ABSTRACT: Tulip Structures in Michigan? A New Look at the Origin and Structural Styles of Some Michigan Basin Oil and Gas Fields
Robert T. Versical
Quantification of the direction and magnitude of shortening strains in Paleozoic limestone units of the Michigan basin by applying a calcite twin-strain analysis constrains the mechanisms by which structures in the Paleozoic section may have developed. The orientation of vertical joint sets and strains calculated from mechanically twinned calcite grains of limestone units are indicative of an overall ESE-WNW- to southeast-northwest-directed compressive stress associated with Appalachian orogenesis. This inferred direction of compression nearly parallels the majority of oil- and gas-producing structures in the basin, providing insight into their origin.
Calculated values for homogeneous shortening in the Paleozoic section should approximate value for shortening in basement unless there is a regional detachment at the basement/cover interface. Since no such detachment surface has been documented, an alternative mechanism for accommodating basement shortening is required. With the relatively low values for shortening strain determined for this study (approximately 1.0%), reactivation of preexisting lines of basement weakness may be a good way to achieve the required shortening. Assuming the trends of these basement faults are essentially parallel to overlying fields, then movement on the basement faults must have a strong component of left-lateral strike-slip to be compatible with the compression direction inferred from the calcite str in analysis.
Models that involve deformation above strike-slip faulting in basement with varying degrees of dip-slip may explain the observed structural geometries of those fields considered in this study, and different structural styles are associated with different trends in basement. "Tulip structures," two concave-upward faults that flank an anticline and merge at depth with the basement fault, result when the direction of compression lies at angles of 20° to 30° from the basement fault.
AAPG Search and Discovery Article #90998 AAPG Eastern Section Meeting, London, Ontario, Canada, September 10-12, 1990