A hydrostratigraphic framework has been developed for southern Ontario consisting of 15 hydrostratigraphic units and 3 regional hydrochemical regimes. Using this framework, the 54 layer 3-D lithostratigraphic model has been converted into a 15 layer 3-D hydrostratigraphic model. Layers are expressed as either aquifer or aquitard based principally on hydrogeologic characteristics, in particular the permeability and the occurrence/absence of groundwater when intersected by a water well or petroleum well. Hydrostratigraphic aquifer units are sub-divided into up to three distinct hydrochemical regimes: brines (deep), brackish-saline sulphur water (intermediate), and fresh (shallow). The hydrostratigraphic unit assignment provides a standard nomenclature and definition for regional flow modelling of potable water and deeper fluids. Included in the model are: 1) 3-D hydrostratigraphic units, 2) 3-D hydrochemical fluid zones within aquifers, 3) 3-D representations of oil and natural gas reservoirs which form an integral part of the intermediate to deep groundwater regimes, 4) 3-D fluid level surfaces for deep Cambrian brines, for brines and fresh to sulphurous groundwater in the Guelph Aquifer, and the fresh to sulphurous groundwater of the Bass Islands Aquifer and Lucas-Dundee Aquifer, 5) inferred shallow karst, 6) base of fresh water, 7) Lockport Group TDS, and 8) the 3-D lithostratigraphy. The 3-D hydrostratigraphic model is derived from the lithostratigraphic layers of the published 3-D geological model. It is constructed using Leapfrog Works at 400 m grid scale and is distributed in a proprietary format with free viewer software as well as industry standard formats.
The underground storage of natural gas and liquified petroleum products in geological formations is a provincially significant industry in Ontario with economic, environmental, and safety benefits for the companies and residents of Ontario. There are 21 active natural gas storage pools in Ontario, with a total working storage capacity of approximately 203 bcf (5.76 billion cubic metres). Most of these pools utilize former natural gas-producing Guelph Formation pinnacle reefs. In addition there are seventy-one solution-mined salt caverns utilized for storage capacity of 24 million barrels (3.9 million cubic metres). These caverns are constructed within salt strata of the Salina A-2 Unit and the B Unit. The steadily increasing demand for natural gas in Ontario creates a continuing need for additional storage capacity. Most of the known gas-producing pinnacle reefs in Ontario have already been converted to storage. The potential value of storage rights is a major incentive for continued exploration for undiscovered reefs in this mature play. There are numerous depleted or nearly depleted natural gas reservoirs of other types with potential for use as storage pools. There is also potential for use of solution-mined caverns for natural gas storage in Ontario.
Abstract Paleokarst and paleokarst aquifers are not as well-documented as shallow karst, and studies of pores and pore networks are rare. Paleokarst lacks the open conduits typical of karst due to compaction, infilling, and diagenetic recrystallization by deep burial, so groundwater movement is through matrix porosity. In this case study, porosity networks in two saline aquifers in carbonate paleokarst of the Silurian Guelph Formation and Salina A-1 Carbonate Unit in southern Ontario (Canada) have been studied in drill cores at microscopic to macroscopic scales, utilizing medical computed tomography (CT) scans, optical petrography, macroscopic core examination, and scanning electron microscopy. The CT scans provided nondestructive three-dimensional visualization and quantification of pore distribution, size and volume, pore connections, and estimates of total porosity, similar to gas porosimetry. In the A-1 Carbonate, 50% of the pore volume comprises layers of macropores associated with algal laminations, with good horizontal connections. In the Guelph Formation, vuggy macropores contribute most of the pore volume. They are connected through subvertical fractures and rubble-filled karst conduits and by abundant small pores and micropores with poor horizontal connections. The Guelph paleokarst represents a longer period of subaerial exposure as evidenced by its greater thickness and geographic extent, former karst conduits, and near-total destruction of primary sedimentary fabrics. The findings provide real-world examples for construction of laboratory scale models of paleokarst aquifers, demonstrate the value of a multi-scale approach to porosity studies, and showcase the value of medical CT scans with its unique ability to visualize pore connections within drill cores.
Abandoned hydrocarbon wells in southwestern Ontario can act as conduits for sulphur water, brines, and hydrocarbons from intermediate to deep Paleozoic bedrock aquifers. Such leakage may pose a threat to shallow groundwater aquifers and the surface environment. Costeffective plugging of these wells requires knowledge of the sources of the leaking fluids. This study characterizes the isotopic compositions (δ 18 OH2O, δ 2 HH2O, δ 34 SSO4, δ 18 OSO4, δ 13 CDIC, 87 Sr/ 86 Sr, δ 37 Cl and δ 81 Br) of groundwater in the region, which show distinctive differences between bedrock formations, allowing determination of unique ‘fingerprints’ of each formation. The geochemical data also improve our understanding of groundwater origin and evolution. A brackish to saline aquifer system containing dissolved H2S is present at intermediate depths of up to 450m, recharged by down-dip infiltration of meteoric water from shallow fresh water aquifers. At greater depths, a series of confined brine aquifers contain residual evaporated Paleozoic seawater, modified by rock-water interaction and other processes. A Bayesian mixing model, SIAR, was applied to these data to develop a tool for identifying the source(s) of leaking fluids. This model determines the possible range of proportions for each source and the probability distribution therein; hypothetical test mixtures and a few real-world examples indicate that it is able to predict proportions with acceptable accuracy.
A regional three-dimensional (3-D) lithostratigraphic model of the Paleozoic bedrock of southern Ontario has been completed. The model encompasses the entire Phanerozoic succession of southern Ontario (110 000 km2), consisting of over 1500 m of sedimentary strata straddling regional arch, or forebulge, zones separating the Appalachian foreland basin from the Michigan structural basin. This initiative provides an unprecedented regional 3-D perspective and digital framework based on an updated regional lithostratigraphic chart. Constructed using Leapfrog Works, an implicit modelling software application, the model format can readily support numeric groundwater-flow modelling. Fifty-four Paleozoic bedrock layers representing 70 formations, as well as the Precambrian basement and overlying unconsolidated sediment, were modelled at a spatial resolution of 400 m. Borehole records in Ontario's public petroleum well database (Ontario Petroleum Data System (OPDS)) were the principal data source, supplemented by Ontario Geological Survey (OGS) deep boreholes, measured sections, control points and Michigan boreholes. A newly revised digital bedrock topography surface combined with revised subcrop geology and digitized 3-D surface polyline and point constraints were used to better align the modelled layers and their extrapolation to the subcrop surface. Model development was an iterative cycle of interim modelling, expert geological appraisal, and quality assurance and control (QA/QC) editing of geological data using geophysical logs, drill cuttings and core, supplemented by manual editing of model layers. The 3-D model provides a robust representation of regional bedrock geology. A properly constructed borehole database and its supporting information is an essential requirement for construction of a 3-D model, but data errors, inconsistencies, data gaps, location errors, etc. can compromise the reliability of the model. From 2015 to 2018, project geologists and geological contract staff of the Oil, Gas and Salt Resources Library completed edits to 30 320 formation tops in a total of 7812 wells, resulting in a revised data set and permanent improvements to the petroleum well database. This report highlights the importance of QA/QC of well data, specifically formation top identification, and summarizes the data improvements made in support of the present 3-D model. No seismic data was available.
