Gert Jan Weltje

Prediction of Geochemical Composition from XRF Core Scanner Data: A New Multivariate Approach Including Automatic Selection of Calibration Samples and Quantification of Uncertainties

A multivariate log-ratio calibration (MLC) model for XRF-core-scanning devices is presented, based on a combination of basic XRF-spectrometry theory and principles of compositional data analysis. The performance of the MLC model is evaluated in comparison with other empirical calibration procedures for XRF core scanners data using two data sets acquired with two different XRF core scanner. The quality of calibration models is assessed by calculating the uncertainties associated with predicted concentrations using cross-validation techniques. Results show that (1) the commonly used direct linear calibration (DLC) methods, which are based on the questionable assumption of a unique linear relation between intensities and concentrations and do not acknowledge the compositional nature of the calibration problem, give poor results; (2) the univariate log-ratio calibration (ULC) model, which is consistent with the compositional nature of the calibration problem but does not fully incorporate absorption and enhancement effects on intensities, and permits estimation of “relative” concentrations only, is markedly better, and (3) the MLC algorithm introduced in this contribution, which incorporates measurement uncertainties, accommodates absorption and enhancement effects on intensities, and exploits the covariance between and among intensities and concentrations, is the best by far. The predictive power of the MLC model may be further increased by employing automatic sample selection based on the multivariate geometry of intensity measurements in log-ratio space. The precision attained by MLC in conjunction with automatic sample selection is comparable to that attained by conventional XRF analysis of heterogenous materials under laboratory conditions. A solution to the long-standing problem of XRF core scanner calibration implies that high-resolution records of sediment composition with associated uncertainties can now be routinely established, which should increase the range of quantitative applications of XRF-core-scanning devices and strengthen inferences based on analysis of geochemical proxies.

The life cycle of crevasse splays as a key mechanism in the aggradation of alluvial ridges and river avulsion: Life cycle of crevasse splays in aggrading and avulsion

Accommodation space in the unconfined distal part of low-gradient fluvial fans facilitates abundant floodplain deposition. Here, the development of crevasse splays plays a key role in the aggradation of alluvial ridges and subsequent river avulsion. This study presents an analysis of different stages in the evolution of crevasse splays based on observations made in the modern-day Río Colorado dryland fluvial fan fringing the endorheic Altiplano Basin in Bolivia. A generic life cycle is proposed in which crevassesplay channels adjust towards a graded equilibrium profile with their lower-lying distal termini acting as a local base level. Initial development is dominantly controlled by the outflow of floodwater, promoting erosion near the crevasse apex and deposition towards the splay fringes. When proximal incision advances to below the maximum level of floodplain inundation, return flow occurs during the waning stage of flooding. This floodwater reflux leads to a temporary repositioning of the local base level to the deeper trunk-channel thalweg at the apex of the crevasse-splay channels. The resultant decrease in the floodplainward gradient of these channels ultimately leads to backfilling and abandonment of the crevasse splay, leaving a subtle local elevation of the floodplain. Consecutive splays form an alluvial ridge through lateral amalgamation and subsequent vertical stacking, which is mirrored by the aggradation of their parent channel floor. As this alluvial ridge becomes increasingly perched above the surrounding floodplain, splay equilibration may cause incision of the levee crevasse down to or below its trunk channel thalweg, leading to an avulsion. The mechanisms proposed in this study are relevant to fluvial settings promoting progradational avulsions. The relatively rapid accumulation rate and high preservation potential of crevasse splays in this setting makes them an important constituent of the resultant fluvial stratigraphy, amongst which are hydrocarbon-bearing successions. Copyright

High-Resolution Reservoir Architecture Modelling of Thin-bedded Fluvial Terminal-splay Sandstone in the Rotliegend Feather-edge Area

Summary Rotliegend aeolian and fluvial sandstones are prolific gas reservoirs throughout the Southern Permian Basin (SPB). Exploration and research efforts concentrated on the sand-dominated southern flank of the SPB, where the major gas fields are located. The reservoir potential of the Rotliegend feather edge in the central part of the SPB, i.e. the area where the south-derived sands pinch out towards the Silver Pit salt lake, has long been underestimated. The sand-starved, claystone-dominated and evaporite-bearing lithology in the central part of the basin hampered the detectability of thin-bedded, potential reservoir sandstones with conventional well logs. The present study uses high-resolution well correlations in a sequence-stratigraphic framework. The employed methodology is a combination of pattern analysis (trends and trend changes) in gamma-ray (GR) logs, and maximum entropy-based (MEM-based) spectral trend curves of GR to graphically evaluate the validity of the pattern analysis. This allowed for the correlation of at least twelve individual thin-bedded sheets (1–2m thick) a 20–25m thick reservoir interval over distances larger than 20km. Core analysis has identified the sheets as unconfined fluvial terminal-splay sandstone. The facies characteristics and sedimentary architecture were corroborated in an outcrop analogue study of a present-day river system in the Altiplano Basin (Bolivia).

Process-based Modelling of Sediment Distribution in Fluvial Crevasse Splays Validated by Outcrop Data

Exploitation of unconventional resources could prolong the gas production in the North Sea. Low-net-to-gross fluvial intervals may have tough-gas reservoir potential in thin-bedded crevasse splays.To assess economic risks associated to the development of these reservoirs, a numerical model can help to predict the sediment distribution. To this end, simulations were conducted with Delft3D process-based modelling software. Input parameters and the validation data sets for these models are derived from outcrop studies in the present-day Rio Colorado fluvial system in the Altiplano Basin, Bolivia. The grain-size trends of the simulated surface sediments for a single flood event show a trend which is consistent with the validation data. For example, grain-size decreases with increasing distance from the channel, which is in line with the physical concept of decreasing sediment size for decreasing flow energy. This shows that numerical models can be used to support sediment trends and depositional mechanisms of a crevasse splay. The combination of numerical models and discrete field data provides a solid case for sediment distribution predications. However, simulations still have a limited accuracy.

Floodplain Aggradation as a Dominant Control on Autocyclic Switching of Low-gradient Dryland Rivers in Endorheic Basins

Floodplain shales are abundant in the distal part of dryland fluvial fans fringing endorheic basins, such as the Rio Colorado fluvial system in Bolivia. In a low-gradient coastal plain, lowstand progradation of a fluvial system creates accommodation space. Long periods of low-flow stage only support a single channel, decreasing in capacity downstream. Flooding during short episodes of peak runoff results in massive overbank deposition and aggradation of the floodplain around the active channel. Assuming a constant flow capacity, this is mirrored by an elevation of the channel floor, combining to form an elongated lobe of up to several kilometres in width. Distributary channels within crevasse splays develop reflux phenomena, effectively decreasing the flow capacity of their parent channel. This, combined with the decreased gradient of the active channel profile, causes an upstream increase in avulsion proneness and is a dominant control on the autocyclic switching of channel belts in a process of large-scale compensational stacking. The resulting stratigraphy is heterogeneous and may have tough gas reservoir potential.

High-resolution Reservoir Architecture Modelling of Crevasse Splay Deposits in Low-net-to-gross Fluvial Stratigraphy

Thin-bedded crevasse splays in low-net-to-gross fluvial stratigraphy were previously not considered as potential reservoir targets. This paper focuses on the construction of a high-resolution static reservoir-architecture model of crevasse splay deposits associated with meandering rivers on the low-gradient coastal plain of endorheic basins. Outcrops of the Miocene Huesca fluvial fan (Ebro Basin, Spain) display low-net-to-gross fluvial stratigraphy, bounded by two large sand-prone channel belts. A static model of the study area is constructed following a sequential macro-to-micro approach. The model shows connectivity between the channel belts through crevasse splays. Connections between separate crevasse splays are present through incisions of younger crevasse splays and channels. Without the connectivity between the channel belts, connectivity would still be present in vertical wells through the extensive crevasse splays originating from the channel belts. This makes the model suitable for reservoirs with less connections as well. Ongoing research on process-based modelling of crevasse splays will yield an improved understanding of the grain-size distribution and can be used to populate the static model. The model will be upscaled to allow for fluid flow simulations in which several production mechanisms will be evaluated in order to assess the economic potential of these secondary tough gas reservoirs.

Improved subsurface property prediction in the Netherlands by integrating stratigraphic forward modelling

Classic geological reservoir characterisation relies on interpolation of high resolution well data with (at best) low resolution seismic derived data. In order to fill the data gap (e.g. in labyrinthine type fluvial deposits) we present a methodology to integrate basin scale information in reservoir scale static models by calibrating output from a Stratigraphic Forward Model (SFM). This project showcases the applicability of the integrated workflow to improve facies and property prediction at different scales. By calibrating the parameterized data from the SFM to independent constraints such as thicknesses from seismic interpretations and well logs the model greatly improve property prediction. Previous studies showed the application to synthetic datasets, this study aims to apply the methodology to the Holocene Rhine-Meuse fluvial deposits in the shallow subsurface of the Netherlands. The extraordinary level of detail in the model of these deposits and the parameterized fluvial sedimentation routine in the SFM used provide an ideal test case for the workflow proposed. The ultimate application of the workflow is intended to improve the geological and property models at greater depth where data coverage is limited. © 2016, European Association of Geoscientists and Engineers, EAGE. All rights reserved.

Reservoir potential of thin-bedded sandstone in continental mudrock successions - The search for hidden treasures

The subsurface of the West European gas province contains up to several hundred meters thick continuous Upper Rotliegend and Lower Triassic mud rock sequences which have to date been labelled as nonreservoir ‘waste zone’. The mud rock formed as fluvial floodplain deposits in a semi-arid climate. The sequences contain thin-bedded porous and permeable sandstone beds of crevasse-splay origin. A core study of Triassic deposits in the West Netherlands Basin shows that the sandstone beds are nested in up to 2-m-thick sand-prone heterolithic stacks with net-to-gross up to 0.5. Because of the heterolithic nature and small bed thickness the sand-prone intervals are not detected by the gamma-ray log. An outcrop analogue study of fluvial sediments in the Altiplano Basin of Bolivia shows that the crevasse-splay sediments amalgamate laterally to extensive sand sheets with surface areas of several square kilometres. Combination of the core and outcrop study suggests that the nested sandstone beds may constitute secondary plays with economically interesting gas reservoir volumes that may help postpone the end of field-life in mature production areas.

Reservoir modelling of Lower Cretaceous West Netherland Basin aquifers for geothermal energy production

This project aims to predict Lower Cretaceous reservoir architecture and reservoir properties of the graben blocks in the West Netherlands Basin for low enthalpy district heating geothermal energy. Horst and pop-up structures in the study area were targets of oil and gas production in the last 60 years. For the recent upcoming geothermal energy production the focus lays on the deeper and warmer graben structures in between the oil and gas fields. Reservoir property predictions like thickness and permeability are currently based on interpolations between oil and gas well measurements on horst and pop-up structures. In order to successfully produce from the current 45 geothermal licences in the province of Zuid-Holland, detailed reservoir models and associated uncertainty maps of the Lower Cretaceous sandstones are required. Goals of the project are to model the reservoir architecture in order to simulate production and determine optimal well placement of geothermal doublets and predict possible doublet interference. Reservoir architecture of these sandstones will be studied by re-evaluating the existing lithostratigraphically based well log correlations, in combination with seismic interpretation and core studies. A palinspastic reconstruction is carried out on a cross section to indicate the paleotopography and the complex reservoir architecture.