Abstract Porosity-depth profiles determined from density logs in numerous wells in the Eugene Island South Addition Block 330 area of the offshore Louisiana Gulf of Mexico show departures from the expected hydrostatic compaction trends at depths ranging from 900 m to 1950 m. At greater depths, porosities may remain constant, increase, or decrease. Although it changes over distances of a few kilometers, the porosity pattern is locally coherent. The cause of porosity departure from the expected compaction related trend (hydrostatic trend) is in all observed cases pore pressures in excess of hydrostatic pressures. Porosity profiles are plotted and analyzed in detail in 40 area wells. Fluid pressure in these wells can be predicted from porosity because porosity is linearly related to effective stress. The depth at which porosity departs from the hydrostatic trend (the porosity-defined top of overpressure) coincides mostly with the 1.27 Ma transgressive Helicosphaera sellii shale that immediately overlies the gas-charged “JD” sand. However, in Block 314, the porosity-defined top of overpressure lies 500 m above the H. sellii surface, passing above two sand units and two transgressive shales. In this paper we derive analytical expressions that were used to interpret porosity profiles and overpressure relationships. One well in Block 314 was analyzed in detail as an example. In this well a seal was developed in the H. sellii shale when the shale lay at ~550m depth. When this lithologically fixed seal was gradually buried to 1430m depth, fluid pressures in underlying strata reached 0.8 of lithostatic and the seal began to deform and leak. Continued leakage while the seal was buried to its present depth of 2020m produced an interval of constant porosity (migrating seal compartment) from 1040 to 2020m. Although other interpretations are admittedly possible, we suggest that the migrating seal compartment formed when hydrocarbon fluids were introduced and capillary barriers developed. Applying the same methods, we identify areas in the Eugene Island South Addition Block 330 area where venting has diminished and areas where it has accelerated. The methods further developed and illustrated could facilitate exploration for higher porosity, more permeable sand reservoirs, address hazards associated with fluid overpressuring, and extract information on the timing and nature of hydrocarbon venting from a new information base: shale porosity profiles.

ABSTRACT Detailed interpretation and mapping of more than 10,000 km of 80-fold 8-second recent seismic data and preliminary interpretation of 2500 km of new 15-second, 6 km streamer data provide a new understanding of the kinematic and stratigraphic development of the southern Louisiana shelf. The new data reveal elongated subsalt basins, separated by nearly vertical salt welds and residual salt walls. Sequential back-stripping of balanced depth sections suggests that the walls were initiated by differential loading of overburden depotroughs and grew primarily by down-building. Sand fairways developed between the salt walls with a primary sediment transport direction from the northeast to the southwest. Time structure maps of the MDB (Miocene D. Berggreni) sequence boundary (top of the Miocene) reveal a subsalt and subweld structure disharmonic with a much younger, extremely extended section. The younger section is characterized by backward-rotated hanging walls overlying listric growth-faults. We propose that the original extent of salt sheets emplaced near the sea floor can be defined by the current location of the extended overburden which formed as a result of secondary salt sheet withdrawal. Windows through the residual salt and salt welds, and a few key deep wells which have penetrated the welds provide biostratigraphic control on the timing of the salt/sediment interactions. The vertical welds and walls, coupled with residual salt sheets and horizontal welds, form a network of surfaces separating largely isolated basins. Each basin seems to have developed independently due to varying episodes of local salt motion. The Mahogany discovery, Ship Shoal 349, is an example of sands trapped against the flank of a northeast-trending nearly vertical salt weld. Maps of the former salt sheets associated with salt walls and welds and maps of subsalt structure below the sheets and welds define prospective areas analogous to Mahogany. Additional attractive structures were also localized by salt-sediment interactions.

Abstract The Eugene Island Block 330 field is currently the largest oil-producing field in federally owned waters of the U.S. outer continental shelf. Located about 170 mi (272 km) southwest of New Orleans, the field was discovered by the Pennzoil 1 OCS G-2115 well in March 1971, after leasing on December 15, 1970. The field includes parts of blocks 313, 314, 330, 331, 332, 337, and 338, Eugene Island Area, South Addition, offshore Lousiana. The field is an anticlinal structure on the downthrown side of a large northwest-trending growth fault. Production is from more than 25 Pliocene-Pleistocene delta-front sandstone reservoirs ranging from Lenticulina to Trimosina “A” zones and located at depths of 4,300 to 12,000 ft (1,290 to 3,600 m). The reservoir energy results from a combination water-drive and gas-expansion system. Recoverable reserves are estimated to be greater than 225 million bbl of liquid hydrocarbons and 950 Bcf of gas. Considerable subsurface data provided by 220 exploration and development wells and several seismic grids form the basis for interpretation of the geology and geophysics of the Block 330 field and its producing zones.