Implications Of Plate Tectonics For Offshore Gulf Of Alaska Petroleum Exploration

Abstract

ABSTRACT The Gulf of Alaska is coincident with a converging plate boundary, along which the Pacific plate under thrusts the North American plate. Evidence of this interaction shows up in the deformational pattern of offshoreTertiary strata, as well as the sedimentary record. A broad belt of compressional structures attributable to plate movement can be traced along the continental shelf and slope of the western Gulf of Alaska, parallel to the Aleutian trench. This field cuts diagonally across the continental shelf of the central Gulf of Alaska, splaying into the Fair weather fault system in an area north and west of Yakutat. Structures within this compressional field show evidence of sequential development - from northwest to southeast. Tertiary sediments record a history of southeastern migration of the "Aleutian" trench, and a tectonic incorporation of early Tertiary oceanic", trench and slope sediments in a late forming continental shelf, all in response to episodic, but persistent plate convergence. The implications of plate tectonics for exploration is significant. Knowledge about timing of structural growth, and distribution of reservoir and source rocks is vital to an exploration program. Plate tectonics is not an oil finding tool but a unifying concept to which one may relate the required interpretations about structure and stratigraphy. INTRODUCTION The concept of global plate tectonics is becoming more and more important to petroleum geologists exploring the Gulf of Alaska. There, two segments of the earth's crust, the Pacific plate and the North American plate, interact in an area where one major O.C.S. lease sale has recently been held, and others are scheduled for the future. The most promising areas for petroleum potential in the Gulf of Alaska are in the northern Gulf where O.C.S. Sale 39 was held in April 1976, and in the western Gulf where O.C.S. Sale 46 is tentatively scheduled for November 1977 (Fig. 1). In both areas, petroleum potential lies in Tertiary sediments. Folding and faulting took place in Tertiary time. Both stratigraphy and structure have been affected by the plate-tectonics environment of the region and the time. The regional geology of this area has recently been described by various U.S.G.S. workers.1, 2 Fig. 2 is a vastly simplified geologic map. Mesozoic rocks border the Gulf of Alaska onshore. They are highly deformed and metamorphosed to some extent, and probably have no petroleum potential in the region. Early Tertiary sandstones, shales and volcanics of deep marine (in the west) and shallow marine and non-marine (in the east) environments form the next outcropping belt seaward. They too, are highly deformed. Petroleum potential in these rocks depends on their being found in a location where the degree of deformation is less, and where there is reasonable expectation of reservoir sand development. Late Tertiary rocks outcrop in the next, more seaward out Crop belt.