陕北榆林气田山2段高自然伽马储集层特征及其成因分析

The reservoir features and genetic analysis of high natural gamma for Member 2 of Shanxi Group in Yulin gas field of North Shanxi

针对榆林气田开发过程中,山2段主力产层段测井资料出现的高自然伽马电测显示,开展了山2段高自然伽马储集层特征及其成因分析,通过区域地质的宏观分析及对随钻录井中的岩心、岩屑等薄片资料的微观研究认为,高自然伽马显示是由同沉积火山凝灰岩及其成岩蚀变产物所引起;火山物质溶蚀产生的次生溶孔和蚀变高岭石晶间孔极大地改善了山2段储集层的储渗能力,是山2段储集层稳产、高产的关键因素;而密集分布的火山凝灰岩沉积薄层,岩性致密、物性差是山2段储集层非均质性强的主要影响因素,由于其单层厚度仅数毫米,最厚处只有5 cm,低于自然伽马测井仪的纵向分辨率,并且在自然伽马曲线形态上往往呈尖峰或弧状,易被看成裂缝或泥岩特征而忽视。因此,只有加强自然伽马测井仪的探测精度和现场录井资料的综合应用,才能使自然伽马储集层得到有效识别和精确解释。

In view of the situation that during Yulin gas field de- velopment,wireline logging data from the main pay interval of Member 2 of Shanxi Group shows high natural gamma, the paper makes analysis of the high natural gamma reser- voir features and their geneses for Member 2 of Shanxi Group.Through conducting macroanalysis of local geology and microscopic study of the data of core section and cut- tings from mud logging while drilling,the high natural gamma was considered to be caused by depositional ash tuff and diagenetic alteration product.The secondary solution hole and alteration marga porcellana intercrystlline hole, which were produced by vulcanian matter solution,greatly improved the ability of the reservoir storage and permeabili- ty for Member 2 of Shanxi Group,which was the key factor for the high production of the reservoirs for Member 2 of Shanxi Group.While the volcanic tuff sedimentary streaks in packed distribution because of the dense lithology and poor physical properties,are the main influence factors on the strong heterogeneity for Member 2 reservoirs of Shanxi Group.Due to only a few millimeters in monolayer thick- ness and only 5 cm in the maximum thickness,which are beyond the natural gamma logger longitudinal resolution are always shown in spike or bogen shape in natural gamma curves,and are easily neglected by considering as the fea- tures of fracture or shale feature.So,only increasing the accuracy of natural gamma logger detection and strengthe- ning in-situ mud logging data comprehensive application, can the natural gamma reservoirs be recognized effectively and interpreted accurately.