ABSTRACT: Terrane Analysis of the Circum-Pacific and Its Implications for Petroleum Exploration

David G. Howell

The Circum-Pacific region, the graveyard of Panthalassa, encompasses nearly half the surface of the earth, yet housed in this vast region is less than 3% of the world's resources of petroleum. The low quantity of petroleum resources stems from the disruptive effects of crustal accretion and dispersion, consequences of long lasting convergent and transcurrent plate motions along the entire region of the Circum-Pacific.

Nowhere in the Pacific is a major petroleum accumulation associated with collisional tectonics. Oil seeps are known from various accretionary prisms and belts of amalgamated terranes, but the likely scenario in every case is one where oil has passed through ephemeral traps. The traps have been breached, owing to a continuation of the tectonics that initially formed them. In contrast, lying continentward of much of the Pacific rim are large fold and thrust belts, with their accompanied foreland basins (North Slope, Alaska; Alberta basin, Canada; Campeche, Mexico; Oriente and Putumayo basins, South America; Suchuan, People's Republic of China), where vergences in these thrust belts are, for the most part, directed away from the Pacific realm toward the Atlantic; therefore, the architect res of these giant petroleum basins are related to ridge-sliding forces eminating from the Atlantic rather than slab-pull forces associated with Circum-Pacific subduction zones. These forms of A-type subduction, in intraplate settings, are more orderly and tectonically less disruptive than the B-type subduction of plate-margin settings that characterize the Circum-Pacific region. Thus, convergent tectonics in intraplate settings provide enough stability for the preservation of oil while convergent tectonics of plate-margin settings are generally too disruptive to allow for long-term preservation of oil.

The one bright spot for oil exploration associated with Circum-Pacific tectonism is along transform fault margins. For example, in China, the Bohai Gulf and Songliao basins lie within the Tan Lu fault system, and in California, numerous petroleum-rich basins lie within the San Andreas fault system. The critical element in these transcurrent fault systems is the coincidence of a complete oil cycle within one tectonic cycle, i.e., transtension leads to basin formation, organic accumulation, and maturation; and transpression leads to deformation, migration, and entrapment.

Undoubtedly, small, subtle, and maybe even unique traps lie undiscovered within the Circum-Pacific. For example, in east-central Alaska, in an area known as the Yukon Flats and Kandik basin, there is little that is alluring for a petroleum geologist. The crystalline basement is commonly ophiolitic, the Tertiary fill may be up to 5 km thick, but is nonmarine and probably gas prone. The Mesozoic strata are penetratively deformed and thermally overmature for oil. However, structurally involved in these unappealing strata are Paleozoic rocks that are both organically rich and thermally mature for oil. Until we understand this area and others like it, we cannot abandon all hope of ever finding more oil in the Pacific's confounding rim involving terrane accretion and tectonic dispersion.