Julien Moreau

Glacial sedimentology interpretation from borehole image log: Example from the Late Ordovician deposits, Murzuq Basin (Libya)

AbstractIn the Murzuq Basin, the Late Ordovician glaciogenic succession forms a very complex clastic reservoir system. Although the structural setting is simple, the architecture of the stratigraphic succession is particularly intricate, and conventional wireline logs display rather homogeneous signatures. However, when exposed, the glaciogenic sedimentary succession indicates a very large range of depositional environments and clear stratigraphic changes. Based on high-quality processing and interpretation of wireline microresistivity image logs over a single well, our method allows the precise recognition of the internal sedimentary structures and supports the interpretation of the depositional environments within the Late Ordovician succession. During interpretation, it is possible to draw a descriptive sedimentological log, similar to a standard log from cores or outcrops. The image log is interpreted like a regular sedimentary log and compared to an outcrop analog from the nearby outcrop area of Ghat...

Relationships between basin architecture, basin closure, and occurrence of sulphide-bearing schists: an example from Tampere Schist Belt, Finland

We present field observations from the Palaeoproterozoic volcano-sedimentary Tampere palaeobasin, where the primary structures have been exceptionally well preserved. We use the observations to construct a new tectonic model for the southeastern margin of the Tampere basin during its inversion and subsequent closure. The observed volcano-sedimentary and structural features suggest a change in the local structural style from thick-skinned inversion to thin-skinned thrusting, to accommodate the crustal shortening during basin closure. Furthermore, it is suggested that there is a genetic relationship between the interpreted palaeothrust and the sulphide-bearing schist horizons in the study area. On a more general note, the results imply that currently subvertical mineralized shear zones may have originally been gently dipping, further suggesting that the mineralized fluids may not necessarily have been sourced from great depths (i.e. from deep within the basement).

Discovery of Lower Cretaceous hydrothermal vent complexes in a late rifting setting, southern North Sea: insights from 3D imaging

We present data for 31 kilometre-scale hydrothermal vent complexes and associated sand remobilization that have formed as a consequence of magmatic intrusions within salt strata (southern North Sea, Dutch Sector). This study uses 3D seismic data tied to three wells and artificial neural networks to describe conical and cylindrical vent structures located in the NE part of the Broad Fourteens Basin. The structures are composites of 58 subvertical pipes that form complexes, and are 951 ± 364 m in height and 487 ± 426 m in diameter. Interactions of magma and fluids at depth triggered successive fluid pressure build-ups in Triassic sandstones, resulting in upward-propagated ruptures, followed by the collapse of overlying deposits and the injection and extrusion of fluidized sand. The relationship between vent tops and their overlying deposits indicates a Mid-Hauterivian age, unusual for volcanic activity in the North Sea area. This event could have a significant impact on hydrocarbon exploration, as the hydrothermal vent complexes bypass a regional source-rock and seal, offering new migration routes and connectivity between potential reservoirs. The intrusions also raise questions about the influence of transform tectonics on the focusing of volcanic activity during the late rifting phase of the North Sea. Supplementary material: Tables of seismic attributes and the geometry of the vent complexes as well as a 3D visualisation of the vents are available at https://doi.org/10.6084/m9.figshare.c.3571593

Early diagenetic evolution of the Chalk in eastern Denmark

The genesis of polygonal faults is an intriguing diagenetic phenomenon. This study discusses their origin in carbonate mudstones together with other associated diagenetic features. In the eastern Danish Basin, at the fringe of the Baltic Sea, the Stevns peninsula offers a unique opportunity to study the early diagenesis of Upper Cretaceous Chalk deposits, buried between 500 m and 1400 m. This paper combines data from onshore and offshore high‐resolution seismic reflection profiles, a fully cored borehole with high‐resolution wireline logs and quarry and coastal cliff outcrops to study early diagenetic features at different scales. Chalk is affected by an extensive polygonal fault system that is detected in onshore and offshore seismic data. Outcrop and core data provide a better understanding of the distribution of contraction‐related features like deformation bands (hairline fractures), stylolites and fluid escape structures. An original model of genetic relationships between these different diagenetic processes is documented for Chalk. The spatial relationships between stylolites and fractures suggest that pressure‐solution processes triggered shear failure that initiated the polygonal fault systems. The early diagenetic processes affect the reservoir properties of Chalk by creating compartments and vertical connections. Taking these features into account will allow for a more detailed understanding of early diagenesis and better models for exploiting drinking water or hydrocarbons hosted in Chalk.