Identification of Subaerial Exposure Surfaces and Porosity Preservation in Pennsylvanian and Lower Permian Shelf Limestones, Eastern Central Basin Platform, Texas
Published:January 01, 1995
J. A. D. Dickson, Arthur H. Saller, 1995. "Identification of Subaerial Exposure Surfaces and Porosity Preservation in Pennsylvanian and Lower Permian Shelf Limestones, Eastern Central Basin Platform, Texas", Unconformities and Porosity in Carbonate Strata, David A. Budd, Arthur H. Saller, Paul M. Harris
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The southwest Andrews area on the eastern side of the Central Basin platform (west Texas) contains cyclic Pennsylvanian and Lower Permian shelfal limestones. Limestones were deposited in shallow marine environments during numerous highstands of sea level, but most cycles are bounded by subaerial exposure surfaces. Reservoir porosity is developed in only 10-45% of those depositional cycles in any given well. The purposes of this paper are to determine: (1) features useful for identifying subaerial exposure surfaces, (2) factors that affect stable-isotope profiles around subaerial exposure surfaces, and (3) circumstances critical to porosity preservation in subaerially exposed limestones.
Features commonly present at or below subaerial exposure surfaces include an abrupt change in depositional lithology, caliche crusts, micritic rhizoliths precipitated around roots, soil-related fractures, breccias, and mottling associated with plant roots. Rhizoliths, caliche crusts, and breccias have developed best in wackestones and packstones. Mottling associated with plant roots is distinct in grainstones and was caused by heterogeneous dissolution and cementation.
The stable isotope signature most characteristic of subaerial exposure is abrupt decreases in δ13C of the carbonate immediately below subaerial exposure surfaces. This signature is displayed best in cycles with: (a) wackestones/packstones at the top, (b) moderate duration of subaerial exposure, (c) limited overprinting by later meteoric diagenesis, (d) little erosion during the subsequent transgression, and (e) negligible effects of late cements on the isotopic composition of the bulk rock.
Ultimate porosity of these subsurface limestones was largely determined by amounts of burial compaction and burial cement, rather than by the amount of porosity created during subaerial exposure. Most of the limestones were porous during shallow burial. Currently porous grainstones have significant early intergranular calcite cement, limited compaction-related features, and minor burial cements. In contrast, nonporous wackestones, packstones, and grainstones had much primary and secondary porosity occluded by compaction and /or cements derived from compaction-related pressure solution. Early marine and freshwater lithification apparently left currently porous strata with a rigid framework that resisted compaction during deeper burial. Early lithification and associated porosity are most common in the upper part of relatively thick depositional cycles.
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Unconformities and Porosity in Carbonate Strata
Looking for insights and methods useful for predicting and identifying subaerial exposure surfaces and associated porosity? This memoir addresses four major topics: Detection of unconformities; Modification of porosity during exposure; Preservation of porosity during burial; and Influence of unconformities on subsequent depositional and diagenetic patterns.