American Oil and Gas Reporter - March 2016 - 65

SpecialReport: Unconventional Resource Science
FIGURE 3
Combined Lower Eagle Ford/ Woodbine Oil in Place (MMbbl/Section)
50
45
40
35
30
25
20
15
10
5
0

Ford source rock.
In the past, operators implemented
small water/acid fractures to enhance the
naturally fractured chalk. As the play
progressed, multilateral wells were drilled
to intersect natural fractures and comingle
multiple horizons. Although a mature
and often-depleted play, considering the
wide distribution of migrated Lower Eagle
Ford oil into the chalk, it is reasonable to
assume that it still can be charged where
the two intervals are in close contact.
There also are pockets of untapped
potential where little drilling has occurred.
Where other plays competed or shifted
through time, the Austin Chalk occasionally was overlooked when shows or calculated log pays were present.
Given the nature of the lithology, we
expect diminished storage capacity, but
with conservative porosity estimates and
a minimal amount of natural fractures,
there is a substantial amount of oil in
place yet to be found in the chalk. With
continued innovation of long horizontal
wells and hydraulic fracturing, there is a
sizable prize remaining in areas where
depletion has not occurred. Using rate
transient analysis, we can estimate the
drainage area per wellbore and assess
the distribution of previously drilled and
unfractured wells.
Once the drainage area distribution is
known, assumptions can be made about
productivity by area based on the density
and storage capability of the natural frac-

ture system. By regionally mapping these
data, areas can be seen that were not effectively drained or had suboptimal storage
capability. Pairing these "poorer-quality"
rock areas with longer laterals and modern
completion technology can lead to an
untapped portion of an old, but very productive field.
Early results make estimating singlewell ultimate recoveries in the play difficult, but given the competence of the
rock, the necessary stimulation should
be less expensive than other plays, yielding
better well economics.
Companies such as Apache, Earthstone
Energy, EnerVest Ltd. and GeoSouthern
Energy are testing the edges of the Austin
Chalk trend with great results. Some operators
are re-entering old wells and implementing
minor new fracs, while others are drilling
new laterals with advanced completion designs. We see similar storage and shows in
northern Brazos and southern Robertson
counties to justify continued evaluation.
Woodbine Sandstone
The variably thick Woodbine is located
below the Austin Chalk and between the
Upper and Lower Eagle Ford. There are
up to five sandstone members, but only
one or two typically develop appropriate
reservoir facies with sufficient storage to
justify as drilling targets.
The best Woodbine reservoir is correlated to heavily bioturbated, quartz-rich
intervals that preserve primary porosity.

With good-quality reservoir properties
and direct contact with the underlying
Lower Eagle Ford, single-well Woodbine
EURs in excess of 200,000 barrels were
common during vertical Woodbine development in the 1970s-80s.
As with many other basins/plays in
the Cretaceous intervals, the edges of
these conventional fields remain charged
with hydrocarbons, but have undesirable
characteristics. Horizontal drilling/completion and hydraulic fracturing allow
operators to access more rock and enhance
reservoir deliverability.
Using recently acquired whole cores
to tie petrophysical properties and an abundant log database, original oil in place in
the Woodbine sand was mapped with confidence throughout the region. More than
12 million barrels of oil per section remain
in place in parts of the play that have not
been developed (Figure 3). While core/log
ties are necessary to understand reservoir
distribution, well-to-well reservoir variability
is difficult to predict.
With Woodbine permeability an order
of magnitude better than unconventional
targets in the area, this play requires
smaller frac jobs (600-1,000 pounds of
proppant and 10-11 barrels of fluid per
foot of lateral), typically has a flatter decline curve, and generally provides superior
economics. Given the nature of the limited
sand bodies, the extent of development
is more restricted. EURs for horizontal
wells in the Woodbine play typically are
greater than 300,000 barrels.
Where the most optimal Woodbine
reservoir properties are found in Leon,
Madison and Brazos counties, companies
such as CML Exploration, Contango Oil
& Gas, MD America Energy, Vess Oil,
New Gulf Resources, and Marlin Resources are achieving favorable economics
by drilling tightly spaced horizontal wells
with relatively small frac jobs.
Since 2004, more than 240 horizontal
wells have been drilled in the play. The
most productive area is located in northeast
Brazos and west-central Madison counties,
where EURs average more than 400,000
barrels. Since the depositional limits of
the sand bodies have been defined, the
delineation and future development of
the play may be limited to untapped areas
in Robertson County, thin reservoirs in
Madison and Brazos counties, and
downspacing within current fields.
Lower Eagle Ford Shale
The carbonate- and clay-rich Lower
Eagle Ford Shale stratigraphically sits
MARCH 2016 65

American Oil and Gas Reporter - March 2016

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