Sandstone Connectivity Research Articles

A regional CO2 containment assessment of the northern Utsira Formation seal and overburden, northern North Sea

AbstractUpscaling Carbon Capture and Storage requires identification of suitable storage sites, with robust reservoir seals. The Utsira Formation in the northern North Sea has been flagged as a target for further storage. However, there are no regional studies of seal variability addressing heterogeneities that could facilitate seal bypass. This study aims to: (a) identify, assess and map the elements that promote or restrict fluid migration, (b) develop a matrix to regionally map containment confidence (CC) and (c) rank the different areas for CO2 containment across the Utsira Formation. The seal and overburden were mapped using a high‐resolution, pre‐stack depth‐migrated 3D broadband seismic reflection dataset and 141 exploration wells. Seal geometry, sandstone presence and sandstone connectivity in the seal and overburden were assigned relative CC scores, which were summed to map overall CC of the Utsira Fm. Indicators for shallow gas and migration were mapped and correlated with the other elements. Areas with the lowest CC are in the west of the Utsira Fm. Here, sandstones within the Seal Interval are connected through the overburden via sandy submarine fans. In the southeast, dipping stratigraphy downlaps onto the Utsira Fm., increasing the potential for connection with glacially‐derived channel‐lobe systems in the overburden. The areas with the highest CC are the central and northeast parts of the Utsira Fm., where the Seal Interval is mudstone‐dominated and parallel to the reservoir, and channel‐lobe systems identified in the Overburden Interval are disconnected from the reservoir. This area coincides with a thick depocentre of the northern Utsira Fm. These results can be used to inform CO2 storage site selection and constrain future CO2 plume simulation analyses for the Utsira Fm. The CC matrix outlined here can also be adapted and applied to regionally assess the containment of other potential CO2 storage reservoirs in any setting.

Digital core analysis: Improved connectivity and permeability characterization of thin sandstone layers in heterolithic rocks

Heterolithic thin beds reservoirs contain significant amounts of hydrocarbons. We characterized the reservoir properties (sandstone connectivity, continuity and permeability) of sandstone layers in heterolithic rocks using three-dimensional imaging and analysis of heterolithic core plugs and mini plugs. The high-resolution three-dimensional models of the samples are smaller than a typical reservoir model grid block and similar the millimetre-to-centimetre beddings in heterolithic rocks. We find that the key to reservoir quality in heterolithic rocks is the connectivity and continuity of thin reservoir sandstone layers. Also, we show that these well connected and continuous thin sandstone layers have higher effective permeabilities that are comparable to a reference reservoir sandstone. These results can provide further information as to why some low resistivity heterolithic intervals with low net-to-gross values of the sandstone produced a significant amount of hydrocarbon despite lower production forecast based on well logs and core plug data. Determining the connectivity and continuity of thin sandstone layers is difficult due to limitations of well logs and core plug analysis that do not resolve and capture the connectivity of thin beddings. We show that 3-D Image Visualization is effective in characterizing the connectivity and continuity of thin sandstone layers in heterolithic rocks. Also, our Digital Core Analysis method computed permeability of thin sandstone layers in 3-D models of mini plugs representing thin sandstone layers in heterolithic core plugs. Our results represent information and reservoir properties at the millimetre-to-centimetre scale that are similar to the thickness of thin sandstone beddings in heterolithic rocks.

Static connectivity of stacked deep-water channel elements constrained by high-resolution digital outcrop models

High-resolution digital outcrop models of stacked deep-water channel elements are constructed from the Laguna Figueroa section of the well-exposed Upper Cretaceous Tres Pasos Formation in Chilean Patagonia. The models are based on greater than 1600 m (>5250 ft) of centimeter-scale measured sections, greater than 100 paleoflow measurements, and thousands of differential global positioning system points (10-cm [4-in.] accuracy) from an outcrop belt that is approximately 2.5 km (∼1.5 mi) long and 130 m (425 ft) thick. The models elucidate the effects bed-to-geobody–scale architecture has on static sandstone connectivity among a series of stacked deep-water channel elements and how that connectivity is altered by grid cell size. Static connectivity analyses show that channel element base drapes can strongly influence sandstone connectivity and that smaller channel element widths are more likely to produce disconnected sandstone geobodies. Net-to-gross (NTG) is not directly correlated with connectivity because of the presence of thin channel element base drapes, which do not significantly contribute to NTG. Upscaling the models consistently increases channel element contact (up to 10%) but decreases sandstone connectivity (up to 2%–3%). Channel element stacking patterns strongly impact connectivity. For example, connectivity is reduced in cases of high lateral channel element offsets. Increasing drape coverage markedly decreases connectivity. Evaluating connectivity in a vertical, along-system profile is critical to understanding flow units and reservoir piping. Ultimately, this work constrains uncertainty related to the impact of subseismic-scale stratigraphic architecture on reservoir connectivity by providing concrete knowledge that can be used to guide the model building process.

The influence of stratigraphic architecture on seismic response: Reflectivity modeling of outcropping deepwater channel units

The size, shape, stacking patterns, and internal architecture of deepwater deposits control reservoir fluid flow connectivity. Predicting deepwater stratigraphic architecture as a function of position along a deepwater slope from seismic-reflection data is critical for successful hydrocarbon exploration and development projects. Stratigraphic architecture from confined and weakly confined segments of a deepwater sediment-routing system is analyzed in outcrop from the Tres Pasos Formation (Upper Cretaceous), southern Chile. Outcrop observations are the basis of two geocellular models: confined channel deposits at Laguna Figueroa and weakly confined channel and scour deposits at Arroyo Picana. Key stratigraphic surfaces and facies relationships observed in outcrop are forward seismic modeled at high to low resolution to bridge the gap in subseismic scale interpretation of deepwater reservoirs and demonstrate challenges associated with identification of varied reservoir architecture. The outcrop-constrained geometry of architectural elements, their stacking arrangement, and the varied internal distribution of facies each impart a strong influence on seismic reflectivity. Key outcomes from the analysis include (1) stratigraphic architecture transitions down-paleoslope from vertically aligned low-aspect-ratio channel elements to a more weakly confined depocenter characterized by a breadth of laterally offset low- and high-aspect-ratio channel and scour elements. Seismic reflections, down to 30 Hz frequencies, record aspects of these stratigraphic changes. (2) Key seismic reflections are often comprised of multiple outcrop-constrained stratigraphic surfaces. Tuning effects result in composite seismic surfaces that are vertically offset from the known position of sedimentary units; this hinders accurate interpretation of stratigraphic surfaces from seismic-reflection data. This is particularly problematic in the weakly confined system in which shifted stratigraphic surfaces, which bound deposits characterized by numerous similar architectural elements, can alter the interpretability of sandstone connectivity within and across zones. Furthermore, misinterpretation of surfaces is problematic when they are flow barriers draped with debris flows, slumps/slides, or thin-bedded turbidites. (3) Tuning effects also impart significant control on volume-based interpretations from seismic data. In particular, calculations of gross rock volume from seismic reflection data that do not consider the tuning or architectural element stacking pattern can overestimate actual volumes by 10%–50%, with implications for reservoir prediction and hydrocarbon volume estimation.

Lateral variation in sandstone lithofacies from conventional core, Scotian Basin: implications for reservoir quality and connectivity 1This article is one of a series of papers published in this CJES Special Issue on the theme of Mesozoic–Cenozoic geology of the Scotian Basin. 2Geological Survey of Canada Contribution 20120021.

Facies analysis in outcrops on land is strongly dependent on the lateral variability of lithofacies. Interpretation of conventional core in wells relies principally on the vertical succession of lithofacies. To better understand depositional environments and reservoir sandstone connectivity, the lithofacies of reservoir sandstones sampled in conventional core were correlated laterally through two sets of closely spaced wells in the Scotian Basin: in the Barremian–Albian succession around the Panuke–Cohasset field and in the Late Jurassic succession west of the Venture field. Regional correlation by gamma logs is confirmed by lithologically similar transgressive units including shelly mudstones or coals. A standard scheme of lithofacies and recognition of three types of parasequences were used for comparisons between wells. Some transgressive surfaces are of limited extent and may represent delta distributary switching and subsidence rather than regional changes in sea level. Major sandstone packets extend at least 10–30 km laterally, but commonly show lateral changes in lithofacies, and some are bounded by the margins of incised valleys. Such packages show poor correlation of lithofacies with porosity and permeability, probably because of the variable effects of diagenesis. Lateral transitions from tidal estuary sandstones in Panuke B-90 to thick-bedded river-mouth turbidites in Lawrence D-14, over a distance of 15 km, demonstrates the scale of delta lobes and confirms that sharp-based sandstone beds are turbidites related to river floods, not storm deposits. Similar lateral transitions in the Venture field are on a similar scale and pass distally into prodeltaic muddy landslide deposits.

Characterization and 3D reservoir modelling of fluvial sandstones of the Williams Fork Formation, Rulison Field, Piceance Basin, Colorado, USA

This study describes the stratigraphic characteristics and distribution of fluvial deposits of the Upper Cretaceous Williams Fork Formation in a portion of Rulison Field and addresses 3D geologic modelling of reservoir sand bodies and their associated connectivity. Fluvial deposits include isolated and stacked point-bar deposits, crevasse splays and overbank (floodplain) mudrock. Within the Williams Fork Formation, the distribution and connectivity of fluvial sandstones significantly impact reservoir productivity and ultimate recovery. The reservoir sandstones are primarily fluvial point-bar deposits interbedded with shales and coals. Because of the lenticular geometry and limited lateral extent of the reservoir sandstones (common apparent widths of ∼500–1000 ft; ∼150–300 m), relatively high well densities (e.g. 10 acre (660 ft; 200 m) spacing) are often required to deplete the reservoir. Heterogeneity of these fluvial deposits includes larger scale stratigraphic variability associated with vertical stacking patterns and structural heterogeneities associated with faults that exhibit lateral and reverse offsets. The discontinuous character of the fluvial sandstones and lack of distinct marker beds in the middle and upper parts of the Williams Fork Formation make correlation between wells tenuous, even at a 10 acre well spacing. Some intervals of thicker and amalgamated sandstones within the middle and upper Williams Fork Formation can be correlated across greater distances. To aid correlation and for 3D reservoir modelling, vertical lithology proportion curves were used to estimate stratigraphic trends and define the stratigraphic zonation within the reservoir interval. Object-based and indicator-based modelling methods have been applied to the same data and results from the models were compared. Results from the 3D modelling indicate that sandstone connectivity increases with net-to-gross ratio and, at lower net-to-gross ratios (<30%), differences exist in the cumulative volume of connected sandstone bodies between the indicator- and object-based lithology models. Therefore, the types of lithology-modelling methods used for lower net-to-gross ratio reservoir intervals are important.

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.

INTERPRETING BIOSTRATIGRAPHICAL DATA USING FUZZY LOGIC: THE IDENTIFICATION OF REGIONAL MUDSTONES WITHIN THE FLEMING FIELD, UK NORTH SEA

The Fleming gas‐condensate field, located on the eastern flank of the Central Graben, UK North Sea, is an elongate stratigraphic pinch‐out whose reservoir is composed of stacked turbidite sandstones of the lower Palaeogene Maureen Formation. The sandstones have a sheet‐like geometry with each sandstone lobe being partially offset into the swales of the preceding lobes. In such depositional environments, the understanding of lateral and vertical sandstone connectivity, a major uncertainty in reservoir modelling and well planning and production strategies, depends upon the choice of depositional model that is applied and the lateral continuity of pelagic mudstones.Previously published work defined a model, based on variations in the composition of agglutinated foraminiferal populations, that could be used to derive a qualitative measure of the level of pelagic influence within mudstones interbedded with turbidite sandstones. It was considered that mudstones with a high pelagic influence are likely to be more laterally extensive than those with a low pelagic index. A fuzzy logic workflow was constructed using this model and was applied to the Fleming field in order to identify laterally persistent mudstones. This approach was combined with high‐resolution correlation of bioevents using graphic correlation. A detailed layering scheme for the Fleming field was defined and this predicted the presence of a field‐wide mudstone.Initial attempts at history matching during reservoir simulation using a simple six‐layer lithostratigraphical scheme were not successful. A revised layering scheme defined by biofacies modelling and graphic correlation was used to produce a 13‐layer model; this was later simplified by combination with the six‐layer model to produce a ten‐layer model. This layering scheme is shown to provide a better understanding of both net‐to‐gross distribution and the dynamic behaviour of the field, and also improved history matching against production data. The biostratigraphical model applied using a fuzzy logic approach is authenticated by the reservoir simulation (fluid flow) and pre‐ and post‐maintenance well pressure tests of well 22/5b‐A3 which showed that the perforated interval in that well is isolated from the perforated intervals in the other producing wells. While history matching during reservoir simulation is important, the predictive capability of the fuzzy logic model proved to be critical to our understanding of the field.

Late highstand patterns of shifting and stepping coastal barriers and washover-fans (late Messinian, Sorbas Basin, SE Spain)

The late Messinian Sorbas Member, up to 75 m thick, consists in its type area of a parasequence set of three prograding coastal barriers (sequences I–III), associated with lagoon and washover sediments. Around the town of Sorbas these strata can be studied exceptionally well due to absence of burrowing by raised Messinian salinities and exposure along a network of up to 30 m deep canyons. Fifteen vertical sections were logged and carefully correlated. This permits to reconstruct and discuss patterns of relative sea-level movements between decimetres, up to 15 m within a parasequence. Excellent examples of non-tidal transgressive facies are characterized by lagoon and washover sediments instead of the usual combination of washover and tidal deposits (channel and flood-tidal delta). Implications for the sandstone connectivity are given. The lower two sequences are deposited in a relatively large, tectonically enhanced wedge-shaped accommodation space. They show both fining-up, deepening sequences, followed by prograding coarsening-up shoaling sequences and can be compared to the classical parasequences of the Western Interior Basin (USA). Progradation of sequence II was interrupted by a major slide event (most likely triggered by an earthquake), which caused more than 400 m seaward slumping of a stretch of 1 km of coastal sands. The architecture of sequence III is more complex due to limited accommodation space characteristic for the late highstand, so that this setting was very sensitive to sea-level fluctuations. This resulted in an intricate pattern of juxtaposed and superposed lagoonal muds, washover fans and swash zones. So-called ‘stranded’ coastal barriers occur, which were left behind after seaward jump of the coastline over more than 1.5 km during forced regression. The pattern of reconstructed sea-level positions is well comparable to the sequential pattern shown by the correlated equivalents along the northern basin margin, which belong to the so-called Terminal Carbonate Complex. The problem of ranking the complex sequence III as one or more parasequences and its consequences for cyclostratigraphy are shortly discussed. Two models of washover formation are given, respectively during more rapid and more slow sea-level rise. The influence of synsedimentary folding on the location of barriers is discussed and also the source area of extra- and intraclasts supplied to barriers and washovers. After deposition of the Sorbas Member the sea withdrew from the Sorbas Basin, probably as a result of the major downdrop in the Mediterranean at the maximum isolation during the Salinity Crisis. It is suggested that the semi-enclosed setting of the basin resulted only in limited, localized erosion, in contrast to the deeper adjacent Vera Basin, which was more open to the Mediterranean.

Evolving Fluvial Style in the Kekiktuk Formation (Mississippian), Endicott Field Area, Alaska: Base Level Response to Contemporaneous Tectonism

The hydrocarbon reservoir at Endicott field, offshore from the Sagavanirktok delta in northeastern Alaska, is found in the Upper Mississippian (Visean) Kekiktuk Formation. The trap is bounded by the northwest-southeast-trending Tigvariak and Mikkelson Bay faults, and the younger east-west-trending Niakuk fault. The Kekiktuk comprises sandstones, siltstones, and mudstones with locally significant coals and conglomerates, and is subdivided into three major reservoir zones based on the relative dominance of these lithologies. Zone 1 (the lowest) comprises mudstones, siltstones, and coals with subordinate isolated sandstones, and represents deposition within a low-lying swamp plain. With very low net-to-gross ratio, this zone is considered to be effectively nonreservoir. Major movement on the Mikkelson Bay and Tigvariak faults created a half-graben with uplift to the north and east and a very significant drop in base level. This change resulted in fairly widespread deposition of zone 2, medium- to coarse-grained sandstones from sandy bed-load braided streams. However, those sandstones thin significantly to the west, away from the faults. Continuing tectonism at this time created local shifts in base level, manifest as diversion of streams, widespread abandonment, and subsidence leading to the creation of a arge lake. The lacustrine deposits are widespread, and effectively divide zone 2 into two subzones. Those subzones are mappable sheet sandstones, and constitute the best reservoir at Endicott field in terms of net-to-gross ratio, porosity/permeability, and sandstone connectivity. Base level shows evidence of rising dramatically at the end of zone 2. This rise is attributed to continuing tectonism within the basin, and is particularly associated with the east-west-trending mid-field fault system at Endicott field, as well as ongoing activity on the Tigvariak/Mikkelson Bay fault system. Thus, at the base of zone 3 (subzone 3A), in the southern and eastern parts of the field, the rocks are mudstones and widespread coals, and contain at least one unequivocally marine mudstone. These rocks are lower delta-plain interdistributary bay-fill deposits, with rare distributary channel sandstones. North and west of the mid-field fault system, the marine mudstone is absent and the facies changes significantly with the loss of dominant coal and a concomitant increase of dist ibutary channel sandstones. Reservoir properties of subzone 3A, including connectivity, are significantly different in this area. Subzone 3B is a thick accumulation of fine-grained alluvial overbank deposits within which occur rare, isolated anastomosed channel sandstones of limited connectivity. This subzone makes minimal contribution to Endicott field reserves. Subzone 3C is characterized (particularly in its upper part) by well-developed fining-upward cycles indicating mature meandering streams. The relationships between these two highest subzones of the Kekiktuk at Endicott field indicate they represent an aggradational episode initiated by tectonically induced avulsion and set against a background of rising base level. To the west, at some distance from the Tigvariak/Mikkelson Bay f ult system, zone 3 is much thinner than at Endicott field, and is dominated by mudstones and coals. This evidence emphasizes the control that contemporaneous tectonism related to those faults had on sediment (especially sandstone) distribution within the Kekiktuk Formation. Base level changes can be identified within the End_Page 1723------------------------------ Kekiktuk Formation in the North Slope subsurface by careful lithofacies analysis and an awareness of key constraining indicators, such as are revealed by palynology, paleontology, and structural geology. By analogy with shallow-marine rocks, sequence stratigraphic concepts may be extended to continental successions. However, one must approach such problems with a full awareness of the impact of tectonism on base level and basin development.