低渗砂岩储层孔喉的分布特征及其差异性成因

摘要

Pore and pore-throat size types and distributions of two low permeability sandstone reservoirs (E1f3 reservoir in the Shanian area of North Jiangsu basin and J2x reservoir in the Niujuanhu area of Santanghu basin) were analyzed to understand their origins using a series of methods such as grain size analysis, thin slices, pore images, SEM, mercury-injection data. The results show that pore type in the E1 f3 reservoir is predominantly carbonate cement and feldspar dissolved pores and that in the J2x is mainly tuffaceous and feldspar dissolved pores, with the former containing more large dissolved pores and the latter containing more small intercrystal pores. The pore-throat size distributions in E1 f3 reservoir are wide unimodal for dissolution-origin pore-throats, thin unimodal for those with compaction or compaction- cementation, no peak for tube-shaped ore-throats, as well as bimodal for those with dissolution and compaction or cementation origins. The pore-throat size distributions of J2x reservoir are wide unimodal for that resulting from dissolution, thin unimodal for tube-shaped pore-throat, and bimodal for that resulting from compaction and cementation. No bimodal distribution pattern was observed in dissolution pore-throat or other pore-throats. Pore size and pore-throat radii in the two reservoirs tend to decrease with mean sizes decreasing, and mean pore radii are less than detritus radii. Under the condition of the same grain sizes, the pore and pore-throat sizes of J2 x reservoir are all less than those in the E1f3 reservoir. The higher maturity of E1f3 reservoir composition and structure, along with early carbonate cementation, preserves primary large intergranular pore-space during its compaction, which is enlarged during late-stage dissolution. The J2 x3 reservoir in the Niujuanhu area contains higher plastic detritus content and syn-sedimentary tuffaceous material, greatly decreasing primary intergranular pore-space during the compaction, and late-stage weak dissolution has no contribution to the reservoir. The higher plastic detritus content and filling to primary pores are responsible for difference of compaction and dissolution, and are the leading factors resulting in pore structural difference in two sandstone reservoirs.%应用筛析粒度、铸体薄片、图像孔隙、扫描电镜与高压压汞等方法,对苏北盆地沙埝地区E1f3和三塘湖盆地牛圈湖地区J2x两类不同低渗储层的孔喉类型和分布特征进行了分析对比,对其差异性成因进行了探讨.研究表明,E1f3储层孔隙类型主要为碳酸盐胶结物和长石溶孔,J2x储层主要为凝灰质和长石溶孔,前者溶蚀成因的粗大孔隙较多,而后者细小的晶间孔较多.E1f3储层喉道分布为溶蚀成因的粗单峰分布、压实或压实胶结混合成因的细单峰分布、管束状喉道的无峰分布,以及溶蚀成因与压实或压实胶结混合成因组成的双峰分布四类；J2x储层喉道为溶蚀成因的粗单峰分布、管束状喉道的细单峰分布,及管束状喉道与压实或压实胶结成因组成的双峰分布三类,未见溶蚀成因喉道与其他成因喉道组成的双峰分布.两区孔隙半径、喉道半径均随粒级变细呈减小趋势,平均孔隙半径也总是小于碎屑颗粒半径；在粒度相近的情况下,J2x储层孔隙、喉道半径均小于E1f3储层.E1f3储层较高的成分成熟度与结构成熟度和早期碳酸盐胶结作用使其在压实过程中保存了较大的原始粒间孔隙空间,且为后期溶蚀作用所扩大,牛圈湖地区J2x储层较高的塑性岩屑含量和同沉积期形成的凝灰质,使储层原始孔隙空间在压实过程中大量减少,后期溶蚀作用较弱.杂基对原始孔隙空间的充填程度及其与塑性岩屑共同造成的压实与溶蚀强度的差异,是造成两区目前孔隙结构差异的主要原因.