断陷湖盆天然气成藏生-运-聚动态模拟——以渤海湾盆地辽河坳陷为例

Generation-migration-accumulation dynamic simulation of natural gas accumulation in rift lake basins: A case study on the Liaohe Depression in the Bohai Bay Basin

有效判识天然气来源和生-运-聚特征对天然气资源潜力评价,以及天然气勘探方向具有重要指导意义。为了解决断陷湖盆烃源岩非均质性强、天然气成因类型多样导致的气源判识和资源评价具有多解性的问题,以渤海湾盆地辽河坳陷欢喜岭地区为例,根据典型断陷湖盆有效烃源岩分类和天然气成因类型判识方法,开展了气源研究,并基于典型剖面烃源岩生气、运聚动态模拟,总结了天然气动态成藏模式,并指明下步天然气勘探方向。研究结果表明:①古近系沙河街组三段中-下亚段和沙河街组四段是主要烃源岩层,不同构造位置形成的烃源岩类型不同,包括腐殖型、腐泥型烃源岩,分别生成了油型气、煤型气;②形成的各类天然气在复杂断裂、砂体和不整合面构成的输导体系中运移、汇聚,形成混合成因天然气气藏;③欢喜岭地区具有2个运聚系统,系统Ⅰ为西部斜坡带-双台子构造带上的砂四上亚段运聚系统,为油型气近源成藏,其余区域为系统Ⅱ,为油型气和煤型气远距离运移成藏;④欢喜岭地区天然气藏为单凹陷、多源晚期成藏,气源岩空间差异分布和复杂的断裂系统共同控制了该区气藏成因类型多样性和分布复杂性。结论认为,断陷湖盆天然气成藏生-运-聚动态模拟明确了辽欢喜岭地区的天然气气源,指明了靠近洼陷内部的东部陡坡带为天然气有利勘探区,其成果也为其他断陷湖盆的天然气勘探提供了参考借鉴。

Effective identification of the source and generation-migration-accumulation characteristics of natural gas is of great guiding significance to evaluating the potential of natural gas resources and determining the direction of natural gas exploration.Rift lake basins are characterized by strong heterogeneity of source rocks and various genetic types of natural gas,which lead to ambiguity in gas source identification and resource evaluation.To solve this problem,this paper investigates the gas sources in the Huanxiling area of the Liaohe Depression,Bohai Bay Basin by the representative effective source rock classification and natural gas genesis type discrimination method for rift lake basins.Then,dynamic simulation of gas generation,migration and accumulation is carried out based on the representative profile,and a dynamic accumulation model of natural gas is summarized.Finally,the future direction of natural gas exploration is pointed out.And the following research results are obtained.First,the Lower-Middle Sub-member of the Third Member and the Fourth Member of Paleogene Shahejie Formation are the main source rock layers.The source rocks formed in different tectonic positions are different in types,and there are humic and sapropelic source rocks,which generate oil-type gas and coal-type gas respectively.Second,various types of natural gas formed in this area migrates and accumulates within a transport system composed of complex fractures,sand bodies and unconformity surfaces,to form mixed-genetic gas reservoirs.Third,there are two migration and accumulation systems in the Huanxiling area,i.e.,System I and II.The migration and accumulation system in the Upper Sub-member of the Fourth Member of Shahejie Formation from the western slope belt to Shuangtaizi structural belt is System I,which is near-source accumulation of oil-type gas.The migration and accumulation systems in the rest zones are System II,which is long-distance migration and accumulation of oil-type and coal-type gases.Fourth,natural gas reservoirs in the Huanxiling area are in the hydrocarbon accumulation model of single sag,multiple sources and late stage.The spatial differential distribution of source rocks and the complex fault system jointly control the genetic diversity and distribution complexity of gas reservoirs in this area.In conclusion,the generation-migration-accumulation dynamic model of natural gas reservoir in rift lake basins shows the gas sources of Huanxiling area,and suggests that the eastern steep slope zone near the inside of the sag is a favorable area for natural gas exploration.The research results provide a reference for natural gas exploration in other rift lake basins.