痕迹化石在识别东濮凹陷古近纪河口湾环境和下切谷中的作用

THE ROLE OF TRACE FOSSILS IN DETECTING THE PALEOGENE ESTUARY ENVIRONMENT AND INCISED VALLEY OF DONGPU DEPRESSION, EAST CHINA

河口湾砂岩储层,在下切谷型油气勘探类型中极具潜在经济价值,因而引起众多沉积环境研究者的关注。河口湾是河水、海水相互作用的场所,具明显的三重沉积结构,即在河口湾内侧,河流淡水沉积具主导;在河口湾外侧,则以潮汐或波浪等海水水流沉积占优;而河口湾的中央地带,是海水和淡水汇合区,表现出咸水水体和低能环境的沉积特征。东濮凹陷沙河街组四段,低位期形成的下切谷和高位期转变成河口湾的过程,由沉积柱子中的海水生物痕迹、咸水生物痕迹及淡水生物痕迹记录下来,这些适应于不同水体的造迹生物所留下的痕迹,弥补了这一地区缺乏标志性实体化石带来的认识障碍,为识别其沉积环境真面目找到了新途径。由Cylindricum-Scoye-nia为代表的淡水痕迹化石组合,以Tigillites和喜盐植物根迹为代表的咸水生物痕迹和以Thalassinoides,Teichichnus为代表的海水生物痕迹,在东濮凹陷沙河街组四段钻取的岩芯中多次出现,证实了那里河口湾沉积的三重结构和下切谷的存在。

Paleogene deposits in Bohai Basin have long been considered of terrestrial origin and the reservoir depositional environments have thus been treated as fluvial and lacustrine related facies. However, evidence of marginal marine trace fossils from Dongpu Depression, the southmost of the Bohai rifted basin, shows that marine water incursion might have happened several times and a Paleogene seaway might have been existed correspondingly during the highstand periods; in lowstand periods, the original very shallow marine sedimentary setting became lakes in the structurally low area and eroded rivers scattered in palaeovalley districts which have developed from incised drainage. Trace fossil records in fresh water, salt water and marine water deposits related to an estuarine environment are found for the first time in this region. The importance of ichnological records associated with sedimentary evidences are strongly emphasized here in terms of prospecting a seaway related new type reservoir.