Abstract

Carbonate buildups that actually or apparently contain lime mud and lack internal support by skeletal framebuilders are given the default name “mudmounds.” Frameless reef is a more realistic and practical name for these buildups, even though it is not euphonic and catchy. These buildups may contain a great variety of frameless depositional fabrics and constituents - not just lime mud - as one might think on examination of the typically dull, gray, weathered surfaces on fine-grained buildups with unaided eye. Frameless reefs may be cementstone buildups, lime mud mounds with peloids, cements, and non-framework skeletal constituents, buildups of micrite precipitated around cold seeps, or bioclastic grainstone - packstone piles deposited as “windrows” of skeletal fragments. Early Mississippian strata contain frameless reefs almost exclusively. They are also common in Upper Carboniferous beds, they dominate the Precambrian to Ordovician section, and frameless seep mounds are common in the Cenozoic section, though predominantly in siliciclastic settings. This variety exists because frameless reefs form in nearly all depositional environments, they may have been influenced by precipitation around submarine vents, irrespective of depositional environment, or they may have developed by early marine cementation and microbial carbonate production. Frameless buildups are usually more complex than they appear on cursory examination, and their lithological complexity is manifested as “reservoir inhomogeneity.” Such diversity in buildup character does not preclude commonality in reservoir characteristics, however. Frameless reef reservoir examples are too numerous to list in detail but some familiar ones are the Lodgepole mounds in the Williston Basin, Mississippian and Pennsylvanian reefs in the Paradox Basin, and Chappel reefs in the Hardeman and Fort Worth Basins. Giant Caspian Basin fields such as Tengiz and Karachaganak include frameless reef reservoir zones and major mineral deposits are common in frameless reefs in the Mississippi Valley of the USA, in Ireland, and England.