Unconformities and Porosity in Carbonate Strata

Abstract

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. 1. 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. 2. The stable isotope signature most characteristic of subaerial exposure is abrupt decreases in d13C 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.