鄂尔多斯盆地永利探区上古生界天然气地球化学特征及气源探讨

Geochemical features and origins of natural gas in the Upper Paleozoic in the Yongli area,Ordos Basin

永利探区位于鄂尔多斯盆地西缘逆冲断带的中部,是一个天然气富集区,但目前对该区烃类气体的地球化学特征和主要来源尚不清楚。为此,通过综合剖析天然气样品的组分和碳同位素特征来确定其天然气地球化学特征、成因及气源。研究结果表明:①该区上古生界天然气具有高甲烷含量、低非烃气体含量的特点;②气藏的重烃含量不高,干燥系数较高,气藏总体上偏干,中二叠统下石盒子组天然气曾经经历较明显的分馏作用;③该区天然气接近于腐殖型有机质形成的天然气(煤成气),成熟度较低,为原生型有机成因气;④气源一方面来自于该区下部太原组和羊虎沟组低成熟—成熟阶段的、以腐殖型干酪根为主的母质烃源岩生成的天然气,另一方面也有可能接受东侧天环向斜早期的侧向供烃。

The Yongli block is located in the center of the thrust zone along the western margin of the Ordos Basin.Although natural gas is abundant in this block,the geochemical features and the origins of hydrocarbon gas are still unclear at present.In view of this,the components and carbon isotopes of gas samples are analyzed to determine geochemical features,origins and source rocks of natural gas in this block.The following conclusions are drawn.(1) Gas in the Upper Paleozoic is characterized by a high content of methane but a low content of non-hydrocarbon gases.(2) The content of heavy hydrocarbons is low and dry coefficient is relatively high,indicating that the gas reservoirs are relatively dry as a whole.Obvious fractionation must have been encountered by the gas in the Lower Shihezi Formation of the Middle Permian.(3) The gas in this block is similar to gas generated by humic organic matters(coal-derived gas) with relatively low maturity,belonging to primary gas of organic origin.(4) The gas was mainly sourced from the low-mature and mature source rocks dominated by humic kerogens in the Taiyuan and Yanghugou formations in the deep study area,with some gas possibly sourced from the Early Tianhuan syncline in the east.