The Impact of Thermal Maturity and Burial on the Pore Size Distribution and Matrix Permeability of Organic-Rich Mudrocks of the Duvernay Formation, Alberta

Abstract

Samples from the Duvernay Formation, Alberta were taken from varying thermal maturity and burial depths to assess the impact of thermal and mechanical diagenesis on the pore size distribution (PSD) and matrix permeability of mudrocks. Samples vary in maturity from immature (Tmax=418, HI=500) to the dry gas window (HI=10) and present day burial depth varies from 1,000 m to over 5,000 m. The PSD as measured by N2 low-pressure gas sorption varies dramatically with thermal and mechanical diagenesis and consequently impacts the matrix permeability. The volume of fine mesopores (2-10 nm) is the least in the most shallow, immature samples and increases dramatically with increasing thermal maturity into the dry gas window which is attributed to the thermal generation of pores hosted within the organic matter fraction. Concurrently, the volume of macropores (> 50 nm) decreases systematically with increasing burial depth and maturity, which is attributed to compaction of the pore structure during burial. The matrix permeability as measured by the confined helium pulse-decay permeability (PDP) method is lowest in oil window samples (1.6E-5 mD) and increases by an order of magnitude for wet gas window (1.8E-4 mD) and dry gas window samples (1.1E-4 mD), despite the shift in pore modality to finer sizes. Samples with a higher volume of coarse mesopores (10-50 nm) have higher PDP values, while finer pores have no obvious impact on the PDP value. Within the wet gas window, the PDP is dominantly controlled by the coarse meso- to macropore size fraction which is correlated with increased total quartz and feldspar content. The PDP for dry gas window samples are controlled by higher overall pore volumes and is correlated to higher clay and quartz and feldspar content. Since the PDP method is biased toward higher permeability fractures and pathways, the confined gas expansion permeability (GEPP) method is a non-routine analysis which probes the total gas uptake by the matrix and is more suited to measuring the permeability of finer pore sizes. The GEPP reveals the fine pore sizes, which are developed with maturity and correlated to the organic matter content, are sufficiently interconnected to yield higher GEPP permeabilities. Samples from the wet and dry gas window, which have greater volumes of fine pores, have GEPP values 40 % higher than oil window samples.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016