ABSTRACT: Are Hydrocarbon Source Rocks Hidden Beneath Volcanic Flows in the Southern Washington Cascades?

W. D. Stanley, W. J. Gwilliam, G. A. Latham

Magnetotelluric surveys in the southern Washington Cascades have outlined a thick wedge of conductive rocks interpreted to represent a previously unmapped complex of marine sedimentary rocks. These postulated sedimentary rocks occur at depths of 1-10 km and with a thickness of up to 10 km. Shallow depths of 1-2 km to the top of the conductive section occur along anticlines that are evident in deep reflection data and are known from surface geological mapping. These anticlines are cored with Eocene marine formations like the McIntosh, Carbonado, and Raging River formations, and we infer that these formations and older, unmapped marine units make up the bulk of the conductive package. The marine rocks are also interpreted to have been deposited in the fore-arc environment o a subduction zone that was forced to reposition westward in the middle Eocene by accretion of a large seamount complex. Other lithologies that may be part of the conductive package are (1) highly altered volcanic flows with geothermal fluids; (2) continental pull-apart basin sedimentary rocks; or (3) pre-Tertiary sedimentary rocks like those in the Rimrock Lake inlier. Factors such as possible organic content, maturity, and reservoir quality have been reviewed for our proposed model. This review shows that the hypothesized marine complex should be mature over most of its extent and that organic carbon content may be high due to anoxic conditions in the assumed, restricted fore-arc environment. Structural traps may occur in several anticlines in the study area.

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