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

Nicholas B. Harris

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 (^dgr¹³C = ~-2^pmil), decreasing stratigraphically upward to -5.5^pmil in the Middle Frio, then increasing to -2^pmil in the Upper Frio. The average calcite composition is identical in sandstones and mudstones at specific stratigraphic levels. This pattern contrasts with ^dgr¹⁸O, in which sandstone and mudstone calcite has different compositions and no stratigraphic control is evident. Two limestone beds have ery light ^dgr¹³C compositions (~-20^pmil); 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 ^dgr¹³C 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, 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 te ted with numerical models.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California