沾化凹陷孤西地区二氧化碳特征及成因

Analysis on characteristic and origin of carbon dioxide,Guxi area,Zhanhua depression

沾化凹陷孤西地区奥陶系和沙四段天然气丰富、性质异常、分布复杂,成因机制和成藏规律有待进一步查明。采用地质和测井方法,确定了孤西地区侵入岩的空间分布;通过化学分析结合岩心观察,对研究区天然气和烃源岩进行了研究。结果表明,孤西地区天然气中二氧化碳含量较高,为2.19%~14.19%,碳同位素很重,为-5.5‰~0.7‰,且均分布于侵入岩附近;同时一些烃源岩烘烤严重、导致成熟度偏高,镜质体反射率可达3.75%。由此可见,孤西地区二氧化碳属于无机成因,主要是地层中碳酸钙因岩浆高温烘烤分解而成。岩石化学成因二氧化碳分析表明,孤西地区烃源岩曾经历了异常高温热演化以及天然气成因和成藏的复杂过程。

Abundant gas exists in the Guxi area of Zhanhua depression with abnormal nature and complex distribution, and the origin and accumulation rule need to be ascertained. Firstly, we ascertained the spatial distribution of intrusive rocks by means of geological and logging method, moreover, the chemical analysis and core observation are applied to study the gas and hydrocarbon source rocks. It is revealed by the study that more CO2(2.19%-14.19% ) with very heavy δ13CO2(-5.5‰-+0.7‰) is found in the gas, especially the gas with higher content of CO2 and more heavy δ13CO2 that is distributed surrounding the intrusive rocks. Moreover, some hydrocarbon source rocks are baked seriously with high maturity (Ro = 3.76% ). Finally, we confirmed that the CO2 derived inorganically from the decomposition of calcium carbonate in the stratum by blazing intrusive rocks. The inorganic CO2 studied herein presented that the source rocks in Guxi area ever experienced an exceptional high temperature history, as well as complex gas occurrence and accumulation of hydrocarbon reservoir.