The Importance of Pore Structural Heterogeneities for Shale Gas Reservoir Evaluation

Daniel J. Ross and R. Marc Bustin
Earth and Ocean Sciences, University of British Columbia, Vancouver, BC, Canada

Elucidating the controls upon gas capacities in fine-grained strata and accurately determining reservoir potential requires knowledge of shale physical structure. However the intricate, heterogeneous pore network of shales is difficult to assess because pore-throats can be smaller than 2 nm. Shale gas reservoir evaluations rely upon scaling laboratory data to regional reservoir magnitudes, but failure to recognize nano-scale heterogeneities will lead to erroneous economic assessments.

Comparisons of Devonian-Mississippian (D-M) and Jurassic strata show that shale pore structure is a function of total organic carbon (TOC) content, mineralogy and thermal maturation. Sorption experiments of D-M shales indicate greater micropore volumes of organic-rich shales than organic-lean shales, thus increasing methane sorption capacity with TOC. The ratio of sorbed gas to TOC is also dependent on thermal maturity. At higher maturation levels, internal surface areas are larger (per wt% TOC), illustrating the thermal maturity effect upon pore-wall chemistry. As such, over-mature D-M shales sorb more gas than Jurassic shales on a wt% TOC basis. Micropore volumes of low maturity Jurassic strata tend to be low, despite TOC contents >30 wt%, due to the structureless nature of matrix bituminite.

Inorganic material influences modal pore size, total porosity and sorption characteristics of D-M shales. Highly mature Devonian shales are both silica- (biogenic) and TOC-rich (up to 85% quartz and 5 wt% TOC) and deemed excellent potential shale gas reservoirs because they are both fracable and gas-charged. However, quartz-rich Devonian shales display tight-rock characteristics, with poorly developed fabric, small median pore diameters and low permeabilities. Hence potential ‘frac-zones’ will require an increased density of hydraulic fracture networks for optimum gas production.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas