鄂尔多斯盆地吴起-安塞地区延长组下部砂岩储层中的铸模孔特征与成因

The Characteristics and Genesis of Moldic Pores in Sandstone Reservoir of the Lower Part of Yanchang Formation in Wuqi-Ansai Area,Ordos Basin

吴起-安塞地区的延长组下组合包括长8、长9和长10共3个层段,普遍以低-特低孔、特-超低渗为主。通过铸体薄片镜下观察、压汞测试、微米CT扫描、包裹体测温、荧光等方法进行分析,发现储层孔隙类型主要有次生的粒间孔、长石粒内孔、粘土矿物晶内孔、颗粒的铸模孔、浊沸石溶孔等。随着储层孔隙度减小、渗透率降低,长石粒内孔和铸模孔所占比例上升,粒间孔比例下降。以铸模孔为主的储层孔隙多呈粗歪度,但不发育较大的粒间孔隙,孔隙喉道少,配位数低,空间形态上呈孤立状,孔壁光滑或偶见极少石英微晶紧贴孔壁生长,内部具有荧光显示。成岩作用分析表明,铸模孔形成于砂岩强烈压实之后和储层致密化之前,随着烃源岩成熟并排出酸性流体,砂岩储层中的颗粒和杂基会逐渐被溶蚀形成孔隙,部分长石颗粒甚至被完全溶蚀成铸模孔。而后在碳酸盐胶结物充填孔隙发生致密化过程中,位于流体运移主通道边缘上的部分粒内孔、铸模孔因被少量油气充填,而在储层致密化过程中保留下来,其余孔隙和喉道大部分被碳酸盐矿物胶结,因此形态孤立。铸模孔的大量出现是储层先成藏后致密的典型标志。

The lower part of yanchang formation in Wuqi-Ansai area consists of three layers of Chang 8,Chang 9 and Chang 10,which are generally featured by low to extra-low porosity and extra-low to ultra-low permeability.Microscopic observation of the casting thin sections in the lower part of research region shows that pore types of the reservoirs mainly include secondary interparticle pores,intraparticle pores of feldspar,intercrystalline pores of clay,moldic pores,laumonite dissolution pores and so on.With the decreasing porosity and permeability of reservoirs,the proportion of intraparticle pores in feldspar and moldic pores keep growing,but the proportion of interparticle pores decreases.The pores of the reservoirs which are dominated by moldic pores are mostly coarse skewness and well-sorted with few pore-throats and low coordination numbers.These pores are isolated in space and rarely connected by throats.It can be seen that quartz microcrystals closely attached to the pore walls and grew along.Diagenesis analysis indicates that moldic pores are formed after sandstone intensely compacted and before the reservoirs densified.As the hydrocarbon source rock matures and discharges acid fluids,the grains and matrix in the sandstone reservoirs are gradually dissolved to form pores,and some of the feldspar particles are even completely eroded intomoldic pores.In the following densification process of cements filling pores,the moldic pores located on hydrocarbon migration pathways will be filled by little oil and gas,so they can be retained after densification.This kind of pores are morphologically isolated because of the cementation of most throats.