f DEFORMATION HISTORY OF THE SIERRA MADRE ORIENTAL, MEXICO, AND ASSOCIATED HYDROCARBON System

An examination of the tectonic development and deformation style of the Sierra Madre Oriental (SMO) was undertaken as the first part of a joint project involving Pemex, the Instituto Mexicano del Petr61eo, and Amoco. This served as a starting point for an evaluation of the petroleum potential of this folded thrust belt. Structural style and amount of shortening along this Laramide thrust belt was found to be a function of the type of decollement: detachment in a weak (evaporite) layer tends to generate box or "fan" folds and few emergent thrusts, whereas detachment in . stronger units (shales, limestones) is associated with fault-bend and fault-propagation folds and emergent thrusts. All of these folds have been, for the most part, deeply eroded and reservoir units tend to be breached. Thermal maturity is also high (dry gas) over most of the SMO, and modeling suggests that in much of the area hydrocarbons were generated and migrated before Laramide deformation.