Relation of Thickness of Mississippian Limestone in Central and Eastern Kansas to Oil and Gas Deposits

Abstract

The thickness of the Mississippian limestones in central and eastern Kansas bears a close relation to the geologic structure features of the state, and thus to the occurrence of the oil and gas deposits. The limestones of this age are widely distributed in the central and eastern portions of the state, though they are absent over large areas in the central Kansas uplift and the northern part of the Nemaha ridge, as well as in many smaller areas in other parts of the state. They attain a thickness of 1,138 feet in Clark County, but in most of eastern Kansas they have a general thickness of only 300 to 450 feet.

The Mississippian limestones include representatives of the Kinderhook, Osage, Meramec, and Chester series. They were deposited on a nearly flat surface on the Chattanooga shale. After their deposition they were gently folded and elevated and the subsequent erosion reduced the surface in pre-Pennsylvanian time to a nearly flat horizontal peneplain. Rocks lowered below the plain of base leveling were preserved; those that had been raised above it were worn away. There is, therefore, a close relation between the thickness of the Mississippian limestones and the folding that occurred during the time interval between the final deposition of the Chattanooga shale and the close of base leveling. In western Kansas the Chattanooga shale was not deposited and the Mississippian limestones rest on an eroded pre-Chattanooga surface, so that the thickness there is not so significant of structure as in eastern Kansas.

During the pre-Pennsylvanian folding of the Mississippian rocks, pronounced anticlines, such as the Nemaha ridge fold, the Voshell anticline and many others of less prominence were initiated. They are expressed on the accompanying thickness map (Plate I) by thinning of the Mississippian limestones. The base leveling appears to have been complete. Over 1,100 feet of rocks were removed from the crest of such steep anticlines as the Burns dome and the Eldorado anticline. Erosion revealed the basement granite on parts of the central Kansas uplift and the Nemaha ridge, but on most anticlines the Mississippian was not entirely removed.

During early Pennsylvanian time in eastern Oklahoma 18,000 to 20,000 feet of Cherokee and earlier Pennsylvanian rocks were deposited in a gradually subsiding basin. The surface of eastern Kansas appears to have remained above sea level during most of this time, for only about 500 feet of the uppermost Cherokee rocks overlapped upon the surface in Kansas. During this epoch, the previously eroded surface of the Mississippian, which had been reduced to base level, was gradually reelevated. The reelevation was accompanied by synclinal folding, which developed the northern extension of the Cherokee basin of Oklahoma and probably also the Forest City basin. During the same time, anticlinal movements rejuvenated the Nemaha ridge fold and there was a relative downward displacement on the east of 100 to 500 feet. Most of the other earlier anticlines and synclines were rejuvenated in varying degree.

The continued advance of the Cherokee sea ultimately covered the region except for the northern part of the Nemaha ridge, the central Kansas uplift, and the intervening area which were not submerged until Marmaton time.

The slow and gradual deformation that was going on during the deposition of the early Pennsylvanian rocks in Oklahoma continued in some degree after the Cherokee sea reached Kansas, and it continued to deform the later Pennsylvanian rocks by small increments of folding during their deposition.

Some erosion affected the parts of the surface that had not yet been submerged. Because the limestones contained many porous zones and the surface was covered by residual chert, and because the gradients of the surface were low, most of the rainfall escaped by underground channels and the surface dissection was relatively small.

The folds of determinable pre-Mississippian age trend in a northwesterly direction. The original folds of the Mississippian rocks trend chiefly toward the north or northeast, although there is some continuation of the northwesterly trending folds. During the Cherokee and later Pennsylvanian time and during the Permian period, both sets of folds were active although northeasterly trending folds predominated.

In some places, particularly along the Nemaha ridge fold, there was a tendency toward en echelon arrangement of secondary anticlines. The areas in which igneous intrusions occurred in Pennsylvanian or later time are shown to have been already slightly domed by pre-Pennsylvanian folding. An ill-defined area in Republic and adjoining counties appears to have no Mississippian rocks. The absence of Mississippian rocks in this area may indicate only a local pre-Mississippian topographic high. It may, on the other hand, indicate a folding that trends northwest, parallel to the central Kansas uplift.

As the structural features are closely related to the thinning of the Mississippian rocks, there is a close relation between thinning and the occurrence of oil and gas on the anticlines in the central and eastern parts of the state. Nearly all fields that produce from anticlines are underlain by thin sections of Mississippian rocks. It is concluded, therefore, that the presence of a thin section of the Mississippian in areas thus far unproductive may in some cases indicate the proximity of incompletely explored structural highs and warrant further investigation of the local conditions. Some prominent anticlines, however, are not productive of oil or gas. A list of unproductive areas where the Mississippian rocks are thin is presented.

The producing zones in Mississippian rocks appear to be independent of the stratigraphic formations. Production is dependent on the porosity of the limestone. The base leveling of the folded rocks brought the various formations of the Mississippian to the surface at different places and subjected them to weathering and leaching. In some places where the ground-water level had been lowered, porous zones are present to a depth of over 100 feet below the surface of the Mississippian. A list of fields that have produced from Mississippian rocks is given and the available production figures are presented.