ResearchFocus Area

Figure 2: Research Focus Area within Bighorn Basin, WyomingTownship and Range designations are overlain onto the geography of the Bighorn Basin (outlined in red). Note how Laramide uplifts (grey, Precambrian rocks) surround the Basin, which has produced many oil and gas fields from Cretaceous and Tertiary reservoirs. Thick blue line highlights study area. Grey dashed line marks approximate trace of Basin’s axial syncline. Adapted from Finn (2010).

UNTAPPED HYDROCARBON PLAYS IN THE MUDDY FORMATION OF THE BIGHORN BASIN’S NORTHWEST REGION

Marjorie Ferrone, Geology, 2015Jane Dmochowski, Ph. D.Abstract

The Bighorn Basin of north-central Wyoming and south-central Montana has been commercially producing hydrocarbons since the early 1900s. The Basin’s geologic history generated a multitude of structural and stratigraphic petroleum plays, including those found in the Thermopolis/Muddy/Mowry complex. While these units have been repeatedly exploited, they most likely still contain profitable plays in the northwest region of the Basin. This conclusion is based on the fortuitous union of certain conditions, such as quality of hydrocarbon and reservoir, formation thickness and depth, and degree and type of structural and stratigraphic relation. Some of this information is drawn from well logs, but the majority depends on previous interpretation of distinct and diverse researchers. Conflicting explanations can create data incongruity, the meaning of which must be interpreted, permitting operator error. This research is a continuation of team work completed at the 2015 ExxonMobil Bighorn Basin Field School. Initially, it was decided that Muddy stratigraphic traps could exist in the Basin’s northwest region, if it were not for the unusual and undesirable facies arrangement. This influenced the team to conclude that it would not be profitable to drill in the area. However, this determination was based on limited and questionable data. I present further analysis based on more extensive research that aims to more accurately define the facies changes and assess their impact on the potential of future drilling.

Introduction• Geographic Focus (Figure 2): Bighorn Basin’s Northwest Region, dimensions

defined as Wyoming’s Township 49 to 57 North and Range 100 to 103 West• Stratigraphic Focus (Figure 1): Thermopolis Shale/ Muddy Sandstone/ Mowry

Shale; these units within the Basin’s stratigraphic column were chosen for study for two reasons: (1) they demonstrate the classic features of a productive stratigraphic trap: source, reservoir, and seal, respectively; and (2) the area does not exhibit a high density of previous drilling

• Problem: All conditions required for productive hydrocarbon (HC) play were met, except for proposed facies arrangement

• Why we care: Proper evaluation of an historically productive petroleum system not only measures potential for future drilling in that area, but also contributes to the cache of knowledge when approaching other potential drill sites

SummaryResearch Question: Are there untapped, potentially productive hydrocarbon plays remaining within the Northwest Region of Wyoming’s Bighorn Basin?Answer: Yes.Reason: Paleo-facies are favorably arranged to constitute a stratigraphic trap.

Figure 1: StratigraphyThis chart is an approximation of the stratigraphic column for the Bighorn Basin’s northwest region. Relative width of rows is NOT indicative of relative unit thicknesses. A transgressive period (sea level rise) occurred during deposition, indicating a deepening of the Basin. Blue outline highlights function of each unit of this study within a proposed petroleum play. Adapted from Summa et al. (2015).

Background• Structural: During the Late Cretaceous, the Laramide Orogeny shaped Western

North America into a borderland terrain of fault-bounded basins and uplifts, thus forming the topography of the Bighorn Basin

• Stratigraphic (Figure 1): Deposition of the Thermopolis/ Muddy/ Mowry occurred beneath the Western Interior Seaway, which transgressed from the Early to Late Cretaceous

Discussion• Depositional Conditions: The transgression of the Western Interior

Seaway during the Middle Cretaceous led to the Bighorn Basin’s flooding. During this time, the Muddy Sandstone was deposited under deltaic conditions represented by the Panther Tongue Model (Figure 3). Once sediment in each of the delta sections lithifies, facies are formed as bodies of rock interpreted to be distinguishable based on characteristics such as sedimentary structures and fossils.

• Structural Conditions: The Bighorn Basin is structurally divided by a syncline trending north-south. This promotes hydrocarbon secondary migration away from the axial syncline and towards the Basin edges, a key component for a successful stratigraphic trap.

References and Acknowledgements• Finn, T. M. (2010). Chapter 4: New Source Rock Data for the Thermopolis and Mowry Shales in the

Wyoming Part of the Bighorn Basin. U.S. Department of the Interior, U.S. Geological Survey. Reston, Virginia: U.S. Department of the Interior.

• Paull, R. A. (1957). Depositional History of the Muddy Sandstone Bighorn Basin, Wyoming. University of Wisconsin, Geology. University of Wisconsin.

• Summa, L., Gray, G., May, S., & Stewart, B. Integrated Basin Exploration Guidebook: Bighorn Basin, Wyoming (1.4 ed.). ExxonMobil Exploration Company, ExxonMobil Upstream Research Company.

• Acknowledgements: I would like to thank the instructors of the 2015 ExxonMobil Bighorn Basin Field School: David Awwiller, Thomas Becker, Tonya Brami, Timothy Garfield, Bob Stewart, Lori Summa, Isabel Varela, and Patricia Walker. I would also like to thank my student teammates at the camp, the efforts of whom much of my work is based upon: Anthony Feldman, Jake Marten, Barry Miller, and Eric Lewandowski.

Figure 3.a: Panther Tongue Model of the Muddy Sandstone, Map View (left)This illustration depicts an idealized model of a deltaic depositional system. A distributary channel, or a branch off a stream, dumps its sediment load into a slower-moving body of water, such as an ocean or lake. This system dominated deposition of the Muddy Sandstone as the Cretaceous Western Interior Seaway steadily flooded the Bighorn Basin.

Figure 3.b: Panther Tongue Model of the Muddy Sandstone, Cross Section (below)This illustration shows the deltaic system in cross section view from D to D’. This system is bounded by the Thermopolis Shale beneath (source rock) and the Mowry Shale above (seal rock). In the proposed stratigraphic trap, Mowry HCs would seep into the porous and permeable distributary channels of the Muddy, and migrate (due to buoyancy) into the prodelta sands as a result of the structural incline caused by the Basin’s axial syncline. Both figures taken from (Summa et al., 2015).

Results• Drilling Recommendation: YES, I recommend drilling in the Muddy Formation of the

Bighorn Basin’s Northwest region• Evidence to Support Recommendation: (1) the region has not been repeatedly drilled

within the Muddy Formation, (2) the deltaic depositional environment of the Muddy provides favorable conditions for HC migration within porous and permeable distributary channels, (3) the structure of the Basin, i.e. the axial syncline is also favorable for HC migration, and most central to my research (4) the facies within the deltaic depositional system are arranged in the expected order (Figure 4.b), a critical aspect to successful HC migration and trapping

Methods• During Exxon Camp: I worked in a team with other students to study

whether or not drilling in the Muddy would be profitable. Our conclusion was negative, due to an illogical facies arrangement (Figure 4.a).

• After Exxon Camp: Since camp, I have essentially completed a literature review to determine whether or that arrangement was accurate, and if not, how that impacts the team’s decision not to drill.

Neritic Deltaic Beach/ Back Bar Bar Sand

Figure 4.a: Approximate Facies Arrangement Proposed by Exxon Student Team (left)My research focuses on why the facies arrangement proposed by my team was out of expected order — placing the deltaic facies (blue) in front of the bar sand (yellow) — and whether or not it is correct. This arrangement is incorrect because it resulted from data misinterpretation.

Figure 4.b: Approximate Facies Arrangement Proposed by Me (right)This arrangement is closer to a correct interpretation of the research data. Facies are in an expected order, allowing HC migration and trapping via lateral pinch-out of units.. Both figures adapted from Finn (2010) and Paull (1957).

✔