The Mississippian Limestone of northern Oklahoma and southern Kansas is comprised of four basic depositional environments. These are the Mississippi “Chat”, Chester, Meramec and Osage. The top of the Mississippi is a major erosional unconformity across Kansas. As a result, Mississippi production in this area is from an Osage chert section at the top of the Mississippian and from a sand lens in the Mississippi section, which is usually referred to as the Kinderhook sand.
Mississippian rocks produce oil in several hundred fields in Kansas from combination structural-stratigraphic traps in which porous chat and overlying conglomerates change to nonporous chat or limestone in an updip directions.
The widespread distribution of Upper and Lower Mississippian production in both large and small fields makes it a potential target horizon in virtually all wildcat wells drilled where Mississippian rocks are present.
The Mississippi “Chat” is the uppermost member at the unconformity between the Pennsylvanian and Mississippian aged rocks and is considered to be a very good hydrocarbon reservoir. These “Chat” reservoirs are very wide spread, vary in gross thickness from a few feet to 80’ thick and are heterogeneous in nature. Net porosity thicknesses greater than 5% range from a few feet to greater than 40 feet. This is caused in part through uplift, alteration, erosion and deposition of the original limestone commonly referred to as Mississippi Lime. The “Chat” is comprised of varying amounts of weathered chert, limestone and dolomite. Porosities range from 3 to 35%. The resistivities are generally low due to the amount of weathered chert and appear to be wet on logs by conventional petrophysical analysis. These reservoirs are in fact not wet but do produce some water associated with the production of hydrocarbons. Water rates vary between wells depending on the actual mineral composition of the formation being completed.
The Mississippi Lime underlies the “Chat” and is comprised of the Mississippi Chester, Manning, Meramec and Osage. It is a deep water to shallow marine limestone sequence with interbedded dolomite facies enhancing porosity and permeability. This zone is also productive in the prospect area in addition to the Chat. Porosities range from 5 to 15% with water saturations ranging from 25 to 60%. Net porosity thickness greater than 5% ranges from 10 to 100’ with an average of 30 to 50’.
Due to the heterogenic porosity development within the Mississippian formations it is advantageous to drill horizontal wells to interconnect a larger percentage of porosity with one well bore. Fracture treatment of the formation during completion in addition to the horizontal placement of the well bore will add significant conduits for production of hydrocarbons adding significant recoverable reserves. These reserves would not be recovered with a conventional vertical well.
A concentration of Middle Pennsylvanian oil production from the Cherokee and Marmaton Groups produce on and immediately west of the Central Kansas Uplift. In western Kansas the Cherokee Group was deposited on an extensive pre-Pennsylvanian erosion surface on the flanks and over the crests of the Uplift where it locally pinches out.
Sandstones occupy the lower Cherokee including those that fill valleys incised into the underlying strata and are best developed in the vicinity of the Circle Star acreage which uplifts in NW Kansas and low-relief structures that were a major source area for these deposits. These Lower Pennsylvanian strata are limited to the lower reaches of the basins.
Marmaton and Cherokee limestones are productive across western Kansas. They are components comprised of high-energy deposits such as limestones or mud-dominated carbonate buildups that are altered and leached by exposure of each cycle.
Seismic, core drilling, and subsurface methods have been employed to find anticlinal closures with Marmaton and Cherokee pay zones in western Kansas. Recent successes in the Marmaton and Cherokee limestones and sandstones in western Kansas are occurring as companies explore low-relief structures in less heavily drilled areas west of the Central Kansas Uplift. Circle Star is aiming to acquire interests in this area across producing trends along the western flanks of the Uplift as well as in the less heavily drilled parts of the region.