神木气田早二叠世太原期海侵事件与砂体展布的关系

Relationship between Early Permian Taiyuan transgression and sandbodies distribution in the Shenmu Gas Field,Ordos Basin

下二叠统太原组是鄂尔多斯盆地神木气田的主力开发层系,其砂体厚度大、多期叠置且各期砂岩体展布特征不明。分析认为,该区太原期海侵事件直接影响了砂岩体的展布,研究二者的关系有利于寻找有利开发层段。为此,根据标志性地层、岩性突变界面等层序界面识别的关键地质信息,以太2—太1段发育的4套石灰岩为二级层序界面,建立了太原期层序地层划分方案;在早二叠世海侵期次和方向研究的基础上,刻画出太原组层序地层格架内3期砂岩体(七里沟砂岩、马兰砂岩和桥头砂岩)的展布特征。分析认为:①太原期发生4次海侵事件,海侵方向为东南向,相应发育的4套石灰岩具有分带性且自北向南发育程度渐好;②海侵范围和期次直接影响到砂岩体的分布范围和期次,在海进和北部地区抬升及物源供给变化的控制下,三角洲平原砂岩体的分布范围逐渐缩小。最后,结合实钻数据,指出最有利开发层段为马兰砂岩段,其砂岩沉积发生在太原期波及范围最大的两次海侵事件之间,受海侵影响,沉积的砂岩颗粒受海水的淘洗作用而岩性更纯,物性也优于其他两套砂岩体。

The Lower Permian Taiyuan Fm is the major pay zone in the Shenmu Gas Field, Ordos Basin. The sandbodies are thick and superimposed, but the distributions of sandbodies formed in each stage are poorly defined. It is believed that the transgression inthe Taiyuan period in the study area directly influenced the distribution of sandbodies, thus establishing such relationship is helpful to the identification of favorable intervals for gas development. A sequence stratigraphic classification scheme was established for theTaiyuan Formation by taking the four limestone layers in the 1st and 2nd members of the Taiyuan Formation as secondary sequence boundaries. Based on the study of the stages and directions of transgression in the Early Permian, a map was drawn of the distribution of sandbodies formed in 3 stages （Qiligou, Malan and Qiaotou sandstones） in the sequence stratigraphic framework of the Taiyuan Formation. The following findings were obtained, a. Four times of transgression in southeast direction occurred in theTaiyuan period, and the corresponding four sets of limestones show clear zonation and get more highly developed from the north to the south, b. The scale and stage of transgression directly influenced the distribution area and stage of sandbodies; the distributionarea of sandbodies of delta plain facies was getting progressively smaller under the control of transgression, northern uplifting and the change of provenance. Finally, according to the actual drilling data, it was pointed out that the Malan sandbody is the most favorabletarget interval of development, which was deposited at a time between the two mostly extensive transgression events in the Taiyuan periods due to the impact of transgression, the deposited sand particles are so pure through sea water washing that its reservoir quality is much better than the other two sandbodies..