Deep-water Reservoirs Research Articles

Seismostratigraphic and sedimentological characterization of deepwater channel systems on the NW Borneo margin: Sediment sources and structurally-controlled routing system

• Late Miocene folding in deepwater NW Borneo influenced submarine channel development. • The Champion sediment-supply system comprised in its onshore part multiple rivers. • The paleo-Padas River was one of the primary rivers supplying lithic-rich sediment. • Variation in catchment-area geology influenced deepwater sediment composition. The northwest (NW) Borneo margin is one of the geologically most complex regions of Southeast Asia characterized by a steep slope gradient and an extensive deepwater fold and thrust belt. This area is economically important because of large-volume hydrocarbon accumulations in the deep-marine sandstones in fold and thrust anticlines. Although much attention has been paid to the distribution of deepwater reservoir rocks, the provenance of these rocks and compositional variations are yet not fully understood. In this study we present geomorphological and petrological analyses of Late Miocene deepwater channel deposits that form important reservoirs in the region. Our results reveal the presence of a northwest-trending deepwater channel system displaying morphological variations induced by syn -sedimentary structural development. Petrological analyses of core samples show a predominant lithic-rich rock composition in the reservoir sandstones, suggesting sediment supply from a lithic-rich Crocker Formation hinterland, probably through a paleo-Padas River. This rock composition differs from the quartz-rich rock composition of an adjacent contemporaneous deepwater reservoir mainly comprising reworked quartz-rich Meligan sandstones. The observed petrological differences are interpreted to reflect a Late Miocene multi-source sediment supply system that drained offshore different NW Borneo hinterland areas. Offshore, sediment transport to and across the slope was dominantly margin-perpendicular, linear or of low sinuosity, in the deepwater locally affected by faulting and folding. This study provides new insights into the complexity of sediment routing systems along continental margins, highlighting the influences of the configuration of the sediment supply system and tectonics on deepwater sedimentation.

3D facies and reservoir property prediction of deepwater turbidite sands; case study of an offshore Niger delta field

The distribution of lithofacies and key reservoir properties such as porosity, net to gross and water saturation is essential to describing reservoir heterogeneity predicted within a clastic deep water reservoir unit in the study area. A systematic analysis of core and gamma-ray wireline logs was used to identify the reservoir unit in this study. The identified reservoir unit was mapped on 3D seismic data, and four lithological and fluid sensitive seismic attributes extracted for the reservoir were fed as inputs into an unsupervised artificial neural network (UANN) analysis to define the depositional pattern of the reservoir unit. Afterwards, a 3D sand facies trend model was built using the facies depositional pattern map and upscaled properties from well data. The generated 3D sand probability model from trend modelling was subsequently used to constrain geostatistical facies modelling of the reservoir unit. The systematic stratal analysis from core, well data and the integration of seismic attributes using UANN reveal the reservoir unit as a system of turbidite lobe sands and turbidite channel levees sands. The 3D Geostatistical facies distribution reveals spatial and vertical facies heterogeneity. It indicates high sand amalgamations and static hydraulic connectivity of turbidite lobe sands in the distal part of the study area. Further geostatistical distribution of reservoir properties conditioned to facies indicated good to excellent reservoir quality within the turbidite lobe sands, moderate to low reservoir quality within turbidite channel sands and poor reservoir quality at the turbidite channel margins and the deep-water shale facies. The 3D facies and reservoir property models built in this study would ensure optimisation of the reservoir unit, enable better estimation of hydrocarbon volumes and serve as input data for flow simulation.

Concurrent 17. Presentation for: The story of Esso Australia’s push to explore the frontier Gippsland Basin with the ultra-deep water Sculpin-1 exploration well

Presented on Wednesday 18 May: Session 17 In late 2018, Esso Australia embarked on the drilling of Sculpin-1. Drilled in 2278 m of water, this is Australia’s deepest water exploration well and the first ultra-deep water well in the Gippsland Basin. Drilling of this well was the culmination of a bold exploration campaign in the VIC/P70 permit at the southeastern margin of the prolific hydrocarbon producing Gippsland Basin, which also saw the drilling of Baldfish-1 and Hairtail-1 in 2018. An east coast gas market with a high demand for additional gas resources combined with Esso Australia’s renewed technical focus on the deep and ultra-deep water sectors of the VIC/P70 exploration permit led to the identification of the Sculpin prospect, a stratigraphic lead premised on a late Cretaceous deep water reservoir system flowing into the south east Gippsland Basin depocentre from southern hinterlands. Technical analysis including integrated seismic toolkits, spectral decomposition and colour-blend imaging, rock properties and amplitude versus offset/direct hydrocarbon indicator modelling were key to Esso’s decision to test the new play with the Sculpin-1 well. Although the well did not encounter hydrocarbons, it did provide insights into reservoir quality, source and migration in the previously untested southeastern margin of the basin. To access the presentation click the link on the right. To read the full paper click here

Log evaluation, borehole image interpretation, and core calibration of deep-water reservoir rocks: A case study in the Gulf of Mexico

Log evaluation, borehole image interpretation, and core calibration of deep-water reservoir rocks: A case study in the Gulf of Mexico

The story of Esso Australia’s push to explore the frontier Gippsland Basin with the ultra-deep water Sculpin-1 exploration well

In late 2018, Esso Australia embarked on the drilling of Sculpin-1. Drilled in 2278 m of water, this is Australia’s deepest water exploration well and the first ultra-deep water well in the Gippsland Basin. Drilling of this well was the culmination of a bold exploration campaign in the VIC/P70 permit at the southeastern margin of the prolific hydrocarbon producing Gippsland Basin, which also saw the drilling of Baldfish-1 and Hairtail-1 in 2018. An east coast gas market with a high demand for additional gas resources combined with Esso Australia’s renewed technical focus on the deep and ultra-deep water sectors of the VIC/P70 exploration permit led to the identification of the Sculpin prospect, a stratigraphic lead premised on a late Cretaceous deep water reservoir system flowing into the south east Gippsland Basin depocentre from southern hinterlands. Technical analysis including integrated seismic toolkits, spectral decomposition and colour-blend imaging, rock properties and amplitude versus offset/direct hydrocarbon indicator modelling were key to Esso’s decision to test the new play with the Sculpin-1 well. Although the well did not encounter hydrocarbons, it did provide insights into reservoir quality, source and migration in the previously untested southeastern margin of the basin.

Remote sensing of column-integrated chlorophyll a in a large deep-water reservoir

The accurate assessment of total algal biomass in large deep-water reservoirs is challenging because the characteristics of thermodynamic stratification in such reservoirs differ noticeably from those of other types of inland waters. To address this challenge, we proposed a novel remote sensing model to derive column-integrated chlorophyll a (CIC), a better indicator than water surface chlorophyll a concentration (Csurf), for evaluating total algal biomass. The proposed algorithm was utilized to construct the patterns of spatiotemporal change in CIC in the Xin’anjiang Reservoir (the largest freshwater human-made reservoir in East China, located in Zhejiang Province) based on 97 cloud-free Sentinel-2 satellite images from 2016 to 2020. The results showed that the best index for developing the CIC remote sensing model was the average chlorophyll a concentration within the euphotic layer, which could be derived from Csurf and euphotic depth. The proposed model demonstrated good performance in deriving CIC in the Xin’anjiang Reservoir from Sentinel-2 images (yielding a mean absolute percent error of 20.7% and a root mean square error of 16.3 mg/m2). The values of the five-year averaged CIC and total algal biomass within the reservoir were 62.38 mg/m2 and 22.31 t, respectively. The CIC in the Xin’anjiang Reservoir displayed noticeable spatial variation, with the highest CIC appearing in the riverine zone and the lowest CIC appearing in the lacustrine zone. Clear seasonal variation in CIC was observed, with CIC exhibiting the order summer > autumn > spring > winter. Although the model was developed for a specific reservoir, we believe that this research can serve as a case study that illustrates the benefits of using CIC for assessing spatiotemporal variation in phytoplankton.

Impact of Grain-Coating Clays on Porosity Preservation in Paleocene Turbidite Channel Sandstones: Nelson Oil Field, UK Central North Sea

The Forties Sandstone Member is an important deep-water reservoir in the Central North Sea. The role of depositional characteristics, grain-coating clays, and diagenesis in controlling the reservoir quality of the sandstones is poorly understood. The main aim of the study is to understand the role of depositional characteristics, grain-coating and pore-filling clays, and diagenesis in controlling the reservoir quality evolution of turbidite-channel sandstones. The study employed a multi-disciplinary technique involving thin section petrography and scanning electron microscopy (SEM) to investigate the impact of grain size, clay matrix content, mode of occurrence of grain-coating chlorite and illite, and their impact in arresting quartz cementation and overall reservoir quality in the sandstones. Results of our study reveal that porosity evolution in the sandstones has been influenced by both primary depositional characteristics and diagenesis. Sandstones with coarser grain size and lower pore-filling clay content have the best reservoir porosity (up to 28%) compared to those with finer grain size and higher pore-filling clay content. Quartz cement volume decreases with increasing clay-coating coverage. Clay coating coverage of >40% is effective in arresting quartz cementation. Total clay volume of as low as 10% could have a deleterious impact on reservoir quality. The Forties Sandstone Member could potentially be a suitable candidate for physical and mineralogical storage of CO2. However, because of its high proportion (>20%) of chemically unstable minerals (feldspar, carbonates, and clays), their dissolution due to CO2 injection and storage could potentially increase reservoir permeability by an order of magnitude, thereby affecting the geomechanical and tensile strength of the sandstones. Therefore, an experimental study investigating the amount of CO2 to be injected (and at what pressure) is required to maintain and preserve borehole integrity. The findings of our study can be applied in other reservoirs with similar depositional environments to improve their reservoir quality prediction.

Characterization of Deep-Water Submarine Fan Reservoir Architecture: AB120 Reservoir in the Campos Basin

The exploration of deep-water submarine fan reservoirs has become a prominent research area for hydrocarbon discovery and recovery. However, the high operational costs render efficient exploration of this type of reservoir crucial, and the pivotal foundation lies in studying reservoir architectures. A case study of deep-water submarine fans in the Albacora Leste oilfield in Brazil is presented along with a characterization of the underground reservoir architecture based on well-log and seismic model fitting. A sedimentary microfacies distribution model is systematically established for each reservoir based on a detailed stratigraphic framework with a well-seismic joint characterization method and various types of data (e.g., geological, well-log, and seismic data). The results are as follows: the sedimentary microfacies of the deep-water submarine fans in the study area can be subdivided into tongue-shaped-lobe, muddy-channel, sandy-channel, and contourite deposits. Owing to data abundance and quality constraints, the architecture of the target layer in the study area was characterized based on Level 4 architectural elements. The AB120 Reservoir consisted of three complex channels (I, II, and III) and two tongue-shaped complex lobes (I and II). Complex channels I and III were the first and last to be deposited, respectively, and complex lobe I was deposited earlier than complex lobe II. Complex channels II and III supply complex lobes I and II, respectively. Vertically, complex lobe II was composed of three single lobes formed at different stages, the youngest of which was distributed over the largest area. Complex lobe II has already been drilled and is currently under development. However, complex lobe I has not yet been developed. This study has great practical significance for the effective development of deep-water submarine-fan reservoirs in this area and is of considerable theoretical significance for the advancement of deep-water sedimentology.

Exploring the predictive power of seismic geomorphology to assess sedimentary characteristics of gravity-flow deposits: examples from offshore East and West Africa reservoirs

Abstract This research seeks to test the hypothesis that the rheology of submarine gravity-flows influences the planform geometry of associated fan lobe deposits. If this control exists, seismic geomorphology has the potential to predict the sedimentary characteristics of these deposits by analysing relatively simple seismic outputs such as time structure maps and amplitude maps. This predictive power could potentially aid the identification of prospective reservoirs through automated processes of seismic interpretation. In this work, Upper Cretaceous and Neogene deep-water reservoirs from East and West Africa with porosity in excess of 20% and multi-Darcy permeability were analysed. The overall external geometry of the reservoirs was described by integrating seismic amplitude extractions, structure and isochron maps derived from high-quality 3D seismic data. The maps were generated from detailed interpretation of the top and base reservoir reflections. Core photos of the reservoirs were integrated into the seismic project and their examination allowed the identification of dominant sedimentary facies, enabling a proposal for the classification of these deposits in terms of flow processes. The research indicates that the confinement of flows exerts an obvious control on the shape of the deposits and results in sedimentary bodies with marked elongation at the seismic scale. However, analysis of the reservoirs at the core scale indicated that they are dominated by massive, poorly sorted, medium- to coarse-grained sandstones, commonly with floating oversize clasts, as well as beds with disrupted laminae and sandy conglomerates. These characteristics may indicate ‘depositional freezing’ from high-density flows with yield strength, which may also have contributed to the limited lateral spread of the deposits analysed. The research also highlights the importance of the non-unique relationship between sedimentary processes and external geometries of the resulting deposits at the seismic scale. This means that deposits with similar geometries may have markedly different sedimentary characteristics. This work highlights the increasing importance of integrating detailed seismic geomorphology studies with an accurate examination of core data to validate models of gravity flow processes and test their predictive value.

Static and Dynamic Bulk Moduli of Deepwater Reservoir Sands: Influence of Pressure and Fluid Saturation

AbstractThe static bulk modulus of unconsolidated sands is essential for predicting the in situ effective pressure to reduce drilling risks in deepwater reservoirs; however, the dynamic bulk modulus is often more broadly available from seismic or well logging data. Therefore, it is tempting to investigate the relationship between static and dynamic bulk moduli. We perform a series of ultrasonic velocity measurements on 21 deepwater reservoir sands from the Gulf of Mexico (GoM) to study the static and dynamic bulk moduli simultaneously. Both room-dry and brine-saturated ultrasonic velocities are measured under hydrostatic stress conditions to derive the dynamic bulk moduli. Under brine-saturated conditions, if the pore pressure is kept constant, the pore volume change with the confining pressure can be monitored accurately by a digital pump, which is subsequently used to estimate the static bulk modulus. The experimental results suggest that both the static and dynamic bulk moduli decrease upon pressure unloading. The pressure-dependent bulk moduli are modeled using the Hertz-Mindlin contact theory at the critical porosity and combined with the modified Hashin-Shtrikman lower bound for other porosities. The results suggest that the theoretical estimates can serve as the lower bound for the dynamic bulk modulus and the upper bound for the static bulk modulus. Under room-dry conditions, the static-to-dynamic modulus ratio decreases from a value approaching 0.8 to approximately 0.25 with decreasing differential pressure. Moreover, the effects of brine saturation on the relationship between the static and dynamic moduli are investigated using Gassmann’s equation. The brine saturation substantially reduces the difference between the static and dynamic bulk moduli, making the static-to-dynamic modulus ratio approach unity, which may be relevant to the in situ reservoir rock properties.

Experimental investigation of rheological properties of water-base mud with zinc oxide nanoparticles using response surface methodology

Special and advanced techniques at drilling and production operations are used to drill and produce hydrocarbons from different eccentric reservoirs such as deep-water reservoirs, shale gas, and shale oil necessitates. However, these advanced techniques are considered to be costly and affecting the environment. Therefore, nanotechnology has been implemented in the drilling fluids to improve the rheological properties, hydrogen concentration, and filtration rate. This study is, therefore, aimed at evaluating the properties of zinc oxide nanoparticles and their previous implementation in drilling fluids, examining the rheological properties of water base mud from atmospheric temperature to downhole condition (90 °C). The experiment was designed to investigate the effect of nanoparticles concentration and temperature variation on the rheological properties of water-based mud using response surface methodology. A rotational viscometer was used to measure viscosity, yield point and gel strength. Moreover, a Consistometer was used to heat the fluid sample to different temperatures reaching up to downhole condition (90 °C). It was observed experimentally that addition of zinc oxide nanoparticles impressively improved the rheological properties of water-base mud by 50% for viscosity and gel strength, and 80% for yield point. It also allowed them to maintain their quality at all temperatures. Furthermore, addition of higher concentrations of zinc oxide nanoparticles of more than 1 wt% (6 g) at low temperatures (below 40 °C) gave the highest and most effective results of the rheological properties. • The effects of zinc oxide nanoparticles on rheological properties of water base mud were investigated. • Regression models were developed via Central Composite Design (CCD) method. • Zinc oxide nanoparticles impressively improved viscosity and gel strength by 50% and yield point by 80%. • Addition of more than 1 wt% (6 g) of zinc oxide nanoparticles at low temperatures below 40 °C was found effective. • The developed models' accuracy was affirmed at optimum point.

Seismic resonance: Wavelet-free reflectivity retrieval via modified cepstral decomposition

Spectral decomposition is a proven tool in seismic interpretation. It helps interpreters highlight channels and map temporal bed thickness and other geologic discontinuities. In spectrally decomposed seismic data, amplitude spectra contain notch patterns. The period of these notch patterns inversely correlates to the reservoir layer’s thickness and/or its interval velocity. Additional cepstral decomposition will directly extract the bed time thickness or arrival time under simple reflectivity setups. Based on these observations, we have adopted a new workflow for reflectivity retrieval in stably phased seismic. There is no need to understand the details of the wavelet in this workflow. We find that the reflector time and its “apparent strength” can be identified in a transformed seismic resonance domain resulting from a modified cepstrum analysis. We find that linear hot spots in this domain lead to direct quantification of the reflectivity series. The timing and strength of the linear hot spots reveals reflector times and scaled reflectivity coefficients. We applied this method for thickness prediction for a target deepwater reservoir with a complex geologic setting. Because of the large reservoir thickness variations and weak impedance contrast with underlying lithology, identification of the base reservoir and therefore the reservoir thickness has been historically challenging in this field. In a blind test, the reservoir thicknesses evaluated from this method are close to the true vertical thickness found in wells.

Machine learning assisted history matching for a deepwater lobe system

High exploration costs resulting in sparse datasets and complicated geological structures in deepwater depositional systems make the reservoir characterization extremely difficult. To meet this challenge, rule-based geostatistical subsurface modeling has been developed to fill this data gap with enhanced integration of geological concepts for deepwater reservoirs based on geological rules constrained by this sparse well data. The rule-based model simulates sediment dynamics through depositional rules to calculate reservoir architecture and the associated rock properties distributions. As a result, rule-based models incorporate conceptual and qualitative information, such as temporal deposition sequence and consequent compensational stacking patterns. Integrating quantitative data, such as fluid production history, is a remaining obstacle to the broad application of rule-based subsurface models. We propose a new machine learning assisted history matching workflow for rule-based models. First, multiple rule-based models are calculated as training data for a Generative Adversarial Network (GAN). The successfully trained GAN enables exploration of the latent reservoir manifold with only a small number of numeric values (i.e., latent random vector); this is a massive reduction in the dimensionality of potential subsurface models. The initial ensemble models, generated by the trained GAN with a range of latent random vectors, are coupled with physics-based reservoir flow simulation to obtain production forecasts. Ensemble Kalman filter (EnKF) updates the latent random vectors of the ensemble by minimizing the misfit (i.e., error norm) between the production forecasts and the production observation over time. Our proposed workflow finds an ensemble of optimized reservoir models that honor realistic geological heterogeneity and production history. Besides, as the GAN's latent random vectors are Gaussian distributed, one of the fundamental mathematical assumptions of EnKF, this workflow effectively alleviates possible artifacts (e.g., filter divergence or overshooting) in EnKF. Moreover, this proposed workflow is more computationally efficient than updating the entire high-dimensional reservoir properties and removes the limitation to only Gaussian simulation models with their limited geological realism. The proposed workflow may be expanded to various reservoir depositional settings. • We propose a machine learning assisted history matching workflow for complex deepwater subsurface models. • We apply generative adversarial networks (GAN) to represent the complex subsurface models by latent random vectors. • We update latent random vectors of the subsurface model to match the production history by the ensemble Kalman filter (EnKF). • By combining GAN and EnKF, we can 1) enhance computational efficiency, 2) remove artifacts, and 3) preserve geologic realism.

Taming the Thunder Horse With Axes and Vectors

The Thunder Horse Field targets Middle Miocene deepwater turbiditic reservoirs. Despite being prolific, the mapping of the ~180 m thick, partly amalgamated reservoir sandstones is challenging. Seismic quality is reduced by the presence of salt structures. The salt overburden and high formation pressure require the use of heavy mud weights and oil-based drilling fluids, which limit the resolution and interpretation potential of borehole image logs (BHI). Halokinetic movements caused significant post-depositional deformation of the already complex gravity-driven sediment stack, and the reservoir beds drape against an E-W oriented salt wall. Consequently, the assessment and removal of the structural dip component are not trivial, and the evaluation of paleo-transport directions is considerably more complicated compared to undisturbed deepwater reservoirs. This paper illustrates the potential of eigenvector methods to BHI from Ruehlicke et al. (2019) for reconstructing the depositional slope and the architecture of mass transport complexes in the case of chaotic depositional settings and uncertain structural dip. Figures from Henry et al. (2018) are used wherein part axial analysis was performed on data from a group of Thunder Horse wells and presented in more detail.

Seismic-informed carbonate shelf-to-basin transition in deeply buried Cambrian strata, Tarim Basin, China

We develop an enhanced interpretation workflow to improve facies interpretation in ultradeep (5000–8000 m), high-velocity (6000–7000 m/s) carbonate rocks using low-frequency (as low as 18 Hz) seismic data. Our study area includes two seismic surveys north and south of the Cambrian platform margin that extends more than 500 km in strike direction. Relative impedance (−90° data) and frequency fusion are applied to predict impedance and lithology in carbonate platform-basin architecture. Outcrop seismic modeling clarifies the facies architecture of subtle subseismic clinoforms. Stratal slices are made for seismic geomorphology. We recognized eight seismic sequences and seven seismic facies in 1200-m prograding sediments. As a strongly prograding accretionary margin, the stratal relationships and transitions from platform interior to shelf margin to slope and basin are consistent throughout the basin. Platform interior is characterized by intertidal, supratidal, and salt flat facies; the shelf margin develops microbial reef and shoal facies; the slope and basin facies are dominantly gravity flow and deepwater sediments. The northern and southern areas of the margin, however, are characterized by different slope-to-basin geomorphology and environments. In the north, the shelf edge was cemented, with little reworking of shallow-water sediments to the slope and basin. Gravity failure–related facies, such as gullies and slump aprons, dominate in the slope. No turbidite fans are found in the sediment-starved deep basin. In the south, a series of submarine channels transported shallow sediments long distance to form slope fans and basin-floor fans. Turbidite sediments contain a significant amount of low-impedance sediments from the shelf edge. The syndepositional tectonic trench faults, by initiating channels, are hypothesized to be a new mechanism for sediment transport in a deepwater environment. Such a mechanism provides a clue to help researchers recognize and map deepwater reservoir facies in the basin.

Addressing the root cause of calcite precipitation that leads to energy loss in geothermal systems

Geothermal sources are an essential energy resource in many cities around the world, in which a deep hot water reservoir has two energetic bases: thermal and hydraulic. The use of this energy can produce both electricity and heat. A geothermal well is a way in which the energy flows from the reservoir to the surface. A known operational problem can directly consume hydraulic energy and indirectly consume thermal energy. Calcium carbonate (CaCO3) precipitation and scaling can reduce and even block the flow area of a geothermal well, deferring, degrading, or even closing the production of water, steam, heat, and power. CO2 degassing, which is intrinsic to the deepwater system, is often the cause of this problem. This relationship is evident in the chemical balance: Ca2+(aq) + 2 HCO3–(aq) ⇆ H2O + CaCO3(s) + CO2(aq). With the head loss provided by the water flow, CO2(aq) changes to the gaseous state (CO2(aq) → CO2(g)), which disfavors the CaCO3 dissolution and implies its mineralization. The CO2(aq) closely links the two mentioned chemical reactions and makes them mutually dependent. Therefore, two causes favor the CaCO3 precipitation: changes in thermodynamic conditions (pressure and temperature) and CO2 exsolution. It takes place even if the hot water is not boiling or flashing. This paper aims to quantify these two causes by applying our techniques in some geothermal fields in different locations in the world. In all scenarios investigated, the release of CO2 contributes from 66.1% to 92.6% of the CaCO3 precipitation. These results can lead engineers to adequately remedy the problem and suitably design and operate a geothermal system as a whole.

Seismic interpretation workflows for deep-water systems: A practical guide for the subsurface

Effective exploration, appraisal and development of deep-water reservoirs requires systematic mapping, description and characterization of the depositional systems that host them. Success in these areas requires that there is a firm understanding of hydrocarbon volumes and uncertainties, facies distributions to locate exploration and appraisal wells, and a framework to build reservoir models. We present a practical seismic interpretation workflow methodology which will be applicable in most cases and with most seismic data sets. The workflows are for channels, sheets and mass transport deposits (MTDs). Each step of the workflow is illustrated with subsurface and/or outcrop examples. The channels workflow guides the interpreter through identification of three hierarchical levels of channel architecture and the recognition of six key seismic facies: channel axis; channel margin; basal lags; slumps; thin-bedded facies (internal levees and terraces); and external levees. The sheet sandstone workflow emphasizes a series of steps including: understanding the regional and local context; understanding the evolution of sea-floor topography at basinal and depositional scales; and generating detailed amplitude maps. Mass transport complexes are rarely reservoirs but can have a major impact on reservoir distribution. The workflow encourages enquiries into these impacts which include: potential erosion of underlying reservoirs; ponding on and around the MTD; diversion and deflection of subsequent reservoir-hosting depositional systems; and their potential as sealing facies. The workflows are designed to be efficient and practical, and will provide an appropriate characterization encompassing exploration and appraisal to early development phases.

Microbial community coalescence: does it matter in the Three Gorges Reservoir?

The microbial community coalescence describes the mixing of microbial communities and the merging of their surrounding environments. Despite its prevalence in natural ecosystems and its potential influence on ecological processes, little research focused on the extent of coalescence between aquatic microbial communities. In this research, we analyzed the microbial communities in the Three Gorges Reservoir, a typical deep-water reservoir on the Yangtze River. The biogeography and the coalescence of microbial communities in water and sediment were illustrated and analyzed based on 16S rRNA gene amplicon sequence variants (ASVs). Differences in composition and diversity were identified between microbial communities in water and sediment, and microbial communities in sediment were more diverse than those in water. Between adjacent communities, by calculating the proportion of overlapped taxa, adopting the SourceTracker algorithm, and quantifying the connectivity of microbial cohesion, we found that the extent of intra-medium coalescence was strong (19.8%) and inter-media coalescence was faint (0.2%). 50 keystone species were selected using the cohesion metric. They displayed a stronger coalescence extent than average, and formed an accumulating pattern from upstream to downstream in the Three Gorges Reservoir, exhibiting their importance in the ecological network. Potential influencing factors of microbial community coalescence in aquatic environments were discussed, including environmental conditions, types of habitats, suspended particles in water, and microscale microbial activities. To summarize, this research depicted the coalescence of microbial communities in a deep-water reservoir and emphasized its ecological importance. We anticipate more attention and further research on the processes of microbial coalescence in the aquatic environment, which might provide new insights into turnover of microbial keystone species and changes in aquatic ecological conditions.

Evolution of a Late Miocene Deep-Water Depositional System in the Southern Taranaki Basin, New Zealand

A long-standing problem in the understanding of deep-water turbidite reservoirs relates to how the three-dimensional evolution of deep-water channel systems evolve in response to channel filling on spatiotemporal scales, and how depositional environments affect channel architecture. The 3-D structure and temporal evolution of late Miocene deep-water channel complexes in the southern Taranaki Basin, New Zealand is investigated, and the geometry, distribution, and stacking patterns of the channel complexes are analyzed. Two recently acquired 3-D seismic datasets, the Pipeline-3D (proximal) and Hector-3D (distal) are analyzed. These surveys provide detailed imaging of late Miocene deep-water channel systems, allowing for the assessment of the intricate geometry and seismic geomorphology of the systems. Seismic attributes resolve the channel bodies and the associated architectural elements. Spectral decomposition, amplitude curvature, and coherence attributes reveal NW-trending straight to low-sinuosity channels and less prominent NE-trending high-sinuosity feeder channels. Stratal slices across the seismic datasets better characterize the architectural elements. The mapped turbidite systems transition from low-sinuosity to meandering high-sinuosity patterns, likely caused by a change in the shelf-slope gradient due to localized structural relief. Stacking facies patterns within the channel systems reveal the temporal variation from a depositional environment characterized by sediment bypass to vertically aggrading channel systems.

Deep-water reservoir distribution on a salt-influenced slope, Santos Basin, offshore Brazil

Studies of near-seabed datasets show that salt tectonics controls the distribution and architecture of deep-water reservoirs in many salt-influenced basins. It is typically difficult, however, to study the distribution and stratigraphic evolution of depositional systems preserved at deeper, economically significant depths, reflecting poor seismic imaging of steeply dipping strata flanking high-relief salt structures. 3D seismic and borehole data from the Santos Basin, offshore Brazil allow us to identify a range of depositional elements that form the building blocks of three main tectono-stratigraphic phases. During the first phase, channel systems and lobes were confined within updip minibasins and to the hangingwalls of salt-detached faults. During the second phase, channel systems and lobes filled updip minibasins to bypass sediment downslope, with coarse clastic deposition then occurring in downdip minibasins, >100 km from the coeval shelf margin. Syndepositional seafloor relief caused: (i) channel system deflection and diversion around salt-cored highs; (ii) channel system uplift and rotation on the flanks of rising salt structures; (iii) lateral and frontal confinement of channel systems. During the final phase, rising salt walls dissected previously deposited deep-water systems, with MTCs deposition becoming increasingly important. Our results have important implications for post-salt prospectivity in the Santos Basin and other salt-influenced sedimentary basins, with a range of reservoirs and trapping styles present in this underexplored interval. More specifically, we show that large volumes of clastic sediment were not trapped behind the ‘Albian Gap’, a salt-controlled depocenter dominating the north-western basin margin, but were instead delivered further basinward.