THRUST STRUCTURE IN MICHIGAN COULD POINT TOWARD MORE TARGETS
David DesAutels
Qatar General Petroleum Corp.
Doha
During the drilling of an Ordovician St. Peter development well in AuGres field of Arenac County, Mich., an unexpected, duplicate section of Silurian A-1 carbonate was encountered.
This duplicate section has been interpreted to have been formed by thrusting that was caused by a nonlinear, right-lateral wrench fault. Wrench faulting in Michigan is well documented, but transpression - resulting in thrusted structures - is rare.
Shear-zone fracturing along this wrench fault has enhanced hydrocarbon accumulation within the A-1 carbonate as well as the A-2 evaporate.
Reservoirs associated with wrenching are not new to Michigan. The state's largest oil field, Albion-Scipio, is producing from fractures and porosity enhancement resulting from wrench movement. Exploring for such features, however, could be shifted to other parts of the Michigan basin.
INTRODUCTION
In December 1989, Petro-Star Energy spudded two wells in 19n-7e of Arenac County, which. (Fig. 1). One was a shallow pool test (SPT), and the other was a deep pool test (DPT) of AuGres field, which produces gas and condensate from the Ordovician St. Peter sandstone.
The SPT, the 1-7 Sims, was drilled to evaluate the Silurian A-1 carbonate gas potential that had been recognized when the 2-12 AuGres development well of the St. Peter pool blew out while drilling above the A-1 carbonate.
The DPT, the 2-7 Sims, was drilled below the St. Peter pool in an attempt to reach Precambrian basement. While drilling through the Silurian strata, it encountered a duplicate section of A-1 carbonate.
The nature and relationship of this duplicate section is now recognized to have implications regarding trap formation and hydrocarbon accumulation.
EVALUATION
The A-1 carbonate potential at AuGres St. Peter field was first realized when the 2-12 AuGres development well blew out while drilling in the A-2 evaporate above the A-1 carbonate.
The 2-12 AuGres was not able to be drilled further due to pressure and lost circulation problems. A completion attempt was made in the salt of the A-2 evaporate that resulted in a 30 day cumulative production of 76.1 MMcf of gas, 2,858 bbl of condensate, and 233 bbl of water before the pressure dropped significantly.
The pressure drop may have been associated with the closing of fractures within the salt interval, thereby reducing flow. The gas within these fractures was probably rising from the A-1 carbonate.
The first three wells in the field, the 1-11, 1-11A, and 1112 AuGres, had not encountered high pressure gas within the Silurian section. Subsequent to the 2-12 AuGres, seismic mapping at the A-1 carbonate level identified anomalous areas and one area in particular that was structurally high (Fig. 2).
The 2-12 AuGres was identified as being on the fringe of the feature. With hopes of gaining structural elevation to the 2-12 AuGres, the 1-7 Sims was located at this mapped high to test the A-1 carbonate.
Both the 1-7 Sims and the 2-7 State Sims had unanticipated results. The 1-7 Sims encountered the A-1 carbonate much lower than expected. The 2-7 State Sims cut a duplicate section of A-1 carbonate (Fig. 3).
The A-1 carbonate sections in these two wells have identical log signatures and are normal to regional thickness and character. Also, the 8 m (27 ft) of salt between the two carbonate beds in the 2-7 State Sims appears to be part of the A-1 evaporate. This is reasoned due to the presence of a thin, radioactive, potassium salt that typically occurs at the top of the A-f evaporate.
The 2-7 State Sims is the second well in the area to have encountered a duplicate A-1 carbonate. The double carbonate was also seen in the Union 1-22 State-Turner and Hugo, which was drilled 8 km (5 miles) north of AuGres. These duplications appear to be thrusted tongues caused by a right-lateral transpressive faulting where the fault shifts towards the east (Figs. 4, 5).
The 1-7 Sims is believed to have been drilled on the footwall of the leading edge of the thrust (Fig. 6). The thrust culminates in a reverse fault south of the 1-7 Sims location.
Shear fractures that would be associated with the wrench zone should be present from .4 km (.25 miles) to 1.6 km (1 mile) on each side of the fault zone, such as is evidenced at the Albion-Scipio trend in southern Michigan (Fig. 1).
The fracture zone at AuGres, which is limited by the nonproductive A-1 carbonate 1-11 AuGres and 1-12 AuGres wells to the west, would delineate potentially productive acreage. The 1-7 Sims was flow tested for 30 days with a cumulative production of 82 MMcf of gas, 3,512 bbl of condensate, and 4,332 bbl of water.
SUMMARY
High-pressure gas in the Silurian A-1 carbonate at AuGres field seems to be associated with shear fracturing due to transpressional faulting.
A better understanding of the nature and extent of this fault system may result in more exploration targets and the delineation of other fault trends in this part of the Michigan basin.
ACKNOWLEDGMENTS
The author thanks Elf Exploration Inc. and PetroStar Energy for permission to publish.
BIBLIOGRAPHY
DeHaas, R., and Denison, D., Geology, and geochemistry, of the Trenton-Black-River formations of central southern Michigan, 23rd annual Ontario Petroleum Institute conference proceedings, 1984, Paper No. 13, 26 pp.
Keith, B.D., Reservoirs resulting from facies-independent dolomitization: Case histories from the Trenton and Black- River carbonate rocks of the Great Lakes area, in G.M. Friedman, ed., Carbonates and Evaporites, Northeast Science Foundation Inc., Vol. 1, No. 1, 1986, pp. 74-82.
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