Episodic Hydrocarbon Migration during Deposition of the Frio Clastic Wedge, Gulf of Mexico: Its Expression and Causes: ABSTRACT

Abstract

A petrographic and geochemical studies of the Oligocene Frio Formation show that the carbon isotopic composition of calcite varies systematically with stratigraphy, due to varying hydrocarbon migration to the sea bed during Frio deposition. In the St. Charles Ranch fields, Aransas County, Texas, compositions in the Lower Frio are relatively heavy ([delta][sup 13]C = [approximately]-2[per thousand]), decreasing stratigraphically upward to -5.5[per thousand] in the Middle Frio, then increasing to -2[per thousand] in the Upper Frio. The average calcite composition is identical in sandstones and mudstones at specific stratigraphic levels. This pattern contrasts with [delta][sup 18]O, in which sandstone and mudstone calcite has different compositions and no stratigraphic control is evident. Two limestone beds have very light [delta][sup 13]C compositions ([approximately] -20[per thousand]); they are analogous to modern carbonate buildups in the Gulf of Mexico forming around methane vents. The carbon isotopic composition of calcite reflects the flux of hydrocarbons to the sea bed: low [delta][sup 13]C values indicate greater incorporation of hydrocarbon into calcite and therefore a large hydrocarbon flux, and vice versa. The data suggest that a pulse of hydrocarbon migration occurred during Frio deposition, peaking during Middle Frio deposition. Two mechanisms for this migration pulse are considered: first,more » deposition of the Frio resulted in sudden hydrocarbon maturation and generation in underlying source rocks; and second, growth faulting, associated with deposition of the Frio wedge, opened migration pathways. As sedimentation rates slowed during deposition of the Upper Frio, movement on growth faults decreased and migration pathways closed. These mechanisms are tested with numerical models.« less