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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Origin of Reservoir Compartmentalization
in Lower Ordovician Karstic Dolostones,
Ellenburger Group, West Texas
By
Ellenburger Group reservoirs constitute a major play in
the Permian Basin of West Texas, with over 1.4 billion bbl
cumulative production through 1985. These reservoirs
typically have been developed by assuming homogeneous
fracture-related pore systems. Examination of core, log, and
production data demonstrates that most Ellenburger
reservoirs are characterized by pronounced vertical and
lateral
heterogeneities created by post-Ellenburger karst
development.
Vertical reservoir compartmentalization in the Ellenburger evolved from development of a laterally-extensive cave system between 100 and 300 ft. beneath the original land surface. Caves were filled by relatively impermeable siliciclastics from the overlying Simpson Group, effectively isolating permeable cave-roof breccias (uppermost Ellenburger) from collapse breccias deposited on cave floors prior to shale infill. This cave-fill-associated vertical reservoir compartmentalization is prevalent throughout a 3,000 square-mile, six-county area, which has produced more than 80% of the oil from all major Texas Ellenburger reservoirs. Historically, most wells were completed in the uppermost (cave-roof) Ellenburger to avoid water-coning, leaving significant mobile oil (as much as 30% of original oil in place) ineffectively drained from the less commonly exploited lower collapse zone.
Lateral
compartmentalization of Ellenburger reservoirs
originated by localized collapse of the cave system both
during karst formation and after burial. In the Shafter Lake
field,
lateral
compartmentalization is the result of a 200-ft.
vertical collapse during deposition of Simpson Group
sands. Abrupt
lateral
discontinuities in the Big Lake and Glasco fields may represent similar collapse-related
features, such as are spectacularly displayed in Ellenburger-equivalent
outcrops of the Franklin Mountains. An estimated
750 million bbl of remaining mobile oil in addition to
conventional reserves, occurs in this mature but complexly
compartmentalized play. Considering this paleokarst model
will aid in further exploitation of Ellenburger reserves.
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