American Oil and Gas Reporter - October 2018 - 58

SpecialReport: The Eagle Ford and Austin Chalk
FIGURE 1
USGS Eagle Ford Shale Assessment Units
-101°

-102°

32°

-100°

-99°

-98°

-97°

-96°

-95°

-94°

-93°

-92°

-91°

-90°

-89°

31°

30°

29°

28°

27°

26°

25°

Source: U.S. Department of the Interior National Park Service

0

EXPLANATION

Eagle Ford Marl Continuous Oil
Eagle Ford Marl Continuous Gas
Submarine Plateau-Karnes Trough Oil
Submarine Plateau-Karnes Trough Gas
Cenomanian-Turonian Mudstone Continuous Oil
Cenomanian-Turonian Mudstone Continuous Gas
Cenomanian Downdip Continuous Gas
Upper Jurassic-Cretaceous-Tertiary Composite
Total Petroleum System Boundary (part)
State Boundary
International Boundary

0

100
100

200 Miles
200 Kilometers

FIGURE 2
USGS Austin Chalk Assessment Units and Major Fields
102°

100°

98°

96°

94°

92°

90°

88°

86°

34°

32°

30°

28°

EXPLANATION

Field Locations
Oil Production

26°

58 THE AMERICAN OIL & GAS REPORTER

Gas Production
AU Boundaries
LC Shelf Edge

Oil migrating through natural microfractures in the Eagle Ford and filling
the tectonic features of the porous matrix
above is the primary source of oil in the
Austin Chalk, observes Scott Martin,
chief executive officer of Blackbrush Oil
& Gas. In the past, operators targeted
naturally fractured zones within the Chalk,
using horizontal drilling to connect fracture
systems and drain the reservoir, but
ignored the primary matrix porosity because of its low permeability. Martin says
that is changing, however, as companies
apply Eagle Ford-style completion technology to the Chalk.
Mirroring the extent of the Eagle
Ford in the Lone Star State (Figure 1),
the Austin Chalk spans from west to
east-from the Pearsall Field near the
Mexican border in Southwest Texas to
the Brookeland Field bordering the
Louisiana state line-but then extends
eastward on into Louisiana, Mississippi
and Alabama (Figure 2).
"The 'quiet' Austin Chalk is more of
a true reservoir," comments Martin, whose
company was the first to successfully
complete Austin Chalk horizontal wells
in Karnes County, Tx. "We call the Chalk
in Karnes County quiet because it is not
as naturally fractured as is the Giddings
Field in Lee and Frio counties, Tx., but it
does have porosity development. It requires
a frac."
He says Blackbrush discovered that
by accident. It was drilling a vertical Edwards well, which historically has produced a stacked pay. The cementer made
a mistake, and the company lost the
bottom of the hole.
"So we started looking uphole," Martin
recalls. "We perforated a short vertical
section in the Austin Chalk, and it came
in at 350 barrels a day. In that area, the
Chalk is much better than the Eagle Ford.
We started developing the Upper and
Lower Eagle Ford intervals, as well as
the Austin Chalk."
Austin Chalk reserves are estimated
at greater than 855,000 barrels of oil
equivalent per well in Karnes County.
Although Blackbrush later sold its
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