Spatial analysis of coal seam gas water chemistry
J. Fennell1, P.C. Smith2, T.J.F. Cook3, J. Sweeney2, G. Wilcox1, E. Haack1 & S. Honicke3 (1 WorleyParsons Alberta, Canada; 2 WorleyParsons Melbourne, Australia; 3 WorleyParsons Brisbane, Australia)
Background
Interpretive Methods
The origins and history of coal seam waters, and their relationship with
different geological formations, can be inferred from their chemical properties.
Results collated from the dataset were used to generate a range of physical
and chemical interpretive assessments. When used in conjunction with one
another, such interpretive assessments may provide an understanding of
how regional groundwater flow characteristics and geology may influence
groundwater chemistry. The primary driver of the present study is to
identify if a unique chemical signature can be identified for each of the major
formations of the Surat and Bowen Basins. The interpretive assessments used
include:
The objective of Activity 1.2 is to collate and interpret groundwater flow
data, in conjunction with groundwater chemistry, in an effort to identify if
unique hydrogeological ‘signatures’ or ‘fingerprints’ exist for each of the major
water-bearing formations of the Surat and Bowen Basins, and to identify the
potential for indicators of connectivity between different formations.
Data Sources
a) Hydraulic (physical) properties:
For this particular project, a series of datasets were accessed and
amalgamated into an integrated database . These data sets were:
•
The Queensland Government Groundwater Database, containing data for
138,534 registered bores;
•
The Geological Survey of Queensland Borehole Database, containing
similar data for 7,362 petroleum wells;
•
Geological Survey of Queensland water bore data, interpreted pre-1987;
and
•
Multiple Geoscience Australia datasets with source data from the Bureau
of Rural Sciences, Geological Survey of Queensland, the Department of
Primary Industries and Resources of Southern Australia and DERM.
To provide context for the hydrochemistry assessment, the regional
groundwater flow patterns and the hydraulic inter-relation between
aquifers needed to be investigated. Contour maps of groundwater levels
or pressure surfaces were produced where data allowed.
b) Chemical Assessments:
Data Limitations and Uncertainties
The chemical datasets used in this investigation were of considerable size,
however some issues and limitations relating to the data were identified
during the assessment process. For example, significant variations in the
chemical concentrations values were observed.
This variability may suggest possible issues relating to sample collection or
bore integrity. However, it is possible that such variability in the dataset relates
to the mixing of groundwater across discrete formations. These are not simple
questions to answer. Nevertheless, in an effort to address such questions,
a series of chemical assessments were performed.
Major Aquifers of the
Bowen and Surat Basins
What is groundwater, where is it and how does it move?
Groundwater is not a static entity and moves under the influence of gravity. Simply put,
precipitation that falls on an upland area, and does not directly run off the landscape, will enter or
recharge groundwater and move to lower lying areas. Streams, rivers and lakes are typically found
in these lower lying areas. Figure 1 provides an example of how this process works. Groundwater
can take days to years, decades or even thousands to millions of years to move large distances.
The speed at which groundwater moves is largely dependant on the permeability of the formation
or the presence of geological structures (i.e. faults or fractures).
Rolling Downs
Group
Surat Silstone
Doncaster Member
Injune Creek
Group
•
ion ratios (i.e. sum of major cations to chloride; silica-fluoride- chloride/chloride);
•
multivariate statistics (i.e. Cluster Analysis, Principle Component
Analysis);
•
isotopes; and
•
spatial analyses (i.e. sodium to chloride ratios; bicarbonate to
chloride ratios, silica + fluoride + chloride to chloride ratios, mineral
saturation indices).
Of the major ion relationships investigated, sodium (Na+) versus chloride (Cl-) was analysed in an
attempt to differentiate groundwater types within the various formations.
Bungil Formaon
Figure 3 shows the concentrations of Na+ versus Cl- for the Rolling Downs and BMO group
formations as identified within the working dataset Results from the Na+ versus Cl- relationships
show a distinct and positive linear correlation and a unique slope to the linear regression lines
through the data for each formation. It was determined, however, that this method alone may not
be sufficient to differentiate the various formations and assess mixing relationships. However, such
relationships may prove useful when used in conjunction with a sophisticated statistical approach.
Kumbarilla Beds
Westbourne Formaon
selected ion relationships (i.e. sodium vs. chloride);
Sodium (Na+) versus Chloride (Cl-):
Coreena Member
Gubberamunda Sandstone
•
Although the complete suite of results can not be presented here, only a few select and
representative findings are discussed and explained. Despite the variability in bore and chemical
results in the working dataset, various interpretive methods are showing very promising results in
identifying regional groundwater flow in relation to chemistry.
Griman Creek Formaon
Orallo Formaon
major anions and cations and their interpretation;
The interpretive assessments used for this study represent a series of methods that geoscientists
use, industry-wide, for better understanding such large-scale systems as the Surat and Bowen
Basins.
Main Range Volcanics
Mooga Sandstone
•
Results of the analysis
Condamine River (and tributaries) Alluvium
Wallumbilla
Formaon
In an effort to identify and associate a groundwater chemical signature to
a particular formation, groundwater data was interpreted using several
chemical techniques. This information was assessed on a formation by
formation basis. The interpretive chemical methods applied included:
Springbok Sandstone
Sodium versus chloride
relationships for the Rolling
Downs Aquitard and BMO Group
(the first value in the equation
for each charts shows the unique
slope of the associated regression
line, and the R2 value indicated
the variance in data points
described by the line, with values
approaching 1 describing the data
variance well).
Walloon Coal Measure
Huon Sandstone
Eurombah Formaon
Evergreen Formaon
Precipice Sandstone
Moolayember Formaon
Figure 3
How can chemistry be used to interpret groundwater flow?
Analysing and interpreting groundwater chemistry can provide valuable insight into the
composition of subsurface geological materials (i.e. aquifers) and the rate and direction of
groundwater flow.
For example, each aquifer is composed of different materials (i.e. clay, sand and rock),
and therefore may have a unique suite of dissolved ‘chemical identifiers’ associated with
its groundwater. The composition of groundwater chemistry is very important to
hydrogeologists in identifying aquifer-types and flow systems. Therefore, investigations
involving groundwater flow (hydraulics), geology and chemistry all assist in the forensic
analysis of natural groundwater systems. This includes how these aquifers or ‘systems’
might function separately, or if there is a connection between them. A change in the
chemistry of groundwater between aquifers can be defined as a chemical signature,
and therefore may be unique to a particular groundwater formation.
Silica (Si+)-Fluoride (F-)-Chloride (Cl-) / Chloride (Cl-) ratios &
Mineral Saturation Indices:
Ion ratios were calculated for selected elements (i.e. F-, Si+ and Cl-). The purpose of
using these elements as ratios is that their concentration in a particular aquifer is
influenced by a limited set of chemical processes. Therefore, ions such as F- relative
to Cl-, and mineral saturation indices, can be used to determine their source
(i.e. identifies mineral dissolution; predominantly from silicate-mineral weathering).
As yet, the approach of using these ion ratios and data for mineral saturation indices
has not provided definitive results in terms of identifying chemical signatures for
any specific aquifer. However, preliminary results do show the possible indication
of mixing between certain formations near faults identified in or near outcropping
areas. Figure 4 shows, spatially, a representative example of the variability of these
ratios for the BMO Group and areas where potential groundwater mixing might be
occurring.
Figure 4 - circles indicate areas
of potential mixing
Statistical Approach (Cluster Analysis):
To assist with the regional forensic interpretation of the groundwater chemistry/quality, groundwater flow, within and
between major aquifers, was assessed. Groundwater flow in a particular aquifer can greatly assist in predicting where
discharge occurs and to assess the direction of groundwater flow it is necessary to relate the depth to groundwater
measured in individual wells. For example, Figure 2 (below) shows the direction of groundwater flow for aquifers of the
Bungil, Mooga and Orallo (BMO) Group, as indicated by the arrows; where the arrow points in the direction of flow.
To further the process of assessment, a number of statistical tests relating to correlation of constituents were employed
to assess differences in groundwater types between formations. A general summary of the results obtained is provided in
Table 2 (below).
Cluster analysis (k-means) identified three potentially unique chemical clusters (types), for each major formation in the Surat
Basin. These relate to:
To determine the potential for groundwater flow between aquifers, it is necessary to understand the head difference,
this includes measuring groundwater levels or pressures. Where aquifers are not separated by a impermeable layer (i.e.
a confining unit) the head differences are said to be near to or in equilibrium and allowing for flow between them. The
opposite occurs where impermeable layers exist. Other controls contributing to flow between aquifers can be caused
by preferential
flow paths such
as faults or poorly
constructed bores
that may have failed.
•
Recently recharged porewater of low Total Dissolved Solid (TDS) content near formation outcrop areas likely due to
variable degrees of mineral weathering;
•
Groundwater with a marine-type chemical signature; and
•
Groundwater indicating an evolved character typical of longer residence time and flow down-gradient of recharge areas.
Dominant alignments
Formation or interval
Suspected cause or reason
Sodium and Chloride
Majority of intervals
Marine influence
Mixed cation, Bicarbonate, Sulfate
Alluvium
Mineral weathering
Mixed cation, Chloride Gubberamunda, Hutton
Mineral weathering;
ion exchange
Mixed cation, Fluoride, Sulfate
Precipice
Mineral weathering
BMO Group, Springbok,
Walloons, Hutton
Mineral weathering
Clematis
Mineral weathering
Upper Permian
Marine influence
Permian coal
Mineral weathering
Lower Permian
Mineral weathering
pH, Bicarbonate, Fluoride
Mixed cation, Sulfate
Bowen Basin
It has been estimated
that inter-aquifer
leakage constitutes
a significant portion
of the water balance
of individual aquifers
within the Great
Artesian Basin.
Surat Basin
Groundwater hydraulics of aquifers of the Bowen and Surat Basins
Mixed cation, Chloride,
Bicarbonate
pH, Bicarbonate, Fluoride, Sulfate, Chloride
Mixed cation, pH, Carbonate, Sulfate
What does it all mean?
The present study is a first-ever approach at integrating and
assessing physical and chemical groundwater data across all
major hydrogeological formations of the Surat and Bowen
Basins.
Results presented here reflect examples of the positive
findings to date which have built upon the current
groundwater chemistry and dynamics of the Surat and
Bowen basins. While promising results are being identified,
the acquisition of additional data would greatly improve the
ability to understand these processes and assign chemical
fingerprints to specific formations. Such data could be
obtained through the development of a coordinated regional
monitoring program.
Figure 2
The Healthy HeadWaters Coal Seam Gas Water Feasibility Study is analysing the opportunities for, and the risks and practicability of, using coal seam gas water to address water
sustainability and adjustment issues in the Queensland portion of the Murray–Darling Basin. The study is being funded by the Commonwealth Government and managed by the
Queensland Department of Environment and Resource Management (DERM). For further information, visit www.derm.qld.gov.au or email [email protected].
Healthy HeadWaters
Coal Seam Gas Water Feasibility Study