American Oil and Gas Reporter - January 2016 - 52

Unconventional Resource Science
FIGURE 2
Modeled Petrophysical Log (A)
And Microresistivity Image Log (B) of Marble Falls Formation
A

B

4,805
4,806
4,807
4,808
4,819
4,810
4,811
4,812

4,800

4,813
4,814
4,815
4,816
4,817
4,818
4,819
4,820

4,832

4,821
4,822
4,823
4,824
4,825
4,826
4,827
4,828

draulic fracs, to increasingly successful
multistage, high-rate, slickwater stimulations with high proppant concentrations.
The latter have produced the best results
to date, are more economical compared
with linear gel or cross-linked systems, and
are capable of sufficiently carrying lowcost natural proppants. The latest generation of completions being applied by
Newark incorporates 2,200 gallons/foot
loadings pumped at 100 bbls/minute with
proppant concentrations up to 2.5
pounds/gallon.
Between January 2010 and January
2015, more than 800 wells were completed in the new Marble Falls play, producing
an estimated 20 million barrels of oil and
250 billion cubic feet of gas in that period.
By comparison, the 3,500 "conventional"
wells drilled prior to 2010 have cumulatively produced 13 MMbbl of oil and 370 Bcf
of gas. With average per-well drilling and
completion costs of less than $750,000, new
vertical wells in the core area continue to
yield favorable economic returns.
Marble Falls Architecture
Newark studied tens of thousands of
52 THE AMERICAN OIL & GAS REPORTER

vintage and modern open-hole well logs,
microresistivity image logs, petrophysical
data, and conventional whole and rotary
sidewall core data to map in detail the internal architecture of the Marble Falls
throughout the Fort Worth Basin and adequately evaluate its regional-scale reservoir potential.
The greatest challenges in developing
a geologic model were creating an accurate stratigraphic framework and achieving a reasonably accurate understanding
of the geographic distributions of historic
and modern wells producing from the Marble Falls. These challenges stem primarily from inconsistencies among stratigraphic nomenclature. Over time, geologists and operators have assigned nearly
a dozen different names to the Marble Falls
section, in many cases erroneously. Ultimately, Newark correlated dozens of individual formation tops in more than
30,000 wells to produce a consistent regional stratigraphic framework.
A Marble Falls petrophysical model
was developed to characterize and map lateral and vertical variations in lithology,
anisotropy, and reservoir characteristics

such as fractures. Nonuniform responses
from tools of multiple vintages and service providers presented a significant challenge because of the geographic scale and
number of logs incorporated. All open-hole
log curves were run through a rigorous,
systematic workflow to normalize data to
a uniform measurement sensitivity based
on the responses in a control set of modern well logs. An artificial neural network
was then used to calculate any common
log data that were absent (such as bulk density), as well as sonic two-way travel times,
sonic porosity, compressional slowness,
and shear-wave slowness based on a
training set of log suites, including modern dipole sonic logs (Figure 2).
Absent a software-based approach, a
two-step process was devised to map the
frequency of natural fractures. First, microresistivity borehole image logs from 90
Newark-operated wells were visually analyzed for natural fractures in the Marble
Falls and a qualitative scale ranging from
0 to 5 was generated to assess fracture frequency relative to each log in the dataset
(with zero representing no visible fractures
and 5 representing intensely fractured).
The Marble Falls was divided from bottom to top into 10-foot intervals, and each
interval was assigned a value from the
scale, which became the basis for a coarse
square wave log ("fracture rank").
In the second step, a training set consisting of the fracture rank curve and several primary and calculated curves (gamma ray, true resistivity, photoelectric effect,
etc.) were run through the artificial neural network to produce a calculated fracture frequency ("FracFreq") curve (sixth
column on the image at the left in Figure
2). The qualitative fracture rank curve and
calculated FracFreq curve exhibited a
reasonably acceptable correlation coefficient for the nature of the inputs. Sum and
average FracFreq values then were mapped
to identify fractured sweet spots in the formation.
Continuous Depositional System
The results show that the Marble Falls
is a single, continuous depositional system
covering nearly 15,000 square miles. It
thickens from subcrop on the Bend Arch
to the west to 500 feet in the Fort Worth
Basin to the east, a trend that generally corresponds with an increase in the percentage of shale facies and represents a basinward shift in depositional environment.
American Oil and Gas Reporter - January 2016

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