Characterization of porosity structure in a biogenic gas shale - the Antrim Formation in the Michigan Basin

Abstract:

Recent studies examining unconventional gas reservoirs have focused primarily on thermogenic gas shales where thermal maturity and burial depth play a major role in gas generation. Conversely, biogenic gas is generated at shallow depth with low geothermal gradient and abundant microbiological activity. The Devonian Antrim Shale is an unconventional biogenic gas accumulation with a technical recoverable resource of 19.9 Tcf. However, major knowledge gaps remain regarding our understanding of the production, storage, and transport mechanisms in biogenic gas shales. This project aims to integrate organic matter properties (quantity, composition, maturity, and morphology), mineral composition and spatial distribution of sediments (vertical and lateral position in the basin) to characterize storage and transport potential of hydrocarbon in the study area.

Core samples from wells penetrating the Antrim Shale members both in the shallower and deeper parts of the basin have been selected along a north – south traverse for this study. Core description and wireline logs have been used to identify and sample the Lachine, Paxton and Norwood members of the Antrim Shale. This study utilized 1) physical core description, 2) Rock Eval pyrolysis, 3) x-ray diffractometry minerals analysis (XRD), 4) scanning electron microscopy (SEM), 5) porosity measurements from helium porosimetry and 6) gas adsorption (CO₂ and N₂), and mercury injection capillary pressure (MICP) for both pore size distribution and pore throat properties.

Preliminary results showed that the Lachine and Norwood members are relatively rich in organic matter (up to 14 wt.% TOC) while the Paxton is relatively lean in organic matter (<6 wt.% TOC). Porosity values are higher in Paxton (about 6%) while Lachine and Norwood are less than 3% with diverse distribution of micro-pores and meso- and macro pores. Organic matter in the Antrim Shale hosts micro-pores instead of meso- and macro pores, while clay minerals host micro-, meso- and macro- pores. Mineral-related pores play a primary role in the total porosity of the Antrim Shale. The biogenic Antrim Shale, therefore, has a different pore structure than typical thermogenic gas shales worldwide.