INTEGRATED EXPLORATION LOCATES CINCINNATI ARCH DOLOMITE BRECCIAS

Dolomite breccias or chimneys are prolific reservoirs found along the Cincinnati arch and adjacent basins from Tennessee to Ontario (Fig. 1).

These reservoirs are characterized by:

a thick reservoir (upwards of 800 ft) of secondary porous dolomite encased in dense carbonates,
a subtle structural depression above,
a general thickening of overlying strata,
association with synclines adjacent to or lying on top of a Precambrian rift system,
Upper Cambrian and Middle and Lower Ordovician rocks, and
basement faulting.

An integrated approach using seismic and surface geochemistry, augmented by subsurface geology and magnetics, has led to a dramatic increase in the number of these fields being discovered in the past 10 years.

Historically the reservoirs have been found by random drilling. Well known examples of fields producing from breccias are the Wood-Hancock complex in Ohio (155210 million bbl of oil equivalent), Albion-Scipio in Michigan (155 MMBOE), and Dover (l MMBOE) in Ontario (Fig. 1).

The nature of the reservoir prevents the use of subsurface mapping and regional seismic to effectively identify the location of these reservoirs. In the early 1980s the use of surface geochemistry, which was eventually integrated with seismic, led to the discovery of Stoney Point field (17 million bbl expected recovery) in Michigan.

During the later half of the 1980s a variety of techniques including seismic led to the discovery of several fields in southwestern Ontario. In the early 1990s numerous new discoveries were made along the southern portion of the Cincinnati arch in Kentucky and Tennessee with varying flow rates between 100 b/d to 6,000 b/d of oil from less than 1,500 ft.'

The overall increased success was due to the use of seismic and surface geochemistry in an integrated approach. By utilizing a variety of surface geochemical methods, high fold seismic data and magnetics, subsurface geology, and geomorphics, the result has been an increase in success rates that were at one time less than one in 50 to one in three.

GEOLOGY

The Middle and Lower Ordovician and Upper Cambrian rocks found along the Cincinnati arch and adjacent basins are predominantly carbonates (Fig. 2).

The majority of these carbonates are lithographic limestones and dense dolomites overlain in the northern area by the Maquoketa shale (Middle Ordovician) and the Chattanooga shale (Devonian) in the southern area.

The section is typically underlain by Cambrian sands.

BRECCIA CHARACTERISTICS

Formation of the breccias or chimneys is still a matter of conjecture, but two mechanisms have been proposed.

The first mode of formation is by karsting as proposed by Sangster.2 The evidence for this is that the breccias found in the southern portion of the Cincinnati arch area lie along and terminate at the base of numerous unconformable surfaces. This concept is further supported by the presence of blocks of younger strata located well into the breccia and the similarity of the geometry of the chimneys along the Cincinnati arch to modern cave systems.

Dolomitization and silicification of the reservoir, which occurs at a temperature range of 85-140 C., is assumed to occur as a later event resulting from the expulsion of hot brines from the Appalachian and Illinois basins. This event was followed by petroleum accumulation and sulfide mineralization.

Sangster 2 admitted that evidence is generally circumstantial and conceded that in none of the breccias have paleosoils been found. There is no indication that Sangster had the benefit of seismic data to evaluate any of the breccias. In addition, the breccias found in Michigan, Ohio, and Ontario clearly have no evidence of karsting as the cause of formation.

A second mechanism proposed by Reimer and Tear 3 utilizes the low temperature hydrothermal nature of the brines as a mechanism in the formation of the breccia itself and is called TSR-HTD. The formation of the breccias is caused by a reaction between the carbonate, petroleum, and CO2 gas present and creates the breccia by the reaction:

[SEE FORMULA]

This mechanism is a hydrocarbon fueled mechanism involving thermal-organic sulfate reaction (TSR) and concurrent dolomitization in a hot aqueous hydrothermal system (HTD) causing the conversion of dense carbonates to porous dolomite. In addition, the reaction also causes:

The formation of significant amounts of sulfide minerals,
An overpressured petroleum phase,
a fracturing of local country rock caused by these increased pressures, and
H2S.

It also explains the high temperature of the formation of the dolomite. The presence of blocks of younger strata found in lower portions can be easily caused by a collapse of the host rock above and adjacent to the breccias during the reaction. The reaction once begun is exothermic, highly acidic, and causes aggressive digestion of the carbonate host rock. This explanation is a consistent and simple mechanism that is applicable to the breccias found from Tennessee to Ontario.

Characteristics of the breccias are secondary dolomite bodies with great vertical height encased in dense carbonate (Fig. 3). The formation of the dolomite causes reduction in rock volume and an increase in porosity. Sulfide mineralization is typically found in significant and occasional minable grades (Tennessee) in the breccias. The collapse in the breccia causes thickening of the overlying strata. The majority of the breccias have tremendous depth, a narrow width, and extensive length. Breccias are generally perpendicular to basin edges. In addition, basement normal faulting on seismic and magnetic anomalies are associated with the chimney. This faulting does not usually breach the overlying shale. Many of the breccias in the Tennessee-Kentucky area seem to be associated with major unconformities or faulting found in the underlying Precambrian rifts. Karsting does not appear to be possible in the formation of the breccias in the northern portion of the Cincinnati arch area, but possibly may have occurred at some time during their formation in the southern portion.

CASE HISTORIES

The use of an integrated approach to finding these breccias has been very successful. The unique nature of the breccia is such that drilling can easily miss the chimney even with a seismicly defined location (Fig. 4). Consequently one test may not condemn the proposed location of the breccia.

In 1992 a field extension to Seventy-Six field (Fig. 1) was discovered by the use of surface geochemistry and aeromagnetics 4 that flowed more than 1,000 b/d of oil from the Stones River formation at 1,006 ft (Fig. 5). Subsequently two more wells were drilled with high flow rates.

The term Seventy-Six field is a misnomer as it represents a group of separate breccias and shallow producers grouped into one field. The iodine surface geochemistry surveys clearly indicate an east-west trending anomaly that outlines the productive breccia.

A seismic line shot in spring 1994 transacts the eastern side of the chimney (Fig. 6). Along the seismic profile are the iodine, propane, and ground magnetic data. The synclinal nature and change is characteristics of the strata can be seen in the area of the breccias. In addition, basement faulting can be seen extending into the terminating in the reservoir.

Petroleum production is usually found in the upper part of the breccia where the adjacent country rock is extensively shattered but not turned to rubble causing the impression of a fracture play. Production extends downward into the interior of the breccia but becomes dominated by significant and occasional minable sphalerite mineralization. In the mining industry the two parts of the breccia are termed the crackle breccia (upper and edges) and the body termed the rubble (central part) breccia.

Another collapsed breccia is indicated in Fig. 7 with the associated basement faulting, a change in seismic character of the Knox, a Precambrian unconformity, and a syncline on top. Iodine, magnetics, and soil gas anomalies are associated with the collapse. The difficulty in mapping the breccia is outlined by the velocity contrast between the Pencil and Mud Cave bentonites and the Stones River formation. A fault can be seen extending from the basement into the breccia.

A second example is near the southern end of the Wood-Hancock complex in Wyandotte County, Ohio (Fig. 8). An iodine surface geochemical survey was done prior to the drilling of the labeled wells. The anomaly clearly outlines the existing production and indicates that it is located in a syncline portion based on the well data that are available. The wells are reported to be pumping 200 b/d from the Knox-Trempealeau formations. Potential drilling based on the iodine anomaly is indicated.

The most notable example of a dolomite breccia or chimney discovered by surface geochemistry and later developed by seismic is Stoney Point field located in southern Michigan (Fig. 1). It is interesting to note that this discovery was the 26th prospect utilizing surface geochemistry and seismic in the drilling program.5 Because, as indicated in Fig. 4, there is the real possibility of missing the breccia, many of these previously drilled anomalies may require reevaluation and additional drilling.

The use of surface geochemistry, seismic, and magnetics has shown an ability to successfully explore and find breccia reservoirs. The play area is extensive, lying along not only the Cincinnati arch and adjacent basins but in areas of rifting in the Midcontinent U.S. where Upper Cambrian to Middle Ordovician carbonates have been preserved.

The size of these reservoirs is significant and represents a potential world class play. The use of regional surface geochemical and magnetic surveys has helped focus seismic surveys quickly, allowing drilling of potential breccias and eliminating numerous areas that have no potential. The use of seismic and magnetics also has allowed a greater understanding of the complexities, geometry, and mechanisms of the formation of breccias.

REFERENCES

Hamilton-Smith, T., et al., High-volume oil discovery in Clinton County, Ky., Kentucky Geological Survey, Information Circular 33, Series XI, 1990.
Sangster, D.F., Breccia-hosted lead-zinc deposits in carbonate rocks, in Paleokarst, James, N.P., and Choquetle, P.W., eds., Springer-Verlag, New York, 1988, pp. 102116.
REIMER, J.D., AND TEARE, M.R., DEEP BURIAL DIAGENESIS AND porosity modification in carbonate rocks by thermal organic sulphate reduction and hydrothermal dolomitization: TSR-HTD, in Subsurface Dissolution Porosity in Carbonates - Recognition, Causes, and Implications, CSPG short course No. 1, 1992 joint CSPG-AAPG convention, Calgary, Alta., p. 41.
Burn, T., Mining boom rages in Clinton county as deeper formations keep paying, Northeast Oil World, 1993, pp. 23-25.
Kiesau, Douglass, 1994, personal communication.

BIBLIOGRAPHY

Anderson, Warren H., Mineralization and hydrocarbon emplacement in the Cambrian-Ordovician Mascot dolomite of the Knox group in south-central Kentucky, Report of Investigations 4, Series XI, Kentucky Geological Survey, 1991, p. 31.