DICKSON, J. A. D., Univ. of Cambridge, Department of Earth Sciences, Cambridge, UK; A. H. SALLER, UNOCAL, Brea, CA; S. A. BOYD, UNOCAL Oil and Gas, Midland, TX

ABSTRACT: Subaerial Exposure of Late Paleozoic Limestones from the Central Basin Platform, West Texas: Controls on Porosity Preservation

The Southwest Andrews area of West Texas contains cyclic Pennsylvanian and Lower Permian limestones deposited on the eastern flank of the Central Basin Platform. Most of these limestones were deposited during high stands of sea level during a time of high amplitude and high frequency sea level fluctuations probably caused by repeated waxing and waning of continental glaciers (Heckel, 1986; Ross and Ross, 1987; Crowley and Baum, 1991). The Strawn to Wolfcamp Reef interval involves 63 cycles, most are capped by subaerial exposure surfaces.

Porosity in these limestones is quite variable vertically and horizontally and is facies selective, cccuring in grainstones (5-15 feet thick) below certain exposure surfaces. The three most important porosity-modifying processes are calcite cementation, compaction and aragonite dissolution. Most limestones underwent substantial alteration early, in freshwater environments. The amount of early cement is critical to subsequent preservation of porosity. Grainstones whose primary intergranular porosity was completely cemented now yield uncompacted rocks with unconnected moldic pores. Grainstones whose primary porosity was only partially cemented possess a rigid framework that resisted compaction. Grainstones that were uncemented early, developed secondary porosity on aragonite dissolution but lost all porosity during compaction of the weak rock.

Stable isotopic analysis of bulk rock samples show distinctive trends related to cycle thickness and extent of early diagenesis. Thicker cycles have fairly consistent (isotope){18}O values (-2 to -5% PDB) but quite variable (isotope){13}C values (-4 to +3 o/oo PDB). Thinner cycles have similar (isotope){18}O values (-3 to -5 o/oo PDB), but consistently low (isotope){13}C values (-3.5 to -6 o/oo PDB). The thicker cycles possess moderate amounts of early cement, retain porosity and show a distinctive (isotope){13}C profile with a switch from -4 o/oo below, to +3 o/oo above, subaerial exposure surfaces. These cycles had shorter periods of subaerial exposure; only the uppermost parts of the cycles accumulated isotopically light, soil-related carbon. Thinner cycles were subjected to prolon ed subaerial exposure causing pervasive alteration and a consistently light (isotope){13}C profile.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.