Crustal Oxidation State, Subduction Slab Segmentation, and Giant Oil Fields

Monte M. Swan, Stanley B. Keith, Thomas L. Thompson

In convergent continental margins geographic coincidence of some giant oil fields with regions of oxygen reduced lithosphere and tears in segmented lithospheric subduction slabs suggests genetic implications. Examples include well known oil fields in Chile, Columbia, Mexico, Western United States, Japan and China.

Crustal oxidation state appears to have a fundamental impact on the long term stability of hydrocarbons. Data for crustal oxidation indicate that petroleum accumulations are not stable in regions where crustal oxidation state is high (i.e. FE₂O₃/FeO>0.6). In such regions petroleum fluids ultimately degrade and oxidize to CO₂+H₂O. The basement with lowest oxidation state is spatially related to petroleum reservoir sweet spots (Highest CH₄/CO₂).

Slab segmentation causes formation of cryptic structural zones transcurrent to the convergent continental margins. Variations in slab segment dips (particularly flat subduction dips) correlate with variations in basin margin geometry, sedimentary facies changes, structural permeability and migration direction. These variations also cause dramatic magmatic arc discontinuities and crustal warps and folds in the continental lithosphere above the tears that bound the slab segments.

Slab tears also may perturb the lower lithosphere and asthenospheric mantle (80-1050 km) sufficiently to cause adiabatic decompression and release of non-biogenic hydrocarbons. Additionally, slab segment dip histories can be correlated with chemical characteristics of igneous rock, not only providing a high-resolution geologic framework for basin modeling, but also a dynamic thermal model of igneous rock induced heat flow.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995