陕北地区奥陶纪马家沟组五段沉积时期岩相古地理演化

Lithofacies palaeogeography evolution of the Member 5 of Ordovician Majiagou sedimentary stage in northern Shaanxi Province

陕北地区马家沟组五段沉积时期(简称马五期)沉积的碳酸盐岩地层是重要的天然气储集层。为明确各沉积期岩相古地理演化特征,在对周缘野外地质剖面考察、研究区内98口井的岩心观察、测井曲线形态分析的基础上,对陕北地区马家沟组马五10—马五1各亚段的岩相古地理演化特征进行了恢复。结果表明,马五10—马五1各亚段以海相碳酸盐岩台地沉积为主,包括局限台地、开阔台地和蒸发台地3种亚相,并进一步细分为:云坪、膏云坪、含膏云坪、泥云坪、灰云坪、灰坪、颗粒滩、云灰坪、泥灰坪、膏盐湖等10种沉积微相;从马五10到马五1岩相古地理单元整体表现为自东向西的环带状分布特征,环带中心主要位于延长、延川和清涧一带,值得注意的是,马五2沉积时期由于受南部较强构造抬升作用影响,古地理单元环带中心呈现出向西北挪移的特征,环带中心位于靖边、子长一带;马家沟组各沉积时期的旋回性环带沉积为储集层叠合发育奠定了物质基础,云坪、膏云坪、含膏云坪、颗粒滩微相易于形成有利储集空间,纵向上的马五1+2、马五4、马五5、马五7、马五9亚段为该类相带较发育的层位,是区域勘探开发的重点领域。

The carbonates of the Member 5 of Majiagou Formation in the northern Shaanxi Province are important natural gas reservoir.In order to clarify the lithofacies palaeogeography evolution of each sedimentary period of Majiagou Formation in the study area,the lithofacies palaeogeography characteristics of M510-M51 submembers were restored based on the investigation of surrounding field outcrops,core observation of 98 wells,and well logging curve analyses.The results show that the marine carbonate platform is the major sedimentary microfacies in each layer of M510-M51,including three subfacies:restricted platform,open platform and evaporate platform,and further subdivided into 10 sedimentary microfacies:dolomite flat,gypsum-dolomite flat,gypsum-bearing dolomite flat,muddy dolomite flat,carbonate-dolomite flat,carbonate flat,grain beach,dolomite-carbonate flat,muddy carbonate flat and gypsum salt lake.The palaeogeography unit of M510-M51 is distributed in a ring belt shape from east to west with the central part located in Yanchang,Yanchuan and Qingjian.It is worth noting that the center of palaeogeography unit ring belt of M52 submember move northwestward to Jingbian and Zichang due to strong tectonic uplift in the south occurred in that period.Dolomite flat,gypsum-dolomite flat,gypsum-bearing dolomite flat,and grain beach microfacies are favorable for the formation of reservoir space.In vertical,M51+2,M54,M55,M57 and M59 submembers are relatively rich in these microfacies,which are the focused areas of regional hydrocarbon exploration and development.