Stuart G. Archer

Seismic imaging of 'broken bridges': linking seismic to outcrop-scale investigations of intrusive magma lobes

Three-dimensional seismic datasets have provided unrivalled insights into magma flow within sub-volcanic systems. One of the key revelations is that sills appear to be constructed of a series of discrete magma lobes that form during the emplacement of magma into host-rock. We focus on a large sill, within the Faroe–Shetland Basin, North Atlantic, that is well imaged on seismic data, and identify the presence of ‘broken bridges’ within the sill, developed between elongate magma lobes, and reveal for the first time in three dimensions the development of broken bridges. Critically, by relating the imaged structures to key outcrop-scale examples we confirm that bridge and broken-bridge structures are oriented perpendicular to the magma flow direction. This work thus demonstrates a key link that can be made between seismic-scale investigation of intrusions and sub-seismic (outcrop-scale) processes, highlighting the seemingly scale-invariant nature of the magmatic emplacement process.

Porosity trends in the Skagerrak Formation, Central Graben, United Kingdom Continental Shelf: The role of compaction and pore pressure history

This paper describes reservoir properties in the Triassic Skagerrak Formation in the Central North Sea. This prolific sandstone reservoir often possesses anomalously high porosity for its depth of burial. Simple statistical analysis of wire-line-log-derived porosity data is used to derive empirical trends as a function of both depth and vertical effective stress that show variations between neighboring hydrocarbon fields and between different parts of the basin. Porosity data from the Josephine (J) Ridge (Quadrant 30 of the United Kingdom Continental Shelf [UKCS]) show a marked degradation with depth, but the porosities are significantly higher than in similarly deeply buried areas such as the Puffin high to the west (Quadrant 29) or the Forties–Montrose high to the north (Quadrant 22). To understand the porosity patterns better the data have been analyzed by plotting against vertical effective stress. This allows a better comparison to be made between fields and wells within the high-pressure–high-temperature (HPHT) realm. High pressure here refers to fluid pressures above 10,000 psi (), whereas high temperatures are above 300°F (149°C). Results show that porosity and fractional effective reservoir (the proportion of net sandstone with a porosity greater than a predetermined cutoff) decrease systematically with increasing vertical effective stress. Data from the different J Ridge fields fall on a common compaction trend even though they are derived from structures with marked variations in present-day depth of burial and static formation overpressure. Trends from the other areas of the Central Graben (the Puffin and Forties–Montrose highs) indicate more indurate reservoir states. The observed porosity trends are independent of fluid type within the reservoir and the absolute magnitude of overpressure. The main observed hydrocarbon effect is the result of buoyancy forces. The analysis supports the contention that, after accounting for facies-related grain-size variations, compaction controls average reservoir properties. Differences in compaction state between areas are postulated to relate primarily to structurally controlled timing of overpressure development relative to burial, and how these affect the resultant vertical effective stress history. Both the Puffin and Forties–Montrose highs are directly attached to the basin margins across stepped faults. These marginal terraces were open to lateral fluid flow for longer probably because across-fault seals were only established late in the burial history when higher temperatures promoted cementation and the destruction of permeability within fault cores. As a result, they developed overpressures in the last 5–10 m.y. or so and are largely normally compacted. The J Ridge horst block is hydrologically more isolated within the basin center by across-fault juxtaposition seals. Here, overpressure development appears to have started earlier, possibly between 50 and 60 Ma, retarding compaction and allowing preservation of higher porosities. Compaction continues to present day driven by the large static vertical effective stress gradients in these deeply buried reservoirs. The observed empirical trends offer a means of predicting average reservoir properties in deep untested exploration targets.

Dominance of lateral over axial sedimentary fill in dryland rift basins

ABSTRACT Facies models for continental rift basins suggest longitudinal (axis-parallel) fluvial activity dominates sediment transport and deposition. In contrast, modern basins in the arid to semi-arid northern Basin and Range Province, USA, show axial drainage development to be characterized by short endorheic systems that contribute little to the basin fill. Mapping and calculation of the proportionate distribution of surficial facies of three representative basins at different stages of rift evolution show that basin fill is dominated by the deposits of transverse catchments, and axial fluvial deposits are restricted to a narrow corridor by the progradation of lateral systems. Drainage integration in these dryland rifts is limited by the moisture-stressed climate, with a resultant reduction in stream power, and the complex, tectonically-induced physiography, which limits potential drainage pathways. River systems that flow through multiple structural basins are rare, restricted to those systems with catchment headwaters lying outside the dryland climatic regime. These data imply that long-range, axial fluvial deposits should not automatically be included as a significant part of dryland rift sedimentary facies models. Sediment routing pathways in ancient dryland rift systems may be much shorter than commonly predicted, so affecting the spatial distribution of lithofacies. Climate, therefore, has a much stronger control on drainage and lithofacies at all stages of rift development than is generally stated.

Challenges of future exploration within the UK Rockall Basin

Nick Schofield, David Jolley, Simon Holford, Stuart Archer, Douglas Watson, Adrian Hartley, John Howell, David Muirhead, John Underhill and Paul Green

Sills in Sedimentary Basins and Petroleum Systems

Our knowledge of igneous emplacement in sedimentary basins has been revolutionised by studies of offshore 3D seismic reflection data, where large scale structures and relationships are realised. These offshore data sets require detailed information from onshore analogues to fully understand subsurface structure of such intrusions and their potential effect on petroleum systems. The Inner Hebrides of Western Scotland, which contains an onshore record of the extensive Palaeogene magmatic activity that affected much of the North Atlantic, allows us to examine some of the classic sill geometries on a seismic to sub-seismic (outcrop) scale. As hydrocarbon exploration moves to more challenging basins, it is clear the need exists for us to fully understand the role in which intrusive volcanism plays in active hydrocarbon systems. Intrusions in general can have major effects on prospective sedimentary basins by forming interconnected low-permeability zones which can compartmentalise significant volumes of source and reservoir rock. We present a series of outcrop case studies which allow the potential influences of sills on what would represent potential source and reservoir rock intervals to be addressed and discuss the wider implications for sill emplacement in such basins.

Salt tectonics, sediments and prospectivity: an introduction

Salt is a crystalline aggregate of the mineral halite, which forms in restricted environments where the hydrodynamic balance is dominated by evaporation. The term is used non-descriptively to incorporate all evaporitic deposits that are mobile in the subsurface. It is the mobility of salt that makes it such an interesting and complex material to study. As a rock, salt is almost unique in that it can deform rapidly under geological conditions, reacting on slopes ≤0.58 dip and behaving much like a viscous fluid. Salt has a negligible yield strength and so is easy to deform, principally by differential sedimentary or tectonic loading. Significant differences in rheology and behavioural characteristics exist between the individual evaporitic deposits. Wet salt deforms largely by diffusion creep, especially under low strain rates and when differential stresses are low. Basins that contain salt therefore evolve and deform more complexly than basins where salt is absent. The addition of halokinetic processes to the geodynamic history of a basin can lead to a plethora of architectures and geometries. The rich variety of resultant morphologies have considerable economic as well as academic interest. Historically, salt has played an important role in petroleum exploration since the Spindletop Dome discovery in Beaumont, Texas in 1906. Today, much of the prime interest in salt tectonics still derives from the petroleum industry because many of the world’s largest hydrocarbon provinces reside in salt-related sedimentary basins (e.g. Gulf of Mexico, North Sea, Campos Basin, Lower Congo Basin, Santos Basin and Zagros). An understanding of salt and how it influences tectonics and sedimentation is therefore critical to effective and efficient petroleum exploration. Within rift basins in particular, salt is seen to orchestrate the petroleum system. Through halokinesis it creates structural traps, counter-regional dips on continental margins, and it can carry or entrain adjacent lithologies via rafting. Salt influences synand post-kinematic sediment dispersal patterns and reservoir distribution and can therefore be important for the creation of stratigraphic traps. It can also form top and side seals to hydrocarbon accumulations and act as a seal to fluid migration and charge at a more regional scale. Salt may also dramatically affect the thermal evolution of sediments due to its high thermal conductivity. A thick layer of salt cools sediments that lie below it while heating sediments above it. This effect cannot be underestimated as it helps provide the favourable conditions for source rock maturation in the deepwater Gulf of Mexico and Santos basins, even though sedimentary overburden may be 5 km or more in thickness. Salt can also impact reservoir quality. The role of salt in the diagenetic history of reservoirs through its control on hydrothermal pore waters is a crucial element in the risking of the deepwater Palaeogene play of the Gulf of Mexico, for example. Salt continues to kinetically evolve through time, not only by the classical roller-diapir-pedestal-canopy/ collapse progression but also with varying rates of deformation, in response to changing sedimentation rates and patterns. The relative timing of salt movement and its impact on source, reservoir, trap, seal and timing often governs the prospectivity in saltrelated basins. Beyond the realm of petroleum, salt is also used as a resource for potash, gypsum and nitrates and has the potential to be employed as a repository for radioactive waste or a top seal to sequestered CO2.

Aviat: a Lower Pleistocene shallow gas hazard developed as a fuel gas supply for the Forties Field

Abstract The search for a gas source near to Apache9s Forties Field in the North Sea was motivated by the prediction of an ever-increasing fuel gas shortfall as the field oil rate declined. The Central North Sea is well known for a large number of shallow gas hazards in the Pleistocene section that have historically caused blowouts during exploration and development. These gas accumulations typically show up as small bright anomalies on seismic data. In 2009, a large gas anomaly was identified to the east of Forties, and the Aviat Field was discovered in 2010 when exploration well 22/7-5 was drilled. The Aviat Field reservoir is interpreted to be a subaqueous glacial outwash fan, consisting of silt-grade, rock flour material, deposited in front of a grounded ice sheet in some 400 m of water. Aviat sits on an overcompacted silty mudstone that was deformed by this ice sheet – the Crenulate Marker. The distribution of this horizon implies that the Early Pleistocene ice sheet covered at least the northern half of the UK North Sea. Although the Aviat reservoir is thin (2–9 m thick), the well tests, pressure profiles and geophysical response demonstrate that the reservoir is well connected, extensive (over 35 km 2 ) with high deliverability (up to 18 MMscfd achieved). Aviat was sanctioned in 2014 for development as a fuel gas supply for the Forties Field, with first gas achieved in July 2016.

Spatiotemporal relationships of deep-marine, axial and transverse depositional systems from the synrift Upper Jurassic of the Central North Sea

Distinguishing axial and lateral sedimentary systems in rift basins is crucial for predicting reservoir distribution and quality, particularly where synrift strata are interrupted by mass transport complexes (MTCs). Upper Jurassic deep-marine synrift successions in the central North Sea have been studied to assess the temporal and spatial relationships of sediments and controls on reservoir quality. In the Late Jurassic, the central graben experienced erosion at rift margins, whereas adjacent grabens were starved and underfilled with marine sediments, supplied by axial and transverse systems. This study focused on sediments adjacent to a major intrabasinal high, the Josephine ridge. Data included seismic, wireline logs from 16 wells, and biostratigraphic and sedimentological analysis of 144 m (472 ft) of core. Synrift strata are dominated by mudstones but include MTCs interbedded with coarse sandstones at the rift margin and fine-grained turbidite sandstones in basinal depocenters. Petrographic and heavy mineral data indicate different provenance between MTCs and basinal turbidites. Turbidites correlate with periods of lowered relative sea level, during the initial rift phase, and record axial sediment supply. The composition of the MTCs corresponds to in situ strata on the adjacent Jade and Judy horsts. The distribution of MTCs implies formation by crestal collapse horsts during the rift climax and represents a transverse system, with no genetic relationship to axial turbidites. In starved deep-marine basins, fine-grained, well-sorted axial systems may provide the most extensive reservoirs. Transverse systems derived from isolated horsts are typically coarse-grained, poorly sorted, and spatially restricted, being unlikely to provide significant reservoir material.

Termination geometries and reservoir properties of the Forties Sandstone pinch-out, East Central Graben, UK North Sea

Abstract As hydrocarbon-prone basins mature through time, stratigraphic traps become increasingly important as hosts for yet-to-find reserves. Explorationists strive to reduce the uncertainty in reservoir distribution and quality, but considerable complications exist in the evaluation of stratigraphic traps owing to the inextricable links between stratigraphy, trap definition and their subsequent risking. This study quantifies the relationships that exist between reservoir geometries and the rates of reservoir property degradation in a turbidite sandstone pinch-out zone. The investigation focuses on the Paleocene Forties Sandstone Member of the Everest and Arran fields of the East Central Graben of the UK North Sea. We utilized standard seismic interpretation techniques and integrated stratigraphic and petrophysical analysis of wireline log data to map deep-water turbidite sandstone terminations and develop a predictive model for reservoir property changes close to the feather edge. The Forties Sandstone Member thins systematically up on to a palaeoramp on the eastern basin margin of the Central Graben. Results reveal that the net reservoir sandstones pinch out after the turbidite flows had traversed 5 km across the palaeoramp. The gross interval is predicted to completely terminate 6.4 km up the palaeoramp. The reservoir properties decrease in concert with the thinning trend in the wedge zone as a function of the interaction of palaeotopography and the hydraulics of the decelerating flows. The inclination of the counter-regional slope is considered to be a key controlling factor that determines the rate of thinning and thus the termination position of the sandstones and their concomitant reservoir property decline. The results of this study demonstrate that characterization of pinch-outs into distinct zones based on a palaeotopographic template can be of utility in stratigraphic and combination trap definition. This work also has wider implications for prospect risking, volumetric analysis, the population of properties and geological modelling of stratigraphic traps.

First recorded occurrence of detrital baddeleyite (ZrO2) in sedimentary rock (Smith Bank Formation, Triassic, Central North Sea)

Baddeleyite (ZrO2) is a comparatively rare mineral in nature. It has been found previously only in igneous and metamorphic rocks, in soils and modern river sands derived from these rocks and, most recently, in deep sea ferro-manganese nodules in the Indian Ocean. Here we report two occurrences of baddeleyite in a siltstone from the Triassic Smith Bank Formation of the Central North Sea, which we believe to be the first record of this mineral as a detrital phase within a clastic sedimentary rock.