Subsurface Examples Research Articles

About this title - Carbonate Reservoirs: Applying Current Knowledge to Future Energy Needs

This Special Publication is a compendium of studies on carbonate petroleum reservoirs from several key areas, together with subsurface examples of renewable energy generation, storage and sequestration. It shows how the tools developed entirely for petroleum reservoir exploitation are now being utilized to enable aspects of our new low-carbon environment.

Predictable time preservation on passive continental margins revealed from forward stratigraphic models

AbstractAlthough the majority of the world's sedimentary basins are located along passive continental margins, the link between their stratigraphy and time architecture has not been fully investigated in three dimensions. Here, using a 3D numerical forward stratigraphic model, we report a previously unidentified trend: an increase in temporal completeness from the shelf to the upper slope and a reduction towards the basin floor. Sedimentation rates from our results are consistent with those measured from a global dataset; our results are also consistent with a chronostratigraphically well‐constrained subsurface example: the Eocene of the Santos Basin, offshore Brazil. Because our results reproduce both globally and locally observed time‐preservation trends, we suggest the use of such models to inform time architecture in the context of surface and subsurface accumulations. Applying these concepts will not only provide a spatiotemporal framework for passive‐margin settings but will provide a more complete understanding of the inherently incomplete geological record.

Digitalization of Legacy Datasets and Machine Learning Regression Yields Insights for Reservoir Property Prediction and Submarine-Fan Evolution: A Subsurface Example From the Lewis Shale, Wyoming

Machine-learning algorithms have long aided in geologic property prediction from well-log data, but are primarily used to classify lithology, facies, formation, and rock types. However, more detailed properties (e.g., porosity, grain size) that are important for evaluating hydrocarbon exploration and development activities, as well as subsurface geothermal, CO2 sequestration, and hydrological studies have not been a focus of machine-learning predictions. This study focuses on improving machine-learning regression-based workflows for quantitative geological property prediction (porosity, grain size, XRF geochemistry), using a robust dataset from the Dad Sandstone Member of the Lewis Shale in the Green River Basin, Wyoming. Twelve slabbed cores collected from wells targeting turbiditic sandstones and mudstones of the Dad Sandstone member provide 1212.2 ft. of well-log and core data to test the efficacy of five machine-learning models, ranging in complexity from multivariate linear regression to deep neural networks. Our results demonstrate that gradient-boosted decision-tree models (e.g., CatBoost, XGBoost) are flexible in terms of input data completeness, do not require scaled data, and are reliably accurate, with the lowest or second lowest root mean squared error (RMSE) for every test. Deep neural networks, while used commonly for these applications, never achieved lowest error for any of the testing. We also utilize newly collected XRF geochemistry and grain-size data to constrain spatiotemporal sediment routing, sand-mud partitioning, and paleo-oceanographic redox conditions in the Green River Basin. Test-train dataset splitting traditionally uses randomized inter-well data, but a blind well testing strategy is more applicable to most geoscience applications that aim to predict properties of new, unseen well locations. We find that using inter-well training datasets are more optimistic when applied to blind wells, with a median difference of 0.58 RMSE when predicting grain size in phi units. Using these data and results, we establish a baseline workflow for applying machine-learning regression algorithms to core-based reservoir properties from well-log and core-image data. We hope that our findings and open-source code and datasets released with this paper will serve as a baseline for further research to improve geological property prediction for sustainable earth-resource modeling.

Channel trajectories control deep‐water stratigraphic architecture

AbstractInterpretation of deep‐water channel deposits is challenging because the spatial arrangement of their constituent lithologies is highly variable. This variability is often thought to be a signature of complex interactions between controlling boundary conditions and processes. A three‐dimensional forward stratigraphic model of a sinuous meandering channel is used to explore the production of channelised deep‐water stratigraphy. This model highlights three stages of stratigraphic evolution for channel belts: (1) an initial phase of rapid growth in mean belt width and variability in belt width driven by increasing channel sinuosity; (2) a subsequent phase of reduced belt‐width growth rate because of cutoff processes; and (3) a mature phase during which repeated bend lifecycles act to produce a statistically stable channel‐belt width. When a trajectory defining the vertical movement of a channel over time is added to the model, commonly recognised patterns of deep‐water channel‐belt stratigraphy are produced. These results demonstrate how forward stratigraphic models provide insights into processes governing the evolution of deep‐water stratigraphy that elude interpretations of static outcrops and seismic images of subsurface examples.

Dinaric up-thrusts in the Pliocene evolution of the Central Apennines thrust belt of Italy: the Montagna dei Fiori structure

AbstractSeveral orogenic belts exhibit regional-scale anticlines characterized by prominent faults in their crestal/forelimb zone. These faults are also a common feature in the Neogene fold-and-thrust belt of the Apennines, where they have been contrastingly interpreted as younger-on-older thrust faults, large-scale strike-slip faults, and pre- or syn-thrusting normal faults. In this study, we analysed a NW–SE-trending fault (Montagna dei Fiori Fault) that affects the hinge-zone/forelimb of the Montagna dei Fiori Anticline. This fold is the outermost exposed contractional structure within the Pliocene–Quaternary antiformal stack of the outer Central Apennines. The integration of stratigraphic and structural data collected during a field geological survey enabled us to reconstruct a multiphase reactivation and deformation along the Montagna dei Fiori Fault. From the novel field data, a different interpretation for the evolution of the Montagna dei Fiori Fault is proposed. The fault originated as a Late Cretaceous – middle Miocene, NE-dipping, Dinaric up-thrust and was later reactivated, displaced and rotated during Pliocene Apennine thrusting and related folding, until assuming a present-day SW-dipping attitude with an apparent normal fault character. This newly proposed Dinaric origin of the Montagna dei Fiori structure is compared with an analogous subsurface example of a Palaeogene–Quaternary structure imaged by seismic reflection profile in the Adriatic foreland. The outcome of this combined field and subsurface investigation provides new elements to unravel the complex evolution of the Apennine thrust belt that developed at the expense of a previously deformed foreland, ahead of the advancing Dinaric chain.

Halokinetic modulation of sedimentary thickness and architecture: A numerical modelling approach

AbstractSubsurface salt flow can deform overlying strata and influence contemporaneous sedimentary systems. Studying salt‐sediment interactions is challenging in the subsurface due to poor imaging adjacent to salt, and in the field due to the dissolution of halite. Discrete Element Modelling provides an efficient and inexpensive tool to model stratigraphy and deformation around salt structures, which is advantageous over other modelling techniques as it realistically recreates brittle processes such as faulting. Six 2D experiments were run representing 4.6 Myr to determine the effect of salt growth on syn‐kinematic stratigraphy. Halokinetic deformation of stratigraphic architecture was assessed by varying sediment input rates through time. Results show the realistic formation and evolution of salt‐related faults which define a zone of halokinetic influence ca. 3 times the width of the initial diapir. Outside of this, early diapiric and syn‐kinematic stratigraphy are undeformed. Within this zone, syn‐kinematic strata are initially isolated into primary salt withdrawal basins, onlapping and thinning towards the salt‐cored high. In most models, syn‐kinematic strata eventually thin across and cover the diapir roof. Thinning rates are up to six times greater within 350 m of the diapir, compared to further afield, and typically decrease upwards (with time) and laterally (with distance) from the diapir. Outputs are compared to a subsurface example from the Pierce field, UK North Sea, which highlights the importance of considering local fluctuations in diapir rise rate. These can create stratigraphic architectures that may erroneously be interpreted to represent increases/decreases in sedimentation rate. Exposed examples, such as the Bakio diapir, northern Spain, can be used to make inferences of the expected depositional facies, below model resolution. Our models aid the prediction of sedimentary unit thickness and thinning rates and can be used to test interpretations arising from incomplete or low‐resolution subsurface and outcrop data when building geological models for subsurface energy.

Mass transport deposits in deep-water minibasins: Outcropping examples from the minibasins adjacent to the Bakio salt wall (Basque Country, Northern Spain)

Recent subsurface studies show that mass-transport deposits (MTDs) in salt-controlled basins may correspond to local or regional bodies induced by either regional tectonics, or diapir growth. These MTDs are commonly considered as muddy bodies but they may alternatively incorporate a high amount of clasts and reworked beds with good reservoir properties and thus they are often challenging deposits in oil and gas exploration. The minibasins adjacent to the Bakio salt diapir, in northern Spain, provide a unique opportunity to study up to seven outcropping MTDs comparable in size to subsurface examples. Detailed structural analysis was used to reconstruct the transport direction for each MTD and to infer their source locations. In addition, facies analyses enabled the estimation of their percent of mud or matrix, allowing for a discussion on their potential reservoir and seal properties. At least six of the studied MTDs correspond to locally-derived MTDs sourced from the Bakio diapir or from the footwall of the adjacent sub-salt extensional faults. The primary trigger for these MTDs may be halokinesis, probably with contributions from other secondary processes, such as carbonate platform aggradation, high sedimentation rates and regional extension. Transport directions together with palaeoflow analysis suggests that regionally-derived turbidites flowed along the minibasin axis, while MTDs were transported laterally from the minibasin margins at high angle with the turbidity flows. We identified three types of MTDs: muddy siliciclastic-dominated MTDs, sandstone clast-rich siliciclastic-dominated MTDs and carbonate-dominated MTDs. Using this classification and subsurface analogs we propose a model of locally-derived MTDs according to the nature of the source area and the sedimentary facies reworked along the MTD downslope trajectories. This model suggests that reservoir and seal properties could be suggested for MTDs in subsurface studies by characterizing the nature of the diapir roof and the facies at the seafloor found along the MTDs trajectories.

Complex stratigraphic fill of a small, confined syn-rift basins: an Upper Jurassic example from offshore mid-Norway

Abstract Extensional rift basin systems have been a focus of study for hydrocarbon exploration and have provided significant economic reserves in recent decades, which has led to advances in our understanding of their tectono-sedimentary evolution. However, with the increasing maturity of such settings, focus is shifting from pre-rift structures to the underexplored syn- and late-rift plays. This change in focus brings with it a significant increase in complexity when trying to develop an integrated understanding of the sedimentological, sequence stratigraphic and structural conditions that control the distribution of syn-rift reservoirs. Here we present a subsurface example from offshore Norway of a small, confined, syn-tectonic basin sourced from a local basement high. 3D seismic data, sedimentary facies analysis and stratigraphic correlations based on well and core data from six key wells constrain the local and regional tectonic controls. The PL586 licence partners drilled several wells between 2014 and 2015 targeting syn-rift Upper Jurassic stratigraphy on the southern Halten Terrace. The wells penetrated a complete syn-rift stratigraphic sequence, which provided an excellent subsurface record of the inherent complexities encountered in small, confined (3 by 5 km) syn-rift basins. Seismic data reveals that the Late Jurassic hanging-wall basin architecture and the footwall hinterland of the Frøya High were controlled by activity on the basin-bounding Vingleia Fault Complex. Facies analysis from well and core data reflects a complete syn-rift succession. The early-rift succession comprises mainly mud and siltstone with minor fine-grained sandstones interpreted to be deposited by sediment gravity flows in isolated mini-basins. The peak-rift deposits comprise coarse-grained sediment gravity flows that were sourced locally from the uplifting footwall and deposited as coarse-grained submarine fan systems in the immediate hanging wall. Significant footwall uplift during the peak-rift phase resulted in fault scarp instability and the emplacement of several large landslide complexes into the basin. The late-rift phase is dominated by a mud-rich sequence with minor coarse-grained apron fans shedding sediment off the basin bounding fault and shoreface deposits flanking the hanging-wall dip-slope and subtle intra-basinal highs. This subsurface study from the southern Halten Terrace is significant in the number of wells that have specifically targeted syn-rift stratigraphy and the level of detail at which it captures the stratigraphic variability of these deposits within a small, confined basin system. Integration of seismic geomorphology and well data with robust provenance, dating and stratigraphic correlation has enabled the development of a tectono-stratigraphic model specific to the scale and local tectonic setting. The model reveals that structural evolution ultimately controlled the development of sediment supply routes, their age and distribution, and allows several conclusions to be drawn: The stratigraphy of small, confined syn-rift basins is broadly comparable with more typically documented larger syn-rift settings, although lateral facies changes occur more rapidly. The cannibalistic nature of syn-rift systems, where a large volume of pre- and syn-rift sediments are uplifted and eroded then rapidly re-deposited makes determining the relative age of laterally variable depositional settings difficult. In order to adequately constrain the timing and depositional extent of syn-rift stratigraphy, it is essential to understand the structural evolution of the system and integrate this information with other disciplines and datasets. This integrated understanding will ultimately improve predictions of reservoir presence and quality.

Deformation–sedimentation feedback and the development of anomalously thick aggradational turbidite lobes: Outcrop and subsurface examples from the Hikurangi Margin, New Zealand

ABSTRACT Concepts of the interaction between autogenic (e.g., flow process) and allogenic (e.g., tectonics) controls on sedimentation have advanced to a state that allows the controlling forces to be distinguished. Here we examine outcropping and subsurface Neogene deep-marine clastic systems that traversed the Hikurangi subduction margin via thrust-bounded trench-slope basins, providing an opportunity to examine the interplay of structural deformation and deep-marine sedimentation. Sedimentary logging and mapping of Miocene outcrops from the exhumed portion of the subduction wedge record heavily amalgamated, sand-rich lobe complexes, up to 200 m thick, which accumulated behind NE–SW-oriented growth structures. There was no significant deposition from low-density parts of the gravity flows in the basin center, although lateral fringes demonstrate fining and thinning indicative of deposits from low-density flows. Seismic data from the offshore portion of the margin show analogous lobate reflector geometries. These deposits accumulate into complexes up to 5 km wide, 8 km long, and 300 m thick, comparable in scale with the outcropping lobes on this margin. Mapping reveals lobe complexes that are vertically stacked behind thrusts. These results illustrate repeated trapping of the sandier parts of turbidity currents to form aggradational lobe complexes, with the finer-grained suspended load bypassing to areas downstream. However, the repeated development of lobes characterized by partial bypass implies that a feedback mechanism operates to perpetuate a partial confinement condition, via rejuvenation of accommodation. The mechanism proposed is a coupling of sediment loading and deformation rate, such that load-driven subsidence focuses stress on basin-bounding faults and perpetuates generation of accommodation in the basin, hence modulating tectonic forcing. Recognition of such a mechanism has implications for understanding the tectono-stratigraphic evolution of deep-marine fold and thrust belts and the distribution of resources within them.

The connection between the Alps and the Carpathians beneath the Pannonian Basin: Selective reactivation of Alpine nappe contacts during Miocene extension

Abstract The Cretaceous basement of the broader NW Pannonian Basin, including the Vienna Basin, has an Alpine nappe structure that can be correlated between the Eastern Alps and the Western Carpathians. For the first time in this region, based on the systematic integration of petroleum industry vintage 2D reflection seismic data with well data from Austria, Hungary and Slovakia, we document various modes of interaction between pre-existing thrust and superimposed normal faults. Whereas in some cases the pre-existing Cretaceous nappe contacts became extensional low-angle normal faults, the typical scenario is that the Miocene normal faults obliquely cut across them. The style of this negative inversion, i.e. contraction followed by extension exploiting the same fault system, is based on the orientation and geometry of the Alpine nappes in relation to the successor extensional basins. We expand our earlier findings made in the Hungarian sector of the NW Pannonian Basin where Neoalpine (Neogene) low-angle normal faults did indeed interact with abandoned Eoalpine (Cretaceous) and Mesoalpine (Paleogene) thrust fault planes, but in a more selective manner than expected before. The subsurface examples of this paper are located in the Austrian part of the Vienna Basin, the Hungarian, Austrian and Slovakian segments of the broader Danube Basin. Our new regional and local scale subsurface observations also helped to define a very large, but so far unrecognized Eocene antiformal nappe stack beneath the Danube Basin. Presently this is a highly dismembered and dominantly subsurface structure concealed beneath the NW Pannonian Basin between the Eastern Alps and the Western Carpathians. This finding has important implications for the understanding of the Alpine evolution of the broader region.

Sedimentology and facies analysis of ancient sand ridges: Jurassic Rogn Formation, Trøndelag Platform, offshore Norway

Sand ridges represent a common type of sedimentary bedform of modern shelves seldom used as analogues to interpret isolated marine sandbodies recognised in the subsurface. Lack of extended literature on outcrop and subsurface examples limits the possibility for their recognition and seems one of the reason behind this underrepresentation. The Draugen discovery made in the early 80's represents an unicum in the Trøndelag Platform, offshore Norway. After more than 30 years the Froan Basin and Frøya High area are still underexplored and the Late Jurassic Rogn Fm play not well understood. Predicting reservoir distribution, and its internal architecture and properties requires the understanding of factors controlling sedimentation (e.g. palaeocirculation, depositional processes). North-south elongated sandbodies pertaining to the Rogn Formation are recognised in the Froan Basin and Frøya High encased within thick shaly deposits. Sandbodies develop above a ravinement or flooding surface (i.e. Callovian Unconformity) of regional extent where local depressions occur with a non-erosional concave-up top. Depressions representing the depositional loci for the accumulation of sand and development of the ridge. The presence of eastward and westward dipping reflections within the sandbodies allows identifying their large-scale architectures. Sediments form coarsening-upward vertical units characterised by a shaly base evolving upwards to medium- and coarse-grained sand forming tabular and trough cross strata. Locally, a fining upward trend characterised by plane-parallel stratification and coarse-grained massive layers is recognised. Sediments results well organised and sorted, which positively affects final porosity and permeability with values up to 30% and 6 Darcy, respectively - typical values for many sand ridges. Accordingly, sand ridges encased within thick shaly deposits can form stratigraphic traps with the potential for large hydrocarbon accumulations. The aim of the present study is to help the understanding of distribution, and internal architectures and properties of the Rogn Fm in the Trøndelag Platform.

A modern analog for meandering rivers of the Mina del Carmen Formation (Albian) at Cañadón Seco oilfield, Golfo San Jorge Basin, Patagonia

Fluvial sandstone reservoirs within the Mina del Carmen Formation (Albian) currently produce up to 10% of the liquid hydrocarbons of the Golfo San Jorge Basin, mainly from isolated channel belts encased in a fine-grained, pyroclastic-rich floodplain. In spite of their economic importance, no modern analogs for the subsurface fluvial reservoirs have been proposed yet, mainly due to the reduced thickness of the channels in relation to the vertical resolution of available 3-D seismic surveys. At Cañadón Seco oilfield (South Flank of the Golfo San Jorge Basin), the planform of a meandering channel belt in Upper Mina del Carmen Formation has been imaged in a 3-D seismic dataset using seismic attributes. The meandering channel belt is up to 13 m thick and 3200 m width, with a sinuosity of 3.61 and has an average channel width of 192 m. Using the measured parameters obtained from the seismic dataset, we tested the applicability of empirical relations developed for modern rivers; the results of the statistical equations showed a close correspondence with seismic measurements and allowed us to estimate data of formative river discharge and upstream catchment area for the subsurface example. Both values were then parallel with a database of river discharge and area of up to 1018 hydrometeorological stations in drainage catchments around the world. Nine rivers of the database show discharge-area values comparable to those obtained using empirical equations. After a review of satellite imagery of the nine areas, a modern analog for the subsurface example was found in the Middle Ebro River (Spain). The analog consists of a meandering fluvial depositional system in a foreland basin setting, with seasonal discharge variability. The approach followed in the research can be of use to any fluvial basin, but considering that fluvial systems are highly variable in scale, geometry, pattern and composition, the extrapolation of results to other regions or even to other strata within the Mina del Carmen Formation, should be done with caution.

Long-lived mega fault-scarps and related breccias at distal rifted margins: insights from present-day and fossil analogues

Thinning of the continental crust during rifting is accommodated by a number of major faults, only a few of which produce long-lived mega fault-scarps. In this paper, we investigate mega fault-scarps and the sedimentary system located at their toe across magma-poor rifted margins. Our approach combines observations from subsurface examples along present-day margins and field analysis of fossil examples exposed in the Alpine Tethys margins. Whereas present-day examples of rifted margins imaged by seismic techniques provide details about the architecture of the top basement and the relationship between faults and sediments, outcrops give access to the sedimentary features of rocks related to mega fault-scarps. Our study shows that (1) mega fault-scarps are preferentially located at rift domain boundaries, implying a topographic escarpment that juxtaposes rift domains of different crustal thickness, and (2) mega fault-scarps are long-lived local sources for syn- and post-tectonic breccia, respectively produced during and after fault activity.

Structural and lithological controls on the architecture of igneous intrusions: examples from the NW Australian Shelf

Rift-related magmatism resulting in widespread igneous intrusions has been documented in various basins, including the Faroe Shetland Basin (UK), the Voring and Møre basins (Norway), and along the NW Shelf of Australia. Seismic mapping, combined with fieldwork, has resulted in greater understanding of subsurface intrusive plumbing systems but knowledge of emplacement style and the mechanisms by which intrusions propagate is limited. The interpretation of a 3D seismic dataset from the Exmouth Sub-basin, NW Shelf of Australia, has identified numerous igneous intrusions where a close relationship between intrusions and normal faults is observed. These faults influence intrusion morphology but also form pathways by which intrusions have propagated up through the basin stratigraphy. The steep nature of the faults has resulted in the intrusions exploiting them and thus manifesting as fault-concordant, inclined dykes; whereas in the deeper parts of the basin, intrusions that have not propagated up faults typically have saucer-shaped sill morphologies. This transition in the morphology of intrusions related to fault interaction also highlights how dykes observed in outcrop may link with sills in the subsurface. Our interpretation of the seismic data also reveals subsurface examples of bifurcating intrusions with numerous splays, which have previously only been studied in outcrop. Supplementary material: Figures showing uninterpreted seismic lines are available at https://doi.org/10.6084/m9.figshare.c.4395974

Fold-related deformation bands in a weakly buried sandstone reservoir analogue: A multi-disciplinary case study from the Numidian (Miocene) of Sicily (Italy)

Deformation bands, usually recognized in association with faults, are here analyzed in relation to a tight synclinal fold developed in the Miocene Numidian turbidites of Sicily. Deposited above a growing thrust-wedge and then buckled during continued deformation, the porous Numidian sandstones form subsurface gas reservoirs elsewhere in the region and are analogues for deepwater systems in general. Structural data have been collected and statistically analyzed to characterize preferred orientations and size parameters (thickness, spacing, length) of deformation bands. Two distinct populations relate to folding: the most recent one is NE-SW oriented, which produced the most prominent structures, whereas an older one is partially obliterated. Microscopic investigation reveals porosity decreases within deformation bands with respect to host rock. The principal deformation mechanisms are grain rotation/sliding and pore-collapse, consistent with folding having occurred under low burial conditions. Within the thrust wedge, near-surface folding is widespread, as indicated by growth strata. Thus we expect early-burial deformation bands of the types (compaction and shear bands) illustrated here to be a component of reservoir damage in subsurface examples.

Experimental studies of the controls of the geometry and evolution of salt diapirs

Experimental models were conducted to study the controls of the evolution and geometry of salt diapirs, and the transition from passive to active diapirism. Both constant and variable sedimentation rate experiments were conducted. Constant sedimentation rate experiments show that the evolution of diapirs is strongly dependent on the rates of sand deposition and the thickness of the source layer. Low rates of sedimentation result in high ratios of salt flow to aggradation, leading to cylindrical diapirs which subsequently develop flared shapes with overhangs, whereas high sediment rates result in low ratios of salt flow to aggradation, resulting in tapered shapes and eventual eclipse and occlusion of the diapirs. Thick source layers result in higher salt flow rates than thin layers. Variable sedimentation rates result in changing shapes of diapirs over time. A small increase in sedimentation rates for flared or cylindrical diapirs result in an initial eclipse followed by tapering and a transition to active diapirism, enabling the diapir to pierce the overlying sediments. The narrower diapir continues to grow by passive diapirism. A large increase in rates results in a permanent eclipse, because the tapered diapir is unable to penetrate the greater thickness of the overburden. Finally, thicker source layers generally result in wider diapirs during both phases of deformation. Tapering and flaring are mechanisms of maintaining equilibrium between the rate of sedimentation and net salt flow rate. The resulting models are directly applicable to the analysis of poorly or partially imaged salt diapirs in natural subsurface examples.

Interpreting syndepositional sediment remobilization and deformation beneath submarine gravity flows; a kinematic boundary layer approach

Turbidite sandstones and related deposits commonly contain deformation structures and remobilized sediment that might have resulted from post-depositional modification such as downslope creep (e.g. slumping) or density-driven loading by overlying deposits. However, we consider that deformation can occur during the passage of turbidity currents that exerted shear stress on their substrates (whether entirely pre-existing strata, sediment deposited by earlier parts of the flow itself or some combination of these). Criteria are outlined here, to avoid confusion with products of other mechanisms (e.g. slumping or later tectonics), which establish the synchronicity between the passage of overriding flows and deformation of their substrates. This underpins a new analytical framework for tracking the relationship between deformation, deposition and the transit of the causal turbidity current, through the concept of kinematic boundary layers. Case study examples are drawn from outcrop (Miocene of New Zealand, and Apennines of Italy) and subsurface examples (Britannia Sandstone, Cretaceous, UK Continental Shelf). Example structures include asymmetric flame structures, convolute lamination, some debritic units and injection complexes, together with slurry and mixed slurry facies. These structures may provide insight into the rheology and dynamics of submarine flows and their substrates, and have implications for the development of subsurface turbidite reservoirs.

Depositional architecture of sand-attached and sand-detached channel-lobe transition zones on an exhumed stepped slope mapped over a 2500 km2 area

The geomorphology and seismic stratigraphy of deep-water clastic systems from slope valleys through channel-levee systems to basin-floor fans have been observed and described in modern and ancient subsurface examples around the world. However, the distribution of sedimentary facies, grain size, and small-scale architectural elements remains poorly constrained. Extensive exposures (>2500 km 2 ) of four stacked deep-water composite sequences have been mapped from heterolithic channel-levee systems on the slope to sand-rich basin-floor deposits. The data set from Units C–F of the Fort Brown Formation in the Permian Laingsburg depocenter of South Africa permits a unique opportunity to document and compare their depositional architecture at a high resolution for tens of kilometers downdip. Isopach thickness maps indicate that compensational stacking across multiple stratigraphic scales occurs on the basin floor, whereas preferred axial pathways were present on the slope, leading to subvertical stacking patterns. Units C and D are sand-attached systems; slope valley systems are mapped to pass transitionally downslope through levee-confined channels to lobe complexes over distances of >30 km. The slope valley fills of Units E and F, however, are separated from their downdip sand-rich lobe complexes by a thin, sand-poor tract several kilometers in length and are termed sand detached. Locally, this sand-poor tract is characterized by a distinctive facies association of thin-bedded turbidites with numerous scours mantled with rip-up clasts, and a top surface that includes megaflutes and remobilized sediments. This assemblage is interpreted to indicate a widespread area of sand bypass. This unique data set provides an exploration-scale insight and understanding of how different segments of a prograding slope evolved over time in terms of gradient, physiography, and hence the degree to which sand was stored or bypassed to the basin floor, and the evolution from sand-attached to sand-detached systems. The development of sand-detached systems suggests that a steeper gradient formed, possibly related to developing underlying structure, that led to the development of a stepped slope profile. The study highlights that updip stratigraphic trapping at reservoir scale can occur with minor bathymetric changes.

Stratigraphic variations control deformation patterns in evaporite basins: Messinian examples, onshore and offshore Sicily (Italy)

Three-dimensional seismic data are increasingly resolving original compositional heterogeneity and structural complexity in evaporitic successions within sedimentary basins. The relationship between basin structure, evaporite composition and its influence on subsequent deformation are investigated here using Messinian examples from the Maghrebian thrust system of Sicily and applied to the adjacent Ionian sea-bed. By integrating outcrop and onshore subsurface data, we demonstrate variations in evaporite stratigraphies deposited across thrust-top basins, and how these variations have controlled subsequent deformation of these basins. Gypsum and carbonate units develop broad single-layer buckle fold trains, with wavelengths reflecting layer thickness. The development of deformation appears limited by bending resistance at fold hinges, which can be overcome by syntectonic erosion. In contrast, the thick halite and K-salt accumulations in growth synclines have deformed with short-wavelength folds and distributed strain. These structures can display rapid lateral variations (on a scale of hundreds of metres). Similar structural styles, with buckle fold trains passing laterally into more homogeneously shortened, short-wavelength folding, are evident on seismic data from the buried Messinian interval beneath the Ionian Sea. Using the Sicilian outcrop as analogues, the structural styles for the Ionian may be used to infer evaporite type in these subsurface examples.

Defining a 3-dimensional trishear parameter space to understand the temporal evolution of fault propagation folds

The application of trishear, in which deformation occurs in a triangular zone in front of a propagating fault tip, is often used to understand fault related folding. A key element of trishear, in comparison to kink-band methods, is that non-uniform deformation within the triangle zone allows the layer thickness and length to change during deformation. By varying three controlling parameters independently (trishear propagation/slip ratio, trishear apical angle and fault dip), we construct a three-dimensional parameter space to demonstrate the variability of resultant geometry feasible with trishear. We plot published natural examples in this parameter space and identify two clusters and show that the most applicable typical trishear propagation/slip ratio is 2 to 3, while the trishear apical angle varies from 30° to 100°. We propose that these findings can help estimate the best-fit parameters for natural structures. We then consider the temporal evolution of specific geometric examples and factors that increase the complexity of trishear including: (1) fault-dip changes and (2) pre-existing faults.To illustrate the applicability of the parameter space and complex trishear models to natural examples, we apply our results to a sub-surface example from the Qaidam basin in northern Tibetan Plateau.