鄂尔多斯盆地耿湾地区长6段古盐度恢复与古环境分析

RECONSTRUCTION AND ANALYSIS OF PALEOSALANITY AND PALEOENVIRONMENT OF THE CHANG 6 MEMBER IN THE GENGWAN REGION,ORDOS BASIN

应用B，Sr，Rb，Sr／Ba比值，Rb／K比值和K＋Na质量分数等微量元素地球化学方法并结合粘土矿物X衍射分析对鄂尔多斯盆地耿湾地区长6古盐度进行综合判别，认为长6期古水介质盐度为0.48‰～4．43‰，平均值为1．87‰，属于淡水微咸水环境，局部为半咸水环境，且自长63沉积期至长61沉积期湖盆水体是逐渐变成的；微咸化的湖水介质和封闭还原的深水环境既有利于优质烃源岩的发育，又能促使砂体早期环边绿泥石胶结形成抗压实-压溶组构，有利于原生粒间孔的保存而对储层发育非常有利；根据长6古盐度定量计算确定0．5％古盐度等值线是淡水河流发育区与微咸水湖盆分界的古湖岸线位置，并划分出淡水、微咸水和半咸水3个古盐度平面分区，为预测岩性地层油藏有利发育区带提供了重要依据。

B,Sr,Rb,Sr/Ba ratio,Rb/K ratio,K＋Na mass fraction and clay mineral X-diffraction analysis were used to reconstruct the paleosalinity of the Chang 6 reservoir in the Gengwan region, Ordos Basin. The results showed that the paleosalinity of the Chang ranged from 0. 48‰ to 4.43‰, with an average value of 1.87‰,indicating a fresh-mildly brackish environment, partly brackish environment. The water became salter gradually from the period of Chang 63 to Chang 61. The mildly brackish and close environment was favorable for the development of high quality source rocks and for the formation of early rim chlorite cementation,which resisted the compaction and dissolution, preserved primary inter-granular pores and provided condition for the development of reservoir. The paleosalinity was contoured and three paleosalinity distribution areas,the fresh water area, mildly brackish water area and brackish water area, were classified. It was considered that the 0.5‰ contour line was the location of paleo lakeshore which was the boundary of fresh water rivers and mildly brackish lake. It was an important evidence for predicting favorable stratigraphic reservoir.