方正断陷不同成因天然气地球化学特征及成藏模式

Geochemical characteristics and accumulation models of different genetic types of natural gas in the Fangzheng Fault Depression

方正断陷天然气成因类型复杂,查明气源对其天然气勘探具有重要意义。根据天然气组分及烷烃碳同位素特征,结合成气地质背景,对天然气的成因类型进行了分析。通过不同构造单元的烃源岩有机质类型及热演化程度差异,对比天然气热成熟度并进行气源探讨。结果表明:方正断陷天然气可以分为3类。A类天然气为干气,成因类型为催化过渡带气,为新安村组-乌云组湖相泥岩所生;B类天然气为湿气,成因类型为热成因煤型气,与腐殖型有机质生成的原油相伴生,为煤系泥岩所生;C类天然气亦为湿气,成因类型以热成因油型气为主,具有一定的腐殖型有机质生气贡献,与腐殖型有机质生成的原油相伴生,为混源气。伊汉通断裂两侧构造的差异活动是造成东、西两个次凹天然气地球化学特征及成藏模式有明显差异的最主要因素。

Because of the complicated genetic types of gas,accurate identification of the sources of the nature gas has great significance to gas exploration in the Fangzheng Fault Depression. First,the genetic types of natural gas were classified based on the gas components,alkane carbon isotopes and in combination with the actual geological conditions. Then,the thermal maturity of gas was correlated with source rocks in different tectonic units to reveal the gas sources based on identification of organic types and thermal evolution degree. The results show that the gas of this area can be divided into three types. Type A is dry gas and its genetic type is catalytic transitional zone gas generated by lacustrine mudstone in the Xin＇ancun-Wuyun Formations. Type B is wet gas and its genetic type is thermal coal-related gas generated by coal measure mudstones. Type C is also wet gas and its genetic type is petroliferous gas generated partly by humic organic matter. It is associated with the oil generated by humic organic matters and is of hybrid sources. Differential structural activity on two sides of Yihantong fault is the main factor leading to the different geochemical characteristics and accumulation patterns of natural gas between the east and west sags.