DIVERSITY IN THRUST-RELATED HYDROCARBON TRAPS--NOT BAD, JUST DIFFERENT

Thrust structures form traps for hydrocarbons throughout the world. Many of the characteristics common to thrust belt structures were initially described in the North America. These relationships were then applied elsewhere, because “rules” of structural geometry are powerful interpretation guides where data is poor or sparse.

Geometry of structural traps is largely controlled by the thickness and mechanical properties of the decollement zone, and by the mechanical stratigraphy of the hanging wall rocks. Pre-existing structure can also play a role. These factors are different in thrust belts around the world, so it is not surprising that structural geometries and histories are different than the “rules” developed in North America.

In Papua New Guinea, the Darai Limestone is high in the section, underlain by thick shale units. The result is large, asymmetric detachment folds. In Bangladesh and Azerbaijan, the entire, thick section is mixed clastics and shortening takes place by symmetrical detachment folds. In Colombia, the hanging wall rocks are thick sections of metasediments that are equivalents of thinner, unmetamorphosed rocks in the footwall. These relationships form by thrust inversion of earlier rift basins. In deep water of passive margins, presence of a salt layer or overpressured shale controls geometry of thrust structures. The role of salt units in determining the width and thickness of thrust belts in cordilleran belts is well documented. The blind application of “rules” is not considered prudent. The recognition of the influence of the structural factors is critical to better predictive models of trap conditions and a better assessment of risk.