因气模型

利用同位素模型预测甲烷、乙烷和丙烷的δ13C值（干酪根／油生成气态物的程序指标），并用指标Ⅱ型干酪根的生气特征，通过把气体的多组份动力学模型与同位素模型相结合，可得到气体组份转化率与生成温度之间的相关性，δ13C1，δ13C2和δ13C3之间的关系式利用天然气的热解模型进行限制，通过西得克萨斯Delaware 和Val Verde盆地海相腐泥型页岩（II型干酪根）生成的气体数据对同位素模型进行验证与校对，这些气体相对不受运移和生物气的混合影响，所以气体碳同位素比值的变化能初步反映成熟度的演化程度，因此，这些数据对解释气体生成温度和来源于II型干酪根的气体δ13C值是有实际价值的。     

费—托合成实验中烷烃气碳同位素分馏机理

40多年来的费—托合成模拟实验表明,只有部分结果出现了烷烃气碳同位素反序现象。通过较为完整的分析发现,其原因可能是:随着模拟实验时间的增加与温度的升高并随着产物转化率的增高,控制烷烃气碳同位素分馏的因素逐渐由动力学机制转变为热力学平衡,烷烃气碳同位素序列将由反序转变为部分倒转再到类似常规天然气的正序分布。只有在较短时间(转化率较低)或者开放体系(随生随排)条件下才遵从动力学分馏。电火花放电合成实验仅代表了理想状态下动力学分馏过程。     

煤成大、中型气田天然气的碳同位素特征

本文根据我国典型大、中型气田的283组碳同位素数据,通过煤成气与油型气的对比,探讨了我国大、中型煤成气田烷烃气的基本特征。研究结果表明,烷烃气碳同位素具有以下特征:甲烷及其同系物随碳分子数的增加,δ¹³C频率区间值展布缩小,主频率值变重;④煤成气与油型气相比,具有低的δ¹³C₁ 主频率峰值,高的δ¹³C₂值;㈣Δ(δ¹³C₂-δ¹³C₁)与δ¹³C₁ 呈负相关关系;煤成气碳同位素系列只有两种形式,即正碳同位素系列和同位素倒转系列;5烷烃气碳同位素系列特征取决于显微组分、演化程度和成藏机制。     

Characteristics of carbon isotopes of organic alkane gases in petroliferous basins of China

Based on analytical data of gas samples from some gas- and oilfields in China, this article describes characteristics of alkane gases (C_1–4) with different origins and occurrences. It is found that δ¹³C values of organic alkane gas increase with increasing maturity (R₀). The δ¹³C values of synchronous and isogenous methane and its homologues are found to increase with the increase in carbon number of alkane gas molecules. The δ¹³C values of methane and its homologues of coal-type gas are heavier than those of corresponding components of oil-type gas, both from source rocks with the same or similar maturity. Part of the components of methane and its homologues were oxidized by bacteria which resulted in heavier carbon isotopes compared to the remaining components.     

Geochemcal characteristics of abiogenic alkane gases

Recently abiogenic alkanes have been found in various locations in the world and other celestial bodies.The chemical composition of abiogenic alkane gases varies widely.The content of methane is low and nearly no C2+ is found in the abiogenic alkane gases from fluid inclusions in volcanic rocks or hot springs in China.In the unsedimented submarine hydrothermal vent system C1/C2+ ratios are much greater than those for the thermogenic gases,mostly >800 and in some cases up to 8,000.In the Songliao Basin,China,C1/C2+ of some abiogenic gases are often less than 150.Abiogenic alkane gases which have been found in nature often have carbon isotopic reversal among C1-C4 alkanes(δ13C1>δ13C2>δ13C3>δ13C4),whereas both regular and reversed hydrogen isotope distribution pattern among C1-C4 alkanes have been reported.The δ13C of abiogenic methane is mainly greater than-30‰ though laboratory synthesized methane can have δ13C as low as-57‰,and its δD1 values vary widely and overlap with biogenic gases.High 3He/4He ratios often indicate the addition of mantle-derived helium and are related to abiogenic gases.However,some biogenic gases can also have high 3He/4He ratios up to 8.The CH4/3He end-member is often lower than 106 for abiogenic alkane gases while greater than 1013 for biogenic gases,and the values between these two end-members often reflect the mixing of biogenic and abiogenic gases.     

Geochemical characteristics of abiogenic alkane gases

Recently abiogenic alkanes have been found in various locations in the world and other celestial bodies. The chemical composition of abiogenic alkane gases varies widely. The content of methane is low and nearly no C2+ is found in the abiogenic alkane gases from fluid inclusions in volcanic rocks or hot springs in China. In the unsedimented submarine hydrothermal vent system C1/C2+ ratios are much greater than those for the thermogenic gases, mostly ＞800 and in some cases up to 8,000. In the Songliao Basin, China, C1/C2+ of some abiogenic gases are often less than 150. Abiogenic alkane gases which have been found in nature often have carbon isotopic reversal among C1-C4 alkanes (δ13C1＞δ13C2＞δ13C3＞δ13C4), whereas both regular and reversed hydrogen isotope distribution pattern among C1-C4 alkanes have been reported. The δ13C of abiogenic methane is mainly greater than -30‰though laboratory synthesized methane can have δ13C as low as -57‰, and its δD1 values vary widely and overlap with biogenic gases. High 3He/4He ratios often indicate the addition of mantle-derived helium and are related to abiogenic gases. However, some biogenic gases can also have high 3He/4He ratios up to 8.The CH4/3He end-member is often lower than 106 for abiogenic alkane gases while greater than 1013 for biogenic gases, and the values between these two end-members often reflect the mixing of biogenic and abiogenic gases.     

不同采样装置及储存时间对非烃气体碳、氢、氧同位素的影响

以CO2和H2为例,探讨了常见的几种气体采集装置对于非烃类气体采集、储存过程中碳、氢、氧同位素的影响。采集装置包括高压钢瓶、锡箔纸袋、玻璃瓶,其中玻璃瓶分为蒸馏水封存和饱和食盐水封存2种,实验时间为样品采集后96 h以内。整个时间段测试结果显示:对于CO2碳同位素和H2氢同位素而言,采样装置的选择和储存时间的长短对碳、氢同位素的测定影响不大,而对于CO2氧同位素,在最初的5 h内,无介质参与时所采集的样品氧同位素基本保持恒定,有介质参与时,受介质的影响,整个测试过程中氧同位素持续变化从而无法获得准确的氧同位素比值,故建议通过现场测定的方式来获取CO2氧同位素。以上实验结果可为科研人员采集非烃类气体样品时提供参考和借鉴。     

On the prediction of solubility of alkane in carbon dioxide using the LSSVM algorithm

The increasing global energy demand and declination of oil reservoir in recent years cause the researchers attention focus on the enhancement of oil recovery approaches. One of the extensive applicable methods for enhancement of oil recovery, which has great efficiency and environmental benefits, is carbon dioxide injection. The CO₂ injection has various effects on the reservoir fluid, which causes enhancement of recovery. One of these effects is extraction of lighter components of crude oil, which straightly depends on solubility of hydrocarbons in carbon dioxide. In order to better understand of this parameter, in this study, Least squares support vector machine (LSSVM) algorithm was developed as a novel predictive tool to estimate solubility of alkane in CO₂ as function of carbon number of alkane, carbon dioxide density, pressure, and temperature. The predicting model outputs were compared with the extracted experimental solubility from literature statistically and graphically. The comparison showed the great ability and high accuracy of developed model in prediction of solubility.     

济阳坳陷煤系烃源岩生物标志化合物与同位素对比研究

在系统分析和总结大量烃源岩分析数据的基础上,对济阳坳陷石炭一二叠系和中生界两套煤系烃源岩的生物标记化合物参数与碳同位素特征进行了详细的对比,对比结果显示石炭-二叠系煤系烃源岩干酪根、族组成碳同位素大于中生界坊子组;生物标记化合物中,石炭-二叠系煤系烃源岩与中生界坊子组相比具有更强的植烷优势、更高的γ蜡烷含量和更高的C₂₇胆甾烷含量,表明石炭-二叠系煤系烃源岩有机质的生成环境、母质组成及保存条件优于中生界。