Petroleum Exploration Aspects of Wrench-Fault Tectonics

Abstract

Many, if not most, of the faults, fractures, and lineaments in the worldwide regmatic shear pattern appear to be wrench faults, along which the dominant motion is horizontal and the fault planes are essentially vertical. The theory of wrench-fault tectonics postulates that lateral compression formed wrench faults in the earth's crust early in geologic history. The shear pattern thus formed has controlled subsequent deformation in a continuing lateral-compression stress field, which has produced second-, third-, and higher order shears. In the third order, however, shear directions become repetitive, hence the ultimate shear pattern in any region contains only eight preferred directions of wrench faulting and four preferred directions of drag folding. A meridional shear direction consisting of two primary systems has been superimposed on a somewhat older equatorial shear system, which is similarly structured. With four primary shear directions, there are six types of shear intersections: intersections of complementary shear sets form angles that approximate 60°; shears of the same sense intersect at 90°; meridional and equatorial shears of opposing sense intersect approximately at 30°. The 30° and 60° intersections have lateral compressive components that favor vertical movements, whereas the 90° intersections show a tendency for block rotation. Wrench-fault tectonics have direct application to petroleum exploration, particularly in delineating sedimentary basins whose history of deformation and sedimentation is critical in the accumulation and preservation of hydrocarbons. Wrench faulting according to the postulated pattern also forms traps of four or more types. Brecciation associated with wrench faulting also may develop fracture-type porosity such as is found in the Lima-Indiana, Scipio-Albion, Panuco (Mexico), and some Ellenburger (Texas) fields. The scale on which wrench faulting operates ranges from such single oil-producing trends as the drag folds of Elk Hills and Kettleman Hills in California, to such regional oil provinces as Venezuela-Colombia, Alaska, and Gulf of Guinea. Wrench-fault tectonics appear to be a prim factor in respect to petroleum accumulation.