The Eagle Ford Shale
Outcrop Studies Related to the Oil and
Gas Potential of a Major Unconventional
Reservoir.
Brian E. Lock
University of Louisiana, Lafayette
Prepared for LGS meeting, September 21, 2011
Lauren Peschier and Nick Whitcomb (UL graduate students)
contributed to the study
Outline
• Resource Plays and the Eagle Ford
• Stratigraphy and Sedimentology of the
Outcrops
• Application of the Outcrops to Understanding
the Eagle Ford Resource Play
Keys to a Successful Resource Play
• Original organic richness and generation
potential
– Kerogen type (from Rock-Eval analysis)
– TOC values (determine TOCo?)
• Maturation (includes primary and secondary
cracking)
– Ro
– Tmax (from Rock-Eval analysis)
– TTI
S2: kerogen
Tmax
S1: existing
hydrocarbons
S3
S3: CO2 from spent
kerogen
HI (Hydrogen Index) = (S2/TOC).100
OI (Oxygen Index) = (S3/TOC).100
PI (Production Index = S1/(S1+S2) S2: kerogen
Tmax:
400-430o
435-450o
>450o
S1: existing
hydrocarbons
immature
oil zone
overmature
S3: CO from spent
S
Values as mg/gm3of sample:
kerogen
2
if S1 >1 mg/g = oil show
if S2 > 5 mg/g = good source rock
Keys to a Successful Resource Play
(continued)
• Retention of oil/gas in the resource reservoir
– Gas adsorption and free oil and gas, vs. primary migration
• Porosity
– Increases with maturation and hydrocarbon generation
• Brittleness
– Mineral composition (from X-Ray Diffraction analysis)
• Quartz
• Carbonates
• Clays
– Open natural fractures (not cemented) or hydraulically
fractured during stimulation
• (note – reservoir quality lower in areas of greater open fractures)
Jarvie et al., AAPG Bulletin, 2007
resource shales
REGIONAL
STRATIGRAPHY
Eagle Ford and Austin considered
a single reservoir unit
Notes:
Maverick Basin Eagle Ford has different tectonic, thermal and
diagenetic history from the Reef Trend Eagle Ford, and is thick
and over-pressured. Thermally mature rocks have been uplifted.
Delaware
Basin
Delaware
Basin
Maverick
Basin
Delaware/Rio Grande
field area
Delaware/Rio
Grande
Aulacogen
EF outcrops
Aulacogen
Maverick Basin
Eagle Ford
Lower Cretaceous
Reef Trend Eagle Ford
Outcrop D
Outcrop G
Lozier Canyon
Outcrop Q
The field study area – Eagle Ford outcrops
Lower member
(“facies A” of BP workers).
unstable slope deposits;
slump folds
debris flows
turbidite
traction deposits - ? contourites
Lowstand Systems Tract
Not present in the subsurface
debris flow
debris flow
Ash/turbidite marker bed extends from Comstock to Lozier Canyon (about 40 miles)
clast
debris flow
ash/turbidite
New LGS
Vice-President
Buda Formation
Sources of authigenic kaolinite
(and silica)?
• numerous ash beds throughout the Eagle Ford are composed
almost entirely of kaolinite (possible source of diagenetic silica)
ash bed in
lower member
kaolinite
dolomite
Authigenic dolomite and kaolinite filling vug in lower member. Note: dolomite
(and de-dolomite) rhombs are almost universally present in Cretaceous carbonates in
southwest Texas.
The productive interval (lower Eagle Ford
in the subsurface)
Road cut G-1
Middle member (productive interval) in outcrop
Road cut G-2
upper member
middle member
lowest chalky limestone
Lozier Canyon – bluffs expose full Eagle Ford section
Lozier Canyon section
Austin Chalk
upper member
middle member
Lower member (obscured)
Buda Limestone
Lozier Canyon middle member:
rock is fresh because of stream
undercutting the bluff.
2,000 ft long outcrop in same stratigraphic interval shows lateral consistency.
Road cut Q
upper beds, middle member
lower beds,
middle member
Note: most of the outcrop is weathered (oxidized iron).
grey patina (result of last 30 years
since road-cut was made)
black color
of fresh
rock
Note laminae that pinch out – evidence for bottom currents (hyperpycnal flow)
Parasequences in highway outcrop, middle member
10 ft
Parasequences in a core gamma ray
log, from a Petrohawk well.
Deep water sedimentation and parasequences
What controls limestone/shale alternations? Eustasy? Climate? Tectonics?
or periodic gravity flows from unstable platform margin?
Includes possible hypopycnal flow
terrigenous clay and silt, suspended carbonate mud
sea level fluctuation
flocculation, pelletization
few feet
water
depth
clay
CaCO3
shoaling
marine snow
parasequence 2
deep water traction currents
(meteorological? geostrophic?)
mfs
parasequence 1
parasequence 2
mfs
parasequence 1
Is the limestone the base or the top of the parasequence?
limestone – brittle
marl (“shale”) - ductile
hundreds
of feet
Smaller scale cycles – result of climate/sea level variations or individual hyperpycnal flows?
Note evidence of current action
Core photos from Petrohawk core
with permission
Inoceramid fragments
Graded bed – white dots are planktic forams
copepod
fecal pellet
Evaluating the Eagle Ford outcrops
• Original organic richness and generation
potential
– Kerogen type
• Rock-Eval analyses
Oil prone
Evaluating the Eagle Ford outcrops
• Original organic richness and generation
potential
– TOC values (determine TOCo?)
Blue: data from BP study (Donovan et al., 2011). Red: from UL study
Maturity
• TTI – estimate of time/temperature burial
history. Not currently available
• Ro – vitrinite reflectance - microscopy
• Evolution on Van Krevelen plot (Kerogen
Transformation Ratio – KTR) – from Rock Eval
• Tmax – from Rock Eval
• Production Index – from Rock Eval
Kerogen Transformation
modified
Van Krevelen Diagram
Van Krevelen Diagram
Evaluating the Eagle Ford Outcrops
• Maturation
Tmax from
Lozier Canyon
samples (UL).
No Ro or TTI
data
available
Close to, but not yet in oil window
Vitrinite Reflectance (Ro) data
• Performed by USGS (Mark Pawlewicz) via Russ
Dubiel, on US Hwy 90 samples.
• Essentially no vitrinite, only solid bitumen
(two generations? – Ro values around 0.6 and
a second bitumen from external source with
higher Ro)
Evaluating the Eagle Ford outcrops
• Retention of oil/gas in the resource reservoir
– Gas adsorption and free oil and gas
• Oil types in the First Shot Field (Austin Chalk)(Corbet 2010) and
general sourcing of other Cretaceous reservoirs from Eagle Ford
demonstrates migration from the Eagle Ford
–
Natural fractures
• Indicate brittleness (good!)
• May have already permitted hydrocarbon
primary migration out of the reservoir (not so
good!).
– Experience in the Barnett Shale indicates that
areas that are most highly fractured are less
productive
• Distinguish open fractures from calcite filled
fractures
Core photos from Petrohawk core
with permission
Evaluating the Eagle Ford outcrops
• Porosity
– Increases with maturation and hydrocarbon
generation
EM images, following Argon milling: Barry Wawak, Core Labs
Note quantities of organic material (om), but Ro data unsatisfactory
Evaluating the Eagle Ford outcrops
• Brittleness
– Mineral composition
• Quartz
• Carbonates
• Clays
UL samples from Hwy 90, analyzed by Core Lab (Barry Wawak)
Our data from Lozier Canyon
THANKS!
• The following ran samples for me or paid to have them run:
– Barry Wawak, Core Labs
– Russ Dubiel, USGS
– Bruce Hart, ConocoPhillips
• The following supported my graduate students (Lauren Peschier,
Nick Whitcomb) with grants:
– GCAGS, GCSSEPM, STGS
• The following helped in the field:
– Dr. Vicky Hover, Ashley Fife, Natasha Jeansonne, Rob Glaser, Aaron
Summerfield
• Dr. Art Donovan (BP) provided access to the Lozier Canyon outcrops
• Mr. Billy Foster allowed us to access his land (Old Hiway road cut)