The occurrence of clay in pore space of impure chalk: An approach from high resolution petrographic studies of the Outer Carpathian marls (Poland)

Abstract

The paper deals with the architecture of clay occurring in the pore space of fine grained carbonate rocks, which are impure chalk in type. Chalk acts as an important reservoir for water and hydrocarbon, a source rock for oil and gas, and as a sealing horizon. In order to understand why petrophysical properties of chalk are so diversified, X-ray diffraction (XRD) was applied for the identification of clay minerals and field emission gun electron microscopy (FEG-SEM) was used for imaging clay in the pore space of impure chalk from the Outer Carpathians (Poland). Fifteen complexes, which differ in age, location in the sedimentary basin, diagenetic history, clay abundance, and mineralogy were represented in this study.XRD revealed that the dominant clay mineral composition of the impure chalk studied consisted of illite-smectite with variable amounts of swelling layers. FEG-SEM shows that intergranular, intragranular (intrabiotic and intracrystal), shelter, and moldic pore space is occupied by clay. Contact, massive, pore-filling, pore-lining and pore-bridging categories of cementing material composed of clay are locally present in all samples studied; however, their abundances depend on clay content. The relationships between clay and carbonate cements suggest that the former locally postdate the later.The clay-hosted porosity, which was evaluated using the microscopic method, varies from almost negligible in parallel-organized clay platelets to potentially high in boxwork and pore-bridging clay assemblages. Thicker and more abundant contact clay is present in the chalk that acts as sealing rock (Węglówka Marls) when compared to those having source rock features (Grybów Marls). Petrographic evaluation of impure chalk porosity measured by Nitrogen-Gas-Adsorption Analyses documents that the organic matter can clog mostly smaller pores, which is expressed by a larger average pore diameter in chalk-type source rock than in chalk-type sealing rock.