Howard D. Johnson

Polycyclic motion history of some Gulf Coast growth faults from high-resolution displacement analysis

This paper describes some simple graphical techniques for analyzing the kinematic evolution of growth faults and presents a case study of the late Pleistocene–recent history of a group of 17 faults from offshore Texas. Throw versus depth plots derived from depth-converted high-resolution seismic data were used to define growth histories. The faults exhibit polycyclic kinematic behavior, with at least three cycles of active fault growth separated by periods of inactivity. This cyclic behavior correlates broadly with three transgressive-regressive cycles along this part of the Gulf Coast. In detail, however, we find that activity of closely spaced faults in the array can either be in phase or out of phase with neighboring faults. This complex group behavior is attributed to the geometry of the detachment surface, pore-fluid pressure distribution, loading, and friction. This study demonstrates that although a general correlation with sediment loading may exist, individual fault activity is unlikely to correlate with periods of maximum sediment accumulation.

Three-dimensional reservoir characterization and flow simulation of heterolithic tidal sandstones

Tidal sandstone reservoirs contain significant intervals of hydrocarbon-bearing heterolithic facies, characterized by the presence of tide-generated sedimentary structures such as flaser, wavy, and lenticular bedding (millimeter to centimeter sand-mud alternations). We have characterized the reservoir properties (sandstone connectivity, effective permeability, and displacement efficiency) of these facies using three-dimensional (3-D) models reconstructed directly from large rock specimens. The models are significantly larger than a core plug, but smaller than a typical reservoir model grid block. We find that the key control on reservoir quality is the connectivity and continuity of the sandstone and mudstone layers. If the sandstone layers form a connected network, they are likely to be productive even at low values of net-to-gross (about 0.30.5). This may explain why the productivity of low net-to-gross, heterolithic tidal sandstones is commonly underestimated or overlooked. Connectivity is the dominant control on the transition between productive (pay) and nonproductive (nonpay) heterolithic facies. However, connectivity is difficult to characterize because core plugs sampled from the subsurface are too small to capture connectivity, whereas two-dimensional outcrop measurements can significantly underestimate the true 3-D value. Our results suggest that core-plug measurements of permeability and displacement efficiency are unlikely to yield representative values at the scale of a reservoir model grid block because the connectivity and continuity of sandstone and mudstone layers varies significantly with length scale.

Shallow marine sand bar sequences: an example from the late Precambrian of North Norway

Five coarsening upward shallow marine sandstone sequences (2–10 m thick), are described from the late Precambrian of North Norway, where they occur in a laterally continuous and tectonically undeformed outcrop. The sequences consist of five facies with distinct assemblages of sedimentary structures and palaeocurrent patterns. Each facies is the product of alternate phases of sedimentation during relatively high- and low-energy periods. Facies 1 to 4 are interpreted as representing prograding, subtidal sand bars. Sand bar progradation occurred during the highest energy periods when unidirectional currents flowed to the northwest, depositing trough cross-bedded sandstones (facies 3 and 4) on the bar crests and flanks, and sheet sandstone beds (facies 1 and 2) in the offshore environments. Weaker northwesterly flowing currents continued during moderate energy fair weather periods. Low energy fair weather periods were dominated by wave processes, which formed largescale, low-angle, westerly inclined surfaces on the bar flanks (facies 4) and wave rippled sandstone beds (facies 2) and flat laminated siltstone layers (facies 1) in the offshore environments. One sand bar was dissected by channels and infilled by tabular cross-bedded sandstones (facies 5). Bipolar palaeocurrent evidence, with two modes separated into two laterally equivalent channel systems, suggests deposition by tidal currents in mutually evasive ebb and flood channels. The inferred processes of these sand bars are compared with those associated with modern storm-generated and tidal current generated linear sand ridges. Both are influenced by the interaction of relatively low and high energy conditions. The presence of the tidal channel facies, however, combined with the inferred strong bottom current regime, is more analogous to a tidal current hydraulic regime.

Upscaling Permeability Measurements Within Complex Heterolithic Tidal Sandstones

We investigate numerically the effect of sample volume on the effective single-phase permeability of heterolithic tidal sandstones, using three-dimensional models reconstructed directly from large rock specimens measuring ∼45 × 30 × 15 cm. We find that both individual and averaged effective permeability values vary as a function of sample volume, which indicates that permeability data obtained from core-plugs will not be representative at the scale of a reservoir model grid-block regardless of the number of measurements taken. However, the error introduced by averaged data may be minimized using the appropriate averaging scheme for a given facies type and flow direction.

Transgressive changes from tidal estuarine to marine embayment depositional systems: The Lower Cretaceous Woburn Sands of southern England and comparison with Holocene analogs

The Lower Cretaceous Woburn Sands (Lower Greensand Group) in southern England constitutes one of the most intensively studied tidal sandstone outcrops for sedimentological and reservoir analog studies. Most recent workers have interpreted the whole 30–60-m (100–200-ft)-thick succession around Leighton Buzzard as representing an ancient tide-dominated estuary. However, unequivocal estuary characteristics are limited to the lowermost part (about 15–20 m [50–66 ft]). We suggest that a significant portion of the Woburn Sands, and most of the middle part, was formed in a tide-dominated marine embayment. Hence, the vertical facies change from the lower to middle part of the Woburn Sands is interpreted as a change from (1) a narrow estuary to (2) a broad marine embayment. The Wash embayment in eastern England is a striking modern analog; it receives most of its sediments and waters from marine sources and is largely filled with nondiluted seawater. Moreover, the Holocene transgressive history of The Wash is remarkably similar to the transgressive evolution of the Woburn Sands. Early estuarine sequence models predict landward translation of facies zones along the valley thalweg during transgression, but eventual facies translation in the strike direction has not been fully documented or discussed. An embayment facies that is not commonly confined in the early incised valley can occur vertically between the estuarine valley fill and marine shelf deposits and is probably underrepresented in current models. However, this segment of the transgression, comprising along-strike bay expansion and the development of a broad marine embayment, may be more important than previously thought.

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.

A sedimentological approach to refining reservoir architecture in a mature hydrocarbon province: the Brent Province, UK North Sea

Improved reservoir characterisation in the mature Brent Province of the North Sea, aimed at maximising both in-field and near-field hydrocarbon potential, requires a clearer understanding of sub-seismic stratigraphy and facies distributions. In this context, we present a regional, high-resolution sequence stratigraphic framework for the Brent Group, UK North Sea based on extensive sedimentological re-interpretation of core and wireline-log data, combined with palynostratigraphy and published literature. This framework is used to place individual reservoirs in an appropriate regional context, thus resulting in the identification of subtle sedimentological and tectono-stratigraphic features of reservoir architecture that have been previously overlooked. We emphasise the following insights gained from our regional, high-resolution sequence stratigraphic synthesis: (1) improved definition of temporal and spatial trends in deposition both within and between individual reservoirs, (2) development of regionally consistent, predictive sedimentological models for two enigmatic reservoir intervals (the Broom and Tarbert Formations), and (3) recognition of subtle local tectono-stratigraphic controls on reservoir architecture, and their links to the regional, Middle Jurassic structural evolution of the northern North Sea. We discuss the potential applications of these insights to the identification of additional exploration potential and to improved ultimate recovery.

Outcrop studies of tidal sandstones for reservoir characterization (Lower Cretaceous vectis formation, isle of wight, Southern England)

The Vectis Formation (Lower Cretaceous) in the Isle of Wight, southern England serves as an excellent outcrop analogue for tidal sandstone reservoirs. It consists of two juxtaposed marginal-marine depositional systems separated by an erosional, regionally extensive sequence boundary. The lower system is characterized by a muddy succession which overlies fluvial strata and lacks tidal indicators. This is interpreted as a low-energy, broad lagoonal or embayment complex. The upper, sandstone-dominated system is characterized by a wide range of heterolithic deposits, with abundant evidence of tidal processes, as indicated by sedimentary structures and palaeocurrent patterns. It is interpreted as a laterally migrating tidal bar and channel complex within the inner part of a mixed-energy estuary (sensu Dalrymple et al., 1992). The Vectis Formation sandstones contain a highly variable array of sedimentary structures and heterogeneities, ranging from the regional scale (tens of kilometres wide, tens to hundreds of metres in thickness) to individual laminae scale (mm in thickness). These have been described and quantified in order to provide a basis for evaluating their impact on fluid flow and hydrocarbon recovery. The large-scale heterogeneities and facies variations are explained within a sequence stratigraphic framework and reflect the transgressive infill of a mixed-energy estuarine complex. The intermediate- to small-scale heterogeneities (mm-m in thickness, representing the scale of individual bedforms and laminae) reflect autocyclic tidal processes and have been examined in detail by ( 1 ) constructing a detailed 2-D facies model directly from the outcrop (measuring 400 m × 6 m), (2) photographing and digitizing small, representative areas (ca. 1 m × 1 m) of 2-D outcrops, and (3) using serial sectioning techniques to reconstruct the 3-D geometry of tidal sedimentary structures (wavy-, lenticular- and flaser-bedding) directly from large (ca. 60 cm × 60 cm × 20 cm) rock specimens. These reconstructed rock models have been used to investigate the validity of using core-plug and well-log derived permeability data to represent heterolithic facies in tidal sandstone reservoir models.

Effective flow properties of heterolithic, cross-bedded tidal sandstones: Part 1. Surface-based modeling

Tidal heterolithic sandstones are commonly characterized by millimeter- to centimeter-scale intercalations of mudstone and sandstone. Consequently, their effective flow properties are poorly predicted by (1) data that do not sample a representative volume or (2) models that fail to capture the complex three-dimensional architecture of sandstone and mudstone layers. We present a modeling approach in which surfaces are used to represent all geologic heterogeneities that control the spatial distribution of reservoir rock properties (surface-based modeling). The workflow uses template surfaces to represent heterogeneities classified by geometry instead of length scale. The topology of the template surfaces is described mathematically by a small number of geometric input parameters, and models are constructed stochastically. The methodology has been applied to generate generic, three-dimensional minimodels (9 m3 [∼318 ft3] volume) of cross-bedded heterolithic sandstones representing trough and tabular cross bedding with differing proportions of sandstone and mudstone, using conditioning data from two outcrop analogs from a tide-dominated deltaic deposit. The minimodels capture the cross-stratified architectures observed in outcrop and are suitable for flow simulation, allowing computation of effective permeability values for use in larger-scale models. We show that mudstone drapes in cross-bedded heterolithic sandstones significantly reduce effective permeability and also impart permeability anisotropy in the horizontal as well as vertical flow directions. The workflow can be used with subsurface data, supplemented by outcrop analog observations, to generate effective permeability values to be derived for use in larger-scale reservoir models. The methodology could be applied to the characterization and modeling of heterogeneities in other types of sandstone reservoirs.

Synthesis of time-stratigraphic relationships and their impact on hydrocarbon reservoir distribution and performance, Bridport Sand Formation, Wessex Basin, UK

Abstract 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.