ABSTRACT
Texture, diagenesis and detrital composition of the Upper Cretaceous (Campanian) basal Belly River reservoir sandstone were studied in core from nine wells in the Pembina-Keystone Belly River ‘B’ pool. The study was undertaken with the aim of explaining the low water/oil production history from a sandstone which, at the time of drilling, had core-analysis water saturations averaging 48 per cent. Study techniques included stereoscopic examination, thin-section petrography, scanning electron microscopy (SEM) and X-ray diffraction analysis.
Authigenic clays (kaolinite, mixed-layer illite-montmorillonite, and chlorite) occur as pore linings and pore fillings. Porosity and permeability have been modified, resulting in porosities of 5 to 23 per cent as measured by Core Laboratory analysis, and less than 1 to 13 per cent as measured by thin-section point counting. Cross plots of these parameters suggest that sandstones with core-analysis porosities of less than 13 per cent will usually not have porosity recognizable by normal thin-section petrography. SEM examination indicates that about 4 per cent porosity is accounted for by submicroscopic pores between clay cement crystals. Sandstone samples with core-analysis porosities of less than 13 per cent, corresponding to observable porosities of less than 1 per cent, have permeabilities of less than 1 md and no oil saturation.
Adsorbed water contributes to the relatively high Core Laboratory analysis porosity and water-saturation values, but is not produced. In order to anticipate maximum well performance, petrographic information should be evaluated along with Core Laboratory analysis data before well completion practices are decided on.
