The hydrocarbon reservoirs consist of upper Paleozoic platformal carbonate rocks in northeasterly verging, imbricate, listric thrust-fault wedges, and in related flexural-slip folds. They formed in Maestrichtian to early Paleogene time, after these rocks had been buried under more than 5 km of Late Jurassic to Paleogene molasse. Both the generation and entrapment of the hydrocarbons result from subduction of the floor of the Cordilleran miogeocline.

Palinspastic reconstructions of the foreland thrust and foldbelt, based on balanced structure sections that take into consideration the deep crustal structure as outlined by seismic refraction, magnetic, gravity, and geomagnetic depth sounding data, show that: (1) there has been about 200 km of net horizontal convergence between the Mesozoic magmatic arc of the eastern Cordillera and the autochthonous cover on the North American craton; (2) the convergence is expressed at a shallow level, in the eastern, more external zone, by horizontal compression and vertical thickening within supracrustal rocks that overlie an unbroken basement of cratonic continental crust; but at deeper levels, in the western, more internal zone, it involved the subduction of the former basement of the miogeocli e; (3) the Cordillean miogeocline is a northeasterly tapering wedge of craton-derived sedimentary rocks that accumulated outboard from the edge of the continental craton, on oceanic or tectonically attenuated continental crust; (4) the foreland thrust and foldbelt is a shallow subduction complex that was tectonically prograded northeastward as the miogeoclinal, platformal, and exogeoclinal supra-crustal rocks were scraped off the underriding slab and accreted to the overriding slab; (5) subsidence in the migrating foredeep was due to flexure of the lithosphere under the weight of the encroaching subduction complex, and of the molasse itself.

The first of two main pulses of subduction occurred outboard from the craton, during Late Jurassic and Early Cretaceous time. It involved outward verging thrusting and folding on either side of the uplifted core of the miogeocline, and it produced a thick wedge of molasse that covered the western craton. The hydrocarbon reservoirs formed during the second pulse, in Late Cretaceous and early Paleogene time, as the cratonic cover rocks were deformed and accreted to the growing subduction complex, while the continental craton moved under the detached miogeocline.

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