It’s not ‘just the Miss …’

Mississippian a Symphony, Not a Solo

American Association of Petroleum Geologists (AAPG)

In the mid-continent Mississippi Lime, it’s the same song and a totally new verse – with a challenging geological chorus.

Horizontal drilling has orchestrated a booming new oil and gas play in northern Oklahoma and southern Kansas, keyed to the presence of chert, tripolite, dolomite and vertical fractures.

Operators drilled oil wells in this area for decades, thousands of them, with hit-and-miss success.

Using a combination of horizontal drilling and hydraulic fracturing, they now bring in wells with hundreds of barrels of oil equivalent (Boe) production per day – and per-well estimated ultimate recovery up to 400,000 barrels.

And those wells can cost less than $2.5 million each.

This new play is a tune of rocks and water.

First Notes of Understanding
Salvatore Mazzullo
Salvatore Mazzullo

For a heavily drilled area, the Mid-Continent Mississippian was surprisingly little understood – or even misunderstood.

AAPG member Salvatore Mazzullo, a professor of geology at Wichita State University and a three-time winner of the A.I. Levorsen Award, started a project in 2009 to study the Mississippian Cowley Formation.

“We could figure out the Cowley – we had cores, but we couldn’t figure out where it fit in the Mississippian,” Mazzullo said.

“So we went to the outcrop. The first day, we realized these rocks are exactly like the cores we see in the subsurface. Not just similar, but exactly alike,” he said.

AAPG member Charles Wickstrom is managing member of Spyglass Energy Group in Tulsa, a player in the Mississippi Lime.

“You can take what we see in the outcrops in this tri-state area (of Missouri, Arkansas and Oklahoma) and just take it directly to the subsurface,” he said.

The Tulsa Geological Society sponsored a three-day field trip to study the outcrops in April. Saying industry interest in the Mississippian runs high would be an understatement.

“We had the field trip planned for one bus with 43 people. It sold out within an hour of the announcement,” Wickstrom said, which caused an expansion to accommodate more than 90 people.

In his studies, Mazzullo found that the accepted depositional model for the mid-continent Mississippian didn’t match up with the reality he was seeing.

“We realized these units weren’t doing what they were supposed to be doing in the model,” he said.

For instance, the model says the Mississippian rocks deepen to the south, but they actually shallow to the south because of tectonic activity, he noted.

Recognition and understanding of the unconformities in the Mississippian are essential, Mazzullo said. Some are eustatic, some tectonic.

“If it’s a tectonic unconformity and you’re looking updip for a reservoir, it’s not going to be there” he said.

In Missouri, the Mississippian outcrops can be found to about 40 miles east of Springfield. They extend south past Harrison in Arkansas and to Tahlequah in Oklahoma, Mazzullo said.

From there the characteristic Mid-Continent Mississippian rocks, up to 400 feet in total thickness, extend 300 miles to the west.

“These rocks have been there for a long time, but people haven’t been able to put it all together,” Mazzullo said.

When he looks at the Mississippian rocks, “they’re telling us, we’ve been looking at the wrong things,” he noted. “We’ve been looking at it in the wrong way.”

Complex Structure

Starting with knowledge of the subsurface was key, according to Mazzullo. That allowed close analysis of the outcrops.

He identified three distinct formations in the Osagian Mississippian, with different members and characteristics: the Burlington-Keokuk, Reeds Spring and Pierson.

The Pierson Formation contains reefs and has dolomite updip, presenting two reservoir objectives, Mazzullo said.

The Reeds Spring is a series of prograding wedges and has tripolite at the top, he noted. It also may contain multiple reservoir objectives. The Cowley is a facies of the Reeds Spring.

Care is needed because “it doesn’t suffice to say ‘Reeds Spring.’ You have to identify which wedge,” he observed.

“Our work on the outcrops has defined clear reservoir objectives within the Mississippian. It used to be you just drilled into the Mississippian,” Mazzullo said.

“Everybody went under the mistaken impression, it’s just the Miss. Well, it’s not just the Miss.

“You need to know which particular reservoir section you’re in. And you have to do sample work,” he added.

Going Out

In a series of papers, Mazzullo and co-author and AAPG member Brian Wilhite have tried to refine the nomenclature used for the Mid-Continent Mississippian, right down to the types of rocks present.

They distinguish among:

  • Chert, which is SiO2, and readily identified by geologists.
  • Tripolite, which is chert of any age that has been highly weathered by meteoric fluids, and is light-weight because of high microporosity formed during subaerial exposure.
  • Spiculate, which is fine-grained rock composed of the siliceous spicules of invertebrates.
  • Chat, which, according to the authors, is an informal, local term that does not denote any specific lithology. They suggest it not be used.

(Some mid-continent operators, however, refer to the new play as the “Mississippi chat play” and identify tripolitic or weathered chert as chat.)

Defining and identifying these rocks has helped Mazzullo identify the characteristics of different Mississippian formations. The Reeds Springs contains very cherty limestones in which spicules have not yet been found.

Wickstrom said the Mississippian often was logged without noting the distinctions among formations.

“Many well site geologists, when they got to the top of the Mississippian, may or may not have looked at the samples,” he said.

Closely defining those characteristics has helped to identify reservoir objectives, even beyond the most frequently targeted zones.

Mazzullo said the lowermost, Kinderhookian part of the Mississippian includes the Compton-Northview sections. The Compton Lime contains reefs that are locally oil-stained and deserves more study, he noted.

In the area being developed by Spyglass Energy, a productive weathered chert layer of lower porosity exists under the target Reeds Spring zone, Wickstrom said. Horizontal drilling also can open up the potential of that type of secondary target.

“I see this eventually being drilled with mutiple laterals in the Mississippian,” he said.

Unconventional Approach

Shane Matson, Spyglass Energy geologist in Tulsa, said he began searching for possibilities in the Mississippian tripolite chert in 2003.

“We weren’t paying too much attention to the other 250 feet of the formation, mostly because the logs were so confusing,” he said.

A breakthrough came in 2009, when the company obtained its first image logs in the formation, he recalled. Matson and Wickstrom were able to correlate what they saw with outcrops in the Ozarks, and now routinely run image logs on their wells.

This new Mississippian play has been called an application of horizontal drilling and fracking to a conventional reservoir. But Matson said he considers it an unconventional play because the geology is outside conventional boundaries.

“One of the challenges people have in this play is evaluating it on traditional neutron porosity logs. The porosity profile is complex,” he said.

Even the operators drilling successful wells don’t always understand the nature of the formations they’re drilling, Matson noted.

“There are multiple facies people are exploiting and much of the time, probably, they don’t don’t know which facies they are exploiting,” he said.

Wells in the Mississippian play typically produce a lot of water. And that’s something operators don’t mind in this play.

“The more water/fluids you’re getting, the more oil you’re producing,” Wickstrom observed.

Coping with the water does take planning, and Wickstrom said a development project starts with drilling an injection well to the bottom of the Arbuckle. That can handle the water from up to eight development wells, he said.

Today, the long-time Mississippian play in Kansas and Oklahoma is back big time. Exploration that was hit-and-miss is now hit-and-produce.

“The fractures were vertical and it just depended on whether you were lucky enough to get in the fractures,” Wickstrom said. “It’s ideal for a horizontal play.”

For Mid-Continent players, that’s music to their ears.

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