The Significance of Isotope Variations in Brine from the Poplar Oil Field, Montana
Oxygen (δ18O) and hydrogen (δ2H) isotopic compositions of connate or formation water are usually correlated but typically deviate from the global meteoric water line (GMWL). Brine samples from 18 of the remaining 42 wells still producing largely from Madison Group carbonate reservoirs in the Poplar Field of eastern Montana define a highly correlated linear array (δ2H = 3.50*δ18O - 78) intersecting the GMWL at -19.6 (δ18O) and -147 (δ2H). Two groups of brine previously recognized on the basis of strontium isotopes are also distinguishable by stable isotopes. Brines with a mean δ87Sr value of +2.39 per mil (87Sr/86Sr = 0.71090) show a limited range in δ18O from -14.4 to -17.0 and plot near the GMWL (Group II). Group I brines with a mean δ87Sr of -1.46 (87Sr/86Sr = 0.70816, consistent with Mississippian carbonates) range from -0.8 to -16.4 in δ18O, and with the corresponding δ2H values, define the linear relationship shown above.
The Group II brines with δ18O and δ2H values clustering close to the GMWL originated as meteoric water recharging clastic rocks where they acquired a dissolved load and δ87Sr values larger than those of the Group I brines. Group II brines are mostly from wells yielding large water-to-oil ratios, which are commonly indicative of long-term production and probably decreasing reservoir pressure. Thus, cross flow from older or younger units may have been induced by pressure changes in the reservoir perhaps localized by faults. Most of the wells bottom in the producing unit, and the inflow of Group II brines also may have occurred via leaky casings through younger units.
Exploration of the Bakken Shale has started in the Poplar Dome. Elsewhere, Bakken production such as in the Elm Coulee field is characterized by water-to-oil ratios typically much smaller than production from conventional reservoirs. Preliminary chemical and isotopic data (Sr and stable isotopes) for Bakken brines differ significantly from those of brines from the Mississippian carbonates, consistent with long-term hydrologic isolation of the Bakken from the adjacent units.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013