American Oil and Gas Reporter - March 2016 - 66

SpecialReport: Unconventional Resource Science
below the South Texas equivalent Eagle
Ford (Upper Eagle Ford) and above the
Buda. Like its younger sibling to the
south, there is abundant carbonate in the
system, which plays a large role in both
the concentration of oil in place and rock
quality, particularly brittleness.
The Lower Eagle Ford is below the
Woodbine and Subclarksville (where present). In the southwestern part of the
Brazos Basin, where there is no Woodbine,
the top of the Lower Eagle Ford is best
defined where resistivity reaches a minimum and organics begin to increase.
The Lower Eagle Ford has a trend of
increasing clay content (low resistivity)
moving eastward toward the Harris Delta.
Interval thickness varies between 100
and 400 feet, and it can be mapped regionally using a series of sequence boundaries. At its most distal setting in Burleson
and Lee counties, total organic carbon
can exceed 4 weight percentage, clay is
less than 20 percent, and oil in place can
reach 20 million-30 million barrels/section.
Like the South Texas play, the condensate-to-gas window is very narrow,
while the black oil window tends to be
much broader. In general, gas-to-oil ratios
are low, but reservoir pressures are high.
The primary target interval, where resistivities are relatively high, is bracketed
by an upper interval with low TOC and
higher clay (frac barrier) and the underlying Maness Shale (Figure 2). With a
different stress profile and higher siliciclastic and clay contents, the Maness
also acts a frac barrier and seal. We conservatively estimate that the Lower Eagle
Ford has the generative capacity to expel
substantial hydrocarbon volumes. Given
the cumulative production observed from
the Austin Chalk, Buda/Georgetown and
Lower Eagle Ford, we calculate a net
surplus in oil identified and sourced by
the Lower Eagle Ford, meaning there are
significant hydrocarbons yet to be found.
The Lower Eagle Ford was explored
originally by Apache as an extension of
the South Texas Eagle Ford play. Initial
production and EUR results were disappointing, but then a few smaller companies
recognized that EURs per foot of stimulated
lateral were actually very attractive, and
activity slowly began increasing. After
Weber Energy drilled the first longer laterals
with positive results in 2011, numerous
operators initiated drilling/leasing programs.
For the most part, the Lower Eagle
Ford has been explored and tested in the
shale facies, but in the eastern part of the
basin, operators have attempted to exploit
66 THE AMERICAN OIL & GAS REPORTER

sandy/silty intervals interfingered in the
shale.
Productivity Controls
The Lower Eagle Ford has varying
amounts of clay, carbonate and oil concentrations. These elements, along with
reservoir pressure, are the primary variables controlling productivity. We have
stacked a variety of maps to not only
show where oil in place exceeds 20
million barrels/section, but also where
fracturable facies are expected to exist in
the greatest thickness.
While clay content is high (greater
than 30 percent by volume) for a shale
play, the total porosity ranks among the
best in North America. Even with low
permeability and fair TOC, the shale can
store a substantial amount of hydrocarbons.
Overall, the Lower Eagle Ford has fair
reservoir properties compared with other
shales. Porosities and permeabilities range
from 4-10 percent and 0.001-0.00001
millidarcies, respectively.
There is little to no faulting in the
shale, and natural fracturing is variable.
In many cores, natural fractures have
been healed or partially filled with calcite.
It is rare to see fractures greater than a
few feet in length where clay content exceeds 35 percent. The shale sits in the
basin with a low monoclinal dip to the
southeast, making it an exceptional candidate for efficient exploitation.
Well EURs average 300,000 barrels
of oil equivalent in the play fairway.
Lateral lengths range between 5,500 and
9,500 feet. With such an expansive play
still in its infancy, operators continue to
refine stimulation methods. We expect
this shale will behave like all the others
in the region, where stacking and tighter
spacing dramatically increase recoveries
and net present value.
Although more than 300 wells have
been drilled over the past six years, the
Lower Eagle Ford is an emerging play,
and as with any other shale play during
the delineation phase, unanswered questions
remain. The exploration pace has decreased
with the decline in oil prices, but operators
intend to better understand the play's full
potential over the next few years.
Buda/Georgetown Carbonates
The Buda/Georgetown have been exploited for years, including with multilateral
wells. These regionally pervasive units
are tight platform carbonates interspersed
with marl and lean shale. Total thickness
is 100 to 500 feet.

The Buda, which sits below the Lower
Eagle Ford and above the Georgetown,
generally has half the thickness of the
Georgetown. They are separated by the
thin Del Rio Shale, which serves as a potential seal. Both carbonates have low
matrix porosities (2-5 percent), but contain
through-going natural fractures that provide
substantial storage and secondary
porosity/permeability.
Characterizing and estimating OOIP
for this interval is complex, because traditional log correlation and mapping is
unable to adequately assess the storage
capacity of a given fracture system. Looking at individual well performance across
the basin provides more detail on potential
flow boundaries and drained areas, which
can assist in identifying the density of
the fracture system. With low primary
porosity and permeability, the success of
this interval depends on predicting fracture
system density.
This type of play is attractive because
the overall well costs can be very low.
Simple acid washes and barefoot completions have been used frequently in
this competent, naturally fractured carbonate reservoir. Paired with low-horsepower rigs, the economics can be spectacular on wells with EURs above 200,000
boe, which can be found throughout the
basin.
Operators who hold legacy positions
in the basin have begun testing hydraulically fracturing in the Buda/Georgetown
to improve matrix permeability. While
still early, stimulated laterals in the eastern
part of the basin in Madison, Walker and
Grimes counties are generating one-year
cumulative totals in excess of 150,000
boe. With only a handful of attempts
west of the Fort Trinidad Field, this play
is at the cusp of potentially unlocking a
big resource.
Shale, Carbonate Sequences
A series of thick (200-500 foot) shale
and carbonate sequences are located beneath the Georgetown, starting with the
Goodland Lime and ending with the
Travis Peak. Within the Brazos Basin,
these combined units can make up 1,5002,500 feet of section. Sporadically riddled
with shows and organic-rich shales, this
interval rarely has been tested, but frequently penetrated by wells targeting gas
in the underlying Bossier and Cotton Valley. The oil in place is difficult to gauge
since downdip well tests are minimal,
but mapping shows gas-filled intervals

American Oil and Gas Reporter - March 2016

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