滨岸含砾砂岩层序地层、沉积相及对优质储层发育的控制作用——以塔里木盆地TZ2井区为例

Sequence stratigraphy and sedimentary facies of pebbled sandstones in coastal system and their controlling effect on the development of high quality reservoirs:Case study of Tazhong 2 well block,Tarim Basin

玛湖大油气区的重大发现掀起砂砾岩储层勘探新热潮,塔里木盆地东河砂岩“石英质砾”储层已获多个油气发现,其油气藏特征及分布规律已基本明朗,而“灰质砾”的成因以及对储层的影响目前还是研究空白区域。以塔里木盆地TZ2井区含砾砂岩段为例,探索从滨岸体系控制下的高精度层序地层学角度来展开研究。综合薄片、岩心、测井、地震等资料,对TZ2井区石炭系含砾砂岩段沉积环境及储层特征进行研究,进而探索灰质砾对储层的影响及储层在地层层序和沉积相序中的划分,总结层序沉积对有效储层展布的控制作用。研究表明:东河砂岩段底至生屑灰岩段顶构成一个具有海侵体系域—高位体系域旋回的Ⅲ级层序,并可进一步划分为4个Ⅳ级层序,在Ⅳ级层序里含砾砂岩段为高位体系域,主要为海侵作用以及奥陶系潜山剥蚀作用下形成的一套滨岸相沉积体,发育前滨亚相和临滨亚相,具有纵向上厚度变化大,横向上非均质性强的特点,其中前滨亚相是储层发育的有利相带。灰质砾在前滨亚相普遍分布,其母岩为奥陶系灰岩,来自于具有多期隆升特征的奥陶系潜山,灰质砾石含量越高,储层物性越差,是影响有效储层发育与否的关键因素。

Great discoveries of oil and gas in Mahu area have set off a new wave of exploration for glutenite reservoir. Several oil and gas discoveries have been made in the“quartz gravel”reservoir of Donghe sandstone,Tarim Basin. The characteristics and distribution of the reservoir are basically clear. But the origin of gray gravel and its influence on the reservoir is still a study blank area. This paper,taking the pebbled sandstone section in Tazhong 2 well block,Tarim Basin as an example,explores this issue by employing high precision sequence stratigraphy controlled by coastal system. Based on thin section,core,well logging,and seismic data,the sedimentary environment and reservoir characteristics of the Carboniferous pebbled sandstone section in Tazhong 2 well block were studied,and then the influence of gray gravel on the reservoir and the division of reservoir in stratigraphic sequence and sedimentary facies sequence are explored,and the controlling effect of sequence deposition on effective reservoir distribution is summarized. The results indicate that it is a third-order sequence from the bottom of Donghe sandstone section to the top of Crumbs limestone section,which contains a transgression system tract and a highstand system tract cycle assemblage. The third-order sequence concludes four four-order sequence,and the pebbled sandstone section belongs to highstand system tract,which is a set of shore facies deposits controlled by transgression and denudation of Ordovician buried hill. The pebbled sandstone section develops foreshore subfacies and shoreface subfacies with the characteristic of large thickness variation and strong transverse heterogeneity,in which the foreshore subfacies is the favorable facies zone for reservoir development.Gray gravel is widely distributed in foreshore subfacies,and its parent rock is Ordovician limestone,which comes from the Ordovician buried hill with multi-stage uplift characteristics. The higher the content of gray gravel,the worse the physical property of the reservoir,which is the key factor affecting the development of effective reservoir.