鄂尔多斯盆地乌审旗气田中二叠统下石盒子组盒8段下亚段滩坝砂体沉积特征

鄂尔多斯盆地乌审旗气田中二叠统下石盒子组盒8段下亚段中下部岩石为石英砂岩类砂岩，即“骆驼脖砂岩”，上部岩石为暗色粉砂质泥岩或碳质泥岩。“骆驼脖砂岩”广布于华北地台，在鄂尔多斯盆地呈“满盆砂”现象，是盆地的主要储层之一。以往学者多认为盒8段下亚段是河流相或三角洲相沉积。通过对众多取心井盒8段下亚段岩心的观察，结合对砂岩岩性、剖面结构、沉积构造、测井曲线特征及砂体空间展布规律的综合研究，提出了盒8段下亚段中下部是滩坝相沉积，上部是沼泽相或沼泽相为主夹小河道沉积。盒8段下亚段可发育一段或两段滩坝相石英砂岩类砂岩，其成分和结构成熟度较高，两段砂岩间夹若干米半深湖相的暗色粉砂质泥岩或沼泽相的碳质泥岩。发育＜10°的冲洗交错层理、互成180°相交的板状和楔状层系组，具丘状和洼状交错层理的风暴岩。自然伽马曲线呈箱形或带齿的箱形。众多厚度较大的砂岩组成的若干列近东西向展布的长条透镜体状砂体，大致平行于古岸线。

Sedimentary characteristics of beach and bar sandbodies in the lower submember of Member 8 of Xiashihezi Formation of Middle Permian in Wushenqi Gasfield, Ordos Basin

Quartz sandstone, namely the “Luotuobo Sandstone” dominated the lower middle parts of the lower submember of Member 8 of the Middle Permian Xiashihezi Formation in the Wushenqi Gasfield in the Ordos Basin, while its upper part is characterized by dark color silty mudstone or carbonaceous mudstone. The “Luotuobo Sandstone” is widely distributed in the North China Platform, is developed in the whole Ordos Basin, and is one of the main reservoirs of the basin. Previous researchers considered that the lower submember consists of fluvial or delta facies. By core observations on the lower submember in many wells, it is proved that the lower middle part of the lower submember consists of beach and bar facies, while the upper part is marsh facies or is dominated by marsh facies intercalated with channel deposits. The composition and texture maturity of the quartz sandstones in the lower submember is high. One sandstone interval or two sandstone intervals were developed in the lower submember. The two sandstone intervals were intercalated with several meters dark color silty mudstones of semi deep lacustrine facies or carbonaceous mudstones of marsh facies.  Sedimentary structures such as flush cross bedding with angle lower than 10°， hummocy cross bedding were developed. The GR curves are box type or tooth like box type. Many thick bedded sandstones make up east west oriented lanticular sandbodies and they approximately parallel to the paleo shoreline.