Upper Devonian Woodford Shale (base) overlain unconformably by Kinderhookian Compton Limestone (crinoid grainstones) with syndepositionally tectonically displaced bryozoan-crinoid reef (above and to the right of person). Compton reefs may be exploration objectives.
A new hydrocarbon play in Kansas and Oklahoma is drawing interest to possibilities in several Mississippian formations.
What should geologists look for in the subsurface Mississippian?
Three principal things at this point, according to AAPG member Sal Mazzullo of Wichita State University.
- First is the fractured but otherwise tight Osagean Reeds Spring cherty limestone reservoir that companies are treating with big frack jobs in northern Oklahoma to produce commercial quantities of oil.
- Second is the locally well-developed tripolite just below the unconformable top of the Reeds Spring that has produced much hydrocarbon in giant fields such as Glick and Spivey-Grabs in south-central Kansas.
- Third is the Osagean Cowley spiculite reservoir, which has produced gas and a lesser amount of oil from several fields in southern and south-central Kansas.
The Cowley spiculite is a facies of the Reeds Spring Formation.
“Siliceous reservoirs within and at the top of the Reeds Spring, and within the Cowley spiculite, may have similar appearances on some logs,” Mazzullo noted.
“They have simply been referred to in the past as ‘Osage,’” he said. “Our studies have indicated that such simplicity is not warranted by Mississippian outcrop or subsurface lithostratigraphy.”
Know Your Targets
Accordingly, the explorationist needs to know which unit is being exploited.
“These reservoirs have been produced for years. However, exploration indeed is still in its infancy, because there has been a fundamental misunderstanding of the geology – which in turn leaves a lot to be explored for by the astute geologist,” said AAPG member Brian W. Wilhite, exploration geologist for Woolsey Operating Co. LLC in Wichita.
Mazzullo and Wilhite have studied the Mississippian with colleague Darwin Boardman, associate professor of event stratigraphy and paleontology at Oklahoma State University in Stillwater.
Left to right: Field trip organizer Shane Matson and trip leaders Beau Morris, Brian Wilhite, Sal Mazzullo, Darwin Boardman and Robert Turner.
Their studies were initially stymied by lack of full core coverage, according to Wilhite.
“Existing 30- and 60-foot cores were not cutting it in our work in understanding the reservoirs. We needed a point of context – a benchmark core where we could view all stratigraphic contacts,” he recalled.
“This initial core, and numerous other full Mississippian cores since this time, along with cuttings work greatly expanded our understanding of the subsurface,” he said.
Wilhite said Woolsey Operating began looking for production from the Cowley years ago.
After measuring over 100 outcrop locations, the researchers were able to establish a depositional framework for application to the subsurface.
“Since that time we have modified our understanding of reservoir units, recognized these systems and discrete objectives in both subsurface and seismic and furthered our exploration efforts,” Wilhite said.
The stratigraphic architecture of the Reeds Spring and Cowley are composed internally of discrete, basinward progradational wedges, Mazzullo said.
Individual wedges within such packages may differ somewhat in their lithology – and therefore in reservoir potential.
“Distinguishing these lithologies is a very easy task if one has well cuttings samples or cores, as they are readily identifiable in such samples,” Mazzullo observed.
“Point is, all three of these Osagean units are proven reservoirs in the subsurface, so they are viable exploration plays,” he said. “But one has to know which Osage is being hunted.”
Outcrop and subsurface studies indicate that syndepositional tectonism has played a major role in the development of unconformities and truncated folds within the Osagean section, and in the deposition of certain reservoir-associated lithofacies.
“Accordingly, one has to tie in lithostratigraphic and syndepositional tectonics in any exploration play for these rocks. Whether these plays are more prone to oil or gas in different areas has not been well established,” Mazzullo said.
“It has been presumed, for example, that gas predominates east of the Nemaha Ridge, but I have identified several Cowley-Reeds Spring fields that produce oil instead of gas in such areas. Much more data are needed in this regard, he added.
Mazzullo cited two additional plays that have emerged from outcrop studies and that will require evaluation of detailed stratigraphic correlation and subsurface sample studies ahead of drilling.
First are reefs in the Kinderhookian Compton Formation and the basal Osagean Pierson Formation, where deposition and diagenesis were strongly affected by eustasy as well as syndepositional tectonism.
Second is the up-dip pinchout of porous dolomites within the Pierson.
“We can no longer simply refer to the Mississippian section as ‘Mississippi chat’ or ‘Mississippi Lime,’” Mazzullo said.
“Explorationists need to know which formation within the Mississippian they are going after, and what type of reservoir they are seeking.”