Estimation of rock properties from drill cuttings is proving valuable in the geologic description of tight gas strata. The characterization scheme includes an integrated analysis of detrital and authigenic mineralogy, pore geometry, and flow and storage capacity using drill cuttings samples calibrated with a limited amount of core data. This methodology has been successfully applied to characterize the fine-grained siliciclastic, shallow marine Nikanassin Formation in the Deep Basin of Alberta.
This workflow is particularly useful for the characterization of undercored low permeability hydrocarbon-bearing intervals from both new and legacy wells. Macroscopic description of drill cuttings samples, coupled with petrographic analysis performed on custom made multi-sample thin sections from the same samples allows a direct correlation between these two observations. The principal detrital and authigenic components are also investigated through microprobe analysis and SEM imaging of selected samples. Porosity values and dominant pore geometries are estimated using laboratory measurements and thin section image analysis. Finally, permeability values are measured using the Liquid Pressure Pulse methodology on drill cutting samples.
Porosity and permeability of the analyzed samples ranges between 2–13%, and 0.01–0.25 mD, respectively. Porosity values from drill cuttings samples are found to be slightly higher than routine core analysis measurements, which in turn usually have higher values than porosity estimated from thin sections. A high degree of reproducibility was confirmed for the porosity values obtained from the saturation method on drill cutting-sized samples, with resultant values comparing very well with measurements from standard nuclear magnetic resonance on the same samples. Reservoir quality within the analyzed samples is highly affected by quartz overgrowth and subsequent carbonate cement, with the former increasing with depth. Compared to the dominant microporosity domain, remnant intergranular porosity significantly enhances the permeability of the samples.
This workflow represents an inexpensive yet comprehensive interpretation tool specially targeted to improve the geological understanding of potential by-passed tight gas formations, which usually lack representative cored intervals. In addition, economic returns can be highly optimized by partial replacement of coring programs by appropriate sampling and preservation of drill cuttings samples in new wells.