Facies Bodies Research Articles

Compression-based Facies Modelling

Simple object- or pixel-based facies models use facies proportions as the constraining input parameter to be honored in the output model. The resultant interconnectivity of the facies bodies is an unconstrained output property of the modelling, and if the objects being modelled are geometrically representative in three dimensions, commonly-available methods will produce well-connected facies when the model net:gross ratio exceeds about 30%. Geological processes have more degrees of freedom, and facies in high net:gross natural systems often have much lower connectivity than can be achieved by object-based or common implementations of pixel-based forward modelling. The compression method decouples facies proportion from facies connectivity in the modelling process and allows systems to be generated in which both are defined independently at input. The two-step method first generates a model with the correct connectivity but incorrect facies proportions using a conventional method, and then applies a geometrical transform to scale the model to the correct facies proportions while retaining the connectivity of the original model. The method, and underlying parameters, are described and illustrated using examples representative of low and high connectivity geological systems.

Simulation experiment of fluid-feldspar sandstone interactions and their implications for tight oil and gas exploration of the Yanchang Formation, Ordos Basin, China

Tight oil and gas reservoirs are generally formed in structurally stable and broad buffer zones, with high-quality source rocks developed in large areas and sand bodies in extensive contact with source rocks. Fluids generated from source rock during the thermal maturation are bound to dissolve or precipitate in the adjacent reservoir, which directly restricts the pore development of the reservoir and the direction of oil and gas exploration. To reveal the effect of the total component fluids from source rock on reservoir transformation during hydrocarbon evolution, taking the Yanchang Formation of the Ordos Basin as an example, a fluid-feldspar sandstone interaction simulation experiment of different geological evolution stages under a closed environment was carried out. The results show that the fluids that formed in the hydrocarbon evolution process contain organic acids, and consist mainly of acetic acid. At the peak period of oil generation (VRo = 0.96%), the fluids are the most acidic, the acetic acid yield is the highest, and the average molar ratio is 56.11%. Under closed conditions, after the fluid-feldspar sandstone interaction, the feldspar minerals are altered and clay minerals are generated in situ, which results in a tight reservoir. The feldspar mineral content decreased from 26.1% to 20.8%–22.3%, clay mineral content increased from 11% to 14.30%–14.37%, porosity decreased 5.18%–28.79%, and permeability decreased 4%–34.16%. In addition, a semi-quantitative evaluation method of feldspar dissolution and porosity was established, which was used to evaluate the effect of fluids on feldspar sandstone reservoirs during hydrocarbon evolution. The local semiclosed or open diagenetic system and acid fluid transport and accumulation area beneficial to the formation of the “sweet spot” of tight oil and gas. However, the gentle tectonic setting and extensive contact of sand and mudstone lead to the retention effect of diagenetic fluid in the Yanchang Formation of the Ordos Basin. Oil and gas exploration should be mainly directed to finding sedimentary sand facies bodies favorable for the formation and preservation of primary pores.

Data Assimilation in Truncated Plurigaussian Models: Impact of the Truncation Map

Assimilation of production data into reservoir models for which the distribution of porosity and permeability is largely controlled by facies has become increasingly common. When the locations of the facies bodies must be conditioned to observations, the truncated plurigaussian model has been often shown to be a useful method for modeling as it allows gaussian variables to be updated instead of facies types. Previous experience has also shown that ensemble Kalman filter-like methods are particularly effective for assimilation of data into truncated plurigaussian models. In this paper, some limitations are shown of the ensemble-based or gradient-based methods when applied to truncated plurigaussian models of a certain type that is likely to occur for modeling channel facies. It is also shown that it is possible to improve the data match and increase the ensemble spread by modifying the updating step using an approximate derivative of the truncation map.

On facies belts and facies mosaics: Holocene isolated platforms, South China Sea

AbstractAlthough the general criteria for recognition and environmental interpretation of different carbonate facies are well‐established, a predictive understanding of the areal extent and spatial patterning of facies bodies and why they might organize into facies belts or facies mosaics is poorly constrained. To explore patterns and process dynamics of facies on isolated carbonate platforms, quantitative analysis of thematic maps derived from remote sensing images of 27 Holocene atolls of the Paracel and Spratly chains in the South China Sea explores variability within and among platforms. On these systems, most annular shelf‐margin reefs are less than 500 m wide on both chains; inboard of the reefs, reef sand aprons range up to 500 m (Spratlys) and 1000 m (Paracels) wide. Around individual platforms, Spratly Chain sand apron widths are wider to the north‐west, whereas apron widths in the Paracel Chain are more symmetrical; collectively, data indicate log‐normal width‐exceedance probability distributions. Platform‐interior patch reefs include area‐exceedance probability distributions and gap size distributions (lacunarity) consistent within chains, but distinct between the chains. To understand the processes underlying distinct distributions, simulations explored distinct growth scenarios. Results suggest that differences may represent distinct process classes: proportional growth processes with multiplicative random effects (reef sand aprons – belts), versus non‐linear, size‐proportional growth of randomly aged and distributed elements (patch reefs – mosaics). The probabilistically distinct sizes and spatial patterns of geomorphic elements within these general process classes are interpreted to represent ‘variations on themes’ related to the different impacts of tropical storms, winter cold fronts and circulation in each chain. The results highlight fundamentally different growth patterns impacting the sizes and distribution of facies belts and mosaics on isolated carbonate platforms. Because these types of bodies ultimately construct stratigraphy, the themes could be applied to understand and predict variability in the architecture of subsurface reservoir analogues.

Physical Oceanographic Influences On Sedimentology of Reef Sand Aprons: Holocene of Aranuka Atoll (Kiribati), Equatorial Pacific

Abstract Depositional facies are the net product of a complex set of processes that impact sediment supply and transport in geomorphic systems. Although the general facies motifs of many isolated platforms throughout the geologic record are well documented, the details of geomorphological and sedimentological patterns, and the physical oceanographical processes that can impact sedimentological differentiation, are less well constrained. On these platforms, accumulations of reef-associated debris in platform-top reef sand aprons can form expansive geomorphic elements, hosting prolific hydrocarbon reserves in geologic analogs. To better understand the nature and scale of reef-sand-apron accumulations, this project integrates remote-sensing, field, petrographical, and granulometrical characterization of surficial Holocene sediments with physical oceanographical observations and modeling of Aranuka Atoll, Republic of Kiribati, in the western equatorial Pacific. Results illustrate trends in hydrodynamics, geomorphology, and sedimentology from the platform margin to the platform interior. Current-meter data and modeling illustrate how the tides (2.5 m spring-tide amplitude) modulate wave energy (open-ocean annual average swell height of ∼ 2 m; distal swell can be larger) to produce dominant on-platform flow (speeds up to 90 cm/s) on the northern reef sand apron. The strongest currents there appear to be impacted by southward-propagating waves related to fronts that pass thousands of kilometers north of the atoll, not local, wind-generated waves. These hydrodynamical influences are interpreted to have led to the development of the expansive northern reef sand apron (> 2000 m wide); the southeastern apron, with currents that reverse with the tides, includes a distinct, narrower sand apron (∼ 1500 m wide). Concomitantly, the hydrodynamical patterns and platformward decrease in energy across the reef sand apron, coupled with changes in biota, are interpreted to control variability in sedimentary structures, bottom types, and sediment attributes. Sediment near the margin on the reef sand apron contains coral and red algal–rich coarse sand and gravel, passing to foraminifera-rich, medium to coarse sand toward the lagoon. The lagoon includes even finer sediment, chiefly fine and very fine sand, and patch reefs. Collectively, the results of this study illustrate that selective winnowing, differentiation of sediment size, type, and sorting, and nature and size of geomorphic elements ultimately are linked to the hydrodynamical patterns across the platform. The sedimentological (depositional porosity and permeability) and geomorphic (facies bodies) variability and the processes active on this reef sand apron provide a general conceptual model that may be applicable to predict trends in ancient reservoir analogs.

Contrasts between wave- and tide-dominated oolitic systems: Holocene of Crooked–Acklins Platform, southern Bahamas

As the stratigraphic record is neither homogenous nor isotropic, a fundamental challenge in sedimentary geology is characterizing and predicting the nature of variability. Although many nearshore to shallow-marine siliciclastic depositional systems have been grouped into wave-dominated and tide-dominated classes, many facies models of carbonate systems have not recognized or applied this distinction. To fill this gap, the purpose of this study is to compare and contrast the hydrodynamic processes, sedimentology, geomorphology, and stratigraphy of an oolitic tide-dominated shoal complex and an oolitic wave-dominated shoreline system from the same area (Crooked–Acklins Platform, southern Bahamas). Field, petrographic, granulometric, remote-sensing, and hydrodynamic data illustrate marked contrasts between the two end members. The wave-dominated shoreface system near French Wells on the leeward margin is influenced by small (Hs <30 cm) waves and only weak, wave-driven currents (<15 cm/s, net southward flow); the passage of Hurricane Irene (in 2011) had only a local impact on the shoreface. The low-energy setting favors generation and alongshore transport of ooids and peloids of fine- to medium-sized sand, and has facilitated nearly 1 km of shoreface progradation, forming geomorphic bodies with margin-parallel geometry. The stratigraphic record of the area includes a coarsening- and shoaling-upward succession. In contrast, the ~7-km-wide, tide-dominated shoal on the southwestern margin is dominated by vigorous tidal currents (in excess of 50 cm/s, and reversing twice daily). These currents generate margin-normal bars and channels consisting of oolitically coated grains of medium to coarse sand size. Stratigraphically, above a basal clast-rich deposit, sediment size and sorting vary across the shoal. The open-ocean and the platformward flanks of the shoal, most impacted by tide and wave energy, include the cleanest oolitic sand, with fines accumulating in the stabilized shoal interior. Overall, the oolitic shoal aggraded and expanded platformward over bioturbated peloid–ooid sand. These results reveal how sediment, geomorphology, and stratigraphy differ between tide- and wave-dominated end members, patterns broadly analogous to several ancient examples. These data collectively illustrate the processes that create multi-scale linkages among stratigraphic patterns, geological facies bodies, and granulometry (e.g., depositional porosity and permeability). These linkages provide information that could be used to predict sedimentological and geomorphical trends in ancient analogues.

Detailed mapping, three-dimensional modelling and upscaling of a mixed-influence delta system, Mitchell River delta, Gulf of Carpentaria, Australia

Abstract Data from satellite imagery, field measurements and analogues were used to construct a three-dimensional (3D) geocellular facies model of the Mitchell River Delta, Australia; a modern mixed-influence delta system. Detailed mapping identified 16 different facies elements and classified the delta as tide dominated, fluvially influenced and wave affected. The 3D model was subjected to varying degrees of upscaling of the horizontal and vertical dimensions and allowed comparison of volume and connectivity changes throughout. The upscaling process, to coarser grid cells up to 100 m horizontally and 4 m vertically, created false compartmentalization of facies bodies and significant changes in facies bulk volumes. The vertically upscaled models produced greater changes when compared to the horizontally upscaled models. Key changes in reservoir facies connectivity and bulk volume due to upscaling are associated with the facies architecture, including the elongate and thin morphology of beach ridge and channel facies in this mixed-influence delta system. Recognition of the defining reservoir features and incorporation into reservoir modelling methodology can improve volumetric estimation and allow for better predictions of reservoir connectivity in ancient delta systems.

DFTopoSim: Modeling Topographically-Controlled Deposition of Subseismic Scale Sandstone Packages Within a Mass Transport Dominated Deep-Water Channel Belt

Facies bodies in geostatistical models of deep-water depositional environments generally represent channel-levee-overbank-lobe morphologies. Such models adequately capture one set of the erosional and depositional processes resulting from turbidity currents traveling downslope to the ocean basin floor. However, depositional morphologies diverge from the straight forward channel-levee-overbank-lobe paradigm when the topography of the slope or the shape of the basin impacts the timing and magnitude of turbidity current deposition. Subaqueous mass-transport-deposits (MTDs) present the need for an exception to the channel-levee-overbank-lobe archetype. Irregular surface topography of subaqueous MTDs can play a primary role in controlling sand deposition from turbidity currents. MTD topography creates mini-basins in which sand accumulates in irregularly-shaped deposits. These accumulations are difficult to laterally correlate using well-log data due to their variable and unpredictable shape and size. Prediction is further complicated because sandstone bodies typical of this setting are difficult to resolve in seismic-reflection data. An event-based model is presented, called DFTopoSim, which simulates debris flows and turbidity currents. The accommodation space on top of and between debris flow lobes is filled in by sand from turbidity currents. When applied to a subsurface case in the Molasse Basin of Upper Austria, DFTopoSim predicts sand packages consistent with observations from core, well, and seismic data and the interpretation of the sedimentologic processes. DFTopoSim expands the set of available geostatistical deep-water depositional models beyond the standard channel-levee-overbank-lobe model.

Reservoir properties and petrophysical modelling of carbonate sand bodies: outcrop analogue study in an epicontinental basin (Triassic, Germany)

Abstract This paper represents the second part of an integrated study that is focussed on the development and distribution of reservoir bodies and properties in epeiric carbonate systems. It is based on outcrop analogue data from Triassic ‘layer-cake’ carbonates in the South German Basin, which were deposited along an epicontinental, very gently inclined carbonate ramp. The reservoir facies consists of skeletal and oolitic carbonate grainstones (Φ max 23%, K max 700 mD), which are organized in a pronounced hierarchy of stratigraphic cycles. Based on outcrops, cores, gamma ray (GR) logs and thin sections, a high-resolution, geocellular 3D facies model was generated, which covers the area of a Middle East giant gas field (25×36 km). The spatial distribution of reservoir properties was systematically investigated on different scales. The lateral distribution of reservoir properties remains in the same order of magnitude for hundreds of metres, within in the same stratigraphic position. However, on a kilometre scale, facies bodies, diagenetic trends and thus reservoir properties show gradual lateral changes. Vertically, in contrast, properties change commonly on a decimetre scale and are largely controlled by stratigraphic cycles. Petrophysical modelling enhanced the understanding of key factors and processes controlling both reservoir quality and quantity.

Seascape Metrics of Shelf-Margin Reefs and Reef Sand Aprons of Holocene Carbonate Platforms

Shelf-margin reefs and reef sand aprons are important geomorphic and facies elements associated with many isolated platforms today and in the stratigraphic record. Although it is generally appreciated that shelf-margin reefs and reef sand aprons have formed at or near breaks in slope at the rims of platforms throughout geologic time, possible environmental causes for variation in their sizes are not well understood. To provide better constraints for assessing possible reasons for such variability in ancient systems, this study relates local, regional, and global patterns of variation in the seascape attributes of Holocene shelf-margin reef and reef sand apron systems to a suite of potential environmental controls. Landsat data from more than 60 isolated carbonate platforms and atolls from the Caribbean Sea, Indian Ocean, South Pacific Ocean, Central Pacific Ocean, South China Sea, and Indonesian Seas were investigated by digitization and spatial analysis of facies belts, and compared with regional environmental parameters. The results of this analysis suggest that: (1) Reefs cover from < 1 to 55% of the spatial extent of lithotopes on platforms (mean = 20%), whereas reef sand aprons represent between 0 and 65% (mean = 20%). (2) The spatial extent of reefs is inversely related to platform size, but reef sand aprons show no comparable trend. (3) Widths of reefs and reef sand aprons are variable within and among platforms, but widths from all platforms and from platforms within different regions are characterized by log-normal size-frequency distributions. (4) Widths of these facies belts vary as a function of environmental factors such as swell period, swell height, tidal amplitude, latitude, and margin orientation relative to direction of wind and wave approach. These data indicate the presence of trends in widths of facies belts within different regions that correlate with regional oceanographic patterns. Nonetheless, marked variability is evident on individual platforms, among platforms within each region, and globally; these variations are not related to any of the parameterized environmental variables. This observation are interpreted to reflect the influences of unknown (and in some cases, unknowable) spatial and historical contingencies such as the depth to bedrock, character of earlier Holocene sea-level change, or changes in physical oceanographic state (waves or tides). The collective influence of these parameters, each of which individually may have the potential to invoke a deterministic response, may be reflected in reef and sand aprons with log-normal width-frequency distributions, interpreted to reflect multiplicative random effects. Collectively, however, these results suggest that accurate, a priori prediction of the details of the sizes of analogous facies bodies in the stratigraphic record may be impossible, especially in the deep geologic past, where biologic and physical controls are even less well constrained.

A Stochastic Object Model Conditioned to High-Quality Seismic Data

We present an approach for modeling facies bodies in which a highly constrained stochastic object model is used to integrate detailed seismic interpretation of the reservoir’s sedimentological architecture directly in a three-dimensional reservoir model. The approach fills the gap between the use of seismic data in a true deterministic sense, in which the facies body top and base are resolved and mapped directly, and stochastic methods in which the relationship between seismic attributes and facies is defined by conditional probabilities. The lateral geometry of the facies bodies is controlled by seismic interpretations on horizon slices or by direct body extraction, whereas facies body thickness and cross-sectional shape are defined by a mixture of seismic data, well data, and user defined object shapes. The stochastic terms in the model are used to incorporate local geometric variability, which is used to increase the geological realism of the facies bodies and allow for correct, flexible well conditioning. The result is a set of three-dimensional facies bodies that are constrained to the seismic interpretations and well data. Each body is defined as a parametric object that includes information such as location of the body axis, depositional direction, axis-to-margin normals, and external body geometry. The parametric information is useful for defining geologically realistic intrabody petrophysical trends and for controlling connectivity between stacked facies bodies.

Mapping facies distributions on modern carbonate platforms through integration of multispectral Landsat data, statistics-based unsupervised classifications, and surface sediment data

Benthic sediment facies maps were constructed for a series of large, isolated carbonate platforms, including (1) Great and Little Bahamas Banks; (2) Caicos Platform, British West Indies; (3) Chinchorro Bank, Mexico; (4) Glovers Reef, Belize; (5) south Cocos (Keeling) atoll, Indian Ocean; and (6) Bu Tini shoal, Persian Gulf. Facies maps were generated by applying statistics-based image analysis algorithms, called unsupervised classifications, to Landsat 7 multispectral satellite data. Classification results were subsequently validated with sediment data to create geologic facies maps. Landsat-derived facies maps demonstrate 82–85% agreement when compared to sediment data. At the platform scale, Landsat-derived facies maps accurately capture the principal depositional facies observed on each platform and are in general agreement with published maps generated through conventional mapping techniques. Examination at more detailed scales reveals that Landsat-derived maps differ from conventional maps with respect to the spatial dimensions and shapes of facies bodies. Landsat-derived maps show a level of complexity and heterogeneity that is more realistic than shown in previously published maps, which are characterized by broad, homogeneous facies belts. These results suggest that application of statistical algorithms to Landsat data, coupled with sediment data, provides a cost- and time-efficient method for quantitatively mapping spatial variability of depositional facies in modern carbonate environments. Landsat-derived facies maps, like the ones presented here, provide depositional analogs for subsurface carbonate reservoirs as well as a global data set for extracting predictive relationships between the occurrence and distribution of carbonate sediments that can aid in global hydrocarbon exploration and production.

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.

Application of three-dimensional seismic texture analysis with special reference to deep-marine facies discrimination and interpretation: Offshore Angola, west Africa

In seismic stratigraphy, a seismic texture is an acoustic expression of the stratigraphic configuration of beds and thin beds that is related to depositional facies and reservoir properties. This study applies an amplitude co-occurrence matrix method to characterize seismic textures in a deep-marine setting offshore Angola (west Africa). Beginning with high-quality three-dimensional seismic data, the algorithm calculates textural homogeneity, contrast, and randomness by evaluating the amplitude co-occurrence matrix of a texture element at each sample location. Based on textural homogeneity, contrast, and randomness, a clustering analysis segments the regular seismic amplitude volume into different categories in a resulting thematic texture class volume. An integrated interpretation of the texture classes and regular amplitude characters, along with contemporary and outcrop analogs of deep-water systems, leads to the identification of several major facies elements such as channel fill, levee, overbank, mass-transport complex, and marine shale in the lower Miocene–upper Pliocene stratigraphic interval. Sequential stratigraphic horizon slicing through the texture class volume indicates that the geomorphology and facies architecture have changed significantly from the early Miocene to the late Pliocene, which is instrumental in analyzing sequence stratigraphy, delineating the play fairway, and evaluating hydrocarbon potential. Subvolume detection of distinct texture classes shows that different facies bodies are complex in extent, geometry, and connectivity in three dimensions, which provide critical seismic-scale constraints for geologically meaningful reservoir modeling and simulation.

Integrating outcrop data and forward computer modelling to unravel the development of a Messinian carbonate platform in SE Spain (Sorbas Basin)

AbstractGeological mapping, definition of facies distributions and reconstruction of platform‐interior growth geometries of the Messinian Cariatiz carbonate platform (Sorbas basin, South Spain), were performed to evaluate the controlling factors in platform growth and to test a 3‐D computer simulation program. For the simulation with the program REPRO, five platform‐related facies were modelled: (1) the reef crest facies by the numerical solution of a Fisher equation; (2) the lagoonal facies by a function of water depth‐dependent carbonate production; (3) the proximal and middle slope facies (breccia and block facies, calcarenite facies) by a subroutine simulating gravity‐driven particle export from the reef crest; (4) a distal slope; and (5) a basinal facies by a pelagic rain function. Development of a fan delta conglomeratic system is simulated by using a siliciclastic point source and gravity‐driven particle redistribution. A best fit between the observed platform growth geometries and modelling results is achieved by assuming that high‐frequency sea‐level changes superimposed onto a longer term sea‐level fall controlled platform growth. For the modelling, a relative sea‐level curve was reconstructed, which is based on a deep‐sea benthic foraminiferal stable oxygen isotope record at ODP Site 926 with a 45 m eustatic sea‐level fall, and a tectonic uplift component of 20 m. The consistency of 3‐D simulation results is corroborated by the coral growth rates provided by the Fisher‐equation subroutine. These rates of 2–8 mm year−1 compare well to the coral growth rates in Recent fringing reefs. We propose that during the early stage of platform evolution the high‐frequency fluctuations were obliquity‐modulated precessional cycles, whereas precessional cycles control later stages of platform growth. REPRO provides a separate visualization of the different facies bodies as a function of time and space, showing the intrinsic pattern of facies distribution in the platform. This is the result of a combination of platform growth and syndepositional subaerial erosion. For example, only the youngest stages of reef framework facies in the development of the Cariatiz carbonate platform are preserved.

Hierarchical dynamic stratigraphy in various Quaternary gravel deposits, Rhine glacier area (SW Germany): implications for hydrostratigraphy

The approach of ‘dynamic stratigraphy’ aims to understand genetic processes that form stratigraphic units in a hierarchy of spatial and temporal scales. This approach was used to investigate Quaternary gravel deposits in terms of their sedimentology and in order to characterize the various sedimentary units in terms of their hydrogeological properties. Facies analysis within 62 gravel pits, laboratory permeability measurements of field samples and geophysical surveys (3-D georadar, 2-D seismic reflection) led to the detection and classification of sedimentary heterogeneity according to the following six scales whereby each scale can be translated into defined hydrostratigraphic units. (1) Particles and pores (micro scale) that reflect depositional and diagenetic fluid dynamics as well as source material behaviour (e.g. grain-size, roundness, lithological composition). This was found to be important for the hydrogeochemistry of groundwater in gravel aquifers (e.g. higher sorption capacity of carbon-rich limestone particles for organic pollutants). (2) Strata (meso scale) contain the recognition of sorting, fabric, texture and stratinomic features, which can give an indication of transport and depositional dynamics. Five major lithofacies groups, for example, were distinguished within fluvial gravel-bed deposits. Their variable hydraulic properties led to their subdivision into 12 hydrofacies types. They form the smallest mappable hydrostratigraphic units, which may result in either preferred pathways for fluid flow or flow barriers. (3) Depositional elements (macro scale) enable reconstruction of sedimentary/geomorphic elements and their dynamics within a depositional system (e.g. gravel-bed braided river systems are dominated by gravel sheet, gravel dunes and scour pool depositional elements). Hydrostratigraphically, the architecture of depositional elements influences the hydraulic connectivity and local permeability structure/distribution within an aquifer body. Five types of depositional elements in fluvial gravel-bed deposits were distinguished and their geometries/dimensions quantified. (4) Facies bodies (mega scale) composed of a stack of depositional elements and strata recording distinct environmental systems and their dynamics (e.g. a coarse-grained prograding delta system). Hydrostratigraphically, facies bodies represent major compartments of an aquifer. Six major types of meltwater-controlled facies bodies were identified in the study area. (5) Genetic sequences (mega scale) reflect the shifts of depositional environments caused by allocyclic changes (e.g. glacial advance recorded by a coarsening upward sequence) or autocyclic changes of landscape shaping events. These sequences may form separate hydrostratigraphic units or aquifer storeys. (6) Basin fill (giga scale) comprising the lateral and vertical stacking of facies bodies and genetic sequences controlled by either long-term glacier dynamics or short term flood events. The regional distribution of permeable gravel units and, for example, less permeable diamicts builds the larger scale hydrostratigraphy.

Apparently homogenous “reef”-limestones built by high-frequency cycles: Upper Jurassic, SW-Germany

Central European Upper Jurassic sponge-microbial “reefs” are commonly regarded as compact, massive and homogenous facies bodies. A continuous study of borehole cores from SW Germany clearly shows a very systematic architecture of these “reefs” built by sedimentary cycles of various hierarchical levels. Altogether five orders of cycles can be distinguished: decimeter-scale microcycles building meter-scale mesocycles, which in turn are stacked to form several-meter thick macrocycles. These high-frequency cycles in turn are parts of basin-wide medium-scale and large-scale cycles reported in a previous paper. This paper documents the high-frequency micro-, meso- and macrocycles. Microcycles (0.1–0.8 m thick) usually have a sharp erosional base covered by reworked sponges, shells and intraclasts, interpreted as storm events. These bioclastic layers are colonised by a succession of thrombolitic and stromatolitic microbialites. Successive microcycles form distinct stacks that record gradually increasing and decreasing depositional energy, thus defining mesocycles (0.5–4 m thick). Several mesocycles construct macrocycles (3–25 m). The cycle stacking patterns were analysed by means of facies-proportion-diagrams. The dominant microscale cyclicity records changes in water agitation, possibly caused by fluctuating storm wave base. The sponge-microbial mounds could probably build up just to average storm wave base. Within the cycles of larger hierarchical levels the successive smaller-scale cycles show systematic trends in their symmetry and in thickness. This may be caused by variations in the ratio of accomodation space (and associated energy level) to carbonate production. External reef geometries (progradation, aggradation, retrogradation, condensation) can be related to internal cycle stacking patterns. To approximate the duration of the high-frequency cycles two different approaches were used. Firstly, the duration of cycles was determined with the help of published chronostratigraphic time scales. Secondly, cycle duration was estimated using published microbial reef growth rates. Both approximations result in ca. 0.4 Ma for the medium-scale cycles and 0.1 Ma for the macrocycles. Both are regionally correlatable, thus appear to be allocyclic in nature and probably are Milankovitch-driven. In contrast, the microcycles are only locally correlatable and thus indicate an autocyclic (storm-induced) nature. This study poses the question if not more of the apparently homogenous “reef bodies” of other settings and geological ages may possibly also show internal high-frequency cycles.

ABSTRACT: Sizes, Shapes, and Patterns of Sediment Accumulations on a Modern Tidal Flat and Stratigraphic Implications: Three Creeks Area, Andros Island, Bahamas

Construction of realistic geologic and simulation models of subsurface reservoirs requires data on the geometry and continuity of flow units, baffles, and barriers, parameters commonly constrained by log, core, seismic, and production data. If the minimum horizontal dimensions of facies bodies is less than the typical well spacing; however, properties will not be accurately described using either deterministic or stochastic methods. In these situations, seismic or production data can provide insights, yet still may include ambiguous characterization.

Integrated reservoir characterization and uncertainty analysis, Heidrun Field, Norway

The Lower Jurassic Åre and Tilje Formations of the Heidrun Field contain some 350 × 10 6 Sm 3 (2160 × 10 6 BBL) of oil in place. The reservoirs are highly heterogeneous and represent deposition in a wide range of fluvial, tidal and marginal marine environments. This, together with a high level of faulting and a relatively viscous oil type, has led to low recovery estimates. To help improve recovery, an integrated reservoir description project including a thorough sedimentological analysis, stochastic modelling of genetic and diagenetic facies, and an associated uncertainty analysis also using stochastic modelling was initiated. Facies architectures were modelled in 17 individual reservoir zones, each zone consisting of up to 10 × 10 6 grid blocks. The Åre Formation is characterized by alluvial plain and low energy deltaic systems, including incised valley fills, fluvial channels, crevasse splays, upward coarsening bayfills and large splay lobe deposits. The overlying Tilje Formation is characterized by mixed tidal and marginal marine depositional systems with tidal channels, tidal sand and mud flats, tidal shoals and shoreface and offshore sands. Several facies were modelled within each zone and the desired facies architecture was often achieved by merging several individual stochastic realizations. Input to the facies modelling was based on well data as well as information from outcrop analogues and modern depositional systems. Petrophysical attributes were distributed stochastically within each facies body type, using frequency distributions from the wells and interpreted variogram functions. The realizations were finally fitted to the structural maps and upscaled for flow simulation. This enabled the building of a ‘best guess’, or most likely, full-field geological and dynamic simulation model. A subsequent uncertainty study integrated and evaluated the full spectrum of geological and petrophysical uncertainties related to the dynamic behaviour of the reservoir including facies geometries and facies volume fractions. Best, worst and intermediate case facies realizations with respect to fluid flow were first generated using stochastic modelling. These were combined with other major reservoir uncertainties (gross rock volume, petrophysical values) and 150 complete geological models were established. The hydrocarbon pore volume in each of these models was calculated, and the models were taken through a simplified flow simulation. The results from this process allowed a ranking of the models and a selection of representative models for further dynamic flow simulations. The uncertainty study shows that the uncertainty related to the gross rock volume is the most significant on a field-wide scale. Uncertainties in the facies input parameters (geometry, facies volume fraction) have relatively little impact at a field wide scale. However, their impact upon individual zones or segments (local scale) can be large.

Abstracts: Deposition of Deep-Water Sands, Pliocene, Niger Delta: Sequence Stratigraphy, Depositional Facies, and Sand Body Geometry and Stacking Patterns

Abstracts: Deposition of Deep-Water Sands, Pliocene, Niger Delta: Sequence Stratigraphy, Depositional Facies, and Sand Body Geometry and Stacking Patterns