American Oil and Gas Reporter - January 2016 - 55

Unconventional Resource Science
Within the core area, the Marble Falls
is dominated by (outer ramp) siliceous spiculitic siltstone, with subordinate (basinal)
siliceous to calcareous mudstones. A significant accumulation of the siliceous
spiculite lithofacies occurs in the northern
Fort Worth Basin in Jack and northern Palo
Pinto counties, and is controlled by an antecedent topographic low, possibly a submarine valley within the underlying Forestburg-Comyn limestone (Figure 3).
Image logs characterize the Marble
Falls formation by layers of high-resistivity rock up to 1.5 feet thick separated by
thin partings of low-resistivity rock. Layers exhibiting high resistivity correspond
to intervals with higher carbonate/silica
values and lower shale volumes (Vshale)
in petrophysical logs, whereas layers
with low resistivity correspond to intervals
with low carbonate/silica and high Vshale.
Highly resistive shales (i.e., source rocks)
are present in minor amounts, but the Marble Falls generally is not considered a selfsourcing reservoir in this area.
Analyses of rotary sidewall cores,
conventional whole core, and thin-sections
show that the siliceous grainstones, siltstones and mudstones within the play fairway contain very low matrix porosity (less
than 3 percent by volume) and permeability (less than 0.015 millidarcies), nearly
irrespective of lithology. Core samples, thin
sections, and resistivity image logs reveal
a complex network of nearly vertical natural fractures that are variably filled with
calcite.
Figure 4 shows images of typical Marble Falls siliceous spiculite facies deposits.
The photographs at left (A) is conventional slabbed core from the EOG House No.
1 well showing abundant natural lithologically bound fractures. At right are photomicrographs of the siliceous spiculite facies (B), multigenerational fractures in the
Marble Falls (C), and oil (fluorescing under ultraviolet light) emanating from a fracture in slabbed core of the Marble Falls
from the Cobra Geer No. 1 well (D).
The natural fractures are fractal in nature, occurring from micro- to megascopic scales, and their size is inversely proportional to their frequency. The aperture
sizes of the fractures show a positive correlation with vertical extent, and overall
fracture heights have a distribution profile
similar to apertures.
The vertical extents of most fractures
are at or less than the scale of individual

FIGURE 3
Geologic Cross-Section of Marble Falls Formation
And Relevant Adjacent Units

Open-Hole Log Curves
Gamma Ray
True Resistivity
Neutron Porosity
Density Porosity

bed forms and are readily discernable in
image logs (Figure 2). These macroscopic fractures characteristically terminate vertically at lithological (mechanical) boundaries within the formation. In image and
open-hole logs, these LBFs are restricted

to the high-resistivity layers, which represent higher mechanical competency.
LBFs typically either initiate or terminate,
or both, at the mechanical boundaries separating thicker, more competent layers and
thinner, less competent layers, and typical-

FIGURE 4
Marble Falls Siliceous Spiculite Facies Deposits

Source: The image at left (A) is courtesy of Klinton M. Farrar from a 2010 Texas Christian
University master's thesis

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American Oil and Gas Reporter - January 2016

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