Spe 08952

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8/9/2019 Spe 08952


PERSPECTIVE ON DEVONIAN SHALE GAS EXPLORATION

commercial development of the natural gaa producing

potential of the organic rich Devonian shales that

underlie some 200,000 square miles of the eastern

United States (Figure 2). Devonian shales are an

unconventional natural gas resource in that,

although they contain vast volumes of gas, they

usually lack sufficient natural permeability to

permit the gas to migrate to the wellbore. His-

torical natural gas production at commercial

rates from the shales has occurred in isolated

areas of the Appalachian, Illinois, and Michigan

Basins. Most of this production can be attributed

to extensive natural fracture systems that act as

interconnected conduits feeding gas desorbed

from the shale matrix to the wellbore. One of

the primary goals of the EGSP is to develop the

capability of creating artificial fractures in

the shale in order to create a permeable link

between the wellbore and such natural fractures

as may exist in the vicinity.

Fracture stimula-

tion technology is not alone sufficient to induce

gas production from the Devonian shales; natural

fracture permeability must be present. Hence,

another primary goal of the EGSP is the formula-

tion of shale specific exploration rationales

characterized by some geological fracture

producing mechanism and identification of the

areas to which these rationales appiy.

Gruy Federal No. 1 Grainger Co. tested an

exploration rationale that predicts intense and

intricate natural fracturing in the Devonian

shale wherever proximally associated with the

major thrust faults of the Appalachian overthrust

belt. In eastern Tennessee the Devonian shale is

represented by the Chattanooga Shale, a formal

stratigraphic unit,

the bulk of which is Upper

Devonian, but the uppermost portion, ia Lower

Fiississippiaii.“’’”’- ‘L- “-”-- -a ‘:~--ILIILII UK vdLL=y aL,U..~U6=

Province it crops out along the northwestern

flanks of two isolated northeast trending syn-

c1ines, the Newman Ridge and Greendale Synclines

(Figure 3).

Both synclines are bounded to the

southeast by major thrust faults of regional

extent,

the Clinchport and Saltville Thrust

Faults, respectively. The Chattanooga Shale

passes into the subsurface beneath these southeast

dipping thrusts. The above stated exploration

rationale is based on USGS Professional Paper

1018 (Harris & Milici, 1977). The authors observe

at those few localities in the Southern Appala-

chians where major bedding plane faults, termed

“decollements”, are exposed that the overlying

rocks are very highly fractured. The most in-

tensely fractured rocks occur in their “broke.ii

formation zone” immediately above a decollement.

The rocks in their overlying “fractured zone” are

still pervasively fractured, but less intricately

so. That the Chattanooga Shale in the Newman

Ridge and Greendale Synclines occurs beneath the

associated major thrust gaults does not invalidate

the exploration rationale if one supposes the

existence of bedding plane faults in the shale

induced by activity on the overlying master thrust.

This is a reasonable supposition for which there

is at least indirect evidence.

In the general

vicinity of Evanston, TN, (Figure 4) Chattanooga

Shale passes under the entire length of a portion

of the upper plate of the Hunter Valley Thrust

isolated by erosion and the more steeply dipping

Clinchport Thrust.

Where it emerges to the

northeast it may be observed in a roadside borrow

pit to be intensely fractured (Milici & Statler,

in press), clearly reflecting historic activity on

the overlying Hunter Valley Thrust, now eroded away

to reveal the shale.

PROSPECT DEVELOPMENT

In 1967 U.S. Energy Research & Development Ad-

-J-J..-..~__

?7RI)A) EQW ~Q~) lec a

three-year

~-kl~-..”.-”..\-----

contract (DE-Ac21-76MC05196) to the Tennessee Divi-

sion of Geology (TDG) to characterize the Chatta-

nooga Shale in the Valley and Ridge Province of

eastern Tennessee and to evaluate its natural gas

producing potential.

The TIN

COiduCi~d Z bible

thrusted research program designed to define two

separate aspects of the resource:

1.

To learn the true thickness of the shale,

its internal stratigraphy, and especially

the relative proportion of organic rich

to organic lean material, they sponsored an

NX core drilling program along the major

outcrop belts of Chattanooga Shale.

Eight

holes were cored, three in the Newman Ridge

Syncline and five in the Greendale Syncline

2. To reveal the subsurface extent of the

nh tta=nng ~ha~e ~~~.~a~h. the Hunter

U,,ake ..”” .

Valley, Clinchport, and Saltville Thrust

Faults they arranged for seismic surveys

to be conducted along two lines, KIS-TC1

and TC2 (Figure 3).

The salient results may be summarized as follows:

.

1. The total stratigraphic thickness of the

Chattanooga Shale varies from several

hundred feet to well over a thousand feet,

a verv subs~antial proportion of which is

.

organic rich.

2.

The Chattanooga Shale extends at least

several miles back beneath the thrusts

before being truncated and is accessible

to exploratory drilling at surprisingly

shallow depths, less than 4,000 feet.

In a memo to DOE EGSP management dated May

18, 1979, this author summarized progress under the

TDG contract,

expounded the exploration rationale

at length,

and identified seven Devonian shale

prospects for exploratory drilling.

These are shown

on Figure 3.

Gruy Federal No. 1 Grainger Co.

EGSP-TN9 was drilled to test prospect P3. Criteri

for .+.-

.aln”ti~~ ~~

.L.= ...=.

~~~~~ of priority were the fol

lowing:

1. Location within the P3 area as defined on

Figure 3.

2,

Proximity to seismic line TCL.

3. Availability of public land.

4.

Potential local consumer of natural gas.

5. Proximity to an all-weather road.

Hence,

the Grainger County Industrial Park was the

natural choice. At this location the well was

spudded in the Cambrian Conasauga Group in the

uPPer Plate of the Saltville Thrust Fault. It

238

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c. s.

penetrated the gently dipping fault and encountered

the Chattanooga Shale in the lower plate. Thrust-

ing Lower Cambrian over Lower Mississippian, the

fault has a stratigraphic throw of over 10,000 feet.

DRILLING AND CORING

Drilling predominantly on air, the well was

spudded in the Rogersville Shale and passed through

two other formation belonging to the Cambrian

Conasauga Group, the Rutiedge Limestone and the

Pumpkin Valley Shale, before entering the Cambrian

Rome Formation (Figure 5). Surface casing was

set at 200 feet. Water entry into the wellbore

was first noticed at 400 feet.

The volume of

water increased markedly at approximately 600 feet

and a strong hydrogen sulfide odor was emitted.

(Richland Valley, in which the industrial park

is situated, is noted for its mineral springs,

associated with which were several health spaa.)

Penetration of the Saltville Thrust Fault

occurred at 667 feet.

Surprisingly, this event

was readily recognized in the samples.

The 650-60

sample contained slightly dolomitic, medium grained

sandstone similar to the preceding five samples of

Rome sandstones, some of which are notably mica-

ceoua. The 660-70 was lithologically similar to

the above, but contained abundant slickensided

cuttings and what the author interprets to be

mylonite.

The 670-80 was gray fossiliferous silt-

stone containing an identifiable brachiopod and

crinoid columnal.

Further along, abundant glau-

conite appeared in ssmplea 710-20 and 720-30, by

which the “glauconite zone” of Hasson (1967) was

recognized. The measured outcrop sections of the

Mississippian Grainger Formation in Kenneth Hasaon’s

(1967) Ph.D. thesis indicate a prominent glauconite

zone near the top of the “middle ahale-siltstone

member”. Recognition of the “glauconite zone” in

the cuttings enabled the accurate prediction of the

top of the Chattanooga Shale, which subsequently

was encountered at 1136 feet. A possibly signifi-

cant aaide,

the position of the glauconite zone

relative to the Saltville Fault in the well indi-

cates approximately 90 feet of migration of the

fault downward through the Grainger Formation from

the outcrop to the well, a distance of about 3/4

of a mile.

That is equivalent to a down-dip loss

of section of 120 feet per mile.

n.=..-.

. ..-J..o-A4n. in . ~~~ Chattannnra ~h~~e

DCL”LC p.ue.==uu.~ . . ...”

“..-----.--=—

an intermediate string of caaing was set to shut

off the flow of water, which was substantial.

This

waa done to protect any fractured reservoirs that

might be encountered in the shale, inaamuch as it

was suspected that they might be underpressured.

From this casing point to T.D. hole was made by

alternately drilling and coring. Cored intervals

are indicated on Figure 5, aa also are the major

lithostratigraphic unita within the Chattanooga

Shale and their probable regional correlation.

Primary coring targets were the most richly

organic intervals within the shale, as previously

determined from two nearby NX holes cored and logged

under the TDG contract.

Core points were accurately

picked on the basis of samples taken at five-foot

intervals,

and additionally in the case of the

Lower Huron, a gamma ray log run for correlation.

Fifty feet of core was recovered from the Sunbury-

Cleveland interval; 132 feet, from the Lower Huron;

and 48 feet, from the Rhinestreet, of which the

last ten feet is actually Wildcat Valley Sandstone.

Ten-foot samples were taken and logged as usual

throughout the drilled intervala.

An unexpected

water bearing zone was encountered at 1410 feet

while drilling through the lower part of the

Chagrin interval.

Initial water production was a

2-inch atream; with time that eventually diminished

to less than l/2-inch.

Sample 1410-20 is ailtstone

containing anomalous quantities of slickenaided

cuttings and coarse crystalline dolomite, indicative

of slickensided and mineralized fractures. The

fhle F~erstia: lthcr Qtec Qss____

a unique marker

fossil, between 1600 and 1610, and identified

bentonite, probably the Center Hill Bentonite,

in sample 1760-70.

Following the extraction of the

laat core, a 53-foot rat hole waa drilled through

the remainder of the Devonian Wildcat Valley

Sandstone and into the Silurian Clinch Sandstone.

The last 20 feet of Clinch is brownish shale of

RockWood aspect.

The base of the Chattanooga

Shale stands at 1856 feet.

Thus, the total

thickness of the Chattanooga proved to be 720

feet.

The aggregate thickness of abnormally

radioactive shale (an indicator of organic con-

tent) as revealed on the gamma ray log (25 API

units above the shale base line) is 480 feet (67

of the total), of which 220 feet was cored.

Indications of hydrocarbons during drilling

and coring operations were meager.

No gas detector

was stationed on the well.

After coring the Lower

Huron, the most prospective interval in the Chatta-

nooga Shale, the rig compressors were shut down

and a futile attempt was made to ignite whatever

gases might be flowing from the well. No natural

flow of any kind was detected. Only when the

compressors were restarted was a flaah observed,

indicating the presence of a small amount of gaa.

Gas bled out of a few fractures in the core

extracted from the Lower Huron and Rhinestreet

intervals.

More impressive natural gas shows in

the Chattanooga Shale may have been precluded by

near-wellbore formation damage (skin effect)

arising from water production at 1410 feet.

Light

gravity, straw colored oil was observed in the

core and cuttings of the Wildcat Valley-Sandstone.

The sandstone appears to be tight, the oil be ng

contained in fracturea.

FORMATION EVALUATION

Virtually all the shale core recovered from

the well is naturally fractured, some of it

intensely so.

In fact, the core from Gruy Federal

No. 1 Grainger Co. (EGSP-TN9) is more highly

fractured by far than any of the 46 other cores

recovered under the EGSP to date.

The greatest

degree of fracturing occurs in the lower part

of the Lower Huron and in the Rhineatreet (Figure

5). Most of the rest of the core is at least

moderately fractured; only 23 is unfractured.

Moderate to low angle slickensided fractures pre-

dominate;

sub-vertical extenaile fractures are

moderately abundant.

The vertical fractures are

mineralized with dolomite. Some of the moderate

to low angle fractures are also mineralized with

dolomite; most are simply slickensided. A few

fractures are related to the growth of septarian

concretions, which are not abundant, and hence

are non-tectonic. A distinct vertical progression

of fracture intensity and style occurs at least

twice, once in the Lower Huron and once in the

Rhinestreet.

Proceeding downward, it first msni-

festa itself as several sub-vertical

, mineralized

239

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PERSPECTIVE ON DEVONIAN SHALE GAS EXPLOUTION

fractures,

These rather abruptly give way to mod-

erate and low angle slickensided fractures, which

increase in abundance until the core is reduced

to slickensided rubble. The bottom of this rubble

zone ia fairly well defined. This progression

is reminiscent of the “zone of fracture” and

“broken formation zone”

observed above decollements

by Harris and Milici (1977).

A full suite of wireline logs* was run through

the Chattanooga Shale.

Tiieyare the fOHOWi~g:

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

Caliper

Gamma Ray

Density, Borehole Compensated (Poros:

on a 2.68 matrix) (dry hole)

Temperature

Sibilation

Simultaneous Compensated Keutron-

Formation Density (wet hole)

Spontaneous Potential

Dual Induction -

Laterolog

Fracture Identification

Borehole Compensated Sonic

ty

Open Hole Amplitude - Variable Density

Copies of the logs rest in the UGR Open File at

the Morgantown Energy Technology Center (METC),

Morgantown, WV, and are available for public

inspection.

The gamma ray log reveals substantial thick-

nesses of highly radioactive shale, 250 API units

or greater, within the Sunbury-Cleveland, Upper

Huron, Lower Huron,

and Rhinestreet intervals

(Figure 5).

As rule of thumb, shale radioactivity

is directly correlated with organic content. The

bulk density curve, however, indicates that the

highly radioactive intervals are not nearly as

organic rich as the gamma ray readinga imply.

The various nuclear porosity logs faintly suggest

some gas filled porosity development in all of

the above named shale intervals. They somewhat

more strongly imply such development in the silt-

stones in the lower part of the Chagrin between

1350 and 1420.

The clay content in the siltstones

makes it difficult to estimate porosity; however,

it does not exceed a few percent.

At 1408 near

the base of the Chagrin the nuclear logs indicate

a narrow zone of liquid (water) filled porosity

that can only be interpreted as a highly porous

fracture zone.

The resistivity measuring logs are

enigmatic.

It should be here cautioned that poro-

sity and resistivity logs were intended to evalu-

ate shale-free formations (sandstonea and carbon-

ates) and their interpretation in shales ia a

qualitative art at best.

tio of the logs, Temperature and Sibilation,

directly indicate gas entry into the wellbore,

in shale presumably from fractures.

WO other

logs, Fracture Identification and Open Hole

Amplitude - Variable Density, were run as frac-

ture finders. There is a reliable sibilation

anomaly and an associated temperature anomaly

in the lower Chagrin at 1363 feet opposite a

four-foot shale break within a 25-foot potentially

* The occasional use of trade names rather than

generic names for various products and services

reflects field usage and does not imply U.S.

Government endorsement of any company, product,

or service.

gas bearing siltstone interval. A second sibilation

anomaly at 1786 feet without associated temperature

anomaly is dubious due to the close proximity of

fluid in the hole.

A broad, shallow temperature

anomaly stretching from 1660 to 1740 across most

of the Lower Huron interval is one of the more hope-

ful indications of producible shale gas in the well.

Activity on the Fracture Identification Log (FIL)

is considerably less than one would anticipate from

the core and correlates only modestly with core

~~~zr\.2~~apd~2~ fy=.+,J~in*>

----- .-= TIM correlation with

fracture indications on the Open Hole Amplitude -

Variable Density log (VDL) is much better. Perhaps

by virtue of tool design and operating principle,

the FIL is better able to detect sub-vertical

fractures than the VDL, while the VDL is better

able to detect sub-horizontal fracturing than the

FIL.

Both logs, however, concur in drawing the

c-.,------

.- -1 -f .” - .l Q inrlQ.

10IAUWAL1~ ~~~t ~.ca~ ~“..-.-.-m---..-

1.

2.

3.

4.

The water bearing zone at 1410 feet is

massively fractured.

The remainder of

the lower part of the Chagrin from 1310

to 1420 feet is heavily fractured,

especially opposite the sibilation-

temperature anomaly at 1363 feet.

The upper part of the Chagrin appears

to be fracture free. This section ia

useful for calibration and comparison

with variably fractured sections.

The interval from the top of the Lower

Huron to the base of the Chattanooga Shale

is nearly continuously fractured, at least

to a moderate degree.

The Sunbury-Cleveland interval is only

lightly fractured.

STIMULATION AND TESTING

Widespread and locally intense fracturing

within the Chattanooga Shale, especially within

the organic rich intervals, observed in the core

and evidenced in the logs vindicates the explora-

tion rationale used in siting a Devonian shale

wildcat well in the P3 prospect area (Figure 3),

Grainger County, TN. Whether the shale will pro-

duce natural gas at a useful rate remains to be

proven through a planned hydraulic fracture

stimulation and subsequent well test. Very few

Devonian shale wells produce without any stimula-

tion;

some good producers had no measurable natural

flow of gas prior to stimulation.

Following drilling operations a production

string of 4.5 inch casing was set at 1895 feet and

cemented back to surface. In April 1980 U.S. DOE

through its contractor Gruy Federal, Inc., intends

to run a cement bond log and perforate the casing

from 1630 to 1850 feet. After formation breakdown

with 1500 gallons of acid and nitrogen gas to

generate 6,000 gallons of foam, the well will be

hydraulically fractured with 50,000 gallons of

75 quality foam and 50,000 pounds of 20/40 mesh

sand pumped at a rate of 25 barrels foam per

minute. After flow back the well will be tested

to measure its deliverability, evaluate fracture

geometry, and compare well performance with other

Devonian shale wells tested under the EGSP well

testing program.

This should result in an

accurate evaluation of the gaa producing potential

240

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C. S. DEAN

——.—

—.—. .

of the Lower Huron, Olentangy, and Rhinestreet

intervals in Gruy Federal No. 1 Grainger CO. (ECSP-

TN9).

Depending on the results, the Sunbury-

Cleveland, Upper Huron, and possibly the Chagrin

intervals may be tested at a later time.

Hydraulic fracturing will overcome the near-

wellbore formation damage that occurred during

drilling and coring operations. This may well

explain the lack of a substantial natural gas

show despite the highly fractured nature of the

Chattanooga Shale. On the negative side, the

slickensided and mineralized fractures encountered

may be insufficiently permeable to create a proper

matrix rechargeable reservoir.

Alternatively,

reservoir pressure may be depleted through gas

leakage to the surface; the shale outcrops only

1.5 miles away from the well.

REFERENCES

1. Harris, L.D., and Milici, R.C.: “Characteris-

tics of Thin-Skinned Style of Deformation in

the Southern Appalachians, and Potential Hydro-

carbon Traps”, U.S. Geological Survey Profes-

2.

3.

4.

5.

.

sional Paper 1018, 1977, 40 p.

Hasson, K.O.:

“Lithostratigraphy of the

Grainger Formation (Mississippian) in

Northeast Tennessee”, unpublished Ph.D.

dissertation, University of Tennessee,

Knoxviiie (1972).

Milici, R.C., and Statler, A.T.:

“Fractures

Related to Major Thrusts--Possible Analogues

to Tectonically Fractured Chattanooga Shale

in Tennessee”, Abstracts with Programs,

S.E. Geological Society of America Meeting,

v. 10, no. 4, 1978, p. 176 (in press).

Milici, R.C., Harris, L.D., Statler, A.T.:

“An Interpretation of Seismic Cross Sections

in the Valley and Ridge of Eastern Tennessee”,

Tennessee Division of Geology, Oil and Gas

Chart 6, Sheet 2 (in press).

Tegland, E.R.:

“Seismic Investigations in

Eastern Tennessee”,

Bull. 78, Tennessee

Division of Geology, Knoxville (1978).

Fi g. 1

- Locati on map.

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K n ow n a s Sh a l e eposi t s

Loca t ed i n t h e Un i t ed St a t es

Fig. 2 -

Easterngas shai edeposi

ts.

/

SCALE 1250 @30 –

5

0

5

10 STATUTE MlLES

/

ExPLANATIW

YOUNGER THAN CHATTANOOGA

: I PRO’+PECT AREAS

CHATTANOOGA SHALE

/“

OLDER THAN CHATTANOOGA

/ ’

Fig. 3 - EGSP exploration prospects related to the cl inchport and Saltvi 1 le

Thrust Faults, Eastern Tennessee and Southwestern Virginia.

8/9/2019 Spe 08952


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Fig.

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Fig. 5 - Summary:

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Spe 08952