Miocene Oil Source Beds in the East Breaks Basin, Flex-Trend, Offshore, Texas

Abstract

Crude oils in Pliocene reservoirs at East Breaks 160/161 field, offshore Texas are partially biodegraded and have compositions typical of continental shelf oils and are unlike most Flex-Trend oils. Geochemical data on samples from a deep well clearly define the presence of oil-generating kerogen which can easily be overlooked by conventional analytical techniques. A model is offered for the deposition of even better quality oil source beds during the Middle Miocene and Early Pliocene transgressive cycles. Hydrogen-rich, marine organic matter is deposited under anoxic conditions on the upper continental slope. Turbidites carried this material to accumulation sites in oxic middle and lower slope basins where rapid sedimentation prevented consumption by benthonic scavengers. Slower sedimentation in interbasin areas, on basin flanks, and on continental rises permitted biodegradation of hydrogen-rich kerogen. Turbidites deposited during regressive cycles contain only low yield, oxidized organic matter. A basin model of a deep well in the field shows that peak oil generation occurs within the Middle Miocene section at depths between about 13 and 15 thousand feet and began only 200,000 years ago. The model indicates that Middle Miocene source beds in the East Breaks intraslope basin depocenter are actively generating and expelling oil and gas at the present time. The heating rate is very high due to rapid burial, convection along growth faults, and proximity to salt. Oil generation and expulsion are, therefore, highly efficient and take place at relatively low maturity levels (0.5 to 0.7 Ro). East Breaks 160/161 field is believed to be a closed petroleum system and contains all of the elements necessary for the generation, migration, and accumulation of oil and gas. Effective Middle Miocene oil source beds are connected to sealed Pliocene and Pleistocene reservoirs by an active growth fault system. Underlying salt appears to isolate the system from possible Early Tertiary and Mesozoic oil source beds. Because the East Breaks oils are correlative with other continental shelf oils, a similar source for all is implied. Structural style controls the distribution of Gulf Coast Late Tertiary oil by providing source bed and reservoir depositional sites as well as vertical migration pathways. When integrated with geology and geophysics, geochemistry can help lower finding costs by identifying prospects with all of the elements necessary for petroleum occurrence.