A Conceptual Model for Sources of Oils in Gulf Coast Cenozoic Reservoirs

Abstract

ABSTRACT A conceptual model, which reconciles and concepts of Gulf Coast source rocks and includes the source rock as an integral part of the system in which hydrocarbon accumulation, structure, and stratigraphy are closely related in time and space, provides for (1) deposition of source sediment; (2) maturation of source rock by deep burial under a younger depocenter; (3) generation and expulsion of hydrocarbons from the source rock; (4) migration into reservoirs in relatively shallow traps in a younger depocenter that overlies the source rock. Primary migration through microfractures in the geopressured shales probably precedes migration through growth-fault systems into traps in younger reservoirs. Although most Cenozoic reservoirs and adjacent shales are in thermally immature and organic-poor intervals, some geologists have assumed that shales in the reservoir facies are source rocks. Assumptions regarding the source rock potential of the thermally immature shales derive from the early timing of trap formation and filling. Geochemical requirements for a source rock demand late migration from the source rock after many thousands of feet of burial. Based on geothermal gradient, age, and depth, it can be shown that thermally mature sediments must be in the deeper, older slope shales beneath producing trends. Geological and geochemical requirements are met if migration out of the source rock is late (long after deposition and deep burial of the older source rock) but migration into the trap is early (soon after shallow burial of a young reservoir/trap system) in a depocenter overlying the source rock. Geochemically immature Cenozoic reservoir sections contain mature oils derived from more deeply-buried sources. Some source rocks of Cretaceous and Tertiary age have been identified and many have been inferred. The proposed model satisfies geological and geochemical constraints, is consistent with analytical findings, provides for multiple sources for the oil in Gulf Coast Cenozoic reservoirs and defines the relationships of the source/reservoir/trap system in time and space.