Abstract

In the fall of 1993, Phillips and partners Anadarko and Amoco reported the discovery of the Mahogany field in block Ship Shoal 349. Discovered oil reserves were said to be at least 200 million barrels of oil. This is a first sign of a major new oil and gas play developing on the Texas/Louisiana shallow-water shelf. Future reserves in this play are estimated to be at least equal to the reserves found so far on the shelf in Late Miocene to Pleistocene reservoirs - over 10 billion barrels of oil and over 130 trillion cubic feet of gas.

This play thus is the largest, most important one in the U. S. today. In economic terms it appears to be considerably more attractive than the Gulf Coast deep-water plays, because of lower development cost, presence of an active pipeline system and market organization, and because of participation by more operators.

The reservoirs in this play will be mainly Early to Middle Miocene (?) and older sandstones. Seals will be interstratified clay and Jurassic salt. Traps are structural and stratigraphic. Structural traps range from simple anticlines to fault closures in a rift-type tensional fault system. Stratigraphic components will play a major role in the distribution of reservoirs and seals. Deep-water sand distribution systems (fans, canyons, turbidites, contourites, etc.) will be present. High porosities and permeabilities have been reported from numerous fields on the shelf in Pliocene and Pleistocene facies reservoirs in spite of fine grain sizes. The prospective section often is over 15,000 feet thick. Typical are the mainly-parallel seismic reflections in the Miocene. They indicate minor syn- and post-depositional deformation in the Miocene section only, in contrast to the more intensely-deformed, younger Plio-Pleistocene beds with their growth-fault and slump tectonics. Multiple reservoirs are expected to occur in the Miocene on a single prospect.

Salt plays only a passive role in this play and can not be considered as a play-forming agent. The term “Sub-Salt Play” thus is incorrect. Salt is mainly a geological, geophysical, and technical (drilling) nuisance, often requiring major efforts to eliminate its effects. All salt is considered to be of Jurassic age (Louann Formation). Vertical salt diapirism and subsequent multi-stage lateral gravitational spreading and secondary diapirism near the top of the sedimentary section led to the present distribution of laterally-extensive salt sheets. They form hydrocarbon traps only in few exceptional cases. Lateral salt layers mark regional unconformities and tectonic detachment surfaces.

The petroleum system in this play involves Jurassic, Cretaceous, and/or Paleogene (? Eocene) marine calcareous shales. No source bed has yet been sampled on the Gulf Coast shelf. All age determinations of potential source beds are based on crude analyses and comparisons between crude properties and onshore source beds in the U. S. and in Mexico. Some or all of the generating source beds may be over-mature today. Major vertical and lateral hydrocarbon migration has occurred.

Prediction of the “best” acreage in this play is possible through integrated efforts of geology, geophysics, and geochemistry. Future exploration will involve the critical factor of preferred hydrocarbon migration pathways. In this regard, the new Gulf of Mexico shelf play is not different from other plays in the world. Two different strategies are foreseen.

In one, all acreage blocks lying in the generally prospective region will be analyzed by 3D-seismic data bases. This will lead to major data acquisition and processing efforts by industry. The volume is large: some 2000 blocks, each one of 5000 acres, are involved. The data acquisition cost alone will lie in the $0.5 billion range.

Another exploration strategy will involve pre-detailed seismic area evaluations, using established principles of regional lateral and vertical hydrocarbon migration. The required deep structure maps can be obtained from gravity, magnetic, and regional seismic surveys that are freely available. In this approach, detailing 3D seismic will be restricted to pre-drilling prospect detailing in high-interest areas and single acreage blocks.

Most future reserves will be located in blocks that overlie deep structures at Pre-Miocene and Pre-Middle Miocene levels. Acreage blocks with such anomalies, in many cases, will include blocks that today are not prospective for lack of traps at shallow (Late Miocene to Post-Miocene) levels.

The new shallow-water play on the Texas-Louisiana shelf is one for thermal oil and gas from source beds that may well be over-mature today. Such anomalous conditions in an active petroleum system are also know or suspected from Subandean basins in South America in Trinidad, Venezuela, Colombia, Ecuador, and Peru.