Identification of clay microporosity in the reservoir characterization of the Cypress Sandstone; implications for petrophysical analysis, reservoir quality, and depositional environment

A petrographic study was initiated to identify microporosity in the Upper-Mississippian Cypress Sandstone of the Illinois Basin. Preliminary well log analyses of incised valley fill (IVF) Cypress reservoirs indicated the presence of an additional source of conductivity, and microporosity in clays was suspected. Clay micropores often contain structurally trapped, immobile water that cause increased formation conductivity. If identified, clay micropores impact effective porosity and permeability, as traditional analytical techniques do not account for microporosity. Characterization of clay mineral morphologies also aids in interpretation of the depositional environment of a reservoir sandstone. The known properties of the IVF coupled with anomalous well log analyses suggest the presence of water-saturated clay micropores. To address microporosity, clay mineral morphologies were identified, volume of clay was determined, and the percent-volume of clay micropores were quantified. Samples from IVF Cypress core were collected for petrographic thin section preparation. Slides exhibiting clay textures, usually in the form of coatings on sand grains, were analyzed using high-resolution scanning electron microscopy with a back-scatter detector. The volume of clay for the IVF Cypress reservoirs, typically about 1 to 4%, was determined. Data for interpreting clay species and morphologies and thus, species specific percent-volumes of microporosity in the IVF Cypress, was collected. Observed morphologies include kaolinite booklets, chlorite rosettes, and illite and illite-smectite fibers. Percent-volumes of microporosity were used to correct petrophysical calculations of water saturation from resistivity logs and calibrate porosity and permeability of the sandstone, as reductions in effective porosity attributed to microporosity can cause significant reductions in permeability. Additionally, observed clay mineral morphologies were categorized as detrital or diagenetic, which aid in the interpretation of the depositional environments. Study of microporosity in clay minerals of the IVF Cypress improves petrophysical calculations of water saturation, enhances understanding of the depositional framework and corrects overestimation of effective porosity and permeability.