Depositional Environments of the Mississippian Chappel Bioherms, Hardeman County, Texas: ABSTRACT

Abstract

Numerous crinoid-fenestrate bryozoan banks are developed within the Mississippian Chappel Formation, located in the Hardeman basin, Hardeman and Wilbarger Counties, Texas. The banks are oval in shape and range in size from 2 to 12 mi (3 to 19 km) in diameter. Stratigraphic, hydrocarbon entrapment in the banks has resulted in cumulative production exceeding 6 million bbl of oil plus 13 bcf of gas. The Chappel Formation is a shallow-water limestone, overlain End_Page 414------------------------------ by the oolitic grainstones of the Mississippian St. Louis Formation. Three distinct Chappel facies can be identified: the bank core, flank, and interbank facies. The bank core is composed of crinoid-fenestrate bryozoan mudstones and wackestones; the flank consists of crinoid-bryozoan grainstones and packstones; and the interbank consists of argillaceous sponge spicule mudstones with minor crinoid and bryozoan debris. Chert is a major constituent of the interbank facies but decreases in amount toward the bank core. Bank growth began in a low-energy environment with the mechanical accumulation of lime mud which was baffled and trapped by crinoids and fenestrate bryozoan. Once the bank core reached wave-base, the crinoid-bryozoan mudstones and wackestones were reworked and redeposited as the crinoid-bryozoan grainstones and packstones of the flank facies. The extensive development of the flank facies, compared with bank core development, indicates that the top of the bank remained at or near wave-base for an extended period of time. Porosity development in the Chappel banks is secondary and results from dolomitization of the micritic bank core, fracturing and leaching. Although the crinoid-bryozoan grainstones of the flank facies were originally porous, primary intergranular porosity is now absent because of epitaxial cementation. The Chappel banks can be located in the subsurface by using isopach maps which help identify interval thickening within the Chappel. In addition, thinning, shown by the isopach intervals on horizons immediately overlying the Chappel Formation, can also be used to delineate the presence of these bank deposits. In subsurface exploration, the bank facies can be differentiated from the interbank facies by petrographic analysis and by noting the more massive and also lower gamma ray log response. The lower gamma ray log response is caused by a lack of argillaceous material in the bank facies. End_of_Article - Last_Page 415------------