Surface to Subsurface Correlation of the Late Ordovician Montoya Group, southern New Mexico and western Texas
POPE, MICHAEL C., Washington State University, Department of Geology, Pullman, WA 99164, HUAIBO LIU, and DAVE THOMAS, Tom Brown, Inc., Midland, TX, 79702
A field-based sequence
stratigraphic framework including hand-held gamma-ray profiles
of the Late Ordovician Montoya Group in southern New Mexico and western Texas was
correlated into the subsurface using wireline logs, and core/cuttings descriptions. This
analysis
indicates that 2nd- and 3rd-order depositional sequences are regionally
traceable. In outcrop the Montoya Group rests unconformably on the Early Ordovician El
Paso Group, but in the subsurface it lies unconformably on the Middle Ordovician Simpson
Group. The 2nd-order TST is characterized by skeletal packstones/wackestones with lower
gamma-ray values. Deepwater cherty facies with relatively higher gamma-ray values,
characterize the 2nd-order maximum flooding zone. The 2nd-order HST is composed of
subtidal carbonates shallowing upward into prograding peritidal carbonate facies; this
unit has a higher gamma ray response at its base and lower values at its top. In the
subsurface the top of the Montoya Group may be
well
-cemented ooid grainstone. The
2nd-order
sequence
boundary separating the Montoya Group from the overlying Fusselman
Dolomite is a subaerial surface in outcrop but is conformable in the subsurface. Montoya
outcrops can be differentiated into six 3rd-order sequences.
Sequence
0 is locally present
within incised valleys or topographic lows on the underlying El Paso karst surface.
Sequence
5 occurs only far down dip being eroded up dip. Sequences 2-4 are regionally
recognizable and correlative into the subsurface. These sequences are equivalent to the
Cable Canyon Sandstone (0 and 1), Upham Formation (1), lower Aleman Formation (2), upper
Aleman Formation (3), and Cutter Formations (4 and 5) respectively. Integrating field
observations, hand-held gamma-ray
log
profiles and subsurface data provides a detailed
sequence
stratigraphic framework to exploit the unusual Montoya Group chert reservoir.