The Ontario Portion of the Michigan Basin:
Exploring for Carbonate Reservoirs

Retread Resources > Publications > Ontario Carbonate Reservoir

by Dennis Nikols, Peter Smit, Georgia L. Hoffman, Harvey R. Spaven, Eric J. Allen
and Vince DiStefano (Retread Resources Ltd) and John Paquette (Paquette Projects Ltd)

This paper is based on a poster of the same title presented at Geoscience Canada 2000 in Calgary, May of 2000.

Abstract

In 1858, North America's first commercial oil production took place in the Ontario portion of the Allegheny Basin at Oil Springs, Ontario. Since that time, exploration and production in Ontario have been concentrated in the Allegheny Basin, but the Ontario portion of the Michigan Basin remains under-explored.

Southern Ontario is situated on the Algonquin Arch, broad high with an NE-SW axis that separates the Michigan and Allegheny Basins. Formations from each basin lap onto the arch from either side.

The major reservoirs in the region include Albion-Scipio, Dover and Arthur. They are hosted in localized, fault-controlled zones of dolomitization in the Paleozoic carbonate formations. In those fields, the controlling faults originate in the basement and were apparently reactivated periodically during early Paleozoic time. Our exploration strategy has therefore focused on locating reactivated basement faults, and identifying areas where they intersect within suitable carbonate formations.

We observed a relationship between the geophysical fabric and the stratigraphic framework within the Dover field. Similar relationships were observed in the study area. A number of exploration targets were identified for further evaluation with high-resolution geophysical surveys.

Introduction

Oil has been continuously produced in southwestern Ontario for over 130 years. Significant production comes from strata of Cambrian to Devonian age, at depths from near surface (60 meters) to depths of 1200 meters or more . The major fields of the region include Dover and Arthur in the Appalachian basin, and Albion-Scipio in the Michigan basin (Figure 1). They are hosted in localized, fault-controlled zones of dolomitization in the Paleozoic carbonate formations. In those fields, the controlling faults originate in the basement and were apparently reactivated periodically during early Paleozoic time (Sanford, 1961).

In this paper, an attempt is made to merge geologic, aeromagnetic, and seismic data from a portion of the Appalachian basin in southwestern Ontario (Study area, Figure 1) to explore for hydrocarbons using nearby fields as analogies. With the integration of these methods, a more comprehensive understanding of the exact structural framework of the Appalachian basin can be identified and further exploration can be carried out.

Methods

A comprehensive database of well data from the study area, including formation tops, was constructed. Geological models were created using standard stratigraphic techniques integrated with interpretation of available aeromagnetic data from government sources. Trend analyses of the Ordovician Trenton and Queenston units were done using the methods of Brigham (1971).

Structural Setting

The discussion below focuses on the trapping mechanisms in Cambrian and Ordovician strata and its application to the study area.

Southwestern Ontario lies on a shelf between the Appalachian Basin to the southeast and the Michigan Basin to the west. Formations from both basins overlap the shelf from both sides. This intervening high is called the Algonquin Arch (sometimes called the arch in this paper); but it seems not to have been a true, active, structural lineament. Rather, it seems to exist simply because the basins subsided on either side of it; and indeed when the basins did not subside significantly, as in the case during the Ordovician, the rocks were deposited across it as if it did not exist. (after Bailey, 1986)

Having a passive role does not preclude deformation. The arch did undergo severe structural deformation and wrenching, which produced a number of major, and many minor stress relief faults. Many of these faults involved the overlying paleozoic veneer, producing or effecting many hydrocarbon traps. It is these traps that we will examine in more detail below.

The Algonquin Arch lines up with another major structure to the southwest, the Findlay Arch. At the approximate intersection of the two arches the rocks are downwarped into a regional syncline, preserving the youngest formation in the province, the upper Devonian Port Lambton shales. This structure is called the Chatham Sag.

Faults with two general orientations, approximately SSE-NNW and WSW-ENE, were identified from stratigraphic and structural data. The SSE-NNW faults persist into the deeper part of the basin, but the WSW-ENE faults seem to be more prevalent along the basin rim (i.e., Huron County, Ontario). South of the Algonquin Arch, in the Allegheny Basin, the SSE-NNW faults are a determining factor in Silurian reservoirs, but north of the arch (in Huron County), the ENE-WSW direction can be dominant.

The Cambrian

It is instructive to briefly review the Cambrian stratigraphic and structural setting. Known Cambrian pools all lie along the northern edge of the Application Basin. For the most part there has been little deep drilling along the Michigan Basin side. The geology indicates that similar rock faces and similar geometry exists on both sides of the arch so potential Cambrian pools should be there. The Cambrian section consists of three basic lithofacies, a basal sandstone called the Potsdam or Mt. Simon, an interbedded dolomite and sandy dolomite unit, called the Theresa or Eau Claire, and an upper clean dolomite, which has been eroded completely from the crest of the arch, called the Trempleau or Little Falls.

The Cambrian section is more or less porous and may be thought of as a single, prospective entity. The types of traps found in the Cambrian are:

fault bounded; a single fault block with up dip and lateral faults as seals. A down dip water leg and top shale commonly complete the trapping situation. This type of trap should show up well on seismic sections.
multiple fault segmented; a series of short displacement faults segments the pool(s) into a number of plates. These plates are in close geographic proximity but have individual pressure systems. This type will be hard to find on seismic due to the lack of fault displacement.
true stratigraphic traps; the pool will be defined by loss of porosity in all directions and covered by a shale seal.
combination traps (structure and stratigraphic); usually the stratigraphic trap is intersected by faults that may or may not be sealed. The porous stratigraphic component will be channel type sands. This will be the most difficult to locates on seismic.

Ordovician

A careful examination of known pools and their locations leads one to several conclusions:

All of the production is directly related to faulting
Porosity development is more or less independent of variation with the host rocks
Traps can occur anywhere in southern Ontario if Ordovician carbonates are present and faulted.

The following descriptions and comments are quoted, essentially unedited, from a presentation by Bruce Bailey (1986):

We will examine only three fields: Arthur, Dover and Albion-Scipio, because they clearly illustrate the Ordovician trapping mechanism.

Arthur: The Arthur Pool was discovered in 1968, and is located 8 kilometers northeast of the town of Arthur, Ontario. The trapping mechanism is both structural and stratigraphic, and the gas reserves which amount to 1.7 Bcf (OGIP) are again found in dolomitized fracture zones associated with faulting in the Ordovician Black River, and also in the sandstones of the Precambrian and Shadow Lake.

The Arthur Pool is significant due to its position east of the study area, as well as its' location far to the northeast of significant production in the Ordovician.

Dover: Discovered in 1917 and located in Kent County, Ontario, it is the most highly publicized of any of the Ordovician pools because itsÕ trapping mechanism is somewhat unusual, but the principles of its accumulation are similar to the other Ordovician pools.

Once again, a fault is present, but in this instance the downthrown block has buckled into a syncline. The rocks were fractured during this process and subsequent dolomitization has resulted in at least 13 small isolated gas accumulations along the axis of the structure.

Dover initially contained over 13 Bcf of reserves but nowadays it is primarily a gas field.

Albion-Scipio: The Albion-Scipio (A-S) trend is a narrow, divergent wrench system located on the south-central flank of the Michigan basin. The trend consists of several linear oil and gas fields that produce from fractured, dolomitized Ordovician Trenton-Black River limestones. The producing area is at no point more than one mile in width, trends northwest-southeast, and is about 35 miles in length. Recoverable reserves are estimated at 150 mmbls of oil and 225 bcf of gas (Clark, 1988).

The A-S field has been described as a stratigraphic trap that is mildly synclinal in an area of uniform regional north dip of 33 feet per mile. Ells noted, after detailed regional mapping that isopach thickening of the middle Devonian over the central syncline, which defined the field, was related to lateral motion of pre-existing basement faults.

Ells suggested that the field resulted from the intermittent movement of these pre-existing basement fault systems throughout the Paleozoic. These reactivated Precambrian zones of weakness created migration pathways in the Cambrian and Ordovician strata. Low temperature hydrothermal fluids brines dolomitized, enhanced porosity, caused structural collapse and deposited base metals. At the same time or after dolomitization, petroleum migrated into many of these corridors. Major fault systems, whether normal or wrench, that were reactivated post-Ordovician, to date, have shown no indications of dolomitization, petroleum accumulation and mineralization. Beuhner and Davis, also Harding, suggested that the mechanism for structural development of the field was a wrench fault system.

The intermittent reactivation of these basement fault blocks specifically during Siluro-Devonian time provides the mechanism for the development of this divergent wrench system. This wrench system appears to be the mechanism for the dolomitization of the Trenton-Black River reservoirs. The vertical seal for the reservoir is the Utica shale, while tight unfractured Trenton limestone acts as the lateral seal. The A-S field of Michigan is analogous to the situation expected in the study area.

First-order trend surfaces of selected horizons in the study area suggest that a distinct clockwise rotation occurred between Middle and Upper Ordovician (Trenton and Queenston) times (Figure 2), which we interpreted to indicate strike-slip movement on the eastern basin margin. Also evident during this time interval was a change in depositional style, with an increase in thickness from north to south. No relative changes in trend surface orientation or depositional style were observed in older or younger units, so we confined our focus to the Trenton which could represent the changing geologic setting at that time.

Second order residual mapping of the Trenton and horizons above and below (Precambrian and Silurian) show areas of possible basement faulting expressed as magnetic lows when geophysical trends are overlain (Figure 2, 3, 4). The selection of these units shows clearly the confinement of faulting to Precambrian and Ordovician times while Silurian units show little correlation with the magnetic interpretation.

Points to Consider

Analogous characteristics to AS and those in Ontario are found in the Michigan and Appalachian basins. Several points should be considered (pers. comm., Inden 1996):

the reservoirs are almost always associated with dolomitization;
the reservoirs function as fracture porosity systems;
the dolomitization is related to basement faulting that has been reactivated through time;
the wrench systems are often cut by cross faults that act to compartmentalize the reservoirs;
the wrench systems can be very long, 25 to 50 miles or more;
lateral seals are a function of host (e.g. Trenton ) porosity (or the lack there of);
shales act as top seals;
water legs are rare or small;
Dolomitization can invade the fractured limestone near the fault system. This porosity enhancement process can and does cause vertical stratigraphic shrinkage or the appearance of a syncline like structure;
anomalies are often recognizable on seismic surveys;
In many instances (in Ohio for example) the underlying basement faults can be located with geophysical techniques that help to target the seismic surveys (Inden, 1996)

In summary then, the Ordovician pools are all contained with open fractures associated with nearby faults (as in the case of many of the smaller fields) or in sporadic, discontinuous lenses of secondary porous dolomite with formed around these fractures. These porous lenses can occur on both the upthrown and the downthrown blocks and cut across formation boundaries.

Water legs are uncommon, and the decline curves exhibit all the characteristics of a fractured reservoir. There is usually a very large initial flow and rapid pressure decline in the initial production period. However, when the flows do stabilize they tend to produce for an unbelievably long period of time. Many of them have been on production for close to 20 years and Dover is still producing small amounts of oil and gas after 64 years!

Like some of the Cambrian traps, these accumulations are best found using seismic, and since the faults that form them cut the underlying Cambrian as well, it is almost to be expected that traps will be found in the future that produce from both the Cambrian and the Ordovician.

Geophysical Methods

The use of existing models, magnetics, and seismic in an integrated approach can be used in an attempt to identify the wrench corridors, junctions of secondary shears and subsequently the location of future petroleum reservoirs (Tedesco). The use of seismic in south-central Michigan has proven that these accumulations of hydrocarbons do in fact occur in accordance with known models. Interpolative modelling and field-testing done in 1982 by Ladd Petroleum, indicated that several seismic anomalies are associated with the dolomitized, fractured portions of the Trenton reservoirs. These include:

resolution of the primary fault which cuts the Trenton limestone reflector,
"sagging" of the overlying stratigraphy,
disruption of the Black River reflector seen as increases in amplitude and decreases in frequency. This was attributed to constructive/destructive interference patterns set up in the fractured, low-velocity, dolomitized portion of the Trenton (Clark et al). All of these characteristics are seen in our seismic line (Figure 7).

The use of magnetics can be used to map the covered basement fault block pattern. It can do this because of the rock type changes (resulting in magnetic susceptibility changes) that occur across the basement faults (Gay, 1999). In addition, base metals transported by brines along fault planes will likely be deposited in areas of dolomitization and therefore will show some susceptibility contrast. Figure 8 shows levelled aeromagnetic data from a portion of the study area, as a black square. This "saddle" shaped zone of intersecting steep gradients could be interpreted as block faulting or structural collapse, and therefore a possible zone of increased porosity. When combined with second order residual maps of the Top of Precambrian, Trenton and Silurian, some similarities are apparent (Figure 2, 3, 4). For the most part topographic lows and highs match magnetic lows and highs respectively.

In the case of the study area, we have identified what we believe to be a basement fault trend that extends for 80 km. We believe that this trend could hold as many as 15 potential pools of which Badger is one.

Bibliography

Bailey, B., The Hydrocarbon Potential of the Cambrian, Ordovician and Devonian of Ontario,Ó Ont. Dept. of Mines, 1986.
Buehner, J. H. and S. H. Davis Jr.,Albion-Pulaski-Scipio Trend Field, Michigan Basin Geol. Soc. Symposium, pp. 37-48, 1968.
Clark, Stacy L. and White, Ron; Seismic anomalies help to locate fractured production, The World Oil, pp 63-68, Dec. 1988.
Ells, G. D., " Structures Associated with the Albion-Scipio Oil Field Trend," Michigan Geol. Surv., p 86, 1962.
Gay, P., "Basement mapping highly crucial", A.A.P.G. Explorer Nov. 1999 vol. 20, no. 11, p.32-33.
Harding, T. P., Petroleum Traps Associated with the Wrench Faults, A.A.P.G. Bulletin, v 58, pp 1290-1304, 1974
Inden, R., Personal communication 1996.
Sanford, B.V., Subsurface stratigraphy of Ordovician rocks in Southwestern Ontario. Geological Survey of Canada, Paper 60-26.
Tedesco, S. (1997): Basement control of reservoir development of middle Lower Ordovician strata, eastern North America. American Association of Petroleum Geologists 1997 annual convention. Annual Meeting Abstracts 6; Pages 115. 1997

The Ontario Portion of the Michigan Basin:Exploring for Carbonate Reservoirs