A New Tool for Ongoing Studies of Unconventional Reservoirs
Recent Wyoming State Geological Survey (WSGS) projects involving correlating Upper Cretaceous subsurface oil reservoirs in the Denver and Powder River basins have stressed the need for a comprehensive overview of statewide stratigraphy. Stratigraphy is the relative order and position of strata and their relationship to the geological time scale.
WSGS oil and gas geologist, Dr. Ranie Lynds, and Joshua Slattery at the University of South Florida collaborated to create a chart correlating Upper Cretaceous strata at 25 generalized locations throughout Wyoming to Western Interior biostratigraphy (fossils preserved in strata, including ammonites, bivalves, vertebrates, and pollen) and the most recent chronostratigraphic data. The new chart (Correlation of the Upper Cretaceous Strata of Wyoming, Open File Report 2017-3), focuses on strata deposited between 100 million and 63 million years ago and covers all Wyoming basins.
This stratigraphic chart will be useful in assessing hydrocarbon potential of correlative unconventional strata throughout the state.
“The Survey believes that the potential for unconventional oil and gas resources in Wyoming is very high and that the understanding of these plays is in the early stages,” says WSGS Director, Tom Drean. “We hope that information provided by the WSGS, such as this stratigraphic chart, will play a helpful role in facilitating this understanding.”
In Wyoming, the working definition of an unconventional reservoir is one in which production is improved through horizontal drilling and enhanced stimulation techniques such as hydraulic fracturing. Currently, most of Wyoming’s unconventional production is from the Powder River and Denver basins. The 2016 production numbers highlight the importance of this chart: seven of the top 10 oil-producing formations are Upper Cretaceous in age and account for nearly 60 percent of total production.
“The marine to transitional marine organic-rich shales and sandstones that were originally deposited as part of the Western Interior Foreland Basin are the heart of Wyoming’s unconventional oil plays,” says Lynds. “The geologists here at the WSGS want to expand upon our knowledge of these current plays to include potential unexplored unconventional reservoirs elsewhere in the state.”
In spring 2017, the WSGS released a study of the unconventional Codell Sandstone reservoir in the Denver Basin, Open File Report 2017-2 by geologists Rachel Toner and Dr. Erin Campbell.
“The Codell Sandstone was deposited at roughly the same time as the Turner Sandstone and Wall Creek Member of the Frontier Formation. Twenty-nine percent of Wyoming’s 2016 oil production was from these three equivalent formations, with the Turner alone accounting for 14 percent,” explains Lynds. “Recognizing the correlation between similar formations is the first step in understanding unconventional reservoir potential. We attempted to lay a solid foundation from which other geologists in industry and academia can build upon for future reservoir evaluations.”
Toward this end, the WSGS is currently investigating completion practices and geology of the Wall Creek and Turner reservoirs in the southern Powder River Basin. This effort is led by Toner, who notes that, “While drilling practices such as well orientation and lateral length are important, initial results suggest they may not be as important as in the Codell study. Basic geology may have more of an influence on the Turner/Wall Creek production trends we see today.”
The WSGS is also working to create a subsurface stratigraphic energy database, which will include all formations interpreted in the subsurface by the WSGS. Initially, the database will deliver currently available interpretations, such as the Upper Cretaceous in the Denver and Powder River basins, the Lance and Fort Union formations in the Washakie and Great Divide basins and coal correlations throughout the Greater Green River Basin. These data will be free for download in summer 2018.
“Right now, if a user wants these type of data they need to pay a private company. This will be the first dataset of this sort where all data has been hand-picked by a WSGS geologist and is free to the public. The data should be invaluable to industry, and we hope that providing this data framework will attract additional exploration in Wyoming,” says Toner.
The new stratigraphic correlation chart is available as a free download and for purchase on the WSGS sales site. It is an open file report and therefore will be supplemented periodically as new information becomes available.
Slattery will present the chart at the Geological Society of America’s annual meeting Oct. 22 in Seattle.