Macro- to Micro-Scale Quantitative Chalk Reservoir-Rock Characterization to Assess CO2 Storage Potential (Harlingen Upper Cretaceous Gas Field, the Netherlands)

Abstract

This study provides a quantitative, multi-scale assessment of the CO₂ storage potential in the chalk of the Upper Cretaceous Ommelanden Formation in the Harlingen Field. Through the integration of seismic data interpretation, well-log and cross-plot analysis, core observations, thin-section microscopic analysis, and micro-CT imaging, the study evaluates the reservoir petrophysical properties, aperture vari ability, and flow behavior across scales. Seismic data define a gently folded anticline with two structural crests separated by a saddle zone with the northern crest preserving a thicker chalk interval (~327 m). No faults or sub-seismic fractures were imaged, but a low-impedance zone indicating gas retention is ob served. Well-log responses are only diagnostic of intervals where the fractures are open and gas-charged. These gas-filled intervals show consistent downward petrophysical trends of density increasing from ~1.9 to 2.25 g/cm³, porosity increasing from ~19 to 26%, and sonic travel times decrease from ~140–100 µs/ft with depth. Cross-plot clustering distinguishes compact, non-reservoir chalk from the more variable responses in gas-charged fractured zones, supporting the interpretation of three fractured intervals in the upper 20 m of the formation. Thin-section and micro-CT imaging show fracture apertures ranging from 5 to 600 µm, with substantial variability in mineral infill, connectivity. Cubic-law flow estimates, assuming a constant pressure differential and representative fracture geometries, yield maximum individual fracture flow rates up to 10,627 m³/day. When scaled using observed stylolite intensities (1.1–6 stylolites/m) and applied to a representative 5 m reservoir thickness, the estimated flow contribution ranges from 4,070 to 53,135 m³/day. These values align with production-tested flow rates from HRL-02 (39120 60000 m³/day), HRL-04 (80509 m³/day), and FRA-01 (22000 m³/day), validating the upscaling approach. The mechanical stability of the reservoir, presence of overpressure, and preserved porosity support its suitability for secure CO₂ injection. Results confirm that flow in the Ommelanden Formation is dominated by gas-filled fractures and is primarily controlled by aperture, rather than fracture intensity or matrix porosity. The outcome of this study advances understanding of how fracture geometry and distribution control fluid flow in fractured chalk reservoirs and supports the evaluation of their feasibility for CO₂ storage.