United States
Environmental Protection
Agency
Robert S. Kerr Environmental
Research Laboratory
Ada OK 74820
Research and Development
EPA-600/S2-83-082 June 1984
SEPA Project Summary
Ground Water—Mineralogy
Relationship for In Situ Oil Shale
Retorting
J.J. Fitzpatrick
Potential ground water problems
associated with modified in situ (MIS)
oil shale retorting need to be examined
in order to minimize or mitigate possible
invasion of spent shale leachates into
ground water reservoirs in actively
mined or mined and abandoned sites.
This project summary describes a
background report on the hydrology,
mineralogy and ground water chemistry
of the Green River Formation in the
Piceance Basin, the full report compre-
hensively discusses what is known
about the three physico-chemical
systems which will interact to produce
ground water quality variations at an
uncontrolled mine site in the Piceance
Basin and more specifically, at either of
the two federal lease tracts Ca and Cb
situated therein.
This Project Summary was developed
by EPA's Robert S. Kerr Environmental
Research Laboratory. Ada. OK, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
The Green River Formation underlies
an area of approximately 6,500 square
kilometers (17,000 square miles) in
Wyoming, Utah, and Colorado and
contains an estimated 1.8 trillion barrels
of oil and shale beds. Structurally, it
occurs in seven basins - the Uinta,
Piceance, Sand Wash, Washakie, Green
River, Great Divide, and Fossil basins.
Stratigraphically, the formation generally
thickeps toward basin centers, and
laterally inter-tongues and is interbedded
with adjacent units toward basin margins.
Sedimentologically, it represents deposits
of two large Eocene lakes, Gosiute and
Uinta, in which oil-bearing shales asso-
ciated with beds of tuff, siltstone,
claystone, sandstone, halite, trona and
nahcolite were deposited. In total thick-
ness, it may exceed a few thousand
meters, and in places it is saturated by
ground water throughout its entirety.
Modified in situ (MIS) processes of
extraction of oil from Green River shales
has received much attention as a potential
method of retorting. The MIS method is
advantageous because shale extraction
costs are reduced, deeper shale beds
can be retorted, less water is required,
and less surface disturbance occurs from
spent shale and retorting processes.
However, the dewatering of the mining
zone and the chemical and mineralogic
alteration of it during in situ retorting will
change the local ground water regime.
Alteration of ground water flow rates
and volumes, and changes in ground water
chemistry will certainly result from an
MIS burn. In addition, changes in local
mineralogy will occur dependent upon
changes in quantities, rates and directions
of water moving through the MIS system.
A new equilibrium of ground water
quality will be established after retorting.
Accordingly, the purposes of this report
are: 1) to review the existing knowledge
regarding the potential effects of the MIS
process on properties of ground water
storage and flow in the Green River
Formation, 2) to review the existing
knowledge of the mineralogy of the
Green River Formation, and 3) to review
the existing knowledge of possible
mineral-ground water interactions during
and after an MIS event.
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Modified In Situ Technology
This section discusses modified in situ
(MIS) technology with major emphasis on
leaching and ground water quality. The
approximate size of one in situ retort will
be 50 meters by 50 meters laterally and
300 meters vertically.
The MIS process consists of two basic
steps. The first involves forming the in
situ retort by removing about 20 to 40% of
the oil shale deposit from the bottom of
the column to be retorted and expanding
by sequential explosions the remaining
shale into the void volume. The second
step is retorting the rubblized oil shale in
place. The heat required for pyrolysis is
supplied by either external or internal
means and the volatilized oil is condensed
and collected by means of a sump below
the retort and pumped to the surface. The
shale oil is in the form of condensable
hydrocarbon vapors which is cooled to
produce a semi-viscous liquid. Along with
this, noncondensable vapors such as
methane, CH4, and ethane, H3C-CH3, are
produced along with other hydrocarbons
and organic compounds which exit with
the off-gas. A considerable volume of
retort water is also collected with the oil.
The retort water comes from the liberation
of free and bound water from the oil shale
maxtrix; the combustion of hydrocarbons,
the dehydration of various oxygen-
containing compounds, and the recombi-
nation of H+ and OH~ ions which are
produced during pyrolysis.
Pyrolysis is defined as thermal decom-
position in the absence of oxygen. As
already mentioned, pyrolysis in an in situ
retort is supplied by external or internal
means. Use of an external heat source
requires the pumping of a hot inert gas
such as nitrogen through the retort. As
the pyrolysis zone moves, a zone of spent
shale containing organic residue is left
behind. When this alternative is used, the
reducing conditions exist throughout the
retort. Use of an internal heat source
implies the combustion of residual
organics on the shale as the heat source
for pyrolysis. The shale is initially ignited
with propane, and air is then continuously
supplied to the retort. The result is the
formation of a pyrolysis zone and a
combustion zone. The pyrolysis zone
moves in front of the combustion zone
leaving behind some organic matter
which serves as fuel for the combustion
zone. Behind the combustion zone is the
spent shale. In this type of retort both
oxidizing and reducing conditions exist at
the same time, but for convenience the
overall environment of the retort is
defined as being oxidizing. Some of the
residual carbon from this process still
remains on the spent shale.
The MIS process will result in alterations
of the ground water regime different from
those obtained from traditional under-
ground mining and surface retorting
processes. Specifically, changes in the
types and distributions of permeability and
thus the rates and volumes of flow will
result. These changes, together with the
altered chemical and mineralogic compo-
sition of the mining zone, will in turn
affect the final chemical quality of the
ground water.
Hydrology
Although investigations have been
made of the chemical and flow properties
of ground water in the oil shale basins, a
thorough assessment of the effects of the
modified in situ process has not been
accomplished, principally because of its
quite recent development as an important
process. However, it is apparent that
underground changes in mineralogy and
water chemistry will be fewer for
standard methods as compared to the
modified in situ process.
Existing models of oil shale basins and
their ground water systems are regional
in scope and emphasize dewatering rates
and effects of underground mining
processes. Such models, constructed and
calibrated for the hydraulics of intergran-
ular permeability, may apply over large
areas, but are not the best predictors of
the local effects of changes from initial
conditions of secondary (fracture and/or
solution) permeability to rubbilize inter-
granular permeability, as will occur
during the MIS retort process. And finally,
no models in existence consider ground
water quality equilibria and their responses
to mining in conjuction flow properties in
the oil shale environment. Models
including processes of hydrodynamic
dispersion and hydrogeochemical atten-
uation in fracture flow systems are
applicable in solving these problems.
Because approximately two-thirds of
the oil shale potential of the Green River
Formation exists in the Piceance basin in
Colorado and because initial steps are
underway to recover oil using the MIS
process at both Colorado federal lease
tracts Ca and Cb, these areas will be
emphasized.
Ground Water in the Piceance
Basin
The richest oil shale in the Piceance
Basin is in a layer known as the
Mahogany zone. In much of the Piceance
Basin it acts as an aquitard restricting the
movement of ground water between two
major aquifer systems, one above and
another below the Mahogany zone. The
upper aquifer includes the Uinta Forma-
tion (formerly called the Evacuation Creek
member), and the upper strata of the
Parachute Creek member of the Green
River Formation, consisting chiefly of oil
shale. The lower aquifer is the section of
the Parachute Creek member below the
Mahogany zone. The garden Gulch and
Douglas Creek members, beneath the
Parachute Creek member, are like the
Wasatch formation below them. They all
consist of clay, shale, and lenticular
sandstone beds. (Figure 1)
Units below the Parachute Creek
member are not aquifers in the Piceance
Basin. Some alluvial deposits along
stream channels are aquifers, but will be
affected only indirectly by MIS retorting;
thus they will not be considered here.
As of 1979, over 100 exploration or
ground water sampling, testing and/or
monitor wells had been drilled in the
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Figure 1. Electric log response showing
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(From Welder and Saulnier,
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Piceance Basin. Approximately 30 wells
exist at each Federal lease tract, Ca and
Cb, with nearly as many additional wells
nearby. Many of the exploration wells
were drilled through both aquifers and
still permit the exchange of water
between them. However, programs of
well recompletion for testing and/or
monitoring have recently been initiated.
The detailed testing of aquifers at
Tracts Ca and Cb have resulted in the
identification of three aquifers at Tract Cb
and two at Tract Ca.
Lower Aquifer
The major individual bedrock aquifer in
the Piceance Basin is the lower aquifer
(leached or low resistivity zone). Primarily,
it has secondary permeability, like that of
both the Mahogany zone and the upper
aquifer. In addition, its fracture permea-
bility has been enhanced by the solution
of nahcolite and other minerals. Its
transmissibility is highly variable and
may reach 250 mVday (2600 ftVday). In
contrast, during pumping tests of indivi-
dual horizons in the lower aquifer at Tract
Cb, low transmissibilities from 0.15to 8.5
mVday (1.6 to 92 ftVday) were obtained.
Storage coefficients of the lower aquifer
in such tests have ranged from 10'3to 10~7.
The variation in transmissibility is a
result of the pronounced anisotropy of
permeability of the aquifer, which is
primarily due to the orientation of
fracture systems (both faults and joints)
and variations between different oil shale
beds. In addition, the preferred orientation
of faults and folds, in northwest, south-
east, and east directions, undoubtedly
has a significant effect on ground water
movement.
Upper Aquifer
The upper aquifer has chiefly fracture
permeability, with little evidence of
significant solution permeability. Its
transmissivity ranges up to approximately
9 mVday (1,000 ftVday) and generally
increases from east to west in the basin.
Most investigators have noted that
hydraulic coefficients of the upper aquifer
also vary widely within small distances. In
general, however, the upper aquifer is
less permeable than the lower. Of course,
the upper aquifer is not confined every-
where in the basin. It will have to be
dewatered to retort oil shale by the MIS
process.
Ground Water Movement
Studies of wells penetrating both the
upper and lower aquifers have shown
distinctly different water levels between
them. In general, potentiometric heads
are higher for the upper aquifer in
recharge areas and higher for the lower
aquifer in discharge areas. Head differ-
ences commonly vary between 15 and 30
meters. The results indicate that the
ground water system is recharged at
higher elevations near the basin margin
on the south, and that discharge from the
ground water system to the streams
occurs near the basin center and to the
north along White and Yellow Creeks.
Mineralogy of the Green River
Formation in Piceance Basin
The Eocene Green River Formation,
host to the rich oil shales of the Piceance
Creek Basin, has long been of interest to
mineralogists due to the occurrences of
rare and unusual minerals within its
strata. In the past 50 years, hundreds of
studies have been carried out by research-
ers on core cuttings and cores from the
Piceance, Uinta, Green River and Washakie
Basins in Colorado, Wyoming and Utah.
Studies, discussed in the full report build
up a picture of an amazingly diverse
formation composed of authigenic car-
bonates, silicates, sulfides, sulfates,
ha I ides, oxides, hydroxides, phosphates
and hydrocarbons in a highly stratified
sequence of sediments covering over
16,000 square miles of a three-state area
and representing the single largest oil
resource known in the United States.
The Green River Formation in the
Piceance Basin of northwestern Colorado
is generally subdivided into three members
and include the lowermost Douglas Creek
Member, which is stratigraphically
equivalent to the Anvil Points Member in
parts of the basin, the Garden Gulch
Member, which in places in time-
equivalent to the underlying and adjacent
Douglas Creek and Anvil Points, and the
uppermost Parachute Creek Member,
host to the rich oil shales of the basin.
Together these units form a complexly
interfingered series of sandstones,
siltstones, mudstones, shales, oil shales,
limestones, marlstones, and evaporites
with occasional tuff beds.
The deposits in -the Piceance Basin
were laid down beneath the waters of
Eocene Uinta Lake. The Douglas Creek
Arch undoubtedly formed a barrier to
communication between the Uinta and
Piceance basins during periods of low
water in the history of Lake Uinta, thus it is
not surprising that the beds of the Green
River Formation in the Piceance Basin are
mineralogically distinct from those of the
Uinta Basin.
Generally speaking, the evaporites
(halite and the sodium carbonates) were
laid down during dry periods in the
deepest parts of the section of Lake Uinta,
which occupied the Piceance Basin.
During wetter periods the marlstone and
organic-rich oil shales accumulated.
The distinguishing feature of the
Piceance Basin is its relatively simple
mineralogy in comparison to the Uinta and
Green River-Washakie Basins. The
Green River beds in the Piceance Basin
have been extensively characterized due
to the rapid development of oil shale
industries seeking to mine in the area. This
is especially true of the Parachute Creek
Member, which contains the Mahogany
zone oil shales, the main oil shale ore of
the basin. A report on all the studies
carried out on the mineralogy of the oil
shales of the Piceance Basin alone would
form a document of formidable size.
Ground Water Chemistry of
Piceance Basin
This section of the full report describes
the major types of chemical reactions
occurring in the aquifers of the Piceance
Basin with consideration given to the
specific mineral assemblages. The con-
centration of dissolved solids in the
aquifers of the Piceance Basin varies over
2 orders of magnitude with values of 400
mg/l at the margins of the basin and up to
40,000 mg/l in the North Central part of
the basin. Along with these lateral
changes, there are also changes in
concentration with depth. These changes
in water quality include an increase in
dissolved-solids concentration, a change
in water type from a mixed cation
bicarbonate water to a sodium bicarbonate
type, oxidation and reduction of sulfur
species, and relatively large increases in
certain trace constituents such as
strontium and fluoride. These changes
reflect the geochemical environment
through which the ground water flows
and that the magnitude of the changes is
affected by the type and concentration of
soluble minerals present in the aquifer,
the type of clay minerals and the amount
of organic material present in the aqui-
fers, and finally, the flow path of the
water through the aquifers.
As the ground water of the upper
aquifer flows from the recharge areas,
the TDS concentration increases and the
water changes to a sodium bicarbonate,
NaHCOa, type. Large increases in TDS
concentrations occur in the Uinta Forma-
tion where concentrations of two to three
times that of the recharge zone are
observed. Also, near the base of the upper
part of the Parachute Creek member, TDS
concentrations are up to ten times that of
the recharge water. The principal source
of this increase is upward flow of water
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with a high dissolved solids concentration
from the lower aquifer to the upper
aquifer. Upward flows also probably
account for the larger dissolved solids
concentrations near Tract Cb and in the
north-central discharge areas of the
basin.
Trace Elements in the Green
River Formation of Piceance
Basin
A knowledge of the trace element
distribution is important so that teachability
can be appropriately modeled. Relative to
average rocks, shale contains much
higher levels of selenium and arsenic,
and moderately higher levels of cobalt,
nickel, chromium, zirconium, and manga-
nese, cadmium, antimony, mercury, fluo-
ride, and boron.
Conclusion
The final report is a compilation of the
general state of knowledge of environ-
mental chemistry in oil shale research. It
offers no conclusive definitions on the
leaching phenomenon from MIS retorts.
Some trends can be seen in the various
investigations; however, the available
data can only beclassified as preliminary.
Completed investigations on inorganic
leachablesarenotin agreement with one
another, and there has been only one
effort with organic leachables. Completed
work does point out which parameters
warrant further study.
The major tool which will be required to
complete the chemistry for this project
will be a geochemical computer model
which simulates mass transfer and
reaction path sequences among a given
mineral assemblage. Once the mineralogy
of spent shale is known m detail, the geo-
chemical computer model can predict the
changes in water quality, and knowledge
of hydrology can show the flow path of
the leachables.
J. J. Fitzpatrick is with the University of Denver, Denver, CO 80208.
B. D. Newport is the EPA Project Officer (see below).
The complete report, entitled "Ground Water—Mineralogy Relationship for In
Situ Oil Shale Retorting," (Order No. PB 84-187 764; Cost: $46.00, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Robert S. Kerr Environmental Research Laboratory
U.S. Environmental Protection Agency
P.O.Box 1198
Ada, OK 74820
•if U.S GOVERNMENT PRINTING OFFICE; 1984 — 759-015/7737
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