Reservoir Zones Research Articles

Diagenetic variability in Tertiary, syn-rift mixed siliciclastic‑carbonate depositional system (Lower Musayr Formation), Red Sea, Saudi Arabia

Mixed siliciclastic‑carbonate depositional systems are prone to differential diagenesis due to lithological heterogeneity. However, unlike pure carbonate and/or siliciclastic counterparts, the diagenesis within mixed clastic‑carbonate successions remains poorly known. This study integrates sedimentological and petrographic analysis with porosity-permeability measurements to understand diagenetic variability and its impact on fluid flow in the Oligocene-Miocene mixed clastic‑carbonate system (lower part of Musayr Fm) in Midyan Basin, Red Sea, Saudi Arabia. Sedimentological observations highlight that siliciclastic intervals comprise conglomerates, coarse- to fine-grained sandstones and subordinate mudstones whereas mixed siliciclastic‑carbonate intervals are composed of shell, ooids and microbialites-dominated facies. Petrographic analysis indicates presence of several diagenetic processes in siliciclastic intervals including dissolution of unstable silicate grains (feldspar), formation of pore filling and/or grain-replacive kaolinite, precipitation of calcite and/or dolomite, and replacement of dolomite by silica. The siliciclastic strata exhibit minimal compaction fabrics with no stylolites and are characterized by higher permeability (average = 1884 md) and porosity (average = 18.7 %). On the other hand, mixed siliciclastic‑carbonate intervals underwent micritization, sparry calcite formation, dolomitization of micrite and bioclasts, and replacement of sparry calcite by pyrite and Fe-oxides. Wavy stylolite seams were also observed in ooid facies reflecting moderate chemical compaction. In addition, the porosity (average = 6.6 %) and permeability (average = 93 md) are magnitudes lower than siliciclastic counterparts. Significant differences between porosity and permeability of siliciclastic and mixed intervals are partly linked to relatively intense diagenetic alteration (higher cementation and chemical compaction) in mixed units. Understanding such diagenetic heterogeneity in mixed systems has important implications for identification of reservoir and non-reservoir zones and may provide useful insights for hydrocarbon exploration and carbon sequestration in the analogous strata in the subsurface.

Accentuating the Reservoir Potentials of Balkassar Field by Delineating the Fracture Trends and Hydrocarbon Prospects with the Aid of Seismic Attributes.

Carbonates constitute a significant proportion of the world's hydrocarbon reserves, accounting for approximately 43%. Despite their substantial potential, accurately characterizing these reserves is a challenging task due to their complex and anisotropic nature. In the upper Indus basin of Pakistan, Eocene carbonates exhibit strong production capabilities. However, the Eocene reservoir (comprising the Chorgali and Sakesar formations) and the deeper Lockhart formation of the Paleocene age present a considerable challenge. The Chorgali formation is dolomitic in composition, featuring both primary and secondary porosity, while the Sakesar formation has only secondary porosity resulting from tectonic activity. The delineation of tectonically induced porosities is a highly demanding task that requires exceptional quality seismic and well data for reliable results. To address the complex heterogeneities present in the Eocene reservoir of the upper Indus basin, a variety of seismic attributes, such as sweetness, instantaneous frequency, amplitude, curvature, similarity variance, lateral continuity, and fault likelihood, have been employed in conjunction with fundamental interpreting techniques. These advanced seismic attributes greatly contribute to delineating fracture zones and identifying sweet spots with remarkable precision. They also enable a focus on high-frequency data content and differentiate between shale beds and reservoir zones based on frequency and amplitude. This helps in the concise marking of fractured zones to enhance our understanding of secondary porosity. Moreover, these attributes help delineate the continuity of reflectors and pinpoint disruptions caused by compression forces and tectonic activities. This study aims to capture the heterogeneity and complexity of reservoir zones to address a critical question regarding the Balkassar structure. The Western lobe of the field, which demonstrates promising oil production, outperforms the structurally higher Eastern part, which lags behind in production. This research seeks to recognize the geological elements contributing to the superior performance of the Western lobe and provide guidance for maximizing the potential of the Eastern lobe through advanced characterization techniques.

Strategies for hydrogen storage in a depleted sandstone reservoir from the San Joaquin Basin, California (USA) based on high-fidelity numerical simulations

Depleted hydrocarbon reservoirs are promising candidates for underground hydrogen (H2) storage (UHS) due to their known (and often high) storage capacity, well-identified geological conditions, reduced cushion gas requirements, and adaptable existing infrastructure. While these reservoirs have shown success in natural gas storage, their performance for pure H2 storage remains untested, necessitating further research. To address this knowledge gap, we conduct high-fidelity multiphase compositional reservoir simulations to evaluate the technical feasibility of UHS in a depleted hydrocarbon reservoir within the Temblor Formation at the North Belridge Field from the San Joaquin Basin, California, USA. This reservoir zone comprises five distinct sand layers separated by four shale intervals and is bounded above and below by thick shales (overburden and underburden, respectively). This research has two primary objectives: (a) to assess the feasibility of two distinct UHS strategies—one that involves storing H2 across all sand layers and another that stores an equal amount of H2 in the single thickest sand layer, and (b) to evaluate the effect of interbedded shale permeability on UHS performance. Our findings indicate that UHS in the Temblor Formation is technically viable, showing enhanced H2 withdrawal efficiency and produced H2 purity over successive cycles with minimal leakage risk. The two storage strategies significantly impact UHS performance in the interbedded sandstone sequence. Storing H2 across all sand layers results in higher H2 withdrawal efficiency, but lower produced H2 purity. In contrast, storing H2 in a single sand layer leads to lower withdrawal efficiency but consistently high H2 purity. In addition, H2 storage in a single sand layer results in higher reservoir pressure buildup, implying greater geomechanical risks. The impact of interbedded shale permeability on UHS performance is negligible across the range of uncertainty for shale permeability. This research not only lays the groundwork for potential UHS implementation in the abundant sandstones of the San Joaquin Basin of California, but also offers valuable insights for other depleted hydrocarbon reservoirs under consideration for UHS that share similar properties.

Drought in play: A grounded socio-hydrological tool to increase social participation in drought plans

Drought management requires a proactive approach and active involvement of stakeholders to enhance effectiveness and equity. However, there are challenges such as power imbalances and knowledge disparities. This paper proposes an innovative methodological tool to facilitate the understanding of key concepts, such as drought states and reservoir zoning, alerts for changes in drought state, risk predisposition, breaking down hierarchical structures of teaching and learning, contributing to more horizontal and participatory processes. We present “Drought in Play” from its creation and design to its application in two water systems in the state of Ceará, located in the Northeast region of Brazil. The game encourages active participation in the creation of a proactive drought plan for water systems and stimulates critical thought and proactive actions regarding drought and water resources management. The game was designed for a multidisciplinary team and tested with researchers that know the drought plans methodology and the challenges to build an effective drought plan. “Drought in Play’’ includes game elements (cards, boardgame) and simulations. We explore the participants’ perceptions when using this tool to facilitate stakeholder participation in building a proactive plan to deal with drought and to enable water users to collectively manage a reservoir that is a common resource to all of them. The results suggest that ”Drought in Play“ can be used as a tool for socio-hydrological modeling, because it combines hydrological elements with social aspects such as decision-making, cooperation, and conflicts among participants. The game has potential to facilitate sustainable water resources management, enhance social learning, and provide a knowledge framework to address complex environmental challenges. It can be easily adapted to other regions with similar situations without requiring further modifications.

Internal-multiple-elimination with application to migration using two-way wave equation depth-extrapolation scheme

Internal multiple interference, affecting both seismic data processing and interpretation, has been observed for long time. Although great progress has been achieved in developing a variety of internal-multiple-elimination (IME) methods, how to increase accuracy and reduce cost of IME still poses a significant challenge. A new method is proposed to effectively and efficiently eliminate internal multiples, along with its application in internal-multiple-eliminated-migration (IMEM), addressing this issue. This method stems from two-way wave equation depth-extrapolation scheme and associated up/down wavefield separation, which can accomplish depth-extrapolation of both up-going and down-going wavefields simultaneously, and complete internal-multiple-elimination processing, adaptively and efficiently. The proposed method has several features: (1) input data is same as that for conventional migration: source signature (used for migration only), macro velocity model, and receiver data, without additional requirements for source/receiver sampling; (2) method is efficient, without need of iterative calculations (which are typically needed for most of IME algorithms); and (3) method is cost effective: IME is completed in the same depth-extrapolation scheme of IMEM, without need of a separate processing and additional cost. Several synthesized data models are used to test the proposed method: one-dimensional model, horizontal layered model, multi-layer model with one curved layer, and SEG/EAGE Salt model. Additionally, we perform a sensitivity analysis of velocity using smoothed models. This analysis reveals that although the accuracy of velocity measurements impacts our proposed method, it significantly reduces internal multiple false imaging compared to traditional RTM techniques. When applied to actual seismic data from a carbonate reservoir zone, our method demonstrates superior clarity in imaging results, even in the presence of high-velocity carbonate formations, outperforming conventional migration methods in deep strata.

Unleashing geological sequestration potential of mature oilfield by enforcing 4-dimensional seismic model-based inversion in Widuri Field, Indonesia

Implementing reservoir characterization by undertaking seismic inversion on time-lapse surveys is very effective for observing the distribution and changes in the hydrocarbon reservoir. In mature oilfields, these changes are most likely influenced by the thermodynamic activities including the injection of fluid into the reservoir, which can change the volume, pressure, and composition of the geological formations. Fluid injection (in this case, water injection) into the reservoir is typically used as an oil and gas booster to increase production through EOR (Enhanced Oil Recovery). However, in light of CCUS (Carbon Capture Utilization and Storage) application, injecting CO2 can be considered as a new EOR strategy development, with the dual objectives of increasing oil and gas production as well as lowering carbon emissions at the same time. The 4-dimensional (4D) seismic data available over Widuri Field covers an area of 125 km2, with 884 inlines and 905 crosslines, acquired in 1991 and 2004 over the same area. The inversion algorithm is using seismic deconvolution to generate an acoustic impedance model before developing a geological reservoir model. Reservoir characterization was conducted in this study to obtain detailed information of the reservoir zones by determining the impact of water injection that replaces hydrocarbons. Intervals and areas with an abundance of water can be considered as potential CO2 sequestration in the future, as a part of the CCUS application. In conclusion, the findings of increasing impedance from inversion data from 1991 to 2004 can indicate the presence of existing porosity and permeability. This evidence could indicate reservoir capability as CO2 storage.

Analysis of communities, with an historical reconstruction from a deep submarine seamount in an oligotrophic area (Valencia Seamount, Balearic Basin, Western Mediterranean)

Seamounts can have strong influence on the dynamics and production of waters surrounding them, sometimes creating a kind of oasis effect. To assess to what depths and under what conditions this effect may persist on seamounts, we analysed the fauna and recent history of the Valencia Seamount (VS), a deep, small seamount (summit depth: 1056 m) from an oligotrophic zone of the Mediterranean Sea. The (living) epibenthic fauna of the VS summit (up to 1300 m) was composed of filter feeders, surface deposit feeders and carnivores (63 species), structured in a trophic chain as indicated by the high positive relationship (r2 0.502) between the δN15and δC13 of the fauna analysed. This trophic linearisation (compared with Catalan and NW Mallorca slope sites at similar depths) may optimise the energy flux reaching the seamount, with rather high %TOC (0.93–1.19%) and C/N (5.0–5.5) in July. The Valencia Seamount can also act as a reservoir zone for fish (grenadiers: Nezumia aequalis, and Hymenocephalus italicus) and shrimps (Plesionika martia), which are distributed at shallower depths on adjacent slopes, where they are exploited. The specimens caught were mainly juveniles, suggesting that these species may rather form pseudopopulations. Tuna and cetaceans were regularly sighted on the surface of VS during a specific period of the year (June–July). Reconstructing the recent history of Valencia Seamount (MC2 sediment core taken at 1151 m in 2021) we evidenced a decline of zooplankton pteropods (primary consumers) since ca. the 1960s (MC2 dating after 210Pb, metals and microplastic analyses), linked to a decline of Si in sediments suggesting a decrease in primary production due to a decrease in rainfall and river runoff. An increase of vorticity (ω+ values), i.e. of cyclonic gyres, and currents after the 1970s over the summit probably increased resuspension of organic matter, which in turn may enhance zooplankton aggegations at VS summit. We hypothesise that this would explain the parallel increase in mesopelagic myctophids in MC2 (from otolith records). Sediments of the Valencia seamount recorded several historical changes. Reconstruction studies carried out in oceanic areas can therefore help to understand the history and improve the management of fragile systems such as seamounts.

Does dam construction increase the biotic homogenization of riparian vegetation along a regulated river?

AbstractDam construction in natural rivers alters the intensity and frequency of water level fluctuations in reservoirs and their downstream river reaches, leading to the biotic homogenization of riparian communities. Although the intensity of dam‐induced water level fluctuations is often described qualitatively, quantitative analysis of its effects on riparian vegetation remains inadequate. This study compared riparian species dissimilarity between the natural Chishui River (as a control) and the regulated Wujiang River (with cascade reservoirs) to assess the level of biotic homogenization under different regulation types and in zones with different riverbank characteristics using the Bray–Curtis dissimilarity index. The reservoirs along the Wujiang River were classified into four different regulation types according to the water retention time, which were daily (0–7 days), weekly (7–30 days), seasonally (30–90 days), and yearly (>365 days). Results showed significant species dissimilarity in riparian vegetation along the Wujiang River compared to that along the Chishui River in the inundation, transitional, and unflooded zones. Furthermore, the species composition and dissimilarity patterns differed in the reservoirs and downstream rivers of the cascade dams in terms of regulation type and zone. For different regulation types, there was high species dissimilarity in both the upstream and downstream inundation zone of the daily reservoirs, while species dissimilarity was lower in both upstream and downstream of the yearly reservoir (Bray–Curtis index <0.5). This study provides a quantitative measure of how dam construction perturbs riparian vegetation communities, emphasizing the importance of the balance between the riparian ecosystem conservation and the economic demands of dam construction.

Pore Structures and Evolution Model of Reservoirs in Different Secondary Structural Zones in the Eocene Shahejie Formation, Chezhen Sag, Bohai Bay Basin.

Although abundant unconventional oil resources have been discovered in conglomerate and sandstone reservoirs in rift basins, the mechanism of differential pore evolution in conglomerates and sandstone reservoirs within different secondary structural zones of rift basins is not yet clear. The pore structures of conglomerate and sandstone reservoirs in the distinct secondary structural zones in the Chezhen Sag were quantified in three dimensions using high-resolution microcomputed tomography (micro-CT). Thin section and scanning electron microscopy observations were used to investigate the differential evolution mechanisms of conglomerate and sandstone reservoirs. Micro-CT analysis of the pore structures of conglomerate and sandstone reservoirs revealed that sandstone reservoirs are superior to conglomerate reservoirs with regard to the pore number and pore connectivity and that sandstone reservoirs are more heterogeneous than conglomerate reservoirs. Triangles dominate the pore and pore throat geometries of sandstone and conglomerate reservoirs, while the sandstone reservoir pores are more regular than conglomerate reservoir pores. The depositional environment, mineral composition, and diagenetic intensity jointly control the quality of the reservoirs. Because of the lengthy transportation distance of their parent rocks, the compositional maturity and sorting behavior of sandstone reservoirs in depression and gentle slope zones are better than those of conglomerate reservoirs in steep slope zones, and thus sandstone reservoirs have a higher initial porosity than conglomerate reservoirs. The rapid compaction experienced by the conglomerate reservoirs in steep slope zones in their early stages creates a closed diagenetic environment, making it difficult to effectively improve reservoir porosity through dissolution. However, the widely developed microfractures in the reservoirs provide channels for fluid migration, promote the development of dissolution pores, and form a tight reservoir dominated by secondary pores. With weak compaction and an open diagenetic environment, the primary pores in sandstone reservoirs in the gentle slope zone are preserved in large quantities. Meanwhile, dissolution expands the secondary pores of the reservoir, resulting in a high-quality reservoir having both primary and secondary pores. In addition, an approach based on primary, secondary, and total porosity was proposed in the study to efficiently evaluate reservoir quality and identify reservoir evolution mechanisms.

Logging-While-Drilling Laterolog vs. Electromagnetic Propagation Measurements: Which Is Telling the True Resistivity?

Summary The electromagnetic propagation (EMP) measurement frequently acquired with logging-while-drilling (LWD) tools in high-angle wells is sensitive to geometrical effects that can mask the true formation resistivity. Less commonly used, the LWD laterolog measurement is sometimes perceived as providing data too shallow to give a true formation resistivity (Rt). In this paper, we presents modeling and actual examples to demonstrate that the laterolog can often provide a superior resistivity measurement for formation evaluation to that of the LWD EMP tool. We examine the laterolog and EMP resistivities in several high-angle wells crossing carbonate formations in 8.5-in. and 6.125-in. hole sizes. In the 8.5-in. sections, producers and water injectors (high- and low-resistivity ranges) were evaluated. In the 6.125-in. sections, one reservoir sandwiched between two very high-resistivity layers and another borehole in a highly fractured reservoir were examined. The laterolog data were corrected for invasion using a 1D inversion of the memory data. Structure-based forward modeling was used to examine and explain the differences between the laterolog and EMP resistivity measurements. In the first example in a thick low-resistivity water reservoir, laterolog resistivity and EMP resistivity agree, showing that the two tools provide the same measurement when no geometrical effects are present. In the first part of the second example, a reservoir zone was initially drilled only with the LWD EMP resistivity measurement. The LWD laterolog was run several days later, and the resistivity data read much lower in the relogged section compared with the EMP resistivity. The laterolog 1D inversion was unable to resolve Rt because of the excessively deep invasion that occurred over the course of several days. In the second part of the second example, the laterolog resistivity showed a clear conductive invasion profile. While the deepest laterolog real-time resistivity data indicated lower resistivity than the EMP resistivity, the true resistivity, Rt (invasion-corrected 1D-inverted laterolog resistivity), matched the EMP Rt resistivity. This result validated both measurements and emphasized that the differences were due to invasion. The first two examples demonstrated that when acquired in normal drilling conditions (within 1–2 hours of drilling the section), the laterolog measurements can provide uninvaded formation resistivity even in the presence of invasion. A reservoir in another example was sandwiched between resistive layers that caused difficult-to-explain elevated EMP resistivity readings. Structural modeling reproduced the elevated behavior of the EMP data and explained the differences between resistivity measurements. This result showed that the laterolog is better suited to evaluate resistivity in thin reservoirs where there is a high-resistivity contrast to the adjacent layer. Finally, fractured reservoir examples are presented, which show that both the laterolog and EMP can be affected by the presence of fracture swarms. The examples presented in this paper demonstrate that in high-angle wells, when acquired under normal drilling conditions, invasion-corrected laterolog resistivity is often nearer to Rt than EMP resistivity. In those cases, the laterolog measurement provides data that are better inputs to water saturation calculations. Multiple examples from high-angle wells illustrate the findings.

Zooplankton community structure and diel migration patterns vary over hours, days, and years in the pelagic and littoral zone of a eutrophic reservoir

Abstract Zooplankton play an integral role as indicators of water quality in freshwater ecosystems, but exhibit substantial variability in their density and community composition over space and time. This variability in zooplankton community structure may be driven by multiple factors, including taxon-specific migration behavior in response to environmental conditions. Many studies have highlighted substantial variability in zooplankton communities across spatial and temporal scales, but the relative importance of space vs. time in structuring zooplankton community dynamics is less understood. In this study, we quantified spatial (a littoral vs. a pelagic site) and temporal (hours to years) variability in zooplankton community structure in a eutrophic reservoir in southwestern Virginia, USA. We found that zooplankton community structure was more variable among sampling dates over 3 years than among sites or hours of the day, which was associated with differences in water temperature, chlorophyll a, and nutrient concentrations. Additionally, we observed high variability in zooplankton migration behavior, though a slightly greater magnitude of DHM vs. DVM during each sampling date, likely due to changing environmental conditions. Ultimately, our work underscores the need to continually integrate spatial and temporal monitoring to understand patterns of zooplankton community structure and behavior in freshwater ecosystems.

Numerical analysis of a pressure distribution field and fluid flow velocity vectors near cumulative perforation holes

Relevance. The need to determine a reliable pressure distribution field and fluid filtration velocity vectors inside the perforation hole and the reservoir rock surrounding it. Aim. On the basis of numerical finite element modeling of the fluid flow inside the perforation holes and its filtration in the surrounding reservoir rock, to reveal the patterns of pressure distribution and fluid filtration vectors in the perforation hole, on its walls and in the near-wellbore zone. Objects. Near-wellbore zone of a limestone reservoir of one of the oil fields in the south of the Perm Region, including perforations. Methods. Numerical finite element method for calculating the flow and filtration of liquid in the near-wellbore zone, taking into account the geometry of perforation holes. Results. The paper considers the main relationships used in numerical simulation of fluid flow and filtration in the ANSYS finite element modeling software package. The authors have developed the finite element scheme of the near-wellbore zone, including cumulative perforation holes and taking into account their geometric parameters, as well as the fact that inside the holes the fluid flow is modeled in open space using the Navier–Stokes equations, and in the surrounding reservoir rock based on the filtration equations and Darcy's law. Numerical calculations were carried out, on the basis of which the distribution of pressure, flow velocities and fluid filtration inside the holes and in the near-wellbore zone as a whole was obtained. Calculations were made with varying pressure in the well (or pressure drawdown), as well as for different values of reservoir permeability. The calculation results showed that for the actual drawdown values of 10 MPa and the reservoir permeability of 50 mD, the value of pressure change inside the perforation holes will not exceed 0.01 MPa, i.e. it can be assumed that it practically does not change inside the hole. It is noted that the maximum value of the filtration rate corresponds to the top of the perforation holes and then its value decreases as it approaches the borehole wall. It is concluded that in further modeling of the stress-strain state of the near-wellbore zone, taking into account the holes of cumulative perforation on the surface of the holes, it is permissible to set a constant pressure value equal to the pressure in the well, and not logarithmic or any other distribution of it.

Integrated petrophysical, sedimentological and well-log study of the Mangahewa Formation, Taranaki Basin, New Zealand

This study aims to address the problem of porosity preservation in the Mangahewa Formation of five main hydrocarbon fields covering onshore and offshore of the Taranaki Basin. An integrated reservoir characterization of the Middle to Late Eocene Mangahewa Formation is achieved through petrophysical evaluation, sedimentological and petrographical descriptions, and well log analysis methods. Petrophysical parameters (porosity and permeability) were acquired from the available core analysis and using mathematical equations to obtain other petrophysical matrices such as normalized porosity index (NPI) and reservoir quality index (RQI). Factors that affected Mangahewa reservoir were studied through thin-section microscopy and well-log analysis helped to measure the reservoir and hydrocarbon potentiality in the Mangahewa Formation. The Mangahewa Formation is dominated by sandstone and a range of marginal to shallow marine facies with varying hydraulic flow units (HFU). The Mangahewa Formation has a high positive correlation in porosity-permeability relationship and has a maximum of 4.67 μm RQI and 20.08 μm FZI (Well Kapuni-14) which reflect potential reservoir. The Mangahewa Formation observed from Wells Kapuni-14, Maui-A1G, McKee-16A, and Mokau-1 are dominated with 59.6%, 56.4%, 79.3%, and 68% of macro- and megapores, respectively. The presence of authigenic clay and calcite cement has greatly reduced the reservoir quality; however, primary and secondary pores are still observed within the Mangahewa sands. Moreover, well log analysis was carried out on four wells in Taranaki Basin, to run a qualitative and quantitative analysis of the Mangahewa reservoir. Eight potential reservoir zones were examined, revealing that the Mangahewa Formation has a very low shale volume of less than 6%, good effective porosity ranging between 11.0% and 13.3%, up to 36.2% of average water saturation and maximum of 69.8% average hydrocarbon saturation. In conclusion, from this comprehensive study, it can be deduced that the Mangahewa Formation possesses fair to good reservoir quality and hydrocarbon potentiality.

Pore Pressure Prediction for High-Pressure Tight Sandstone in the Huizhou Sag, Pearl River Mouth Basin, China: A Machine Learning-Based Approach

A growing number of large data sets have created challenges for the oil and gas industry in predicting reservoir parameters and assessing well productivity through efficient and cost-effective techniques. The design of drilling plans for a high-pressure tight-sand reservoir requires accurate estimations of pore pressure (Pp) and reservoir parameters. The objective of this study is to predict and compare the Pp of Huizhou Sag, Pearl River Mouth Basin, China, using conventional techniques and machine learning (ML) algorithms. We investigated the characteristics of low-permeability reservoirs by observing well-logging data sets and cores and examining thin sections under a microscope. In the reservoir zone, the average hydrocarbon saturation is 55%, and the average effective porosity is 11%. The tight sandstone reservoirs consist of fine- to extremely fine-grained argillaceous feldspathic sandstone. The mean absolute error for reservoir property prediction is 1.3%, 2.2%, and 4.8%, respectively, for effective porosity, shale volume, and water saturation. Moreover, the ML algorithm was employed to cross-check the validity of the prediction of Pp. Combining conventional and ML techniques with the core data demonstrates a correlation coefficient (R2) of 0.9587, indicating that ML techniques are the most effective in testing well data. This study shows that ML can effectively predict Pp at subsequent depths in adjacent geologically similar locations. Compared to conventional methods, a substantial data set and ML algorithms improve the precision of Pp predictions.

Revealing a deep condensate gas reservoir in an Archeozoic metamorphic buried hill by ultrasonic image logs, spectroscopy logs, and core data, Bohai Bay Basin, Eastern China

The Archeozoic metamorphic buried hill reservoir of the Bozhong Sag in the Bohai Bay Basin has a proven condensate gas reserves of over 100 billion cubic meters. A clear understanding of buried hill vertical zone variation is vital to characterize reservoir quality and reservoir distribution. Conventional core sample analysis is far more sufficient to characterize the complexity and diversity of basement reservoirs. Weathering, lithofacies, and fracture networks also play important roles in controlling the reservoir property. The fractures in the target area are unevenly developed due to multitectonic activities and stress field rotation during the initial period of basement evolution. In addition, the mineral composition of the lithofacies is complex due to metamorphism, alteration, and weathering, which further affect the reservoir quality. Characterizing the effective fracture distribution of the buried hill formation across the area is crucial to classifying good reservoir zonation. However, identifying fractures by an electrical resistivity borehole image endures high uncertainty as conductive fractures are possibly filled by clay, iron minerals (pyrite or siderite), or mud filtrate. Ultrasonic image logs are used to identify fractures in combination with amplitude and transit time images, which can clearly identify the effective or noneffective fractures. In total, four types of fractures are identified: closed fractures, fully open fractures, partially open fractures, and vuggy open fractures. Based on the lithofacies, fracture development, and volumetric minerals from spectroscopy logs and conventional logs, the Archeozoic metamorphic buried hill is revealed zone by zone: the strongly weathered zone, subweathered zone, and insider buried hill zone. The reservoir quality, lithofacies identification, multiwell fracture development analysis by three buried hill zones, and production controlling factors are fully demonstrated.

Spontaneous Imbibition and an Interface-Electrostatics-Based Model of the Transition Zone Thickness of Hydrocarbon Reservoirs and Their Theoretical Interpretations

The transition zone (TZ) of hydrocarbon reservoirs is an integral part of the hydrocarbon pool which contains a substantial fraction of the deposit, particularly in carbonate petroleum systems. Consequently, knowledge of its thickness and petrophysical properties, viz. its pore size distribution and wettability characteristic, is critical to optimizing hydrocarbon production in this zone. Using classical formation evaluation techniques, the thickness of the transition zone has been estimated, using well logging methods including resistivity and Nuclear Magnetic Resonance, among others. While hydrocarbon fluids’ accumulation in petroleum reservoirs occurs due to the migration and displacement of originally water-filled potential structural and stratigraphic traps, the development of their TZ integrates petrophysical processes that combine spontaneous capillary imbibition and wettability phenomena. In the literature, wettability phenomena have been shown to also be governed by electrostatic phenomena. Therefore, given that reservoir rocks are aggregates of minerals with ionizable surface groups that facilitate the development of an electric double layer, a definite theoretical relationship between the TZ and electrostatic theory must be feasible. Accordingly, a theoretical approach to estimating the TZ thickness, using the electrostatic theory and based on the electric double layer theory, is attractive, but this is lacking in the literature. Herein, we fill the knowledge gap by using the interfacial electrostatic theory based on the fundamental tenets of the solution to the Poisson–Boltzmann mean field theory. Accordingly, we have used an existing model of capillary rise based on free energy concepts to derive a capillary rise equation that can be used to theoretically predict observations based on the TZ thickness of different reservoir rocks, using well-established formation evaluation methods. The novelty of our work stems from the ability of the model to theoretically and accurately predict the TZ thickness of the different lithostratigraphic units of hydrocarbon reservoirs, because of the experimental accessibility of its model parameters.

Deterministic and probabilistic analysis of a multi-sliding zone reservoir bank landslide influenced by the combined effects of reservoir water level fluctuation and precipitation

Deterministic and probabilistic analysis of a multi-sliding zone reservoir bank landslide influenced by the combined effects of reservoir water level fluctuation and precipitation

Monitoring dynamics of Kyagar Glacier surge and repeated draining of Ice-dammed lake using multi-source remote sensing

Glacier surges, a primary factor contributing to various glacial hazards, has long captivated the attention of the global glaciological community. This study delves into the dynamics of Kyagar Glacier surging and the associated drainage features of its Ice-dammed lake, employing high temporal resolution optical imagery. Our findings indicate that the surge on Kyagar Glacier began in late spring and early summer of 2014 and concluded during the summer of 2016. This surge resulted in the transfer of 0.321 ± 0.012 km3 of glacier mass from the reservoir zone to the receiving zone, leading to the formation of an ice-dammed lake at the glacier's terminus. The lake experienced five outbursts between 2015 and 2019, with the largest discharge occurring in 2017. And the maximum water depth during this period was 112 ± 11 m, resulting in a water storage volume of (158.37 ± 28.32) × 106 m3. On the other hand, our analysis of the relationship between glacier surface velocity and albedo, coupled with an examination of subglacial dynamics, revealed that increased precipitation during the active phase of the Kyagar Glacier results in accumulation of mass in the upper glacier. This accumulation induces changes in basal shear stress, triggering the glacier's transition into an unstable state. Consequently, glacier deformation rates escalate, surface crevasses proliferate, potentially providing conduits for surface meltwater to infiltrate the glacier bed. This, in turn, leaded to elevated basal water pressure, initiating glacier sliding. Furthermore, we postulated that the repetitive drainage of Kyagar Ice-dammed lake was primarily influenced by the opening and closing of subglacial drainage pathways and variations in inflow volumes. Future endeavors necessitate rigorous field observations to enhance glacier surge simulations, deepening our comprehension of glacier surge mechanisms and mitigating the impact of associated glacial hazards.

Elevation-related variations of soil disintegration and its driving forces in the water level fluctuation zone of the Three Gorges Reservoir, China

Soil disintegration is a key process of the early stages of soil erosion. The water level fluctuation zone (WLFZ) in reservoirs undergoes seasonal changes involving periodic alternations in inundation and exposure that can affect soil disintegration. In this study, we conducted experiments on pre-wetted undisturbed soil samples obtained from different elevations ranging from 155 to 180 m a.s.l. and different soil depths (0–10 and 10–20 cm) in the WLFZ of the Three Gorges Reservoir (TGR), upper Yangtze River, China. We investigated how soil disintegration varies depending on elevation within the WLFZ and used principal component analysis (PCA) and stepwise regression to identify the dominant variables affecting soil disintegration. Results show the following: (1) The disintegration ratio, which is a measure of the amount of soil disintegration that occurs during experiments, varied in the range of 0.45 %–4.79 % over 60 min, with 79 %–100 % of the disintegration occurring within the first 30 min. The disintegration ratio increases with decreasing elevation in the WLFZ. Soil samples from a depth of 0–10 cm disintegrated 0.43 % less on average than those from a depth of 10–20 cm. The relationship between disintegration rate and elevation can be expressed using an exponential function with R2 > 0.80. (2) The main variables loading onto the first three principal components (explain 91.4 % of the variation in all variables) identified by PCA were those involving soil aggregates, the plant root system, and soil particle-size, respectively. (3) Stepwise regression identified >0.25 mm water-stable aggregate content (Ws0.25) and root mean diameter (Droot) as the strongest predictors of soil disintegration and yielded a linear regression equation with R2 = 0.88. Ws0.25 and Droot were the most influential variables on the elevation-dependent occurrence/degree of disintegration. Such information is essential for understanding the occurrence and prevention of bank erosion in reservoir areas.

Hydrothermal alteration mechanisms of an Archaean metamorphic buried hill and the models for reservoir zonation, Bozhong depression, Bohai Bay Basin, China

Multiple metamorphic buried-hill hydrocarbon reservoirs have been recently discovered in the Bozhong Depression, Bohai Bay Basin. Among them, the buried-hill reservoirs in the BZ13, BZ19 and BZ26 regions have the largest condensate gas reserve in eastern China. The hydrocarbon-bearing reservoir has a large thickness with a maximum of ∼1 km, making the oil-equivalent reserve exceed 5 × 108 tons. This study used petrography, isotopes, gas composition, zircon U–Pb geochronology, X-ray diffraction, electronic probe and hydrothermal dissolution experiments to discuss the formation mechanisms of the metamorphic buried-hill reservoirs. The results indicate that the reservoirs 500 m below the surface of the buried hill have been significantly altered by the mantle-derived CO2. This is supported by the carbon isotope of CO2, He isotope of the helium gas, strong negative excursions in δ18OPDB, positive excursions in δ13CPDB, Co/Ni ratio greater than 1 in pyrite and secondary characteristic minerals such as dawsonite. Hydrothermal dissolution experiments using CO2 result in the precipitation of kaolinite, ankerite and quartz cements, which is consistent with petrographic observation. The mantle-derived CO2 reacts with feldspars, hornblende, biotite and carbonates, creates secondary dissolution pores and significantly improves reservoir quality. Moreover, partial carbonate cements exhibit negative excursions in δ13CPDB and the depletion of light rare earth elements, indicating the influence of organic acids-bearing low-temperature hydrothermal fluids. Therefore, three development models illustrating the formation of metamorphic buried-hill reservoirs were proposed. This study may serve as a reference for the hydrocarbon exploration in metamorphic buried-hill reservoirs.