Loucks: Characterizing the Three-Dimensional Architecture of a Coalesced, Collapsed-Paleocave System

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.

Characterizing the Three-Dimensional Architecture of a Coalesced, Collapsed-Paleocave System in Lower Ordovician Carbonates by Integrating Ground-Penetrating Radar, Shallow-Core, and Outcrop Data: Application to Potential Deep Knox Reservoirs in Mississippi
Robert G. Loucks, Senior Research Scientist
Bureau of Economic Geology, 
John A. and Katherine G. Jackson School of Geosciences,
UT at Austin, Austin, Texas; \

Paul Mescher, Geological Resources Company, Plano, TX; and

George McMechan, UT at Dallas, Richardson, TX

EXTNDED ABSTRACT:

INTRODUCTION
A three-dimensional study of a Lower Ordovician coalesced, collapsed-paleocave system was completed near Marble Falls in central Texas. This area was chosen because it is adjacent to a quarry face and ties into a two-dimensional study completed within the quarry. The shape of the study area is an irregular polygon. The greatest north-south dimension is approximately 2500 ft and greatest east-west dimension is approximately 3300 ft.

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 FROM CORE
The rock types and facies seen in the cores are similar to those described from the quarry faces.

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.

GROUND-PENETRATING RADAR FACIES
The data from the GPR lines were characterized in the context of the continuity, regularity, and strength of the reflections. Three classes of GPR reflections are apparent: (1) strong, continuous, undisturbed reflections imaging the undisturbed strata/undisturbed host, (2) relatively continuous reflections as much as tens of feet long, characterized by faults and folds, imaging the disturbed strata/disturbed host, and (3) chaotic reflections having little to no perceptible continuity imaging cave-related breccias. Integration of data from the cores, GPR survey, and outcrop provides a three-dimensional analysis of the paleocave system. Large-scale facies and structural patterns are readily apparent from the GPR data, and rock textures and pore types are provided by the core and outcrop data.

DISCUSSION
The study volume was divided into three 3-m-depth slices (~10 ft) in order to map out the geometry and areal extent of the collapsed-paleocave facies. Each slice displays a series of paleocave breccias trending to the northeast. The breccias are separated by disturbed- and undisturbed-host rock. The breccia bodies that outline the trend of coalesced former passages are as much as 1100 ft wide. The intervening area between the breccias is as much as 650 ft wide. The best reservoir quality is within the collapsed-paleocave trends and consists of chaotic and crackles breccia porosity.

APPLICATION TO DEEP KNOX CARBONATE RESERVOIRS
The coalesced, collapsed-paleocave system analog developed from this study can be used as a universal model to understand the potential reservoir types and their distribution in karsted carbonates. It might be especially useful in understanding potential deep Knox reservoirs in Mississippi. The first insight to gain from the analog is that paleocave reservoirs are extremely heterogeneous and the pore networks are complex being composed of both fracture and large interclast pores. The best zones for interclast pores are the collapsed passage zones that may be up to several hundred meters wide. Crackle breccia fracture pores will also be important in this zone. The collapsed passage zones can have permeabilities in the tens to hundreds of millidarcys.

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.