耿湾—史家湾地区长6段微量元素地球化学特征及古盐度分析

Paleosalinity analysis and trace element geochemistry of Chang 6 member in Gengwan-Shijiawan area,Ordos Basin

应用B、Sr、Rb、K+Na质量分数和Sr/Ba、Rb/K比值等微量元素地球化学方法,结合粘土矿物X衍射分析结果,对鄂尔多斯盆地耿湾—史家湾地区长6段古盐度进行综合研究,认为长6期湖水介质古盐度为0.014‰～6.47‰,平均值为2.56‰,属于淡水—微咸水环境,局部为半咸水环境,且自长63时期至长61时期湖盆水体是逐渐变咸的。湖水介质古盐度研究有2个重要的意义:①在长6古盐度定量计算基础上,编制了古盐度等值线图,从中划分出淡水、微咸水和半咸水3个古盐度平面分区,确定0.5‰等值线是淡水河流与微咸水湖盆分界的古湖岸线位置;②鄂尔多斯盆地长6期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。长6段古盐度研究为岩性地层油藏有利区带预测提供了重要依据。

The trace element geochemistry methods such as B, Sr, Rb and K＋Na mass fraction, Sr/Ba ratio, Rb/K ratio combining with clay mineral X-ray diffraction analysis are applied to analyze the paleosalinity of Chang 6 in Gengwan-Shijiawan area. The result shows that in the period of Chang 6 stage, it is fresh-mildly brackish environment, partly brackish environment with the paleosalinity from 0.014‰ to 6.47‰, average of 2.56‰, and the water is saltier gradually from the period of Chang 63 tO Chang 61. The paleosalinity isolines are mapped after recovering the paleosalinity quantitatively, in which three paleosalinity zones are divided, including fresh water zone, mildly brackish water zone and brackish water zone. And 0.5‰ isoline is determined to be the position of lake shoreline which is the boundary of fresh water river and mildly brackish lake. The mildly brackish water and close environment are favorable for the development of the high quality source rocks, and could promote the forming of the early chlorite rim cementation to retard the compaction and dissolution which is helpful for preserving primary intergranular pores. So it provides important evidence for the favorable target prediction of lithostratigraphic reservoirs.