ALBERT

Description: Clinoform surfaces control aspects of facies architecture within shallow-marine parasequences and can also act as barriers or baffles to flow where they are lined by low-permeability lithologies, such as cements or mudstones. Current reservoir modeling techniques are not well suited to capturing clinoforms, particularly if they are numerous, below seismic resolution, and/or difficult to correlate between wells. At present, there are no modeling tools available to automate the generation of multiple three-dimensional clinoform surfaces using a small number of input parameters. Consequently, clinoforms are rarely incorporated in models of shallow-marine reservoirs, even when their potential impact on fluid flow is recognized. A numerical algorithm that generates multiple clinoforms within a volume defined by two bounding surfaces, such as a delta-lobe deposit or shoreface parasequence, is developed. A geometric approach is taken to construct the shape of a clinoform, combining its height relative to the bounding surfaces with a mathematical function that describes clinoform geometry. The method is flexible, allowing the user to define the progradation direction and the parameters that control the geometry and distribution of individual clinoforms. The algorithm is validated via construction of surface-based three-dimensional reservoir models of (1) fluvial-dominated delta-lobe deposits exposed at the outcrop (Cretaceous Ferron Sandstone Member, Utah), and (2) a sparse subsurface data set from a deltaic reservoir (Jurassic Sognefjord Formation, Troll Field, Norwegian North Sea). Resulting flow simulation results demonstrate the value of including algorithm-generated clinoforms in reservoir models, because they may significantly impact hydrocarbon recovery when associated with areally extensive barriers to flow.

Description: Permeability contrasts associated with clinoforms have been identified as an important control on fluid flow and hydrocarbon recovery in fluvial-dominated deltaic parasequences. However, they are typically neglected in subsurface reservoir models or considered in isolation in reservoir simulation experiments because clinoforms are difficult to capture using current modeling tools. A suite of three-dimensional reservoir models constructed with a novel, stochastic, surface-based clinoform-modeling algorithm and outcrop analog data (Upper Cretaceous Ferron Sandstone Member, Utah) have been used here to quantify the impact of clinoforms on fluid flow in the context of (1) uncertainties in reservoir characterization, such as the presence of channelized fluvial sandbodies and the impact of bed-scale heterogeneity on vertical permeability, and (2) reservoir engineering decisions, including oil production rate. The proportion and distribution of barriers to flow along clinoforms exert the greatest influence on hydrocarbon recovery; equivalent models that neglect these barriers overpredict recovery by up to 35%. Continuity of channelized sandbodies that cut across clinoform tops and vertical permeability within distal delta-front facies influence sweep within clinothems bounded by barriers. Sweep efficiency is reduced when producing at higher rates over shorter periods, because oil is bypassed at the toe of each clinothem. Clinoforms are difficult to detect using production data, but our results indicate that they significantly influence hydrocarbon recovery and their impact is typically larger than that of other geologic heterogeneities regardless of reservoir engineering decisions. Clinoforms should therefore be included in models of fluvial-dominated deltaic reservoirs to accurately predict hydrocarbon recovery and drainage patterns.

Description: :  Stratigraphic architectures are fundamentally controlled by the interplay at different temporal and spatial scales of accommodation and sediment supply, modulated by autogenic responses of the sediment routing system and its constituent segments. The flux and caliber of sediment supply is a function of climate, catchment area, and tectonics in the source regions, and unraveling these forcing mechanisms from the observed stratigraphic architecture remains a key research challenge. The mid-to-late Eocene Escanilla sediment routing system had its source regions in the south-central Pyrenean orogen, northern Spain, and transported sediment from wedge-top basins along tectonic strike to marine depocenters. By constructing a volumetric budget of the sedimentary system, it has been demonstrated that there were marked changes in the grain-size distribution released from the sediment sources and also in the position of the gravel front, across three ~ 2.6 Myr time intervals from 41.6 to 33.9 Ma. Classical sequence stratigraphic interpretations would relate the movement of depositional boundaries such as the gravel front to changes of base level, either in isolation or in combination with sediment supply. Herein, we explore the possibility that the position of the gravel front was primarily driven by variability of grain-size distributions released from the source regions as a result of changes in catchment uplift rate and/or surface run-off. Using a simple model of sediment transport that captures first-order processes, we simulate the lateral movement of gravel deposition in the proximal part of the Escanilla sediment-routing system. Movement of the gravel front is a function of both accommodation generation and the transport capacity of the sediment routing system. We assume that the transport capacity is a linear function of the local slope and the water flux. By assuming that the observed thickness of deposits is equivalent to the accommodation available during deposition, we then use the stratigraphic architecture to constrain the change in catchment size and water flux over the three time intervals of the Escanilla paleo–sediment-routing system. Multiple scenarios are investigated in order to find the most plausible tectonic and climatic history. Model results indicate that during the mid-Eocene there was an increase in catchment length and sediment flux, most likely driven by tectonic uplift in the Pyrenean orogen. Subsequent marked progradation of the gravel front during the late Eocene was the consequence of reduced transport capacity due to a reduction in surface run-off. The latter model result is in agreement with records of pollen taxa that indicate increased climatic aridity in the late Eocene. The combination of a sediment transport model with a full sediment budget makes it possible to test the non-uniqueness of these results.

Description: Numerical models and recent outcrop case studies of alluvial-to-coastal-plain strata suggest that autogenic avulsion can control the stacking density and architecture of channelized fluvial sandbodies. The application of these models to subsurface well data was tested by the analysis of upper coastal plain deposits of the late Bajocian Ness Formation in the Brent Field reservoir, UK North Sea. These coastal plain deposits accumulated during the progradation and retrogradation of the wave-dominated ‘Brent Delta’. Sedimentological facies analysis and palaeosol characterization in cores were used to interpret the styles of palaeochannel avulsion. These results were then compared with the dimensions and distributions of channelized fluvial sandbodies that had been quantified by spatial statistical tools (lacunarity, Besag's L function) applied to interpretative correlation panels between closely spaced wells. The results indicate that the distributions of channelized sandbodies may plausibly have been generated by avulsions and that they influence sandbody connectivity and pressure depletion patterns. Intervals of upper coastal plain strata with relatively wide sandbodies that display some clustering in their stratigraphic architecture are associated with a high proportion of avulsions by incision and annexation in core samples. Such intervals display relatively good vertical pressure communication and relatively slow, uniform pressure depletion.

Description: Data from a large-scale outcrop analogue (Upper Cretaceous Blackhawk Formation, Wasatch Plateau, central Utah, USA) were used to construct three-dimensional, object-based reservoir models of low to moderate net-to-gross (NTG) ratios (11–32%). Two descriptive spatial statistical measures, lacunarity and Ripley’s K function, were used to characterize sandbody distribution patterns in the different models. Lacunarity is sensitive to sandbody abundance and NTG ratio, while Ripley’s K function identifies clustered, random and regular spacing of sandbodies. The object-based modelling algorithm reproduces sandbody dimensions and abundances, but patterns of sandbody distribution generated by river avulsion are poorly replicated because pseudo-well spacing provides only limited constraint on sandbody positions. In common with previous studies, the connected sand fraction in the reservoir models increases with increasing NTG ratio and increasing range of sandbody orientations, but there is significant stochastic variation around both of these trends. In addition, low NTG reservoir models in which sandbodies exhibit strong clustering may also have a low connected sand fraction across the model volume because the sandbody clusters are widely spaced and, thus, tend to be isolated from each other. Consequently, connected sand fraction could be overestimated if avulsion-generated sandbody clusters are not identified and replicated in models of such reservoirs.

Description: A bstract :  Conceptual and quantitative models of fluvial stratigraphy typically argue that alluvial architecture is driven by a combination of allogenic processes (e.g., tectonic subsidence, sea level, sediment supply), and autogenic behaviors (e.g., avulsion). We test these models via analysis of a large (c. 200 m thick by 100 km depositional-strike extent) alluvial-to-coastal-plain succession that records shoreline progradation in response to decreasing tectonic subsidence (Late Cretaceous Blackhawk Formation, Wasatch Plateau, central Utah, U.S.A.). Oblique aerial photographs and architectural panels of six nearly linear, nearly vertical cliff face "windows" were used to characterize the dimensions (apparent widths, thicknesses) and spatial distributions of channelized fluvial sandbodies. Two statistical measures, raster-based lacunarity and point-based L -function, are used to investigate whether the distribution of channelized sandbodies show significant regularity, randomness, or clustering. Over 490 channelized fluvial sandbodies are identified in the six cliff-face "windows." From base to top of the Blackhawk Formation, sandbodies broadly increase in width and decrease in overall abundance per unit area. Clustering of sandbodies occurs relatively frequently in lower-coastal-plain strata (〈 50 km from the coeval shoreline). Spatial regularity of sandbody spacing is consistently more apparent in upper-coastal-plain and alluvial strata (〉 50 km from the coeval shoreline). A strong negative correlation between lacunarity and stratigraphic position is also observed, such that a wider range and greater variety of spatial gaps occurs between sandbodies in lower-coastal-plain strata relative to upper-coastal-plain and alluvial strata. Comparison with numerical modeling studies of fluvial stratigraphic architecture implies the predominance of an avulsion-generated pattern of sandbody distribution that includes an element of compensational stacking on the upper coastal plain and alluvial plain, for a range of distances from the coeval shoreline (c. 40–130 km) and tectonic subsidence rates (c. 40–700 m/ Myr). On the lower coastal plain (〈 50 km from the coeval shoreline) localized clustering of channelized sandbodies is interpreted to have formed by avulsion of deltaic distributary channels downstream of delta-apex avulsion nodes, possibly modulated by low-amplitude (〈 30 m) sea-level fluctuations.

Description: The Lower Jurassic Bridport Sand Formation records net deposition in the Wessex Basin, southern UK of a low-energy, siliciclastic shoreface that was dominated by storm-event beds reworked by bioturbation. Shoreface sandstones dip at 2–3° to define (subaerial?) clinoforms that pass distally into a near-horizontal platform, and then steepen again to form steep (2–3°) subaqueous clinoforms in the underlying Down Cliff Clay Member. The overall morphology indicates mud-dominated clinoforms of compound geometry. Compound clinoforms are grouped into progradational sets whose stacking reflects tectonic subsidence and sediment dispersal patterns, and also controls basin-scale reservoir distribution and diachroneity of the formation. Each shoreface clinoform set consists of an upward-shallowing succession that is several tens of metres thick with a laterally continuous mudstone interval at its base. The successions are punctuated by calcite-cemented concretionary layers of varying lateral continuity, which formed along bioclastic lags at the base of storm-event beds. Concretionary layers thus represent short periods of rapid sediment accumulation, while their distribution likely results from variations in storm-wave climate, relative sea-level, and/or sediment availability. The distribution of impermeable mudstone intervals that bound each clinoform set and concretionary layers along clinoform surfaces controls oil drainage in the Bridport Sand Formation reservoir.

Description: Multiple techniques are available to construct three-dimensional reservoir models. This study uses comparative analysis to test the impact of applying four commonly used stochastic modeling techniques to capture geologic heterogeneity and fluid-flow behavior in fluvial-dominated deltaic reservoirs of complex facies architecture: (1) sequential indicator simulation; (2) object-based modeling; (3) multiple-point statistics (MPS); and (4) spectral component geologic modeling. A reference for comparison is provided by a high-resolution model of an outcrop analog that captures facies architecture at the scale of parasequences, delta lobes, and facies-association belts. A sparse, pseudosubsurface data set extracted from the reference model is used to condition models constructed using each stochastic reservoir modeling technique. Models constructed using all four algorithms fail to match the facies-association proportions of the reference model because they are conditioned to well data that sample a small, unrepresentative volume of the reservoir. Simulated sweep efficiency is determined by the degree to which the modeling algorithms reproduce two aspects of facies architecture that control sand-body connectivity: (1) the abundance, continuity, and orientation of channelized fluvial sand bodies; and (2) the lateral continuity of barriers to vertical flow associated with flooding surfaces. The MPS algorithm performs best in this regard. However, the static and dynamic performance of the models (as measured against facies-association proportions, facies architecture, and recovery factor of the reference model) is more dependent on the quality and quantity of conditioning data and on the interpreted geologic scenario(s) implicit in the models than on the choice of modeling technique.

Description: A bstract :  Sequence stratigraphic models emphasize the role of external, allogenic controls such as relative sea level, tectonic subsidence, and climate on alluvial to coastal plain stratigraphic architecture, at the expense of internal, autogenic behaviors such as avulsion. These models are tested via sedimentologic analysis of fine-grained floodplain deposits and their relationship to channelized fluvial sandbodies in a well exposed alluvial to coastal plain succession developed under progressively increasing distance from the coeval shoreline and an accompanying decrease in the rate of accommodation creation (Late Cretaceous Blackhawk Formation, Wasatch Plateau, central Utah, U.S.A). The studied alluvial to coastal plain succession contains fourteen lithofacies, which are grouped into four associations and eight architectural elements. Facies associations consist of: 1) channelized fluvial sandstone bodies; 2) nonchannelized fluvial sandstone and siltstone bodies; 3) fine-grained floodplain deposits; and 4) brackish floodplain deposits. Paleosols are immature, and are classified as entisols, inceptisols, and histosols. Integrated analysis of paleosol character, facies analysis, and architectural-element analysis indicates deposition on a rapidly aggrading floodplain that was subject to frequent crevassing and channel-belt switching. The character, relative abundance, and vertical stacking of the paleosol types, architectural elements, and the first three facies associations exhibit little variation stratigraphically, from base to top of the Blackhawk Formation (c. 250 m), or paleogeographically between the northern and southern limits of the outcrop belt (c. 100 km). Local stratigraphic architectures record three styles of avulsion, representing avulsion by reoccupation, avulsion by progradation, and avulsion by incision. Avulsion style displays no consistent pattern with stratigraphic position or paleogeographic location. In combination, these results suggest that floodplain sedimentation and local stratigraphic architecture are dominated by autogenic avulsion. The three observed avulsion mechanisms operated in apparent uniformity over a range of tectonic subsidence rates (c. 80–700 m/Ma) and distances from the coeval shoreline (c. 0–100 km).

Description: Understanding the factors controlling the development of accommodation above collapsing salt diapirs and their influence on reservoir distribution is critical in reducing exploration risk in salt-influenced sedimentary basins. In this study, we use an integrated subsurface data set (three-dimensional and two-dimensional seismic reflection, wire-line-log, core, and biostratigraphic data) from the Upper Jurassic of the Cod terrace, Norwegian North Sea, to understand the influence of rifting on accommodation creation and shallow-marine deposition during the initial-stage collapse of salt diapirs. We demonstrate that rifting resulted in the rise and fall of salt diapirs, and the formation of supra-diapir minibasin-style depocenters that became sites for deposition and preservation of up to 500 m (1640 ft) thick net-transgressive shallow-marine sandstone reservoirs. Maximum thickness is recorded in the axis of minibasins with a reduction in thickness of up to 65% noted on their flanks. The stratigraphic architecture of individual minibasins is variable. Proximal-to-distal facies variations from shoreface to offshore shelf and commensurate changes in reservoir quality occur over scales larger than individual minibasins. These deposits contain large sand volumes, and are not confined to areas of localized sandstone subcrop. In combination, these features suggest that the minibasins formed a linked network supplied by regional sediment-routing systems. The results of this study provide a new tectono-stratigraphic model for prediction of reservoir presence, thickness, and continuity in diapir-collapse minibasins along salt walls in the Central North Sea, and in other less mature, data-poor basins where reservoirs have been identified in depocenters above salt walls.