Page 246 - Geology of Carbonate Reservoirs
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DIAGENETIC RESERVOIRS 227
coast of South Andros Island in the Bahamas. The shape and extent of many cave
systems is dictated by preexisting fracture systems, and this association may be
especially useful for exploring and developing paleocave reservoirs. The caves of
northeast Yucat á n, South Andros, and in the Guadalupe Mountains are related to
fracture systems. It is certainly worth considering that paleocave reservoirs and
aquifers may follow preexisting fracture patterns.
Two field examples of hybrid reservoirs with diagenetic attributes dominant were
selected for this chapter. One of them illustrates diagenetic microporosity (neomor-
phic stabilization diagenesis) in a grainstone reservoir at Overton Field, Texas. The
other is an example of pore enlargement, new pore formation, and moldic porosity
in a Permian grainstone and reef - rubble buildup at Happy Field, Garza County,
Texas. These examples were chosen because they represent different styles of dia-
genesis and pore types.
8.4.2.1 Overton Field
Location and General Background Overton Field is one of the largest gas fi elds
in the East Texas basin (Figure 8.10 ). It produces from altered oolite grainstones in
the Jurassic Cotton Valley Limestone. Diagenesis affected only the oolitic facies on
tops of paleostructural highs while relatively unaltered, nonproductive oolite grain-
stones are present lower (older) in the stratigraphic section. The relatively unaltered
oolite facies are also missing on paleostructural highs in the producing fairway.
OKLA
N 0 10 20 TEXAS
mi
MEXA-TALCO FAULT ZONE ? ? ? GRAINSTONES Kilgore ARK
OOLITIC
LA
? ? ANCESTRAL
SABINE
UPLIFT
MUD-RICH LS.
OVERTON
McSwane
OUTER RAMP
CARBONATES
Bear Grass
Bald Prairie
Figure 8.10 Location of Overton Cotton Valley (Jurassic) gas field in East Texas. Note that
the field is on the edge of an ancient structural feature known as the Ancestral Sabine Uplift.
To the west, off the edge of the ancient uplift, is the East Texas salt basin where salt tectonics
has created uplifts, domes, diapirs, and withdrawal basins.