塔里木盆地古城地区下古生界碳酸盐岩储层硅化作用特征及对储层的影响

Characteristics of the silicification and its effect on the Lower Paleozoic carbonate reservoirs in Gucheng area,Tarim Basin

塔里木盆地古城地区下古生界碳酸盐岩储层中局部存在显著的硅化现象,硅质主要发育于寒武系,奥陶系蓬莱坝组、鹰山组三段—四段和一间房组。显微镜下岩石学特征表现为2种形式:以隐晶硅质交代碳酸盐岩并充填孔隙和以自生石英的方式充填孔洞。流体包裹体均一温度、主微量元素和硅氧同位素等实验分析测试表明:石英中的流体包裹体均一温度较高,介于121~206℃,主要集中在150~175℃之间;硅质具有高Fe、Mn,低Al的特征;硅质的δ^(30)Si值介于0.1‰~3.6‰,δ^(18)O值介于12.7‰~27.9‰,计算得到硅质交代流体的δ^(18)O值在8.23‰~12.65‰之间,与中—酸性岩浆水相当。综合分析认为古城地区下古生界碳酸盐岩中的硅质来源可能为中—酸性岩浆热液。早期交代成因的隐晶硅质对储层起破坏作用,而晚期自生石英往往代表酸性流体背景下的产物,酸性热液对储层起到了一定的建设作用。早期硅化作用主要发育于寒武系、奥陶系蓬莱坝组和鹰四段、鹰三段,而晚期硅化作用主要发育于寒武系和鹰三段。硅化作用对寒武系和鹰三段储层的改善作用相对更大。

The Lower Paleozoic carbonate rocks in Gucheng area of Tarim Basin are mainly developed in limited platform,semi limited platform,and open platform.There is significant silicification in some parts,which is mainly developed in the Cambrian,Ordovician Penglaiba Formation,Yingshan Member 3 and 4,and Yijianfang Formation.Based on thin sections,there are two forms of petrological features under the microscope:cryptocrystalline siliceous replacement and filling of pores,and authigenic quartz filling of pores.Experimental analyses including homogeneous temperature of fluid inclusions,major and trace elements,and silicon and oxygen isotope show that the homogenization temperature of fluid inclusions in quartz is high,ranging from 121℃to 206℃,mainly ranging from 150℃to 175℃;siliceous rock has the characteristics of high Fe,Mn and low Al;δ30Si of siliceous rock is 0.1‰-3.6‰,δ18O is 12.7‰-27.9‰,and the calculatedδ18O value of siliceous metasomatic fluid is 8.23‰-12.65‰,which is equivalent to mediumacid magmatic water.It is concluded that the silicon in the Lower Paleozoic carbonate rocks in Gucheng area may be derived from the medium-acid magmatic hydrothermal fluid.Most of the early cryptocrystalline siliceous materials filled pores or metasomatic protolith in a large area,which should be the products of high temperature and high salinity fluid.This silicification has a destructive effect on the development of reservoir space at this stage.The late authigenic quartz often represents the product of acidic fluid background.This hydrothermal solution can cause significant dissolution of carbonate minerals,forming dissolution pores and fractures.As the concentration of siliceous fluid decreases,the siliceous fluid begins to crystallize slowly and gradually into quartz in the areas with sufficient pore space.At this stage,silicification plays a certain role in reservoir construction.The early silicification mainly occurred in the Cambrian,Ordovician Penglaiba Formation,Yingshan Member 4 and Member 3,while the late silicification mainly occurred in the Cambrian and Yingshan Member 3 with reservoir improvement.