--> --> Lithostratigraphic Framework of the Wolfcamp and Spraberry of the Midland Basin

AAPG Southwest Section Annual Convention

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Lithostratigraphic Framework of the Wolfcamp and Spraberry of the Midland Basin

Abstract

The greater Permian Basin of west Texas and southeastern New Mexico represents a world-class petroleum province. The Midland Basin, the eastern sub-basin of the region, contains numerous large conventional oil and gas fields, including the giant Kelly Snyder and Spraberry Trend fields. Recently the Wolfcamp and Spraberry Formation have emerged as two of the nation’s leading unconventional shale plays. Despite the fact that these intervals have been extensively studied for conventional reservoir potential, recent coring and horizontal drilling is shedding new light on the Wolfcamp and Spraberry operational units and sub-units as unconventional petroleum systems. The Wolfcamp (WC) shale consists of five operations units and nine subunits. The oldest of these, the WC-D (Cline), the product of the late Pennsylvanian icehouse, consists of a series of semi-starved, thinly-bedded basinal cyclothems. These grade upward into clay-rich shales of the WC-C2 which are overlain by more organic-rich, calcite-rich mudstones of the greenhouse WC-C1, WC-B, and WC–A units. A high frequency of thin volcanic ash beds is noted within the WC-B and WC-A. The Wolfcamp shale is directly overlain by the Dean sandstone, which marks a significant change in basin evolution with the initial influx of sand- and silt- rich deep-water fan systems into the basin. The overlying Spraberry interval consists of five operational units, including from youngest to oldest, the Leonard shale (Lower Spraberry shale), Jo Mill sand, and Lower, Middle, and Upper Spraberry. The Jo Mill represents a second major submarine fan phase, and the Upper Spraberry represents a third. Minor fan systems are noted with the Lower and Middle Spraberry. Regional sea-level and tectonic analyses tie specific operational sub-units to particular periods of long-term lowstands and highstands -- a result of eustatic changes, regional tectonic and structural events, and sediment supply. Spraberry depositional rates display a ten-fold increase over Wolfcamp units. Integration of core, thin- sections, mineralogical, reservoir pressure and petrophysical data indicate that each Wolfcamp and Spraberry operational sub-unit is unique, differences that are ultimately revealed during subsequent drilling and completion operations.