Invited speaker presentation for the Mississippi Geological Society 2002 "Spring Symposium on Recent Activity and Trends in the Mississippi-Alabama Oil Patch" held on May 16, 2002, in Jackson, Mississippi.
The basis for the study is a closely spaced grid of ground-penetrating radar (GPR) data. Seven and eight-tenths miles of GPR data was acquired and processed. An initial geological interpretation of the GPR data was completed to pick locations for twenty-nine 2.5-inch-diameter cores. Locations for coring were selected to analyze different GPR reflection patterns, while generating a core distribution that would facilitate the construction of cross sections.
The coalesced, collapsed-paleocave system analog developed from this study can be used to understand the potential reservoir types that may be encountered in the deep-Knox carbonate section in Mississippi. At depths greater than 10,000 ft. the reservoir will be composed mainly of crackle breccia fractures and interclast pores. Only minor matrix porosity will be present. Porosities will likely be between 5 to 10%, but permeabilities may be in the tens to hundreds of millidarcys.
ROCK TYPES DESCRIBED
Undisturbed-Host Rock: The host rock is composed of a variety of packstones and grainstones.
Disturbed-Host Rock: The rock types in the disturbed-host rock are the same as those in the undisturbed-host rock. The disturbed-host rock is commonly crackle brecciated. The brecciation is attributed to minor settling into deeper chambers. Some tilting is noted in the cores. Within the disturbed-host rock are small pockets of breccia and sediment a few inches to a few feet thick. The breccias are both clast and matrix supported.
Matrix-Free, Clast-Supported Breccias: These breccias are composed of clasts ranging in size from a few inches to several feet. Thicknesses of the breccias extend from less than a foot to 15 feet. Clasts are composed of different lithologies of the Ellenburger strata. Pore types include interclast (between clasts), vug (within clasts), and crackle breccia (within clasts) porosity. The finer, better-sorted clasts are probably deposited by transport through a passage. The larger clasts are most likely the result of collapse from the cave roof or walls.
Clast-Supported Breccias with Matrix: The poorly sorted clasts are similar to those seen in the matrix-free, clast-supported breccias and range in size from a few inches to more than 6 ft. The amount of matrix ranges from a few percent to 40% and consists of silt- to granule-sized sediment. This rock type was deposited within cave passages or in dolines.
Matrix-Supported Breccia: The clasts and matrix are similar to those in the clast-supported breccias with matrix. The amount of matrix ranges from 40 to 80%. Clast size ranges from a few inches to a foot in diameter, and the clasts are poorly sorted. Compaction and slump features are present. Some upward-fining and -coarsening sequences are present and are probably the result of deposition as debris flows in caverns.
Sediment Fill: The cave sediment fill associated with the Ellenburger cave system is composed of silt- to granule-sized carbonate sediment with generally less than 10% of the material larger than granules. Siliciclastic clay is rare.
DEEP KNOX CARBONATE RESERVOIRS
Between the higher
permeability zones are disturbed host and undisturbed host rock on the
scale of several hundreds meters. The undisturbed host rock will be relatively
tight (>1 md) whereas the disturbed host rock may have open crackle
breccia fracture porosity with good permeability (tens of millidarcys).
If the reservoirs are shallower than 9,000 ft, then some large vug or
cavernous pores may still be open. Dolostone reservoirs will have higher
quality rock at similar depths than limestone reservoir, because dolomite
is more chemically and mechanically stable with burial.