American Oil and Gas Reporter - January 2016 - 60

Unconventional Resource Science

Project Models Eagle Ford Properties
By David Hull,
Philip Chapman,
Dave Miller,
David Ingraham,
Nicole Fritz
and Nicholas Kernan
OKLAHOMA CITY-Devon Energy
Corporation has undertaken a regional
study to analyze core from the eastern
Karnes Trough area, outcrop, petrophysical
data, literature, well logs, seismic and
other data to evaluate both the Lower
Eagle Ford Shale (the dominant target in
South Texas) and the more locally deposited Upper Eagle Ford Marl. This information is incorporated into 3-D static
earth models, from which geologic inputs
drive engineering testing and adaptations.
The modeling was grounded in a core
study from Lavaca and DeWitt counties,
which yielded six stratigraphic surfaces
and eight facies between the top of the
Upper Eagle Ford and the Buda formation.
These stratal surfaces guided the structural
framework of the earth model, and the
facies drove 3-D property models. The
distribution and shape of these surfaces
is controlled primarily by paleogeography,
particularly the interplay of the San
Marcos Arch, the Woodbine deltaic system,
the Rio Grande Embayment, and the positions of the Sligo and Edwards Reef
margins.
The eight facies grade from chalk/limestone to marl to clayey mudstones. In the
modeling effort, the defined core facies
were tied to well log signatures, which
then were used as predictive tools for
facies in wells with no core data.
The Lower Eagle Ford Shale reservoir
facies is dominated by organic-rich, relatively low clay, foraminifera-rich, coccolith mudstones/marlstones. The Upper
Eagle Ford Marl interval is extremely
low in clay and rich in carbonate, composed of highly interbedded, organic-rich
carbonate mudstone and bioclastic carbonate mudstone.
On the foundations of this geologic
model, regional changes occurring in
both reservoirs from DeWitt County to
Lavaca County were investigated, including facies variability and other petrophysical/geologic properties.
60 THE AMERICAN OIL & GAS REPORTER

Static earth models driven by geologic
inputs are most functional when they are
grounded in multiple integrated datasets.
The integration starts with a regional paleogeographic framework and includes
core data, chronostratigraphic well correlations, core-calibrated petrophysics,
and seismic data. Statistical methods and
geologic concepts then are applied to
build 3-D geocellular models. The model
outputs include facies, reservoir properties
(water saturation, oil saturation, mineralogy, etc.), and deterministic and/or probabilistic estimates of pore volumes, hydrocarbon pore volumes and original oil
in place volumes.
These models provide the basis for
asset development plans, including predrill
well planning, completion design, and
post-drill production performance evaluation. During the asset development cycle,
model properties and OOIP volumes provide backstops for recovery factor analysis,
EUR estimates, and reserves.
Two Models
Because of computational limitations,
time constraints and other logistical factors,
two separate, but slightly overlapping
models, were developed. The Dewitt
County model covers 297 square miles,
and includes 11 petrophysical wells, four
cores and 290 square miles of 3-D seismic.
The Lavaca County model covers 630
square miles, and includes 27 petrophysical
wells, five cored wells and 315 square
miles of 3-D seismic.
Both models concentrate on an interval
in the Upper Cretaceous that starts at the
Pepper/Maness Shale overlying the Buda
and includes the Lower Eagle Ford Shale,
Upper Eagle Ford Shale, and the Upper
Eagle Ford Marl underlying the Austin
Chalk. The two models are largely congruent, especially with respect to the
types of input data, parameter settings,
and algorithms.
Understanding the paleogeography of
the region is key to understanding differences between the two models. Both
areas are on the San Marcos Arch. This
was a paleo-high, underlain by more
stable continental crust. The paleo-high
was created by increased subsidence in
the Maverick Basin to the southwest and

the East Texas Basin to the northeast.
Both of these basins are underlain by
weaker transitional continental crust.
The productive Eagle Ford in this area
was deposited in the paleo-low Karnes
Trough, situated on the nose of the arch
and created by Gulf of Mexico extension.
Paleo-highs set up on the relative stability
of the Edwards Reef trend to the southeast
and farther up the arch to the northwest
provided increased restriction. Movement
of extensional faults along the northern
margin of the Karnes Trough is syndepositional with the Eagle Ford.
The model areas are located near the
northeastern margin of the Eagle Ford
producing trend. These areas experience
variable influence from Woodbine-related
sediments sourced from the East Texas
Basin. In Lavaca County, marine sediments
and organic matter concentrations tend
to be diluted by the influx of clay. In DeWitt County, conditions of sediment starvation resulted in significant differences
in reservoir properties.
3-D Property Models
After the 3-D geocellular framework
was established and validated, 3-D property models that included facies, petrophysical properties, and geomechanical
properties were developed using data
analysis for variograms (or multipoint
statistics) and geostatistical mapping.
Rock facies and properties were extrapolated based on the paleogeography, as
well as log properties and correlations.
Variograms were developed to extrapolate
the facies, and the facies then were used
to weight log-based properties.
In order to develop the log-derived
facies (from wells with petrophysics but
no core), we started with core facies descriptions from all of the available Eagle
Ford cores in the model areas. In these
areas, as in most of the Eagle Ford trend,
there are six volumetrically significant
facies between the Buda and Austin Chalk.
These facies are somewhat distinct from
one another in terms of physical properties,
helping differentiate them in log responses.
The best quality reservoir facies are
organic marls and bioclastic limestones.
In the organic marl facies, total organic

American Oil and Gas Reporter - January 2016

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