The porosity cutoff is critical porosity for a required permeability which is commonly obtained from a porosity-permeability plot. It also indicates the sedimentary and diagenetic factors characters, such as reservoir rocks structure(the content of matrix or mud, particle size and sorting), composition of authigenic mineral (such as the content of authigenic minerals as the pore-lining presence), dolomite content and reservoir pore structure (interparticle, intercrystalline, intraparticle, moldic, macropores, mesopores and micropores, etc.) and so on. The porosity cutoff of different oil layers of the Triassic Yanchang Formation in eastern Gansu of Ordos Basin changes between 7% to 12% at 0. 1×10-3 μm2 permeability, which is mainly controlled by the meteoric action and secondary porosity formation in the interval of Indosinian exposure. If 0. 1μ 10-3 μm2 permeability is used as the lower limit of sandstone, that of the corresponding porosity changes generally between 7%~12%, which will obviously affect the calculation of hydrocarbon reserves of different oil formations. The authigenic minerals mainly presenting in the form of pore-lining cement can protect porosity space of buried sandstones, but the increasing content of these cements will lead to porosity cutoff increasing, the reservoir quality is worse than that with similar porosity but lack of pore-lining cement. Porosity cutoffs show significant variability among the different pore types of carbonate reservoirs, ranging from 5.4% to 31.3% at 1 μ 10-3 μm2 permeability. The carbonate reservoir mainly consist of intercrystalline and intraparticle pores has minimum porosity cutoff and better reservoir quality. The carbonate reservoir mainly consists of mudstone micropores has maximum porosity cutoff and worst reservoir quality. The carbonate reservoir mainly consists of intraparticle and moldic has medium porosity cutoff and reservoir quality.%截止孔隙度可从储层孔隙度-渗透率关系曲线获取,是要获得某一特征渗透率所需要的孔隙度临界值,也是储集岩的岩石结构(杂基或灰泥的含量、碎屑的粒度和分选性)、自生矿物构成(如以孔隙衬里方式存在的自生矿物的数量)、白云石的含量和储层孔隙构成(粒间孔、晶同孔、粒内孔、铸模孔、大孔、中孔和微孔等)等各种沉积、成岩因素的表征.鄂尔多斯盆地陇东地区三叠系延长组不同油层组储层与0.1×10-3 μm2渗透率对应的截止孔隙度大致变化在7％～12％之间,主要受印支期暴露时间间隔中大气水作用与次生孔隙形成的控制,如果以0.1×10-3μm2渗透率作为渗砂岩下限,相应孔隙度的下限也大致变化在7％～12％之同,这显然会影响不同油层组烃类的储量计算；以孔隙衬里方式存在的自生矿物对埋藏条件下储集空间具有保护作用,但这些胶结物数量的增加会导致储层截止孔隙度增加,储层质量比具有类似孔隙度但缺乏孔隙衬里胶结物的储层差；由不同类型孔隙构成的碳酸盐岩储层1×10-3μm2渗透率对应的截止孔隙度变化在5.4％～31.3％之间,粒间孔、晶间孔为主的碳酸盐岩截止孔隙度较小,储层质量较好,灰泥岩微孔为主的碳酸盐岩的截止孔隙度最大,储层质量最差,粒内孔和铸模孔为主的碳酸盐岩具有中等的截止孔隙度、中等的储层质量.
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