甲烷碳同位素在天然气勘探中的应用

本文只讨论以烃类组分为主的热成因天然气中甲烷碳同位素的问题。用于天然气勘探的甲烷碳同位素方法包括两个方面的内容:一是测定勘探区内表层沉积物和探井岩心(或岩屑)的吸附甲烷的碳同位素异常变化以及气显示中甲烷碳同位素组成特征,用以指导勘探;二是对已发现的天然气藏测定其甲烷碳同位素,提供有关成因、类型、成熟度、运移、次生变化等多方面的信息,以便对天然气进行综合评价。甲烷碳同位素组成、天然气组分、源岩有机质类型和成熟度等是目前天然气勘探中广泛使用并且行之有效的地球化学指标。本文除用实例讨论了前人提出的由温度和母质类型决定的甲烷碳同位素组成特征之外,还根据碳同位素分馏理论提出用同源的甲烷与原油以及甲烷与干酪根的碳同位素差值作为天然气类型及成熟度的指标,并对煤层甲烷碳同位素数据的应用提出了自己的见解。

METHANE CARBON ISOTOPE AND ITS APPLICATION IN NATURAL GAS PROSPECTING

Methane carbon isotopic composition can provide information on origin,maturity and secondary modification of natural gas.Therefore,it can serveas a useful geochemical clue for gas prospecting.The focus of this study isconcentrated on discussing the variation of methane carbon isotope in thermo-genic natural gas.1.It is the temperature that influences the maturity of organic mattersand determines isotopic fractionation range.The fractionation range is largewhen the maturity is lower with methane generated contains light carbonisotopic composition;the isotopic fractionation is decreased,when the methanecontains obviously heavier Carbon isotopic composition.The range of carbonisotopic fractionation depends also on different primary textures of organicmatters,giving rise to methane with its own properties.The isotopic differencein methane and kerogen can manifest the maturity of organic matters.2.The carbon isotopic composition of methane in gas generated from crudeoil become heavier with the increase of cracking of crud oil.The differencein carbon isotope of methane and crude oil can be used to distinguish theoil-cracked gas and the associated one.3.Because of either the emergence of additional gas derived from thelater maturity of organic matters in source rocks or carbon isotopic exchangeunder high temperature,the natural gas affected by high temperature mayproduce an anomalous phenominon of δ