Quantifying Microporosity in Clay Minerals of the Cypress Sandstone: Implications for Petrophysical Analysis And Diagenesis

Abstract

A petrographic study was conducted to characterize clay mineralogy and quantify clay microporosity in the incised valley fill facies of the Upper-Mississippian Cypress Sandstone in the Illinois Basin. Clay micropores contain immobile water that causes increased formation conductivity. Microporosity also impacts estimates of clay mineral volume and water saturation, thus microporosity should be included in calculated estimates. Additionally, observed clay mineral morphology and occurrence provides useful information pertaining to the formative diagenetic processes. This study utilizes electron beam methods to investigate whether water-saturated clay mineral micropores are causing anomalous well log analyses results, and to characterize micro-scale clay mineralogy.

Thirty-five petrographic thin sections of Cypress core exhibiting clay textures were analyzed using scanning electron microscopy, which was supplemented by back-scatter imaging (BSE) and energy dispersive X-ray spectroscopy. Observed clay mineral occurrences and morphologies include pore-filling kaolinite booklets, vermicules, chlorite clusters, illite mats, filamentous illite-smecite, grain-coating chlorite rosettes, and pore-bridging hairy illite. Percent-volumes of microporosity particular to clay mineral species were determined using BSE image analysis. Average values of microporosity in kaolinite, chlorite, illite, and illite smectite, were 40%, 55%, 65%, and 65%, respectively. Effective clay mineral volumes including micoroporosity showed a >2-fold increase over X-ray diffraction determined-estimates. To evaluate the accuracy of petrophysical techniques, samples with effective clay mineral volume were compared against volume of shale calculations based on neutron density-porosity and gamma ray logs. Petrographic analysis also revealed textural relationships indicative of diagenetic replacement between clay minerals. Mats of illite, for example, surround kaolinite booklets in pore-space, indicating an illitization process of a kaolinite precursor. Multiple morphologies of kaolinite, chlorite, and illite imply formation by multiple diagenetic processes, such as replacement of framework grains, or by direct precipitation from ion-rich pore-water.

As part of a multidisciplinary study to identify residual oil zones in the Illinois Basin, detailed petrographic study has resulted in quantification of clay mineral microporosity and additional details about diagenetic processes that affect reservoir quality. Together, these findings will enhance petrophysical estimates of water saturation, improve understanding of the diagenetic history of the Cypress Sandstone, and highlight the importance of studying micro-scale mineralogical properties during reservoir characterization.

AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016