Sandstone Formation Research Articles

Influence of Henna Extracts on Static and Dynamic Adsorption of Sodium Dodecyl Sulfate and Residual Oil Recovery from Quartz Sand.

The application of surfactant flooding for enhanced oil recovery (EOR) promotes hydrocarbon recovery through reduction of oil-water interfacial tension and alteration of oil-wet rock wettability into the water-wet state. Unfortunately, surfactant depletion in porous media, due to surfactant molecule adsorption and retention, adversely affects oil recovery, thus increasing the cost of the surfactant flooding process. Chemical-based materials are normally used as inhibitors or sacrificial agents to minimize surfactant adsorption, but they are quite expensive and not environmentally friendly. Plant-based materials (henna extracts) are far more sustainable because they are obtained from natural sources. However, there is limited research on the application of henna extracts as inhibitors to reduce dynamic adsorption of the surfactant in porous media and improve oil recovery from such media. Thus, henna extracts were introduced as an eco-friendly and low-cost sacrificial agent for minimizing the static and dynamic adsorption of sodium dodecyl sulfate (SDS) onto quartz sand in this study. Results showed that the extent of surfactant adsorption was inversely proportional to the henna extract concentration, and the adsorption of the henna extract onto the quartz surface was a multilayer adsorption that followed the Freundlich isotherm model. Precisely, the henna extract adsorption on quartz sand is in the range of 3.12-4.48 mg/g (for static adsorption) and 5.49-6.73 mg/g (for dynamic adsorption), whereas the SDS adsorption on quartz sand was obtained as 2.11 and 4.79 mg/g at static and dynamic conditions, respectively. In the presence of 8000 mg/L henna extract, SDS static and dynamic adsorption was significantly reduced by 64 and 82%, respectively. At the same conditions, the residual oil recovery increased by 9.2% over normal surfactant flooding. The study suggests that the use of henna extracts as a sacrificial agent during SDS flooding could result in the reduction of static and dynamic adsorption of surfactant molecules on quartz sand, thus promoting hydrocarbon recovery from sandstone formations.

Quantitative characterization of fracture spatial arrangement and intensity in a reservoir anticline using horizontal wellbore image logs and an outcrop analogue

Using image logs from horizontal wells in the asymmetric East Painter Reservoir anticline in Wyoming, scanlines and petrographic observations from nearby outcrops, and methods that allow statistical quantification of fracture spatial arrangement patterns (including clustering), we show that in early Jurassic Nugget Formation sandstone degree of clustering and intensity of fractures varies with structural position. In the backlimb, fractures are markedly more clustered than random, and hierarchical and regularly spaced clusters are present, but intensity is low. In the forelimb, arrangements are mostly indistinguishable from random, but intensity is high. Based on image log response, the backlimb contains a higher proportion of open fractures, although the forelimb has greater numbers of open fractures. Spatial arrangement and intensity patterns are similar in a forelimb outcrop. Petrography reveals that quartz seals fractures less than 0.15 mm wide. Wider fractures are open with quartz rinds or are quartz lined and sealed with calcite or filled by quartzose cataclasite. The proportion of open fractures depends on quartz and calcite deposits and closure by reactivation (shearing) of pre-existing quartz-lined fractures. Based on increased abundance and greater strike dispersion, results suggest that in forelimbs, high fracture intensities reflect shear on preexisting fractures, possibly resulting in more numerous but less spatially correlated open fractures. The most prolific gas and water production wells are in the backlimb, suggesting that clustered but sparse quartz-lined open fractures are more effective fluid conduits than closely spaced partly damaged/sheared arrays.

Кварцевые песчаники сосновской толщи палеогена Ульяновско-Сызранского Поволжья (восток Русской плиты): вещественный состав и происхождение

The Paleogene quartz sandstones of the Sosnovka formation are widely occurred in the Uljanovsk-Syzran Volga region and are used as valuable building materials. The articlecomprises an improvement for thecontinental origin of the Sosnovkasandstones based on the results of a comprehensive study of the features of the mineral composition and microstructure of quartz sandstones of the Sosnovka formation obtained by using petrographic, XRD and electron-microscopic analyses. Therelevance of thestudy is based on the debatability of the origin ofsandstones, the depletion ofthemineralresource base ofthistype ofmineralrawmaterials andthe needto develop new approachesto its prediction. Accordingto theresults ofthestudy, itwasfoundthat opokas and diatomites ofthe Lower Syzransubformation, brought uptothe denudationsurface,could be the source of free silica for the cementation of sand grains of the Sosnovka sandstones. The deficit of siliceous cement and its extremely irregular distribution among the clastic grains indicate the local and restrict occurance of the source of solutions saturated with free silica, which, most likely, was controlled by paleorelief. Electron microscopic images and microprobe analysis of individual sandstone microcomponents indicate strong bacterial activity and frequent change of the land paleoenvironments to extensive marshy ones. The formation of pyrite framboids in the cementitious mass is a sign of oxygen deficit in the continental basin and the activity of magnetotactic bacteria. Whereas the decay of framboids is associated with dissolution of organic matter inside the framboids due to oxidation of the paleobasin. The results of the study leave no doubt about the non-marine origin of the sandstones studied, hence, we should not assume the presence of extended bodies of sustained thickness when solving prognostic-mineralogical problems. The Sosnovka sandstones are of extremely irregular distribution in the sandy sequence and formation in extremely continental pedological conditions will allow us to solve the important task of improving approaches to the prediction of new objects of this valuable mineral resource in the Ulyanovsk-Syzran Volga region.

The Early Paleocene Ranikot Formation, Sulaiman Fold-Thrust Belt, Pakistan: Detrital Zircon Provenance and Tectonic Implications

This study reports on the detrital zircon provenance of the sandstones of Early Paleocene Ranikot Formation exposed in the Fort Munro section, Sulaiman fold-thrust belt, Pakistan. This marks the Cretaceous-Tertiary boundary sequence. The detrital zircon U-Pb ages reported are mainly clustered around ~460–1100 Ma, ~1600–1900 Ma and ~2300–2600 Ma. The age cluster ~460–1100 Ma is mainly matched well with the Tethyan Himalaya. However, the age clusters ~1600–1900 Ma and ~2300–2600 Ma matched fairly with the lesser Himalayas and Higher Himalayas. In addition, the sandstone petrography suggests the craton interior provenance. The two younger Cretaceous zircon ages may be derived from the Tethyan Himalaya volcanic rocks as supported by a high (>0.3) Th/U ratio. Furthermore, the absence of the ophiolitic component ~115–178 Ma suggests that the western ophiolite may be emplaced at the same time as Ranikot Formation deposited or later. Moreover, the absence of the Eurasian (zircon with ages <100 Ma) in the Ranikot Formation excludes the possibility of the early collision along the western margin, as reported in earlier studies.

Automatic Multiwell Assessment of Flow-Related Petrophysical Properties of Tight-Gas Sandstones Based on the Physics of Mud-Filtrate Invasion

Summary Petrophysical interpretation of borehole geophysical measurements in the presence of deep mud-filtrate invasion remains a challenge in formation evaluation. Traditional interpretation methods often assume a piston-like radial resistivity model to estimate the radial length of invasion, resistivities in the flushed and virgin zones, and the corresponding fluid saturations from apparent resistivity logs. Such assumptions often introduce notable inaccuracies, especially when the radial distribution of formation resistivity exhibits a deep and smooth radial front. Numerical simulation of mud-filtrate invasion and well logs combined with inversion methods can improve the estimation accuracy of petrophysical properties from borehole geophysical measurements affected by the presence of mud-filtrate invasion. We develop a new method to quantify water saturation in the virgin zone, residual hydrocarbon saturation, and permeability from borehole geophysical measurements. This method combines the numerical simulation of well logs with the physics of mud-filtrate invasion to quantify the effect of petrophysical properties and drilling parameters on nuclear and resistivity logs. Our approach explicitly considers the different volumes of investigation associated with the borehole geophysical measurements included in the interpretation. The new method is successfully applied to a tight-gas sandstone formation invaded with water-base mud (WBM). Petrophysical properties were estimated in three closely spaced vertical wells that exhibited different invasion conditions (i.e., different times of invasion and different overbalance pressures). Available rock-core laboratory measurements were used to calibrate the petrophysical models and obtain realistic spatial distributions of petrophysical properties around the borehole. This approach assumes that initial water saturation is equal to irreducible water saturation. Based on the calibrated petrophysical models, thousands of invasion conditions were numerically simulated for a wide range of petrophysical properties, including porosity and permeability. Based on the large data set of numerical simulations, analytical and machine-learning (ML) models were combined to infer unknown rock properties in each well. Mean-absolute-percent errors (MAPE) of the analytical and ML models for the estimation of water saturation in the virgin zone are 5% and 2%, respectively, while the MAPE of the analytical models for the estimation of residual hydrocarbon saturation is 10%. Synthetic and field examples are examined to benchmark the successful application and verification of the new interpretation method. Estimates of water saturation in the virgin zone using the new method are in good agreement with core-based models.

Groundwater Amphipods of the Hyporheic Interstitial: A Case Study from Luxembourg and The Greater Region

Hyporheic interstitials are ecologically dynamic and often neglected aquatic environments. In this study, groundwater amphipods (Niphargidae, Pseudoniphargidae and Crangonyctidae) were sampled in hyporheic interstitials throughout Luxembourg and The Greater Region and specimens were analyzed by DNA barcoding. Sites characterized by gravel or coarse sand and high flow velocities of incoming water were the most species- and specimen-rich. A total of 11 species were detected, of which the Niphargus aquilex lineage EF of the N. aquilex cryptic species complex and juveniles of N. schellenbergi dominated the data set, but an unknown lineage of the N. aquilex-complex was also found. Some regions appeared to be devoid of groundwater amphipods. We hypothesize that underlying sandstone formations resulting in low sediment porosity may prevent physical colonization, but also that historical water pollution may have a long-lasting effect, either through the persistence of contaminants in the sediment or low recolonization rates of affected populations. In summary, our approach expanded regional species inventories, confirmed known occurrences, and validated previously questionable or historical morphology-based detections. In addition, the collection of absence data provided valuable insights into local extinctions. Finally, DNA-based distribution data are needed to gather information on the ecological affinities of groundwater amphipods to understudied hyporheic interstitial environments.

Hydrocarbon indication in Rio Bonito Formation sandstone: Implication for CO2 storage in São Paulo, Brazil

São Paulo State has witnessed CO2 storage-based investigations considering the availability of suitable geologic structures and proximity to primary CO2 source sinks related to bioenergy and carbon capture and storage (BECCS) activities. The current study presents the hydrocarbon viability evaluations and CO2 storage prospects, focusing on the sandstone units of the Rio Bonito Formation. The objective is to apply petrophysical evaluations with geochemical inputs in predicting future hydrocarbon (gas) production to boost CO2 storage within the study location. The study used data from eleven wells with associated wire-line logs (gamma ray, resistivity, density, neutron, and sonic) to predict potential hydrocarbon accumulation and fluid mobility in the investigated area. Rock samples (shale and carbonate) obtained from depths >200 m within the study location have shown bitumen presence. Organic geochemistry data of the Rio Bonito Formation shale beds suggest they are potential hydrocarbon source rocks and could have contributed to the gas accumulations within the sandstone units. Some drilled well data, e.g., CB-1-SP and TI-1-SP, show hydrocarbon (gas) presence based on the typical resistivity and the combined neutron-density responses at depths up to 3400 m, indicating the possibility of other hydrocarbon members apart from the heavy oil (bitumen) observed from the near-surface rocks samples. From the three-dimensional (3-D) model, the free fluid indicator (FFI) is more significant towards the southwest and southeast of the area with deeper depths of occurrence, indicating portions with reasonable hydrocarbon recovery rates and good prospects for CO2 injection, circulation and permanent storage. However, future studies based on contemporary datasets are required to establish the hydrocarbon viability further, foster gas production events, and enhance CO2 storage possibilities within the region.

Density-Driven Convection for CO2 Solubility Trapping in Saline Aquifers: Modeling and Influencing Factors

Industrial development has significantly increased the concentration of CO2 in the atmosphere, resulting in the greenhouse effect that harms the global climate and human health. CO2 sequestration in saline aquifers is considered to be one of the efficient ways to eliminate atmospheric CO2 levels. As an important mechanism, the solubility trapping greatly determines the efficiency of CO2 sequestration in saline aquifers, and this depends, in turn, on the density-driven convection that occurs during the sequestration. Density-driven convection is influenced by multiple factors. However, existing discussions on some of these influential factors are still ambiguous or even reach contradictory conclusions. This review summarizes the common modeling approaches and the influence of factors on density-driven convection. We suggest that saline aquifers with high values of depth, permeability, pH, and SO2 impurity concentration are the ideal CO2 sequestration sites. A certain degree of porosity, fractures, stratification, slope, hydrodynamic dispersion, background flow, and formation pressure are also considered advantageous. Meanwhile, the geological formation of the Permian White Rim Sandstone or carbonate is important, but it should not contain brine with excessive viscosity and salinity. Finally, we discuss the contents in need of further research.

Petrography of quartz sandstone in Nanggulan formation at Sermo area, Kulon Progo Regency, Daerah Istimewa Yogyakarta, Indonesia: implications on provenance and basin origin

Provenance can be used to determine how the sedimentary rock formed, as reflected in the mineral composition of lithology as the composition of sedimentary minerals. The Research area is located in the Hargowilis area, Kokap district, Kulon Progo Regency, Daerah Istimewa Yogyakarta Province. This Study uses a surface geological mapping method supported by laboratory analysis in the form of petrographic analysis. Based on the analysis of the sandstone provenance in the study area, the results show the research area is included in the Recycled orogenic provenance zone, foreland uplift sub-zone to continental block provenance with a dominant quartz percentage (92%). The tectonic setting in the research area was controlled by an orogenous process during the Eocene showing that the analysis is very relevant to the provenance in the research area which shows the source of recycled rock due to the orogenous process which is then deposited in the basins formed as a result of the process.

Repurposing a Geothermal Exploration Well as a Deep Borehole Heat Exchanger: Understanding Long-Term Effects of Lithological Layering, Flow Direction, and Circulation Flow Rate

In the drive to achieve net-zero carbon emissions, decarbonisation of heating is essential. This can be facilitated by geothermal energy, but drilling geothermal wells is associated with high risks and costs. The use of preexisting wells (e.g., exhausted hydrocarbon wells or failed geothermal exploration boreholes) offsets this cost while potentially turning liabilities into assets. The Newcastle Science Central Deep Geothermal Borehole (NSCDGB) is a geothermal exploration well that was drilled to target the Carboniferous Fell Sandstone Formation at 1418.5 to 1795 m depth. However, low hydraulic conductivities prevented the development as a conventional “wet” geothermal abstraction well; therefore, new alternative methods of development are being explored. This work investigates the repurposing of the NSCDGB as a deep borehole heat exchanger (DBHE), focusing on the sustainable operation of the system in the long term by employing a constant heat load designed to contribute to local buildings or a heat network. Numerical modelling was undertaken by using OpenGeoSys software to analyse the thermal and hydraulic performance of the system. Both homogeneous and heterogeneous models were developed to compare the influence of lithological layering in contrast to a homogeneous (nonstratified) subsurface geological model. Results from homogeneous simulations modelling the DBHE to a depth of 922 m show that a 50-kW heat load can be supported for a lifetime of 25 years. This corresponds to a 65-kW building load when coupled to a heat pump with a coefficient of performance of 4.33. Thus, the DBHE could meet up to 72% of the heat demand of the adjacent urban sciences building. Rather than being a purely hypothetical case study, this work considers a real existing borehole, adjacent to a building cluster which could make use of the geothermal heat. Heterogeneity, which has been considered for the first time at the NSCDGB site, exhibits a minor impact in comparison to homogeneous simulation results. Flow direction and mass flow rate also exhibited small effects on the system performance, whereas if the exploration well could be repurposed to increased depths, the heat load could be increased. This is the first study of a coaxial DBHE at the NSCDGB site considering long-term effects of mass flow rate, heterogeneity, and flow direction. The study evaluates the feasibility of repurposing an exploratory geothermal well in the UK as a DBHE that can be used as a low-carbon heat source for space heating, thus converting liabilities into potential “green energy” assets.

Study on mechanical properties and wellbore stability of deep sandstone rock based on variable parameter M-C criterion

With the depletion of shallow oil and gas resources and the increase in fossil energy consumption, deep oil and gas resources have become the main driving force of economic development. However, the physical and chemical properties and rock mechanics characteristics of the deep sandstone stratum are remarkable, and there is little research on the rock failure law of the deep sandstone stratum. Therefore, a batch of sandstone with similar homogeneity was selected, and several triaxial mechanical experiments with variable confining pressure were conducted for such sandstone. The mechanical constitutive model of rock samples under different confining pressures, the macroscopic failure characteristics of rock samples, and the evolution law of wave velocity and internal energy of rock samples during mechanical loading were analyzed. According to the experimental results of triaxial mechanics, a calculation model of wellbore collapse pressure in deep sandstone formation was established based on the variable parameter Mohr–Coulomb criterion. The results show that the crack propagation process of rock samples under mechanical loading can be divided into four stages. The characteristic strength, peak strain, elastic modulus, and deformation modulus of rock samples were positively correlated with confining pressure and showed obvious nonlinearity in high confining pressure areas. The failure mode of the rock sample changed from brittle failure to plastic brittle failure with the increase in confining pressure. According to the characteristics of the stress-strain curve, the variation of acoustic wave velocity during triaxial mechanical loading can be divided into three different types. The variation trend of acoustic wave amplitude of rock samples is consistent with the stress-strain curve. With the loading of axial stress, the elastic energy curve was a concave function, the curve of dissipated energy started increasing at the cracking stress of rock samples. After destroying the rock sample, the decline slope of the dissipated energy curve decreased with the increase in confining pressure. Considering the limited failure of the borehole wall, an evaluation model of borehole wall stability in deep sandstone formation based on variable parameter M–C criterion was established. The equivalent density of borehole collapse pressure predicted using the M–C strength criterion method with variable parameters was smaller than that predicted with traditional methods. The research results provide a theoretical basis for safe and efficient drilling in a deep sandstone formation.

The Early Cretaceous Tsagaangol formation sandstone provenance and paleoclimate

Early Cretaceous Tsagaangol formation refers to Zoolen terrain. This study attempts to make classification of the sandstones of the formation, define its sources and identify its alteration and paleoclimate condition. The sandstones of the formation are rich in mineral fragments, especially, quartz (comprising 70-75% of the entire content) and the composition is identified to be varying from subarcose to lithic arcose. As plotted on the source defining diagrams, they have been identified to be of continental block and mixed orogen and to get intensely altered or weathered under warm humid climate condition.

Numerical Simulation and Field Test Research on Vibration Reduction of PDC Cutting of Pebbled Sandstone under Composite Impact Load

Downhole vibrations caused by rock breaking when drilling through pebbled sandstone formations negatively affect the rate of penetration (ROP) and the safety of downhole tools. Therefore, it is of great significance to study the cutting characteristics of pebbled sandstone and find a method of reducing the drilling vibrations of pebbled sandstone formations. Based on the DEM (discrete element method), a simulation model of pebbled sandstone considering the random filling of high-strength gravels was established by using the random polygon distribution method. The influence of gravel content on the strength parameters and the breaking state of the pebbled sandstone samples was analyzed. Additionally, a DEM model of PDC cutting rocks loaded by a spring–mass system was established, and the Stribeck effect of contact friction between the PDC cutter and the rock was analyzed. The periodic vibration and the stick–slip phenomenon of the cutting system during the drilling process were presented by this model. The model was employed to simulate and explore the influence of composite impact load on stick–slip vibration during PDC cutting of pebbled sandstone. The simulation results showed that the composite impact load had a more obvious effect on mitigating the vibration of PDC cutting of pebbled sandstone under the condition of a higher horizontal impact amplitude coefficient (qh = 40%). Based on the simulation results, a composite impactor with a large impact angle α = 70° was selected to conduct the field tests in the pebbled sandstone formation of Well T1. The results showed that, compared to conventional drilling, the average WOB (weight on bit) of the section drilled with the composite impactor decreased by 57.13%, the standard deviation of the WOB decreased by 57.29%, and the average ROP increased by 98.31%. The employing of composite impactors in pebbled sandstone formations can significantly reduce drilling vibration, improve ROP, and protect bits and downhole instruments.

Zircon U-Pb age of tuffaceous sandstone in the southern margin of Turpan-Hami Basin and its geological significance

The Dongdagou area of Yamansu in the southeastern margin of Turpan-Hami basin, which is located to the north of the Kangguertag collision zone, is one of the representative areas to discuss whether there is a Precambrian basement problem in the Junggar Turpan-Hami massif. As a consequence, zircon LA-ICP-MS U-Pb dating has been carried out for the shallow metamorphic tuffaceous fine sandstone in the Yamansu formation of the Lower Carboniferous in this area. The results show that the ages of 30 clastic zircons range from 365 ±3 Ma to 2451 ±39 Ma, with multi-peak distribution characteristics. The ages of clastic zircons are mainly concentrated in 365~496 Ma, the main peak is 365~436Ma, the secondary peak is 440~496 Ma, and the ages of 8 clastic zircons are between 765~2451 Ma. The study shows that the formation age of the sedimentary rock is not the early Carboniferous but the late Middle Devonian. Based on the isotopic dating data of the two-cloud quartz schist in the Xiaohuangshan area in the northeast of the study area, it is considered that the lower limit of the formation period of this set of strata is not earlier than the Devonian. Combined with the age data of ancient zircons in diabase and fused tuff in the study area, and the comprehensive analysis of regional sedimentary characteristics, it is concluded that there may be Precambrian crystalline basement in the sediment source area-Junggar-Turpan-Hami block.

Capillary pressure equilibrium theory mapping of 4D seismic inversion results to predict saturation in a gas-water system

Understanding the effect of fluid concentration in rocks is crucial to characterize a hydrocarbon reservoir. Monitoring of fluid concentration specifically becomes challenging during a sequestration/enhanced oil recovery program because the objective is to understand the amount of initial fluid replaced by liquid/gas injection and identify the probability of leakage with time. The prior assumption of uniform or patchy type of distribution of gas in a rock-physics theory leads to large uncertainty in the prediction of saturation. As a result, we have used the capillary pressure equilibrium theory (CPET) for creating a reservoir model that matches the physics of capillary-induced fluid invasion and avoids the uncertainty related to the type of distribution of gas in pores. We create a CPET model using the reservoir parameters of the clean and unconsolidated sandstone formation of the Sleipner field, North Sea, which is the world’s first industrial-scale CO2-injection project, assuming that there is no significant change in the rock frame throughout the field. The model is then used to analyze the fluid content from the time-lapse seismic inversion results of the Sleipner field. CO2 at higher quantities, according to our research, is analogous to a uniform distribution, whereas CO2 at lower concentrations is mostly in between patchy and uniform distribution or slightly patchy type. We predict maximum CO2 saturation from a quantitative interpretation of six time-lapse seismic data from 1999 to 2010 using the CPET as 75% of the pore space, and the footprint of the CO2 plume in the topmost layer is spreading from zero in 2001 to 7 × 105 m2 in 2010. Our model finds that injected CO2 from all layers below will migrate to the top layer approximately 50 years after the commencement of the injection.

The main factor influencing reservoir quality of K1yc tight sandstones within volcano-sedimentary succession in the Dehui fault-depression of Songliao Basin, NE China

The main factor influencing high-quality reservoirs of the Lower Cretaceous Yingcheng Formation (K1yc) sandstones in the Dehui Fault-Depression of the Songliao Basin, NE China within volcano-sedimentary succession is still obscure. In the study area, K1yc sandstone reservoirs has exhibited stronger heterogeneity and more complicated diagenetic alteration than terrigenous clastic sandstones. In this paper, K1yc sandstone reservoirs have been characterized through the aspects of petrology, physical property, and pore-types and -throat size. Accordingly, methods including core analysis, thin section, X-ray diffraction, scanning electron microscopy have been used in the study. Finally, the main factor controlling high-quality reservoirs within volcano-sedimentary successions has been concluded. The results reveal that the abundance of authigenic illite clays with forms of fibrous pore-bridging or lath-like pore filling primarily and rapidly reducing the reservoir performance. The chlorite coatings of framework grains should not effectively retard cementation process. Volcanic rock fragments are widespread within the volcano-sedimentary successions and tend to be dissolved by pore fluids, as then illitization should be the most important origin of authigenic illites. It has a critical significance for predicting high-quality reservoirs in the K1yc tight sandstones.

SHRIMP U–Pb geochronology of Mesoproterozoic basement and overlying Ocoee Supergroup, NC–TN: dating diagenetic xenotime and monazite overgrowths on detrital minerals to determine the age of sedimentary deposition

Sedimentary rocks of the Ocoee Supergroup crop out in the Appalachian Blue Ridge of western North Carolina and eastern Tennessee. This ∼12 km thick sequence of strata was mapped as nonconformably overlying Mesoproterozoic crystalline basement (about 1.20–1.02 Ga) and beneath the lower Cambrian Chilhowee Group, leading to the traditional interpretation that the Ocoee Supergroup was deposited between about 1.02 and 0.54 Ga. This interpretation was challenged by Unrug and Unrug (1990, Geology, 18: 1041–1045) and Unrug et al. (2000, Geological Society of America Bulletin. Vol. 112, pp. 982–996), who claimed that Paleozoic fossils were recovered from rocks of the Walden Creek Group (upper Ocoee), and thereby created a new tectonic model for the origin of Laurentia. To resolve the age controversy concerning the time of deposition of the Ocoee Supergroup, diagenetic xenotime overgrowths on detrital zircon and diagenetic monazite on detrital monazite were dated by the U–Pb method. Sensitive High Resolution Ion Microprobe ages of diagenetic xenotime from three samples of sandstone (Thunderhead Sandstone, Cades Sandstone, and Shields Formation) indicate that the Ocoee Supergroup was deposited in the interval of about 580–550 Ma. A sample of Sandsuck Formation (uppermost Ocoee) yielded xenotime overgrowths of about 410 Ma, probably related to post-depositional low-grade metamorphism. Various origins of xenotime overgrowths may be distinguished by trace element distributions. Ages and trace element concentrations of monazite overgrowths support the xenotime age results, although concordia systematics are complicated by high concentrations of common Pb and inheritance of ages (∼1.2 and 0.6 Ga). These age data support the observed field relations for a late Neoproterozoic depositional age of the Ocoee Supergroup.

Investigating the Impact of Sieve Analysis on the Choice of Gravel Pack Design for Wells in Unconsolidated Sandstone Formations in the Niger Delta

Sand production is major problem for the oil and gas industry and solutions to this problem is a continuous process as new challenges arise with time. Various sand control methods have been proposed for tackling the sand production challenge, and research and experience have shown that the use of mechanical sand control methods are more suitable, with gravel packing being the most effective. Gravel packs are proven to be an effective mechanical sand control technique, and a good gravel pack completion design is of great importance to exclude sand from the wellbore while enhancing well productivity. Implementation of sand control by gravel packing in the Niger Delta is usually found to require importation of commercial gravel for the purpose of sand control which is a challenge in terms of high purchasing and transportation costs with import taxes. A solution is presented in this study which involves sourcing for gravel locally and investigating its suitability for sand control by gravel packing. Locally sourced gravel was compared with commercial gravel using sieve analysis and results showed that the locally sourced gravel closely met the requirements of the commercial gravel depicted by slot widths that are close to that obtained by the commercial gravel. Based on the results of this study, favorable performance is expected when locally sourced gravel is used for sand control by gravel packing in unconsolidated formations in the Niger Delta. It is recommended to source for different gravel types from different locations and evaluate using sieve analysis and Laser Particle Size Distribution Analysis to determine suitable gravel types for these type of formations.

A screening assessment of the impact of sedimentological heterogeneity on CO 2 migration and stratigraphic-baffling potential: Sherwood and Bunter sandstones, UK

Abstract We use a combination of experimental design, sketch-based reservoir modelling and flow diagnostics to rapidly screen the impact of sedimentological heterogeneities that constitute baffles and barriers on CO 2 migration in depleted hydrocarbon reservoirs and saline aquifers of the Sherwood Sandstone Group and Bunter Sandstone Formation, UK. These storage units consist of fluvial sandstones with subordinate aeolian sandstones, floodplain and sabkha heteroliths and lacustrine mudstones. The predominant control on effective horizontal permeability is the lateral continuity of aeolian-sandstone intervals. Effective vertical permeability is controlled by the lateral extent, thickness and abundance of lacustrine-mudstone layers and aeolian-sandstone layers, and the mean lateral extent and mean vertical spacing of carbonate-cemented basal channel lags in fluvial facies-association layers. The baffling effect on CO 2 migration and retention is approximated by the pore volume injected at breakthrough time, which is controlled largely by three heterogeneities, in order of decreasing impact: (1) the lateral continuity of aeolian-sandstone intervals; (2) the lateral extent of lacustrine-mudstone layers; and (3) the thickness and abundance of fluvial-sandstone, aeolian-sandstone, floodplain-and-sabkha-heterolith and lacustrine-mudstone layers. Future effort should be focused on characterizing these three heterogeneities as a precursor for later capillary, dissolution and mineral trapping.

Controls on anomalously high porosity/permeability of Middle Jurassic deeply buried tight sandstones in the Taibei Sag, Turpan-Hami Basin, northwestern China: Implications for reservoir quality prediction

The lower member of the Middle Jurassic Xishanyao Formation (J2x1+2) sandstones are significant exploratory targets for hydrocarbon resources in the Taibei Sag of the Turpan-Hami Basin, northwestern China. Formation of anomalously high porosity/permeability in deeply buried J2x1+2 sandstones and reservoir quality prediction were investigated using a variety of petrographic analyses. These results show that the J2x1+2 sandstones are mostly feldspathic litharenites and litharenites, which are characterized by moderate-to good sorting, silty to medium granularity, and point- to long grain contacts. The J2x1+2 sandstone has low porosity (avg 6.0%) and permeability (avg 1.12 mD), but shows anomalously high porosity/permeability at depth interval of 3850–4050 m. There is a noticeable variation in pore types and sizes from intergranular-intragranular dissolution pores with a size mode of 2.0 μm in anomalously high porosity (AHP) sandstones, to major intragranular dissolution pores with a size mode of 0.5 μm in normally high porosity (NHP) sandstones, to most micropores with a size mode of 0.25 μm occurred in the normally low porosity (NLP) sandstones. The compaction is the main cause of porosity destruction, resulting in an average porosity loss of 89.6%. However, the authigenic minerals have relatively little influence on reservoir quality. The combination of nappe tectonics and well-sorted particles alleviated the compaction and thus preserved more intergranular pores in the AHP sandstones. Dissolution further increases the porosity and eventually forms the AHP sandstones. The strong carbonate-cemented facies (SCC), tightly compacted facies (TC), moderately compacted with moderate dissolution facies (MCMD), and weakly compacted with strong dissolution facies (WCSD) are determined in the J2x1+2 sandstones. The sandstones with SCC and TC are recognized as the NLP reservoirs. The MCMD and WCSD correspond to the NHP and AHP reservoirs, respectively. The reservoir quality predicted using the constructed diagenetic facies charts is in good agreement with the photomicrograph observations and physical property tests. Additionally, the AHP reservoirs are still developed in the deeply buried sandstones with depths larger than 4000 m.