CHARACTERISTICS OF PALEO-FLUID AND FLUID FLOW IN THE
PENNSYLVANIAN-PERMIAN FOUNTAIN FORMATION
Introduction:
The Pennsylvanian Fountain Formation is an arkosic sandstone within the
Front Range region of Colorado. Bleached white zones extend laterally
throughout the formation and have sharp contacts with the surrounding red
sandstone. Cross laminae pass through the red and white contacts, indicating
that it was bleached after deposition (Hubert, 1960). Features such as these are
typically related to zones of migrating reducing fluid (Turner, 1980). The purpose
of this project is to determine the nature, behavior, and pathway of the fluid. The
first goal is to determine composition of the reducing fluid. The second goal is to
determine what conditions were present during fluid movement and associated
diagenesis. This includes temperature, pressure, and depth. The third goal is to
determine the path of the fluid and what constraints were on the system to cause
it to take this particular path.
Purpose and importance to the Rocky Mountain region:
The purpose of this project is to solve a local geological problem and to
provide better understanding of large-scale fluid movements through sandstones.
The bleached zones in the Fountain Formation have been noted in several
papers but there is no definite conclusion on how they originated. Early work
suggested that the bleached zones were a result of a process dependent on
flood plain deposition (Hubert, 1960). A later paper states that the bleached
zones also occur in marine sediments in the Fountain (Garner, 1963). Hubert’s
flood plain origin for the reducing fluid is not plausible when applied to a marine
setting. Garner does not propose a solution but merely suggests that more work
is needed. Widespread bleached zones are now thought to indicate large-scale
fluid flow (Turner, 1980), (Parry, Chan, & Beitler, 2004), (Beitler, 2005). The fact
that both marine and continental sediments are bleached over a large geographic
range point to a diagenetic event dealing with a large-scale movement of
reducing fluid through the Fountain Formation.
Understanding fluid flow through sandstones has economic importance for
both hydrocarbon exploration and groundwater management. Pathways and
factors that control flow are important in exploration. The diagenetic process is
very important as well. Fluid inclusions will be used to estimate the temperature,
pressure, and depth that the reaction took place at. Once this information is
known it will be possible to determine what reactions were actually taking place
by analyzing thin sections. Understanding the depth at which certain reactions
take place within reservoirs is vital to understanding how these systems operate.
Methods:
1. Determine the composition of the reducing fluid:
This will be accomplished by analyzing fluid inclusions. Fluid inclusions are
pockets of trapped liquid and gas that form during diagenesis (Goldstein, 2003).
The inclusions to be considered were trapped in the cement that precipitated
during the reduction event and contain the reducing fluid. The inclusions will be
observed under fluorescent light to identify hydrocarbons. Hydrocarbon and
groundwater inclusions can be analyzed by cooling them on a gas flow
temperature stage. The temperature at which they freeze at can be used to
determine the fluid’s salinity, which may offer insight into its origin (Goldstein,
2003). Compositional changes can be measured between inclusions from
different sites to see how the fluid was changing as it migrated through the
sandstone.
2. Determine the conditions that the event took place in:
This will be accomplished using fluid inclusions and thin sections. As the fluid
inside the inclusion cooled its volume decreased but the volume of the inclusion
itself remained the same. This causes a bubble to be present in the inclusion.
The temperature that the inclusion was formed at can be determined by heating
the inclusion until it reaches its original volume, which is the point at which the
bubble disappears. This method can be used to determine what the temperature
and pressure was during entrapment (Goldstein, 2003) which can be used to
estimate depth. Thin sections will be used in order to look for diagenetic changes
between the bleached and non-bleached zones.
3. Determine what controlled the path of fluid flow:
This will be accomplished by studying both large-scale features in outcrops and
small-scale features in thin sections. Porosity and permeability differences
between bleached and non-bleached zones will be studied in outcrops. The
lateral continuity as well as other large-scale patterns of the bleached zones will
be investigated. Small-scale features will be studied under a polarizing
microscope. Thin sections will be made from bleached and non-bleached
samples to determine what features are important in controlling the path of fluid
flow. Features to be taken into account include the texture and composition of
grains, porosity and permeability, clay content, and cement type.
Thin sections and doubly polished fluid inclusion sections will be
necessary for this project. Four fluid inclusion sections and ten thin sections will
be used for each of the seven study sites. Fluid inclusion samples will be taken
from adjacent white and red zones. Thin sections will be from samples within
white zones, at the boarder of white zones, and in red zones at specific distances
from white zones. Samples for fluid inclusion and petrographic analysis will be
taken from seven study sites covering 50 miles along the northern Front Range.
The southern proposed sampling site is near Lyons, CO and the northern site is
near Owl Canyon, 20 miles north of Fort Collins, CO. Five other sampling sites
will be located between these two points. The field work will be conducted during
Summer 2012, with data acquisition and analysis during the following Fall and
Winter, and thesis writing during Spring 2013.
Importance of Larimer County Open Spaces:
Devil’s Backbone and Horsetooth Mountain have many excellent outcrops
of Fountain Formation and are located in the middle section of the study site.
Mapping of the white zones can be done fairly easily at these locations because
of the large outcrops and continuous exposures. Samples of Fountain Formation
sandstone are required to make thin sections and fluid inclusion sections. Ten to
fifteen samples are needed from each of the three sample sites.
References cited:
Beitler, B. (2005). Fingerprints of Fluid Flow: Chemical Diagenetic History of the
Jurassic Navajo Sandstone, Southern Utah, U.S.A. Journal of Sedimentary
Research, 75(4), 547-561. doi:10.2110/jsr.2005.045
Garner, H. (1963). Fountain Formation, Colorado: A Discussion. New York,
(October), 1299-1301.
Hubert, J. F. (1960). Syngenetic Bleached Borders on Detrital Red Beds of the
Fountain Formation, Front Range, Colorado. Geological Society of America
Bulletin, 71(1), 95. doi:10.1130/0016-7606(1960)71[95:SBBODR]2.0.CO;2
Parry, W. T., Chan, M. a, & Beitler, Brenda. (2004). Chemical bleaching indicates
episodes of fluid flow in deformation bands in sandstone. AAPG Bulletin,
88(2), 175-191. doi:10.1306/09090303034
Turner, P. Continental Red Beds. New York: Elsevier Scientific Publishing
Company. 1980.