Fluvial Channel Sandstones Research Articles

Lithology and Lithofacies Characteristics of Deeply Buried Clastic Reservoirs in Alternating Wet-arid Climate and its Hydrocarbon Significance

There have been series of HPHT (high temperature and high pressure) reservoirs with good quality and high productivity discovered in the Triassic Skagerrak Formation in the Central Graben of the UK North Sea. The bottlenecks for further E&P activity are the complicated depositional facies and high reservoir heterogeneity. The main reservoir space is a mixed porosity network composed of primary pores, micro-pores, secondary pores, secondary micro-pores and micro-fractures. Reservoirs of the Skagerrak Formation have greatly varied lateral connectivity as well as poor vertical connectivity. In the case of sufficiently high pressure differentials, hydrocarbon can be fed from high-potential area to low-potential area. There is no simple linear relationship between depth and reservoir quality. The primary depositional texture (grain size/sorting/clay content, etc.) linked to facies has a fundamental control on reservoir quality. Clean and well sorted cross laminated sandstone has the best reservoir quality, while the reservoir quality of planar laminated sandstone decreased due to increased degree of compaction caused by mica drapes. The reservoir quality of the fine-grained ripple cross laminated sandstone is even poorer than that of the planar laminated sandstone, due to abundant mica and clay drapes. Mottled bioturbated sandstone is fine grained with high clay content, which is more muddy because of containing burrows and insect larva. Pedoturbated sandstone has the poorest reservoir quality due to severe dolomite cementation. The dominant detrital material of the Skagerrak Formation is feldspar that provides abundant secondary porosity by alteration and dissolution. Chlorite coats are commonly developed and stained with residual oil or bitumen, which clearly inhibits quartz cementation and feldspar overgrowth. The presence of these clay coats contributes to the preservation of anomalously high porosity at depth in the cleaner and coarser grained fluvial channel facies. The Skagerrak formation facies is a dryland fluvial-lacustrine depositional system which is controlled by fluctuations of depositional base level due to salt mobilisation. The Skagerrak formation consists of several cycles that are vertically separated by lacustrine mudstone. The reservoir permeability system is composed of active fluvial channel sandstones, terminal splay sandstones, abandoned fluvial channel sandstones, minor channel sandstones connecting fluvial channel and terminal splay, as well as low-permeability faults, while the active fluvial channel sandstone is the “sweet spot”. The analysis of controls on deeply buried reservoir quality is significant to precisely predict “sweet spot” in the Skagerrak Formation.

Sedimentary facies analysis, seismic interpretation, and reservoir rock typing of the syn-rift Middle Jurassic reservoirs in Meleiha concession, north Western Desert, Egypt

The Middle Jurassic syn-rift sediments of the Khatatba Formation contain not only the principle source rock organofacies in the north Western Desert (NWD), but also several potential sandstone reservoir targets. However, controls on the reservoir quality of the Khatatba facies and spatiotemporal evolution of the reservoir rock types (RRTs) are still blurred. This study integrates seismic, wireline logs, and conventional core data from some wells in the Meleiha development lease in the NWD to investigate the petrophysical characteristics of the Khatatba reservoir facies, and determine the different RRTs. On seismic, Khatatba Formation consists of high reflectivity, continuous reflections displaying an upward decrease in seismic amplitude corresponding to a fining-upward clastic succession of fluvial–tidal facies. These facies display a large-scale pore system heterogeneity, thereby comprising three different RRTs. RRTI and RRTII rocks have a well-connected pore network dominated by capillary and super-capillary pores (> 10 µm), whereas sub-capillary pores (< 10 µm) are more common in RRTIII sediments. RRTI rocks consist of quartzose sandstones with a clean pore network (cement < 20%). The open pore system of RRTI allows a displacement of more than 80% of the pore water at entry pressure levels < 500 psi. Carbonate and clay components are more common in RRTIII rocks (average = 5.6% and 13%, respectively). These components adversely impact the pore system connectivity of RRTIII sediments. Fluvial channel sandstones host the principal fluid flow conduits where RRTI sandstones account for more than 95% of the reservoir flow capacity. The present study provides the first reservoir rock typing for the Khatatba clastic reservoir facies and highlights the link between depositional, compositional attributes and the spatial distribution of the different RRTs.

Soft‐sediment deformation structures in the Late Messinian Abu Madi Formation, onshore Nile Delta, Egypt: Triggers and tectonostratigraphic implications

The Upper Messinian Abu Madi Formation of the Nile Delta constitutes sediments deposited during the final stage of the Messinian salinity crisis (MSC). Several levels of soft‐sediment deformation structures (SSDS) were observed in the transgressive heterolithic fluvial facies deposited adjacent to the deep‐seated faults. The most common deformation features include diapiric structures, intraformational lithoclastic breccia, small‐scale normal faults, slump folds, and liquefied beds. Such association of SSDS typically resembles those described elsewhere as generated by a complex interplay of gravity‐driven and seismically induced liquefaction processes. The pore pressure measurements revealed high pore fluid density ~ 9.1 ppg equivalent density values which reflect a mild pressure disequilibrium in Abu Madi sediments. Pressure disequilibration is observed all over the studied section of the Abu Madi Formation and is not only restricted to the fluvial channel sandstones deposited during rapid loading. Therefore, allogenic seismic activity has been proposed as the main trigger for pressure disequilibrium and the development of SSDS in the Abu Madi Formation. The heterolithic nature of the Abu Madi sediments as well as the scarcity of bioturbation provides the favourable conditions for the preservation of the seismically induced deformation. The lateral variation in thickness suggests deposition of the Abu Madi Formation during periods of active subsidence which promotes the generation of the SSDS. Late MSC tectonism likely controlled the evolution and sedimentary facies variability of the Abu Madi canyon‐infill system, and therefore the distribution of Abu Madi facies varies significantly over very short distances. Abu Madi SSDS follows the spatial distribution observed in other “lago‐mare” deposits (e.g., Foes Formation and SE Spain), characterized by spatial variability and vertical rhythmic alternation between deformed and non‐deformed layers. Accordingly, regional tectonic instability in the circum‐Mediterranean margins during the late stage of the MSC is proposed.

Integrated modelling framework for enhancement history matching in fluvial channel sandstone reservoirs

Abstract Modelling lithofacies and petrophysical properties are the challenging processes at the beginning of exploration and production of hydrocarbon reservoirs. However, the limited amount of well data and core data are the main issues facing conventional modelling processes. In this study, Artificial Neural Network (ANN), Sequential Gaussian Simulation (SGS), Co-kriging and object-based modelling (OBM) were integrated as the enhancement framework for lithofacies and petrophysical properties modelling in the fluvial channel sandstone reservoir. In the OBM, multiple fluvial channels were generated in the lithofacies model. The result of this model represented all the characteristic of the fluvial channel reservoir. The model was then distributed with channels, crevasse, and leeves depositional facies with background shale. Multiple geological realizations were made and cross-validation to select the most suitable lithofacies distribution. This model was cross-validated by modelling the porosity and permeability properties using Sequential Gaussian Simulation. Thereafter, the modelling process continued with Artificial Neural Network. Petrophysical properties (mainly porosity and permeability) were predicted by training various seismic attributes and well log data using the ANN. Applying the co-kriging algorithm, the predicted ANN model was integrated with OBM simulated lithofacies model to preserve the fluvial features of the geological system. To achieve full field history matching, the final geological model was upscaled to serve as input data in dynamic history matching. An excellent and nearly perfect history matching with a least mismatch was obtained between the measurement and simulated bottom hole pressure from well test and production history. The results indicated that an efficient integrated workflow of ANN and other geostatistical approaches are imperative to attaining an excellent history matching.

Sequence stratigraphic controls on reservoir characterization and architectural analysis: A case study of Tovo field, coastal swamp depobelt, Niger Delta Basin, Nigeria

The decline in reservoir pressure and rate of production have prompted a series of studies to better characterize and define reservoir architecture in anticipation of ultimate recovery of the hydrocarbon deposits by a fluid-injection program in the Tovo field located in the coastal swamp depobelt of the Niger Delta Basin. This study utilizes the integration of well logs, biostratigraphic and 3D seismic data sets to establish a third-order sequence stratigraphic framework consisting of fourth-order sequences. It facilitates the construction of predictive depositional models that elucidate reservoir facies distribution, and architecture. Three types of reservoir deposits that are generally composed of channel and shoreface sandstone facies have been considered: lowstand fluvial sediments with a bulging geometry, marine-influenced sandstone deposits defined by strike-oriented tabular geometry, and highstand fluvial channel sandstone deposit characterized by dip-oriented lobate geometry. Intrareservoir shale, discontinuous shale stringers and/or shale lensoids, which are products of channel avulsion, overbank sedimentation, and marine flooding define the internal reservoir architecture. Their overall reservoir quality is very good. However, the fluvial-dominated sandstone deposits show a wide range of porosity and permeability across individual wells. Conversely, the marine-influenced reservoir deposits show better lateral and vertical sand connectivity as suggestive of near similar in the range of porosity and permeability values across the field, implying a better oil sweep efficiency and fluid-injection potential. The results of this study suggest that the depositional environment of sandstone reservoirs of the basin is complex but not infinite in facies distribution. The spatial distribution of these sandstone reservoir facies is related to changes in local accommodation space in the basin. This study, therefore, demonstrates the importance of evaluating the impact of sequence stratigraphic controls on reservoir characterization and architecture of hydrocarbon-bearing sandstones for optimum exploration and production in clastic depositional settings.

Geochemistry of the middle-upper Miocene Bathan Formation, Al-Rehaili area, Jeddah, Saudi Arabia: provenance, tectonic setting, and paleoweathering implications

The present work investigates geochemical and mineralogical signatures preserved in the fluvio-lacustrine sediments of the middle-upper Miocene Bathan Formation at Al-Rehaili area, northern Jeddah, Saudi Arabia to constrain the composition and tectonic setting of the source area and to appraise the influence of weathering. Sixteen sediment samples were collected from the lacustrine delta plain and delta front sandstones (SS I), lacustrine offshore mudrocks, and the younger fluvial channel sandstones (SS II). The sandstones are compositionally immature first cycle. The sandstones (SS I) and the mudrocks of the first depositional stage were derived from an intermediate igneous source rock, whereas the sandstones (SS II) of the second depositional stage were derived mainly from a felsic igneous source rock with subordinate contribution from an intermediate igneous source rock. They indicate an active continental margin to oceanic arc tectonic setting. The values of the Chemical Index of Alteration (CIA) and the Plagioclase Index of Alteration (PIA) as well as the Al2O3/Na2O, Rb/Sr, and Al2O3/(CaO+Na2O) ratios indicate that the source rocks experienced low to moderate degree of weathering consistent with arid to semi-arid climate that prevailed during the Miocene. Subaerially exposed lacustrine offshore mudrocks displayed excessive depletion in the concentration of mobile elements comparing to the bedrock. The low values of the paleoweathering indices are further explained by the enhanced uplift of the Red Sea Escarpment after the cessation of the first stage of the Red Sea floor spreading at 14–15 m.y. ago.

Linking between sequence stratigraphy and reservoir quality of Abu Madi Formation utilizing well logging and seismic analysis at Abu Madi and El Qar’a fields, Nile Delta, Egypt

This work has been carried out to interpret the link between sequence stratigraphy of the incised valley deposits with the reservoir quality in the Abu Madi Formation in the Abu Madi and El Qar’a Fields, onshore Nile Delta, Egypt based on borehole and seismic data. The distribution of sand-body in the Abu Madi Formation revealed that the fluvial channel sandstones are not randomly distributed but are controlled by the fluctuation of the sea level (regression and transgression). Interpretation of sandstones distribution of the Abu Madi incised-valley within a sequence stratigraphic framework allows a better understanding of their characterization during burial. The Abu Madi Formation is made up of two sequences: the lower sequence-1 (SQ 1) and upper sequence-2 (SQ 2). Sequence-1 forms the basal part of the Abu Madi Formation. It consists of two systems tracts in all the studied wells, in which the lowstand systems tract is overlained by a transgressive systems tract (TST-1), except at AM-12 and AM-15 wells in the western and eastern sides of the studied area, respectively where it consists of only transgressive system tract (TST-1). The upper sequence-2 (SQ2) is recorded in all the studied wells except at AM-12 and AM-15 wells. It is composed of two systems tracts (LST-2 and TST-2); the lower lowstand systems tracts contains thin sandstone channel-fill interpreted as LST-2, above this sandstone channel-fill the tidal deposits interpreted as TST-2. The lowstand systems tracts of the studied Abu Madi Formation is considered good reservoir target in the studied area, its porosity ranged from 15 to 22%. It was formed due to a rapid fall in relative sea level below the shelf edge formed valley incision due to subaerial exposure of the continental shelf. The floodplain deposits of TST represent a good seal separating LST reservoirs and water bearing zone of lower sequence (SQ1) and upper sequence (SQ2) in each well.

Architectural and facies characterization of the Aptian fluvial Barbalha Formation, Araripe Basin, NE Brazil

Abstract The Aptian Barbalha Formation records the beginning of the post-rift stage of the Araripe Basin. It consists predominantly of sandstones and mudstones interbedded with thin layers of bituminous black shales and conglomerates. The depositional and architectural features of the alluvial succession of the Barbalha Formation were characterized by detailed study and descriptions of the selected outcrops and analysis of well core data. In this study, two main depositional sequences were identified. The lower depositional sequence is more than 100 m thick and comprises a vertical facies succession composed of amalgamated, multistorey, braided fluvial channel sandstone bodies overlain by a widespread lacustrine black shale up to 10 m thick. The lacustrine black shales–carbonate mixed interval is known as the Batateira Beds and constitutes a regionally important stratigraphic correlation marker in the basin. This interval records the establishment of a large lake that experienced severe water-level fluctuations and anoxic events. The upper depositional sequence is 60–95 m thick, and mainly consists of thin, yellowish, medium- to fine-grained sandstones and variegated shales. The upper sequence rests unconformably on the lacustrine black shales of the Batateira Beds. Thin and discontinuous conglomerate beds at the base of the upper sequence laterally grade into coarse-grained sandstones. These coarse-grained sandstones are overlain by interbedded sandstones and mudstones organized in fluvial cycles. The upper and lower sequences of the Barbalha Formation are separated by an erosive unconformity, traceable throughout the study area, formed during a period of stratigraphic base-level lowering. This surface marks a change in the lower sequence from a dominantly fluvial depositional style, with amalgamated multistorey braided fluvial channel sand bodies, to a lacustrine system in the top to an eminently fluvial sedimentation, which in the basal section comprises amalgamated, multistorey, braided fluvial channel sand bodies, and in the superior section the amalgamated fluvial channels are overlain by floodplain and overbank sandstone bodies with fixed fluvial channel deposits, interpreted as a suspended-load-dominated fluvial system in the upper sequence. This change in the depositional style is accompanied by a reduction in grain size and a change in the fluvial regime, suggesting that the drainage system was restructured due to tectonic movements in the basin and climatic variations. In addition to the restructuring of the drainage basin, the characteristics of the discharge of the river system have changed, probably because of the more humid climatic conditions that dominated during the deposition of the upper sequence. The fluvial deposition in the lower sequence is associated with more ephemeral river systems, while the facies architecture of the upper sequence is associated with perennial systems and is suggestive of a suspended-load-dominated fluvial system. This fluvial system is capped by lacustrine deposits of the Crato Formation. The upper sequence grades upwards into the Crato Formation. The boundary between these two units is delineated by the presence of greenish calciferous shales that are covered by lacustrine laminated limestones and shales of Neoaptian age. Palaeocurrent readings from the fluvial deposits of both sequences display a consistent palaeoflow to the SE. Sedimentological and palaeontological evidence indicates a tectonic control on sedimentation and humid to subhumid climate conditions.

캐나다 앨버타분지 하부 백악기 층서와 남부 평원 지역 하부 맨빌층군 석유시스템에 대한 고찰

The Manville Group in the Alberta Basin, Canada, contains large amounts of hydrocarbon resources and has long been a target of petroleum exploration and production. However, there exists no standard stratigraphic classification system that can be applied for the basin-wide underground reservoirs and petroleum systems, mainly due to the limitations that most stratigraphic nomenclatures were defined by information obtained from underground boreholes. In this review, after extensive literature survey we reinterpret the spatio-temporal relationships of the Mannville Group and its correlative strata on the basis of the depositional history and stratigraphic context in the Alberta Plain region. It seems appropriate to distinguish the Deville Formation lying immediately above the sub-Mannville unconformity as a stratigraphic unit separated from the overlying Mannville Group and the underlying Jurassic strata based on its unconformable relationship both above and below and its depositional origin. The Basal Quartz, commonly used unofficially, is a stratigraphic unit referring to the lower Mannville strata beneath the Ostracod Beds. The Bluesky and the Wabiskaw sandstones in the Mannville Group are very similar to the Glauconitic Sandstone in terms of lithology, but their depositional histories are different from the latter. The former sandstones had been deposited during the overall transgressive stages, whereas the latter is a deposit that was formed during the following regressive stage after the deposition of the former sandstones. The main reservoirs in the southern Alberta region are the Basal Quartz sandstones, which represent fluvial channel sandstones that filled the main channels having a N-S orientation and dendritic tributary channels connected to the main channels. The main source rocks are interpreted to be organic matter-rich marine shales of the Jurassic Fernie Group. The Ostracod Beds are very poor in permeability and thus form a regional seal, forming stratigraphic traps in numerous areas.

Reservoir quality of fluvial sandstone reservoirs in salt-walled mini-basins: an example from the Seagull field, Central Graben, North Sea, UK

The Triassic fluvial sandstones of the Skagerrak Formation were deposited in a series of salt-walled mini-basins and act as important hydrocarbon reservoirs for several high-pressure, high-temperature (HPHT) fields in the Central Graben, North Sea. The HPHT reservoirs exhibit excellent reservoir quality considering their depth of burial and hence have been of high interest for hydrocarbon exploration. This research uses a multidisciplinary approach to assess the Skagerrak Formation fluvial reservoir quality from the Seagull field incorporating core analysis, petrography, electron microscopy, XRD analysis, fluid inclusion appraisal and burial history modelling. Halokinesis and salt withdrawal at the margin of the salt-walled mini-basin induced early disaggregation bands and fractures at shallow burial and led to increased influx of meteoric water and clay mineral infiltration from overlying sedimentation. The density of disaggregation bands correlates with the occurrence and magnitude of pore-filling authigenic clay minerals, concentrated along the margin of the salt-walled mini-basin. The fluvial channel sandstones of the Skagerrak Formation are subject to strong intra-basinal spatial reservoir quality variations despite diagenesis and low vertical effective stress having played a favourable role in arresting porosity loss.

Understanding the geometry and distribution of fluvial channel sandstones and coal in the Walloon Coal Measures, Surat Basin, Australia

Understanding the controls on coal seam distribution and geometry is fundamental for planning coal seam gas production. In the Jurassic Surat Basin of South East Queensland, Australia, the spatial continuity of coal seams in the Walloon Coal Measures is highly variable and often difficult to map and predict, even with closely spaced (<1000 m) drillings. This paper investigates the frequency and location of thick sandstone in relation to thick coal seams or plies across three broad stratigraphic divisions, Upper Juandah (UJ), Combined Lower Juandah-Taroom (CLJT) and Condamine Coal Measures (CCM), within the Walloon Coal Measures. Basic depositional facies, e.g. channel, floodplain, marginal mire, and coal mire, were interpreted from geophysical logs. An in-house code was used to count the number of coal plies thicker than 2 m, and channel sandstones thicker than 5 m for the UJ and CLJT and 3 m for the CCM at each borehole. Isopleth maps of the numbers of both coal plies and channel sandstones were generated across the basin for the three subdivisions. Results show that there is an upward stratigraphic trend from thick to thin, and then to thick stacked coal plies. This corresponds to a similar vertical thickness change in channel sandstones. The incidence of thick coal and thick sandstone is associated with rising base level within an early transgressive systems tract which was followed by a high stand prior to a regional erosive event above the UJ. Thick and stacked coal plies have a marked tendency to occur in belts adjacent to the thick channel thoroughfares in the basin.

The role played by carbonate cementation in controlling reservoir quality of the Triassic Skagerrak Formation, Norway

Anomalously high porosities up to 30% at burial depth of >3000 m along with varying amounts and types of carbonate cements occur in the fluvial channel sandstone facies of the Triassic Skagerrak Formation, Central Graben, Norway. However, porosities of the Skagerrak Formation are lower in the Norwegian sector than in the UK sector. In this study, petrographic analysis, core examination, scanning electron microscopy, elemental mapping, carbon and oxygen isotope, fluid inclusion and microgeometry analysis are performed to determine the diagenesis and direct influence on reservoir quality, with particular focus on the role played by carbonate cementation. The sandstones are mainly fine-grained lithic-arkosic to sub-arkosic arenites and display a wide range of intergranular volumes (2.3%–43.7% with an average of 23.6%). Porosity loss is mainly due to compaction (av. 26.6%) with minor contribution from cementation (av. 12.1%). The carbonate cements are patchy in distribution (from trace to 20.7%) and appear as various types e.g. calcretes (i.e. calcareous concreted gravels), poikilitic sparite and sparry ferroan dolomite, and euhedral or/and aggregated ankerite/ferroan dolomite crystals. This study highlights the association of carbonate precipitation with the remobilisation of carbonate from intra-Skagerrak calcretes during early burial stage i.e. <500 m. During deeper burial, compaction is inhibited by carbonate cements, resulting high intergranular volume of up to 32% and 29% for fine- and medium-grained sandstones, respectively. Carbonate cement dissolution probably results from both meteoric water flow with CO2 during shallow burial, and organic CO2 and carboxylic acid during deep burial. The maximum intergranular volume enhanced by dissolution of early carbonate cements is calculated to 8% and 5% for fine- and medium-grained sandstones, respectively. Compaction continues to exert influence after dissolution of carbonate cements, which results in a loss of ∼6% intergranular volume for fine- and medium-grained sandstones. Reservoir quality of the Norwegian sector is poorer than that of the UK sector due to a lower coverage of clay mineral coats e.g. chlorite, later and deeper onset of pore fluid overpressure, lower solubility of carbonate compared to halite, and a higher matrix content.

Using precise CA-TIMS ages of volcanic air-fall tuff beds in correlating the Walloon Coal Measures of the Surat Basin, Australia

The Upper Jurassic Walloon Coal Measures of the Surat Basin is one of Australia’s largest and most productive gas provinces. Despite the drilling of over 8500 wells and numerous publications, the stratigraphic framework is poorly defined. The laterally discontinuous nature of the sedimentary facies, including coals and fluvial channel sandstones, makes correlation difficult. The abundance of volcanic air-fall tuff beds within strata across the basin provides a unique opportunity to independently verify existing stratigraphic frameworks. Using the high-precision chemical abrasion thermal ionisation mass spectrometry technique, zircon grains from 28 tuff beds have been successfully dated within an error margin of less than 100 kyr. These dates substantially revise biostratigraphic and lithostratigraphic frameworks. Lithostratigraphic units are diachronous across the basin. In addition, the sparsity of key spore–pollen taxa limits the application of biostratigraphy. The complex interplay of climate and subsidence on facies distributions can now be documented over a time frame of ~4 Ma. Syntectonism played an important role in variable palaeodrainage patterns across the basin, the frequency of fluvial avulsions and preferential sites of peat accumulation through time. The new stratigraphic framework should aid in future exploration for coal seam gas in the area. Dating tuff beds using high-precision dating techniques should also assist in correlation of non-marine strata elsewhere in the world.

Early Mississippian sandy siltstones preserve rare vertebrate fossils in seasonal flooding episodes

AbstractFlood‐generated sandy siltstones are under‐recognised deposits that preserve key vertebrate (actinopterygians, rhizodonts, and rarer lungfish, chondrichthyans and tetrapods), invertebrate and plant fossils. Recorded for the first time from the lower Mississippian Ballagan Formation of Scotland, more than 140 beds occur throughout a 490 m thick core succession characterised by fluvial sandstones, palaeosols, siltstones, dolostone ‘cementstones’ and gypsum from a coastal–alluvial plain setting. Sandy siltstones are described as a unique taphofacies of the Ballagan Formation (Scotland, UK); they are matrix‐supported siltstones with millimetre‐sized siltstone and very fine sandstone lithic clasts. Common bioclasts include plants and megaspores, fish, ostracods, eurypterids and bivalves. Fossils have a high degree of articulation compared with those found in other fossil‐bearing deposits, such as conglomerate lags at the base of fluvial channel sandstones. Bed thickness and distribution varies throughout the formation, with no stratigraphic trend. The matrix sediment and clasts are sourced from the reworking of floodplain sediments including desiccated surfaces and palaeosols. Secondary pedogenic modification affects 30% of the sandy siltstone beds and most (71%) overlie palaeosols or desiccation cracks. Sandy siltstones are interpreted as cohesive debris flow deposits that originated by the overbank flooding of rivers and due to localised floodplain sediment transport at times of high rainfall; their association with palaeosols and desiccation cracks indicates seasonally wet to dry cycles throughout the Tournaisian. Tetrapod and fish fossils derived from floodplain lakes and land surfaces are concentrated by local erosion and reworking, and are preserved by deposition into temporary lakes on the floodplain; their distribution indicates a local origin, with sediment transported across the floodplain in seasonal rainfall episodes. These deposits are significant new sites that can be explored for the preservation of rare non‐marine fossil material and provide unique insights into the evolution of early terrestrial ecosystems.

Importance of vertical effective stress for reservoir quality in the Skagerrak Formation, Central Graben, North Sea

The complex fluvial sandstones of the Triassic Skagerrak Formation are the host reservoir for a number of high-pressure, high-temperature (HPHT) fields in the Central Graben, North Sea. All the reservoir sandstones in this study comprise of fine-grained to medium-grained sub-arkosic to arkosic sandstones that have experienced broadly similar burial and diagenetic histories to their present-day maximum burial depths. Despite similar diagenetic histories, the fluvial reservoirs show major variations in reservoir quality and preserved porosity. Reservoir quality varies from excellent with anomalously high porosities of up to 35% at burial depth of >3500 m below seafloor to non-economic with porosities <10% at burial depth of 4300 m below seafloor.This study has combined detailed petrographic analyses, core analysis and pressure history modelling to assess the impact of differing vertical effective stresses (VES) and high pore fluid pressures (up to 80 MPa) on reservoir quality. It has been recognised that fluvial channel sandstones of the Skagerrak Formation in the UK sector have experienced significantly less mechanical compaction than their equivalents in the Norwegian sector. This difference in mechanical compaction has had a significant impact upon reservoir quality, even though the presence of chlorite grain coatings inhibited macroquartz cement overgrowths across all Skagerrak Formation reservoirs. The onset of overpressure started once the overlying Chalk seal was buried deeply enough to form a permeability barrier to fluid escape. It is the cumulative effect of varying amounts of overpressure and its effect on the VES history that is key to determining the reservoir quality of these channelised sandstone units. The results are consistent with a model where vertical effective stress affects both the compaction state and subsequent quartz cementation of the reservoirs.

Exceptional reservoir quality in HPHT reservoir settings: Examples from the Skagerrak Formation of the Heron Cluster, North Sea, UK

As the exploration of hydrocarbons moves into more complex and deeper basinal settings the need to understand the effect of high temperatures and high pressures on reservoir quality and rock properties becomes even more important. The fluvial channel sandstone reservoirs of the Skagerrak Formation in the Central North Sea exhibit anomalously high porosities and permeabilities considering their depth of burial (>3500 m below sea floor). Despite the complex depositional setting, diagenetic history, high overpressure and temperatures encountered in the Skagerrak Formation, hydrocarbons are currently being produced. The Skagerrak Formation reservoirs used in this study have encountered overpressures of >40 MPa and temperatures up to ∼185 °C at present day maximum burial. To identify the role played by the high pressure and high temperature encountered in the reservoir sandstones a multidisciplinary approach involving petrographic, fluid inclusion, and burial history modelling studies has been adopted. Our interpretation of the results is that the generation of shallow overpressure in these fields limited mechanical compaction and also played an important role in minimizing pressure solution in the chemical compaction regime as evidenced by reduced quartz cementation. Fluid inclusions found in the quartz overgrowths indicate late-stage development with temperatures of formation in the range 130 to 170 °C coincident with late-stage deeper burial. Hydrocarbon emplacement occurred after quartz cementation and has had little to no effect on porosity preservation. The formation of well-developed authigenic chlorite (>70% surface coating) and, to a lesser extent illite clay coats with burial had a positive effect on porosity preservation even though permeability was marginally reduced in the illite-rich sandstones. A schematic porosity and quartz cement evolution model has been developed which allows for pre-drill prediction of reservoir quality in the Heron Cluster and provide valuable insights for other complex high-pressure high-temperature reservoirs.

Predicting geometry and stacking pattern of thin beds by interpreting geomorphology and waveforms using sequential stratal-slices in the Wheeler domain

I used a seismic sedimentology-based approach for interpreting the spatial geometry and stacking pattern of superimposed, seismically thin beds. The method was applicable to conventional low-frequency seismic data and required knowledge of the basic shape of the seismic wavelet and lithology-impedance relationship (impedance model). It was primarily aimed at those beds that were spatially extensive enough to be resolved horizontally, but so thin as to only be detected vertically with a given bandwidth, and it assumed that the data had been converted to stratal slices in the relative geologic time (or Wheeler) domain. A simple one-bed model illustrated that a thin-bed depositional system can be characterized by a seismic-geomorphologic pattern of the same spatial shape on sequential relative geologic-time (stratal) slices, but the amplitude, phase, and polarity would vary depending upon the known (estimated) seismic wavelet. This phenomenon could be captured and evaluated in the thin bed’s response window (RW) in the Wheeler domain. If multiple thin-bed units were present in the RW, the seismic responses from vertically adjacent units would interfere with the “true” seismic-geomorphologic pattern of any single thin-bed unit. The composite waveform for each of the units could be restored in variable quality, depending on its geomorphologic character, thickness, and stratigraphic position. Relative traveltime differences of thin-bed waveforms revealed the depositional history (stacking pattern) of the thin-bed sequence. A field-data test confirmed that within a stratigraphic interval of a composite seismic event ([Formula: see text] or 15 m) at least two, and possibly three, thin fluvial channel sandstones (2–6 m each) could be identified and their spatial localities and distribution could be unraveled.

Reservoir characterization of the Lower Abu Madi Formation using core analysis data: El-Wastani gas field, Egypt

The Messinian (Upper Miocene) sandstones of the Lower Abu Madi unit are the main reservoirs of the El-Wastani (EW) gas field, located in the onshore Nile Delta of Egypt. Reservoir quality is highly variable and represents the major risk in locating new delineation and development wells in the field. Four lithofacies (F1, F2, F3 and F4) are characterized based on textural attributes and sedimentary structures. Facies F4, which is a medium to coarse-grained fluvial channel sandstone, exhibits the best reservoir rock quality. Four hydraulic flow units (HFU 1 through HFU 4) were delineated in the studied wells with flow zone index (FZI) values indicative of a broad range of hydraulic properties. The cementation factor (m) value, was calculated from the inverse relationship between porosity and formation factor for EW-4 and EW-6 wells, agrees with the published values of clean sandstone reservoirs. The results of grain density analyses is close to the published value of grain density of quartz, and also supported by the results of petrographic study, where quartz is the dominant framework mineral. Generally, both the permeability and porosity can be linked to the two derived electrical properties (formation resistivity factor and electrical tortuosity) in the clastic reservoirs of the Lower Abu Madi unit. This correlation is particularly evident in the western and central parts of the study area at EW-4 and EW-6 wells, where facies F4 is dominant.

Quantitative analysis of fluvial sandbodies of the Toolachee Formation, Cooper Basin, Australia

The Cooper Basin produces oil and gas from numerous fluvial channel sandstone bodies deposited within several formations such as the Late Permian Toolachee Formation; however, the geometry of these fluvial deposits are not well constrained. Five sandstone bodies (SS-1 to SS-5) have been identified within the Toolachee Formation. These sandstone bodies are characterized mainly by fining-upward packages, which have been interpreted to be deposited by high-sinuosity channels. This study aims to determine and interpret the size, geometry, and architecture of these fluvial sandstone bodies identified within the Toolachee Formation from well logs in the Meranji Field. The morphometric parameters examined in this study include channel width (w), channel depth (d), and meander-belt width (mbw). All of these parameters have been determined using empirical equations driven from modern rivers. The estimated maximum bankfull depth (d) ranges from 3.3 to 6 m, and the estimated channel width (w) ranges from 42 to 105 m, which indicates that these sandstone bodies were deposited by relatively small fluvial channels. The estimated meander-belt width (mbw) ranges from 800 to 2000 m, which indicates that some of these sandstone bodies (e.g., SS-1, SS-4, and SS-5) are connected and can be correlated between the adjacent wells within the Meranji Field. Furthermore, this indicates that these sandstone bodies are excellent reservoirs due to their good lateral extent. The empirical equations used in this study have improved the quantitative estimations of the channel dimensions and connectivity of the sandstone bodies. These equations can help to reduce the number of wells that are planned to be drilled in the Meranji Field.

Distribution of Diagenetic Alterations in Fluvial Channel and Floodplain Deposits in the Triassic Narrabeen Group, Southern Sydney Basin, Australia

Abstract In the Narrabeen Group, floodplain shale and siltstone are abundant in the Wombarra Claystone, Stanwell Park Claystone and Bald Hill Claystone whereas the Newport Formation is a fine-grained lagoonal deposit. Medium-and coarse-grained fluvial channel sandstone is common in the Coalcliff, Scarborough and Bulgo. Sandstone composition includes mostly lithic grains with minor quartz, feldspar, mica and heavy minerals; litharenites and sublitharenites are common and quartzarenites are rare. The diagenetic alterations include compaction, quartz overgrowths, authigenic clay minerals, carbonate cement and authigenic feldspar. Quartz overgrowths are present in the channel, floodplain, lagoonal and shallow marine facies but are more common in the channel facies. Also, the channell facies are characterised by pore-filling carbonate cement, grain-coating carbonate cement and carbonate replacement of grains. Carbonate cement, pore-filling clays and grain-coating clays occur in all the channel, floodplain, lagoonal and shallow marine facies. Fluvial channel and floodplain facies contain feldspar overgrowths which disappear in the lagoonal and shallow marine facies. Dissolution of unstable grains is observed in the channel, floodplain and lagoonal facies but is more abundant in the channel facies in the Narrabeen Group. Mechanical and chemical compaction is conspicuous in the channel facies but also occurs in the floodplain and lagoonal facies.