东濮凹陷古近系沙河街组沙三段沉积期湖岸线的变化及岩相古地理特征

Shifting of lake shoreline and lithofacies palaeogeographic characters during sedimentary period of the Member 3 of Shahejie Formation of Paleogene in Dongpu Sag

东濮凹陷沙三段地层为深水相暗色泥岩与砂岩频繁互层及深水相暗色泥岩与盐膏岩频繁互层,这种砂体是东濮凹陷很重要的含油气储层。过去这种砂体往往按照静态的模式被解释为重力流成因,随着勘探的不断深入,越来越多的证据表明这些砂体中存在着牵引流和暴露地表的标志,其成因和沉积模式需要重新认识。东濮凹陷沙三段沉积时期,湖平面变化频繁,有长周期,也有短周期。根据录井、电性及岩心等资料识别出湖平面变化的5级周期,其变化频率约为10 0 0次/Ma。根据计算,东濮凹陷沙三段沉积时期湖水最大深度为30m。高水位沉积期,湖岸线距盆地中心较远,盆地沉积中心发育暗色泥岩夹薄层碳酸盐岩,盆地边缘发育粗碎屑岩沉积;水位下降期,边缘相砂体不断向盆地中心进积;低水位沉积期湖岸线向盆地中心退缩了10～30km ,盆地沉积中心发育盐膏岩沉积,盐膏岩的周围发育低位域三角洲、扇三角洲和滨湖滩坝相的粗碎屑岩体。正是由于湖平面变化引起的湖岸线迁移,导致高水位和低水位沉积期湖盆的岩相古地理面貌发生了很大的变化,根据这种动态的模式来预测砂体的分布,在指导油气勘探中产生了明显的效果。

In the Dongpu Sag,the sediments of the Member 3 of Shahejie Formation con sisted of dark-colored mudstone of deep water facies interlayered with sandston e and halite-gypsum,and this kind of sandstone is the important oil-bear ing reser voir in the Dongpu Sag and its origin was explained to be deep water gravity flo w s ediments according to the static sedimentary model. During the process of explo ration,more and more evidence is found to indicate that there are traction flow and exposure-to-air markers in the sandstone. So,it is necessary to reunders t and its origin and sedimentary models.In the Dongpu Sag,during the sedimentatio n of Member 3 of Shahejie Formation,the lake 1evel fluctuated frequently with lo ng-scale cycle and short-scale cycle.According to the data of well log,respon se of r esistivity and core,cycles of 5 scales were recognized,and the frequency of th e fifth scale cycle is 1 000 times/Ma.The minimum extent of lake level fluc tuati on is calculated according to the thickness of single halite bed or single gypsu m bed,and the maximum water depth is 30 m.Combining with the palaeotopogra phy character,the reason of large transference of lake shoreline in landscape orientation is explaine d. On the basis of the study,a dynamic sedimentary model is established,and it is revealed in this model,that the delta and beach sandbody were deposited alon g the shor eline and black mud with thin layer carbonate rock was deposited in the sediment ation center during the highstand period,and during the lowstand period,the shor eli ne shifted about 10～30 km toward the basin center.The sandbody early deposite d along the shoreline was eroded and was prograded toward the sedimentation cent er of the basin.Because of the fluctuation of lake level and the shifting of la ke s horeline,the palaeogeography of highstand period and that of lowstand period are very differe nt.According to this dynamic sedimentary model to predict the distribution of s andbody,there will be more clear effects in petroleum exploration.