Extensional Forced Folds Research Articles

Petroleum system in supra-salt strata of extensional forced-folds: A case-study from the Basque-Cantabrian basin (Spain)

Abstract Hydrocarbon accumulations in the Plataforma Burgalesa are located in structural traps that resulted from Mesozoic extensional forced folding. Pre-rift Triassic evaporites favoured the structural decoupling between faulted basement rocks and their supra-salt cover, which includes carbonate and fluvial reservoirs. These reservoir levels record severe syn-extension fracturing, related to the stretching of the supra-salt layers as extensional forced folding proceeded. The observed syn-extension fracture pattern has been partly modified by subsequent Cenozoic shortening. The availability of a large data set of seismic profiles, well data, oil shows, and the exposures of reservoir and seal levels, make the Plataforma Burgalesa an outstanding analogue for petroleum plays in extensional settings with supra-salt reservoirs. Here, we provide a reassessment of recently published structural data from the area, including geometry, kinematics, and fracture pattern of the partly inverted extensional forced fold system. We then compare structural features and occurrence of hydrocarbon accumulations, to discuss the implications for structural traps formation in rift basins with an intermediate decollement level.

Rift-margin extensional forced folds and salt tectonics in the eastern Basque-Cantabrian rift basin (western Pyrenees)

Abstract In this paper we present and discuss Cretaceous extensional folds of the eastern Basque-Cantabrian Basin (Northern Spain). Geometry and kinematics of folds is constrained by means of structural and sedimentological fieldwork integrated with geological mapping, revealing an intimate link between coeval folding, extensional faulting, and salt mobilization. In detail, the Mesozoic succession of the northwestern and southern borders of the Palaeozoic Bortziriak-Cinco Villas massif (eastern margin of the BCB) host late Albian–early Cenomanian syn-rift forced folds. The studied forced fold axes trend parallel and are located above main and inferred Cretaceous syn-sedimentary basement faults. Structural data indicate that these folds formed during the late Albian − early Cenomanian interval. The presence of Upper Triassic clay and evaporite outcrops along and/or close to the axes of folds and their stratigraphic relationship with Upper Cretaceous strata indicate their halokinetic origin and extrusion during folding. The trigger of salt tectonics is attributed to a basement extensional pulse during the Bay of Biscay – Pyrenean rifting. Related high subsidence-rates allowed salt to flow towards faults, forming salt walls and causing the inflation and folding of the overburden.

Basement structure of the Hontomín CO<sub>2</sub> storage site (Spain) determined by integration of microgravity and 3-D seismic data

<p><strong>Abstract.</strong> A multidisciplinary study has been carried out in Hontomín (Spain) to determine the basement structural setting, its geometry and the geometry of the sedimentary succession of an area aimed to be the first Spanish pilot plant for CO<sub>2</sub> storage. An integration of coincident 3-D seismic results, borehole data and unpublished microgravity data were used to reproduce the deep structure and topography of the basement and to quantify the thickness of the Triassic Keuper evaporites. The subsurface structure is characterized by a half-graben setting filled with Keuper evaporites (up to 2000<span class="thinspace"></span>m thick), forming an extensional forced fold. All data sets clearly identify two main fault systems, compartmentalizing the main structural domain into three differentiated blocks. These faults have been interpreted to be reactivated normal faults that have led to the formation of the Hontomín dome.</p>

Basement structure of the Hontomín CO<sub>2</sub> storage site (Spain) determined by integration of microgravity and 3D seismic data

A multidisciplinary study has been carried out in Hontomín (Spain) to determine the basement structural setting/ geometry and that of the sedimentary succession of an area aimed to be the first Spanish pilot plant of CO<sub>2</sub> injection. An integration of coincident 3D seismic results, borehole data and unpublished microgravity 5 maps aims to reproduce the deep structure of the basement and to quantify the thickness of the Triassic Keuper evaporites. All datasets manage to clearly identify two main fault systems compartmentalizing the main structural domains into three differentiated blocks. These have been interpreted to be reactivated normal faults that have led to the formation of 10 the Hontomín dome. The general structure is characterized by a half-graben setting filled with thick Keuper evaporites (up to 2000 m thick) forming an extensional forced fold.

Tectono‐sedimentary development of early syn‐rift deposits: the Abura Graben, Suez Rift, Egypt

AbstractThe thickness and distribution of early syn‐rift deposits record the evolution of structures accommodating the earliest phases of continental extension. However, our understanding of the detailed tectono‐sedimentary evolution of these deposits is poor, because in the subsurface, they are often deeply buried and below seismic resolution and sparsely sampled by borehole data. Furthermore, early syn‐rift deposits are typically poorly exposed in the field, being buried beneath thick, late syn‐rift and post‐rift deposits. To improve our understanding of the tectono‐sedimentary development of early syn‐rift strata during the initial stages of rifting, we examined quasi‐3D exposures in the Abura Graben, Suez Rift, Egypt. During the earliest stage of extension, forced folding above blind normal fault segments, rather than half‐graben formation adjacent to surface‐breaking faults, controlled rift physiography, accommodation development and the stratigraphic architecture of non‐marine, early syn‐rift deposits. Fluvial systems incised into underlying pre‐rift deposits and were structurally focused in the axis of the embryonic depocentre, which, at this time, was characterized by a fold‐bound syncline rather than a fault‐bound half graben. During this earliest phase of extension, sediment was sourced from the rift shoulder some 3 km to the NE of the depocentre, rather than from the crests of the flanking, intra‐basin extensional forced folds. Fault‐driven subsidence, perhaps augmented by a eustatic sea‐level rise, resulted in basin deepening and the deposition of a series of fluvial‐dominated mouth bars, which, like the preceding fluvial systems, were structurally pinned within the axis of the growing depocentre, which was still bound by extensional forced folds rather than faults. The extensional forced folds were eventually locally breached by surface‐breaking faults, resulting in the establishment of a half graben, basin deepening and the deposition of shallow marine sandstone and fan‐delta conglomerates. Because growth folding and faulting were coeval along‐strike, syn‐rift stratal units deposited at this time show a highly variable along‐strike stratigraphic architecture, locally thinning towards the growth fold but, only a few kilometres along‐strike, thickening towards the surface‐breaking fault. Despite displaying the classic early syn‐rift stratigraphic motif recording net upward‐deepening, extensional forced folding rather than surface faulting played a key role in controlling basin physiography, accommodation development, and syn‐rift stratal architecture and facies development during the early stages of extension. This structural and stratigraphic observations required to make this interpretation are relatively subtle and may go unrecognized in low‐resolution subsurface data sets.

Early synrift reservoir development on the flanks of extensional forced folds: A seismic-scale outcrop analog from the Hadahid fault system, Suez rift, Egypt

Forced folds typically develop above the tips of propagating normal faults in rifts that contain thick, prerift salt or mudstone sequences. This structural style is associated with the deposition of wedge-shaped synrift deposits that thin and onlap toward monoclinal growth folds overlying the vertically restricted fault tips. Subtle stratigraphic traps may develop on the flanks of these folds although, because of limited seismic resolution and sparse well data, the architecture, thickness, and distribution of these early synrift reservoirs are difficult to predict. To improve our understanding of early synrift reservoir development on the flanks of forced folds, we focus on seismic-scale outcrop analogs along the Hadahid fault system, Suez rift, Egypt. Our data indicate that forced folding dominated during early rifting and that the onset of folding was diachronous along strike. Fluvial systems incised the rotating monocline limbs, leading to the formation of valley-like erosional relief along the base synrift unconformity. Reservoir-prone fluvial facies are only locally developed along the forced-fold flank, with their distribution related to the degree of sediment bypass downdip into the adjacent basin. Early synrift relief not filled by fluvial strata was backfilled by transgressive, tidally influenced, reservoir-prone facies, with carbonates being locally developed in areas of low clastic sediment supply. Further extension and fault-tip propagation led to amplification of the forced folds, and deposition of shallow marine-to-shelf parasequences that became thinner toward the growing folds. Although displaying greater strike continuity than the underlying fluvial or tidal reservoirs, shoreface sandstone reservoirs amalgamate onto the flanks of the forced folds and may be absent toward the fold crest. This seismic-scale outcrop analog helps us better understand the subseismic stratigraphic architecture and facies distributions of early synrift reservoirs on the flanks of extensional forced folds. Observations from this and other well-exposed outcrop analogs should help reduce subsurface uncertainty and risk when exploring for hitherto under-explored, subtle, early synrift stratigraphic traps.

Early synrift reservoir development on the flanks of extensional forced folds: A seismic-scale outcrop analog from the Hadahid fault system, Suez rift, Egypt

Early synrift reservoir development on the flanks of extensional forced folds: A seismic-scale outcrop analog from the Hadahid fault system, Suez rift, Egypt

Transpressive inversion of a Mesozoic extensional forced fold system with an intermediate décollement level in the Basque‐Cantabrian Basin (Spain)

AbstractIn the Basque‐Cantabrian Basin (Spain), normal faulting and associated folding occurred during Late Jurassic to Early Cretaceous rifting. Cenozoic Pyrenean thick‐skinned transpressive inversion in the western parts of the basin preserved the first‐order extensional architecture. Integration of geological maps and seismic profiles has permitted to fully constrain the style of extensional deformation and subsequent inversion in the western portion of the Basque‐Cantabrian Basin. Extensional faults offset the Paleozoic basement up to Lower Triassic rocks. The presence of an efficient décollement level represented by Triassic evaporites produced the decoupling between basement rocks and the Upper Triassic to Middle Jurassic prerift cover sequence. Extensional forced folding occurred in the cover, driven by basement faulting and the migration of evaporites toward the hanging wall of the extensional faults, with salt welds developing away from them. Upper Jurassic to Lower Cretaceous syn‐rift sediments deposited synchronously with forced folding, which led to the development of extensional growth geometries associated with both master faults and nearly‐transverse faults. Syn‐rift growth sequences are characterized by downlap and onlap relationships with the underlying prerift units, interpreted as the result of along‐strike variations of master fault extensional displacement rate. Cenozoic Pyrenean contraction generated the right‐lateral transpressive inversion of basement master faults and the almost dip‐slip reactivation of transverse extensional faults.

Determining the uniqueness of best-fit trishear models

We show the application of a simulated annealing algorithm to trishear inverse modeling. The algorithm traverses the parameter space in search for best-fit models without being trapped in local minima, and thus sampling for more possible solutions globally. Simulated annealing is a robust and efficient technique to determine the uniqueness of best-fit trishear models; the spread of possible trishear models that can fit a structure. We first apply the algorithm to a decameter-size, contractional fault-propagation fold in west-central Taiwan, for which there is an exceptional exposure of pre-growth and growth strata. Simulated annealing shows that even for this complete fold dataset with low uncertainties, there is a range of models and fault slip/uplift histories that can fit the data, with the consequent implications for the assessment of seismic hazard. We then apply the algorithm to a kilometer-size, extensional fault-propagation fold, the Hadahid monocline, Gulf of Suez Rift, Egypt. In this monocline there is only surface coverage in the footwall anticline areas and the algorithm was used to delimit the range of possible models that can fit the data and their uncertainties, thus avoiding biases in the interpretation. Simulated annealing suggests that the along-strike structural variability of the monocline can result from along-strike variability in fault slip, fault propagation to fault slip ratio and depth of fault nucleation. Both examples illustrate the benefits of searching for a possible range of models rather than a precise best-fit model when modeling fault-propagation folds. In an attempt to understand which parameters control fault development, and also how the spread of possible solutions varies with fold growth, we apply the algorithm to four sequential stages of a published, analog clay model of an extensional forced fold. The inversions of the natural examples and the analog model suggest that the spread of the possible models is a manifestation of the data uncertainties, the suitability of the trishear model, fold evolution, and rock mechanical properties.

Deformation of the northeastern Venezuelan Andes. Relationships with the Caribbean overthrusts

The Merida Andes (Venezuela) formed in the middle Miocene due to oblique convergence between the South American plate and the Maracaibo block [Audemard et al., 2002] (figs. 1A and 1B). The study area corresponds to the so-called "Barbacoas platform" [Renz, 1960], which constitutes the northeastern termination of the belt, NE of Valera (fig. 2). It is located in the northeastern part of Trujillo block [Hervouet et al., 2001], considered as an independent block separated from the main Maracaibo block along the Valera fault. According to Stephan [1982], the N170°E-trending Caribbean compression developed in this area from late Cretaceous to Eocene. It was followed by a N105°E-trending compression older than middle Miocene, and finally by the NW-SE Andean compressional stage that lasted till now in most of the chain. However, east of El Empedrado fault, a NNE-SSW compression presently occurs that is oblique to the classical Andean stage. The tectonic evolution of the Andean stage is not well understood. The Merida Andes are mainly composed of Precambrian and Paleozoic rocks. The northern part of the belt only comprises a complete and continuous Jurassic to Paleogene cover. This lithologic pattern is probably a consequence of the tectonic escape of the Maracaibo block, and more particularly of the smaller Trujillo block. The Merida and Carribean belts being close to each other, the influence of the Andean deformation on the Caribbean allochthonous must be taken into account. In order to make a structural analysis at regional scale, we privileged the use of remote sensing data (Landsat, Spot and Radar images) and aerial photographs. This was complemented by structural data obtained in the field, allowing the study of geometric and chronological relationships between the tectonic structures.

Jurassic tectono-sedimentary evolution of the Northern Lusitanian Basin (offshore Portugal)

The Lusitanian Basin is one of a series of rift-related basins developed during the rifting that led to the break-up of Iberia and Canada. The strong influence of halokinesis on the depositional evolution of its northern sector provides an analogue for the development of other evaporite-prone rift basins on Atlantic-type passive margins. In this paper, the pre-Valanginian evolution of the Northern Lusitanian Basin is documented based on regional seismic stratigraphy, isochron and time-structure maps tied with sonic and stratigraphic data from 10 exploration wells. The pre-salt structure of the study area is marked by two sets of faults: (i) a N–S to NE–SW striking set active throughout the Jurassic period and (ii) a NW–SE striking set active during the Late Jurassic. The latter defines five Late Jurassic sub-basins northeast of a basin-margin structure, the Marinha-Grande Fault. The development of half-graben basins in the Northern Lusitanian Basin during the Jurassic was overprinted by halokinesis over the Marinha-Grande Fault and other major marginal basement faults. Halokinesis climaxed during the Oxfordian–Kimmeridgian rifting with the formation of a 10 km long salt ridge over the Marinha-Grande Fault. The ridge limited the southward progradation of fluvial/deltaic units derived from hanging-wall drainage systems and restricted the syn-rift sediments to a 15 km long×5 km wide bowl-shaped depocentre located 10 km west of the present coastline. In order to better characterise the Jurassic tectono-sedimentary evolution of the Northern Lusitanian Basin, the interpreted seismic data is compared with physical models for extensional forced folds above active normal faults and with stratigraphic information from well and outcrop.

An experimental study of the secondary deformation produced by oblique-slip normal faulting

We have used scaled clay models to define the secondary deformation produced by oblique-slip normal faulting. In the models, the master fault beneath the clay layer dips 45 degrees and strikes at a 45 degrees angle relative to the heave direction (i.e., the horizontal component of the displacement direction). Thus, the master fault has both normal dip-slip and strike-slip components of displacement. The modeling results show the following. (1) The fault patterns produced by oblique-slip normal faulting vary significantly with depth. Secondary faults that strike obliquely to the master-fault trend are more abundant near the top of the clay layer, whereas secondary faults that are subparallel to the master-fault trend are more abundant at depth. (2) A single episode of oblique-slip normal faulting produces two populations of secondary faults that have different trends and ages. Secondary faults that strike obliquely to the master-fault trend are more abundant during the early stages of the experiments, whereas secondary faults that strike subparallel to the master-fault trend are more abundant during the later stages of the experiments. (3) Relay ramps between overlapping secondary synthetic normal faults are wide and temporally persistent in oblique-slip models. The ramps are cut by numerous small-scale normal faults that are subparallel to the ramp-bounding faults. Cross faults are uncommon and begin to develop only during the final stages of the experiments. (4) Both map and cross section data are necessary to distinguish among the deformation patterns produced by strike-slip, oblique-slip, and dip-slip faulting. The map views of oblique-slip models closely resemble those of strike-slip models; in both models, many of the secondary faults strike obliquely to the master-fault trend. These map views, however, differ considerably from those of dip-slip models, in which most of the secondary faults strike subparallel to the master-fault trend. Alternatively, the cross sectional views of oblique-slip models are similar to those of dip-slip models; in both models, a highly faulted extensional forced fold develops. These cross sectional views are dissimilar to those of strike-slip models, which show no appreciable folding and no change of regional level.

Active Normal Faulting Beneath a Salt Layer: An Experimental Study of Deformation Patterns in the Cover Sequence

Scaled experimental models show that the presence of a viscous layer, such as salt, facilitates the development of extensional forced folds above active normal faults. The geometries of the extensional forced folds and their associated secondary fault patterns depend on the thickness and viscosity of the viscous layer, the thickness of the cover sequence, the strength and ductility of the cover sequence, and the magnitude and rate of displacement on the underlying master normal fault. Increasing the thickness of the viscous layer and the cohesive strength and ductility of the overburden enhances the decoupling between the deep and shallow deformation. Alternatively, increasing the viscosity of the viscous layer, the thickness of the overburden, and the magnitude and rate of displacement on the master normal fault reduces the decoupling between the deep and shallow deformation. Enhanced decoupling facilitates the formation of broad extensional forced folds and the development of detached secondary faults both near and far from the master normal faults. The model-predicted deformation patterns closely resemble those observed in the Gulf of Suez, the Haltenbanken area of offshore Norway, and the Jeanne d'Arc Basin of the Grand Banks, offshore southeastern Canada.

Experimental models of extensional forced folds : Withjack, M O; Olson, J; Peterson, E Bull Am Soc Petrol GeolV74, N7, July 1990, P1038–1054

Experimental models of extensional forced folds : Withjack, M O; Olson, J; Peterson, E Bull Am Soc Petrol GeolV74, N7, July 1990, P1038–1054

Experimental Models of Extensional Forced Folds (1)

We have used single-layer and multilayer clay models to study the development of forced folds above normal faults. Our modeling results show that the deformation patterns associated with extensional forced folding depend on the dip of the underlying normal fault and the presence of layer-parallel detachments. In single-layer clay models, extensional forced folds are upward-widening monoclines. Anticlinal axial surfaces dip in the same direction as underlying master normal faults, and synclinal axial surfaces dip in the opposite direction of master normal faults. Most secondary faults are upward-steepening normal faults. If master normal faults are steeply dipping, however, many secondary normal faults become high-angle reverse faults at shallow depths. The propagation and linkage of secondary faults into through-going normal faults terminates the development of extensional forced folds. More folding occurs prior to fault linkage if the master normal fault is steeply dipping rather than gently dipping. Most dipping beds and secondary faults are preserved in the hanging walls of the through- oing normal faults. In multilayer clay models with layer-parallel detachments, extensional forced folds are also upward-widening monoclines. Slip on the lowest detachment laterally transfers extension induced by normal faulting and forced folding from the master normal fault to the detachment edge. Slip on overlying detachments accommodates minor thickness changes associated with upward-widening of the fold. Secondary faults include low-angle normal faults near the anticlinal axial surface, minor thrust faults near the synclinal axial surface, and high-angle normal faults above the detachment edge. The model-predicted deformation patterns are similar to those of extensional forced folds from the Gulf of Suez and offshore Norway. This similarity suggests that our modeling results apply to extensional forced folds and can provide guidelines for interpreting field, well, and seismic data.

Experimental and Analytical Models of Extensional Forced Folds: Applications to Gulf of Suez and Offshore Norway: ABSTRACT

The authors have studied the structural development of extensional forced folds (i.e., folds that form above normal faults) using single-layer and multilayer clay models and analytical models of dislocations in an elastic half-space. The single-layer clay models and dislocation models show that extensional forced folds are narrow, monoclinal flexures with steep limb dips near the tip of the underlying master normal fault. Folds widths increase and limb dips decrease upward. Secondary faults initiate at the tip of the master normal fault, are subparallel to it, and curve 25/degree/-35/degree/ into its hanging wall as they propagate upward. Consequently, secondary fault patterns depend on the dip of the master normal fault. If the dip is less than 60/degree/, secondary normal faults near the tip of the master normal fault become high-angle reverse faults at shallow depths. The propagation and eventual linkage of these secondary faults terminates the development of extensional forced folds. Less folding occurs prior to fault linkage if the master normal fault is gently rather than steeply dipping. In multilayer clay models that allow interlayer slip, extensional forced folds are wider and limb dips are gentler than those in the single-layer models. These modeling results provide guidelines for interpreting seismic,more » well, and field data from extensional forced folds in the Gulf of Suez and offshore Norway.« less