Sand-body Geometry Research Articles

Mapping palaeoshorelines of river-dominated deltas in lacustrine ramp settings: Application of sedimentological analyses to the Triassic Yanchang Formation (Ordos Basin, China)

In the preserved successions of clastic lacustrine systems receiving substantial sediment input from fluvial sources, the boundary between delta-plain and delta-front facies belts marks the position of ancient lacustrine shorelines. Placing and predicting the position of this boundary in nearshore lacustrine deposits of the Triassic Yanchang Formation (Ordos Basin, China) is of considerable applied significance, since sandstone deposits of this succession host numerous geological resources; yet, delta-top and delta-front deposits differ fundamentally in terms of sandstone fraction, sandbody geometries, and petrophysical properties. However, established sedimentological criteria for shoreline identification relevant to marine environments are only partially applicable to this lacustrine setting. For example, in lacustrine systems, (i) water salinity can be considerably lower, (ii) tidal processes are negligible, (iii) wave activity is often limited and of local significance (e.g., distal and lateral delta-front fringes; transgressive periods of delta-lobe reworking), (iv) seasonal fluctuations in water chemistry and biogenic productivity may be common, and (v) the rate, frequency and magnitude of oscillations in base level differ from marine counterparts. Collectively, these factors hinder the differentiation of delta-plain and delta-front deposits in the Yanchang Formation. The establishment of effective criteria with which to make such distinction is important for palaeogeographic restorations, and for resource exploration and exploitation. On the basis of detailed sedimentological descriptions of well cores and outcrop exposures of the Yanchang Formation in the southeastern Ordos Basin, sedimentary facies accumulated as part of different sub-environments and facies belts are studied. Delta-plain deposits can be readily differentiated from those of delta-front origin on the basis of criteria relating to sediment texture, sedimentary structures, palaeontological content, well-log profiles, and vertical facies successions. These criteria are summarized and applied to better characterize the deltaic setting and to reconstruct the evolution of lacustrine shorelines during accumulation of the Yanchang Formation, particularly for the northeastern area of the basin. A key outcome of this study is the development of a broadly applicable model for the recognition of sub-environments in river-dominated lacustrine deltaic successions. A generalized workflow is established to demonstrate how techniques in sedimentary facies analysis can be employed to reconstruct siliciclastic lacustrine delta shorelines, and thereby predict reservoir characteristics.

A phased workflow to define permit‐ready locations for large volume CO2 injection and storage

AbstractTo‐date, only two UIC Class VI permits have been issued by the US Environmental Protection Agency. We illustrate a four‐phase workflow to first identify regional storage resources and then down‐select sites to yield permit‐ready locations that can accept and store large volumes of CO2. Specific permit requirements should guide objectives and define deliverables of respective workflow phases. In the first phase we used available regional data and screened structure and injection zones to locate resources that match CO2 volumes planned to be captured. Available data were also used to assess presence and depth of usable groundwater, the key resource being protected via permitting. We then used advanced, closed‐form, analytical solutions (EASiTool) to estimate CO2 injectivity into each hydrologically connected injection compartment. In the second phase we acquired and conditioned additional wireline logs and leased available seismic datasets. We interpreted the depositional systems from wireline well‐log character and mapped sandbody geometry to interpolate injection and confining‐zone distribution. Using available data, we mapped faults and locations of freshwater and overpressure (or other capacity‐limiting geologic parameters) in more detail. In the third phase, we used the augmented geologic data to develop a static model for the selected area, extracted the areas of highest interest, and generated and ran dynamic (flow) models. In a fourth phase, we reduced major uncertainties identified in earlier phases. Our case study indicates that to complete preparation of a permit application requires (1) improved lithologic characterization information (thicknesses and horizontal and vertical connectivity) and (2) better definition of poorly defined local faults. © 2023 The Authors. Greenhouse Gases: Science and Technology published by Society of Chemical Industry and John Wiley & Sons Ltd.

Frequency domain electromagnetic calibration for improved detection of sand intrusions in river embankments

Sand intrusions pose significant risks to river embankments due to potential flow pathways that can lead to instability during flood events. Visual inspection is a first step to recognize critical segments, but it does not deliver information about the subsurface. In this context, the electromagnetic induction (EMI) technique is a useful method for preliminary zoning at regional scale while the electrical resistivity tomography (ERT) method, widely used for hydrological purposes, is considered among the most reliable techniques for local subsurface imaging. A major sand intrusion within the levees of the Brenta River, located near Venice (northern Italy), resulted in water seeping during seasonal floods and posed severe threats to embankment stability. ERT and EMI techniques, along with geotechnical investigations, were the best survey choices to address the problem. Resistivity profiling successfully imaged the sand body geometry within and underneath the levee, and results correlated nicely with borehole stratigraphy. A first multiarray EMI device, which represented a faster and less expensive survey, was deployed to map further anomalies along nearby levees, but results were not satisfactory because the inverted profile failed to image the known intrusion. A second multiarray EMI device, with larger coil spacing, was also tested. Although it performed better in detecting the intrusion, results were still below expectations. A calibration procedure based on Pearson's coefficients and using ERT as a reference was then devised and implemented to correct the EMI data prior to carrying out inversion. The procedure was successful for both EMI data sets, leading to realistic subsurface resistivity in the inverted sections. EMI measurements could then be recovered and interpreted correctly to estimate subsurface textures. The possibility of calibrating EMI data and obtaining subsurface resistivity images comparable to standard ERT profiling is an important improvement for cost-effective EMI surveying of river embankments to mitigate flood hazards.

The value of integrated core–CT–BHI data in characterizing aeolian dune geometries and effects of cross-cutting deformation bands: implications for permeability architecture

Abstract Three-dimensional circumferential CT-scans have transformed how core is described and calibrated with borehole image (BHI) datasets to refine reservoir rock typing and facies description. This paper focuses on the value of circumferential CT-scans in the assessment of plug and bed-scale heterogeneities. It shows how careful re-orientation and calibration with borehole images can help unravel sandbody geometries and orientation, and the potential effects of cross-cutting deformation bands on permeability architecture and sweep efficiency. This is demonstrated using aeolian-dominated core examples, supported with circumferential CT-scans, minipermeability data, conventional logs and BHI data, taken from the Jurassic Norphlet Formation from producing fields in the Gulf of Mexico. The formation overlies Early Jurassic Louann Salt, and syn-depositional halokinesis significantly influenced depositional accommodation space, facies distribution, and preservation potential. Furthermore, deposition during active salt tectonics has resulted in complex deformation band networks within these clean sandstones. CT-scan density contrasts highlight stratification types and deformation bands not always visible on slabbed core. Furthermore, BHI re-orientated CT-scans provide high-resolution dip/azimuth data and aid aeolian bedset bounding surface definition, which is important for determining dune geometry and stacking patterns. Hence, an integrated approach using core, circumferential CT-scan and calibrated BHI has been essential for deciphering the complexity of these deposits.

Vertical grain-size trend of mouth bar in lacustrine fan delta: Flume experiments

Field observational previously indicated a mouth bar of a fan delta could exhibit a fining- or coarsening-upward trend, which bring a new challenge to the identification of mouth bar in subsurface studies due to the lack of morphological descriptions. Previous studies have indicated that effluent behavior in river-mouth system can affect the vertical grain-size trend of mouth bar, but the drivers and magnitude of this phenomenon are not understood. We conducted flume experiments to investigate the mechanism and controlling factors of vertical grain-size trend of mouth bar. Experiment with a steeper slope of the substrate layer, greater discharge, higher sediment/water ratio, and coarser sediment induced a fining-upward trend of mouth bar, because the effluent was dominated by strong inertia. Mouth bar in the experiment with a gentler slope of the substrate layer, smaller discharge, lower sediment/water ratio, and finer sediment exhibited a coarsening-upward trend dominated by the friction-dominated effluent. The relationship between the vertical grain-size trend of mouth bar and the gradients of foreset bedding in small-scale flume models and the cut-off of 15°–18° are applicable in natural systems. Identifying depositional setting to infer depositional process in river-mouth system and analyzing the plane geometry of sandbodies are two steps in the interpretation of ancient fan deltaic rock record.

Depositional trends and reservoir geometries of the Early Cretaceous Lower Goru Formation in Lower Indus Basin, Pakistan: evidence from sequence stratigraphy

Lower Goru sand intervals of Early Cretaceous age are proven reservoirs in the Lower Indus Platform area. The delineation of reservoir geometries is of utmost importance while searching for stratigraphic and combination traps in clastic reservoirs. The data of ten wells and two 2D seismic lines from the northern part of the Lower Indus Basin are incorporated in this study. These reservoir intervals of the Lower Goru Formation are correlated in a third-order sequence stratigraphic framework to understand the depositional architecture and reservoir geometries. The Lower Goru Formation was deposited in westerly prograding river-dominated deltas developed by river system drained through the Indian Shield from east and south-east during Cretaceous time. Due to auto-cyclic switching of river distributaries, different lobes might have formed which if identified can point out the possible locations and configurations of stratigraphic and combination traps in this highly prospective area of Lower Indus Basin. The Lower Goru Formation (Barremian to Cenomanian) consists of one full second-order sequence of 20 Ma duration, having all the three fundamental system tracts in which eleven sequences of the third order, i.e. LG-1 to LG-11, were interpreted. Proven reservoir intervals of the Lower Goru Formation are concentrated in various third-order lowstand system tracts. The net sand and porosity maps of lowstand system tracts, gross depositional map and depositional cross section of the Lower Goru Formation point out the potential sites of good sandstone reservoirs and hint at the possible sand-body geometries.

Implications of palaeosols in low net-to-gross fluvial architecture reconstruction: Reservoir analogues from Patagonia and Spain

The changes in the geometry, density and distribution of the fluvial reservoir sand-bodies have been modelled both in the laboratory and from outcrop analogues. On the other hand, fine-grained floodplain deposits, often called “background” sedimentation, have not been in the spotlight. However, they provide a very important database for fluvial architecture reconstruction and therefore constitute a key tool in characterizing, correlating and modelling heterogeneous fluvial reservoirs. The methodology used in this work combines classical sedimentological and palaeopedological data. Palaeosols were identified in outcrops of the Cretaceous of Patagonia and Triassic red beds of Spain based on the main pedofeatures such as colour, soil structures, mottles, nodules, clay illuviation cutans, slickensides and rhizoliths. Palaeosol horizons, thickness and types of contact were described. The palaeosols classification was made through the comparison with the Soil Taxonomy. For the characterization of lateral palaeosols variability, several palaeopedological logs were constructed following the same stratigraphic level at both margins of the main channels. Small-scale or high-frequency palaeosol variations, identified in both fluvial succession are represented by the lateral and vertical superimposition of Inceptisols/Entisols, Vertisols and hydromorphic Vertisols/swamp deposits. We recognized a decrease in the drainage conditions within the palaeosols catena moving away from the main channel bodies. The most waterlogged palaeosol is located at 200 to 300 m from the main channels. Lateral changes of palaeosols are interpreted as the result of intrinsic factors to the depositional systems, such as the relative position within the floodplain and the distance from the main channels, that condition the particle size of parent material, the sedimentation rate and the palaeotopographic position which state the drainage conditions of palaeosols. Vertical stacking of different palaeosols is linked to avulsion processes and the relatively abrupt change in the distance to main channels as the system aggraded. The study of these two environmentally similar examples (low-gradient meandering fluvial systems subjected to a high rate of aggradation), although located in different tectonic and chronostratigraphic contexts, shows that the type of fluvial system acted as the main control of sedimentary and pedogenic processes; as well as the scale at which they occur. Thus, when choosing an outcrop fluvial reservoir analogue, the type of fluvial system and climate should be more important than the age or the tectonic framework of the succession to be studied.

Seismic reservoir characterization of potential CO2 storage reservoir sandstones in Smeaheia area, Northern North Sea

Evaluating any subsurface CO 2 storage site comprises the reservoir, seal, and overburden investigation to mitigate injection and storage-related complications. The Upper-Middle Jurassic Sognefjord, Fensfjord, and Krossfjord formation sandstones are potential CO 2 storage reservoirs at the Smeaheia area, northern North Sea. The Smeaheia area is located east of the Troll oil and gas field. The Upper Jurassic organic-rich Heather and Draupne Formation shales are the main seals for the sandstone reservoirs. In this study, we carried out a prestack seismic inversion to obtain elastic property cubes of acoustic impedance ( AI ), velocity ratio ( Vp/Vs ), and bulk density ( RhoB ). From these elastic cubes, we obtained the reservoir properties such as porosity ( Phi ), shaliness ( V sh ), and permeability ( k ) of Sognefjord, Fensfjord, and Krossfjord formations. We introduced two new equations to extract the shale volume and porosity cubes from the inverted elastic cubes in the present study. These equations are nonlinear based on the AI versus Vp/Vs rock physics template. Both equations are correlated first with the well log data and then applied on the elastic property cubes ( AI versus Vp/Vs ) to obtain the V sh and Phi property cubes. An additional porosity cube ( PhiD ) was generated from the inverted RhoB for comparison. Finally, using an empirical equation, permeability was extracted from the porosity cube. The reservoir properties we derived from 3D seismic, in addition to the well log, revealed the vertical and lateral variations of porosity, shaliness, and permeability in the Smeaheia area. We used these reservoir properties to infer the depositional environment and the viability of reservoirs for CO 2 storage. The depositional environments identified were shoreface and deltaic from the extracted sand-body geometries. We found the Sognefjord Formation possessing the best reservoir properties, followed by the Fensfjord Formation as the secondary storage potential. In contrast, the Krossfjord Formation owed the lowest reservoir quality in the Smeaheia area. • Our new equations helped to reveal the lateral variations in reservoirs and their depositional environment. • The Sognefjord is the best, and Fensfjord Formation is the second-best quality reservoir in the area for CO 2 storage. • The Krossfjord Formation is the least suitable for CO 2 storage in the Smeaheia area.

Application of seismic attribute analysis in fluvial seismic geomorphology

Seismic attributes can be important predictors, either qualitative or quantitative, of reservoir geometries when they are correctly used in reservoir characterization studies. This paper discusses seismic attribute analyses and their usefulness in seismic geomorphology study of Moragot field of Pattani Basin, Gulf of Thailand. Early to Middle Miocene fluvial channel and overbank sands are the reservoirs in Pattani Basin. Due to their limited horizontal and vertical distribution, it is not always possible to predict the geometry and distribution of these sands based on the conventional seismic interpretation. This study utilized various seismic attributes, e.g., RMS amplitude analysis, spectral decomposition, semblance and dip-steered similarity, RGB blending to image the geometry and the spatial distribution of sand bodies in horizon and stratal slices at different stratigraphic intervals. Attribute analyses reveal, at shallow stratigraphic levels, RMS and semblance can successfully identify channel-shaped sand bodies and mud-filled channels associated with channel belts. On the other hand in deeper stratigraphic intervals, sand distribution can be imaged more effectively by using spectral decomposition and dip-steered similarity volumes. High-frequency spectral decomposition slices can image thin sands, and low-frequency slices can image thick sands quite effectively in deeper intervals. RGB blending of different frequency slices is particularly useful in delineating channel systems of various dimensions at deeper intervals. These images show the distribution of sands and mud-filled channels at various stratigraphic levels. The width of channel belts varies from 200 m to 3 km. These channel belts are N–S or NW–SE oriented. From the channel pattern and their dimensions, depositional environments can be predicted. Mud-filled channels identified in the horizon slices will act as a connectivity barrier between sand bodies at either side of the channel. They can also act as lateral and up-dip seal to form stratigraphic traps. The seismic attribute analyses clearly show the geometry and spatial distribution of sand bodies. Hence, this method for predicting sand body geometry might help in field development planning as well as in reducing exploration risk.

Mesh model building and migration and accumulation simulation of 3D hydrocarbon carrier system

Abstract Migration and accumulation simulation of oil and gas in carrier systems has always been a difficult subject in the quantitative study of petroleum geology. In view of the fact that the traditional geological modeling technology can not establish the interrelation of carriers in three dimensional space, we have proposed a hybrid-dimensional mesh modeling technology consisting of body (stratum), surfaces (faults and unconformities), lines and points, which provides an important research method for the description of geometry of sand bodies, faults and unconformities, the 3D geological modeling of complex tectonic areas, and the simulation of hydrocarbon migration and accumulation. Furthermore, we have advanced a 3D hydrocarbon migration pathway tracking method based on the hybrid-dimensional mesh of the carrier system. The application of this technology in western Luliang Uplift of Junggar Basin shows that the technology can effectively characterize the transport effect of fault planes, unconformities and sand bodies, indicate the hydrocarbon migration pathways, simulate the process of oil accumulation, reservoir adjustment and secondary reservoir formation, predict the hydrocarbon distribution. It is found through the simulation that the areas around the paleo-oil reservoir and covered by migration pathways are favorable sites for oil and gas distribution.

The influence of diagenesis on the heterogeneity of sandstone reservoir

Diagenesis plays an important role in controlling the quality and heterogeneity of clastic reservoirs. The variation of diagenesis is usually reflected in the variation of sedimentary porosity and permeability. The effects of the types and distribution of sedimentary rocks and clastic sedimentary evolution and sequence stratigraphic framework provide a powerful tool to predict the prediction effect on the distribution of rock mass and the heterogeneity of the control effect. The heterogeneity of sandstone reservoir, it determines the volume, velocity and hydrocarbon recovery, the sand body geometry and internal structure, grain size, sorting, bioturbation degree, origin, and diagenesis types, volume and distribution.

Understanding reservoir heterogeneity through water-saturation prediction via neural network — A case study from offshore Nile Delta

In complex geologic settings with a great degree of heterogeneity in reservoir properties, such as submarine channel complexes as in the Nile Delta province, we face the challenge of characterizing the reservoir based on availability of different seismic attributes. Amplitude variation with offset (AVO) analysis and prestack inversion techniques show impressive results in delineating the gas-bearing reservoirs, especially in clastic systems. However, a shortage of available wells and/or seismic data leads to a challenge in applying AVO and any prestack seismic inversion approaches. In addition, quantitative prediction of water saturation (Sw) from seismic is ambiguous because of its independent nonlinear relationship with conventional seismic attributes and inversion products. Water-saturation prediction away from the well is essential in order to characterize the reservoirs effectively. Therefore, probabilistic neural network (PNN) analysis has been implemented to predict Sw 3D volume using full-stack seismic data and Sw logs. In this case study, we applied the proposed neural network workflow over one of the late-Pliocene gas-sandstone reservoirs, Sequoia Field, in the West Delta Deep Marine (WDDM) concession, offshore Nile Delta, Egypt. The resulting volume then was tested using two blind wells that haven't been used in the analysis. The predicted Sw volume contains fine details that were used with variance and spectral-decomposed volumes to understand the reservoir's internal architecture in terms of sand body geometries and connectivity. The resulting volumes were used to better define the reservoir and optimize a new development well location.

Architecture and quantitative assessment of channeled clastic deposits, Shihezi sandstone (Lower Permian), Ordos Basin, China

Lower Permian Shihezi sandstone in Ordos Basin is the largest gas reservoir in China. Architecture elements of channel, overbank and floodplain facies of braided channel deposits were identified through an outcrops survey, and their proportion of channel facies have been quantitatively estimated from well logging. Characteristics of architecture elements, such as sand thickness, bounding surfaces and lithofacies were investigated through outcrops and core. Petrology of Shihezi sandstone has also been studied in detail. Analysis on sandstone components shows that monocrystalline quartz with approximately 76% bulk volume, and lithic up to 5%–45% bulk volume, are the two main components. Litharenite and lithic quartz sandstone are the main rock types. Compaction is concluded by former researchers as the control factor of low permeability. Examination through thin section reveals that secondary pores developed well in coarse sand. Inter-granular dissolution is included as the positive effect to increasing porosity, and is concluded as the control factor to the generation of net pay. Scale of coarse grained channel fills and channel bar sandstone bodies are quantitatively estimated. Strike-oriented, dip-oriented, and vertical distribution of channel fills and channel bar sandstone bodies have been investigated. The geometry of sand bodies can be depicted as an elongated lens. Subsurface mapping reveals that channel sandstone bodies distribute widely from both lateral and longitudinal cross section profiles, and are poorly connected.

The geometry, distribution and development of sand bodies in the Miocene-age Frimmersdorf Seam (Garzweiler open-cast mine), Lower Rhine Basin, Germany: implications for seam exploitation

AbstractThe Cenozoic-age Lower Rhine Basin is located in the NW part of the European Cenozoic Rift System. In Miocene times, a combination of warm climatic conditions and basin subsidence resulted in the deposition of up to 100m of lignite (i.e. Main Seam of the Ville Formation). The Main Seam can be subdivided into the Morken, Frimmersdorf and Garzweiler seams, separated by two intercalated transgressive sand units, namely the Frimmersdorf and Neurath sands, deposited in a shallow-marine, tide-dominated environment. The lignite seams of the Ville Formation are currently worked by RWE Power AG, in the Garzweiler II open-cast mine. In the Frimmersdorf Seam (between the Frimmersdorf Sand and the Neurath Sand), the presence of small-scale sand bodies, together with their variable dimensions, affects the industrial exploitation of the seam. Moreover, their irregular distribution complicates their precise and early recognition. Indeed, so-called barren lignite (≥ 17% of sand) and completely clean units can occur within a few metres of each other. Initial classification of these highly variable sand bodies suggests a variety of both pre- and post-depositional causal mechanisms, providing evidence of an extremely complex depositional and post-depositional system. Syn-depositional sand bodies were deposited in a swamp area that was located in the fluvial-dominated sub-environment of an extended tidal estuary. The post-depositional formation of sand bodies is related to the intrusion of fluidized sands from the underlying Frimmersdorf Sand. These sand injectites within the Frimmersdorf Seam are considered to be linked to seismic activity within the Lower Rhine Basin.

Transgressive successions of the Mahakam Delta Province, Indonesia

Abstract Sedimentological and stratigraphic analysis of outcrops and subsurface data in the Mahakam Delta province, combined with concepts of transgressive sedimentary processes derived from a study of the modern Mahakam Delta, indicate that a significant portion of the palaeo-Mahakam Delta succession was deposited during transgressive phases. Some of the transgressive successions resulted from major transgressions and are regionally extensive, but many reflect small, short-lived transgressions within dominantly progradational phases. Two sandstone facies with significant reservoir potential are common within the transgressive successions. Backfilled distributary sandstones occur in outcrop as 10–20 m thick, fining-upward channel sands that become more marine upward and in the subsurface as elongate, coastline-perpendicular sand bodies with a back-stepping stratigraphic architecture and highly variable thickness. Transgressive shoreline sandstones also fine upward, but they are shoreline-parallel, their lateral extent is controlled by the pre-transgression delta morphology and their thickness depends on the rate of relative sea-level rise. Both types of transgressive sandstone can be difficult to distinguish from specific progradational sandstones with wireline log data, especially when they occur within predominantly progradational successions. However, their sand body geometries, volumetrics and connectivity are much different from the stratigraphically adjacent sandstones deposited during progradational phases of deltaic deposition.

Sedimentological interpretation of an Ediacaran delta: Bonney Sandstone, South Australia

ABSTRACTThe type section of the late Ediacaran (ca 565 Ma) Bonney Sandstone in South Australia provides an opportunity to interpret a succession of Precambrian clastic sediments using physical sedimentary structures, lithologies and stacking patterns. Facies models, sequence stratigraphic analysis, and process-based architectural classification of depositional elements were used to interpret depositional environments for a series of disconformity-bounded intervals. This study is the first detailed published work on the Bonney Sandstone, and provides additional context for other Wilpena Group sediments, including the overlying Rawnsley Quartzite and its early metazoan fossils. Results show that the ∼300 m-thick section studied here shows a progressive change from shallow marine to fluvially dominated sediments, having been deposited in storm-dominated shelf and lower shoreface environments, lower in the section, and consisting primarily of stacked channel sands, in a proximal deltaic environment near the top. Based on the degree of influence of wave, tidal or fluvial depositional processes, shallow marine sediments can be classified into beach, mouth bar, delta lobe and channel depositional elements, which can be used to assist in predicting sandbody geometries when only limited information is available. Sediments are contained within a hierarchical series of regressive, coarsening-upward sequences, which are in turn part of a larger basin-scale sequence that likely reflects normal regression and filling of accommodation throughout a highstand systems tract. Paleogeographic reconstructions suggest the area was part of a fluvially dominated clastic shoreline; this is consistent with previous reconstructions that indicate the area was on the western edge of the basin adjacent to the landward Gawler Craton. This research fills in a knowledge gap in the depositional history of a prominent unit in the Adelaide Rift Complex and is a case study in the interpretation of ancient deposits that are limited in extent or lacking diagnostic features.

Geomechanic models using finite element methods show how Cooper Basin structure and sand body geometry impacts stress variation and hydraulic fracturing results

Unconventional reservoirs such as tight sands and shales require hydraulic fracture stimulation to improve productivity. The success of reservoir stimulation is controlled by the local stress field but decisions are often made knowing only the average stress field. This study uses geomechanical modelling to help explain lateral stress variability using structural geology, lithology contrast and boundary conditions. Changes in vertical and horizontal stresses are related to depth, lithology and structural position, yet these effects are not always accounted for. This is evident in the Cooper Basin, Australia, where, for example, unexpected changes in minifrac pressure are commonly observed in adjacent wells in a field. This study presents results from conceptual geomechanical models to help explain such variations in stress. Model scenarios are constructed using finite element package to investigate the impact of structural position, rock mechanical properties and stress regime on the patterns of horizontal and vertical stress magnitudes in a layered antiform sequence. Key findings suggest that: stress magnitude is affected by structural positioning; different patterns of stress exist across different lithologies; and, stress regime impacts on patterns of stress, especially in combination with curvature of structures. These challenge traditional methods of one-dimensional mechanical earth models and show that, rather than employing methods developed for simple layer-cake geology in extensional basins, geomechanical models should be constructed in two- or even three-dimensions. Results of this study highlight part of the solution to the unconventional resource potential of the Cooper Basin. Improved prediction of field-scale stress variations should enable further optimisation of hydraulic fracture stimulation treatments.

Reservoir stratigraphic heterogeneity within the Lower Cretaceous Muddy Sandstone in the Powder River Basin, northeast Wyoming, U.S.A.: Implications for carbon dioxide sequestration

The Muddy Sandstone in the Powder River Basin (PRB), northeast Wyoming, is a promising reservoir for CO 2 sequestration because: (1) existing wells for hydrocarbon production can be used for CO 2 injection when a field is depleted; and (2) data are available to assess the ability and capacity to trap CO 2 . Here, I provide new data and compile published results to: (1) characterize four oil and gas fields (the Amos Draw, Kitty, Hilight, and Sand Dunes fields) in the PRB with respect to lithofacies, sedimentary environment, sandstone composition, sand-body geometry, and porosity and permeability; and (2) assess the controls on reservoir heterogeneity and the CO 2 sequestration potential. Five lithofacies are recognized based on core description and log responses. They are interpreted as offshore, lower–middle wave-dominated shoreface, weathering zone, fluvial incised-valley, and tide-influenced estuarine depositional environments. The Muddy Sandstone contains predominantly quartz, with total quartz higher than 70 percent of the total framework grains. The percentage of feldspar is generally less than 5 percent, except for the Rozet Member in the Amos Draw Field, which is up to 22 percent. Sandstone petrographic examination also shows that the Muddy Sandstone can be divided into four groups based on the relative abundance of pore space, carbonate cement, and matrix. Sandstone with high porosity up to 23 percent is found in the shoreface lithofacies in the Amos Draw and Hilight fields and is also found in the estuarine lithofacies in the Kitty Field. The incised-valley lithofacies is of particularly low porosity due to high matrix content and carbonate cementation. The measured porosity in the sandstone varies between 1 percent and 23 percent, and the permeability is generally less than 10 millidarcys (mD). The variation of porosity is consistent with the observation in thin sections. XRD results show that the pore-filling clay minerals include kaolinite, chlorite, illite, and smectite. Core and well log correlation show that sandstone formed in lower–middle shoreface environments is laterally extensive and of uniform thickness, whereas sandstone of fluvial and estuarine origins is more variable in lateral extent and thickness. Based on examination of lithofacies, sandstone geometry, and thin section petrography, I suggest that the best reservoir interval for CO 2 sequestration in the Amos Draw Field is the lower Rozet Member, in the Sand Dunes and Hilight fields is the Springen Ranch Member, and in the Kitty Field is the Ute Member. Variables examined in this study provide important inputs for calculating CO 2 capacity potential and predicting chemical reactivity after CO 2 injection. Reservoir quality of the Muddy Sandstone is highly heterogeneous, and the complexity may be attributed to a combination of depositional environment, history of relative sea-level change during deposition, and type and extent of diagenetic alteration. The Muddy Sandstone, along with the overlying Mowry Shale, represents one third-order depositional sequence. Diagenetic processes include feldspar and lithic dissolution, secondary clay formation, quartz overgrowth, and four stages of carbonate cementation, which are early dolomite overgrowth, secondary calcite filling, dolomite replacement, and spotty siderite cementation. Carbonate cementation is interpreted as early diagenetic products formed during marine transgressions.

Analysis Of Fluvial Architecture In the Blackhawk Formation, Wasatch Plateau, Utah, U.S.A., Using Large 3D Photorealistic Models

Abstract There are numerous allocyclic and autocyclic factors which influence fluvial sandbody geometry. Unraveling the relative importance of such factors is challenging because fluvial systems can migrate and avulse across significant distances on the floodplain, so that collecting datasets that cover a large enough area with sufficient detail is difficult. This problem has been addressed by acquiring an oblique helicopter-mounted lidar survey over 16 km of exposed sections from Cretaceous coastal-plain deposits cropping out in the Wasatch Plateau of east-central Utah. The exposures of the Blackhawk Formation occur in steep cliffs, which are continuous for several tens of kilometers, and have been previously inaccessible for study. The recorded lidar data were used to build detailed photorealistic models (virtual outcrops) from which quantitative dimensional data on fluvial architectural elements (mostly sandstone channel bodies) were collected. Those channelized bodies show significant variation in width and thickness. 26% of the sandbodies were identified as being multistory channel bodies, with a thickness of 9.1 m (± 4.67 m; 1 σ) and a width of 142.6 m (± 180.7 m), which are significantly larger than the measured single-story sandbodies (5.9 m and 96.9 m, respectively). 50% of all sandbodies have aspect ratios smaller 13∶1 and the highest values reach 78∶1. The spatial constraint of the 397 fluvial sandbodies enabled the recognition of a series of vertical trends within the dataset. These trends include a subtle upward increase in width and thickness of both the mean and maximum values. This is coupled with a general upward decrease in the number of sandbodies and results in a broadly constant channel-to-overbank ratio. A clear correlation between wider channels with increased distance to the shoreline could also be documented. These observations can be explained by considering the entire Blackhawk Formation as the deposits of a large distributive fluvial system. In this model the key control on channel architecture is the distance to the shoreline, contrasting with previous interpretations of the Blackhawk depositional system in which the rate of accommodation creation in the coastal plain was thought to be the key control on channel geometry and stacking patterns.

Depositional Environment of Shallow-Marine Sandstones from Outcrop Gamma-Ray Logs, Belait Formation, Meragang Beach, Brunei Darussalam

The outcrop of shallow marine sandstone, Belait formation at Meragang Beach, Brunei Darussalam have long been regarded as tide-, wave- and storm-dominated environment that reveals the reservoir heterogeneities and variation in sand body geometry. Intensive outcrop study methods were available from previously conducted work on Belait Formation. But for this study the priority was to acquire gamma-ray profile at 5-10 cm scale by using Digital Total GR Count Detector and re-logged of Sedimentological log (in cm) of 311.8 m thick outcrop to describe the depositional environment and sedimentary facies architecture. The sediments deposited in a tide-dominated environment mostly interpreted in all cycles which were characterized by sub-depositional environment which includes; tidal channel, muddy tidal flat and mixed tidal flat sub-environment with sedimentary facies of planner cross-bedding, trough cross-bedding, herringbone cross-stratification, wave ripples and thin parallel lamination with coal fragmented bed and carbonaceous material. The gamma-ray profile for tide dominated environment shows different shapes; block left-boxcar, bell shape, funnel shape and irregular trend. In wave- and storm-dominated are wave ripples, thin parallel lamination with mud drapes, large planner cross-bedding, trough cross-bedding, highly bioturbated (skalithos) sandstone and hummocky cross-stratification. Whereas, the gamma-ray profile shows different shapes; left-boxcar, right-boxcar, funnel shape and irregular trend and in storm-dominated it show only left-boxcar with no sharp edges on top and bottom. This approach combines excellent data quality to interpretate different depositional environment and its impact on sandstone quality which implies that the outcrop gamma-ray log can use to characterize the rock deposited in different depositional environment in different facies. It also reveals that the changes in gamma-ray profile with in the sandstone depend upon the environment in which the sandbody deposited that reveal in reservoir heterogeneities in subsurface well correlation.