Keywords: 
• porosity type /
• quartz dissolution diagenesis /
• Biyang depression /

Abstract: 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.