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New Carbonate Plays in the Mississippian Lime of the Bend Arch Area, Texas

Abstract

A new drilling campaign is currently underway to assess the potential of the “Mississippian Lime” (Chapel Formation) of the Bend Arch area of Texas. Traditionally Waulsortian mounds were targeted in this area; whereas, new drilling targets porous and often silica-rich, outer-ramp and inter-mound strata for oil production. Based on six continuous cores and well logs from Shackleford, Stevens, Young, and Throckmorton Counties this study aims to (1) delineate the facies patterns and stratigraphic architecture of the Miss Lime, and (2) characterize the origin and distribution of chert. Within the Mississippian section two distinct stratigraphic units are recognized, referred to herein informally as the M1 and M2 sequences. The M1 sequence, traditionally referred to as the Chapel Formation, begins with a succession of argillaceous crinoidal floatstones and lime mudstones which grade upward into a progradational package of crinoid and bryozoan rudstones and grainstones. Waulsortian Mounds are common within the M1 sequence and are best developed in mid- to outer-ramp settings. The M1 sequence tapers basinward and transitions from grain-dominated to increasingly mud-dominated fabrics from updip to downdip. The M2 sequence begins with a landward-tapering package of skeletal-peloidal grainstones and packstones, and spiculitic lime mudstones. The M2 lime unit also transitions from grain-dominated fabrics updip to mud-dominated fabrics downdip. The M2 limestones grade upward into the Barnett Shale. The best reservoir potential exists where porous skeletal-peloidal grainstones and packstones of the M2 sequence are juxtaposed against the Barnett Shale. Chert is interpreted to have formed early in the Miss Lime, and is strongly correlated with the presence of sponge spicules and inversely correlated with early-cementation trends. As such, there is a spatial control on chert, which is best developed in mud-dominated, spicule-rich, strata of the middle-to outer-ramp. Chert is absent in the Waulsortian mounds, and rare in grain-dominated crinoidal facies. Chert distribution is also temporally restricted, and is best developed bracketing the M1-M2 boundary. The stratal architecture and silica distribution described here strongly resemble the Lake Valley Formation in Sacramento Mountains of New Mexico. Whereas, the similarities between the Chapel system and Mississippian “chats” of Oklahoma are less compelling and caution is needed when comparing these two systems.