Abstract The title ‘The Value of Outcrop Studies in Reducing Subsurface Uncertainty and Risk’ might suggest that we expect new information to improve prospect risk, but this is not correct. Gaining new information generally does change our estimate of prospect risk: the change may be up or down, and the average of all possibilities is zero change. You cannot acquire data for the purpose of increasing the probability of success. We should expect that: (a) the expected value of the risk of a single prospect, post-data, is equal to the prior (pre-data) value; and (b) risk should become worse for the majority of prospects. New information adds value, not from changing pre-drill risk, but from decisions made as a consequence. The main value added is from identifying prospects not to drill, thereby saving the cost of likely dry holes, and by choosing the ones to test first, accelerating revenue from a successful outcome. Further value is added if the new information leads to the identification of new prospects or new plays, or suggests follow-on potential elsewhere in the region. Value may also be added if the new information is negative for a whole region, enabling us to focus our attention elsewhere.

ABSTRACT Many passive margins have deep-water, contractional fold belts that formed above salt or shale. Margin failure, accommodated by proximal extension and distal shortening, is caused by some combination of gravity gliding above a basinward-dipping detachment and gravity spreading of a sedimentary wedge with a seaward-dipping bathymetric surface. Gravitational failure is inherently self-limiting, and sedimentation patterns provide fundamental control of deformation. Continued shortening is driven primarily by shelf and upper-slope deposition, which maintains the bathymetric slope and the gravity potential, and by increased basin-ward tilting. Deformation is retarded or halted by distal thickening of the overburden caused by the folding itself or by lower-slope and abyssal sedimentation. Net shortening amounts and deformation rates are lower than in collisional/accretionary fold belts, because the driving forces are weaker than those induced by lithospheric plate motions. Structural styles vary but depend largely on the nature of the děcollements layer, not the driving forces. Fold belts detached on shale typically comprise basinward-vergent thrust imbricates and associated folds because of the relative strength and frictional behavior of the plastic shale. Deformation does not occur until there is sufficient overburden, and it is facilitated by high fluid pressures. In contrast, salt is a viscous material with essentially no strength, which leads to symmetrical detachment folds and early deformation beneath only a thin overburden. Moreover, the surface slope can be reduced by proximal subsidence into salt and distal inflation of salt, and much of the shortening can be accommodated by lateral squeezing of diapirs and salt massifs and by extrusion of salt nappes.

Abstract Salt along a passive margin facilitates and accommodates gravitational failure of the margin in several key ways. First, it serves as the basal detachment for a linked system of updip extension and downdip contraction that develops due to a combination of gravity gliding and gravity spreading of the sediment carapace. Second, proximal subsidence into salt and distal inflation of salt reduces the bathymetric slope and the associated gravity potential. Third, preexisting salt diapirs and massifs accommodate basinward translation of the overburden by lateral squeezing and the consequent extrusion of allochthonous salt. Fourth, allochthonous canopies can serve as additional detachment levels for gravitational failure. Deep water environments are where most of the shortening occurs, which is manifested as salt-cored folds, reverse faults, squeezed diapirs, inflated massifs, and extrusion of allochthonous salt. Extensional and strike-slip faults and associated salt deformation are also present, as are loading-induced features such as turtle structures and passive diapirs.

Abstract Recent major discoveries at Mad Dog, Atlantis, and Neptune have demonstrated a working petroleum system in the western Atwater fold belt (WAFB). Understanding the nature and scale of the petroleum system is important in determining an appraisal strategy for the discoveries and for pursuing other exploration opportunities. The source interval in the western Atwater fold belt is interpreted to be Upper Jurassic to Lower Cretaceous carbonates and marls. Hydrocarbon generation and migration began in the Upper Miocene and continues to present day. Hydrocarbon reaches the structural traps by a combination of vertical and carrier bed migration. The traps in the western Atwater fold belt are very large, salt-cored, compressional anticlines cut by both normal and reverse faults. These structures began to form in the upper Miocene and growth continued through the Pliocene. Erosion of the growing structures removed varying amounts of the Miocene section. There is generally less erosion to the southwest and increasing depth of erosion to the northeast. The reservoir rocks in the western Atwater fold belt are lower and middle Miocene submarine fan lobes deposited at or near the base of slope. Pre-existing structural and depositional topography controlled sand deposition. The fan lobes display a compensational stacking pattern and the reservoir sands are interpreted to be amalgamated and layered sheet sands. Interbedded Miocene shales seal the reservoirs.