Facies Indicators Research Articles

Outcrop gamma-ray logging of siliciclastic turbidites: Separating the detrital provenance signal from facies in the foreland-basin turbidites of the Moravo-Silesian basin, Czech Republic

Standard and spectral gamma-ray (GRS) logs are widely used as indicators of facies in the subsurface. In siliciclastics, however, the gamma-ray signal is often influenced by changes in the provenance of the K, U and Th-bearing detrital minerals. In this paper, we have compared outcrop and laboratory GRS with modal and chemical sandstone and mudstone composition and facies in an approximately 2.5km-thick siliciclastic turbidite system of the Moravo-Silesian Culm Basin (Lower Carboniferous), Czech Republic. The aim was to separate the facies signal from the detrital provenance one. The siliciclastics have moderately high outcrop gamma-ray values (174 API on average) and slightly lower laboratory values (127 API). Both the outcrop and laboratory data show low sensitivity to facies, which is demonstrated by the low contrast between the K, U and Th concentrations in the seven facies types ranging from proximal to distal turbidites. Markedly higher GRS variability is observed between equivalent facies at different stratigraphic levels. Major carriers of the GRS signal include K-feldspars, muscovite, sericite, biotite and albite for K, zircon, apatite, monazite and xenotime for U and monazite, thorite, REE secondary minerals, xenotime, apatite and zircon for Th. With the effect of facies filtered out, the GRS values reveal a stratigraphic variability, which coincide with the changes in the sandstone modal composition. A shift from the low-grade metamorphic and volcano-sedimentary provenance to predominantly magmatic sources with ultrapotassic plutonites in the early Late Viséan is associated with a marked increase in U and Th concentrations and generally higher sandstone radioactivity compared to mudstones. Another provenance shift to high-grade metamorphic sources with granulites in the latest Viséan is associated with a rapid decrease in Th, U and partly K concentrations and an increase in the GRS contrast between sandstone and mudstone facies. The GRS data sensitively reflect the extremely rapid exhumation of mid-crustal and deep-crustal rocks in the major source area, the Moldanubian Zone of the Bohemian Massif.

Ensemble Kalman Filter Predictor Bias Correction Method for Non-Gaussian Geological Facies Detection

The Ensemble Kalman Filter (EnKF) is a Monte-Carlo based technique for assisted history matching and real time updating of reservoir models. However, it often fails to detect facies boundaries and proportions as the facies distributions are non-Gaussian, while geologic data for reservoir modeling is usually insufficient. It is convenient to represent distinct facies with non-Gaussian categorical indicators; we implemented discrete cosine transform (DCT) to parameterize the facies indicators into coefficients of the retained cosine basis functions that are Gaussian. For highly complex and heterogeneous models, though observed data were matched, it failed to reproduce realistic facies distribution corresponding to reference variogram and facies proportion.In this paper we propose a new ensemble filtering method in-between of EnKF and PF, where EnKF as predictor combines the advantages of accurate large updates with small ensembles and corrector for non-Gaussian distributions followed by EnKF again for analysis step. Correction is performed by regenerating new realizations using a new pilot point method. The ensemble members that are more consistent with the early production history and the available geological information are considered as high weight particles and used for the applications.Combination of DCT-EnKF and regenerating new realizations using the new pilot point method demonstrates reasonable improvement and reduction of uncertainty in facies detection. Incorporating the new step in the procedure assists the filter to honor the reference distribution and experimental variogram during the history matching process and presents an important potential in improved characterization of complex reservoirs.

Diverse microbially induced sedimentary structures from 1 Ga lakes of the Diabaig Formation, Torridon Group, northwest Scotland

The siliciclastic lacustrine rocks of the ~ 1000 Ma Diabaig Formation, northwest Scotland, contain a remarkable diversity of macroscopic structures on bedding planes that can be compared with various kinds of microbially induced sedimentary structures (MISS). Field sedimentological investigations, combined with laboratory analysis of bedding planes and petrographic study of thin sections have allowed us to characterise a range of depositional environments and document the spectrum of biological structures. MISS are reported from frequently subaerial environments, through commonly submerged facies, and down to permanently sub-wavebase settings. Palaeoenvironmental conditions (water depth, exposure, hydrodynamic energy) control the distribution of MISS within these facies. This demonstrates that mat-forming microbial communities were arguably well adapted to low light levels or periodic exposure. Some MISS from the Diabaig Formation are typical of Precambrian microbial mats, including reticulate fabrics and ‘old elephant skin’ textures. In addition to these, a number of new and unusual fabrics of putative microbial origin are described, including linear arrays of ridges and grooves (cf. ‘Arumberia’) and discoidal structures that are comparable with younger Ediacaran fossils such as Beltanelliformis. These observations indicate that benthic microbial ecosystems were thriving in freshwater lake systems ~ 1000 Ma, and indicate how microbially induced sedimentary structures may be applied as facies indicators for Proterozoic lacustrine environments. The discovery of structures closely resembling Ediacaran fossils (cf. Beltanelliformis) also serves to highlight the difficulty of interpreting simple discoidal bedding plane structures as metazoan fossils.

Upper Triassic foraminifers from Panthalassan carbonate buildups of Southwestern Japan and their paleobiogeographic implications

Detailed sampling of the Upper Triassic atoll-type carbonates of the Sambosan Accretionary Complex throughout Southwest Japan yielded highly abundant and diversified porcelaneous, microgranular, agglutinated and hyaline foraminifers of Carnian-Rhaetian age, as well as some microproblematica and ostracods. The foraminiferal assemblages were collected from shallow-water carbonates originated upon volcanic seamounts surrounded by deep-water radiolarian chert in a mid-open oceanic realm of the Panthalassan Ocean during Triassic time. Because most studies of the Upper Triassic microfauna come from the former Tethys, counterparts of the Panthalassan Ocean are pivotal to decipher the micropalaeontological biodiversity of the western circum Pacific, as well as to evaluate the distribution patterns of organisms and their evolution trends throughout the Tethys and Panthalassa. This study reports on 42 genera and 60 species whose associations can be used as sedimentary facies indicators of carbonate buildup environments. Japanese specimens show a strong Tethyan affinity, and especially with the Peri- and Southern Tethyan forms. A palaeobiogeographic distribution analysis using a large foraminiferal database is led, in order to evaluate the extraordinary spreading of these Upper Triassic foraminifers between the Neo-Tethys and the Panthalassa. Data are finally integrated in a new plate tectonic model, where six faunistic provinces are defined, each containing a characteristic foraminiferal assemblage. This map provides for the first time a useful and visual synthesis of the Upper Triassic foraminifer palaeobiogeographic distribution.

Garnet–chloritoid–kyanite assemblages: eclogite facies indicators of subduction constraints in orogenic belts

AbstractThe assemblage garnet–chloritoid–kyanite is shown to be quite common in high‐pressure eclogite facies metapelites from orogenic belts around the world, and occurs over a narrowly restricted range of temperature ∼550–600 °C, between 20 and 25 kbar. This assemblage is favoured particularly by large Al2O3:K2O ratios allowing the development of kyanite in addition to garnet and chloritoid. Additionally, ferric iron and manganese also help stabilize chloritoid in this assemblage. Pseudosections for several bulk compositions illustrate these high‐pressure assemblages, and a new thermodynamic model for white mica to include calcium and ferric iron was required to complete the calculations. It is extraordinary that so many orogenic eclogite facies rocks, both mafic eclogites sensu stricto as well as metapelites with the above assemblage, all yield temperatures within the range of 520–600 °C and peak pressures ∼23±3 kbar. Subduction of oceanic crust and its entrained associated sedimentary material must involve the top of the slab, where mafic and pelitic rocks may easily coexist, passing through these P–T conditions, such that rocks, if they proceed to further depths, are generally not returned to the surface. This, together with the tightly constrained range in peak temperatures which such eclogites experience, suggests thermal weakening being a major control on the depths at which crustal material is decoupled from the downgoing slab.

New crustacean microcoprolites of the Upper Triassic limestones of the Sambosan Accretionary Complex, Japan

We report here the first crustacean microcoprolites from the Sambosan Accretionary Complex (AC) in Japan. Three species of crustacean (order Decapoda) microcoprolites - Payandea japanica n. sp., Favreina tosaensis n. sp., and a species belonging to the genus Parafavreina? sp. are described from the Late Carnian to Rhaetian shallow-water limestones of the Sambosan AC, which were originated on extinct and subsiding volcanoes forming a mid-oceanic atoll-type buildup in the Panthalassan Ocean. Coprolites, which occur within a lagoonal to back-reef facies, provide new insight into the micropaleontology and sedimentology of the Sambosan limestones. Their presence in this tropical shallow-water depositional setting heightens their status of sedimentary facies indicators of such environments. Furthermore the occurrence of these Tethyan coprolites genera in Japan improves our knowledge of the distribution of Triassic crustacean decapods across the Panthalassan Ocean.

Pyrope and Cr-diopside as indicators of mantle structure and diamond depth facies in the Kola region

PT parameters of crystallization have been determined for pyropes and Cr-diopsides from loose sediments of the Kola region, taking into account the chemical compositions of these minerals. Being either deep-seated xenocrysts or constituents of mantle xenoliths in kimberlites, pyropes and Cr-diopsides bear information on composition of the lithospheric mantle and its diamond resource potential. It was established that pyropes belong to the lherzolitic (45%), harzburgitic (30%), and eclogitic (25%) mineral assemblages. The Ni thermometry of pyropes yielded their formation temperature at 650–1250°C, which corresponds to a depth interval of 75–190 km. The distribution of different pyrope-bearing assemblages and their trace element composition allowed us to suggest a layered structure of the Kola lithospheric mantle. Its shallow unit (75–110 km) is mainly composed of depleted lherzolite; the medium-deep unit (110–170 km) consists of harzburgite, and the deep unit (170–190 km), of both lherzolite and harzburgite. About 16% of lherzolitic-harzburgitic pyropes were derived from the diamond mantle facies, i.e., from a depth of 140–190 km. Cr-diopsides are subdivided into two genetic groups: eclogitic (high Al2O3 and Na2O, low MgO and CaO) and ultramafic (high MgO, CaO, and Cr2O3; low Al2O3 and Na2O). The crystallization parameters of Cr-diopside from deep-seated ultramafic group were determined using the Cr-in-Cpx barometer and En-in-Cpx thermometer. Most samples fall into the graphite stability field (20–45 kbar and 700–1150°C). If these minerals were derived from kimberlites, this implies that the latter were constituents of carbonatite-ultramafic intrusions. Cr-diopsides may also be derived from diamond-free ultramafic xenoliths contained in alkaline ultramafic dikes. Nevertheless, 15% of Cr-diopside compositions fall in the field of diamond stability (55–60 kbar and 1000–1100°C). These conditions fit the geotherm characterizing a low heat flow. The results support the high resource potential of the Kola region for diamonds.

Two-Dimensional Permeability of Breccia Facies in the Athabasca Oil Sands

Abstract Breccia is a lithological facies present in the McMurray Formation in the Athabasca region of northeast Alberta. It consists of mud or shale clasts embedded in clean sands. The mud content of this facies varies from 0 to 100% and the clasts range in size from millimetres to metres with varying degree of angularity and sorting. Breccia is common in bitumen reservoirs. From an in situ recovery perspective, breccia can impede fluid flow and reduce the overall oil saturation due to the presence of the mud clasts. To properly characterize the reservoir flow properties, it is necessary to develop a systematic procedure to capture the impact of these clasts to a scale that is suitable for numerical flow simulation. This is commonly referred to as upscaling in reservoir characterization. The procedure described here involves digitizing core photographs of breccia intervals and assigning a sand-mud facies indicator based on the presence or absence of mud clasts at the pixel level of the core photograph. The resulting indicator map is transferred to a numerical flow simulator. The 2D effective permeability of the breccia zones is determined numerically from single-phase flow simulations by assigning a constant permeability to the sand and zero permeability to the mud clasts. The results indicate that breccia permeability closely follows an averaging power law in terms of the mud content. A 2D mathematical theory is developed to calculate the permeability of breccia facies by treating the mud clasts as elliptic obstacles to the flow through the embedding sand of known permeability. The results from the theory are shown to be in satisfactory agreement with those from the flow simulation. The paper concludes with guidelines for systematic permeability assignment in the breccia zones of oil sands, as well as other similar formations. Introduction Fluvial/estuarine channel sandstones and associated lithofacies host the oil sands deposits of the lower Cretaceous McMurray Formation in the Athabasca region of northeast Alberta. Channel deposits occur where there has been incisement into pre-existing sediments, creating accommodation space for subsequent deposition. Mud clast breccias are commonly associated with these channel deposits. They are mechanically formed through the cutting and bank collapse of the pre-existing sediments as the channels migrate across the depositional area. These breccias are a product of an active energy system. As the channel sand bars migrate, 'ripup' mud clasts that were eroded during the channel action become embedded. The mud clast breccias can vary considerably in length, width and height. They can also vary considerably in the amount of mud clasts in the host sandstone matrix. Mud clast breccias display favourable characteristics to be included as part of a reservoir unit. They are permeable as is evident from the fact that oil migrated into the sand portion of the facies through and around the mudstone clasts. Analysis of core samples reveals the bitumen saturation in the sandstone fraction of the breccias is equal to the bitumen saturation in mud-free sandstones.

Comparative Taphonomy and Paleoecological Reconstruction of Two Microvertebrate Accumulations from the Late Cretaceous Hell Creek Formation (Maastrichtian), Eastern Montana

Although microvertebrate accumulations are commonly used for paleoecological reconstructions, taphonomic processes affecting the final taxonomic composition of an accumulation are often ignored. This research explores the effects of abiotic taphonomic processes on the taxonomic composition of terrestrial microvertebrate accumulations by comparing a floodplain and a channel lag deposit from the Maastrichtian Hell Creek Formation in eastern Montana. Distribution of skeletal elements with specific physical attributes and relative abundance of taxa correlate with the hydraulic indicators (i.e., grain size, sedimentary structures) of the depositional facies. Transport distances, hydraulic equivalencies of dominant skeletal elements, amount of hydraulic sorting and reworking, and degree of time averaging vary between deposits and significantly affect taxonomic distributions. Relative abundance data, in conjunction with chi-square test results and rank-order analysis, show that size, shape, abrasion, and taxonomic compositions vary significantly between assemblages. The fine-grained assemblage is dominated by tabular, low-density elements, such as cycloid scales and fish vertebrae. Dense, equidimensional elements, such as teeth and ganoid fish scales, dominate the sandstone assemblage. Rank-order analysis results demonstrate that relative abundance of hydraulically equivalent skeletal elements from morphologically similar organisms can be compared regardless of accumulation in nonisotaphonomic deposits. Statistical comparisons were made among osteichthyans using ganoid scales, caudates using vertebrae, ornithischians using teeth, and testudinates using shell fragments. Results show that portions of the assemblage analyzed using hydrodynamically equivalent elements are not significantly different, despite different depositional environments.

Enhanced imaging workflow of seismic data from Chicontepec Basin, Mexico

Strecker et al. (2003) show a recent example of cascading workflows to progressively mine seismic data for information. Thus, for multiple seismic attribute volumes, calculated from a 3D data set, data mining can lead to a seismic facies indicator that best discriminates lithology, fluid, and geometry, and finally produces a volume calibrated to rock properties.

The middle Eocene Márkushegy subbituminous coal (Hungary): Paleoenvironmental implications from petrographical and geochemical studies

Samples from two coal seams of the Márkushegy underground mine (Hungary) were investigated for variations in maceral composition, petrography-based facies indicators and various geochemical parameters, using three seam profiles. Both seams originate from a topogenous mire and evolved within a peneplaned coastal area covered with eutrophic swamps. The presence of foraminifera and marine algae in the marly layers is indicative of marine ingressions. Coal facies characterisation from these parameters is combined with the results from biomarker analyses of soluble organic matter in order to reconstruct the depositional environment and to relate petrography-based indicators to the molecular composition. The contents of macerals of the liptinite group are positively correlated with soluble organic matter (SOM) yields and Hydrogen Index (HI). Consistent with maceral composition and high HI values, enhanced proportions of n-alkanes of intermediate molecular weight (n-C21–25), which are predominantly found in macrophytes, are obtained from samples of the marly shales. The final drowning of the mire is reflected by decreasing pristane/phytane ratios, due to the rise of the (ground)water table and the establishment of anaerobic conditions. The observed positive correlation between pristane/phytane ratios and inertinite percentages suggests that, at least in the Márkushegy deposit, both parameters reflect variations in redox conditions of the mire. The predominance of angiosperms in the peat-forming vegetation of the Márkushegy coal is evidenced by the high relative proportions of angiosperm-derived triterpenoids. The influence of the relative proportion of fossil wood on tissue preservation (TPI) is indicated by the positive relationship of the concentration of land plant-derived terpenoid hydrocarbons and TPI. The degree of gelification of plant tissue (GI) is governed by the microbial activity in the mire, as indicated by a negative relationship between GI and the steroids/hopanoids concentration ratios. The variation of the δ13C values of coal is obviously governed by the relative contribution of plant lipids to the biomass. The obtained negative relationship between δ13C of the coal and GI is explained by the effect of cellulose decomposition and microbial activity on the isotopic composition of the organic matter.

Geostatistical Simulation and Numerical Upscaling, to Model Ground-Water Flow in a Sandy-Gravel, Braided River, Aquifer Analogue

Abstract An integrated approach combining sedimentological, geostatistical, and numerical analyses has been applied to model groundwater flow through a glaciofluvial, braided-stream, gravel–sand aquifer of Pleistocene age, exposed in a gravel pit in the eastern Piedmont (northern Italy). The aquifer under investigation consists of a stack of braided bar and minor channel fills and scour fills, where different gravel–sand bedforms migrated. It represents an analogue of buried sediments, which host some of the most important aquifer complexes and water resources of northern Italy. The work has been performed with three main steps: (1) drawing and discretization of the geological models of two orthogonal exposures and estimate of conductivity of every facies; (2) geostatistical simulation of the 3D facies heterogeneity, based on semivariogram analysis of the geological models; (3) application of a 3D numerical model of flow to upscale the conductivity field. Semivariogram analysis developed from facies indicator functions has shown that: (a) the variability in the data set in the horizontal direction is a representation of the oriented distribution of the sedimentary facies, corresponding to the average N–S elongation of sediment units, according to the paleoflow direction; (b) the vertical correlation length approximates the average thickness of the bedsets bounded by fourth-order erosional surfaces (unit bars, scour fill, and small channel fill associations) and may represent the thickness of an individual hydrostratigraphic unit; (c) the conductivity and facies distributions are more variable in the vertical direction than in the horizontal one, with a ratio between vertical and horizontal ranges of about 1:8. The 3D conductivity distribution within the considered deposits has been determined using a sequential indicator simulation, whereas transition-probability analysis has been used to confirm the correlation model obtained with semivariogram analysis and to check the result of the simulation. The equivalent hydraulic-conductivity tensor has been computed with finite-difference modeling of groundwater flow for cubic blocks of the aquifer analogue with side length of about 13 m. At this scale the medium is almost homogeneous, but anisotropic, with the lowest values of hydraulic conductivity in the vertical direction and the greatest values along the elongation of the sedimentary units, i.e., almost parallel to average paleoflow.

Relations of petrographical and geochemical parameters in the middle Miocene Lavanttal lignite (Austria)

Abstract Samples from two lignite seams (Lower Seam, Upper Seam) of the Lavanttal basin (Austria) and additional xylite were investigated for variations in maceral composition, petrography-based facies indicators, bulk geochemical parameters, and molecular composition of hydrocarbons. Both seams originated in a topogenous mire and evolved within a transgressive setting. The final drowning of the mire is indicated by sapropelic shales. Whereas the sapropelic shale overlying the Lower Seam was deposited in a freshwater lake, the sapropelic shale above the Upper Seam represents a brackish lake. Numerous relationships are found between petrography-based facies indicators and the geochemical composition of organic matter. The contents of macerals of the liptinite group are positively correlated with soluble organic matter (SOM) yields and hydrogen index (HI). Consistent with maceral composition and high HI values, enhanced proportions of short-chain n-alkanes, which are predominantly found in algae and microorganisms, are obtained from samples of the sapropelic shales. The final drowning of the mire is reflected by decreasing pristane/phytane ratios, due to the rise in (ground)water table and the establishment of anaerobic conditions, as well as by decreasing ratios of diasterenes/sterenes, indicating increasing pH values in the mire. The degree of gelification of plant tissue (gelification index) is governed by the microbial activity in the mire, as indicated by the hopanes concentration. The differences in floral assemblage during the formation of the Lavanttal lignite seams are reflected by major differences in tissue preservation. Preservation of plant tissue (TPI) in the Lavanttal lignite is obviously controlled by the presence/absence of decay-resistant gymnosperms in the peat-forming vegetation, and additionally influenced by the relative contribution of wood to coal formation. The results provide evidence that valuable information for coal facies characterization could be obtained by petrography-based and geochemical facies indicators. An influence of the floral assemblage (gymnosperms/angiosperms ratio) and of the contribution of algal biomass on carbon isotopic composition of the organic matter (δ13C = − 24.2 to − 28.6‰) is proposed. Carbon cycling during biogeochemical decomposition of plant tissue by bacteria is suggested to affect the δ13C values of the coal. The chemotaxonomical classification of the xylites as gymnosperm remnants, based on the molecular composition of terpenoid biomarkers, is corroborated by the carbon isotopic composition of the xylites (mean δ13C = − 24.1‰) and the extracted cellulose (mean δ13C = − 20.2‰). The higher isotopic difference of about 3.9‰ between cellulose and total organic carbon of the xylites, compared to the difference between cellulose and wood found in modern trees, is explained by the smaller effect of decomposition on δ13C of cellulose.

Depositional environment of the Late Miocene Hausruck lignite (Alpine Foreland Basin): insights from petrography, organic geochemistry, and stable carbon isotopes

Lignites and fossil wood from two boreholes in the late Miocene Hausruck district (Alpine Foreland Basin, Austria) were investigated with respect to organic carbon and total sulfur contents, ash yields, maceral composition, organic geochemistry, and stable carbon isotope ratios. The lignites from the two sampled profiles differ in sulfur contents and petrography-based facies indicators (gelification index [GI], tissue preservation index [TPI]). The results point to drier and more acidic conditions in the part of the mire sampled at drill site Lukasberg, whereas the lignites from Kalletsberg formed under near neutral conditions due to a raised (ground) water table. The different depositional environments are supported by the paleogeographic positions of the boreholes: borehole Lukasberg is located within a paleo-valley, whereas borehole Kalletsberg is located at the deeper part of a basin situated south and southwest of the Hausruck area. Environmental changes with time within the mire are confirmed by the molecular composition of the soluble organic matter (SOM). Differences in pristane/phytane ratio, isomerisation of hopanes at position 17, concentrations of hopanes, and in the ratio of hopanes to hop-17(21)-enes are related to the extent of gelification of plant tissue, sulfur content and pH values, caused by differences in (ground) water table. The variable contents of individual terpenoid hydrocarbons derived from land plants indicate that angiosperms and conifers contribute to peat formation in the Hausruck district in variable proportions. Overall, angiosperm biomarkers slightly predominate. The tissue preservation index (TPI) within the Kalletsberg profile is probably governed by the proportion of decay-resistant conifers, as indicated by the correlation between TPI and biomarker composition of the coals. Correlations between δ 13C values of the lignites (between −24.9‰ and −27.4‰) and their non-hopanoid terpenoid composition indicate the major influence of changes in peat-forming vegetation on the carbon isotopic composition of the coals in the Hausruck district. Carbon isotope data of macrofossils are consistent with their taxonomical classification as conifers ( δ 13C between −22.7‰ and −25.1‰) and angiosperms ( δ 13C between −25.5‰ and −26.6‰), respectively. The δ 13C data of the extracted cellulose reveal clear differences between fossil wood from gymnosperms (average δ 13C=−21.0‰) and angiosperms (mean δ 13C=−23.4‰).

Organic geochemistry of the Lower Miocene Oberdorf lignite (Styrian Basin, Austria): its relation to petrography, palynology and the palaeoenvironment

In the Oberdorf trough at the northwestern margin of the Styrian Basin (Austria), an up to 36-m thick Lower Miocene (Ottnangian) lignite seam occurs within the fluvial Köflach–Voitsberg Formation. Previous results indicate that the lignite originated in a topogenous mire within a wet-forest swamp palaeoenvironment. Differences in soluble organic matter (SOM) yield of borehole samples from the lignite seam are related to free lipids present in plant waxes or resins and the presence of thermally labile polymers indicated by the contents of liptinite macerals (i.e., resinite, suberinite, and liptodetrinite). The n-alkane distribution patterns are dominated by high-molecular weight lipids with marked odd over even predominance, originating from plant waxes. A contribution of algae to the even numbered short-chain lipids, observed in considerable amounts in the footwall sediments is proposed. Terpenoid biomarkers characteristic for conifers and angiosperms were detected, consistent with palynomorphic spectra which are dominated by forest swamp pollen (Taxodiaceae–Cupressaceae) and pollen from plants living in a mixed mesophytic forest. Excellent relationships between higher plant biomarker composition and palynologic data indicate that the organic geochemical results reflect the varying inputs of coniferales vs. angiosperms. The data further imply that cuparene may be used for taxonomical differentiation of coniferales families. Organic geochemical data provide evidence that the content of land plant-derived biomarkers is controlled by the input of resin-rich conifers. Concentrations of hopanoids, dominated by hop-17(21)-ene, generally increase towards the top of the lignite, a section characterised by high extents of aromatisation of triterpenoid biomarkers. The results suggest that the aromatisation of angiosperm-derived biomarkers may be governed by the activity of anaerobic rather than by aerobic bacteria. However, changes in the depositional environment and palynologic facies during peat formation probably influenced the origin and fate of angiosperm-derived triterpenoids. The results indicate how biomarker composition may contribute to the evaluation of environmental changes. Cross-correlations of organic geochemical parameters with palynological data and petrography-based facies indicators will provide further insights in the peat-forming vegetation and biogeochemical activities in the mire.

Mat-related sedimentary structures in Neoproterozoic peritidal passive margin deposits of the West African Craton (Anti-Atlas, Morocco)

Proterozoic inliers in the central Anti-Atlas mountains expose predominantly siliciclastic sedimentary successions deposited in peritidal zones along the Neoproterozoic continental margin of the West African Craton (WAC). The low-grade metamorphic and modestly deformed sediments contain a wealth of sedimentary structures related to the former presence and activities of microbial mats and respective physicobiological processes. The well-preserved structures include wrinkle structures, erosion marks, microbial sand chips, spindle-shaped and subcircular microbial shrinkage cracks, and possibly gas domes and cabbage-head structures. Thin sections exhibit mat fragments and dispersed grains of hematite/limonite after pyrite in fine-grained quartzitic storm deposits. Post-storm layers frequently consist of matrix-supported sand-sized to silt-sized grains and are overlain by argillaceous veneers including isolated silt-sized grains and black carbonaceous laminae. The muddy veneers are considered to represent compacted stacks of microbial mats (biolaminites), which colonized and biostabilized storm and post-storm layers, and thus prevented them from eroding. In the absence of grazing and burrowing organisms and at suitable depositional and hydrodynamic conditions, it may be expected that Proterozoic microbial mats extensively grew from the supratidal to the intertidal zones and occasionally, e.g. behind protective barriers, in the subtidal zone and beyond. Mat-related structures, however, need specific conditions for their formation and preservation: Wrinkle structures, erosion marks, and microbial sand chips require tractional currents and soon deposition and burial, respectively. Such conditions are preferably met in intertidal and supratidal zones. Spindle-shaped and subcircular cracks require mat shrinkage due to either desiccation or “syneresis”. Crack propagation implies progressive shrinkage, while superposition of crack generations indicates repeated alternation between mat exposure and flooding. Respective conditions prevail in the upper intertidal zone. Gas domes and cabbage-head structures are related to the production of gas from decaying organic material beneath a sealing cohesive mat. They may be some of the first features formed during genesis of petee structures in the intertidal to lower supratidal zones. Mat-related structures may serve as sensitive facies indicators once their modes of formation are revealed.

The Upper Permian Sedimentary Facies and Its Role in the Dajing Cu‐Sn Deposit, Linxi County, Inner Mongolia, China

Abstract: Exhaustive investigations were launched for confirming the upper Permian host rocks of the Dajing Cu‐Sn Deposit, probing into the possibility that Dajing is a Sedex type deposit during cosedimentation, complementing the deficiency of previous researches and going further into substantiating the role of the upper Permian strata in the control of ore distribution. After more than two years work, we reclassified the sedimentary facies in the Dajing area and its periphery as shallow fresh water lake and delta. Indicative sedimentary structures, such as ripple marks, rain marks, and mud cracks combined with contemporary fossils, were revealed. Having measured the flow directions, performed chemical comparison, and analyzed various sediments from sourceland in the Dajing area by XRF, we consequently redivided the strata into four sedimentary members, among which P2l1 and P2l2 were concluded as significant ore‐hosted strata. The upper Permian basin was a lateral rift basin. The water and sediments in the basin are much deeper and thicker in the north than those in the south.The indicators of special sedimentary facies, such as gravity flow, brine pool and synchronogenic stratiform structure of the ore cannot be found in the Dajing area. There was no growth fault, assemblage of sulfide and sulfate, and no zonation as well.On the basis of study in this area, taking into account the paleosedimentary environment as capriciously flowing shallow lake, which approximated the state of oxidation, we figured that the paleogeography made it prohibitively difficult to form stratiform sulfide deposits which are prone to form in deoxidized environment. It can be ruled out the possibility that the Dajing deposit is a syngenetic deposit during sedimentation.

Mapping lithofacies and pore‐fluid probabilities in a North Sea reservoir: Seismic inversions and statistical rock physics

Reliably predicting lithologic and saturation heterogeneities is one of the key problems in reservoir characterization. In this study, we show how statistical rock physics techniques combined with seismic information can be used to classify reservoir lithologies and pore fluids. One of the innovations was to use a seismic impedance attribute (related to the [Formula: see text] ratio) that incorporates far‐offset data, but at the same time can be practically obtained using normal incidence inversion algorithms. The methods were applied to a North Sea turbidite system. We incorporated well log measurements with calibration from core data to estimate the near‐offset and far‐offset reflectivity and impedance attributes. Multivariate probability distributions were estimated from the data to identify the attribute clusters and their separability for different facies and fluid saturations. A training data was set up using Monte Carlo simulations based on the well log—derived probability distributions. Fluid substitution by Gassmann’s equation was used to extend the training data, thus accounting for pore fluid conditions not encountered in the well. Seismic inversion of near‐offset and far‐offset stacks gave us two 3‐D cubes of impedance attributes in the interwell region. The near‐offset stack approximates a zero‐offset section, giving an estimate of the normal incidence acoustic impedance. The far offset stack gives an estimate of a [Formula: see text]‐related elastic impedance attribute that is equivalent to the acoustic impedance for non‐normal incidence. These impedance attributes obtained from seismic inversion were then used with the training probability distribution functions to predict the probability of occurrence of the different lithofacies in the interwell region. Statistical classification techniques, as well as geostatistical indicator simulations were applied on the 3‐D seismic data cube. A Markov‐Bayes technique was used to update the probabilities obtained from the seismic data by taking into account the spatial correlation as estimated from the facies indicator variograms. The final results are spatial 3‐D maps of not only the most likely facies and pore fluids, but also their occurrence probabilities. A key ingredient in this study was the exploitation of physically based seismic‐to‐reservoir property transforms optimally combined with statistical techniques.

Integration of seismic attribute map into 3D facies modeling

Clastic reservoir characterization typically starts with the modeling of the facies distribution and geometry. The architecture of the reservoir, governed by the facies geometry, is a major source of heterogeneity in such clastic systems. Seismic data potentially provide valuable information about the areal distribution of different facies. However, seismic data are available only at coarse vertical resolution, more closely representing interval average rock properties, whereas well log facies data more closely represent point rock properties. This scale difference or “volume support” difference between the seismic data and the facies data available along the wells makes direct integration difficult. A recently developed algorithm based on the concept of cokriging with block average data is woven into probability field simulation for building facies models. The seismic data at its coarse vertical scale, equivalent to attribute maps, can be fully accounted for without any implicit or explicit vertical duplication to match the fine vertical scale of geologic modeling. The cpu-speed advantage of probability field simulation is also retained. The algorithm is demonstrated on a clastic petroleum reservoir. The results are compared with those obtained from facies indicator simulation without integrating seismic data and those using seismic data duplicated along the vertical direction.

Biogeography of Late Silurian and Devonian acritarchs and prasinophytes

The palaeobiogeography of Late Silurian-Devonian acritarch and prasinophyte microfloras is assessed using qualitative data, similarity indexes and cluster analysis. Based on the recent palaeogeographic reconstructions, the northwards movement of Gondwana initiated during the Ludlovian with the progressive narrowing of the Rheic ocean, resulted in North Africa and south European regions being closer to the equatorial zone and hence in a warmer climate. An apparent homogeneity of Late Silurian assemblages from regions of southern Baltica and the North Gondwanan margin could be explained by such a configuration. The phytoplanktonic associations could also have benefited from the equatorial currents system for trans-Rheic connections. In addition, the qualitative study and the cluster analysis allow for delineation of a biogeographic unit, including part of South America, North Gondwanan and perigondwanan regions, which is comparable to the cold Malvinokaffric realm of the southern hemisphere, based on invertebrates. During the same time, major differences recognized in the distribution of acritarchs from near shorelines to deep oceans, emphasize the potential of mazuelloids (previously referred to as acritarchs) as indicators of deep-water facies and outer margin sites. The Early Devonian shows a geographic restriction of several acritarch genera and species, with a pronounced lateral differentiation between North Gondwanan and eastern North American microfloras, that seems to have been controlled by physical barriers rather than climatic differences. The similarities in acritarch assemblages between northern Europe and the North Gondwanan margin argue for a decrease in acritarch provinciality on both sides of the Rheic ocean, and precludes the presence of a wide Devonian ocean north of western Gondwana.