Stratigraphic Heterogeneity Research Articles

Ground ice, organic carbon and soluble cations in tundra permafrost soils and sediments near a Laurentide ice divide in the Slave Geological Province, Northwest Territories, Canada

Abstract. The central Slave Geological Province is situated 450–650 km from the presumed spreading centre of the Keewatin Dome of the Laurentide Ice Sheet, and it differs from the western Canadian Arctic, where recent thaw-induced landscape changes in Laurentide ice-marginal environments are already abundant. Although much of the terrain in the central Slave Geological Province is mapped as predominantly bedrock and ice-poor, glacial deposits of varying thickness occupy significant portions of the landscape in some areas, creating a mosaic of permafrost conditions. Limited evidence of ice-rich ground, a key determinant of thaw-induced landscape change, exists. Carbon and soluble cation contents in permafrost are largely unknown in the area. Twenty-four boreholes with depths up to 10 m were drilled in tundra north of Lac de Gras to address these regional gaps in knowledge and to better inform projections and generalizations at a coarser scale. Excess-ice contents of 20 %–60 %, likely remnant Laurentide basal ice, are found in upland till, suggesting that thaw subsidence of metres to more than 10 m is possible if permafrost were to thaw completely. Beneath organic terrain and in fluvially reworked sediment, aggradational ice is found. The variability in abundance of ground ice poses long-term challenges for engineering, and it makes the area susceptible to thaw-induced landscape change and mobilization of sediment, solutes and carbon several metres deep. The nature and spatial patterns of landscape changes, however, are expected to differ from ice-marginal landscapes of western Arctic Canada, for example, based on greater spatial and stratigraphic heterogeneity. Mean organic-carbon densities in the top 3 m of soil profiles near Lac de Gras are about half of those reported in circumpolar statistics; deeper deposits have densities ranging from 1.3–10.1 kg C m−3, representing a significant additional carbon pool. The concentration of total soluble cations in mineral soils is lower than at previously studied locations in the western Canadian Arctic. This study can inform permafrost investigations in other parts of the Slave Geological Province, and its data can support scenario simulations of future trajectories of permafrost thaw. Preserved Laurentide basal ice can support new ways of studying processes and phenomena at the base of an ice sheet.

Integrated outcrop and subsurface geomodeling of the Turonian Wall Creek Member of the Frontier Formation, Powder River Basin, Wyoming, USA

Inter-well heterogeneities influencing fluid migration in deltaic reservoirs are controlled by lateral lithofacies changes and vertical complexities such as low permeability thin-beds (i.e. mudstones). Subsurface tools cannot adequately predict the spatial and stratigraphic organization of these architectural elements, nor their influence on effective reservoir properties and connectivity. Our outcrop-based geomodeling study of the Turonian Wall Creek Member of the Frontier Formation in the Powder River Basin, Wyoming, USA, integrates subsurface production and flow simulation data to quantify the impact of multi-scale stratigraphic heterogeneity on analogous reservoir behavior and horizontal well design. The upscaled representation of thin-bed complexity in our 500 m × 715 m x 15 m geomodel is derived through flow simulation of internally nested, facies specific, centimeter-scale models (basic depositional element models or BDEMs). These BDEMs are populated with quantitative data of mudstone thin-bed geometries directly derived from digital outcrop measurements and measurements in the field, and aided by subsurface (core) petrophysical properties. Reservoir simulations are performed using various landing zone and completion configurations, and under both single-phase and multi-phase flow conditions history matched to subsurface production data.Results from our workflow demonstrate that, in heterolithic settings, the permeability of mudstone thin-beds impart the most significant impacts toward upscaled reservoir behavior, and deliver unique Kv/Kh flow anisotropies dependent on the governing depositional environment. Thin, laterally continuous and planar mudstone beds in the wave-and river-dominated distal delta front setting have the greatest negative effect on Kv and a minimal impact on Kh, promoting laterally continuous and vertically baffled flow units. Conversely, the more curvilinear and intersecting architectures of mudstone beds in the tidal dune facies maintain better vertical connectivity but decreased Kh compared to the wave-influenced facies. Reservoir flow is only sensitive to changes in engineering design (landing zone, well orientation, completion size) where thin-beds act as complete barriers rather than baffles to flow. Layer cake subsurface models failing to account for high-resolution stratigraphic architectures are likely to overestimate reservoir production where thin-beds act as barriers capable of appreciable reservoir compartmentalization, which in our models is more characteristic of the distal delta front setting. Synthesized results of this study are immediately applicable toward development planning in similar tight sandstone reservoirs.

Investigation of Groundwater In-rush Zone using Petrophysical Logs and Short-offset Transient Electromagnetic (SOTEM) Data

The water burst from the Ordovician limestone underlain by the Permo-Carboniferous coal seams have potential to trigger coalmine hazards in Northern China. Therefore, it is crucial to identify and accurately map the water enrichment zones and delineate coal seams using an integrated approach based on surface TEM and subsurface wireline log information to avoid water-inrush hazard and ensure safe production of coal. We inverted surface based TEM data using 1-D Occam inversion to identify the conductive anomaly and then further quantified the zone of interest by gamma and resistivity logs. 1-D Occam inversion results show conductive zone around 370 m while higher resistivity and lower gamma ray log signatures were observed against coal seams. Groundwater inrush zone falls within the mid-range gamma ray and resistivity interval as shown on the petrophysical logs. The distinct log signatures (low gamma-ray and high resistivity values) clearly indicated coal seams at depth of 410 and 470 m and subsequently the log trends were used to distinguish between coal units and more permeable sands. The magnitude and the variability of these parameters in the borehole are attributed to the subsurface stratigraphic heterogeneity. They can be key clues for interpretation of depositional facies of coal-bearing sequence and may also be used as a constraint in characterization of groundwater enrichment zone.

Impact of Geological Variables in Controlling Oil-Reservoir Performance: An Insight from a Machine-Learning Technique

Summary Predicting oilfield performance is extremely challenging because of the large number of variables that can influence and control it. Traditional methods such as decline-curve analysis have been commonly used but have been shown to have significant shortcomings. In recent years, advances in machine learning (ML) have provided a new suite of tools to tackle complex multivariant problems such as understanding oil-reservoir performance and predicating the final recovery factor. In this study, the application of a random-forest algorithm to train three predictive models and investigate the influence of the various input variables was investigated. To train the algorithm, a database was built that includes information on 32 variables from 93 reservoirs from the Norwegian Continental Shelf. These variables control or potentially influence field performance and include factors that are a function of geology, subsurface conditions, fluids, and the engineering decisions taken in field development. In addition to these controlling parameters, data were also recorded for the fields that record performance. These included information on the estimated recovery factor and production rates. Eighty percent of the data were input into the random-forest algorithm to train the models, whereas 20% were retained to blind test the subsequent models. Model accuracy was measured by comparing actual and predicted observations for each prediction metric using an R2 score, mean square error, and root mean square error. The production-rate model had a mean square error of 0.004, whereas the mean square error for recovery factor was 0.024. Estimates of average monthly depletion rate have a mean square error of 0.0104. Predictor importance estimates indicate that geology/depth-dependent variables such as stratigraphic heterogeneity, reservoir depth of burial, average porosity, and diagenetic impact are among the variables with high importance in predicting recovery factor. When predicting reservoir-oil rate, the most important variables are related to field size, such as cumulative oil produced, number of wells, oil in place (OIP), and bulk rock volume. In this study, we provide data-driven insight into understanding the relationship between subsurface and engineering conditions of reservoir producibility; we also provide a tool for predicating reservoir performance within a basin or region.

Seismic refraction study, single well test and physical core analysis of anthropogenic degraded Peat at the Badas Peat Dome, Brunei Darussalam

The Badas peat dome located in the Belait district is one of the biggest peat domes in Brunei. Over the years, this area has seen a noticeable loss of peatland due to fires and infrastructure development. In this paper, the anthropogenic effects on the Badas peat dome were studied for three survey areas using seismic refraction surveys, core analysis and single well tests. Satellite images were used to confirm the extent of past peat fires in the area. Using these methods, this study has identified multiple peat, sand and clay layers demonstrating the stratigraphic heterogeneity that can be present in tropical peatlands. In particular, a sand horizon underlying the peat deposits was identified which has the potential to be a significant hydrogeological flow unit. Groundwater level interpretations from 2D velocity data were correlated to groundwater levels from coring. In order to determine the degradation impact on peat through lowering of groundwater levels using these methods we produced a subsurface reconstruction at two locations with different levels of peat degradation. Single well tests conducted in this study showed that disturbed peat had lower hydraulic conductivities with an average of 1.73 × 10−5 m/s compared to undisturbed peat with an average of 6.66 × 10−5 m/s. A combination of these methods (seismic refraction, single well test and coring) are suitable for identifying different levels of peat degradation. In addition, these methods can be suitable for profiling in tropical peatlands, for identifying groundwater levels as wells as identifying significant hydrogeological flow units.

Impacts of stratigraphic heterogeneity and release pathway on the transport of bacterial cells in porous media

In order to manage and control the pathogen release from waste streams of various municipal, industrial, and agricultural pollution sources, it is crucial to investigate the impact of release pathways of such contaminants on their fate and transport in groundwater, especially in respect to natural heterogeneities encountered in aquifers. In this laboratory scale study, we investigate the impacts of different release scenarios of Escherichia coli bacteria, including spatially distributed surface recharge and single-point deep injection, as well as mono-pulse and continuous injection on the transport of Escherichia coli within both single-layered and multilayer aquifers.The results demonstrate earlier arrival of bacteria breakthrough curve (BTC) than conservative solute within a single-layer system with textural and continuum scale heterogeneities, attributed to size exclusion mechanism and preferential flow paths. Size exclusion may be responsible for multiple peaked BTCs observed in all cases of mono-pulse injection of bacteria through both single layer and multi-layer systems. The higher breakthrough of bacteria suspension introduced through a distributed source compared to the point source injection at the same flow rate (19% and 53% in middle and top layers, respectively) suggests that natural hydrologic events such as storm may be more influential in the transport of pathogens in soils than point injections of bacteria in engineering applications such as bioremediation. Moreover, our results reveal that the concentration of the semi-steady state breakthrough formed under distributed and continuous injection condition increases significantly with an increase in the recharge flow rate. This would suggest that a variation in hydrologic conditions can significantly mobilize pathogens which are already deposited in soils.

Combined chronological and mineral magnetic approaches to reveal age variations and stratigraphic heterogeneity in the Yangtze River subaqueous delta

Delta deposits show large spatial heterogeneity in terms of depositional rate and age, which is critical to the study of delta erosion in response to the declining fluvial sediment load observed at many river mouths in the world. In this study, we show that the magnetic susceptibility (χ) can be an indicator to reveal age variations and stratigraphic heterogeneity in the Yangtze River subaqueous delta. Ages of three short sediment cores (<2 m) collected at 20–32 m water depth from the Yangtze River subaqueous delta were determined using 210Pb, 137Cs, and optically stimulated luminescence (OSL) dating. In addition, depth variation of χ, which is influenced by post-depositional diagenesis and hence age, was used to roughly infer sediment ages among the cores in a simple way. The profiles of 210Pb, 137Cs, and OSL dating results indicate the spatial variability of ages, ranging from the last 100 years to >1700 years. The cores at shallow water depths are younger than those from deeper sites. Modern deposits (i.e., <100 years old) occur primarily at water depths shallower than ca. 30 m. The core in the northern part of the subaqueous delta shows much older ages than the core at the southern site with similar water depth, which is caused by their longer distance relative to the mouth of active sediment discharge distributary. Profile of χ confirms such spatial variation of ages in terms of depth distribution pattern and χ value. Older sediments (>800 a) show lower and uniform χ values due to the reductive dissolution of ferrimagnetic minerals, while younger sediments (<350 a) show higher χ values in the top layer but they decline with increasing depth. Considering the quick way of magnetic measurement, stratigraphic correlation based on χ can be used first to screen for cores before they are subjected to more detailed dating. This study shows that the methodological approach of combining sediment dating with magnetic measurement has great potential in revealing heterogeneous deltaic deposits.

Facies and stratigraphic architecture of the Upper Devonian–Lower Mississippian Sappington Formation, southwestern Montana: A potential outcrop analog for the Bakken Formation

This study describes the sedimentology and stratigraphy of the Devonian–Mississippian Sappington Formation in exceptional outcrop exposures in southwestern Montana. The goal was to assess the extent to which these outcrops could be used to define stratigraphic heterogeneity of time-equivalent Bakken Formation reservoir and source rocks of the Williston Basin. Facies analysis of the Sappington Formation shows three depositional sequences deposited along a storm- and wave-influenced coastline. The first sequence encompasses the lower Sappington shale, dominated by organic-rich mudstones that represent an anoxic to dysoxic lower-shelf environment. The second sequence is marked by a basal transgressive surface and initial siltstone deposition. These siltstones are abruptly overlain by basinward-dipping, low-angle clinoforms (<1°) that represent a storm- and wave-influenced shoreface. Facies distributions along clinoforms change along depositional dip over approximately 17 km (∼11 mi) from proximal to distal across the study area. The best reservoir facies are present in the upper foresets of these clinoforms. The upper shale of the Sappington Formation forms the lowest part of the third depositional sequence that continues into the overlying Lodgepole Limestone. The outcrop-based stratigraphy of the Sappington Formation provides insights into the scale of stratigraphic heterogeneity that is likely to be present in the time-equivalent Bakken Formation over the length of a typical approximately 3-km-long (∼2-mi-long) horizontal well. Geosteering horizontal wells in the Bakken along bedding surfaces, that is, along gently dipping clinoforms, is likely to cause variations in rock properties from the toe to heel of a horizontal well.

Porosity prediction of lower cretaceous unconventional resource play, south Indus Basin, Pakistan, using the seismic spectral decomposition technique

Incised-valley shale systems are renowned as the fruitful exploration domains. However, the stratigraphic heterogeneity is significant, and thus, identifying the porous compartments within the thin-bedded and fractured shale system is one of the unconventional stratigraphic challenges. We present an innovative scheme for the exploration of the porous unconventional resources within the south Indus basin, Pakistan. The continuous wavelet transforms (CWT) exclusively detects the porous shales within the complete fractured system as compared to the full spectrum seismic data interpretation tool. The seismic amplitude, instantaneous frequency, and reflection strength tools show some tuning effects due to lithology and fluids. However, the tuning block of 28 Hz envelope sub-bands in conjunction with instantaneous phase tool accurately mark out the shale-plugged pinch out reservoirs. The synthetic wedge model based on the conventional seismic wavelet resolves the shale bed (incised-valley shale-filled lens) of about 23 m thickness, but shows limited imaging capability due to frequency band limitation for the porous shale locale. The acoustic impedance based on instantaneous spectral porosity modeling using the tuning volume of 28 Hz CWT has spectacularly resolved the thin-beds (gas shows) within the porosity range of 20 to 30%, which may have implications for future exploration of unconventional stratigraphic plays.

Internal architecture of mass‐transport deposits in basinal carbonates: A case study from southern Italy

AbstractSubmarine mass‐transport deposits represent important stratigraphic heterogeneities within slope and basinal sedimentary successions. A poor understanding of how their distribution and internal architecture affect the fluid flow migration pathway may lead to unexpected compartmentalization issues in reservoir analysis. Studies of modern carbonate mass‐transport deposits mainly focus on large seismic‐scale slope failures; however, the near‐platform basinal depositional environment often hosts mass‐transport deposits of various dimensions. The small‐scale and meso‐scale (metres to several tens of metres) carbonate mass‐transport deposits play a considerable role in distribution of sediment and therefore have an impact on the heterogeneity of the succession. In order to further constrain the geometry and internal architecture of mass‐transport deposits developed in near‐slope basinal carbonates, a structural and sedimentological analysis of sub‐seismic‐scale mass‐transport deposits has been undertaken on the eastern margin of the Apulian carbonate platform in the Gargano Promontory, south‐east Italy. These mass‐transport deposits, that locally comprise a large proportion (50 to 60%) of the base of slope to basinal sediments of the Cretaceous Maiolica Formation, typically display a vertically bipartite character, including debrites and slump deposits of varying volume ratios. A range of brittle and ductile deformation styles developed within distinct bed packages, together with the presence of both chert clasts, folded chert layers and spherical chert nodules, suggest that sediments were at different stages of lithification prior to downslope movement associated with mass‐transport deposits. This study helps elucidate the emplacement processes, frequency and character of subseismic‐scale mass‐transport deposits within the basinal carbonate environment, and thereby reduces the uncertainties in the characterization of subsurface carbonate geofluid reservoirs.

Sedimentological and Ichnological Characterization of the Middle To Upper Devonian Horn River Group, British Columbia, Canada: Insights Into Mudstone Depositional Conditions and Processes Below Storm Wave Base

Abstract The Middle and Upper Devonian Horn River Group in northeastern British Columbia, Canada, consists predominantly of organic-rich mudstones that are typically described as black shales. This stratigraphic unit has seen substantial exploration and development for natural gas during the last decade. Although black shales such as the Horn River Group have historically been interpreted as deposited in anoxic deep-water basins, detailed sedimentological and ichnological analyses of eleven Horn River Group cores indicate that the depositional conditions varied significantly with respect to paleoenvironments, redox conditions, and physical processes, resulting in distinctive sedimentary facies. This study integrates data on lithology and physical and biogenic sedimentary structures from detailed core descriptions combined with total-organic-carbon content to better understand depositional processes and conditions. Ten mudstone lithofacies and three lithofacies associations are identified. Massive mudstones and pyrite-rich mudstones display very rare planar lamination and graded beds and scarce bioturbation (BI 0–1), and are interpreted to represent anoxic waters, below storm wave base (SWB), with conditions of relatively low to moderate energy. Heterolithic, laminated, and more bioturbated lithofacies are interpreted to represent oxygenated waters below SWB with relatively high-energy conditions. Overall bioturbation intensity varies between moderate to intense (BI 3–6). Where these lithofacies are not intensely bioturbated, oxygenated lithofacies show combinations of well-preserved physical sedimentary structures, including horizontal parallel lamination, soft-sediment deformation, double mud drapes, graded beds, and current ripples. A third lithofacies association represents conditions intermediate between anoxic and oxygenated lithofacies associations, mainly based on local changes in the ichnological characteristics. These lithofacies show either intense or sparse bioturbation (BI 4–6 or BI 0–1, respectively) depending on the core location, possibly indicating local changes in the depositional conditions and processes (e.g., variations in the oxygen content and/or sedimentation rate). They can also be found in thick anoxic and oxygenated deposits. These lithofacies display intervals of faint lamination, amalgamated current ripples, and normally graded bedding. The cores collectively display significant spatial and stratigraphic heterogeneity in lithology, sedimentary structures, and bioturbation that are related to relative water depth, oxygen content, sediment input, and deep-water currents. We interpreted bioturbation intensity to vary mainly as a function of oxygen content related to proximity to the basin margin and sea–level fluctuations, although secondary stress conditions (e.g., nutrient content and sedimentation rate) may also be important. Unbioturbated and massive fabrics represent anoxic to dysoxic conditions. Current-generated structures are present, mainly in the central and distal parts of the basin, and are interpreted to represent the presence of contour currents. In the proximal parts of the basin, laminated and heterolithic units are more common and more bioturbated than in the central and distal basin. This study thus can provide significant insights into other shale basins that show similar lateral and vertical variations. Integration of multidisciplinary studies, particularly sedimentology, ichnology, and total organic carbon aids in understanding and modeling the complexity of these systems.

The Cenomanian-Turonian Boundary Event (CTBE) in northern Lebanon as compared to regional data – Another set of evidences supporting a short-lived tectonic pulse coincidental with the event?

Constrained by both foraminifera and isotope data, the Zeitoun and Nahr Ibrahim sections in northern Lebanon record a steady flooding of the Cenomanian carbonate platform during the uppermost Cenomanian-lower Turonian interval. The flooding began just before the inception of the CTBE δ13C positive shift. It is marked by the deposition of finely bedded to laminated, fine-grained limestone overlying platformal carbonates. This scenario is entirely different to what has been published in northern Israel, where repeated emergences are instead recorded during the event. A survey of published data from nearby Middle East countries to North Africa suggests that the CTBE is accompanied on a larger scale, especially along the Syrian Arc, by tectonic disturbances responsible for sequence stratigraphic heterogeneity and palaeogeographic changes. Detailed analysis of available data makes it possible to extract from the sedimentary record four sequence stratigraphic scenarios implying two phases of heterogeneity bounding the event, and responsible for the outphasing of relative sea level changes in some sensitive areas. These disturb a stepped early Cenomanian to early Turonian overall transgressive trend recorded in some large stable areas, often used to support the concept of Mesozoic eustasy. The discussion focuses on the global processes that are possibly involved.The hypothesized tectonic pulse does not explain the CTBE. It is just another set of data to be taken into account in the search for causes.

Structural significance of an evaporite formation with lateral stratigraphic heterogeneities (Southeastern Pyrenean Basin, NE Spain)

We run a series of analogue models to study the effect of stratigraphic heterogeneities of an evaporite formation on thin-skinned deformation of the Southeastern Pyrenean Basin (SPB; NE Spain). This basin is characterized by the existence of evaporites, deposited during the Early-Middle Eocene with lateral variations in thickness and lithological composition. These evaporites are distributed in three lithostratigraphic units, known as Serrat Evaporites, Vallfogona and Beuda Gypsum formations and acted as décollement levels, during compressional deformation in the Lutetian. In addition to analogue modeling, we have used field data, detailed geological mapping and key cross-sections supported by seismic and well data to build a new structural interpretation for the SPB. In this interpretation, it is recognized that the basal and upper parts of the Serrat Evaporites acted as the main décollement levels of the so-called Cadí thrust sheet and Serrat unit. A balanced restoration of the basin indicates that thrust faults nucleated at the stratigraphic transition of the Serrat Evaporites (zone with lateral variations of thickness and lithological composition), characterized by a wedge of anhydrite and shale. The analogue models were setup based on information extracted from cross-sections, built in two sectors with different lithology and stratigraphy of the evaporites, and the restored section of the SPB. In these models, deformation preferentially concentrated in areas where thickness change, defined by wedges of the ductile materials, was inbuilt. Based on the structural interpretation and model results, a kinematic evolution of the SPB is proposed. The kinematic model is characterized by the generation of out-of-sequence structures developed due to lateral stratigraphic variations of the Serrat Evaporites. The present work shows a good example of the role of stratigraphic heterogeneities of an evaporite formation which acts as décollement level on structural deformation in a fold-thrust belt. The results of this work have implications for hydrocarbon exploration and are relevant for studying structural geometry and mechanics in shortened evaporite basins.

Pre-stack separation of PP and split PS waves in HTI media

Separation of PP and split PS waves in transversely isotropic media with a horizontal axis of symmetry is crucial for imaging subsurface targets and for fracture prediction in a multicomponent seismic survey using P-wave sources. In conventional multicomponent processing, when a low velocity zone is present near the surface, it is often assumed that the vertical Z-component mainly records P modes and that the horizontal X- and Y-components record S modes, including split PS waves. However, this assumption does not hold when the ubiquitous presence of azimuthal anisotropy makes near surface velocity structures more complicated. Seismic wavefields recorded in each component therefore generally represent a complex waveform formed by PP and split PS waves, seriously distorting velocity analysis and seismic imaging. Most previous studies on wave separation have tended to separate P and S modes using pre-stack data and to separate split S modes using post-stack sections, under the assumption of orthogonal polarization. However, split S modes can hardly maintain their original orthogonal polarizations during propagation to the surface due to stratigraphic heterogeneity. Here, without assuming orthogonal polarization, we present a method for pre-stack separation of PP, PS1 and PS2 waves using all three components. The core of our method is the rotation of wave vectors from the Cartesian coordinate system established by Z-, R- and T-axes to a coordinate system established by the true PP-, PS1- and PS2-wave vector directions. Further, we propose a three-component superposition approach to obtain base wave vectors for the coordinate system transformation. Synthetic data testing results confirm that the performance of our wave separation method is stable under different noise levels. Application to field data from Southwest China reveals the potential of our proposed method.

Middle Eocene paleohydrology of the Dongying Depression in eastern China from sedimentological and geochemical signatures of lacustrine mudstone

Previous understanding that stratigraphic variability in lacustrine sequences is primarily caused by depositional processes governed by regional tectonics and climatic change remains to be tested on microscopic scale. Here we study the upper fourth Member (Es4U) of the Eocene Shahejie Formation, the major source rock of the Dongying Depression, eastern China, in order to understand stratigraphic heterogeneity and tectonic and climate controls on depositional processes in lacustrine mudstone. Our study integrates core descriptions, microscopic observations, and studies of mineralogy, TOC and chemical weathering proxies of CIA* and Ln(Al2O3/Na2O) from two full-bore cores and correlation of gamma ray logs from six wells across the depression. We identified four major lithofacies, including the interbedded evaporite and mudstone deposited in a shallow salt lake; the silt-bearing clay-rich mudstone deposited in a pro delta environment within a shallow open lake; the laminated calcareous mudstone deposited in a deep, stratified meromictic lake with frequent earthquake activities; and the massive calcareous mudstone deposited in a shallow and oxygenated lake. The Es4U Member changed from the interbedded evaporite and mudstone-dominated lithofacies association to silt-bearing clay-rich mudstone-dominated lithofacies association, and then laminated calcareous mudstone-dominated lithofacies association, suggesting basin-wide changes in depositional processes, lake level, lake water chemistry, and paleohydrology. We suggest that these changes reflect balance between sediment and water supply and potential accommodation of the lake basin, which were governed by paleoclimate in the lake drainage and extensional tectonics. We infer that during the deposition of the Es4U Member, the lake basin evolved from an underfilled lake basin with salty lake water under arid climate to a shallow balanced-fill lake basin when climate became humid, then to a deep balanced-fill basin with stratified bottom lake water when climate became more humid and the basin was deepened by tectonic activity. This study demonstrates that climate and tectonics both control paleolimnological changes and depositional processes of lacustrine mudstone.

Excursions along the Lennard Shelf Devonian carbonates, Canning Basin, Western Australia

ABSTRACT The world-class Middle–Upper Devonian carbonate outcrops of the Lennard Shelf, Canning Basin, Western Australia, offer a unique opportunity to examine reefal carbonate shelf-to-basin evolution in response to multiple coeval extrinsic and intrinsic drivers. Variable styles of carbonate stratigraphic architecture and heterogeneity developed as a function of the interplay between long hierarchical accommodation trends, global biological crises, greenhouse-to-transitional climatic changes, and syndepositional tectonics. The linkage of reservoir-scale, shelf-to-basin carbonate expression to regional, seismic-scale architectures made possible by these pristine exposures allows the generation of conceptual models and predictive relationships that are relevant to steep-sided carbonate hydrocarbon reservoirs and plays. Furthermore, exquisite exposure of syndepositional fracture systems and their association with carbonate facies and position along the depositional profile provide templates for characterization of subsurface nonmatrix flow. In particular, the Lennard Shelf outcrops are excellent analogs for the age-equivalent Canadian Alberta Basin and Carboniferous reservoirs of the Pricaspian Basin in Kazakhstan; however, the breadth of insight contained within the Lennard Shelf outcrop belt is applicable to the greater understanding of reefal carbonate systems in general.

Seismic architecture and seismic geomorphology of heterozoan carbonates: Eocene-Oligocene, Browse Basin, Northwest Shelf, Australia

Seismic characterization of Eocene-Oligocene heterozoan carbonate strata from the Browse Basin, Northwest Shelf of Australia, defines marked progradation of nearly 10 km. Stratal terminations and stacking subdivide the succession into mappable seismic units. Stratal architecture and seismic geomorphology varies systematically through the succession.Individual surfaces, discerned by toplap, onlap, and truncation, outline sigmoidal to tangential oblique clinoforms with heights of ranging from 350 to 650 m and maximum gradients between 8 and 18°. Sigmoidal clinoforms can include aggradation in excess of ∼200 m, prograde more than 500 m, and have slopes characterized by inclined, wavy to discontinuous reflectors that represent ubiquitous gullies and channels. In contrast, the overlying tangential oblique clinoforms include downstepped shelf margins, limited on-shelf aggradation (<100 m) and toplap, subdued progradation (<500 m), and continuous parallel inclined reflectors on the slope. Wedges of basinally restricted reflectors at toe of slope onlap surfaces of pronounced erosional truncation or syndepositional structural modification. The succession includes repeated patterns of seismic units that onlap, aggrade, and prograde, interpreted to represent sequence sets and composite sequences.The associations of shelf aggradation, shelf-margin progradation, and slope channeling within sigmoidal seismic units and the less marked progradation and channeling within tangential oblique seismic units contrast with the classic sequence model in which sediment delivery to the slope and pronounced progradation is favored by limited shelf accommodation. This distinct divergence is interpreted to reflect the prolific heterozoan production across the shelf during periods of rising and high base level when the shelf is flooded, perhaps enhanced by downwelling. Comparison with purely photozoan systems reveals similarities and contrasts in seismic stratigraphic heterogeneity and architecture, interpreted to be driven by distinct characteristics of heterozoan sedimentary systems.

Detailed 3-D depositional architecture of Late Jurassic carbonate–anhydrite cycles (Brightling Mine, Weald Basin, UK)

Quantifying the geometries of evaporite deposits at a <1 km scale is critical in our understanding of similar ancient depositional systems, but is challenging given evaporite mineral dissolution at surface conditions. A high-resolution stratigraphic study of the basal Purbeck Beds in Brightling Mine, UK, provides insight into the three-dimensional architecture, lateral continuity and vertical heterogeneity within an evaporite seal. We conducted a field mapping study, combined with X-ray diffraction, petrographic microscopy, and δ13C and δ18O isotope analysis. The stratigraphic interval contains five facies. In stratigraphic order, these include supratidal porphyritic nodular evaporite, shallow subtidal peloidal packstone with evaporite and two overlying rhythmic sequences of intertidal microbial laminite, subtidal shale, and subtidal laminar marl, capped by nodular anhydrite. The interpreted environment of deposition is a supratidal sabkha subject to periodic flooding in which intertidal (tidal flat) facies and subtidal (shallow marine) facies laterally passed into the evaporative sabkha. The cycles are interpreted as meter-scale shoaling-upward sequences, likely controlled by localized high-frequency changes in relative sea level and/or sabkha hydrology. Spatial patterns in the geometries of key stratigraphic surfaces reveal a subtle depression towards the central western region of the mine seam. The variation in stratal geometries is interpreted as paleotopography and is a function of individual or composite processes related to dissolution, eolian processes, and coastal erosion. These observations indicate a similar mode of deposition to the modern-day sabkha of the Persian Gulf. We conclude that the dynamic process of evaporite deposition led to subtle stratigraphic heterogeneities and changes in bed thicknesses, but largely continuous lateral bedding at an interwell-scale.

Multi-level stratigraphic heterogeneities in a Triassic shoal grainstone, Oman Mountains, Sultanate of Oman: Layer-cake or shingles?

ABSTRACT A fundamental question in the correlation of 1-D sedimentologic data is whether to use a layer-cake or shingled correlation approach. The resulting reservoir geometry has important implications for the characterization of reservoir heterogeneities and fluid flow. On the Saiq Plateau in Oman, epeiric carbonate ramp deposits of the Triassic Sudair Formation are well exposed and can be investigated in detail over several kilometers. There, reservoir heterogeneities on different scales have been documented by creating various outcrop wall panels and 2-D correlations. Multi-level architectural elements with different depositional geometries were discovered, which were linked to a sequence-stratigraphic hierarchy consisting of three levels. Level 1: A “layer-cake”-type stratigraphic architecture with minor thickness variations over several kilometers becomes apparent when correlating fourth-order cycle set boundaries. Level 2: The correlation of fifth-order cycle boundaries reflects horizontally continuous geometries, within which, however, internal grainstone layers were discovered to be arranged in a shingled fashion. Muddy layers in between these shingles illustrate sixth-order mini-cycle boundaries. Level 3: Within sixth-order mini-cycles another scale of a shingle-like architecture can be observed. Amalgamated cm-thick grainstone units form thin wedges with subtle but clearly inclined dipping geometry. Fourth-order cycle sets and fifth-order cycles can be traced over several kilometers, and therefore assumed to be related to allocyclic stratigraphic processes. The internal shingle geometries within fifth-order cycles are traceable over 100s of meters and presumably reflect an autocyclic lateral migration of a shoal complex. Cm-thick shingling grainstone wedges within sixth-order mini-cycles are interpreted as storm-related spill deposits. Their event-driven character is reflected by frequent amalgamation and reworking of the preceding deposits. The results of this study of epeiric carbonate ramp deposits suggest that a “layer-cake” correlation approach is appropriate when correlating 10s of m-thick grainstone units over a distance of several kilometers. However in the documented example, these thick grainstone units consist internally of small-scale architectural elements, which show inclined geometries and require a shingled correlation approach. These small-scale heterogeneities within an overall “layer-cake” architecture might have an impact on fluid flow in similar subsurface reservoirs and should be taken into account for detailed reservoir correlations and static reservoir models.

The dynamic behavior of shallow marine reservoirs: Insights from the Pliocene of offshore North Trinidad

The Pliocene reservoirs of the North Coast Marine Area (block NCMA-1) comprise shallow marine shoreface to shelf sandstones (up to 30 m [98 ft] thick) and interbedded shelfal mudstones organized into stacked upward-coarsening parasequences (ca. 25–70 m thick). Production behavior is strongly conditioned by the interplay of subregional sand distribution and aquifer connectivity, and intrafield reservoir architecture complexity, commonly in the form of clinoforms. The M2 parasequence is a well-connected reservoir unit and forms part of a late highstand systems tract with a strongly progradational architecture. Large-scale reservoir connectivity can be demonstrated by pressure communication in producing fields up to 10 km (6 mi) apart. The widespread nature of the sand also links the reservoir to a significant aquifer that provides pressure support during field depletion. In contrast, clinoforms that dominate the M2 reservoir architecture at a variety of scales reduce lateral connectivity. Wells situated in individual sand bodies show significant pressure interference during production, whereas wells that straddle bounding clinoforms (possible parasequence boundaries), although in pressure communication, show more subtle pressure interference. In the Chaconia field, clinoforms that bound lobate sand bodies in the M2 reservoir have also apparently slowed the onset of water cut within the field by several years and therefore increased reserves recovery. Similarly, in the M6 reservoir of the Ixora field a significant bounding clinoform has partitioned the field through much of its production life, but production data are now suggesting breakdown and late-stage pressure support across the same clinoform. The M6 parasequences form part of a lower quality early highstand systems tract and are more discontinuous than their M2 counterpart due to well-developed bounding clinoforms. As such they are not characterized by large active aquifers and do not benefit from such prominent long-term pressure support. The M4 reservoir is an isolated sand-body complex that forms part of a high-frequency lowstand complex. It shows similar stratigraphic heterogeneity to that displayed in the M2, with variations in pressure decline exhibited within individual wells of the same reservoir unit. Its isolated geometry means it is not connected to a regional aquifer and is therefore undergoing volumetric decline with no water breakthrough.