Porosity
evolution associated with a karst horizon occurs in highstand
systems tract grainstone. Significant porosity destruction
exists within a foot of the unconformity. Both calcite and
chalcedomy occlude porosity. Actual cavernous porosity was
filled with lime mud during subsequent transgressive carbonate
deposition. A progressive loss of carbonate cementation can
be observed below the karst surface. Solution-enlarged interparticle
porosity and maximum permeability developed approximately
five to six feet below the unconformity. |
Previous
diagenetic studies recognized that anomalously high porosity
log readings exist within some Arab-D dolostones because of
intracrystalline dolomite porosity. Initial interpretations
attributed this form of pore space to dedolomitization processes
associated with subaerial exposure. More recent diagenetic
investigation demonstrate hat the formation of intracystaline
dolomite porosity represents a fabric selective diagenetic
dolomite growth process rather than the dissolution of dolomite.
Although dolomite rhombs have cores riddled with separate
holes that could be interpreted as an early phase of a dedolomitization
process, the presence of dolomite rims that follow the skeleton
of fossils strongly argues against this. Further, dedolomitization
observed in outcrop samples illustrate that the dissolution
of dolomite generally proceeds by corroding dolomite rhomb
edges or by leaching selectively unstable zones within the
crystal to produce continuous rather than isolated intracrystalline
pores.
Although
variable, the stratigraphic distribution of porous dolomite
rhombs is also not consistent with dedolomitization related
to subaerial exposure. Porous dolomite crystals are common
at the base and never at the top of dolostone intervals as
would be expected from diagenetic process related to an unconformity.
Instead, stratigraphic and petrographic observations suggest
the development of porous dolomite rhombs is more a case of
incomplete dolomite crystal development. These observations
are also more consistent with the overall stratigraphy, which
does not exhibit other digenetic features, cycle stacking
patterns or breaks in deposition indicative of subaerial exposure.
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