Miocene reservoir rocks: pore throat size distribution as a strong controller on petrophysical attributes is a reflection of facies change

Abstract

Pore throat size distribution (PSD) is considered a finger print for each rock type and has strong effects on many petrophysical parameters, so a comparative study was done on irregular subsurface rock samples that belong to Kareem and Rudeis Formations of middle and early Miocene, respectively. These samples were collected from San El-Hagar-1 well which is located to the East of the Nile Delta to show the effect of the different lithologies on pore throat size distribution and hence many related reservoir parameters such as displacement pressure, macroporosity, microporosity, mean hydraulic radius and permeability distribution function (PDF). For achieving the previous purpose, helium porosity and capillary pressure by mercury injection were carried out. Permeability measurements were canceled due to the non-cylindrical irregular nature of the tested samples. Porosity showed a great difference between the results of the samples of the two formations. Capillary pressure results showed the presence of characterized pore throat size distribution and hence capillary pressure-derived parameters for samples of each formation. PDF showed that samples of Kareem Fm. have better reservoir flow properties (permeability) in contrast to those of Rudeis Fm. which have tight reservoir flow properties. In addition, petrographical study through thin sections and scanning electron microscope displayed the presence of two facies which are feldspathic quartz wacke (sandstone facies) of Kareem Fm. and bioclastic sandy wackestone (calcareous facies) of Rudeis Fm. All results were consistent and confirmed the influence of facies change on PSD and hence the reservoir parameters.