Abstract

Placing regional lithofacies maps into their Late Paleozoic latitude and plate orientation can assist in explaining and predicting basin sedimentation patterns. Paleogeographic reconstructions indicate that west Texas was in the low latitudes during the Late Paleozoic. A low latitude location meant that the region was an ideal area for carbonate sedimentation. Carbonate sedimentation during the Pennsylvanian was influenced by trade winds in a similar fashion as observed in the modern tropics near the equator.

Paleogeography contributed to the formation of broad carbonate platforms in the Midland basin. One such platform, called Horseshoe atoll for its paleogeomorphology, is the site of numerous important oil reservoirs. Paleolatitude reconstructions of the platform place it very near the equator throughout its existence. In addition, it is clear that from 312 Ma to 280 Ma the atoll and basin were rotated 43 degrees to the northeast relative to current coordinates. Shelf edges in the Delaware and Midland basins, presently oriented 0 to 15 degrees, were in fact oriented 40 to 60 degrees northeast during the Late Paleozoic. Carbonate facies distribution on the Horseshoe atoll suggests that paleolatitude and paleogeography greatly influenced sedimentation by controlling the prevailing winds, tropical climate, ocean currents, and platform orientation. During the Pennsylvanian the eastern edge of the atoll faced prevailing northeasterly winds. This orientation, when combined with the regional winds and ocean currents, affected the distribution of carbonate facies along the atoll.

Carbonate facies in the Cogdell field mimic the regional variations of carbonates facies on the Horseshoe atoll. Lithofacies at Cogdell clearly illustrate the influence paleogeography and prevailing winds had on sedimentation. Located on northeast area of the atoll, Cogdell exhibits an environmental transition from high energy in the northeast to low energy in the southwest. The percentages of grainstones are highest in the northeast and decrease to the southwest. The grainstones form a belt which runs the length of the field in a northeast to southwest direction. Algal-sponge boundstones and wackestones are found just west of the grainstones belt in leeward areas. The past orientation and paleolatitude of the carbonate shelves must be determined and combined with the direction of prevailing winds to better understand lithofacies distribution. Understanding basin paleogeography and prevailing paleowind direction enables the prediction of the distribution of carbonate grain types and carbonate sand body geometry and location.