Page 247 - Geology of Carbonate Reservoirs

228 SUMMARY: GEOLOGY OF CARBONATE RESERVOIRS

Natural gas is produced from intraparticle microporosity that developed after meta-
stable ooids were altered to microporous, microcrystalline grains that retained the
size and shape of the parent ooids but lost their original microstructure and mineral-
ogy. The microcrystalline microporosity is interpreted to be the result of neomorphic
stabilization (a type of recrystallization) that transformed metastable, probably Mg -
calcite, ooids into spheroidal masses of stable, microrhombic calcite microcrystals
with micrometer - scale, intercrystalline porosity. Because microporosity occurs only
in oolite grainstones on the crests of paleo - highs, it is facies selective and because
only the ooids were altered — other constituents were not similarly altered — the

reservoir is also fabric selective. These attributes define the reservoir as a hybrid
with diagenetic attributes dominant. But diagenesis only affected oolite facies on

paleostructural highs — not all oolite facies in the field. This hybrid reservoir is a
special case in which porosity distribution is the result of diagenetic alteration that
occurred only on certain paleostructures, meaning that oolite grainstone facies in
general are not proxies for porosity. Instead, paleostructure is the key element in
the geological concept for finding reservoir porosity. Only the upper Cotton Valley

oolite facies on paleostructural highs are productive. Other oolite facies in the for-
mation are not productive and although those oolites may occur higher on present
structure than oolite facies in the productive fairway, they were not on highs when
pore - forming diagenesis created the reservoir.

Structural Setting Present structure at Overton Field (Figure 8.11 ) reveals several
NE – SW elongate highs along the western perimeter of a basement feature known

as the Ancestral Sabine Uplift, along with some higher, circular structures west of
the uplift. Drilling revealed that the smaller, circular highs are salt domes. Wells that
tested these structures were dry holes that did not penetrate porous sections of the
Cotton Valley Formation. The takeaway lesson is that present structural highs can
be “ high and dry. ” In this case it is because the Cotton Valley Limestone on the salt
anticlines contains stratigraphically older oolite facies that were not in the paleo-
structural position to be affected by microporosity - forming diagenesis. The more
oval, elongate structures on the edge of the Ancestral Sabine Uplift refl ect the
underlying basement of the Ancestral Sabine Uplift where wells that tested the
Cotton Valley Limestone are productive. Paleostructural mapping, or interval
isopach mapping of the overlying Bossier Shale Formation, revealed that the pro-
ductive wells are higher on paleostructure than the wells on the salt domes to the
west, as illustrated by the isopach map of the overlying Bossier Shale (Figure 8.12 ).
Shale thicks over present structural highs (the salt domes) reveal that the domes
were paleostructurally lower than the ancestral uplift when the microporosity was
formed. Even though salt movement created environments favorable for oolite
formation in the older part of the Cotton Valley Formation, it did not put the older
oolite facies in the diagenetic setting that created microporosity. Bossier Shale
isopach thins reveal the antecedent highs on the underlying basement surface where
intraparticle microporosity (Figure 6.7 ) was formed.

Depositional and Diagenetic Characteristics Unaltered oolites from dry holes
drilled on the salt anticlines west of the Ancestral Sabine Uplift are also older than
those in the producing wells on the basement highs. The older oolites were deposited
in agitated water as oolite bars on bathymetric highs at a lowstand of Jurassic sea