SOURCE-ROCK AND ORGANIC PETROLOGY STUDIES OF THE DEVONIAN ANTRIM SHALE, MICHIGAN BASIN

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 source rock potential (quantity, composition, maturity), organic facies assemblages and paleo-depositional conditions of the Antrim Shale members.

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) Rock- Eval pyrolysis, 2) organic petrology, 3) petrographic thin-section studies, 4) x-ray diffractometry minerals analysis (XRD), and 5) scanning electron microscopy (SEM) for Antrim Shale geochemical evaluation.

Preliminary results showed that the Lachine and Norwood members are relatively rich in organic matter (up to 14wt%) while the Paxton is relatively lean in organic (<6wt%). Organic matter is mainly dominated by marine Type II kerogen while the Paxton member contain both Type II and Type II/III. Reflected light microscopy showed an abundance of solid bitumen, alginite, amorphous matrix bituminite, and sparse zooclast. Thin sections petrography confirmed the dominance of solid bitumen, matrix bituminite and marine alginites, which is in agreement with Type II kerogen identified from Rock-Eval pyrolysis and marine depositional environment as interpreted based on sedimentology. Major organic matter types identified from SEM studies involve stringy OM, OM-mineral admixture and particulate OM. This is the first step towards understanding the hydrocarbon generating potential and heterogeneity of the Antrim Shale in Michigan Basins.