DETAILS CONFIRM GULF OF MEXICO DEEPWATER AS SIGNIFICANT PROVINCE

David T. Lawrence
Shell Offshore Inc.
New Orleans

Roger N. Anderson
Lamont-Doherty Earth Observatory
Palisades, N.Y.

Extraordinary, multicompany data on the geology, geophysics, biology, and hydrocarbon habitat of the deepwater Gulf of Mexico were disclosed last month at the American Association of Petroleum Geologists annual convention in New Orleans.

In his treatise on the structure of scientific revolutions several years ago, Thomas Kuhn revealed the paradigm shifts required for true technical and scientific growth. These paradigm shifts may be seeded by new data, insights, technology, and/or methods.

For the past three decades, the earth science profession has been fortunate to have had new paradigms revealed at key conventions. Plate tectonics in the late 1960s-early 1970s, black smokers and chemosynthetic communities at mid-ocean ridges in the early 1980s, and sequence stratigraphy in the late 1980s produced memorable convention environments.

New paradigms emerged once again, not only because of new data technology and methodology but also because of a new spirit of industry synergy and cooperation that combined to unveil the spectacular geology of the deepwater gulf.

DEEPWATER POTENTIAL

Well data from 160 rank wildcat wells, more than 110 appraisal and field wells, and seismic data acquired through more than 1,000 proprietary seismic crew months portray the deepwater as a diamond in the rough.

Large salt-related structures, prolific middle Miocene to Upper Pleistocene turbidite sand deposits, widespread and rich source rocks, and a favorable thermal regime are key factors that add the deepwater gulf to the select list of the world's giant oil provinces.

Thirty-two confirmed hydrocarbon discoveries have been drilled with announced discovery volumes of at least 1.5 billion bbl of oil equivalent. Scoping volumes for the deepwater are placed at 8-10 billion BOE, with the possibility that continued evaluation may reveal volume potential approaching that of the Gulf of Mexico shelf.

Yet to date development plans have been publicized on only six projects. Significant economic risks and technical hurdles still need to be overcome to profitably develop even large accumulations in the deepwater.

SALT STRUCTURES

Salt is the dominant structural element of the deepwater.

Large allochthonous salt thrust sheets, driven by the huge Plio-Pleistocene sediment dump of the Mississippi River, dominate the slope to the Sigsbee escarpment. Salt movement is recorded by large, stepped counter-regional growth faults and down-to-the-basin fault systems soling into evacuated salt surfaces.

New methods linking palinspastic reconstructions to subsidence analysis were presented at the conference to track the history of salt movement. Horizontal velocities of salt movement are in centimeters per year, making this supposedly passive margin as tectonically active as most plate boundaries.

The present day high relief sea floor of the slope pockmarked by minibasins and dissected by canyons is a product of salt movement in response to sediment supply (Fig. 1).

DEEPWATER RESERVOIRS

The Mississippi River and its smaller neighbors to the east and west have supplied vast quantities of sand to the deepwater.

The sediment has been trapped in salt bounded minibasins that eventually overflow and allow the sand to be transported to unconfined settings downdip.

Gravels have been discovered not only in cores taken from the Mississippi fan in Leg 96 of the Deep Sea Drilling Project but also from Miocene rocks at a depth of more than 17,000 ft in Mississippi Canyon Block 657 more than 100 miles downdip from the time-equivalent shelf margin.

More than 440 net ft of Miocene and Pliocene oil bearing sand in 14 stacked pay horizons in the Mars basin (Mississippi Canyon blocks 763 and 807) amply support a model for voluminous sand supply to the deepwater.

Convention speakers commonly cited porosities of more than 30% and permeabilities greater than 1 darcy in deepwater reservoirs. Compaction and diagenesis of deepwater reservoir sands are minimal because of relatively recent and rapid sedimentation.

Sands at almost 20,000 ft in Auger field in Garden Banks Block 426 still retain a porosity of 26% and a permeability of almost 350 md. Pliocene and Pleistocene turbidite sands in Green Canyon 205 field have reported porosities ranging from 28-32% with air permeabilities between 400 md and 3 darcies. Extended production tests reported to date are very encouraging, even in thin-bedded turbidite sands. Flow rates from centimeter thick beds in Tahoe field in Viosca Knoll Block 783 were 29 MMcfd of gas and 974 b/d of condensate.

Nine wells from thicker Pliocene turbidite sands at Bullwinkle field in Green Canyon Block 65 have consistently produced more than 4,000 b/d. Flow rates greater than 3,000 b/d were reported from wells in Jolliet field in Green Canyon Block 184.

Shallow analog studies constrained by high resolution seismic and borehole data and outcrop analog studies presented at the AAPG convention were used to develop models of deepwater reservoir architecture and connectivity. Connectivity in sheet sands, amalgamated sheet sands, and amalgamated channel sands is predicted to be high with recovery efficiencies in the 40-60% range.

HC CHARGE, TRAP

Hydrocarbon source rocks are rich and widespread in the Gulf of Mexico.

Thermal and maturity modeling demonstrate that probable Mesozoic source rocks have entered the oil window throughout most of the deepwater. Thermally mature hydrocarbons migrate along vertical entry points at salt flanks and faults until permeable sands for lateral migration are encountered.

Hydrocarbons are trapped in a variety of structural and stratigraphic settings in the deepwater. Salt flank (Bullwinkle), salt/fault (Auger), fault (Green Canyon 205-Jolliet), stratigraphic (Ram-Powell), and combination salt/fault/stratigraphic (Mars) traps were described.

The hydrocarbon mix is fairly even between oil and gas. Much of the free gas discovered in the deepwater to date is bacterial methane, though large thermal gas accumulations have been tested in the Viosca Knoll area.

In contrast to shelf crudes, most deepwater oils are moderately sour (0.5-2.0 wt % sulfur). Gravity reported at the conference ranges from 15-60 API, with most crudes falling between 25-35.

Natural hydrocarbon seeps and mud volcanoes at the sea floor attest to present day migration of hydrocarbons and fluids along faults. More than 180 seeps from across the western Gulf of Mexico slope were reported (OGJ, Apr. 19, p. 64).

A natural seep was reported at the sea floor above producing Jolliet field in Green Canyon Block 184. Hydrocarbon seeps are not restricted to the recent. The world's first Pliocene paleoseep was discovered at Auger field based on isotopic, petrographic, paleontologic, and stratigraphic evidence.

TECHNICAL COOPERATION

Papers were presented from virtually all of the oil companies, academic institutions, and government organizations studying the deepwater Gulf of Mexico.

The technical disclosures described above are a result of the clear realization in the oil industry that a change from a proprietary technical culture to technical cooperation is required to profitably bring the hydrocarbons of the deepwater on line.

Synergistic appraisal, development, and transportation strategies within geographic corridors will link the hydrocarbon potential of the deepwater to existing and planned for infrastructure. This ongoing paradigm shift is as significant and difficult as any of those creating technical breakthroughs but will similarly create new opportunities for discovery and growth as industry moves forward into the deep water.