东营凹陷古近系天然气成藏条件及其主控因素

CONDITIONS AND MAIN CONTROLLING FACTORS OF NATURAL GAS POOLING IN THE PALEOGENE IN DONGYING SAG

东营凹陷古近系深层裂解气勘探呈现出良好的态势,但目前对其成藏条件及其主控因素认识不清,因而制约了该区深层天然气的进一步勘探。对研究区大量的地球化学生烃指标、天然气组成指标及地层压力指标等进行了综合分析研究,结果认为东营凹陷古近系深层拥有古近系渐新统沙河街组沙三下亚段、沙四段以及古近系古新统—始新统孔店组二段共计3套烃源岩层系,形成复合天然气生烃系统,深层裂解气形成于新近系上新统明化镇组中晚期,属于晚期成藏。由于沙四段中部大套膏盐岩的分割,古近系深层超压体系具有盐上、盐下两个次一级超压"封存箱"系统,相应形成盐上、盐下两套天然气成藏系统。各类扇体及次生孔隙是该区深层气主要的储集体及储集空间。该区沙三段中、下部沉积的深湖相灰色泥岩、油页岩,沙三段中上部的中厚层暗色泥岩夹钙质泥岩,以及沙四段中部的大套膏盐层构成了多套优质区域盖层,与弱的后期构造运动相互配置,形成良好的裂解气保存条件。古近系深层发育断层气藏、构造—岩性气藏、各类沉积扇体形成的岩性气藏,其中后者应为该区近期深层天然气勘探的主要目标。

Cracked gas exploration in the Paleogene in deep of Dongying sag is highly prospective,but current limited understandings about the controlling factors and conditions of gas pooling constrained further exploration of gas in deep of the sag.Analyses of the geochemical indexes of hydrocarbon-generation,gas compositions and formational pressure reveal a composite hydrocarbon generation system that is composed of three sets of source rocks in the Paleogene in deep of Dongying sag,including the lower third member,the fourth member of the Shahejie Formation and the second member of the Kongdian Formation.The deep cracked gas was generated in the middle and late stages of deposition of the Minghuazhen Formation and is characterized by late accumulation.The thick evaporates in the middle of the fourth member of the Shahejie Formation compartmentalizes the overpressure system in the Paleogene into a pre-and a post-salt compartment,resulting in a pre-and a post-salt gas system correspondingly.The reservoirs are composed mainly of various fans,while the pore space is dominated by secondary pores.The grey mudstone and oil shale in the middle and lower intervals of the third member of the Shahejie Formation,the medium-to-thick dark mudstone with calcareous mudstone interbeds in the upper-to-middle Shahejie Formation,and the thick evaporates in the middle of the fourth member of the Shahejie Formation are regional seals of high quality.These seals in combination with the weak late tectonic movement created favorable conditions for preservation of the cracked gas.Fault block gas reservoirs,structural-lithologic gas reservoirs and lithologic gas reservoirs are recognized in the Paleogene and the lithologic gas reservoirs are the major targets of deep gas exploration in the near future.