Salt Dome Research Articles

Evidence of new diapiric structures in the southern Adria Plate (Eastern Margin of Tethyan Ocean): Implications for Triassic paleogeography and evaporites remobilization during subduction/collision (Northern Ionian Sea, Central Mediterranean)

It has long been recognized that the Adria Plate, as well as the Dinaric-Hellenic sector, contains a large volume of Triassic salt associated with diapirism, and it is well known that Triassic evaporites in the Mediterranean Sea region were deposited over epicratonic platforms surrounding the Mesozoic passive margin of Tethys Ocean. However, the real extent of these evaporites is still unknown; especially in the deep-water offshore area where publicly available seismic reflection profiles are scarce and petroleum exploration wells are missing. Several authors highlighted the presence of Triassic salt structures, mostly triggered by Meso-Cenozoic compressions and associated with fold-and-thrust belts. Both the lack of data and the later compressions contribute to hiding how a diapiric structure, formed in a passive margin and characterized by dome's geometry, is remoulded to assume a teardrop geometry typical of diapirs in fold-and-thrust belts. Two end members of diapiric structures are discussed: extensional structures formed in a passive margin, and compressional structures formed in a fold-thrust belt. The transformation of one end-member into the other is not well known. In the Northern Ionian Sea, the Apulian foreland, belonging to the southernmost termination of the Adria Plate and considered one of the largest preserved Mesozoic passive margin block of the Tethys Ocean, is a deep-water area associated with the opposing fronts of the nearby Calabrian Arc/Apennines and Dinaric-Hellenic Chains since the Cretaceous. Therefore, it represents an ideal area for investigating both the real extent of Triassic evaporites and the transformation of the diapiric structures under different regional geodynamic contexts.Based on seismic reflection profiles but no petroleum exploration wells, we present evidence of Triassic evaporites in the deep-water portion of the Apulian foreland, as indicated by the interpretation of two diapiric structures never described previously. These two new diapirs likely formed from inherited Mesozoic salt structures such as pillows and/or salt domes. The identification of halokinetic-related sequences affecting the Plio-Quaternary foreland sediments allows constraining the polyphaser evolution of the two diapirs. We suggest they are reactivated in response to the compressive stresses transmitted by the nearby Calabrian Arc/Southern Apennines and Hellenides. But, after the Middle Pleistocene, extensional tectonics, related to Adria Plate flexuring, acted again, and the diapirs were cross-cut by normal faults. These observations and findings are integrated into the geological context that controls the fundamental features of the diapirs, providing new insights on the fate of diapiric structures formed in a passive margin, later involved in a subducting and colliding plate, and before they become part of a fold-and-thrust belt. Furthermore, the study provides new constraints on the Triassic paleogeographic reconstruction of the Northern Ionian and Central Mediterranean Seas at the continental margin of the Tethys Ocean.

Geometry and kinematics of the Middle to Late Miocene salt tectonics, central Egyptian Red Sea margin

The Red Sea basin includes a thick Middle to Late Miocene evaporitic succession that underwent halokinesis and caused intensive reshaping of the seafloor and the development of salt-tectonic structures. However, the geometry and kinematics of these structures are still poorly understood. This study uses 2D and 3D seismic surveys and well data of the northern Egyptian Red Sea to systematically describe the distribution and morphology of salt structures, discuss their initiation, and construct a kinematic model for their origin. Our results indicate that the massive salt layer developed into five major NW-SE to NNE-SSW trending salt walls, characterized by relatively irregular crests and moderately dipping flanks. In addition, several symmetrical and asymmetrical folds and two categories of normal faults (subsalt and suprasalt) have been recognized. Based on our observations, salt mobilization in the study area started in the Late Miocene, during the precipitation of layered evaporites, and continued until the present day. In the northern Egyptian Red Sea, seismic interpretation indicates that halokinesis was triggered by a combination of thin- and thick-skinned systems, where the latter played a major role. The salt layer was welded during the Quaternary as several sags and grabens developed above the salt diapirs. Thick-skinned physical models are compatible with our observations, supporting the impact of basement faulting on Red Sea diapirism.

The Quaternary activity of the Estella diapir from the uplift record of fluvial terraces, pediments and cave sediments in the Western Pyrenees, Spain

This work analyses the Quaternary landforms, geomorphic changes and stratigraphic evidences of evaporite dissolution and halokinesis in the Estella salt diapir (Basque-Cantabrian Basin). The mapping of Quaternary deposits demonstrates that karstic subsidence and diapirism happen at the same time. The diapir geometry is characterised by a prominent 4.5 km-long fault scarp attributed to the uplift of the diapir and a fluvial gypsum escarpment along the northern and eastern diapir edges. The dissolution of the gypsiferous caprock is responsible for the development of a large number of collapse sinkholes, the local thickening of terrace deposits and a network of endokarstic conduits connected to saline springs that cause a notable increase in the salinity of the Ega river across the diapir (70% electrical conductivity increase). The 40 m vertical displacement of T4 terrace and P5 pediment deposits, the anomalous longitudinal profile of the old terraces, the deflection of infilled-valleys at the base of the fault scarp, the occurrence of wind gaps and the continuous lateral migration of the Ega river away from the diapir support salt upwelling since 644 ± 81 ka ago at a long-term, average uplift rate of 0.05–0.07 mm/yr. The 12 m offset of T8 terrace, the steepening of the Ega river floodplain and the tilting of the upper level and the antigravitative erosion genesis of the Longinos cave point to a faster diapir rising rate of 0.23–0.57 mm/yr in the last 36 ± 15 ka coinciding with a period of increasing river incision. The lack of tectonic activity in the region suggests that diapirism is enhanced by erosional unloading related to fluvial entrenchment. The thickening of pediment P5 supports the north-northeast flow of salt towards the river valley.

Re-examination of the Aïn Allegua-Aguiba celestite-Pb-Zn salt diapir-related Mississippi Valley-type deposits (Nappes zone, Northwestern Tunisia): New conceptual model

The celestite-rich and Pb-Zn-poor Aïn Allegua deposits are located at the northeastern extremity of the Dougas diapir in the Nappes zone of northern Tunisia. The Sr-Ba-Zn-Pb mineral system studied in this paper is enclosed in a Triassic breccia, labeled cap rock, located between the Triassic diapir and the surrounding Cretaceous to Lutetian marine sediments. It consists of evaporite dissolution float breccia where Triassic dolostone clasts are floating in a Triassic matrix of anhydrite and argillites. Five mineralizing stages are recognized, which are from early to late: celestite (stage 1); hydrothermal dolomite (stage 2); crosscutting galena-barite or sphalerite-galena-barite-calcite (stage 3); (4) fracture and karst filled by barite, celestite, and galena (stage 4); and late supergene mineralization (strontianite, smithsonite, and cerussite) (stage 5). The present study shows that the celestite and Pb-Zn mineralization formed from the circulation in the cap rock of at least three fluids: (1) fluid A (100–150 °C; 10 to 25 wt% NaCl eq.) deposited the celestite; (2) fluid B (125 to 185 °C; 29 to 34 wt% NaCl eq.) deposited the hydrothermal dolomite, and (3) fluid C (temperatures around 90 °C; 14 to 22 wt% NaCl eq.) deposited the calcite that accompanied sphalerite and galena precipitation. The δ34S values for sphalerite and galena fall within a narrow range from + 10.4‰ to + 18.9‰ (avr. 13‰) indicating a homogeneous source of sulfur. The δ34S values of celestite and barite are also relatively homogeneous from + 14.7 to + 23‰ (ave. + 16.5‰) and match those of Tunisian Triassic gypsum and Triassic marine sulfates (δ34S = +15‰). The temperature of deposition together with sulfur isotope data indicate that the reduced sulfur in sulfides was derived through thermochemical sulfate reduction of Triassic sulfate. The lead isotopes in galena show that Pb, and probably Zn and Ba were extracted from the upper crustal rocks. Part of the strontium is derived from the Triassic anhydrite and the other part is probably transported to the cap rock in formation waters. The tectonic setting, the carbonated nature of the host rocks, the epigenetic style of the mineralization, and the mineral associations, together with fluid inclusions data and sulfur and lead isotopes show that the Aïn Allegua deposit belongs to the diapir-related Mississippi Valley-type (MVT) deposits of the Eastern Maghreb salt diapirs province, that formed, as the most of the MVT deposits and districts in the world, from basinal brines with no mixing with metamorphic and/or magmatic fluid or involvement of a heat source from a concealed sill or magmatic intrusion.

AHP algorithm used to select suitable abandoned underground mines for energy storage infrastructure – iCAES technology. A specific case study for León (Spain)

In the energy transition, the promotion of renewable sources entails the development of storage technologies to manage the mismatch between energy production and demand. In this scenario, the use of CAES (Compressed Air Energy Storage) technology enables the efficient and cost-effective storage of large amounts of energy. However, this technology is developed in salt domes who have an inherent risk associated of underground exploration phase. To address this, we propose to develop an infrastructure (iCAES) in abandoned underground mines, where the exploration phase is completed and well known. For its implementation, this paper defines a structure hierarchization method gathers the technical and socio-economic criteria. It involves a multi-criteria problem, and the correct selection of the location must be based on specific mathematical algorithms. For this case the Analytic Hierarchy Process (AHP) from multi-criteria decision making (MCDM) methods allows quantified by means of a scientific and mathematical scale and the assignment of weights, so that it is possible to evaluate different alternatives. This is possible thanks to the application of the AHP model in absolute terms. The information gathering has been based on the specific case study of coal basins in the north of Spain, in the region of León. Considering the proposed methodology, the most suitable alternative locations to implement iCAES in the region of León were identified.

Assessment of prokaryotic communities in Southwestern Atlantic deep-sea sediments reveals prevalent methanol-oxidising Methylomirabilales

Continental slopes can play a significant contribution to marine productivity and carbon cycling. These regions can harbour distinct geological features, such as salt diapirs and pockmarks, in which their depressions may serve as natural sediment traps where different compounds can accumulate. We investigated the prokaryotic communities in surface (0–2 cm) and subsurface (18–20 or 22–24 cm) sediments from a salt diapir and pockmark field in Santos Basin, Southwest Atlantic Ocean. Metabarcoding of 16 samples revealed that surface sediments were dominated by the archaeal class Nitrososphaeria, while the bacterial class Dehalococcoidia was the most prevalent in subsurface samples. Sediment strata were found to be a significant factor explaining 27% of the variability in community composition. However, no significant difference was observed among geomorphological features. We also performed a metagenomic analysis of three surface samples and analysed the highest quality metagenome-assembled genome retrieved, which belonged to the family CSP1–5, phylum Methylomirabilota. This non-methanotrophic methylotroph contains genes encoding for methanol oxidation and Calvin Cycle pathways, along with diverse functions that may contribute to its adaptation to deep-sea habitats and to oscillating environmental conditions. By integrating metabarcoding and metagenomic approaches, we reported that CSP1–5 is prevalent in the sediment samples from Santos Basin slope, indicating the potential importance of methanol metabolism in this region. Finally, using a phylogenetic approach integrating 16S rRNA sequences assigned to Methylomirabilota in this study with those from a public database, we argued that CSP1–5 public sequences might be misclassified as Methylomirabilaceae (the methanotrophic clade) and, therefore, the role of these organisms and the methanol cycling could also be neglected in other environments.

Emplacement and associated sedimentary record of the Jurassic submarine salt allochthon of the Wurzeralm (Eastern Alps, Austria).

A fossil salt sheet emplaced in the Jurassic in submarine conditions is described in the Eastern Alps of Austria, providing unique insights into the emplacement of similar submarine structures and their potential control on depositional systems. The salt sheet is a plug-fed extrusion emplaced due to squeezing of a salt diapir under compression. The preserved mylonitic shear fabric in the evaporites indicates radial, south-directed emplacement of the salt sheet. Tectono-sedimentary relationships record the evolution of the salt structure, from initial diapiric growth, to salt sheet extrusion and posterior collapse. Syn-extrusion sediments record the variable bathymetry of the extruding salt sheet, with reefal carbonates building up on the crestal bulge while their deeper water equivalents accumulated on the extruding salt lobe. This is the first description of a salt allochthon still linked to its source diapir in the Eastern Alps.

Palynology of the Triassic–Jurassic transition of the Danish Basin (Denmark): a palynostratigraphic zonation of the Gassum–lower Fjerritslev formations

The Upper Triassic–Lower Jurassic succession in the Danish Basin is penetrated by many deep wells that were drilled during former hydrocarbon exploration campaigns, but it is today targeted for geothermal energy and storage of CO2. The Stenlille salt dome on Sjælland sandstones of the Gassum Formation, sealed by the overlying Fjerritslev Formation mudstones, has been used for decades as a seasonal storage for natural gas. With its comprehensive dataset of seismics, geophysical well logs and conventional core data from 20 wells, the Stenlille succession serves as a model for other salt domes currently being evaluated as potential CO2 storage sites in the basin. Over the last decade the cored Triassic–Jurassic boundary succession has contributed to the understanding of environmental and palynological events during the end-Triassic mass extinction. Core, sidewall core and cutting samples from several of the closely situated Stenlille wells are here used to establish a high-resolution palynostratigraphic zonation scheme covering the entire Rhaetian to Sinemurian succession by integrating new analyses with previously published data. The palynological dataset has allowed the recognition of nine formally described spore-pollen zones, of which eight are new, while two previously described dinoflagellate cyst zones are subdivided into three informal subzones each. The palynological zonation is integrated with a sequence stratigraphic framework and will form the basis for the dating of future well sections in the Danish Basin and other basins and for correlation to outcrops. The large palynological dataset further shows that the vegetation around the Danish Basin was remarkably stable during the early to middle Rhaetian, but that events related to the emplacement of the Central Atlantic Magmatic Province accelerated ecosystem changes for c. 175 ky in the late Rhaetian and earliest Hettangian, including ∼25 ky of successional recovery before the terrestrial ecosystem had again stabilised.

Comment on “Genesis of Zn‑Pb‑(Ba‑Sr) mineralization in the peridiapiric cover of Jebel El Akhouat, Ech Chehid salt dome, Northern Tunisia” (2022) by N. Jemmali, L. Rddad, M. Sośnicka, E. Rahali, F. Souissi and E. J. Carranza

Comment on “Genesis of Zn‑Pb‑(Ba‑Sr) mineralization in the peridiapiric cover of Jebel El Akhouat, Ech Chehid salt dome, Northern Tunisia” (2022) by N. Jemmali, L. Rddad, M. Sośnicka, E. Rahali, F. Souissi and E. J. Carranza

High‐Resolution Local Earthquake Tomography Beneath the Zagros Simply Folded Belt (SFB): Implications for an Inhomogeneous Low‐Velocity Layer and Diapirism in the Upper Crust

AbstractHormuz salt formation is considered the origin of the evaporated salt deposits in the Zagros and its ductile behavior has been known as the reason for the inhomogeneous deformation in the Zagros. However, our knowledge about this formation has been limited to the salt domes on the surface. In our study, local earthquakes recorded by a temporary dense seismological network of 17 stations, deployed in the southern margin of the Zagros Simply Folded Belt (SFB) in southwestern Iran, have been used for seismic imaging. The high‐resolution earthquake local tomography revealed the first geophysical evidence about the Hormuz salt layer and its extension at depth. There is an uneven layer located at a depth of 8–12 km as the origin of extruded salt in this region through a shear fault zone and the production of the Dashti Salt Dome at the Kuh‐e‐Namak Mountain. Based on the obtained results, this layer is characterized by low seismic velocity volumes with significant Vp/Vs ratio variations. The seismic image is also consistent with a major NW‐trending NE‐dipping reverse fault, probably responsible for the 9 April 2013 Kaki earthquake. At depth of around 4 km, smaller scale high velocity anomalies, characterized by a high Vp/Vs ratio, may be related to the fluid saturated sediments in the uppermost sedimentary layer.

APPLICATION OF MUMMIFIED TURK SALTY FIBER CONCRETE AS PROTECTION AGAINST SALTY DOMES AND DESERT INTRUSION

Turk salty fiber concrete is designed to isolate retaining walls against saline materials. In 2018, TSFC mixture design was applied to the salt domes in the GOTVAND reservoir dam, SE of Iran in order to isolate them enormous salt ridges were discovered inside the dam reservoir before construction operations began in 2005 which are also identifiable through the use of geotechnical maps. The designed mixture of TSFC contains salt stone (S), water (W), cement type II (CII), bentonite (B), clay (C) and cotton or synthetic fibers (F). In the laboratory, TSFC is produced taking into account the variable of saline water. The figure ζ is defined by the proportion of the weight of the salt and water over the weight of the combined mixture of cement, bentonite, clay and fibers. The maximum value ζ has been obtained as being a bit over 6.5 for laboratory samples. It means that more than 87% of the sample weight is related to saline water and 13% is related to other materials (CII, B, C and F). The axial load test is the main index to compare resistance with friction of salt formations. The results obtained show the axial resistance of broken TSFC samples to be more 12,000 KPA. SEM images were taken to study TSFC adhesiveness between the salt and fibers. The SEM showed cubic crystallization which in itself supports the hypothesis of salty mummification due to the integrating of the salt concrete and fibers.

Comprehensive numerical modeling of gas storage in Firouzabad salt dome based on multistage creep test and operational parameters

Comprehensive numerical modeling of gas storage in Firouzabad salt dome based on multistage creep test and operational parameters

Static Reservoir Simulations and Seismic Attributes Application to Image the Miocene Deep-Water Reservoirs in Southeast Asia

Globally, deep-water reservoir systems are comprised of a variety of traps. Lateral and downdip trapping features include sand pinch-outs, truncation against salt or shale diapirs, and monoclinal dip or faulting with any combination of trapping designs; the potential for massive hydrocarbon accumulations exists, representing significant exploration prospects across the planet. However, deep-water turbidites and submarine fans are two different types of traps, which are developed along the upslope and the basin floor fans. Among these two traps, the basin floor fans are the most prolific traps as they are not influenced by sea-level rise, which distorts the seismic signals, and hence provides ambiguous seismic signatures to predict them as hydrocarbon-bearing zones for future explorations. Therefore, the deep-water channel-levee sand systems and basin floor fans sandstone define economically viable stratigraphic plays. The subsurface variability is significant, and hence, characterizing the thick (porous) channelized-basin floor fans reservoir is a challenge for the exploitation of hydrocarbons. This study aims to develop seismic-based attributes and wedge modeling tools to accurately resolve and characterize the porous and gas-bearing reservoirs using high-resolution seismic-based profiles, in SW Pakistan. The reflection strength slices better delineate the geomorphology of sand-filled channelized-basin floor fans as compared to the instant frequency magnitudes. This stratigraphic prospect has an area of 1180 km2. The sweetness magnitudes predict the thickness of channelized-basin floor fans as 33 m, faults, and porous lithofacies that complete a vital petroleum system. The wedge modeling also acts as a direct hydrocarbon indicator (DHI) and, hence, should be incorporated into conventional stratigraphic exploration schemes for de-risking stratigraphic prospects. The wedge model resolves a 26-m thick hydrocarbon-bearing channelized-basin floor fans lens with a lateral distribution of ~64 km. Therefore, this wedge model provides ~75% correlation of the thickness of the LSL as measured by sweetness magnitudes. The thickness of shale that serves as the top seal is 930 m, the lateral mud-filled canyons are 1190 m, and the thick bottom seal is ~10 m, which provides evidence for the presence of a vibrant petroleum play. Hence, their reveals bright opportunities to exploit the economically vibrant stratigraphic scheme inside the OIB and other similar global depositional systems.

Pre-Stack Imaging for the Messinian Sub Salt Sequences in the Levantine Basin of the East Mediterranean: A Case Study, Offshore Lebanon

The Mediterranean Sea has a progressive geological significance; it is considered a vast extensive promising basin for gas exploration. Herodotus and Levantine basins are the main basins in the eastern Mediterranean basin, which are similar in their evolution stages and accommodate similar sedimentary sequences. The recent geophysical and petrophysical studies proved that Herodotus and Levantine basins are promising for gas with possible great reserves exceeding 2,000 bcm. Recent discoveries and increased exploration activities including seismic acquisition and offshore drilling made the Eastern Mediterranean a hot area for gas exploration, especially after the discovery of Zohr gas Field in 2015, offshore Egypt. However, the regional extension of the complex Messinian salt beds and domes make the exploration of high risk. The subsalt seismic imaging in a time domain and the resolution of the underlying carbonate/clastic reservoirs' primary reflections are a big challenge in this area. Therefore, the main objective of the present study is to improve sub-salt imaging with much more accurate interval velocity estimation. In the study area, offshore Lebanon, the delineated structures in the sub-salt imaging after the Pre-stack Depth Imaging were more significant compared to the Pre-Stack Time Imaging which provided optimum sub-salt imaging and enabled the accurate interpretation of the main geologic features which may increase the certainty level of the hydrocarbon exploration and discoveries.

Geochemical characteristics and oil–oil correlation of the upper Cretaceous oils from the Iranian part of the Persian Gulf Basin

The Cenomanian Sarvak oil reservoirs are distributed over large areas of the Persian Gulf basin. The purpose of this study is analyzing the geochemical characteristics of the Sarvak oil reservoirs and their inter-relationships in the Persian Gulf, classification of the Sarvak oil samples and investigation of the possible causes of the genetic difference in oil families. In the previous studies, limited samples of Sarvak oil reservoir in scarce oilfields were studied individually and local interpretations are made accordingly. The current study employs a more complete set of geochemical from the Iranian part of Persian Gulf and regional interpretations are drawn. To achieve this goal, the geochemical data of 41 oil samples from 10 oilfields were collected and assessed based on gas chromatography (GC), gas chromatography–mass spectrometry (GC–MS), and stable carbon isotope analysis. It was demonstrated with the evaluations that the oils accumulated in the Upper Cretaceous Sarvak reservoirs in the Persian Gulf basin originate from different source rocks. The oil samples are genetically classified into four oil families based on the similarities and differences of parameters related to the depositional environment-dependent parameters using hierarchical cluster analysis (HCA), star diagram, and stable carbon isotope diagram. The source rocks were mainly deposited in anoxic marine carbonate environments. The thermal maturity of the oils was evaluated using steranes and trisnorhopanes. Oil families 2 and 4 (located at the center of the Persian Gulf) have the highest thermal maturity compared to the other samples; in contrast, oil family 1 (located at the west of the Persian Gulf) has the lowest thermal maturity. The possible source rocks of oil family 1 and family 3 (located at eastern Persian Gulf) using C28/C29 steranes are the upper Cretaceous successions; in contrast, the possible source rocks of oil families 2 and 4 are the upper Triassic and Lower Jurassic formations. Results of the study show a high similarity between the oils of families 2 and 4, which are located at the center of the Persian Gulf; these results also recognize the significant difference between these two oil families and oil families 1 and 3. The significant issue raised in this study is to find out the reason for this difference. The structural analysis of the central Persian Gulf shows that huge vertical faults created by salt domes intrusion provided the migration pathways for trapping oil in Sarvak reservoirs. The oils of the Surmeh, Fahliyan, and Dariyan reservoirs have migrated upwards through these faults to be trapped in the Sarvak Formation across the central Persian Gulf.

Extent and variability of Mesozoic‐Cenozoic multi‐stage salt diapirs in the Southern Permian Basin, Southern North Sea

AbstractSalt structures can be used as an archive for tectonic and depositional processes as all salt structures respond distinctively. Few salt tectonic studies have investigated the evolution of multi‐stage salt structures, nevertheless, no previous study had systematically identified, mapped, nor classified, the evolution of multi‐stage salt structures in a regional study. Decades of hydrocarbon exploration and the heavily dense 3D seismic data available, make the Southern North Sea one of the best natural laboratories to investigate the evolution of salt structures. The Southern North Sea salt basin is a Late Permian Zechstein salt mega‐basin containing a myriad of salt structures. The complex tectonic evolution of the Southern North Sea created diverse Mesozoic structural sub‐basins with different tectonostratigraphic evolutions. We defined a nomenclature, linked to the mega‐stratigraphic sequences, for the classification of salt structures. We used a Two‐Way‐Travel‐Time 3D seismic reflection dataset and time‐thickness variations around salt structures to systematically analyse the evolution of salt structures across the diverse structural sub‐basins of the Southern North Sea. Multi‐stage salt diapirs were triggered halokinetically in the Early Triassic and are linked to regional palaeo‐depocentres controlled by the sub‐Zechstein structural configuration. Multi‐stage salt diapirs in the different sub‐basins evolved through three different regional phases and up to five distinctive local stages. The most complex salt diapirs developed in the Central Graben, Sole Pit High and Silver Pit Basin, where multi‐stage salt diapirs showed 4–5 local stages of salt diapirism. The multi‐stage evolution of salt structures should be thoroughly investigated to reduce risks and uncertainties in the energy sector and net zero projects, such as in carbon capturing and storage projects, and energy storage in man‐made salt caverns.

Interaction between salt and mobile shale in the East Breaks foldbelt, northwestern Gulf of Mexico

The East Breaks foldbelt comprises a series of dominantly slope-parallel folds and thrusts detached on a mostly evacuated salt canopy on the northern Gulf of Mexico slope. The canopy was emplaced near the end of the Eocene. The foldbelt formed mostly in the Oligocene, although minor amounts of shortening continued through the Miocene. We attribute the anomalous trend of structures in the foldbelt to convergent gravity gliding produced by the bend in topography in the northwest corner of the Gulf of Mexico. We suggest that convergent deformation was localized in the East Breaks foldbelt because the area comprised a shale-dominated slope between two sand-prone slope aprons during the Oligocene. These aprons were stronger than the shalier East Breaks area in between, allowing convergent deformation to be partitioned into weak shales.The fact that the Oligocene sequence in the area is shale prone means that shortening was accommodated on multiple detachment levels with fishtail thrusts, as well as in mobile shales, which occur alongside salt diapirs sourced from the canopy. The coincidence of salt structures and mobile shales allows a comparison of their behavior during shortening. Mobile-shale deformation is most intense near a minimally shortened salt massif, leading us to conclude that salt behaved more competently than mobile shale during shortening. This conclusion is counter to the general assumption that salt, where present, is the weakest part of any geologic system.

Normal faulting and landsliding in morpho-structural domes related to buried salt stocks, Zagros Mountains, Iran. Insights into salt breakout

Normal faulting in domal structures above diapirs has important implications in hydrocarbon geoscience, but studies on surface faulting on exposed growing domes are scarce. Moreover, there is limited work addressing the erosional processes involved in the unroofing of active diapirs preceding salt emergence. This investigation analyses two morpho-structural domes (Gavbast Dome: GD; Kooh Bedoo Dome: KBD) associated with growing anticlines related to the rejuvenation of precursor buried salt stocks in the Zagros Mountains. The structural relief of the domes (1330 m in GD and 400 m in KBD) and the available chronological data on fold development indicate long-term uplift rates of around 0.1–0.4 mm/yr. GD displays three domains with different fault patterns: (1) fold-normal grabens in one half; (2) polygonal faulting with multiple enclosed graben depressions in the crest; and (3) fold-parallel faulting on the other half. KBD shows a major fold-normal graben restricted to the crest and one flank of the dome. The lack of the typical radial pattern related to multidirectional hoop extension can be attributed the suppression of extension in the fold-normal direction by regional shortening. Other factors such as complexities in the folding structure, outer-arc extension in the anticline crests, and plan geometry seem to play a local control on fault patterns. The studied domes display an extraordinarily high density of large slope movements, indicating that they play an instrumental role in the unroofing process. The slope failures, with sliding planes mainly developed in argillaceous units overlain by thick limestone packages, include incipient planar rock slides more than 5 km2 in area expressed as buckle folds in the lower part of the slopes. The main preparatory factors involved in the development of these slope collapses include rock mass weakening by faulting and slope over-steepening, both related to doming. Our detailed mapping and the examination of a number of salt diapirs in the Fars Arc suggest that episodic landsliding and more gradual fluvial dissection, often controlled by grabens, are the main erosional processes involved in the unroofing of active diapirs leading to salt emergence.

Palaeostress state around a rising salt diapir inferred from seismic reflection data

A 3D seismic volume and borehole data from the Dutch North Sea are used to investigate the palaeostress state around a rising salt diapir. The results show radial, polygonal and keystone faults around the diapir of interest, which are separated into eight zones based on their geometry, strike and overall distribution. Principal fault families include: a) 400–3500 m long radial faults developed in flanking and corner areas of the salt diapir, and known to have accommodated the stretching resulting from its rise, b) 200–1500 m long polygonal faults providing a record of the stress conditions farther from radial faults, and c) 1000–1800 m long keystone faults controlled by the rise of a buried salt pillow during Paleogene tectonic inversion. Stress inversions for 10,401 interpreted faults reveal that the maximum principal palaeostress (σ1) is close to vertical, whereas intermediate and minimum principal palaeostresses (σ2 and σ3) are sub-horizontal, though variable in their magnitudes and orientations. Palaeostresses in flanking zones of the salt diapir show marked differences when compared with corner zones, but the combination of minimum principal palaeostresses from all flanking and corner zones formed a triangular stress ring around the salt diapir. Importantly, the width of this stress ring was not only associated with the rise of the salt diapir, but also largely influenced by adjacent salt structures. Additionally, minimum principal palaeostresses estimated from polygonal fault systems are nearly normal to the boundary between polygonal and radial faults, implying that the rise of the salt diapir influenced the stress field in the outer part of its flanks. Stress inversion is shown here as a key method to understand the palaeostress state around the salt diapirs with triangular geometry and multiple growth stages, providing insights into their evolving stress states. This work also has implications to the analysis of salt diapirs, and their adjacent strata, in areas posed for carbon sequestration, gas storage, and the production of oil and gas.

Quantitative analysis of the structural evolution of salt diapirs and their impact on sediment routing and the architecture of deep-water channel reservoirs, Venus field offshore Angola

The interaction between deep-water channels and topography on the slope due to faulting, folding or diapirism is well known. However, most studies show only qualitative observations on the development of the structures. In the Block 31 area, offshore Angola, previous studies have established a chronostratigraphic framework where ten regional horizons can be assigned absolute ages from 89 Ma to 9.7 Ma utilizing biostratigraphic markers correlated to a global chronostratigraphic framework. This study focusses on the area around the Venus structure in Block 31 and over the 51-13.3 Ma time interval. The Venus structure is a salt-cored anticline with a syncline and a further salt-cored anticline to the east.Shortening analysis has been performed on 15 dip lines across the Venus structure for each of the ten horizons. During the focus interval, the Venus structure is divided into three areas; the crest of the structure which has a cumulative shortening of 680 m and interval shortening rates between 10 and 25 m/Myr, the northern low which has cumulative shortening of 180 m and interval shortening rates of 2–8 m/Myr and the southern low which has cumulative shortening of 450 m and interval shortening of 5–18 m/Myr. Over the same intervals eight main channel systems are identified (CS10-80). Each of the channels has been imaged on amplitude maps from spectral decomposition processed seismic data. The channel systems enter the study area from the east and take three routes down-dip; passing to the SW along the syncline between the two salt-cored anticlines or crossing the Venus structure through the northern of southern lows.Integrating the shortening analysis and the channel system routes shows that the channel systems are only able to cross the Venus structure through the southern and northern lows when the interval shortening rates in those areas are less than 10 m/Myr. At other times the channels are directed to the southwest along the synclinal axis. Off-set stacking of subsequent channel systems can be observed both at the entry point of the systems and through the northern and southern lows. This may be due to more subtle shifts in structurally controlled sea-floor topography or could also be due to stratigraphic compensational stacking. Within the CS20 system the sinuosity of higher order channel complexes and elements appear to be controlled by crestal faults extending into the southern low.