Porosity–permeability and textural heterogeneity of reservoir sandstones from the Lower Cretaceous Putaohua Member Of Yaojia Formation, Weixing Oilfield, Songliao Basin, Northeast China

Abstract

With the granulometric analyses, mineral composition X-ray diffraction (XRD) analyses, porosity and permeability measurements, scanning electron microscopy (SEM) observations and mercury porosimetry measurements of the reservoir sandstones from four wells in the Weixing oilfield (Songliao Basin, NE China), the porosity–permeability and pore-structural heterogeneity features of the Putaohua Member sandstone of the Lower Cretaceous Yaojia Formation have been investigated in this paper. The sandstones are mostly characterized by a similar deposition in continental detrital environments, with quartz and feldspar taking the majority (∼97%) and argillaceous minerals (illite, kaolinite and chlorite) the minority (∼3%). However, their granule size, sorting index, porosity and permeability vary significantly. In contrast to the generality that high porosity–permeability often corresponds to large grain sizes and sorting indexes, the porosity and permeability here show no good correlations to the grain size and sorting index, suggesting the presence of a remarkable textural heterogeneity. In terms of sequential change in permeability along the well-depth, cyclic sedimentation modes have been identified in some sub-layers despite the porosity data showing little response to those scenarios. This may be ascribed to the sandstone's peculiar intergranular pore structures, in which the cementation occurred in pore-lining and -filling manners to different extents depending on their depositional environments. The distributary channel facies tends to have lowest detrital clay content and rather open packed texture in pore-lining cementation manner, whereas sand sheet facies has pore-filling manner. The Hg-porosimetry analytical data are in fairly good agreement with the heterogeneous pore-structures observed by SEM. Large-sized pores and channels connecting them, though only accounting for a low percentage of the total porosity, are believed to contribute mostly to the effective permeability in the sandstone reservoir. Such textural heterogeneities should be closely associated with the dynamic changing sedimentary facies of sandstones in the continental environment.