砂岩孔隙成因对孔隙度/渗透率关系的控制作用--以鄂尔多斯盆地陇东地区三叠系延长组为例

鄂尔多斯盆地陇东地区三叠系延长组砂岩储层分布于印支期不整合面之下,砂岩次生孔隙的成因与印支期暴露时间间隔中大气淡水的溶解作用有关,因而越靠近不整合面的上部油层组,砂岩物性越好,同时储集空间中次生孔隙的比例也越大.孔隙的这种成因模式和构成方式直接影响了砂岩的孔隙度/渗透率关系,越靠近不整合面的上部油层组,要获得相同的渗透率所需要的孔隙度越大,砂岩的孔隙度/渗透率关系越差.基于这种原理,在考虑陇东地区三叠系储层砂岩的孔隙度下限时(如在定义渗砂岩时或在进行与孔隙度下限有关的储量计算时),不同油层组应有不同的标准,假定以0.1×10-3 μm2 的渗透率作为标准,对陇东地区而言,长1-长2油层组的孔隙度下限可能应在11%左右,而长8油层组可能只要8%左右就足够了.

Control of origin of pores over relationship of porosity to permeability in sandstone reservoir:A case study from Yanchang sandstone of Triassic of eastern Gansu, Ordos Basin

Yanchang sandstone in eastern Gansu, Ordos Basin lies beneath the unconformity at the base of the Jurassic section. The origin of secondary pores is related to the leaching by meteoric water during the time of subaerial exposure. The reservoirs in the upper member of Yanchang Formation near the unconformity, such as Chang 1 and Chang 2, have higher porosity and permeability, but also have a larger proportion of secondary pores. The pattern of origin and constitution of pores directly influences the relationship of porosity to permeability. To get a same permeability, the upper reservoirs near the unconformity should pay higher porosity, while the relationship of porosity to permeability is worse. Based on the principle, when considering the limit of porosity for defining a permeable sandstone or for calculating reserves, we should have different porosity standards. For instance, for the members of Chang 1 and Chang 2 in eastern Gansu area, the limit of porosity should be around 11%, but for the member of Chang 8, 8% of the porosity is enough.