Delineating Depositional Environment through Lithostratigraphy and 2D Sequence Stratigraphy of a Typical Ramp Succession: In the Obom Field Niger Delta

Modelling the most appropriate depositional environment is essential in the reservoir characterisation and 3D modelling of oil bearing sands and the integration of various workflows reduces the uncertainty in deciding the appropriate depositional model which serves as a precursor into petrophysical property distribution during 3D modelling. This paper elaborates a robust study of the integration facies analysis, 2D sequence Stratigraphy and biostratigraphy data in depicting the environment of deposition of the OBOM field. The lithological description of the G8 to the F5 reservoirs ranged from finning upward sequence and blocky shaped sequence as channel sands and coursing upward shoreface deposits. Mineralogical descriptions of the penetrated sands were also carried out, especially on the F5 reservoir in which the presence of radioactive minerals was decisive to constrain the depositional environment to lower shoreface. In the Sequence stratigraphic analysis two 3rd Order depositional cycles was identified from top to bottom in the field. This is substantiated by the facies trend, facies cross plot and cycles indicators like maximum flooding surfaces identified by regional marker shales, biofacies population and biodiversity charts and sequence stratigraphic methods like sequence thickness, bed stacking patterns and facies depositional patterns with regards sea level change. It was noticed that reservoir thickness reduces from the bottom to the top with the proximal channel sands in deep intervals gradually overlain by distal upper shoreface sands and lower shoreface sands at the shallower intervals. The gross depositional environment was a transgressive marine settings ranging from the lower shoreface and channelized upper shoreface deposits. The results from the integration of facies analysis, biofacies, seismic analysis and sequence stratigraphy results reduces uncertainty in depositional environment models.

The impact of compaction of clastic sediments on fluvial-dominated delta morphology

<p>Only a limited amount of data is available to quantify the impact of syn-depositional compaction on delta depositional patterns. In this study, we investigate numerically how different scenarios for compaction rate (0 - 10 mm yr<sup>-1</sup>) drives morphological variations in mud- and sand-rich fluvial-dominated deltas. To do this, a 1D grain-size dependent compaction model was implemented into the open-source Delft3D. This implementation allows deposited sediment to decrease in thickness over time due to the accumulation of newly deposited sediments above. The resultant sedimentary deposits of a prograding delta are post-processed to highlight the changes in depositional patterns under different compaction scenarios. Deposits are classified into sub-environment (e.g., delta top, delta front, and pro delta). The delta top geometry (e.g., area, shape, and rugosity) and the distribution of sediment between different sub-environments are compared. The modeling results verify that the larger compaction-induced subsidence affects accommodation provision. We show that this results in more significant sediment deposition and more evenly distributed sediment across the delta top. Larger compaction results in a smaller area with a more semi-circular shape and less rugose delta top. The modeling results presented here bridges the knowledge gap on the effects of syn-depositional compaction on delta morphology evolution.</p>

Subsurface heterogeneity patterns that emerge from interacting depositional processes

<p>Emergent structures define organizational patterns that spontaneously develop due to interactions between component properties or behaviors of complex dynamic systems, rather than being a simple compilation of the individual parts observed within the system at any one time. Traditional facies models used to predict subsurface lithic variations focus on defining the distribution of depositional environments on Earth’s surface and relating the hierarchy of preserved bedding units to different scales of surficial bedforms. It is increasingly recognized that such static models fail to predict the geometry and character of many types of preserved lithic bodies and discontinuity surfaces unless these observations are placed within the context of the overall evolving system. Numerical depositional process models are presented to show links between evolving depositional patterns and preserved facies patterns within different settings.</p><p>     Channel deposit internal variations tend not to be channel shaped, but rather sweet spots within the deposit resemble a string of beads, each formed as individual channel segments meander. Mouth bar deposits generally do not to have the circular to elliptical shape of a modern channel-mouth bedform, but rather tend to be more elongate fingers cut by a diachronous channel filled as river flows are choked off by loss of gradient during progradation. Although the final channel basal erosion surface appears continuous, timelines cross this surface along the length of the deposit. Deltaic shorelines that look identical at a given time preserve very different deposits when the feeding river avulses at different frequency, a condition that can change within an individual deposit formed alternately during periods of sea level rise and fall. Even major stratigraphic surfaces, like lowstand fluvial incision surfaces and wave-ravined falling stage and transgressive surfaces, are likely to gradually emerge from the migration of localized areas of erosion that were never as extensive at any one time as the preserved surface. Such surfaces may be regionally diachronous, with deposits of the same age locally preserved variably above and below the surface. Understanding emergent lithic bodies and internal heterogeneity patterns are fundamental to understanding how deposition is recorded in the rock record and for facies models used to predict how subsurface fluids move through shallow marine deposits.</p>

Combining process-based models and multiple point geostatistics for improved reservoir modelling

The construction of subsurface reservoir models is typically aided by the use of outcrops and modern analogue systems. We show how process- based models of depositional systems help develop and substantiate reservoir architectural concepts. Process-based models can simulate assumptions relating to the physical processes influencing sedimentary deposition, accumulation and erosion on the resultant 3D sediment distribution. In this manner, a complete suite of analogue geometries can be produced by implementing different sets of boundary conditions based on hypotheses of depositional controls. Simulations are therefore not driven by a desired/ defined outcome in the depositional patterns, but their application to date in reservoir modelling workflows has been limited because they cannot be conditioned to data such as well logs or seismic information.In this study a reservoir modelling methodology is presented that addresses this problem using a two-step approach: process-based models producing 3D sediment distributions, which are subsequently used to generate training images for multi-point geostatistics.The approach has been tested on a dataset derived from a well-exposed outcrop from central Utah. The Ferron Sandstone Member includes a shallow marine deltaic interval that has been digitally mapped using a high resolution Unmanned Aerial Vehicle (UAV) survey in 3D to produce a virtual outcrop (VO). The VO was used as the basis to build a semi-deterministic outcrop reference model against which to compare the results of the combined process/Multiple Point Statistics (MPS) geostatistical realizations. Models were compared statically and dynamically by flow simulation.When used with a dense well dataset, the MPS realizations struggle to account for high levels of non-stationarity inherent in the depositional system that are captured in the process-based training image. When trends are extracted from the outcrop analogue and used to condition the simulation, the geologically realistic geometries and spatial relationships from the process-based models are directly imparted onto the modelling domain, whilst simultaneously allowing the facies models to be conditioned to subsurface data.When sense-checked against preserved analogues, this approach reproduces more realistic architectures than traditional, more stochastic techniques.

The Influence of Confining Topography Orientation on Experimental Turbidity Currents and Geological Implications

Turbidity currents distribute sediment across the seafloor, forming important archives of tectonic and climatic change on the Earth’s surface. Turbidity current deposition is affected by seafloor topography, therefore understanding the interaction of turbidity currents with topography increases our ability to interpret tectonic and climatic change from the stratigraphic record. Here, using Shields-scaled physical models of turbidity currents, we aim to better constrain the effect of confining topography on turbidity current deposition and erosion. The subaqueous topography consists of an erodible barrier orientated 1) parallel, 2) oblique and 3) perpendicular to the incoming flow. An unconfined control run generated a supercritical turbidity current that decelerated across the slope, forming a lobate deposit that thickened basinwards before abruptly thinning. Flow-parallel confinement resulted in erosion of the barrier by the flow, enhanced axial velocities, and generated a deposit that extended farther into the basin than when unconfined. Oblique confinement caused partial deflection and acceleration of the flow along the barrier, which resulted in a deposit that bifurcated around the barrier. Forced deceleration at the barrier resulted in thickened deposition on the slope. Frontal confinement resulted in onlap and lateral spreading at the barrier, along with erosion of the barrier and down-dip overspill that formed a deposit deeper in the basin. Acceleration down the back of the barrier by this overspill resulted in the generation of a plunge-pool at the foot of the barrier as the flow impacted the slope substrate. Observations from ancient and modern turbidity current systems can be explained by our physical models, such as: the deposition of thick sandstones upstream of topography, the deposition of thin sandstones high on confining slopes, and the complex variety of stacking patterns produced by confinement. These models also highlight the impact of flow criticality on confined turbidity currents, with topographically-forced transitions between supercritical and subcritical flow conditions suggested to impact the depositional patterns of these flows.

On the Fringe: Sheepdogs and Their Status Within Bronze Age Ontologies in Scandinavia

This contribution draws mainly on images of dogs, humans and sheep from Nordic Bronze Age rock art sources, but living arrangements within the household and depositional patterns of dog bones on settlements are also considered to extrapolate an understanding of the physical reality and ontological role of sheepdogs within the social aspects of the practice of herding. I use theories from the interdisciplinary field of human-animal studies to understand how socialisation, habituation and trust create a seamless choreography between human, dog and sheep.

Depositional patterns constrained by slope topography changes on seamounts

Slope topography is known to control the spatial distribution of deposits on intraplate seamounts; however, relatively little is known about how slope topography changes constrain those depositional patterns. In this study, we analyse data on four lithotypes found on seamount slopes, including colloidal chemical deposits comprising mainly cobalt-rich crusts, and examine the relationships between the spatial distribution of these lithotypes and current slope topography. We use these relationships to discuss depositional patterns constrained by slope topography changes. Some depositional units in drill core samples are interpreted to have resulted from past topographic changes that created the current slope topography. Two or more types of deposits that accumulated at the same location implies that the slope topography changed over time and that the depositional patterns on seamount slopes are constrained by changes in slope topography.