Since 1972, numerous large and giant fields have been discovered in Chiapas and Tabasco States, southern Mexico, and in the offshore platform west of Campeche. Most of these fields produce from fore-bank talus, now largely dolomitized, of Late Jurassic and Early to middle Cretaceous ages. Drilling depths to the tops of the reservoirs generally are 3,800 to 4,500 m. Offshore discoveries include fields which also are productive in fore-bank talus of Paleocene age. The petroleum source materials for the Jurassic-Cretaceous fields are believed to be mainly Jurassic. Proved reserves in these new fields are 20 billion bbl.

Although some porosity and permeability are primary, most is secondary--the result of solution, dolomitization, and intense microfracturing. The original trap for the Late Jurassic-middle Cretaceous fields was stratigraphic, but the present traps are fractured, faulted, domal salt pillows created during the Laramide orogeny.

The basis for the discovery of the fields was the widespread presence on the Yucatan Peninsula, and in the states of Campeche, Chiapas, and Tabasco, of Cretaceous through Tertiary back-reef or lagoonal facies--carbonates, anhydrites, and some halite. In addition, more than 200 oil seeps were known in a linear zone along the foot of the Sierra Madre, adjacent to the coastal plain. By analogy with the Golden Lane, it was concluded that a great fore-bank talus deposit should lie gulfward from the lagoonal facies. With this geologic concept in mind, seismic work was commenced, and drilling during 1971-72 led to the dual discoveries of the Cactus and Sitio Grande fields in 1972.

The great carbonate bank of Yucatan is believed to continue northwestward into Veracruz State, where several discoveries have been made in carbonate rocks of Early to middle Cretaceous age in thrust sheets buried beneath the coastal plain. We believe that large, subthrust, anticlinal structures underlie the thrust sheets of the Veracruz basin and that, when drilled, these also may be the sites of giant-field discoveries.

Although the potential for this large area is great, it is too early to speculate on the potential reserves of the numerous but still untested structures of the region.

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