Geologic evaluation of the Parkman sandstone, Powder River Basin, Wyoming

Abstract

The Powder River Basin in Wyoming has yet again entered a new stage of oil and gas production and reserve growth. Oil production in the basin has grown from 46,900 BOPD in 2009 to a peak rate of 190,000 BOPD in 2019. This dramatic increase in production is due to the expanded application of horizontal drilling and large volume, multi-stage hydraulic fracturing. These technologies are now being applied to a Cretaceous age resource base that has historically been uneconomic due to the limited reservoir contact of vertical well completions. The Parkman Sandstone Member of the Mesaverde Formation is one of several Cretaceous sandstones and source rocks currently targeted for horizontal development in the basin. Horizontal drilling in the basin is targeting reservoirs in the Teapot, Parkman, Sussex, Shannon, Niobrara, Turner, Frontier (Wall Creek), Muddy and Mowry. The Parkman is not a prolific and widespread, tight oil play like the Frontier and Turner sandstones, or a continuous source rock shale play like the Niobrara or Mowry formations. In these types of plays, hydrocarbon saturated reservoirs (or facies) are present over much of the Powder River Basin geographic area and wells can be drilled with a repeatable well bore design. Production from the Parkman is more localized, with sweet spots that can be extremely economic. For clastic reservoirs, such as the Parkman, possible geologic constraints on economic production can be related to stratigraphy, facies distribution (i.e. depositional environment), structure, or diagenetic factors. Interpreting the stratigraphic and facies constraints on hydrocarbon production in the Parkman is the focus of this research. The Parkman and the Ferguson Member of the Parkman Sandstone can be divided into four facies associations based on sixteen individual facies. There are two marine facies associations and two non-marine facies associations. The marine associations include a coarsening upward marine shelf to shoreface progradational succession and a reworked transgressive succession. The non-marine associations include a coastal plain facies association and a fluvial facies association. Three of the four facies (coarsening upward shoreface, coastal plain, and fluvial) are present along the western margin of the basin in outcrop. The fourth facies (reworked transgressive sediments of the Ferguson) is present only in the subsurface. The depositional model for the Parkman Sandstone and the Ferguson Member of the Parkman Sandstone is based on abundant well control, sequence stratigraphic concepts, oil and gas production, good outcrops, core data, correlation of bentonite marker beds, and principles based on the understanding of modern depositional systems. The Parkman is a progradational parasequence set comprised of both non-marine coastal plain deposits and coarsening upward marine shelf to shoreface deposits, which step basinward to the east. The Ferguson Member of the Parkman Sandstone is a retrogradational parasequence set backstepping onto the Parkman. A sequence boundary separates the Parkman from the Ferguson. Along the western margin of the Powder River Basin the sequence boundary can be defined at the base of an incised valley which has eroded into the coastal plain deposits. In other areas along the western margin of the basin the sequence boundary is defined by the juxtaposition of marine shales over the coastal plain deposits. In these area the sequence boundary is co-planar with a flooding surface. Further to the east in the subsurface the sequence boundary is defined by an erosional surface that has been modified by transgression. In the eastern most portions of the study area the sequence boundary becomes conformable. Correlation, mapping, and core description indicates that production in the Parkman is from multiple parasequences and facies. In the expanded study area, the Parkman is productive from two marine facies: the structureless sandstone facies and the horizontal-planar laminated sandstone facies. Porosity for the two facies range from 6-20%, while permeability ranges from 0.001 – 100 md, which is over five orders of magnitude. Since the overpressured compartment in the Powder River Basin described by Surdam (1997) does not include the Parkman, charging of oil into Parkman facies is dependent on a buoyancy mechanism. For individual Parkman facies, Mercury Injection Pressure Tests show that high-quality rocks (horizontal – planar laminated sandstone facies) can easily be charged from the Niobrara. However, tighter rocks (bioturbated sandstones and mudstones) are not as likely to be charged due to lesser reservoir quality. Lower quality facies are more likely to retain original formations waters which can cause production problems.