Abstract

During the Late Pennsylvanian through the Early Permian the Midland Basin was very near the equator. Being located in the low latitudes meant the basin was an ideal location for carbonate sedimentation throughout the Late Paleozoic. An abundance of reef forming organisms, a warm tropical climate, and lack of clastics led to the accumulation of a considerable thickness of carbonates. These factors contributed to the formation of a broad carbonate platform in the middle of the basin. Called the Horseshoe Atoll for its paleogeomorphology, this platform is the site of numerous important oil reservoirs.

The platform began developing in the Early Strawn (Desmoinesian) while the basin was on the equator. Later during the Canyon (Missourian), carbonates were accumulating on the platform from 1 to 2 degrees north latitude. As North America (Laurentia) migrated northeastward, carbonate sedimentation continued during the Cisco (Virgilian), occurring from 2 to 4 degrees north latitude. Carbonate sedimentation on the platform ended during the Wolfcampian when at 6 degrees north latitude it was choked by influx of fine-grained clastics. From 312MA (Early Strawn) to 306MA (Middle Canyon) the relative motion of the platform and basin was north 63 degrees east. Later, from 298MA (Early Cisco) to 280MA (Wolfcampian) the direction of movement changed to north 24 degrees east. From 312MA to 280MA the North American (Laurentia) plate was oriented so the platform and basin were rotated 43 degrees to the northeast relative to current coordinates.

Carbonate distribution in the Midland Basin suggests paleolatitude and paleogeography greatly influenced sedimentation by controlling the prevailing winds, tropical climate, ocean currents, and platform orientation. When north of the equator, 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. Along the eastern edge of the platform in a northeast to southwest direction, the general facies distribution is oolitic grainstones, algal-sponge boundstones, and tidal-flat mudstones. The percentages of grainstones are highest in the northeast and decrease to the southwest where there are predominately mud dominated carbonate facies.

Carbonate facies on the platform indicate an environmental transition from high-energy in the northeast to low-energy in the southwest. This reflects the directional energy of the northeast prevailing winds and associated ocean currents as they struck the platform. The northeast-southwest depositional setting of the platform carbonates in the Late Paleozoic differs from their present north-south orientation.

In exploration and production geology it is necessary to understand how regional paleogeography and paleolatitude, among other factors, influence the distribution of carbonate facies. This approach gives a more complete and thorough understanding of the many controls on carbonate facies distribution and assists in the prediction, location, and exploitation of reservoir facies.