Late Pennsylvanian and Early Permian Sedimentation on the Central Basin Platform and Implications to the Wolfberry Deposition in the Western Midland Basin
¹Cobalt International Energy
In the late Pennsylvanian and Permian, deposition of the Wolfberry interval in the Midland Basin was controlled by sediment supply from the adjacent shelves and platform. Depositional systems on the eastern Central Basin Platform supplied sediment to the western Midland Basin (Figure 1), however those sediments varied greatly through time depending on rate of subsidence, eustatic sea level fluctuations, and climate. The Permian Basin was dominated by a humid to subhumid climate from the middle Pennsylvanian to the early Wolfcampian causing siliciclastic systems to be dominated by shales transported to the basin by rivers (Figure 2). Limestone was the dominant carbonate during that time. The climate in the region became increasingly arid in the Permian resulting in middle to upper Permian carbonates dominated by dolomite and evaporites, and siliciclastics dominated by feldspathic sand and silt transported by wind (Figure 2). The middle Pennsylvanian to early Permian was a time when large continental glaciers in the southern hemisphere repeatedly grew and melted causing repeated large amplitude rises and falls of sea level (50-100 m fluctuations every 110,000 years) similar to the Pleistocene. As a result, the Strawn to lower Wolfcamp interval contains more than 90 depositional cycles with highly variable depositional facies and common subaerial exposure surfaces. The Southwest Andrews area of the eastern Central Basin Platform (Figures 1,3) was studied with core, logs, and seismic data to interpret the depositional history of Strawn through Wolfcamp strata, and that information was used to infer depositional processes for the Wolfberry in the adjacent Midland Basin. Strawn and Canyon strata at the Southwest Andrews Area are dominated by limestones with relatively thick cycles that were capped by subaerial exposure surfaces (Figures 4, 5). Although most cycles included periods of deposition and subaerial exposure, most of the time represented by each cycle was spend underwater with carbonate deposition. The Strawn and Canyon are associated with a vertically building platform margin with modest amounts of micritic and grain-rich carbonate expected to be transported off the platform and deposited as channels, aprons and fans on the adjacent slope and basin floor (Figures 2, 6).
The Cisco to early Wolfcamp at Southwest Andrews is dominated by thin cycles with interbedded carbonates and shales (Figures 4, 5). Sinkholes filled with shale are present in upper Canyon and Cisco carbonates (Figures 7, 8). Shale-filled fluvial valleys occur in the siliciclastic-rich lowest Wolfcamp (Figure 9). The Cisco and earliest Wolfcamp cycles are thin and experienced intense subaerial exposure (Figures 4, 5) indicating that most of the time represented by each cycle was spent subaerially exposed (above sea level). As a result of being dominated by low sea level, substantial amounts of siliciclastics from the interior of the Central Basin Platform were transported across the platform margin and deposited in the western Midland Basin (Figure 10). Hence, late Cisco and earliest Wolfcamp are expected to be times when large amounts of shale accumulated in the western Midland Basin during many different eustatic cycles. A general transgression and deepening of environments followed the "maximum regression" or period of lowest relative sea level which occurred near the Cisco-Wolfcamp boundary (Figures 4, 5). The "Wolfcamp Reef" is an interval of widespread shallow-marine limestone deposited during part of that transgression (Figure 2). Limestone detritus and mud was probably transported off the platform and into the basin during deposition of the "Wolfcamp Reef". A major drowning and backstepping of the platform margin followed "Wolfcamp Reef" deposition, and the Southwest Andrews Area was covered by deep-water shales as the Abo/Wichita platform margin grew to the west. The western Midland Basin was likely starved during much of Abo/Wichita time (Figure 2).
Middle to late Permian strata were affected by a more arid climate and less continental glaciation. As a result, eustatic sea-level fluctuations generally had relatively low amplitudes (maybe 2-10 m) with more long-lived periods of high and low sea level (Figure 2). The middle to upper Permian is characterized by thick intervals of prograding carbonate platforms, and thick intervals of sandstones and siltstones in the basin. The prograding carbonate shelves and platform are dominated by dolomite and evaporites that were probably deposited during times of high relative sea level. Sands and silts were probably deposited in the basin during long periods of generally low sea level when sand and silt were blown across the exposed shelves and platforms, and then deposited in the basin as onlapping packages. Thin sandstones and siltstones were deposited repeatedly on the shelves and platform generally during transgressions following lowstands of sea level.
AAPG Datapages/Search and Discovery Article #90190©AAPG Southwest Section Annual Convention, Midland, Texas, May 11-14, 2014