Deepwater Subsalt-Suprasalt Middle-Lower Slope Sands and Reservoirs of the U.S. Gulf of Mexico: The Evolution of an Exciting Giant Field Concept

摘要

For almost 70 years, the U.S. Gulf of Mexico has been an incubator of new depositional system models, and more recently, the focus for complex and dynamic subsalt-sediment trapping styles. Early passive margin models applied to the Gulf of Mexico suggested short slopes between delta fronts and basin floors, and concluded that most slopes were sand "bypass" areas. Subsequent study of the Gulf of Mexico's present-day salt-supported slope, which extends nearly 125 miles from its present-day shelf edge to the basin floor, suggested possible new sand depositional models applicable to the Cenozoic sediments below if the slope was similarly salt-supported at time of deposition. Today, well control has dispelled this sand "bypass" thinking by demonstrating the presence of thick sand sequences and, when combined with seismic stratigraphy and depth imaging that shows the ancestral salt sheets, yields a robust potential for both lowstand and highstand sands in many subsalt Cenozoic cycles. Most importantly, the Miocene confined minibasin sand bodies of amalgamated fans, amalgamated channels, and amalgamated channel levees of the ancestral mid-lower-slopes, have proven sandstone reservoirs that have produced significant oil and gas fields. Undoubtedly, there are more of these to be discovered and developed. This is not the first time that geologists in the Gulf of Mexico have pioneered new depositional concepts. When the authors started their Gulf of Mexico geoscience careers in the 1970s, industry was starting to apply Miocene delta depositional models seaward. These models had been pioneered in the 1950s and 60s, near shore. By 1970, Plio-Pleistocene deltas were hypothesized seaward of the Miocene deltaic fields, and billions of dollars of lease bonus monies were invested in the 1970s lease sales. Those massive Plio-Pleistocene deltaic sandstone reservoirs deposited in the last million years were discovered to contain billions of barrels of oil equivalent, all generated from oil and gas shale source rocks deposited in the Lower Cretaceous Aptian (125-113 Ma) and Upper Jurassic Tithonian (152-145 Ma). One of these most notable giants on the present-day outer shelf, Eugene Island 330 Field (the 3rd largest single field in the Gulf of Mexico as per the Bureau of Ocean Energy Management [BOEM] with an estimated ultimate recovery [EUR] of 770 million barrels of oil equivalent [MMBOE]), was discovered and produced suprasalt, in about 300 feet of water just 20 miles north of the edge of the Outer Continental Shelf. During the first 40 years of offshore Gulf of Mexico industry exploration, all petroleum reservoir objectives were "suprasalt," that is above all sheets or beds of salt, like Eugene Island 330 Field. The original bedded deep salt (autochthonous) was first deposited soon after the opening of the Gulf of Mexico, during mid-Jurassic Callovian "Louann" time (165-163 Ma), and all horizontal salt sheets (allochthonous) originated from that Jurassic salt. The old concepts of vertical-only salt dome structuring have evolved toward dual-structuring models of both the traditional vertical doming AND the concept of horizontal translation of salt sheets across broad areas of the ancestral shelf and slope, similar to today's salt-supported slope. Exploratory drilling, both on the shelf and deepwater, has demonstrated that most salt bodies in the Gulf of Mexico are part of these extensive allochthonous salt sheets that have translated more horizontally than vertically throughout the ancestral shelfs and slopes. Much of what the mobilized salt has covered are, thick untested sedimentary sections containing reservoir quality sand bodies and effective sealing shales. Petroleum liquids preservation is also greatly assisted by the heat transfer properties of these extensive, subsurface lateral salt sheets.