ABSTRACT: Regional Factors Affecting Petroleum Potential of the Aleutian Accretionary Prism

David W. Scholl, A. J. Stevenson

Two first-order factors influencing the petroleum potential of large accretionary prisms are (1) their structural contact (backstop) with the margin's bedrock framework, and (2) that beneath a master decollement, the volume of the sediment that has underthrust the prism and passed through the petroleum-generation window (PGW).

In the Aleutian prism, the backstop contact slopes landward because the accretionary body has underrun and elevated the seaward edge of the ridge's arc massif. Within the thickened landward part of the prism, the overlying lid of the underrun bedrock slab and its mantile of thick (2-4 km) forearc deposits has served to constrain vertical fluid movements and enhance the thermal environment and circumstances that favor the generation of hydrocarbons and their channelized migration toward higher structural levels.

The thickness of the underthrusting or subdecollement section commonly is greater than 1.0 km. Most of this sequence of fine-grained sediment is transported below the landward-dipping slab or forearc basement to depths in excess of 8 km. A substantial mass is transported farther landward and deeper, and is subducted beneath the base of the arc massif. Deeply underthrust and subducted sedimentary deposits thus are transported through the PGW--a circumstance that should sustain the continuous generation of petroleum from deep within and below the accreted pile.

During the past 5-6 m.y., more than 50% of the 1 to 3-m-thick sedimentary section entering the subduction zone (at roughly 80 km/m.y.) has been subducted and, therefore, passed through the PGW. Evidently, since the late Miocene, larger volumes of hydrocarbons have been produced and possibly entrapped in high-level structures within either the accretionary complex or the forearc section mantling the underthrust basement slab.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990