Browsing by Subject "Salt deposits"

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Physical modeling of normal faults and graben relays above salt
(2002-05) Le Calvez, Joel Herve; Marrett, Randall; Vendeville, B. C. (Bruno C.)
In salt basins, thin-skinned extension is typically accommodated by graben development in sedimentary overburden and viscous flow in the evaporitic layer. This study presents a series of physical models in which grabens propagate along strike and interact in relay zones whose geometry depends on graben across-strike separation. I first describe the basis for physical modeling, including scaling analysis. I then review various experimental designs used by previous authors to model the interaction between laterally offset faults, finally I introduce a design in which faults nucleate at predetermined locations above small initial instabilities but that permits faults to propagate freely thereafter. I used series of systematic experiments to investigate the 3-D geometry and evolution of two laterally offset grabens interacting in a relay zone. Results x allow the determination of the influence of key geologic parameters on the evolution of relay zones. The main parameter, the across-strike offset between the grabens, controls the size (along both strike and dip) of the relay zone and its structural style. Where the offset is large, fault traces curve gently toward one another and fault throw and amount of diapir rise below the graben decrease progressively toward the relay zone. Some distance from that zone, old faults cease to slip as new faults form. In the relay zone old fault segments remain active and no new faults form. Some new faults hard-link along strike with active segments of older faults. Offset grabens and faults start interacting only when they have propagated along strike far enough to overlap. Syntectonic sedimentation can suppress new-fault formation during extension. Another experiment shows that inversion of graben relays by late shortening forms reverse faults rooted at depth on the crest of underlying diapirs. A second series of experiments, motivated by a field example in Canyonlands National Park, focuses on the genesis of graben arrays having regionally arcuate traces. Curvature is explained in terms of shear stresses between extending domains of the sedimentary cover, which is underlain by an evaporitic décollement, and lateral nonextending domains, a setting common along salt-bearing passive margins during thin-skinned extension.
Pressure and stress at Mad Dog Field, Gulf of Mexico
(2012-12) Merrell, Michael Phillip; Flemings, Peter Barry, 1960-; Hudec, Michael R; Hesse, Marc; Walker, Chris
Hydrocarbon exploration involves drilling into or near salt deposits in the Gulf of Mexico, Brazil, Egypt, and the Middle East. Drilling these systems has proven to be quite dangerous, challenging, and expensive due to the pressure and stress perturbations that exist around the salt. My study focuses on characterizing the pressure and stress distribution at the Mad Dog field, which is a large oil field below an allochthonous salt body in the deepwater Gulf of Mexico. The Mad Dog field lies beneath the Sigsbee Escarpment, which represents the surface and seaward-most indicator of a mobile salt in Green Canyon blocks 781, 782, 825, and 826, 190 miles southwest of New Orleans in 4,500-6,500 feet of water. I characterize the pressure distribution within the Lower Miocene sandstone reservoir which has produced over 100 million barrels to date. I map the reservoir horizon using 3D seismic data and that the reservoir is a complex regional anticlinal structure that is separated by numerous normal faults that cause it to be segmented into compartments. The in-situ pore pressures show that the compartments are not in pressure communication across the field and that multiple aquifer phase pressures are present. The in-situ pore pressure measurements are used to characterize the pressure distribution in the Miocene sediments below the salt body and in front of the mobile salt body. These measurements show that between the upper Miocene to middle Miocene there is an absolute pressure decrease and between the middle to lower Miocene there is a large pressure increase. This pressure distribution is seen both within the Miocene sediments below salt and in front of salt. A porosity and effective stress relationship from shallow Pleistocene sediments was developed to predict the pressure behavior observed within the Miocene and compare the predicted pressure with in-situ pore pressure measurements. The mudstone pressure prediction overestimates the in-situ sand pore pressure. The mudstones bounding regional sandstone have a constant porosity throughout the field, suggesting that the vertical effective stress is constant. These observations can be used to estimate the mudstone pore pressure in a new well location. If the vertical effective stress in an offset well is known and given knowledge of the total vertical stress in the new well location, the mudstone pore pressure can be estimated.