Influence of Grain Sorting and Grain Shape/Elongation on the Intergranular Porosity of Cubic Packing for Sedimentary Rocks

Abstract

Porosity, being the main storage parameter in porous reservoir rocks, relates to the microscopic void spaces in rocks where oil and gas are accumulated. Diagenetic processes may damage porosity since they affect the systematic arrangements of grain particles and cause grain rearrangements. The extent to which the grain rearrangements may occur depends on the original packing of the grains and how extensively the packing has now been modified. The present study deals with the geometry of cubic packing of grains for a sandstone and carbonate rock core samples obtained from different parts of India which are known for undertaking hydrocarbon activities. Cubic packing is studied including the theoretical model along with the obtained model from the digitally analyzed image through Field Emission Scanning Electron Microscopy (FESEM) with the help of MATLAB coding. A set of particles (idealized as spheres of varying radii) was added to the ideal cubic assemblage of eight spherical particles, and thus, the effect of grain sorting on intergranular porosity was observed. The same model was taken into account to estimate the effect of degree of elongation. But the only difference was a set of particles (idealized as spheres of varying radii) that was added to the ideal cubic assemblage of eight elongated particles. Thus, it is concluded that the nature of pore systems at micrometer to millimeter scale gets affected by the grain sorting. Digital images acquired from FESEM-backscattered electron imaging gives promising inputs for grain size distribution which in turn discloses the frequency of available grain sizes corresponding to different size ranges. Porosity obtained from cubic assemblage influences the shape and degree of elongation of the grains responsible for the formation of interstitial pore spaces which may affect the storage capacity of porous reservoir rocks.