RICHARDS, JAMES A., ANNA M. MARTINI, and LYNN M. WALTER, The University of Michigan, Ann Arbor, MI; and CHARLES J. KAISER, Shell Western Exploration and Production, Inc., Houston, TX

ABSTRACT: Hydrogeochemistry of the Upper Devonian Antrim Shale, Michigan Basin: An Unconventional Gas Reservoir, A Complex Aquifer

Recent extensive drilling for Antrim gas presents an unique opportunity to evaluate utility of hydrogeochemistry to determine flow-patterns in a fractured-shale gas-reservoir. The trend is on the northern rim of the basin and production results from combined occurrence of the organic-rich black shale facies and a regionally extensive fracture system. Co-produced water samples collected from wells in the Antrim Shale gas-producing trend were analyzed for chemical and isotopic composition to identify: (1) area of recharge, (2) source(s) of water and solutes, and (3) relation of large-scale flow-patterns to regional fractures.

Formation waters are highly variable in salinity over small distances and isoconcentration maps over the producing trend show well-defined gradients. Solute concentrations increase with lateral distance away from the subcrop and with depth. In contrast, carbonate alkalinity and total inorganic carbon (TIC) decrease with increasing salinity. Although shallow groundwater samples from glacial deposits overlying the Antrim subcrop are depleted in TIC the most dilute endmember Antrim water has the highest TIC. This requires rapid and extensive organic carbon decomposition within the Antrim reservoir. Stable isotopic composition (O, D) also vary with salinity. Minor elements such as Ba exhibit local centered enrichment, apparently unrelated to the regional flow system.

Chemical variations across the Antrim trend indicate that meteoric water recharges at the subcrop. The source of salinity for Antrim waters is more complex; Cl-Na-Br systematics indicate that salinity is derived, in part, from local salt dissolution, and from mixing with Br-enriched Ca-Cl brines from subjacent formations. Geochemical gradients parallel the dominant northeast-southwest fracture trend in the Antrim.

Meteoric recharge, coupled with flow through fractures within the Antrim leads to the complex pattern in fluid chemistry. Better documentation of fluid chemistry variations and their sources may prove useful in identifying areas with potential production problems and, coupled with gas-production data, improve exploration and development of this unconventional gas resource.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.