Silurian Reefs
Location
Plate 1 (green triangles on map below)
References
Bretz 1939, Lowenstam and DuBois 1946, Lowenstam 1948, 1950, 1952, 1957, Smoot 1958, Ingels 1963, Droste and Shaver 1980, 1987, Howard 1963a, b, 1964, Bristol 1974, Willman and Atherton 1975,Shaver et al. 1978, Whitaker 1988
Description
Although they are not of tectonic origin, Silurian reefs are structural features and responsible for doming of overlying strata. A number of named anticlines and domes originally thought to be tectonic structures have been shown to be the result of differential compaction over reefs. For these reasons Treworgy (1981) listed reefs in her structural compendium, and in this document reefs are listed in Table 6 and shown on plate 1.
| Table 6 Silurian reefs in Illinois and cumulative oil production (thousand barrels) | |||
|---|---|---|---|
| Reef | Location | County | Oil production |
| Baldwin | T4S, R6W | Randolph | 16.4 |
| Bartelso | T1N, R3W | Clinton | 4,426 |
| Bartelso East | T1N, R3W | Clinton | 1,026 |
| Boulder | T2N, R2W | Clinton | 8,299 |
| Brubaker | T2N,R3E | Marion | 637 |
| Chicago Heights | T35N, R14E | Cook | 0 |
| Coulterville North | T3S, R5W | Washington | 45.6 |
| Darmstadt | T2S, R6W | St. Clair | 0 |
| Elbridge | T12N, R11W | Edgar | 15,463 |
| Frogtown North | T2N, R3W | Clinton | 2,242 |
| Germantown East | T1N, R4W | Clinton | 2,174 |
| Lillyville North | T9N,R7E | Cumberland | 132 |
| Lively Grove | T3S, R5W | Washington | 0 |
| Marine | T4N, R6W | Madison | 12,637 |
| McKinley | T3S, R4W | Washington | 797 |
| Nashville | T2S, R3W | Washington | 2,315 |
| Nevins | T12N, R11W | Edgar | 0 |
| New Baden East | T1N, R5W | Clinton | 320 |
| New Memphis | T1S, R5W | Clinton | 3,522 |
| New Memphis South | T1S, R5W | Washington | 0.7 |
| Okawville | T1S, R4W | Washington | 64.7 |
| Okawville North | T1S, R4W | Washington | 216 |
| Patoka | T4N,R1E | Marion | 14,777 |
| Patoka East | T4N,R1E | Marion | 6,241 |
| Raccoon Lake | T1N,R1E | Marion | 4,316 |
| St. Libory | T1S, R6W | St. Clair | 0 |
| Sandoval | T2N,R1E | Marion | 6,480 |
| *Springfield East | T15N, R4W | Sangamon | 366 |
| State Line | T12N, R10E | Clark | 0 |
| Stony Island | T37N, R14E | Cook | 0 |
| Thornton | T36N, R14E | Cook | 0 |
| Tilden | T4S, R5W | Randolph | 4,921 |
| Tilden North | T3S, R6W | St. Clair | 1,085 |
| Tonti | T3N,R2E | Marion | 14,125 |
| *Wapella East | T21N,R3E | De Witt | 3,325 |
| Weaver | T11N, R10W | Clark | 2,574 |
| Unnamed | T5N, R11W | Crawford | 0 |
| Unnamed | T17N, R2E | Macon | 0 |
| Unnamed | T19N, R11W | Vermilion | 0 |
| Unnamed | T20N, R11W | Vermilion | 0 |
| Unnamed | T2S,R5E | Wayne | 0 |
| Unnamed | T2S,R6E | Wayne | 0 |
| Information from Treworgy (1981) and *Whitaker (1988). Oil production figures from B.G. Huff, personal communication 1992. Cumulative production is given in thousands of barrels through January 1, 1993. Not all production is necessarily from reef rock or reef-related structure. | |||
Pinnacle reefs of Niagaran (middle Silurian) age are widespread in the northeastern United States and eastern Canada. They form a belt or archipelago around the Michigan Basin and around the north and west margins of the Illinois Basin. In Illinois, reefs crop out near Chicago and in the northwestern part of the state. They also are found in the subsurface along a broad zone trending northeastward from St. Clair to Clark and Edgar Counties. Individual reefs are generally less than 2 miles (3 km) in diameter and as much as 1,000 feet (300 m) thick. In map view they are roughly circular or ovoid. The Marine Reef (pl. 1, H-4), the first and largest reef discovered by drilling, is horseshoe-shaped and has been interpreted to be an atoll (Lowenstam 1957).
Reefs generally contain a core (fig. 67) of massive, often vuggy or fractured limestone or dolomite composed of the skeletons of corals, algae, stromatoporoids, and numerous other types of invertebrates. Surrounding the core and dipping away from it at angles of 20° to 45° are reef flank beds of detritus shed from the core. These interfinger outward with normal interreef fades of fine grained, often highly silty or argillaceous carbonates. Satellite reefs and huge slump blocks of reef core material may be found on the reef flanks (Ingels 1963).
The reef core rises from 20 feet to more than 150 feet (6-45 m) above the top of adjacent Silurian strata. Original relief probably was even greater, but the tops of many reefs were truncated by post-Silurian erosion. Overlying Devonian beds drape over the reef core and conform to its upper surface. Mississippian and Pennsylvanian strata likewise are arched above reefs; the structural relief gradually diminishes upward. For example, the Marine Reef has about 120 feet (36 m) of relief and produces 110 feet (34 m) of closure on the top of the Devonian. Closure is reduced to 70 feet (21 m) on a Mississippian marker 900 to 950 feet (270-290 m) above the reef, and to 60 feet (18 m) on Pennsylvanian markers 400 to 800 feet (120-240 m) higher (Lowenstam 1948). It is even possible that the present ground surface may be slightly domed above some reefs. Geologists have prospected for reefs by searching for radial drainage patterns of small creeks (Whitaker 1988); however, most buried reefs have been discovered by gravity and seismic methods or by contouring of shallow subsurface horizons.
Oil production in Illinois from fields containing Silurian reefs approached 100 million barrels as of 1992. The Marine Field has yielded more than 12 million barrels. Even greater production is reported from the Elbridge and Patoka Fields, but here structural factors not directly related to reefs play a role in trapping hydrocarbons (Smoot 1958). Oil is found in porous reef core and reef flank carbonates, Devonian sandstone and fractured carbonates, and various Mississippian and Pennsylvanian pay zones draped over the reefs.
References
Figure(s)
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