Lisa K. Goetz, J. Gary Tyler, Roger L. Macarevich, Dave Brewster, Jagadeesh Sonnad
Conoco Inc.
Oklahoma City
Conoco Inc. is drilling one of the deepest rank wildcats ever in the southern Illinois basin. The Conoco 1 Turner at Carter Coordinates 21-M-29, is being drilled by Helmerich & Payne International Drilling Co. Rig No. 79 near the town of Livermore.
The well will test the updip end of a large tilted basement horst block located in McLean County, Ky. The well is permitted to 14,800 ft and will test the Precambrian(?)-Cambrian carbonates and clastics infilling the Rough Creek graben, the deep precursor of the Paleozoic Illinois basin.
Only seven other tests have been drilled deeply enough to penetrate the top of this sequence in the 7.5 million acre area of the graben.
The Rough Creek graben (RCG) is a part of a major Precambrian cratonic transform fault system that forms the southern edge of the Illinois basin and the northern terminus of the Reelfoot rift (Mississippi embayment). Recurrent motion along faults located in southeastern Missouri, southern Illinois, and northwestern Kentucky has produced a very complicated and deep graben with more than 25,000 ft of intriguing stratigraphy (Fig. 1).
Current production is limited to the late Paleozoic formations within the uppermost few thousand feet of section.
The authors believe that there is very good potential for a major gas play in the poorly tested Precambrian(?)-Cambrian horizons at depths of less than 18,000 ft.
The following geologic summary incorporates the results from analyzing more than 2,500 miles of seismic data, aeromagnetic and gravity data, regional tectonic modeling, source rock geochemistry, and hydrodynamic modeling,
TECTONIC SUMMARY
The RCG is an east-west striking, 150 mile long by 30 mile wide wedge-shaped graben.
The northern bounding faults of the RCG appear to be an en echelon series of reverse, normal, and wrench faults that begin just west of the Cincinnati arch and relay westward through the St. Louis sag into Missouri, where they die out into smaller northwest trending faults and folds. These faults from east to west include the Rough Creek, Shawneetown, Cottage Grove, and Ste. Genevieve.
The southern boundary of the graben complex is defined by the Pennyrile fault system, the northern edge of the Mississippi embayment, the Simms Mountain fault system, and the Ozark dome.
The bounding faults of the RCG are part of a transform fault system that is one of the oldest mappable features cutting the Precambrian craton.
The regional tectonics from late Precambrian to the Jurassic are illustrated in Fig.2.Northwest oriented basement-controlled features line up in a great circle line extending from the northwestern corner of Wyoming to central Missouri, where the trend merges with 38th parallel lineament.'
The 38th parallel lineament continues east-west from the faults north of the Ozark dome to the RCG and then northeast through the Rome trough, where it vanishes beneath the Appalachian thrusts. The Midcontinent rift, Ozark dome, Reelfoot rift (Mississippi embayment), and Cincinnati arch all appear to be offset or terminated by this fault system.
The intersection of the Reelfoot and Rough Creek fault systems resulted when the Southeast Craton block (defined here as the Precambrian craton south and east of these faults) rifted, drifted east-southeastward (in today's coordinates), and subsided when the proto-Atlantic Ocean opened during the late Precambrian.
The linked half grabens of the RCG complex and Rome trough formed by left transtension across the 38th parallel lineament. Based on structural geometries, the probable total horizontal displacement along the Rough Creek fault system was 10 miles or less.
The younger Reelfoot rift may have initially formed as an aulacogen (a large mantle plume-generated rift) which propagated northward to merge with the RCG as the proto-Gulf of Mexico opened. Until divided by Pennsylvanian/Permian uplift of the Pascola arch, the Reelfoot rift and the RCG remained tied to each other in the same manner as the modern Red Sea rift and Dead Sea transform fault zone.
Quiescent tectonics marked the early Paleozoic (Upper Cambrian-Ordovician) history of RCG. Only subtle arching and regional unconformities recorded the inboard response to the accretion of the Appalachian terranes and the resultant northwest shifting of the southeast craton block. The RCG was largely infilled prior to the initial development of the Illinois basin during the Ordovician.
Strong right transpression along the northern RCG culminated when the late Mississippian Appalachian collision reactivated the transform and produced large uplifts with complex flower structures.
These structures were amplified and uplifted by strong north-south compression during the late Pennsylvanian-Permian Ouachita orogeny.
They were lowered but not destroyed by subsequent extension and minor right transtension produced by the Mesozoic opening of the Gulf of Mexico (Fig. 3). Tectonic adjustments continue today as evidenced by the seismic activity on the New Madrid fault zone.
STRATIGRAPHY
Initial graben subsidence
was relatively fast.
Thousands of feet of sands intercalated with basinal shales, and high energy carbonates were deposited along the active fault fronts (Fig. 4).
An estimated 16,000 ft pre-Knox section can be observed on the regional seismic lines. Though most of these Precambrian(?)-Cambrian sequences are unnamed, the authors have identified and regionally correlated three major basin fill packages.
Through time the rate of basin subsidence decreased. As clastic influx into the basin slowed, sedimentation gave way to Bonneterre/Eau Claire biohermal and ramp carbonates and then to Knox shelf carbonates, which were in turn replaced by epeiric sea and continental deposits at the end of the Paleozoic.
Regional beveling and/or nondeposition continued from the mid Permian through the mid-Cretaceous, when the subsidence of the Mississippi embayment caused the renewal of clastic deposition in the reactivated Reelfoot rift area.
TRAPPING AND SOURCING
Throughout the latest Precambrian to Paleozoic depositional history, the northern flank of the RCG acted as a fulcrum to the facies tracts prograding and regressing across it.
This resulted in numerous possible stratigraphic traps forming along the bounding faults.
Large structural traps with basinal shale and/or tight carbonate seals could also have developed (Fig. 5). Timing of the tectonic movements suggests that many of the present structural highs were in place prior to hydrocarbon migration.
Subsequent tectonic adjustments have not breached or substantially changed the overall geometry of the early Paleozoic structures. Several younger structural highs have been formed by the post Permian tectonic movements.
None of the previous wells in the RCG have fully tested the early depositional sequences for indigenous hydrocarbon sourcing (Fig. 6).
The few wells drilled in the RCG tested only the uppermost portion of the sequences, and most were poorly located to look for source rock.
Periodic restrictions of circulation in the RCG probably led to the development of Precambrian and Cambrian shale basins richer in organics than those in the open Reelfoot rift. Though still unproven, paleotectonic reconstructions suggest that the Proterozoic graben fill may be very similar to the prolific late Precambrian source rocks of the eastern Siberia and the southern Oman oil fields.
Additional sourcing may come from the Cambrian formations.
Cambrian carbonates in surrounding areas have been found to contain bitumen and hydrocarbons when analyzed.'
Recent analysis of Knox reservoired oil in the Illinois basin by the Illinois State Geological Survey indicates a possible Ordovician or older source. 3
Hydrocarbon shows have also been reported in the Cambrian clastics of the Reelfoot rift and the Rome trough.
Since the early depositional history of the RCG appears to be very similar to that of the Rome trough, it seems likely that the RCG contains similar sources and reservoirs.
The tectonic history and source rock geochemistry suggest that several pulses of hydrocarbon maturation and possible long distance migration took place, beginning as early as in the late Cambrian. Oil typing work done in the shallower Illinois basin suggests peak oil generation and migration occurred at the end of the Pennsylvanian. 4
Large scale reverse faulting along the Rough Creek fault system during the Pennsylvanian and Permian would have juxtaposed the early Paleozoic reservoirs of the RCG with the well known New Albany and Maquoketa source beds of the Illinois basin. Thermal maturation modeling suggests that most of the RCG is still within the dry gas window with areas in the condensate/wet gas window.
EXPLORATION POTENTIAL
The Rough Creek graben has the appearance of a mature oil province; however, a closer look reveals a basin with good potential still untried in the deeper horizons.
The authors believe that this basin may have several large hydrocarbon traps and that several different prospect types will need to be tested in order to fully evaluate the potential of the Rough Creek graben and the related southern flank of the Illinois basin.
Many of these possible traps cover large areas (tens of square miles), leading to the exciting possibilities of large reserves yet to be discovered in an area with good infrastructure and close to major gas markets.
ACKNOWLEDGMENTS
The authors acknowledge the help and support received from coworkers at Conoco Inc. They also acknowledge the outstanding work done by the U.S. Geological Survey and the Illinois, Indiana, Kentucky, and Missouri state geologic surveys. Their continued cooperation and help has enabled Conoco to bring this exploration concept to fruition as a drilling exploratory well.
BIBLIOGRAPHY
- Heyl, Allen Y., The 38th Parallel Lineament and its Relationship to Ore Deposits, Economic Geology, Vol. 67, 1972, pp. 879-894.
- Marikos, Mark A., Laudon, Robert C., and Leventhal, Joel S., Solid Insoluble Bitumen in the Magmont West Ore Body, Southeast Missouri, Economic Geology, Vol. 81, 1986, pp. 1,983-88.
- Illinois Ordovician-Cambrian Oil Source Found, OGJ, Feb. 17, 1992, p. 83.
- Davis, Harold G., Pre-Mississippian Hydrocarbon Potential of the Illinois Basin: in Interior Cratonic Basins, AAPG Memoir 51, 1990, pp. 473-489.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
