一种新的储层孔隙成因类型--石英溶解型次生孔隙

石英作为碎屑岩储层中的一种难溶组分,普遍认为它和次生孔隙的形成关系不十分密切。研究认为泌阳凹陷核桃园组储层中的碎屑石英颗粒存在明显的溶解现象,并形成以石英直接溶解型孔隙为主的储集空间特征。石英颗粒被溶解的部分在薄片中所占的范围为 2 %～ 7%者常见,高者达 8%以上,在总孔隙中所占的相对含量也多数在10 %～ 35 %之间,早成岩B期是其最主要形成期。石英溶解型次生孔隙的大量存在为碎屑岩储层中SiO₂ 胶结物及次生孔隙成因等问题的解释以及储层预测和评价提供了新的可能性。

A New Type of Secondary Porosity--Quartz Dissolution Porosity

As one of the most stable petrogenetic minerals in clastic reservoir, quartz isgenerally considered to be indissoluble to directly form secondary porosity during diagenesis, and the dissolved rim of quartz in reservoir has been thought to be the result of dissolution of cements that substituted quartz formerly. In this study, quartz was found to have been dissolved directly in the reservoir of Hetaoyuan formation in Biyang depression, and form a new genetic type of porosity, i. e. quartz dissolution porosity, which is one of the most important porosity types in reservoir in the studied area. The dissolution of quartz reaches 2%～ 7%, and sometimes up to more than 8 %of microscopic porosity in the reservoir rock,, which accounts for 10%～ 35 %of total porosity.Several lines of microscopic evidence for quartz dissolution are listed as follows:( 1)Partial dissolution of clastic quartz. Quartz was dissolved partially to form irregularly rim, isolated grains, intercrystalline pores, sometimes quartz grains can be dissolved partly, and even completely, and result in the for-mation of stretched pores. ( 2)Dissolution of quartz develops along contact seam, the opening of contact seams can form irregular pore throat, which couldnot be the result of dissolution of quartz substituting cements. ( 3)Partial dissolution of quartzose detritus, the erosion of the detritusis obvious, and sometimesit can form honeycombed mi-cropores. ( 4)Shape of carbonate components, both carbonate cements and debritus do not show much dissolution in reservoir rocks of the studied area, however, adjacent quartz grains were dissolved obviously, this further reveals that quartz was dissolved directly.Quartz dissolution in the studied area is the result of sedimentary environment, diagenesis, and evolution of formation water. Biyang depression was an arid to semi-arid subtropical enclosed fault-trough lake during deposi-tion, and the depositional environment of which was an alkaline lake. The original formation water of the reservoir was alkaline to strong alkaline. As a result, together with the influence of alkaline strata, formation water remains its alkalinity mostly during the stages of burial diagenesis. Under such environment, quartz was unstable and easily dissolved. However, early diagenetic stage B is the most important formation period of quartz dissolution and such genetic type of porosity.The existence of aboundant quartz dissolution pores provides new possibility for the interpretation of quartz dissolution itself, SiO ₂ cement, secondary porosity formation, as well as for reservoir prediction and evaluation in clastic reservoir.