Lithofacies Distribution Research Articles

Mesophotic coral buildups in a prodelta setting (Late Eocene, southern Pyrenees, Spain): a mixed carbonate–siliciclastic system

AbstractLower Priabonian coral bioherms and biostromes, encased in prodelta marls/clays, occur in the Aínsa‐Jaca piggyback basin, in the South Central Pyrenean zone. Detailed mapping of lithofacies and bounding surfaces onto photomosaics reveals the architecture of coral buildups. Coral lithosomes occur either isolated or amalgamated in larger buildups. Isolated lithosomes are 1 to 8 m thick and a few hundred metres wide; clay content within coral colonies is significant. Stacked bioherms form low‐relief buildups, commonly 20 to 30 m thick, locally up to 50 m. These bioherms are progressively younger to the west, following progradation of the deltaic complex. The lowermost skeletal‐rich beds consist of bryozoan floatstone with wackestone to packstone matrix, in which planktonic foraminifera are abundant and light‐related organisms absent. Basal coral biostromes, and the base of many bioherms, consist of platy‐coral colonies ‘floating’ in a fine‐grained matrix rich in branches of red algae. Corals with domal or massive shape, locally mixed with branching corals and phaceloid coral colonies, dominate buildup cores. These corals are surrounded by matrix and lack organic framework. The matrix consists of wackestone to packstone, locally floatstone, with conspicuous red algal and coral fragments, along with bryozoans, planktonic and benthonic foraminifera and locally sponges. Coral rudstone and skeletal packstone, with wackestone to packstone matrix, also occur as wedges abutting the buildup margins. Integrative analysis of rock textures, skeletal components, buildup anatomy and facies architecture clearly reveal that these coral buildups developed in a prodelta setting where shifting of delta lobes or rainfall cycles episodically resulted in water transparency that allowed zooxanthellate coral growth. The bathymetric position of the buildups has been constrained from the light‐dependent communities and lithofacies distribution within the buildups. The process‐product analysis used here reinforces the hypothesis that zooxanthellate corals thrived in mesophotic conditions at least during the Late Eocene and until the Late Miocene. Comparative analysis with some selected Upper Eocene coral buildups of the north Mediterranean area show similarities in facies, components and textures, and suggest that they also grew in relatively low light (mesophotic) and low hydrodynamic conditions.

Widespread inflation and drainage of a pāhoehoe flow field: the Nesjahraun, Þingvellir, Iceland

This study describes the emplacement of the Nesjahraun, a basaltic lava flow that entered the lake Þingvallavatn, SW Iceland. High-resolution remotely sensed data were combined with fieldwork to map the flow field. Onshore, the Nesjahraun exhibits a variety of textures related to the widespread inflation and collapse of a pāhoehoe flow field. Its emplacement is interpreted as follows: Initially, the eruption produced sheet pāhoehoe. In the central part of the flow field, the lava has a platy-ridged surface, which is similar to some other lava flows in Iceland and on Mars. Here, the texture is interpreted to have formed by unsteady inflation of the brittle crust of stationary sheet pāhoehoe, causing it to break into separate plates. The ridges of broken pāhoehoe slabs formed as the plates of crust moved vertically past each other in a process similar to the formation of shatter rings. Upstream, fresh lava overflowed repeatedly from channels and tubes, covering the surface with shelly pāhoehoe. Formation of a 250-m-wide open channel through the flow field allowed the inflated central part of the flow to drain rapidly. This phase produced ‘a‘ā lava, which eroded the channel walls, carrying broken pāhoehoe slabs, lava balls and detaching large (>200 m long) rafts of compound shelly pāhoehoe lava. Much of this channelized lava flowed into the lake, leaving a network of drained channels and tubes in the upstream part of the flow. As in other locations, the platy-ridged texture is associated with a low underlying slope and high eruption rate. Here, its formation was possibly enhanced by lateral confinement, hindered entry into the lake and an elevated vent location. We suggest that formation of this type of platy-ridged lava, where the plates are smooth and the ridges are slabs of broken pāhoehoe, can occur without significant horizontal transport, as the surface crust is broken into plates in situ. This reconstruction of the emplacement of the Nesjahraun also demonstrates that high-resolution aerial survey data are extremely useful in the mapping and measurement of lithofacies distributions in large flow fields, but that fieldwork is still necessary to obtain the detailed textural information necessary to interpret them.

Modeling of petroleum generation in the Vietnamese part of the Malay Basin using measured kinetics

Two-dimensional (2-D) modeling of source rock maturation and hydrocarbon (HC) generation histories were conducted for the Malay-Cho Thu Basin in the Gulf of Thailand and South China Sea. The source rocks comprise Oligocene synrift lacustrine mudstones and coals, postrift coals, and terrigenous-influenced mudstones. Three different lacustrine mudstone source rocks and coals were considered in the 2-D modeling, and each source rock was assigned specific generation potential and kinetics. Measured kinetics for bulk petroleum generation determined on two thermally immature upper Oligocene oil-prone lacustrine mudstones and on a terrigenous-influenced mudstone derived from wells were used together with published coal kinetics. The onshore Krong Pa graben, a small-scale rift basin in central Vietnam, was used as a conceptual model for the distribution of lithofacies, including source rock types, in the 2-D models. Exploration targets and drilled direct HC indicators (DHIs) are mainly located in the tectonically disturbed postrift succession. Modeling results suggest that the risks related to this play are (1) timing of petroleum generation from the oil-prone lacustrine synrift deposits relative to structural trap formation, (2) complex migration pathways through strongly faulted strata, and (3) relatively small kitchen areas. Oil generation from uppermost synrift and postrift coaly source rocks with minor contribution from lacustrine deposits after principal trap formation is consistent with the prominent terrigenous geochemical signature of oils in the Malay-Cho Thu Basin. A new untested play is proposed based on mapping of potential DHIs associated with sandstone bodies in tectonically undisturbed synrift traps.

Chapter 6 Phanerozoic palaeoenvironment and palaeolithofacies maps of the Arctic region

Abstract Sixty-three maps illustrate geodynamic evolution and development of palaeoenvironments and palaeolithofacies of the Circum-Arctic region during Phanerozoic times. After the break-up of Rodinia and Pannotia in the Early Palaeozoic, the major Arctic plates Baltica, Siberia and Laurentia drifted from their original position around the South Pole towards the Supercontinent Pangea, which existed in the equatorial position during Late Palaeozoic and Early Mesozoic times. During the Mesozoic and Cenozoic plates gathered around newly formed Arctic Ocean. Large continental masses were assembled from major plates and numerous small plates and terranes on the northern hemisphere and around the North Pole. All the continents were by now connected. Carbonates were abundant in Siberia and Laurentia during Palaeozoic times. Clastic sedimentation prevailed during Mesozoic and Cenozoic times. The distribution of lithofacies shows climatic change associated with continental assembly and disassembly as well as with the steady northward drift of the continents.

Alluvial-deltaic model of the middle-upper Jurassic oil-bearing JV1 productive horizon in the east of the Shirotnoe Priobye

Based on core, log data and seismic, alluvial-deltaic model of the JV12 oil-bearing layer from the one of the regions of the Shirotnoe Priobye in Western Siberia was suggested. To establish the vertical and lateral variation in sediments, the section types were revealed based on electrometric facial models and detailed core description. Coherence and acoustic inversion results allowed authors to reveal the areal distribution of lithofacies. Perspective areas for exploration and exploitation drilling were selected.

Capturing stratigraphic and sedimentological complexity from submarine channel complex outcrops to digital 3D models, Karoo Basin, South Africa

ABSTRACT Submarine slope channel-fills form complicated stratigraphy and lithofacies distributions through repeated phases of erosion and deposition. This provides a challenge to accurate 3D modelling, particularly in representing lithofacies transitions within sand-poor areas. In this paper, traditional (sedimentary logs, palaeocurrent measurements, architectural panels) and non-conventional technologies (Light Detection and Ranging; Ground Penetrating Radar) were integrated to quantitatively describe lithofacies distributions and sedimentary architectures from two large-scale outcrops, one base of slope, high sandstone content system (Unit B) and one from a mid-slope, more mixed lithology system (Unit C), in the Laingsburg Formation, Karoo Basin, South Africa. The workflow described in this study combines digital structural restoration and extrapolation of major stratigraphic surfaces, grouped palaeocurrents and architectural geometries observed at outcrop to create 3D digital models. The model was divided into zones along major stratigraphic discontinuities and populated using lithofacies associations that were adjusted for outcrop rugosity and palaeodispersal direction. Observed channel margin asymmetries, distribution of lithofacies and stacking patterns were all honoured in the digital models. The Unit C slope-channel system differs from many exposed submarine channels due to the low proportion of sandstone present within the infill. Thin-bedded channel margin lithofacies are preserved through the lateral stepping of channels and allow the correlation of stratigraphy from channel axis to margin and on to overbank areas. In the older, sandier Unit B base-of-slope system, the stratigraphic change in stacking pattern, channel aspect ratio, lithofacies of channel-fills and stratigraphic hierarchy were all captured. This research captured the architectural complexity observed at outcrop to generate more realistic models than could be constructed normally using limited subsurface data.

Dominance of lateral over axial sedimentary fill in dryland rift basins

ABSTRACT Facies models for continental rift basins suggest longitudinal (axis-parallel) fluvial activity dominates sediment transport and deposition. In contrast, modern basins in the arid to semi-arid northern Basin and Range Province, USA, show axial drainage development to be characterized by short endorheic systems that contribute little to the basin fill. Mapping and calculation of the proportionate distribution of surficial facies of three representative basins at different stages of rift evolution show that basin fill is dominated by the deposits of transverse catchments, and axial fluvial deposits are restricted to a narrow corridor by the progradation of lateral systems. Drainage integration in these dryland rifts is limited by the moisture-stressed climate, with a resultant reduction in stream power, and the complex, tectonically-induced physiography, which limits potential drainage pathways. River systems that flow through multiple structural basins are rare, restricted to those systems with catchment headwaters lying outside the dryland climatic regime. These data imply that long-range, axial fluvial deposits should not automatically be included as a significant part of dryland rift sedimentary facies models. Sediment routing pathways in ancient dryland rift systems may be much shorter than commonly predicted, so affecting the spatial distribution of lithofacies. Climate, therefore, has a much stronger control on drainage and lithofacies at all stages of rift development than is generally stated.

Blocking Moving Window algorithm: Conditioning multiple‐point simulations to hydrogeological data

Connectivity constraints and measurements of state variables contain valuable information on aquifer architecture. Multiple‐point (MP) geostatistics allow one to simulate aquifer architectures, presenting a predefined degree of global connectivity. In this context, connectivity data are often disregarded. The conditioning to state variables is usually carried out by minimizing a suitable objective function (i.e., solving an inverse problem). However, the discontinuous nature of lithofacies distributions and of the corresponding objective function discourages the use of traditional sensitivity‐based inversion techniques. This work presents the Blocking Moving Window algorithm (BMW), aimed at overcoming these limitations by conditioning MP simulations to hydrogeological data such as connectivity and heads. The BMW evolves iteratively until convergence: (1) MP simulation of lithofacies from geological/geophysical data and connectivity constraints, where only a random portion of the domain is simulated at every iteration (i.e., the blocking moving window, whose size is user‐defined); (2) population of hydraulic properties at the intrafacies; (3) simulation of state variables; and (4) acceptance or rejection of the MP simulation depending on the quality of the fit of measured state variables. The outcome is a stack of MP simulations that (1) resemble a prior geological model depicted by a training image, (2) honor lithological data and connectivity constraints, (3) correlate with geophysical data, and (4) fit available measurements of state variables well. We analyze the performance of the algorithm on a 2‐D synthetic example. Results show that (1) the size of the blocking moving window controls the behavior of the BMW, (2) conditioning to state variable data enhances dramatically the initial simulation (which accounts for geological/geophysical data only), and (3) connectivity constraints speed up the convergence but do not enhance the stack if the number of iterations is large.

Preliminary assessment of ichnofacies in the Gething Formation of NE British Columbia

Abstract The Gething Formation in the Kahntah River–Ring/Border region of northeastern British Columbia is a variably thick, heterolithic succession of facies deposited in continental and marginal-marine depositional settings. This unit was deposited on the southwestern flank of the Keg River Highlands in the northern Western Canada Foreland Basin (approximately 58°N, 120°W). Although Gething Formation sandstone bodies are exploration targets further south, it is not considered to be a primary exploration target in the study area due largely to its complex and laterally discontinuous nature. However, in the study interval several of the lithofacies possess good reservoir properties, and the overlaying Wilrich Shale provides a potential stratigraphic seal. Hydrocarbon sources are available in both the Wilrich Shale and the Triassic Montney Formation into which the Gething Formation incises. An understanding of the overall palaeogeography and lithofacies distribution of the Gething Formation in the study area is necessary for efficiently exploiting these hydrocarbon resources and will facilitate characterization of other Gething Formation reservoirs that may flank the Keg River Highland to the east and west. Subsurface mapping in the area of Ring and Kahntah River gas fields shows that the Gething Formation in the study area occurs within three sinuous incised valleys that debouch southward into a large structural basin. In their northernmost (upstream) reaches, Gething Formation strata are dominated by unbioturbated, high-angle cross-bedded and current-ripple laminated upper-fine-grained sandstones, rich in organic detritus (primarily coal flakes). These are interpreted as fluvial channel and bayhead delta deposits. Lithofacies deposited in continental settings grade southwards into bioturbated, low- and high-angle cross-bedded, lower to upper-fine-grained sandstones with thin, bioturbated siltstone and mudstone intervals. Rhythmic bedding, mud flasers and mud/sand couplets characterize these lithofacies. The trace fossils Planolites, Skolithos, Cylindrichnus, Arenicolites, Teichichnus, and fugichnia are common. These strata are interpreted to represent brackish-water bay deposits. South of this, the Gething Formation is dominated by interstratified, sporadically bioturbated sand, silt and mud, sedimentologically similar to the aforementioned brackish channel deposits, but with more diverse and robust ichnofauna (the previous assemblage as well as large Thalassinoides, rare Helminthopsis and Phycosiphon). These deposits are ascribed to tidal channels in a larger, more marine bay.

Dynamic Carboniferous climate change, Arrow Canyon, Nevada

The Phanerozoic9s longest-lived and most widespread glaciation, the late Paleozoic ice age, is undergoing a resurgence in interest. Long-held models of the timing, duration, and magnitude of glaciation are being reevaluated due to emerging evidence from former high latitudes, evidence that the late Paleozoic ice age was punctuated by long-lived glacial minima or possibly ice-free times. The history of the late Paleozoic ice age is archived within the biostratigraphically well-constrained, carbonate-dominated succession of Arrow Canyon, Nevada, United States. In this paleo-tropical succession, the distribution of lithofacies, flooding surfaces, and subaerial exposure horizons and their stacking into meter-scale cycles record a detailed climate history. The onset of this phase of glaciation during the middle Mississippian was followed by a dynamic evolution of glacioeustasy through the late Mississippian to late Pennsylvanian. Moderate- to high-amplitude glacioeustasy was likely interrupted by an earliest Pennsylvanian short-lived glacial minimum, but otherwise appears to have persisted through the middle Pennsylvanian. Upper Pennsylvanian strata record low- to moderate-amplitude relative sea-level changes, suggesting a long-lived interval of diminished ice volume. This proposed glacial minimum is coincident with a notable minimum in glaciogenic sedimentation near the former southern pole, aridification across paleo-tropical Pangea, and significant floral and faunal turnover, suggesting a link between tropical environmental change and high-latitude glaciation. These conclusions, however, are at odds with those traditionally inferred from Euramerican cyclothems, i.e., persistent high-amplitude glacioeustasy driven by a single, large ice sheet. Rather, the Arrow Canyon archive of varying depositional facies and cycle stacking patterns records major changes in the magnitude of short-term glacioeustasy. This finding contributes to recent and growing near- and far-field evidence for a more dynamic glaciation history than previously inferred from the classic Euramerican cyclothems.

Changes in the architecture of fluvial deposits in the Paganzo Basin (Upper Paleozoic of San Juan province): an example of sea level and climatic controls on the development of coastal fluvial environments

Paganzo Group exposures (Tupe Formation) in the Huaco area provide an excellent opportunity for assessing the role of sea level and climatic changes on the morphology and nature of coastal fluvial systems deposited in areas of limited tectonic activity. The paleogeographic position of Huaco, close to a coastal region within the Paganzo Basin, allows identification of the effects of sea level change on fluvial architecture. Despite the fact that the Huaco area was dominated by coastal fluvial systems, three marine incursions flooded this part of the basin during the Namurian, Early Pennsylvanian and Late Pennsylvanian respectively. During deposition of the Paganzo Group, climatic conditions evolved from glacial (Namurian) to hot and dry (Late Cisuralian). Five types of fluvial deposits were recognized on the basis of architectural element analysis, lithofacies distribution and type of fluvial bounding surfaces present. Fluvial system 1 (FS1) constitutes the lower part of the Tupe Formation and consists of stacked multi-storey channel-fill complexes formed on large braided alluvial plains dominated by channel-avulsion processes. FS2 consists of multi-storey channels alternating with floodplain deposits including coal beds and organic-rich mudstones. This fluvial system is interpreted as the deposit of an anastomosed network of sandy channels. FS3 occurs between the Early Pennsylvanian and Late Pennsylvanian marine transgressions and consists of sandstones and some conglomerates that form stacked channel complexes. Sporadically, very fine-grained sandstone and mudstone floodplain deposits appear as thin intercalations. FS3 likely formed on braided alluvial plains with channels dominated by transversal bars. FS 4 corresponds to an anastomosed fluvial system that was dominated by two types of braided channel belts that were separated by narrow floodplains. Finally, FS5 is composed of fining-upward cycles ranging from gravely sandstones at the bottom of channels to muddy floodplain deposits at the top. The whole FS5 succession was deposited by high-sinuosity meandering rivers. Detailed stratigraphic analyses clearly suggest that both, sea level and climate changes were first-order controls on fluvial system configurations. In this way, braided systems belonging to FS1 correspond to a low-accommodation system tract. Whereas, coal beds of FS2, which resulted from high water-tables, correspond to a high accommodation system tract that was likely associated with advanced stages of the Late Pennsylvanian transgression. A significant change in the nature of fluvial deposits took place prior to the Late Pennsylvanian sea level rise when braided fluvial systems (FS3) with very scarce floodplain deposits prevailed. Towards the top of the Late Pennsylvanian transgressive deposits, a high relief fluvial incision surface was carved into the underlying marine deposits. This surface was later mantled by anastomosed rivers (FS4) corresponding to low-accommodation deposits formed in a lowstand or during the early stages of the ensuing sea level rise. In later phases of this transgression, high accommodation conditions prevailed and fluvial sedimentation was dominated by high-sinuosity rivers (FS5). These fluvial deposits are considered as an inland equivalent to the shallow-marine deposits exposed in the neighboring Agua Negra Formation located to the west.

Evolution, architecture and hierarchy of distributary deep‐water deposits: a high‐resolution outcrop investigation from the Permian Karoo Basin, South Africa

AbstractSea floor and shallow seismic data sets of terminal submarine fan lobes can provide excellent planform timeslices of distributive deep‐water systems but commonly only limited information on cross‐sectional architecture. Extensive outcrops in the Tanqua depocentre, south‐west Karoo Basin, provide these three‐dimensional constraints on lithofacies distributions, stacking patterns, depositional geometries and the stratigraphic evolution of submarine lobe deposits at a scale comparable with modern lobe systems. Detailed study (bed‐scale) of a single‐lobe complex (Fan 3) over a 15 km by 8 km area has helped to define a four‐fold hierarchy of depositional elements from bed through to lobe element, lobe and lobe complex. The Fan 3 lobe complex comprises six distinct fine‐grained sandstone packages, interpreted as lobes, which display compensational stacking patterns on a 5 km scale. Between successive lobes are thin‐bedded, very fine‐grained sandstones and siltstones that do not change lithofacies over several kilometres and therefore are identified as a different architectural element. Each lobe is built by many lobe elements, which also display compensational stacking patterns over a kilometre scale. Thickness variations of lobe elements can be extremely abrupt without erosion, particularly in distal areas where isopach maps reveal a finger‐like distal fringe to lobes. Lobe deposits, therefore, are not simple radial sheet‐dominated systems as commonly envisaged.

On the value of lithofacies data for improving groundwater flow model accuracy in a three‐dimensional laboratory‐scale synthetic aquifer

Improvement of the prediction accuracy of groundwater flow models has been receiving substantial attention from many researchers through the development of enhanced characterizations of the structure of subsurface lithofacies and of the distribution of hydraulic conductivity. In this study, we investigated how incorporating increasing amounts of lithofacies data into the construction of a conceptual model of aquifer heterogeneity helps to reduce prediction error and uncertainty in groundwater flow models. An approach based on both laboratory experiments and numerical simulations was tested using data from an intermediate‐scale synthetic heterogeneous aquifer. The heterogeneous aquifer consisted of five lithofacies, corresponding to five test sands. Three pumping tests were conducted and provided experimental data to perform groundwater flow model calibration and validation. The pumping tests were also simulated numerically in order to provide a series of error‐free synthetic hydraulic data sets. On the basis of Markov chains models of transition probabilities, a total of 901 random realizations of the heterogeneous distribution of lithofacies were created using varying amounts of conditioning lithofacies data sampled along randomly placed hypothetical boreholes. For each realization and for two other simplified lithofacies models, parameter estimation was performed to estimate the hydraulic conductivity of the lithofacies using the experimental and synthetic hydraulic data from the three pumping tests. The results generally showed that the use of more lithofacies data in the construction of the lithofacies realizations led to an improvement in groundwater flow model prediction accuracy. When using the error‐free synthetic hydraulic data, the calibration‐prediction error and uncertainty decreased drastically when the mean borehole spacing was on the order of twice the horizontal correlation length or less. When the experimental hydraulic data were used, this drastic improvement in the calibration‐prediction error was somewhat obscured and, in some cases, exhibited a local minimum. This local minimum, although beyond practical limits, corresponded to an optimal number of boreholes. Finally, the effect of incorporating more lithofacies data for the construction of lithofacies realizations was found to have a similar impact on the quality of model calibration and on the quality of predictive simulations conducted using the calibrated model.

Controls and predictability of carbonate facies architecture in a Lower Jurassic three‐dimensional barrier‐shoal complex (Djebel Bou Dahar, High Atlas, Morocco)

AbstractSpatial information on lithofacies from outcrops is paramount for understanding the internal dynamics, external controls and degree of predictability of the facies architecture of shallow‐water carbonate‐platform tops. To quantify the spatial distribution and vertical stacking of lithofacies within an outer‐platform shoal‐barrier complex, integrated facies analysis and digital field technologies have been applied to a high‐relief carbonate platform exposed in the Djebel Bou Dahar (Lower Jurassic, High Atlas, Morocco). The outer platform is characterized by subtidal, cross‐bedded, coarse grainstone to rudstone grading into supratidal, pisoidal packstone‐rudstone with tepees that together formed a 350 to 420 m wide shoal‐barrier belt parallel to the margin. This belt acted as a topographic high separating a restricted lagoon from the subtidal, open marine region. Low‐energy tidal flats developed on the protected flank of the barrier facing the lagoon. Lithofacies patterns were captured quantitatively from outcrop and integrated into a digital outcrop model. The outcrop model enabled rapid visualization of field data and efficient extraction of quantitative data such as widths of facies belts. In addition, the spatial heterogeneity was captured in multiple time slices, i.e. during different phases of cyclic base‐level fluctuations. In general, the lateral continuity of lithofacies is highest when relative water depth increased during flooding of the platform top, establishing low‐energy subtidal conditions across the whole platform, and when the accommodation space was filled with tidal flat facies. Heterogeneity increased during deposition of the relief‐building bar facies that promoted spatial diversification of depositional environments during the initial phases of accommodation space creation. Cycles commonly are composed of a thin transgressive tidal flat unit, followed by coated‐grain rudstone bar facies. Lateral to the bar facies, pisoidal‐grainstone beach deposits accumulated. These bar and beach deposits were overlain by subtidal lagoonal facies or would grow through the maximum flooding and highstand. There the bars either graded into supratidal pisoidal facies with tepees (when accommodation space was filled) or were capped by subaerial exposure (due to a sea‐level fall). Modified embedded Markov analysis was used to test the presence of common ordering in vertical lithofacies stacking in a stationary interval (constant depositional mode). Analysis of individual sections did not reveal any ordering, which may be related to the limited thickness of these sections. Composite sections, however, rejected the null hypothesis of randomness. The addition of stratigraphically significant information to the Markov analysis, such as exposure surfaces and lateral dimensions of facies bodies, strengthens the verdict of unambiguous preferential ordering. Through careful quantitative reconstruction of stratal geometry and facies relationships in fully integrated digital outcrop models, accurate depositional models could be established that enhanced the predictability of carbonate sediment accumulation.

Modeling and interpreting the seismic-reflection expression of sandstone in an ancient mass-transport deposit dominated deep-water slope environment

Subsurface mass-transport deposits (MTDs) commonly have a chaotic seismic-reflection response. Synthetic seismic-reflection profiles, created from a precise lithological model, are used to interpret reflection character and depositional geometries at multiple frequencies. The lithological model was created from an outcrop of deep-water lithofacies where sandstone deposition was influenced by mass-transport deposit topography. The influence of MTD topography on sandstone distribution should be considered in reservoir characterization and modeling when MTDs underlie the reservoir, especially if the reservoir is thin relative to the scale of the topography. MTD topography up to several tens of meters in both the horizontal and vertical dimensions (relative to local elevation) compartmentalizes significant quantities of sandstone and is not resolved at lower seismic-reflection frequencies. The resolvability of thick (up to 70 m) sandstone packages is hindered when they are encased in MTDs of at least equivalent thickness. Lateral and vertical changes in seismic-reflection character (e.g., amplitude, polarity, geometry) of sandstone packages in the synthetic profiles are due to lithology changes, tuning effects, resolution limits, and depositional geometries, which are corroborated by the lithological model. Similar reflection-character changes are observed in an actual seismic-reflection profile, of comparable scale to the synthetic profiles, from the Gulf of Mexico, which demonstrates similar lithofacies distributions. Synthetic profiles, when constrained by a precise lithological model, are particularly useful analogues for interpretation of lithofacies relationships, and depositional geometries, in complicated depositional environments, such as deep-water slope deposits.

A Geologically Based Markov Chain Model for Simulating Tritium Transport With Uncertain Conditions in a Nuclear-Stimulated Natural Gas Reservoir

Summary Nuclear-stimulation technology, which used subsurface nuclear detonation to increase permeability of tight natural gas reservoirs, was evaluated in the late 1960s and early 1970s. The Rulison site, located in the Piceance basin, Colorado, is one of three sites in the US where the technology was tested. An increase in exploration and production for natural gas in the basin has led to a need to quantify the extent of radionuclide (mainly tritium) migration after the detonation and potential migration under likely production scenarios. To meet this need, a numerical model was developed to simulate gas flow and tritium transport toward a hypothetical production well. A crucial problem in the model development is that limited on-site data are too sparse to quantify uncertainty of subsurface properties. This problem is partly resolved by using indirect data and information, such as parameter measurements from a nearby site and geological information regarding lithofacies geometry. In particular, a geologically based Markov chain model was developed to simulate spatial distribution of the sandstone lithofacies. This paper presents an application of the numerical model for simulating tritium transport from the nuclear chimney toward the production well at a likely location producing at a rate typical for the basin. The results show that under the circumstances considered in this paper, tritium will not reach the production well with a confidence level of 95%. The results also show that the lithofacies structure is more critical in controlling tritium transport than parameters of the sandstone and hydraulically fractured sandstone. The parameters become important only when the connectivity of sandstone lenses exists to support tritium transport from the chimney to the production well. The developed modeling framework can be updated as additional subsurface data are collected. The framework can be used to support establishment of drilling restrictions that protect public health and the environment for different production well scenarios.

Distinguishing Milankovitch‐Driven Processes in the Rock Record from Stochasticity Using Computer‐Simulated Stratigraphy

Repetitive patterns of facies recurrence are frequently reported from the shallow‐water sedimentary rock record and are postulated to have been driven by orbital forcing on eustatic sea level. Consequently, multiple statistical techniques have been developed to evaluate whether patterns of stratigraphic succession are more consistent with a periodic sea level signal or are stochastic. Previous studies focused on development and/or application of such methods to test empirical geological records. However, the character of such records cannot be known a priori, as deposition, erosion, and preservation influence the resultant composition, nor can those records be readily manipulated to explore the sensitivity, robustness, and overall validity of statistical methods. Here we simulate carbonate layers using computer‐modeled successions generated by periodic sea level changes. The resulting stratigraphic records were then evaluated statistically. Thickness distributions of simulated lithofacies were compared to distributions predicted for Poisson processes, which by definition are not driven by cyclical sea level changes. Our results suggest that periodic processes produce stratigraphic thickness frequencies that are difficult to distinguish from random frequencies except under high‐magnitude sea level fluctuations. Similarly, autocorrelation fails to correctly recognize cyclic patterns in such simulated records. Models with high‐magnitude sea level fluctuations (icehouse conditions) had thickness frequencies that are suggestive of orbital forcing, whereas low‐magnitude sea level fluctuations (greenhouse conditions) appeared independent even though they were modeled using Milankovitch orbital forcing. The increasing evidence based on spectral data from real rock successions suggests that Milankovitch drivers are common in both icehouse and greenhouse periods. Because statistical approaches are unsuccessful in recognizing the cyclic driver of these simulated records, we infer that it is difficult to disprove independence from real stratigraphy even when orbital forcing is controlling the rock composition. Even in the necessarily simplified world of computer simulations, the numerous factors involved in depositing stratigraphic successions work to complicate or mask any periodic signal, thus generating the appearance of stochasticity in some successions.

Lacustrine system evolution during early rifting: El Castellar Formation (Galve sub-basin, Central Iberian Chain)

A detailed sedimentological study of the first synrift deposits recorded in the Galve sub-basin of NE Spain during the Early Cretaceous aided in the reconstruction of climatic and tectonic influences. The El Castellar Formation is composed of siliciclastics and carbonates (unit 1), claystones, gypsum, and carbonates (unit 2), and marls and limestones (unit 3). Unit 1 facies formed in alluvial and palustrine plains, a local alluvial fan, and low-energy shallow lake subenvironments. Low-energy lacustrine facies characterised unit 2. In contrast, palustrine, low- and high-energy lake facies were identified for unit 3. The spatial and temporal distribution of lithofacies representing the several environments were used to propose a general lacustrine system evolution from isolated ponds and marshes, to low-energy shallow lakes, and finally to an extensive, high-energy lake. Lake evolution is related to changes in the subsidence pattern, from local- to basin-scale subsidence, which was ultimately related to the transition from the initial rift to rift climax stage. Shallowing-upwards lacustrine successions have been interpreted as climatically forced and linked to sediment and water supply. Both tectonics and climate determined the change from an overfilled to a balanced-fill lake-basin through time.

Evolution of a High-Relief Carbonate Platform Slope Using 3D Digital Outcrop Models: Lower Jurassic Djebel Bou Dahar, High Atlas, Morocco

Abstract Seismic-scale continuous exposures of an Early Jurassic carbonate platform located in the southern High Atlas of Morocco provide detailed quantitative information about the lithofacies and stratal geometries of an aggrading, retrograding, and prograding steeply inclined slope system that evolved in an active intracratonic rift basin. Aggradational, backstepping, and progradational outer-platform to slope transects are each characterized by distinct lithological features and stratal patterns. A digital outcrop model was constructed using real-time kinematic global positioning system and lidar, storing information on recorded stratal surfaces including lithofacies information. From these data 3D models of the slope system could be built. In the study area, four stages of slope development are recognized. Stage 1 corresponds to a low-relief carbonate platform characterized by widespread sub-wave-base depositional conditions. Stage 2 developed as an aggrading to retrograding platform, during which the platform built considerable relief. In addition, a massive organically bound upper slope fringe formed, dominated by coral–sponge–microbial boundstone. The in situ boundstone consisted of irregular wedges that accreted on the upper slope down to ~ 140 m below the platform break. After a period of sediment starvation Stage 3 involved a major backstepping of sediment deposition. Seventy-meter-high clinoforms prograded from the center of the platform across the platform top until reaching the relict platform break (of Stage 2). Subsequently, progradation occurred basinward, as indicated by the spilling of lobes down the existing slope deposits of Stage 2. Limited progradation, along with the reestablishment of the in situ coral–sponge–microbial boundstone on the upper slope, led to the progressive steepening of the slope profile to dips of 23°. Clinoforms are planar in the upper part of the slope, and concave upwards in the lower part; with a total relief of over 460 m. Stage 4 is observed only in the lower slope and adjacent basin. Coarse detrital carbonate deposits form lens-shaped aprons that grade basinward into kilometer-scale lobes that alternate with thick wedges of basinal fallout sediments. Syndepositional extensional faults operating in the subsurface affected sedimentation along the lower slope profile throughout the four stages and are expressed in the stratal patterns through growth strata in monoclinal folds. Three-dimensional digital outcrop models reconstructing the architecture of the slope system allowed quantification of slope geometries and stratal relationships and spatial distribution of sedimentary bodies. The digital model enhanced the understanding of the depositional changes during slope evolution and also led to observations on, for example, angular relationships that would have been difficult to detect without the digital approach. The evolution from aggrading to retrograding, prograding, and retrograding was strongly influenced by synsedimentary tectonics within an intracratonic rift basin. The tectonics generated irregular distribution of accommodation space, variably interfering with eustatic sea-level movement. This in combination with the character of lithofacies types and style of sedimentary processes along the slope, such as in situ boundstone growth and gravity-driven resedimentation processes, directly controlled carbonate slope architecture and stratigraphy of the Djebel Bou Dahar and influenced stratal anatomy and lithofacies distribution.

Orbital cycles in a Late Cretaceous shallow platform (Iberian Ranges, Spain)

The late Turonian to early Coniacian 3rd-order sequence in the northern Iberian Range is composed of a thin-bedded, inner platform carbonate succession. Based on the vertical facies trend and on the recognition and correlation of internal discontinuities, up to 27 meter-scale 6th-order parasequences are recognised, each showing shallowing upwards trends. These parasequences are hierarchically grouped into 5th-order and 4th-order parasequences. This stratigraphical arrangement suggests the presence of cyclic processes that controlled the sedimentary record. The vertical lithofacies distribution is analysed in three different sections; the spectral analysis of these data, using the Lomb–Scargle algorithm, indicates the presence of three orders of cycles (12.76 to 14.05 m, 3.11 to 3.28 m and 1.32 to 1.53 m), very close to the mean thicknesses of the 4th-, 5th- and 6th-order parasequences, respectively. There is also a fourth prominent peak of 3.91 to 4.14 m, but it is not represented in the field successions as a distinct order of parasequence. The correlation with the orbital periodicities in the Milankovitch frequency band using relative ratio sets shows that the spectral periodicities can be related to the long eccentricity cycle of 400 ka, the short eccentricity cycle of 95 ka, and the obliquity cycles of 39 and 50.6 ka, respectively, whereas the 3.91 to 4.14 m spectral periodicites can be related to the short eccentricity cycles of 123 ka. Thus, it is suggested that orbital forcing acted as a major factor that controlled climate and sea-level, which in turn influenced the depositional environments in the late Turonian to early Coniacian of the Iberian Ranges. These high-frequency changes were superimposed on a long-term, 3rd-order evolution of the relative sea level.