鄂尔多斯盆地延长组致密砂岩不同尺度孔喉分形特征及其控制因素

Fractal characteristics and its controlling factors of pore-throat with different scales in tight sandstones of the Yanchang Formation in the Ordos Basin

微观孔喉结构是影响致密砂岩油藏特征的重要因素。致密砂岩孔喉结构复杂,非均质性强,需结合分形理论对其进行研究。选取鄂尔多斯盆地延长组致密砂岩进行铸体薄片、扫描电镜、恒速压汞实验,分析致密砂岩孔隙和喉道的分布特征。在此基础上,结合分形原理和方法,对致密砂岩孔隙和喉道分形维数及影响因素进行研究。结果表明,鄂尔多斯盆地延长组致密砂岩分形曲线存在明显的转折点,根据其对应的进汞压力,可以将致密砂岩孔隙和喉道分成大尺度和小尺度2种类型。利用线性拟合斜率计算孔隙和喉道分形维数(D),不同尺度孔隙和喉道该值差异较大。小尺度孔喉压实作用和胶结作用强烈,以晶间孔、剩余粒间孔和缩颈状喉道为主。该尺度孔喉非均质性弱,孔喉空间变形较少,分形维数较小(2<D<4)。大尺度孔喉溶蚀作用强烈,以弯片状喉道、溶蚀孔隙和复合孔隙为主。该尺度孔喉储集空间大,孔喉变形明显。因此大尺度孔喉非均质性强,分形维数较大(D>7)。压实作用、胶结作用和溶蚀作用严重影响着致密砂岩孔喉储集空间的大小和形状,决定着不同尺度孔喉的分形特征。

Pore-throat structure is one of the important factors affecting the characteristics of tight sandstone reservoirs.Pore-throat structure of tight sandstone is complex and with strong heterogeneity,thus,conjunction study with fractal theory bas become a promising approach.In this study,casting thin section,scanning electron microscopic observation and rate-controlled porosimetry were performed on tight sandstone samples of the Yanchang Formation in the Ordos Basin to study the distribution of pores and throats.With fractal principle and method,the fractal dimensions and influencing factors of tight sandstone pores and throats were studied.Results showed that there are clear inflection points on the fractal curves of lgS_(Hg) versus lgP_(c),and according to the mercury pressure corresponding to the inflection points of fractal curves,pores and throats of tight sandstones can be classified to be large and small ones respectively.Fractal dimensions(D)were calculated by the means of slope of the straight part of each curve,and the fractal dimensions of pores and throats greatly varied between large pore-throats and small pore-throats.For the small pore-throats,compaction and cementation effects are strong,mainly developing intercrystalline pores,residual intergranular pores and necked throats.The heterogeneity of small pore-throat is weak,the pore-throat space is less deformed,and the fractal dimensions are small(2<D<4).For the large pore-throats,dissolution effect is strong,mainly developing curved lamellar throats,dissolved pores and composite pores.The storage space of large pore-throats is large,and the deformation of large pore-throats is significantly affected by diagenesis.Thus,the heterogeneity of large pore-throat is relatively stronger and the fractal dimensions of large pore-throat are higher(D>7).It was indicated that diagnosis,including compaction,cementation and dissolution,greatly affected the size and shape of pore-throat in tight sandstone,and determined the fractal characteristics of pore-throat with different scales.