Stratigraphy, Exploration and EOR potential of the Tensleep/ Casper Formations, Southeast Wyoming

There are two main themes of this poster. The first theme is to provide an update on the stratigraphy and sedimentology of the Tensleep/Casper Formation of Southeast Wyoming (and parts of Northern Colorado), and oil production from these rocks. We incorporate new measured sections, stratigraphic analysis and petrographic work undertaken by the authors. To this end we created a new database in ArcGis (geographic information software) of tops and other information that updates the historical well database of the Wyoming Oil and Gas Commission archived in Casper. This new database has been used to create Common Risk Segment (CRS) maps of the Upper Tensleep oil play in Southeast Wyoming. These CRS maps indicate trends in Tensleep reservoir, charge and trap that are useful in planning further exploration. It is possible that use of advanced seismic techniques applied over the complex structural terrains in the identified high potential areas of SE Wyoming will produce new leads and ultimately, new discoveries.

The second theme of this poster is to describe the occurrence in outcrop and core, of various flow units and discontinuities produced by reservoir heterogeneity associated with depositional facies in the Tensleep. We have created simple models of the mechanics of the impact of eolian primary strata, mainly ripple and avalanche strata, on recovery factors in eolian reservoirs. Ultimately we plan to improve our models, undertake experimental work and based on our data develop engineering formulas that allow more reliable estimates of recovery factors. These will take into account the impact of primary strata anisotropy, and crossbed dip direction. We continue to compile data on the abundance and arrangement of primary eolian strata – as well as small scale flow units- in outcrops and core in the Tensleep. This descriptive work is a necessary step in order to control digital flow models run in Petrel, Eclipse or other software, or to develop new software that will improve estimates of recovery factors. The history of Wyoming includes much downspacing of oil fields to produce bypassed reserves; and implicitly to correct originally over-optimistic estimates of recovery factors that stipulated widely spaced wells and inappropriate EOR (usually waterflood) designs.

Eolian petroleum reservoirs are of course, not unique to Wyoming although this state has an abundance of them; for example the Tensleep, Leo, Minnelusa Casper, Leo, Nugget and Sundance Formations. Eolian reservoirs worldwide commonly provide long-lived, high-volume production of both oil and gas, and importantly for Wyoming, provide new ideas from around the world that can be applied to similar reservoirs in Wyoming. As with any geological rock unit, each oil/gas field has production characteristics peculiar to its geological history. However, certain common factors link most eolian reservoirs. Cross-stratification due to bedform migration can create preferred sweep directions and thus impact recovery factors. Moreover, stacking of sand seas or bedforms through geological time will create distinctive flow units in subsurface petroleum reservoirs. We hope our work will enable these general observations to be evaluated quantitatively.

Conclusions: Tensleep Stratigraphy Southeast Wyoming

• The Tensleep and Casper Formations comprise the same rocks in SE Wyoming.
• The Tensleep Formation can be divided into a mainly eolian Upper Tensleep member and a carbonate rich, paralic and eolian Lower Tensleep Member.
• The Upper and Lower Tensleep of this study probably correlate to the Hyatt Ranch (Upper Tensleep) and Medicine Lodge (Lower Tensleep) Members of the Big Horn Basin, although that is not confirmed by this study.
• The Upper Tensleep is mainly Permian, the Lower Tensleep is Pre-Permian. Lithological change between Upper and Lower Tensleep was probably driven by climate changes from Upper Carboniferous time into Permian times.
• The Upper and Lower Tensleep Members are recognized regionally in SE Wyoming.
• The “Fountain” inter-bedding with Tensleep occurs mainly in the Lower Tensleep, close to pre-Permian uplifts.
• Oil Production is mainly from the Upper Tensleep in structural traps.
• Shallow marine carbonates inter-tongue with the Upper Tensleep (Permian) occurs along the Laramie Range.

Conclusions: Petroleum exploration, production and sedimentology Southeast Wyoming

• Common Risk Segment analysis of SE Wyoming Tensleep oil shows, structure and reservoir quality has identified four regions near existing production that may still have opportunities for further exploration using improvements exploration technology.
• Oil production in SE Wyoming occurs in a variety of stages from primary through tertiary (EOR), as well as various states of IOR (mechanical upgrades). There may development opportunities in existing fields.
• The State of Wyoming well database is a valuable asset. EORI has used this trove create to a digital database of tops and other data that updates older (historical) tops in the State of Wyoming database, using modern stratigraphic concepts. This database
will be made public, and may lower informational barriers to oil industry activity in Southeast Wyoming.
• Much oil in the Tensleep may be trapped in microscopic, or very small stratigraphic traps created by primary eolian strata and flow units derived from small dunes.
• EORI is working on ways to calculate oil trapped by eolian primary strata, and devise strategies to produce it. We are using static and dynamic models, and rock experiments along with natural outcrops and field studies. We hope this will help to guide down-spacing and other strategies both old and new to improve recovery of oil from Wyoming fields.