鄂尔多斯盆地南部二叠系山西组及下石盒子组盒8段沉积相

Sedimentary facies of the Shanxi Formation and Member 8 of Xiashihezi Formation of Permian in southern Ordos Basin

鄂尔多斯盆地南部指环县—延安—蒲县以南、西安以北地区,其山西组和下石盒子组盒8段是重要含气层。通过古水流方向、超稳定重矿物含量和长石含量分析以及古水系恢复,认为山西期和下石盒子期的鄂尔多斯盆地南部发育6个古水系,对应于6个长石含量分区和6个沉积相区。分别为石板沟—环县分区,平凉分区,淳化—耀县分区,韩城—澄城分区,大宁—Pul井分区和华池—永宁分区。在此基础上重点分析了沉积相分布规律,认为山2期发育小型三角洲沉积,主要分布在西南部平凉、南部淳化—耀县和韩城—澄城一带:山1期三角洲沉积分布范围比山2期明显扩大,北部延安—大宁发育三角洲前缘,湖泊相分布范围缩小。盒8期主要发育辫状河三角洲相,分布范围十分广泛,湖泊仅分布在镇原—环县—华池—正宁一带和大宁—Jil井一线。实践证明有效地区分不同的古水系,有利于沉积相带展布规律的预测。

The southern Ordos Basin refers to the areas south of Huanxian-Yan＇an-Puxian and north of Xi＇an. The Shanxi Formation and the Member 8 of Xiashihezi Formation are important gas-bearing intervals. Based on the analyses on the direction of palaeowater current, the content of super-stable heavy minerals and feldspars, it is concluded that six palaeowater systems were developed during the deposition of the Shanxi and Xiashihezi formations, which respectively corresponded to the six feldspar content subdivi- sions and six sedimentary facies. Six feldspar content subdivisions are Shibangou-Huanxian, Pingliang, Chunhua-Yaoxian, Hancheng-Chengcheng, Daning-Well Pul, Huachi-Yongning. The types and distribution of sedimentary facies are analysed. During the deposition of Member 2 of Shanxi Formation, smallscaled deltas were developed, which were mainly distributed in Pingliang region of the south western area, Chunhua-Yaoxian and Hancheng-Chengcheng region of the south area. During the deposition of Member 1 of Shanxi Formation, the distribution of the delta was larger than that during the deposition of Member 2. The delta front was developed in Yan＇an-Daning in the north area, while the lake size reduced. During the deposition of Member 8 of Xiashihezi Formation, the braided fluvial delta was developed and was widely distributed. Lacustrine facies was located in Zhenyuan-Huanxian-Huachi-Zhengning and Daning-Well Jil. It has been proved that the effective division of different water systems is favourable for the prediction of sedimentary facies belts distribution.