Tectonic Guidelines to Oil and Gas Prospectivity of Continental Margins
Authors: Thompson Thomas L. 
Published: October 2008

This review of concepts regarding the structural, stratigraphic, and temperature history of continental margins suggests that the prospective offshore area of the world, extending to water depths of 8,000 feet or 2,500 meters reasonably could exceed the prospective onshore area including regions already explored and producing. The formation of continental rifts and divergent (Atlantic type) margins by a real-extension tectonics (including normal and oblique-slip faulting, and divergent triple junctions) encourages exploration in Mesozoic and Cenozoic sediments because high earth temperatures (favorable for generation and explosion of oil and gas) reasonably occur during or soon after shallow-water sedimentation that may form sources, reservoirs and impermeable traps prior to subsidence and burial under younger sediment and deep water.

Convergent (Pacific type) margins reasonably include areas of high potential where Cenozoic sedimentation favors the formation of sources, reservoirs, and traps in regions of abnormally high earth temperatures in back-arc basins of the Western Pacific and southeast Asia; and in areas where seafloor spreading axes have encountered Andean subduction trends as in southern California and coastal Peru. The prospectively of subduction thrust-fault trends depends on special circumstances (such as offshore east of Trinidad) where the influx of sediment from the continent provides reservoirs and unruptured traps for oil and gas above distorted older sediments that generally favor oil and gas generation and expulsion.

The overwhelming oil and gas reserves of the Middle East and the Soviet Urals suggest that collision margins offer the greatest potential for giant field reserves (greater than 100,000,000 barrels).

INTRODUCTION

Plate tectonics, the hypothesis of multiple crustal plates floating on viscous material called asthenosphere provides reason for viewing the earth's outer shell or lithosphere as a dynamic system of shifting continents and changing ocean basins that has shaped world geography during the past 200 million years concurrent with the accumulation of roughly 90 percent of known reserves of oil and gas. Although most known reserves of oil and gas occur onshore (under the present land) nearly all the reservoirs have an offshore (marine) origin, implying that the present offshore areas are prospective also.

The rudiments of oil and gas accumulation include:a source of unoxidized organic material disseminated in the sedimentary rock sequence;time and heat to change this organic source material to oil and gas;porous and permeable rocks for migration paths and reservoir storage;impermeable (sealed) traps to localize and prevent the escape of oil and gas to the earth's surface, and;the right time and place relationships of items 1 through 4 above.

Thickness estimates of unmetamorphosed sediment of the world suggest that offshore oil and gas resources could exceed the onshore resources, including regions already explored and producing. The importance of biotic proliferation and input of land-derived sediment in the formation of reservoirs suggests that nearly all of the offshore resources will be discovered in water depths of less than 8,000 feet, about 2,500 meters.