Abstract An important question in the exploration of salt basins is whether or not salt welds can seal hydrocarbons in subweld traps. Any answer must start with the observation that many supraweld traps throughout the world are charged with hydrocarbons from subweld source rocks, requiring migration through welds. However, not all welds are the same, and this conceptual paper examines various factors that may influence the sealing capacity of salt welds. The probability of weld seal is enhanced by: (1) the presence of remnant evaporite along the weld; (2) relatively impermeable lithologies across the weld; (3) subweld reservoirs that are encased in shales rather than in contact with the weld; (4) the presence of clay gouge or smear generated during faulting; and (5) an original base-salt geometry that creates divergent subsalt hydrocarbon migration pathways. Another factor that must be considered is the timing of overburden deformation with respect to that of hydrocarbon generation and migration. Although weld seal is certainly a risk, traps that invoke weld seal should not be summarily discarded. Instead, each prospect should be evaluated separately in light of the factors presented here in order to derive a better assessment of the inherent risk. In addition, the ideas presented here need to be tested with observations obtained from surface exposures and a combination of subsurface well and seismic data.

Abstract Oryx was prospecting in the Gulf of Mexico subsalt, where deep targets (about 5 km) require costly drilling programs (Fig. 1). To minimize risk team members decided to process a 3D seismic data set through prestack depth migration with a velocity model built from iterative updating of image gathers. As usual in such cases, both geological and geophysical information was required to build a competent velocity model (Fig. 2). The interest of this case study to the course is the redefinition of the geological interpretation, based on the results of the velocity modeling and updating. What was originally interpreted as a monolithic salt body based on the best 3D imaging data available at the time was altered to a pair of salt welds, with sediment lying between them. This altered interpretation was due in large part to the quantitative velocity analysis involved in updating the velocity model during the iterative velocity model-building process.

Abstract Pre-salt (Aptian) lacustrine shales are the primary source rocks that charge late Cretaceous and Tertiary reservoirs in the petroliferous (15 billion barrels of oil recoverable) Campos Basin. As a result, an understanding of salt weld timing is a critical component in assessing charge risk and high-grading deep-water play fairways. The traditional method of assessing regional structural timing with isochron maps, although useful, often yields erroneous results when weld timing is being ascertained. Mini-basins that have welded and display onlap stratal geometry at their margins may have the same isochron map pattern as actively subsiding basins which have growth wedge stratal architecture. Utilizing a regional grid of 2D seismic lines, we have developed a methodology which identifies salt weld timing by classifying the stratal geometries of third order sequences within welded mini-basins. These seismic facies accurately map the evolution of weld timing within each Campos minibasin. A basin-wide “weld timing” map has been constructed and compared to regional sedimentation patterns and hydrocarbon charge modeling. At the local scale, the growth of individual welds can be compared with mini-basin structural geometries through time to assess drainage potential. We find that salt weld timing is positively correlated with regional sediment isopach thickening but true weld timing and mini-basin evolution can only be understood by systematically mapping stratal architecture. Understanding salt weld timing has profound implications on basin modeling and future exploration risk assessment within the Campos and similar basins.

Abstract La Popa Weld in La Popa Basin, Mexico, is a 24 km long near-vertical structure with a prominent bend approximately halfway along its length. Halokinetic folding, local unconformities and diapir-derived detritus in flanking strata document a precursor salt wall. Shortening during the latest Cretaceous to Eocene Hidalgoan Orogeny squeezed the salt wall to form the weld. Deformation varies significantly along the weld. The northwestern third has remnant gypsum (including a diapir at the northwestern end), little large-scale folding of flanking strata and only background fracture intensity. Directly NW of the bend are pods of gypsum linked by complete welds, a large-scale cuspate anticlinal geometry and significant fracturing within 5–10 m of the weld. The southeastern half is completely welded with no remnant gypsum, a prominent cuspate anticlinal geometry and a 50 m wide damage zone. The variable deformation was controlled by the original width of the salt wall and the amount and direction of shortening. Where orthogonal to the wall, shortening locally closed the diapir but little further deformation took place. Where oblique, shortening caused post-weld dextral strike-slip movement and significant fracturing and shearing of the wall rock. The resulting deformation variability likely impacted the sealing capability of the weld.

Abstract Isotopic and fluid inclusion analyses of veins and host rocks constrain the compositions, temperatures and sources of palaeofluids along the La Popa salt weld. Most veins formed after the salt was evacuated from the precursor salt wall; veins are generally more abundant on the downthrown side of the weld and near a significant bend in the trace of the weld. The spatial distribution of fluid types and temperatures suggests the weld served as a vertical fluid conduit and a horizontal baffle. Stable isotopes indicate there was significant fluid–rock interaction and little vertical fluid communication between rock units in areas away from the weld. Fluid temperatures along the weld ranged from 84 to 207 °C, salinities ranged from 4 to 25 wt% NaCl equiv. and methane was abundant in the weld zone and on the downthrown side of the weld. Strontium isotopes suggest that some of the vein-forming fluids were derived from the evaporites that once occupied the weld. Our results suggest the sealing potential of similar welds may be related to the presence of abrupt changes in weld geometry such as cusps or bends, the amount of shortening across the weld and the amount of vertical displacement across the weld.

Abstract The Eocene Carroza Formation in La Popa Basin, Mexico, represents fluvial sedimentation in a shortening-influenced salt-withdrawal minibasin, termed the Carroza Syncline. The Carroza Syncline lies adjacent to the La Popa salt weld, which was formerly a passively-rising salt wall that was shortened during the Hidalgoan Orogeny in Late Cretaceous and Palaeogene time. The Carroza Formation displays distinct upsection changes in fluvial facies distribution and geometry of halokinetic drape folding. Fluvial channel distribution changes upwards from widespread thin, broad channels with variable palaeocurrents in the lower part of the formation to thick, stacked channels concentrated in the hinge of the Carroza Syncline with weld-parallel palaeocurrent directions in the upper part. The upper and middle members of the Carroza contain debris-flow facies derived from diapir roof strata and the diapir itself. The style of halokinetic drape fold upturn and thinning towards the weld changes upsection from a broad (800–1500 m) to a narrow (50–200 m) zone, where upper Carroza strata are overturned and in direct contact with remnant gypsum along the weld. The upsection changes in fluvial facies distribution and geometry reflect an overall decrease in local sediment-accumulation rates relative to salt-rise rates controlled by both Hidalgoan shortening and passive diapirism.

Abstract Salt tectonics in the northern Gulf of Mexico involves both vertical diapirism and lateral silling or flow of salt into wings and tablets (sheets). Combinations of these two modes of salt deformation, concurrent with sediment loading and salt evacuation, have produced complex structures in the coastal and offshore region of southeastern Louisiana, a prolific oil and gas province. Many large growth faults and salt domes in the study area root into intra-Tertiary salt welds that were formerly occupied by allochthonous salt tablets. Two end-member structural systems involving evacuation of former tabular salt are recognized: roho systems and stepped counter-regional systems . Both end-member systems share a similar multi-staged evolution, including (1) initial formation of a south-leaning salt dome or wall sourced from the Jurassic salt level; (2) progressive development into a semi-tabular allochthonous salt body; and (3) subsequent loading, evacuation, and displacement of the tabular salt into secondary domes. In both systems, it is not uncommon to find salt displaced as much as 16-24 km south of its autochthonous source, connected by a horizontal salt weld to an updip, deflated counter-regional feeder. Although both end-member structural systems may originate before loading of allochthonous salt having grossly similar geometry, their final structural configurations after loading and salt withdrawal are distinctly different. Roho systems are characterized by large-displacement, listric, south-dipping growth faults that sole into intra-Tertiary salt welds marked by high-amplitude reflections continuous with residual salt masses. Salt from the former salt tablets has been loaded and squeezed laterally and downdip. Stepped counter-regional systems, in contrast, comprise large salt domes and adjacent large-displacement, north-dipping growth faults that sole into intra-Tertiary salt welds before stepping down again farther north. Within the large salt-withdrawal basins north of the counter-regional faults are south-dipping strata that terminate onto subhorizontal salt welds. Recognition of these more complex, deep-seated salt geometries should be factored into an analysis of hydrocarbon charge, migration, and trapping in light of the strong correlation between oil and salt-structural systems in the Gulf Coast.

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 Phillips Petroleum South Timbalier South Addition 260 #1 well, Teak prospect, tested a subsalt closure covering parts of three offshore blocks. The structure was formed by the truncation of southeast dipping strata on the north and west by salt welds. These welds, which juxtapose strata of different ages and geometries, are collapsed salt feeder systems that were the conduits for emplacement of the salt sheet overlying the Teak prospect. The well encountered eight hydrocarbon shows over 5000’ of subsalt section in predominantly thin-bedded turbidite sandstones interpreted as overbank-levee and splay deposits. Pay intervals are confined to a zone directly below the salt and deeper in the section below an unconformity surface. Salt welds proved to be sealing surfaces, but other parameters also influenced the vertical distribution of hydrocarbons. The hydrocarbons directly below the salt are trapped within a tectonically disturbed zone where reservoirs probably have limited lateral continuity due to faulting and sediment disruption caused by movement of the overlying salt. The deeper unconformity is interpreted as an older salt weld below the present-day salt sheet because of missing stratigraphic section and the presence of a condensed section above it. A tectonic model for the area indicates three phases of salt movement. The first phase was a near-vertical emplacement of areally-restricted, wedge-shaped salt massifs related to large counter-regional faulting which occurred during the late Miocene. This was followed by rapid deflation of the massifs and large sediment accommodation during the early Pliocene which emplaced the salt over Teak, forming the bounding weld surfaces. Finally, a period of thin-skinned extension during the late Pleistocene remobilized the salt sheets into secondary piercements, forming sub-horizontal welds extending away from the present Teak salt sheet.

Abstract In the core Brazilian Campos and Espírito Santos Basins the petroleum system is complex with post salt reservoirs and structures linked to the to the pre-salt rift basins through salt welds, basement faults and rift geometries. In addition, basement lineaments have influenced the deepwater sand delivery system. The primary source rocks are rift and sag sediments in the Lagoa Feia Formation. Numerous giant oil fields such as Roncador and the recent Jubarte discovery occur along the Atlantic Hingeline where there has been Lagoa Feia charge focus. Along the basin margins the evacuation of salt and extensional faulting provides charge access from these underlying rift sediments. Hingeline focusing is accomplished both by basement faulting and by the taper of a sag and rift facies wedge. Salt welds within the outboard diapir provenance are also thought to provide charge access, although there are notable failures such as the deepwater BMC10 block. Paleo-migration geometries or residual sub-seismic salt may have prevented charge of large BMC10 four-way structures. Understanding paleo-migration patterns, differentiating effective salt welds from noneffective welds are critical exploration elements. Basement lineaments also influence deepwater channel and canyon orientation. Liniment focused channels are recognized as important elements in the development of the recent Espírito Santo Golfino discovery and in the outboard Espírito Santo Rio Doce Canyon system. With the existing 2D grid this relationship provides a framework for seismic facies interpretation. Successful exploration requires an understanding of the relationship between the rift and post-salt section. Only a petroleum system approach which incorporates an understanding of the underlying rift section and the relationship to the post salt drift section can be seen to result in successful exploration. The link between the underlying rift section and the drift post-salt section is an important element charge focus and reservoir presence The petroleum system is further complicated by the overprint of salt tectonics on charge from the rift basins, reservoir distribution, and structuration.