American Oil and Gas Reporter - January 2016 - 50

Unconventional Resource Science
Activity is concentrated in Jack and
northern Palo Pinto counties, but the tectonic and lithologic factors that enhance
reservoir quality and production extend well
beyond the development area, and operators are working to expand the frontiers of
this economically attractive resource play.
The Marble Falls is a unique reservoir
in the Fort Worth Basin in that it is characterized by ubiquitous natural fractures
occurring on multiple scales, but is dom-

inated by lithologically bound fractures
(LBF) concentrated within a siliceous
spiculitic lithofacies, the distribution and
thickness of which define the play's fairway (Figure 1). LBF orientations have significant impact on the effectiveness of hydraulic fracture treatments and well productivity.
The Marble Falls was historically considered a "conventional" reservoir, in that
it was confined to small fields character-

FIGURE 1
Regional Map of Fort Worth Basin and Marble Falls Play
Marble Falls Modern Wells

0 5 10

20

30

40

Marble Falls Historic Wells

50
Miles

Barnett Shale Wells
Marble Falls Spiculite Limit
Pre-Marble Falls
Marble Falls Outcrop

Marble Falls Spiculite
Gross Thickness
450 - 500
400 - 450
350 - 400
300 - 350
250 - 300
200 - 250
150 - 200
100 - 150
50 - 100
0 - 50

50 THE AMERICAN OIL & GAS REPORTER

ized by restricted porosity/permeability
trends and associated with discrete structural/stratigraphic hydrocarbon traps.
Moreover, only the uppermost part of the
Marble Falls formation was reputed to
have the requisite characteristics to trap and
produce hydrocarbons. Therefore, the
majority of wells intentionally drilled
into the Marble Falls penetrated only
into the top of the unit.
Historical production from formations
below the Marble Falls was even more limited, with only a small percentage of
wells drilled prior to 2000 penetrating formations deeper than the Marble Falls. Fortunately, a wealth of modern open-hole
petrophysical data were acquired over the
past decade in association with the Barnett
Shale play.
Redefining An Old Play
In late 2007, EOG Resources discovered a small, anomalously oil-rich field
within the Barnett Shale in northeast
Palo Pinto County. To date, it has produced
2.2 million barrels of oil and 56 billion cubic feet of gas. EOG drilled some 40 horizontal wells in the field, many of which
have laterals just beneath the Marble
Falls/Barnett contact.
In 2009, EOG completed the Marble
Falls formation in the Richards Ranch No.
9 vertical well in central Jack County with
a small slickwater frac (9,000 gallons of
freshwater and 75,000 pounds of sand),
which went essentially unnoticed amid all
the nearby Barnett development activity.
That well, now operated by Newark E&P
Operating, has since produced 90,000
barrels of oil and 650 MMcf of gas.
A year later, initial production of more
than 400 bbl/d of oil was reported for the
Johnson No. 8, a deviated Marble Falls
well drilled by Halek Energy in central
Jack County and completed with 5,000
gallons of hydrochloric acid. Reports of the
success of the Johnson 8H (which has produced more than 120,000 barrels of oil and
850 MMcf of gas), and the recognition that
the Marble Falls represents a much more
continuous, unassociated hydrocarbon
accumulation, induced a flurry of leasing
and drilling activity.
Completions in the "new" unconventional play have taken several evolutionary steps, mostly by trial and error, from
the low-volume acid treatment of the
Johnson 8H to marginally successful nitrogen foam and low-rate slickwater hy-
American Oil and Gas Reporter - January 2016

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