Page 250 - Geology of Carbonate Reservoirs

DIAGENETIC RESERVOIRS 231

paleo - highs, and that it was later modified by deep - burial neomorphism and partial
cementation.
The depositional succession illustrated in the Cotton Valley oolite buildups at
Overton Field is similar in many respects to the tidal bar complexes of the Schooner
Cays on the Great Bahama Bank. They are tidal bar or sand wave buildups com-
posed of well - washed grainstones deposited on and along bathymetric slope changes
(slope breaks). The Schooner Cay tidal bar belt is associated with the shelf edge of
the Bahama Banks and the strong tidal effects that sweep across the shelf margin
from the depths of Exuma Sound. Oolite bar buildups at Overton Field were depos-
ited on bathymetric highs over salt anticlines and basement features related to the
Ancestral Sabine Uplift and the East Texas Salt Basin. Oolites deposited on the
more stable and apparently higher - relief basement structures formed in shallow -
water environments that would be the first to undergo exposure to micropore -

forming diagenetic waters during relative sea - level retreat and early burial. Oolites
deposited on salt - related highs west of the Ancestral Sabine Uplift are interpreted
to have remained in the marine phreatic environment without undergoing the early
diagenesis that produced the microrhombic microporosity.

Reservoir Characteristics Porosity at Overton Field is both fabric and facies selec-
tive. Pore - forming diagenesis occurred only in the oolite buildups on the tops of
basement highs, which are, in turn, reflected as thin zones on interval isopach maps

of the Bossier Shale that overlies the Cotton Valley Limestone in the fi eld area.
Exploration and development strategies should be based on locating paleo - highs

that are reflected in interval isopach (or isotime) thins in strata just younger than
the reservoir target zones.

Geological Concept The depositional succession for the Cotton Valley oolite
buildups is interpreted to be a sand wave complex that conformed to slope breaks
or slope changes from shallower to deeper water. Sedimentary structures in cored
sequences reveal sequences almost identical to the lower part of the beach and
barrier island standard succession: that is, slightly muddy lower segments that passed
upward into ripple - bedded, mud - free grainstones. The muddier segments commonly
contain skeletal fragments and, in some cases, whole coral skeletons up to 10 – 15 cm

in diameter. Paleogeographically, the sand wave complex is located along the slope
changes around the Sabine Uplift so that it is assigned to the slope - break standard
depositional succession. Tidal bars, or sand waves, are elongate parallel to the slope
break in this case, and they may extend inboard from the slope break for a few
kilometers. Because diagenesis was fabric and facies selective but affected facies
only in certain paleostructural situations, the combination of facies and paleostruc-
tural maps provides the key elements for exploration and development at Overton
Field. This knowledge gives the explorationist a model — a geological concept — with
which to proceed. Understanding the origin and timing of pore - forming diagenesis
enables development of the reservoir in a cost - effective manner.

8.4.2.2 Happy Field

Location and General Information Happy Field is located in the northern Midland

Basin about 50 miles southeast of Lubbock, Texas (Figure 8.13 ). The reservoir is at