Mark G. Rowan

Salt-Related Fault Families and Fault Welds in the Northern Gulf of Mexico

Salt-related faults and fault welds in the northern Gulf of Mexico are classified based on the three-dimensional geometry of the faults or welds, deformed strata, and associated salt. Kinematic or genetic criteria are not used in the classification. Only documented fault styles are considered; those styles produced by experimental or numerical modeling, but not yet observed in the Gulf, are not included. Extensional faults comprising symmetric arrays include peripheral faults, which occur at the landward margin of the original salt basin; crestal faults, which are growth faults rooted in reactive diapirs; and keystone faults, which occur at the crests of anticlines. Asymmetric arrays of normal faults are grouped according to the dominant dip direction. Faults that dip primarily basinward include roller faults, which are listric growth faults that sole into a subhorizontal salt layer; ramp faults, which extend upward from the landward margin of bulb-shaped salt stocks; and shale-detachment faults, which sole into a shale decollement that merges into a salt layer. Counterregional faults are landward-dipping asymmetric arrays that link cylindrical, basinward-leaning salt stocks. Asymmetric arrays with variable dip direction include flap faults, whose footwalls comprise diapirs with uplifted and rotated roof strata, and rollover faults, which occur at the hinges of monoclinal folds. Two families of contractional faults are described: toe thrusts, which are basinward-vergent thrusts that ramp up from a salt or shale decollement, and break thrusts, which are high-angle reverse faults that cut one or both limbs of detachment folds. Fault arrays that strike parallel to the regional dip direction are termed lateral faults. Six types of fault welds are defined: primary welds are those at the autochthonous level; roho welds are subhorizontal, allochthonous welds into which roller faults detach; counterregional welds comprise both subhorizontal and landward-dipping segments beneath growth monoclines; bowl welds are elliptical and upwardly concave; thrust welds are landward-dipping surfaces that separate repeated stratigraphic sections; and wrench welds are steep and strike parallel to the regional dip direction. Groups of geometrically classified fault families and fault welds are kinematically and genetically linked to each other and to associated salt bodies and welds. Linked fault systems can contain extensional, contractional, and strike-slip components. Extensional fault families are formed by basinward translation, subsidence into salt, or folding. Those fault families that accommodate basinward translation are balanced by salt extrusion or contractional fault families. Strike-slip fault families commonly provide hard links, although various fault components also can be soft linked. We illustrate five associations of linked fault systems that are directly related to five types of salt systems: autochthonous salt, stepped counterregional, roho, salt-stock canopy, and salt nappe.

Near-salt deformation in La Popa basin, Mexico, and the northern Gulf of Mexico: A general model for passive diapirism

Strata adjacent to exposed diapirs in La Popa basin, northeastern Mexico, comprise stacked halokinetic sequences consisting of unconformity-bounded packages of thinned and rotated strata cut by radial faults. Deformation results from shallow drape folding over the flanks of the rising diapirs and not from deep drag folding in diapir-peripheral shear zones. Subsurface analogs from the Gulf of Mexico have diapir-flanking geometries ranging from similar, wide zones of upturned and thinned strata to undeformed, constant-thickness strata. Subhorizontal salt tongues display little subsalt deformation and thinning.We propose a general model for passive diapirism and flank deformation that includes (1) gradually varying salt-flow rates, (2) superposed episodic sedimentation that results in changing bathymetric relief, (3) rotation of near-surface strata as salt inflates relative to the adjacent basin, (4) failure and erosion of strata in the steepening bathymetric halo, and (5) bedding-parallel slip surfaces that converge on unconformities and onlap surfaces. A primary factor influencing flank geometries is the width of the bathymetric high extending beyond the diapir edge. This is largely dependent on the thickness of the halokinetic sequence onlapping the diapir, which in turn is controlled mostly by the interplay between salt inflation/deflation rates and sedimentation rates. Other factors include the amount of concurrent shortening, which produces a wider but less intense zone of deformation, and the position on the scarp of salt breakout and extrusion.Our model is important for exploration and production in diapir-flank and subsalt settings because of its implications for trap size and geometry, reservoir distribution, trap compartmentalization and pressure seals, and hydrocarbon charge. It can help in explaining complex and enigmatic well data and in better assessing risk in areas of poor seismic imaging.

The Perdido fold belt, northwestern deep Gulf of Mexico; Part 1, Structural geometry, evolution and regional implications

The Perdido fold belt is a frontier petroleum exploration province located in deep waters of the northwestern Gulf of Mexico. The anticlines are northeast-southwest-trending, symmetrical to asymmetrical, with concentric folds usually bounded on both flanks by steep reverse faults. The folds are interpreted as detachment folds cored by autochthonous Middle Jurassic Louann Salt. The fold belt overlies rifted transitional crust characterized by northeast-southwest-trending basement highs and northwest-southeast transverse structures that controlled the original salt thickness and subsequent fold geometry. Upper Jurassic-Eocene strata were folded during the early Oligocene (36-30 Ma), with deformation possibly continuing into the earliest Miocene. Postkinematic sediments gradually buried the folds, with younger strata progressively onlapping the highest structures. Some folds were reactivated during the middle Miocene, and a late phase of broad uplift during the Pliocene-Pleistocene is attributed to loading of the Louann Salt by the advancing Sigsbee salt nappe. The Perdido fold belt marks the basinward margin of a complex, linked system of gravitational spreading above salt. Updip Paleogene sedimentary loading and associated extension were accommodated downdip primarily by salt canopy extrusion. The 5-10% shortening and folding occurred only after canopy feeders were evacuated and closed off. Subsequent loading and deformation were concentrated at higher, allochthonous levels.

Cross Section Restoration and Balancing as Aid to Seismic Interpretation in Extensional Terranes

Structural cross sections in extensional terranes can be improved significantly by restoration and balancing principles originally developed for fold-and-thrust belts. In rift basins and passive continental margins, use of restoration techniques in analysis of growth faults, elongate salt swells, and other extensional structures results in geologically valid seismic interpretations. Sections must be in depth (as opposed to two-way time) and oriented in the direction of material transport, and decompaction must accompany restoration. Hanging-wall deformation is characterized by different processes depending on the mechanical properties of the rocks being extended, and can be approximated by various geometric and kinematic models: antithetic faulting is analogous to antithetic shear, combined antithetic and synthetic faulting is approximated by vertical shear, domino-style extension corresponds to rigid body rotation, and flexural slip/flow is modeled by bed-length balance methods. Selection of the appropriate algorithm is crucial for proper restoration and balancing. Applications of these techniques, illustrated here using seismic data from the Gulf Coast and North Sea, include (1) evaluating and adjusting seismic interpretations, (2) projecting listric fault trajectories to depth, (3) delineating more accurately the geometry and extent of hydrocarbon reservoirs, (4) identifying new exploration leads, (5) determining the deformation history of an area, and (6) constraining the timing and direction of hydrocarbon migration and entrapment.

Three-dimensional geometry and evolution of a segmented detachment fold, Mississippi Fan foldbelt, Gulf of Mexico

Abstract The frontal fold of the deep-water Mississippi Fan foldbelt is used to investigate the relationships between folding and faulting in detachment folds. Seismic coverage shows the entire three-dimensional geometry, from termination to termination, and the deformation history as recorded by asymmetric growth strata on fold limbs. The fold is a salt-cored detachment fold cut by reverse faults on both limbs. Its three-dimensional geometry is complex, consisting of four separate culminations, each associated with a distinct fault or fault segment. Consequently, profile geometries vary significantly, but unsystematically, along strike. Data analysis and structural restoration suggest a three-stage evolution during Mio-Pliocene shortening: (1) preexisting, en-echelon salt pillows served as buckling instabilities for the initiation of detachment folds that experienced relatively minor lateral propagation during growth and linkage; (2) an increase in shortening rate was accommodated by break-thrust folding; and (3) the faults became inactive upon a decrease in shortening rate, such that further fold amplification occurred by rotation and uplift of the backlimb. There is a direct correlation between fold and fault geometries, and abundant evidence indicates that the geometries of individual fold segments dictated fault geometries.

A systematic technique for the sequential restoration of salt structures

Abstract A method is described for the sequential restoration of cross sections in areas of salt tectonics where deformation is confined to the salt and higher layers. The subsurface geometry evolves with time through the interaction of various processes: sedimentation, compaction, isostatic adjustment, thermal subsidence (if present), faulting, and salt withdrawal/ diapirism. The technique systematically calculates and removes the effects of each of these processes during specified time intervals defined by the interpreted horizons. It makes no assumptions about salt kinematics and generally results in the area of the salt layer changing through time. The method is described for restoration of extensional terranes, but it is also suitable for areas of contractional salt tectonics with only minor modifications. After converting an interpreted seismic profile to depth, the top layer is stripped off and the underlying section is decompacted according to standard porosity-depth functions. A deep baseline, unaffected by compaction or deformation, is used to restore any isostatic compensation or thermal subsidence. Isostasy is calculated according to the Airy model, and differential sedimentary loading across a section is shown to be approximately balanced by changes in salt thickness so that the load is evenly distributed. After these processes have been reversed, the resulting geometry and the seismic data are used to create the sea-floor template for structural restoration. Fault offsets are removed and the layers down to the top salt are restored to this template, while the base salt remains fixed. The resulting space between the restored top salt and the fixed base salt defines the restored salt geometry. In addition, the difference between the sea-floor template and a fixed sea level provides a measure of the change in water depth (ignoring eustatic changes in sea level). The technique is applied to an interpreted seismic profile from the eastern Green Canyon/Ewing Bank area of offshore Louisiana. The section is characterized by a variety of salt structures, including salt rollers, a diapiric massif, a remnant salt sheet, and a salt weld, which are shown to have derived from an originally continuous salt sheet which has been modified by sedimentary loading. Early loading created vertical basin growth that was accommodated primarily by salt withdrawal and associated diapiric rise through the process of downbuilding. Once the salt weld formed, continued sedimentation was accommodated by a lateral increase in basin size caused by down-dip extension on listric growth faults.

Salt-Sediment Interaction, Northern Green Canyon and Ewing Bank (Offshore Louisiana), Northern Gulf of Mexico

Structural and sequence stratigraphic interpretations of two-dimensional seismic and well data from northern Green Canyon and Ewing Bank were integrated to evaluate how salt deformation influenced the distribution of Pliocene-Pleistocene facies in time and space. Two techniques were employed. First, twelve palinspastic maps of near-surface structure were constructed. These were combined with maps of interpreted depositional environments to show how shallow salt diapirism created bathymetric relief that influenced the configuration of sediment transport systems and depocenters through time. Second, tectonostratigraphic packages comprising multiple sequences were defined based on external geometry. Different stacking patterns of these packages characterize four types of minibasins, each with a distinct history of salt evacuation from underlying salt stocks and sheets. Interpreted seismic facies were analyzed within this minibasin framework to evaluate how deep-salt withdrawal influenced the distribution of depositional systems. The results show that both structural and sedimentological variables influenced lithofacies development. External factors dictated the volume and type of systemwide clastic input. Regional factors, such as nearby salt structures and the position of deltas, controlled the dispersal of clastics. Local factors, such as the thickness of underlying salt, influenced minibasin-specific evolution. These factors interacted at three scales: (1) a broad transition from sand-rich ponded settings to shale-dominated bypass settings during the Pliocene-Pleistocene, (2) fluctuations over periods of several sequences that created highly variable stratigraphic stacking patterns, and (3) a progression from ponded to bypass facies within individual sea level cycles. Analysis of these various factors can improve the prediction of reservoir distribution within slope minibasins, and thereby reduce the risk in subsalt and deep-water exploration.

Concepts in halokinetic-sequence deformation and stratigraphy

Abstract Halokinetic sequences are unconformity-bound packages of thinned and folded strata adjacent to passive diapirs. Hook halokinetic sequences have narrow zones of deformation (50–200 m), >70° angular discordance, common mass-wasting deposits and abrupt facies changes. Wedge halokinetic sequences have broad zones of folding (300–1000 m), low-angle truncation and gradual facies changes. Halokinetic sequences have thicknesses and timescales equivalent to parasequence sets and stack into composite halokinetic sequences (CHS) scale-equivalent to third-order depositional cycles. Hook sequences stack into tabular CHS with sub-parallel boundaries, thin roofs and local deformation. Wedge sequences stack into tapered CHS with folded, convergent boundaries, thicker roofs and broad zones of deformation. The style is determined by the ratio of sediment-accumulation rate to diapir-rise rate: low ratios lead to tabular CHS and high ratios result in tapered CHS. Diapir-rise rate is controlled by the net differential load on deep salt and by shortening or extension. Similar styles of CHS are found in different depositional environments but the depositional response varies. CHS boundaries (unconformities) develop after prolonged periods of slow sediment accumulation and so typically fall within transgressive systems tracts in shelf settings and within highstand systems tracts in deepwater settings. Sub-aerial settings may lead to erosional unroofing of diapirs and consequent upward narrowing of halokinetic deformation zones.

The Perdido Fold Belt, Northwestern Deep Gulf of Mexico, Part 2: Seismic Stratigraphy and Petroleum Systems

Analysis of 12,000 km of two-dimensional multifold seismic data shows a thick succession of Mesozoic and Cenozoic deep-water strata in the Perdido fold belt, northwestern deep Gulf of Mexico. These strata differ in seismic facies, areal distribution, and reservoir/petroleum potential. Mesozoic strata are interpreted as dominantly fine-grained carbonates and show minor thickness changes. Cenozoic strata are largely mud-dominated siliciclastic turbidite deposits and vary considerably in thickness across the fold belt. These changes reflect the shifting position of Cenozoic marginal-marine depocenters. Mesozoic reservoir potential consists of fractured Upper Jurassic and Cretaceous deep-water carbonates. Cenozoic reservoir potential consists of siliciclastic deep-water turbidites. Portions of the Paleocene to lower Eocene strata are sand-prone and are the downdip equivalents of the lower and upper Wilcox shallow-marine depocenters. These strata are all incorporated within the folds. Lower to middle Oligocene strata coincide with the main growth phase of the fold belt. Potentially sand-prone middle Oligocene to lower Miocene strata are the downdip equivalents of the Vicksburg (early Oligocene), Frio (Oligocene), and Oakville (early Miocene) shallow-water depocenters. These strata form potential stratigraphic traps against the folds. Mesozoic source potential was modeled assuming Oxfordian, Tithonian, Barremian, and Turonian source beds. One-dimensional thermal maturation modeling showed these sources reached peak oil generation between 51 and 39 Ma, 39 and 8 Ma, 32 and 2 Ma, and 26 and 8 Ma, respectively. Cenozoic source potential was modeled using an Eocene source. Modeling showed this source reached only early oil generation in the basinward half of the fold belt. Thermal maturation was reached by source beds at different times in different locations due to changes in burial depth, amount of structural uplift, and underlying thickness of autochthonous salt. All of these factors indicate that seal and reservoir carry significant risk, but that the potential exists for large petroleum accumulations.

The Evolution of Allochthonous Salt Systems, Northern Green Canyon and Ewing Bank (Offshore Louisiana), Northern Gulf Of Mexico

The discovery that major episodes of subhorizontal, allochthonous salt flow have occurred in the Gulf of Mexico Basin requires a means of quantifying the evolution of allochthonous salt and associated structures to conduct both basin and petroleum systems analyses. Sequential structural restorations of allochthonous salt systems provide an evolving structural framework for integrating stratigraphic, geophysical, and geochemical data sets. In this study, interpretation of more than 10,000 km (6200 mi) of multifold seismic data, and sequential restoration of eleven profiles, were used to determine the geometry and evolution of allochthonous salt structures within Ewing Bank and northern Green Canyon protraction areas. The results illustrate the complex geometry of the multilevel salt system and the types of interactions between counterregional and salt-stock canopy models of allochthonous salt system evolution. Sedimentary loading is accommodated by salt sheet extrusion, gravity spreading, gravity gliding, extension, salt evacuation, and contraction. Salt geometry commonly changes dramatically through time because it provides much of the accommodation for sediments and absorbs much of the extension and contraction within its overburden. The positioning and kinematics of extensional and contractional structures are controlled by salt body geometries, salt system interactions, and, most importantly, the topography of the base salt or equivalent salt weld. The structural restorations also constrain the timing of salt sheet and salt weld formation and document the positive correlation among sedimentation rates, salt flow, and structural deformation. Cross-sectional salt area generally decreases through time in areas of salt evacuation and minibasin formation, but increases in sections crossing growing salt bodies. Three-dimensional restoration is required to determine the three-dimensional kinematics and balance of allochthonous salt tectonics.