烃类与非烃综合判识干酪根与原油裂解气

Hydrocarbon and non-hydrocarbon characteristics for comprehensive identification about kerogen pyrolysis gas and oil cracked gas

在天然气成因类型研究中,如何有效识别干酪根与原油裂解气一直是一个难题。选取不同类型干酪根、不同性质原油开展半封闭—半开放体系的热压生排烃模拟实验及其产物的地球化学分析研究,并对典型的干酪根、原油裂解气(田)进行了地球化学统计和比对。研究表明,干酪根热解气与原油裂解气中烷烃组分及其碳同位素组成显示相似的演化特征,Ln(C_2/C_3)值均呈早期近似水平和晚期近似垂向变化特征,在高过成熟阶段Ln(C_2/C_3)值与δ^(13)C_2-δ^(13)C_3差值具有快速增大的趋势,二者趋同性变化特征指示了生气母质的高温裂解过程,但这些指标不是干酪根与原油裂解气的判识标志,提出天然气中烷烃分子及同位素组成的有机组合是判断有机质(干酪根、原油)高温裂解气的可靠指标,却并不能直接识别干酪根热解气或原油裂解气;非烃组分的演化特征具有明显的差异性,干酪根热解气以高含氮气(N_2)为主,原油裂解气往往高含硫化氢(H_2S),N_2、H_2S含量作为一项重要指标可以与烷烃气同位素组成相结合有效区别干酪根与原油裂解气,分析结果与四川盆地、塔里木盆地不同油气田的地质实际相吻合。天然气中烃类和非烃组成的综合分析为有效判断干酪根与原油裂解气提供了新的途径。

How to effectively identify between kerogen pyrolysis gas and oil cracked gas is always a puzzle in the genetic study of natural gas from marine strata.The hydrocarbon generation and expulsion experiments of different types of kerogen and oil were carried out in a semi-closed and semi-opened system.The simulating products were analyzed and typical cracked gases of kerogen and oil were compared.It indicates that hydrocarbon components and carbon isotope compositions of kerogen and oil cracked gas show similar characteristics,namely Ln（C2/C3） changing rules occur approximately horizontal in early thermal evolution and vertical in late evolution.In the high-over thermal evolution stage,Ln（C2/C3） values and δ^13C2-δ^13C3 values show rapid enlargement.This denotes the high thermal cracking process of source rocks,which is not an identifying sign about kerogen pyrolysis gas and oil cracked gas.The hydrocarbon composition and isotope compositions of natural gas are effective indicators to identify cracked gas in high thermal evolution,but are not a direct distinguishing index between kerogen pyrolysis gas and oil cracked gas.The evolutionary characteristics of non-hydrocarbon of kerogen pyrolysis gas and oil cracked gas indicate obvious difference,namely high nitrogen are predominant in kerogen pyrolysis gas and high hydrogen sulfide always occurs in oil cracked gas.So nitrogen and hydrogen sulfide abundance combed with hydrocarbon gas isotope composition can be important indicators to distinguish between kerogen pyrolysis gas and oil cracked gas.The above cognitions are coincided with geological reality of the gas field in Sichuan Basin and Tarim Basin.The synthesized analysis on hydrocarbons and non-hydrocarbons of natural gas obtains a new method to effectively distinguish kerogen pyrolysis gas from oil cracked gas.