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The Mississippian Chappel Limestone constitutes an important oil-bearing unit that occurs along the western margin of the Fort Worth basin in north-central Texas. It has long been assumed that the Chappel is productive from pinnacle reefs. Stratigraphic relations of the "Chester," which in part overlies the Chappel, and the Chappel have remained enigmatic.
Study of cores reveals that the Chappel represents a mound-core facies consisting predominantly of fenestrate bryozoans which, soon after death, became rigidly stabilized by submarine aragonitic-fan druse cements; these provided sufficient wave resistance for the mounds to be termed "reefs." The "Chester" represents a flank facies, consisting predominantly of crinozoan components and siliceous sponge spicules, that is time-equivalent to Chappel core facies. Flank dips range up to 35°. Thus the term "pinnacle" is no longer applicable to these reef complexes.
The core facies consists predominantly of bryozoan boundstones and very coarse grainstones ("slabstones"); the flank facies contains grainstones, packstones, and wackestones. Slope instability precluded formation of submarine cements in grainstones of the flank facies.
The reef complex was subjected to complex, multicyclic diagenesis which records several episodes of subaerial exposure and freshwater diagenesis followed by submergence, recolonization, and renewed reef growth.
Nearly all preserved porosity is in the reef core and represents primary voids among erect and broken bryozoan "fronds"--voids which have been filled to varying degrees by submarine and vadose internal sediments, submarine cements, and meteoric vadose and phreatic cements. These materials were cyclically emplaced during submergent and emergent phases.
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