ABSTRACT

The Bowes Member, upper member of the Middle Jurassic Sawtooth Formation, produces oil at the NE Rabbit Hills Field in Blaine County, north-central Montana. To date, the field has produced 1.83 million barrels of oil. Available geologic data, comprising outcrops, cores, well logs and thin sections of the Bowes Member, have been integrated with respect to the occurrence and distribution of reservoir-quality rock across the field area to derive a geologic model. This report is part of an ongoing petroleum reservoir characterization at the NE Rabbit Hills Field conducted by a research team of the Montana University System and sponsored by the United States Department of Energy.

The Bowes Member is composed of quartzose sandstone and mudstone, lime mudstone and a variety of coarse­grained limestones. In outcrop in the Bears Paw Mountains these lithologies record a coastal tidal setting. However, in the NE Rabbit Hills Field area, 34 mi (56.4 km) to the north, the Bowes Member is subdivisible informally into lower and upper units, the former of which is considered correlative with the entire Bowes Member in outcrop. The upper unit is interpreted as an aggradational braid plain fill deposited in a valley that was locally incised into the lower unit during a sea-level lowstand. The predominance of ooid and bioclastic grains indicates that the braid plain was developed at sea level.

Oil production is from the upper Bowes unit, in which quartzose sandstone, ooid limestone and bioclastic limestone are interbedded within the aggrading coastal braid plain. Cores, core analyses and thin-section studies indicate that production is closely associated with the bioclastic limestone; the other two lithologies have generally lower porosities, much lower permeabilities and minimal oil stain in core. Bioclastic limestones have high intergranular and intragranular porosity as well as excellent permeability related to dissolution of shell fragments and calcite cement and to limited dolomitization. In contrast, ooid-dorninant limestones show high intragranular porosity but limited intergranular pore space, and quartz-dominant sandstones are generally tightly carbonate cemented. Reported core analyses indicate an average porosity of 15.1% in the bioclastic limestone and 10.6% in the other two lithologies. Core permeabilities average 79.6 md in the bioclastic limestone, but average only 13.7 md in the other lithologies.

As determined from core, well log and production data, reservoir continuity may be quite high across the field due to the relative abundance of the bioclastic limestone, but is unpredictable because distribution of this lithology is essentially random. A subtle structural high in the area indicates that the field is a structurally enhanced stratigraphic trap.