鄂尔多斯东部下二叠统山西组2段储层评价及勘探前景

Reservoir evaluation and exploration prospect of the Member 2 of Shanxi Formation of Lower Permian in eastern Ordos

针对鄂尔多斯东部下二叠统山西组2段低渗透油气藏的特点,通过高分辨率层序地层的划分对比,将研究区山2段划分为3个长期旋回。结合425块薄片观察、扫描电镜、阴极发光等储层微观特征分析研究,认为山2段是一套在岸线背景下具有强侵蚀作用的远源砂质辫状河沉积,受物源分异和水动力条件控制,榆林、神木、府谷3条河流的沉积储集特征差异明显,榆林河河道砂体为一套具有石英含量高、杂基和软岩屑含量较低、几乎不含长石的石英砂岩,储层的储集空间主要是以粒间孔为主的复合型孔隙,具有良好的储集性;神木和府谷河河道砂体与之相反,为石英含量低、杂基和软岩屑含量高、普遍含少量长石的岩屑石英砂岩,储层储集性差,仅见少量粒内溶孔。成岩演化分析认为山2段储层进入晚成岩B期,此期压实、压溶作用已非常强烈,榆林地区由于烃类的早期充注才使大量原生孔隙得以保存。通过对山2段储层沉积成岩演化分析,总结了石英砂岩与岩屑石英砂岩沉积储层特征的差异,预测出鄂尔多斯东部下步勘探方向即以榆林气田为代表的榆林河沉积区山2段高渗透石英砂岩发育的河道砂体。

The Member 2 of Shanxi Formation of Lower Permian in the eastern Ordos Basin is characterized by low permeability. Based on the analysis of high resolution sequence stratigraphic division and correlation, the Member 2 of Shanxi Formation in the study area can be divided into three long term cycles. The analyses of 425 thin sections observation, SEM, cathode luminescence, etc. indicate that the Member 2 was a set of sandy-braided deposits which was distant to the source of sediment and was developed in the strong erosion setting at the shoreline. Influenced by the differentiation of source of sediment and the hydrodynamic conditions, the sedimentary characteristics of the Yulin, Shenmu and Fugu Rivers were obviously different. The channel sandbody of the Yulin River was a set of quartz sandstones with a high content of quartz, a low content of matrix and soft lithic-fragment, and little feldspar. The reservoir was dominated by compound pore in which the inter-granular pore was mainly developed. Its reservoir was favorable. On the contrast, the channel sandbodies of the Shenmu and Fugu Rivers were lithic quartz sandstones with a low content of quartz, a high content of matrix and soft lithic-fragment, and a low content of feldspar generally. The reservoir quality was poor with a few amounts of intra-granular solution pores being developed. Diagenetic analyses indicate that the Member 2 of Shanxi Formation belongs to the late diagenetic B stage. At this stage, the compaction and pressure solution were very strong and a great deal of the primary pore in Yulin area was preserved due to the Member 2 reservoir of Shanxi Formation. The differences of sedimentary reservoir characters between quartz sandstone and lithic sandstone are generalized. The next exploration target in the study area is predicted, i.e. the channel sandbody in which high-permeable quartz sandstone is developed in the Member 2 of Shanxi Formation in the Yulin River sedimentary area, which is characterized by the Yulin Gasfield.