MOZYNSKI, DON C., Consultant, Dallas, TX, ALISTAR REID, Consultant, Midland, TX, and DALE A. WALKER, Mobil E & P U.S. Inc., Midland, TX

ABSTRACT: The Effect of Paleogeography on Erosion of Pennsylvanian Lowstand Carbonates in the Northern Midland Basin, with Modern Analogies: a New View of an Old Trend

In the Pennsylvanian, shallow-water lowstand carbonates were deposited seaward of the Horseshoe atoll in the northern Midland basin. During subsequent sea level fluctuation, the lithified sediments were subaerially exposed and then chemically eroded into high-relief tower karst terrains with excellent dissolution porosity. After mechanical erosion and burial under younger slope to basin sediments, the paleokarst surface became a trend of fields with stratigraphically entrapped oil. This concept contrasts with the conventional view that the majority of these buildups were deeper water pinnacle reefs.

The morphology of individual tower karst terrains was a function of prevailing northeasterly winds and the geographic position relative to the emergent Horseshoe atoll, which was a large positive feature during lowstand deposition and erosion. Fields such as Perriwinkle Canyon in Martin County, which were in the lee of the atoll, tend to be composed of symmetrical tower karst features analogous to those in Phang Na Bay, Thailand, or those in southern China. These features have near vertical sides with steep slopes in all directions. Fields such as B. C. Canyon in Howard County, which were outside the lee of the atoll, are composed of asymmetrical tower karst features similar to the mogotes of Puerto Rico. These features have steep to near vertical slopes on the lee side of the feature and relatively gentle slopes on the windward side. The judicious application of paleokarst terrain models can be very beneficial in predicting the geometry and areal extent of paleotopographic features and reservoir facies in the lowstand trend.

AAPG Search and Discovery Article #90994©1993 AAPG Southwest Section Meeting, Fort Worth, Texas, February 21-23, 1993.