The Medicine Hat Gas Field: A Large Upper Cretaceous Reservoir in Southeastern Alberta

MARTIN, IAN, Martin Petroleum & Associates, Calgary, Alberta, Canada

The Medicine Hat gas field is the largest in Alberta, with estimated initial gas reserves of 3485 bcf. About half the reserves occur in the Medicine Hat A sand. This formation grades upward from a basal mudstone into siltstone and very fine grained sandstone, often capped by a thin layer of coarser granules which mark the base of overlying transgressive strata. The present shallow burial (450-525 m) and limited compaction are key factors in reservoir quality.

Porosity is mainly of primary intergranular and interparticle micropore types, with core values ranging from 20 to 37%. Permeabilities range from 10 to more than 500 md. Divergence between core analysis porosity values (higher) and thin-section porosity values (lower) is attributed to high volumes of microporosity in the lightly compacted clay fabrics of the matrix and mudstones. This microporosity is read by logging tools as well as core analysis and results in high irreducible water saturation.

Interstitial water saturation, derived from oil-based cores, ranges from 38 to 52.5%. Net pay was determined using an 8-ohm resistivity cutoff after normalizing to shale; then a net-to-gross ratio was established for each well with CNL-CDL logs using a density-neutron crossplot.

This paper describes a gas reserve evaluation conducted volumetrically using log, core, and selective perforation test data and compares such estimates to those from production performance and material balance.

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)