Gary J. Hampson

Three-dimensional modeling of a shoreface-shelf parasequence reservoir analog: Part 1. Surface-based modeling to capture high-resolution facies architecture

Conventional reservoir modeling approaches are developed to account for uncertainty associated with sparse subsurface data but are not equipped for detailed reconstruction of high-resolution geologic data sets. We present a surface-based modeling procedure that enables explicit representation of heterogeneity across a hierarchy of length scales. Numerous surfaces are used to construct complex facies-body geometries and distributions prior to generating a grid, allowing sampled and conceptual data to be fully incorporated within field-scale models. Our approach is driven by the improved efficiency that surfaces introduce to reservoir modeling through their geologically intuitive design, rapid construction, and ease of manipulation. Cornerpoint gridding of the architecture defined by the surfaces reduces the number of cells required to represent complex geometries, thus preserving geologic detail and rendering upscaling unnecessary for fluid-flow simulations. The application of surface-based modeling is demonstrated by reconstructing the detailed three-dimensional facies architecture of a wave-dominated shoreface-shelf parasequence from a rich outcrop data set. The studied outcrop data set describes reservoir architecture in a generic analog for many shallow-marine reservoirs. The process of model construction has demonstrated the function of (1) shoreface-shelf clinoforms, (2) paleogeographic changes in shoreline orientation, and (3) storm-event-bed amalgamation in controlling facies architecture. These subtle geometric features cannot be accurately represented using conventional stochastic reservoir modeling algorithms, which results in poor estimation of facies proportions and associated hydrocarbon volumes in place. In contrast, the surface-based modeling approach honors all data and captures subtle geometric facies relationships, thus allowing detailed and robust reservoir characterization.

The application of sequence stratigraphy to Upper Carboniferous fluvio-deltaic strata of the onshore UK and Ireland: implications for the southern North Sea

Stratigraphical correlations and fades interpretations of Upper Carboniferous fluvio-deltaic strata have been based traditionally on cyclothems bound by marine flooding surfaces (marine bands). The recent recognition of major, regionally extensive erosional unconformities (Exxon-style sequence boundaries) within selected cyclothems questions their validity as units of genetically related strata. U sing examples from the Carboniferous of the onshore UK and Ireland, we present sedimentological criteria for the recognition of sequence boundaries, placing particular emphasis on the regional context of these surfaces. Sequence boundaries comprise widespread, deeply eroded surfaces at the base of major fluvial sandstone complexes, and laterally equivalent palaeosols developed on interfluves at the margins of the fluvial complexes. These sequence boundaries define units of genetically related strata (sequences) which contain other key surfaces of time-stratigraphic significance, including marine bands and regionally extensive coals. The recognition of key surfaces enables the construction of a high resolution stratigraphic framework within which coeval facies relationships can be interpreted. Sequence boundaries can be correlated between individual basins in the onshore UK, by reference to their position in relation to a particular marine band. For example, the sequence boundary at the base of the Farewell Rock in the South Wales Basin can be correlated with that at the base of the Rough Rock in the Pennine Basin, northern England, since both these sandstone bodies are directly overlain by the Subcrenatum Marine Band. Interbasinal correlations of this nature imply that potential fluvial sandstone reservoirs within major incised valley fills in the Upper Carboniferous strata of the southern North Sea can be predicted by correlation with the onshore UK. The stratigraphical framework can be extended and tested using core and well-log data, particularly spectral gamma-ray data, which are able to identify key sequence stratigraphic surfaces.

Application of sequence stratigraphy to coal-bearing coastal plain successions: implications for the UK Coal Measures

Abstract Advances in the understanding of coal depositional environments have led to the notion that thick, low ash deposits in close proximity to siliciclastic sediment inputs are most commonly restricted to the products of raised mires. These mires are initiated, sustained and preserved in conditions of slowly rising base level (relative sea-level). Hence it is possible to consider the stratigraphic significance of economic coal seams within the concept of unconformity bounded depositional sequences (sensu Vail et al.). Time-equivalent clastic deposits tend to be minor (commonly with a heterolith fill) channel deposits, with brackish to freshwater lake and crevasse splay sediments, excluded from the peat mire either via the topographically raised nature of the mire or by the landward dislocation of fluvial facies. In this paradigm, spatially related major fluvial channel deposits, major seam splits and even washout deposits may not be time-equivalent to the coal seam. Regional correlations suggest that thick coal seams are time-correlative to significant flooding surfaces at the coeval coastline. Thus coals may be correlated with initial flooding surfaces over incised valley systems and sometimes with parasequence and parasequence set boundaries. This factor may be important in coal exploration. Application of these ideas to the classic Westphalian ‘Coal Measures’ of England indicates that the ‘marine bands’ may be regarded as flooding surfaces or condensed sections, with several being good candidates for maximum flooding surfaces. Between these marine bands the stratigraphy is punctuated by thick, stacked, regionally extensive fluvial units, incised into previously deposited sediments. These are interpreted as incised valley complexes. Systems tract assignment, based on changing accommodation space, indicates that the main coal zones fall dominantly within transgressive systems tracts.

Three-dimensional modeling of a shoreface-shelf parasequence reservoir analog: Part 2. Geologic controls on fluid flow and hydrocarbon recovery

Wave-dominated, shoreface-shelf parasequences are commonly modeled as simple layer-cake reservoirs, yet analysis of modern and ancient analogs demonstrates that these intervals contain a more complex physical stratigraphy. We investigate the impact of depositional and diagenetic heterogeneity associated with gently dipping clinoform surfaces on fluid flow and recovery during water flooding, using a three-dimensional model reconstructed from a well-exposed outcrop analog. We demonstrate that the volume of oil in place is affected by variations in facies thickness associated with interfingering along clinoforms, whereas waterflood sweep efficiency is affected by barriers to flow along clinoform surfaces, such as calcite-cemented layers, mudstones, and siltstones. Sweep efficiency is low when water flooding is down depositional dip because oil is bypassed at the toe of each clinothem as water flows preferentially through high-quality sandstone facies in the upper part of the parasequence. Sweep efficiency is higher when water flooding is up depositional dip because the gravity-driven, downward flow of water sweeps poorer-quality sandstone facies in the lower part of the parasequence. In both cases, injectors may offer limited pressure support to producers. Water flooding along depositional strike yields good pressure support but poor sweep because the gravity-driven flow of water into the lower part of the parasequence is significantly reduced. This yields highly variable fluid saturations but a uniform pressure gradient, which is consistent with pressure and fluid saturation data from the mature Rannoch Formation reservoir, Brent field, United Kingdom North Sea. Simple layer-cake models fail to capture the range of flow behaviors described above and overpredict recovery by up to 20% as a result.

Controls on fluviolacustrine reservoir distribution and architecture in passive salt-diapir provinces: Insights from outcrop analogs

Fluviolacustrine strata host significant hydrocarbon volumes in basins characterized by syndepositional growth of passive salt diapirs. An understanding of salt-sediment interaction is critical to the prediction of reservoir distribution and architecture in these strata. Large-scale stratal geometries and thickness changes resulting from salt movement are commonly apparent on seismic data, but to date, there are few predictive models for facies architecture at subseismic, reservoir scale. This article uses a high-quality outcrop data set of fluviolacustrine strata in an exhumed salt basin (Upper Triassic Chinle Formation, Paradox Basin, Utah) as an analog for improved understanding of subsurface data sets of similar structural and sedimentological setting. Salt-sediment interaction in the Chinle Formation is expressed by localized lateral variations in stratigraphic thickness, angular stratal relationships, and changes in facies architecture. Based on these criteria, there is evidence for salt-sediment interaction across a series of syndepositional salt structures, including anticlines above buried salt pillows, salt walls exposed at surface, and salt-withdrawal minibasins. Stratigraphy and facies architecture across these structures reflect the following controls: regional subsidence, localized differential accommodation space, and localized paleogeomorphology. Both localized controls were driven by syndepositional salt movement, which exhibited subtle spatial and temporal variations during the deposition of the Chinle Formation. The outcrop data set is used to develop generic predictive models of facies distributions and architectures resulting from different conditions of regional tectonic subsidence and/or fluvial energy. Analysis of stratigraphic expansion across syndepositional passive diapirs suggests that the outcrop-derived models are applicable to many subsurface data sets.

Characterization of stratigraphic architecture and its impact on fluid flow in a fluvial-dominated deltaic reservoir analog: Upper Cretaceous Ferron Sandstone Member, Utah

Fluviodeltaic stratigraphic architecture and its impact on fluid flow have been characterized using a high-resolution, three-dimensional, reservoir-scale model of an outcrop analog from the Upper Cretaceous Ferron Sandstone Member of central Utah. The model contains two parasequence sets (delta complexes), each with five or six parasequences, separated by an interval of coastal plain strata. Each parasequence contains one or two laterally offset teardrop-shaped delta lobes that are 6 to 12 km (4–7 mi) long, 3 to 9 km (2–6 mi) wide, 5 to 29 m (16–95 ft) thick, and have aspect ratios (width/length) of 0.4 to 0.8. Delta lobes have a wide range of azimuthal orientations (120) around an overall east-northeastward progradation direction. In plan view, delta lobes in successive parasequences exhibit large (as much as 91) clockwise and counterclockwise rotations in progradation direction, which are attributed to autogenic lobe switching. In cross-sectional view, parasequence stacking is strongly progradational, but a small component of aggradation or downstepping between parasequences reflects relative sea level fluctuations. We use flow simulations to characterize the impact of this heterogeneity on production in terms of the sweep efficiency, which is controlled by (1) the continuity, orientation, and permeability of channel-fill sand bodies; (2) the vertical permeability of distal delta-front heteroliths; (3) the direction of sweep relative to the orientation of channel-fill and delta-lobe sand bodies; and (4) well spacing. Distributary channel-fill sand bodies terminate at the apex of genetically related delta lobes and provide limited sand body connectivity. In contrast, fluvial channel-fill sand bodies cut into, and connect, multiple delta-lobe sand bodies. Low, but non-zero, vertical permeability within distal delta-front heteroliths also provides connectivity between successive delta-lobe sand bodies.

Facies architecture of a net transgressive sandstone reservoir analog: The Cretaceous Hosta Tongue, New Mexico

Net transgressive sandstones form a significant component of many shallow-marine reservoirs, but their shale-poor character commonly masks complex facies architecture and stratigraphy associated with significant permeability variations that impact reservoir drainage patterns and ultimate recovery. In this article, the controls on net transgressive sandstone reservoir architecture are investigated through a detailed analysis of the Cretaceous Hosta Tongue of the Point Lookout Sandstone (informally termed Hosta sandstone in this article) outcrop in New Mexico. Mapping of facies architecture within a series of adjacent canyons has enabled a quantitative three-dimensional reconstruction of key stratigraphic surfaces and sand body distributions from an updip pinch-out to a downdip pinch-out of the net transgressive sandstone complex.The Hosta sandstone contains a complex arrangement of wave- and tide-dominated facies associations arranged in an overall transgressive pattern. Tidal channel-fill sandstones, tidal sheet-form sandstones, and heterolithic tidal-flat and lagoonal deposits comprise the stratigraphy in the updip part of the system. These deposits pass abruptly downdip into wave-dominated shoreface sandstones. The facies composition indicates that the Hosta sandstone represents a wave-dominated barrier shoreline and a tide-dominated back-barrier lagoon. Facies associations are partitioned both vertically and laterally by a hierarchy of transgressive erosion (ravinement) surfaces cut by wave and tidal processes. Reconstructing the geomorphology and spatial organization of these surfaces is critical to understanding sand body distribution and facies architecture at high-resolution (intrareservoir) scale. The exceptional quality of the Hosta Sandstone outcrops has enabled (1) improved understanding of patterns and controls of facies architecture in net transgressive sandstone reservoirs, (2) construction of predictive templates of facies architecture in interwell volumes, and (3) quantification of geobody dimensions and spatial distribution patterns. In combination, these data provide appropriate qualitative and quantitative conditioning for reservoir models.

Large sea, small tides: the Late Carboniferous seaway of NW Europe

Debate surrounds the extent of tidal influence in Palaeozoic shallow epi-continental seas. In the absence of analogical reasoning, numerical modelling provides a quantitative means of investigating tidality in the geological record. Herein a new finite element model, tested for accuracy on the present-day Mediterranean, is used to predict an exceedingly low tidal range (<10 cm) in the epi-continental seaway that covered much of NW Europe during the Late Carboniferous. This small bulge may have been amplified to c. 1 m in estuaries, leading to the localized deposition of cyclic rhythmites, agreeing with geological observations. Extremely low tidal ranges in ancient epi-continental seas may be one mechanism to prevent water-body mixing, enhancing stratification and promoting anoxia.

Continental-scale sequence stratigraphy of the Namurian, Upper Carboniferous and its applications to reservoir prediction

The search for Upper Carboniferous reservoirs can be aided by the development of a chronostratigraphic framework combining detailed sedimentological information with a template using the diagnostic ammonoid-bearing marine bands from onshore analogues derived from European, American and Canadian basins. Analysing Namurian successions constrains key sand-prone intervals, which include the upper Kinderscoutian, lower Marsdenian and top Yeadonian, from a number of depositional settings. A controversial issue for the Upper Carboniferous is the relative importance of high frequency and high magnitude glacio–eustatic sea-level fluctuations as a driving mechanism in the development of basin fill over the controls exerted through the prevailing tectonic regime, climate and sediment supply. The recognition and characterization of time-equivalent sea-level rises and, with greater significance for hydrocarbon exploration, sea-level falls, from a number of European basins attests to their influence. Candidate reservoirs can be constrained at various temporal resolutions and may also be restricted geographically. Significant candidate reservoirs include multistorey fluvial incised valley fills and deepwater sand-rich successions. The most productive onshore reservoirs are coarse-grained/conglomeratic fluvial intervals located in the base of incised valleys where a significant proportion of the subsequent valley fill is fine-grained estuarine sediments. There is potential for large hydrocarbon reservoirs within sand-rich deep water successions but their occurrence in tectonically complex or deeply buried areas has thus far precluded their exploitation. A chronostratigraphic framework established for the UK and Ireland can be applied to European basins and, in a more limited sense, to North America, where marine horizons can be matched. The application of this framework to the subsurface lies in the recognition of key candidate reservoirs and their likely occurrence, both geographically and temporally.

Use of spectral gamma-ray data to refine subsurface fluvial stratigraphy: late Cretaceous strata in the Book Cliffs, Utah, USA

High-resolution (sub-seismic) stratigraphic correlation in fluvial strata is extremely difficult using conventional subsurface core and wireline-log data (e.g. gamma-ray, neutron, density, sonic logs). Spectral gamma-ray logs provide additional data on lithological composition that can aid identification of chronostratigraphic surfaces in a high-resolution sequence stratigraphic correlation framework. The application of spectral gamma-ray data in constructing such a framework is demonstrated using data from exposures and the subsurface from the Cretaceous non-marine Blackhawk Formation and Castlegate Sandstone of the Book Cliffs area, Utah, USA. These strata provide an ideal test case for the use of spectral gamma-ray data in fluvial stratigraphy, because interpretations can be compared directly with a high-resolution sequence stratigraphic framework from coeval shallow-marine strata exposed nearby. The stratigraphic abundance of coals and relatively uniform Th/K ratio, which indicates uniform clay mineralogy, in mudstone sections of the non-marine Blackhawk Formation implies a humid, subtropical climate throughout its deposition. Variations in facies architecture and stratigraphy are therefore interpreted to have been driven principally by fluctuations in relative sea level and hinterland tectonics. Fluvial and fluvio-tidal sandstones within incised valleys, which lie above erosional sequence boundaries, have a highly variable spectral gamma-ray character, but locally are marked by distinctive heavy mineral lags and concentrations (Th consistently >3 ppm). On the interfluves of incised valleys, sequence boundaries are marked by palaeosols that locally have a distinctive leached, potassium-depleted character (Th/K ratio >17). Lateral variability in spectral gamma-ray character of sequence boundaries is consistent with facies and sequence stratigraphic models of the non-marine Blackhawk Formation and Castlegate Sandstone, but sequence boundaries do not exhibit a clear, diagnostic character in every location. In addition, high Th/K ratios of the type noted at interfluve palaeosols may be produced by potassium depletion beneath coal seams as a result of acid groundwater percolation during early diagenesis, and by thorium enrichment in heavy mineral lags within foreshore sandstones. These alternative interpretations can be eliminated if high Th/K ratios occur in intervals lacking coal seams and shallow-marine sandstones, respectively. Thus, within the context of a sound understanding of detrital mineralogy, diagenetic history, facies and appropriate sequence stratigraphic models, spectral gamma-ray data allow sequence boundaries to be identified and correlated in non-marine fluvial strata at least 80 km from the interpreted coeval shoreline.