Hydrothermal Dolomites in Central Kentucky:Possible Analogs For Hydrocarbon Gas Reservoirs throughout the Midcontinent

Clay A. Wilcox III1, David C. Harris2, James A. Drahovzal2

clayw26@insightbb.comharris@kgs.mm.uky.edu

drahovzal@kgs.mm.uky.edu(1) Dept. of Geological Sciences, University of Kentucky, Lexington, KY 40506(2) Kentucky Geological Survey, University of Kentucky, Lexington, KY 40506

AbstractThe occurrence of discrete dolomite bodies in Upper and Middle Ordovician limestone strata of central Kentucky has been well documented by the cooperative U.S. Geological Survey–Kentucky Geological Survey geologic mapping program in the 1970’s. These isolated dolomite bodies, which occur as both coarsely crystalline dolomite and saddle-shaped dolomite cement and vug linings, are spatially related to mapped faults, and have been interpreted as products of hydrothermal fluids that moved along faultconduits, replacing adjacent limestone. Trace occurrences of Mississippi Valley Type (MVT) ores and small hydrocarbon deposits have been iden-tified in outcrops of the hydrothermal dolomite in central Kentucky.

There is new economic interest in fault-controlled dolomitization as the result of significant natural gas discoveries in the Ordovician Trenton and Black River Formations in central New York and possibly in West Virginia. Gas production from these formations is attributed to hydrothermal dolomite, localized along faults. The hydrothermal dolomite bodies that crop out in central Kentucky may serve as analogs to dolomite reservoirs found in West Virginia, Michigan, New York, and other parts of the eastern Midcontinent. Although we cannot state with certainty that the same processes formed dolomites in both Kentucky and New York, the dolomite bodies share strik-ing similarities in structural control, geometry, and stratigraphic occurrence. Understanding the formation of hydrothermal dolomites in central Kentucky may lead to more accurate reservoir prediction and more discoveries throughout the eastern midcontinent.

Fluid inclusion, electron microprobe, and stable isotope analyses will be performed on samples taken from outcrops and cores of the hydrothermal dolomite in central Kentucky. The data gathered from these analyses will be used to characterize the dolomitizing fluid and dolomitization process in a three-dimensional model. Furthermore, the relative direction of fluid migra-tion will be investigated by comparing the homogenization temperatures of fluid inclusion from each location. Could the hydrothermal dolomites and MVT ores have precipitated from a single fluid that migrated from the east as a result of the Alleghanian orogeny?

Goals of Study Determine controls on the distribution of

fault-controlled dolomitization Determine timing of dolomitization and

integrate with structural geology Determine relationships of dolomite and

hydrocarbons Use Kentucky dolomites as an analog for

subsurface Trenton-Black River reservoirsin New York

Develop a predictive model for structural andstratigraphic controls on tectonic dolomitization

Analytical Techniquesto be used on dolomite: Cathodoluminescence petrography 13C and 18O stable isotope analysis Electron microprobe ananlysis Fluid inclusion geothermometry

Stratigraphy

Units affected by fault-controlled dolomitization

Study Area: Ordovicianoutcrops in Clark Co., Ky.

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#S

0.5 0 0.5 1 1.5 MilesScale: 1:24,000

Geologic map of a portion of the WinchesterQuadrange showing distribution of tectonic

dolomite, Clark County, Kentucky

Stratigraphic Units

Alluvium depositsHigh-level fluvial depositsDolomite of uncertain age and originCrab Orchard Formation & Brassfield DolomiteDrakes FormationAshlock FormationCalloway Creek LimestoneGarrard SiltstoneClays Ferry FormationUpper part of Lexington LimestoneTanglewood Limestone Member No. 4Tanglewood Limestone Member No. 3Tanglewood Limestone Member No. 2Brannon Member, Lexington LimestoneTanglewood Limestone Member No. 1Lower part of Lexington LimestoneTyrone LimestoneArtificial fill

Formation ContactsContactConcealed contactInferred contact

FaultsFaultConcealed fault

Dolomite

%RailroadsRoads

#S Boreholes

Hydrology

NGeology by Douglas F.B. Black, 1974Digital compilation by Michael Murphy

"lightbulb"dolomite

[

Boonesborough Fault(Kentucky River Fault System)

Stoner Branchtectonic dolostone

<

USGSAllen #1

Ky. 6

27

<

Stoner Branch

Texaco No. 1 Williams (0.6 mile south)TD: 4937 ft, Precambrian, D&A, 1969

<

Geologic Map of theWinchester Quadrangle, Ky.

Maps showing the generalized surface geology and major fault zoneswithin the study area. The location of the study area and outcropsof the Ordovician dolomite lie within the box.

Ordovician outcropbelt

Roadcut along KY Highway 627. Disturbed bedding in the center of the photograph is hydrothermal dolomite that has replaced bedded Lexington Limestone (Trenton). A small fault controlled the movement of the dolomitizing fluids into the limestone.

Ordovician dolomite from the Stoner Creek locality inClark Co., Ky. Fossil molds and vugs contain liquid oil.

Edge of dolomite body showing sharp transition intobedded limestone (Lexington Ls.).

Limestone

Dolomite

Photo by B.C. Nuttall

Photo by B.C. Nuttall

Photo by B.C. Nuttall

Limestone

We gratefully acknowledge research funding from the New York State Energy Research and Development Authority (NYSERDA),the U.S. Department of Energy, and Triana Energy, Charleston, WV. We also appreciate the cooperation of

the New York State Museum and Geological Survey.

Dolomite outcrops as analogsfor Trenton-Black River gas reservoirs

Ordovician dolomite outcrops in central Kentucky can provide analogsfor subsurface Trenton-Black River gas reservoirs in the Appalachian Basin:

Key features to be compared with New York reservoirs:

Degree of stratigraphic (facies) control on dolomitization Timing of dolomitization Relationship of dolomitization to hydrocarbon migration/source Regional extent of tectonic dolomitization in the Middle and Upper

Ordovician in the Appalachian Basin

Trenton/Black River Subsurface Gas ReservoirGlodes Corners Road Field, Steuben Co., NY

Gas field is a linear fault-bounded graben, with dolomite distribution controlled by faults

Map courtesy of Columbia Natural Resources

North-south cross section through the Trenton/Black River interval in the Glodes Corners Road Field,Steuben County, N.Y., showing fault-bounded graben, and fault-controlled distribution of tectonic dolomite.

Diagram courtesy of Columbia Natural Resources

Coring of tectonic dolomites in KentuckyAs part of the study, Triana Energy will drill 2 continuously-cored boreholes in 2 of the dolomite bodiesin central Kentucky. Cores will be cut through the Trenton (Lexington Ls.), Black River (High Bridge Group),into the top of the Knox Group, to about 1,200 ft (400 m) total depth. The cores will provide lithologic dataand sample control vertically within the dolomite bodies. Petrographic and geochemical data collected fromthe cores will document any vertical trends in dolomite texture or chemistry, which may help constrain a model.

Trenton-Black River discovery well, Cottontree Field,Roane Co., West Virginia

SummaryThis two-year project will involve field and laboratory characterization of fault-controlledOrdovician dolomites in central Kentucky. These dolomites have many similarities with dolomitizedsubsurface natural gas reservoirs in upstate New York in the same stratigraphic interval. We hopeto develop a model for fault-controlled dolomitization in central Kentucky that will help refine explorationmodels for these reservoirs in the subsurface.