Martin R. Wells

CIRCULATION IN LARGE ANCIENT EPICONTINENTAL SEAS: WHAT WAS DIFFERENT AND WHY?

At times in the geological past, vast epicontinental seas flooded the continents. Almost everything we know about pre-Jurassic (200 million years ago) marine life and environmental change originates from such settings. Most of the truly oceanic sediments deposited before this time have either been subsequently subducted or metamorphosed (Allison and Briggs, 1993). An understanding of ancient epicontinental seas is clearly essential to our interpretation of past ecological and environmental change. Epicontinental seas, however, really have no suitably scaled modern counterparts. They were typically shallow, on the order of 10 to 200 m deep, but of vast extent, covering areas of up to ∼106 km2 (Wells et al., 2005a, 2005b). Such modern seas as the Baltic are of similar depths but are considerably smaller. Without suitably scaled modern analogues our understanding of these important water bodies is handicapped. Since the depth and scale of these ancient seaways makes them so different, their response to storm and tidal waves has been questioned (Hallam, 1975). Specifically, it has been argued that wave energy was attenuated in ancient epicontinental seas by the large distances traveled by waves (Keulegan and Krumbein, 1949; Shaw, 1964; Irwin, 1965). This contention can be rejected in the case of wind and storm waves on the basis of wave height data from modern isolated seas. Wind waves are affected by fetch, duration, and intensity of wind (e.g., see Jonsson et al., 2002, 2005). Substantial waves are regularly documented in even the small epicontinental seas of today. Wave heights—crest-to-trough height—of larger than 6 m are documented in the Baltic Sea almost annually. They result from strong winds (15–20 m/s) blowing over deep water for at least 6 hours. Annual storms can generate wavelengths of 80 m, leading to sediment reworking down to 40 m water …

Large sea, small tides: the Late Carboniferous seaway of NW Europe

Debate surrounds the extent of tidal influence in Palaeozoic shallow epi-continental seas. In the absence of analogical reasoning, numerical modelling provides a quantitative means of investigating tidality in the geological record. Herein a new finite element model, tested for accuracy on the present-day Mediterranean, is used to predict an exceedingly low tidal range (<10 cm) in the epi-continental seaway that covered much of NW Europe during the Late Carboniferous. This small bulge may have been amplified to c. 1 m in estuaries, leading to the localized deposition of cyclic rhythmites, agreeing with geological observations. Extremely low tidal ranges in ancient epi-continental seas may be one mechanism to prevent water-body mixing, enhancing stratification and promoting anoxia.

A systematic approach to unstructured mesh generation for ocean modelling using GMT and Terreno

A systematic approach to unstructured mesh generation for ocean modelling is presented. The method optimises unstructured meshes to approximate bathymetry to a user specified accuracy which may be defined as a function of longitude, latitude and bathymetry. GMT (Generic Mapping Tools) is used to perform the initial griding of the bathymetric data. Subsequently, the Terreno meshing package combines automated shoreline approximation, mesh gradation and optimisation methods to generate high-quality bathymetric meshes. The operation of Terreno is based upon clearly defined error measures and this facilitates the automation of unstructured mesh generation while minimising user intervention and the subjectivity that this can introduce.

Deciphering multiple controls on reservoir quality and inhibition of quartz cement in a complex reservoir: Ordovician glacial sandstones, Illizi Basin, Algeria

Abstract Late Ordovician (c. 445 Ma) glacial sandstones form important gas reservoirs in the Illizi Basin, SW Algeria. These reservoirs have a high degree of depositional and diagenetic complexity, such that understanding and predicting reservoir quality (RQ) presents a major challenge to their economic development. Porosity is typically 1–10%, but reaches up to 15% and permeability is typically <10−15 m2 (<1 mD), but locally reaches >10−13 m2 (>100 mD). The key questions addressed herein concern the development and distribution of this RQ variability, specifically why has good RQ been locally preserved? Primary depositional fabric exerts a strong control on RQ. Muddy sandstones are either highly compacted or pervasively cemented by quartz and microporous illite, and have very poor RQ. Only fine- to medium-grained, moderately well sorted, clean sandstones can contain good RQ, but texturally and mineralogically similar sandstones span a wide range of porosity and permeability. This range is primarily driven by the degree of quartz cementation, with incomplete cementation resulting in the best RQ. Quartz overgrowths in incompletely cemented clean sandstones are patchy and non-luminescent in scanning electron microscopy with cathodoluminescence (SEM-CL), possibly indicating slow growth rates. There is tentative evidence to link incomplete quartz cementation with oil charging of the reservoir. An alternative or additional explanation of RQ preservation may be related to limited silica supply in the centres of the thickest, stacked, clean sandstones, where the better RQ tends to reside. The results of this study imply that sustained high-energy depositional processes, coupled with an early oil charge, are prerequisites for retaining the best RQ. This has important implications for the exploration and development of Late Ordovician glacial sandstones in the Illizi Basin, and potentially similar plays elsewhere.

Provenance of Lower Cretaceous clastic reservoirs in the Middle East

Some of the worlds largest oilfields contain reservoirs in Lower Cretaceous (Hauterivian–Albian) clastic successions of the Middle East. They comprise sandstone-prone intervals of hundreds of metres thickness deposited in non-marine to paralic settings on a very shallow gradient, otherwise carbonate-dominated, passive margin. This study integrates petrography, X-ray powder diffraction, heavy mineral analysis, mineral chemistry, zircon geochronology and palaeodrainage basin scale calculations to determine the likely provenance of the clastic deposits. Regional geological considerations are then used to suggest the likely driving mechanisms for the clastic influx. The sediments are highly texturally and chemically mature with low-diversity heavy mineral assemblages, suggesting extensive weathering and sediment recycling. The mixture of heavy minerals derived from metapelitic, metamafic and granitic domains at variable metamorphic grades is consistent with erosion of a complex heterogeneous basement or recycling of multiple earlier cycle sediments. Comparison with published data suggests an Arabian Shield provenance, with input from Precambrian basement and Palaeozoic sedimentary cover both possible. This is consistent with drainage basin scaling calculations. Large-scale tectonic reorganization associated with opening of the South and Equatorial Atlantic together with mantle plume related uplift in the northern Arabian Shield is postulated as the main driving mechanism of clastic influx.

Discussion on large sea, small tides: the Late Carboniferous seaway of NW Europe

Roger Higgs writes: Wells et al . (2005 a , b ) produced an innovative and thought-provoking numerical model of tides in the Late Carboniferous seaway of NW Europe, predicting very low tidal ranges (<1 m), consistent with the scarcity of sedimentary evidence for (weak) tides reported in the corresponding rocks. Based on the model, Wells et al . (2005 a ) argued intuitively that tidal range was minimum during (glacioeustatic) highstands, and maximum during lowstands (and transgressions). I would argue the converse. During highstands, not only was the water volume (mass) greater, leading potentially to larger tides, but also any tide in the adjacent ocean would necessarily be passed into the seaway, adding to the locally generated tide. This is a weakness of the model: the oceanic tidal influence was ‘not incorporated for simplicity’ (Wells et al . 2005 a , p. 418). In contrast, during lowstands, the seaway became a freshwater lake (or chain of lakes), perched at the level of the outflow sill, and bordered by incised highstand prograded deltas and alluvial plains (‘Lake Bude’ of Higgs 1991, 2004). Disconnected from the global ocean, the lake would have had an extremely low tidal range (<5 cm), as in all lakes, because of the relatively small water volume (Talbot & Allen 1996). Indeed, no evidence for tides has been reported in Lake Bude deposits (Ross, Crackington and Bude Formations of Ireland and England; Higgs 1991, 2004, and references therein). However, given the (scarce) evidence reported elsewhere in the seaway, sedimentary structures indicative of weak tidal currents (e.g. ripples with foreset slack-water mud drapes; rhythmic lamination; opposed ripples) may eventually be discovered in the Lake Bude formations, probably associated with the volumetrically minor shale bands with brackish and marine fossils (Higgs 1991, 2004), representing transgressions and highstands, when …

Introduction to the sedimentology of paralic reservoirs: recent advances

Paralic reservoirs reflect a range of clastic depositional environments developed along or near coastlines, including deltas, shoreline–shelf systems and estuaries. Such reservoirs provide the backbone of production in many mature basins around the world, and contribute significantly to global conventional hydrocarbon production. Strata that host these reservoirs are shaped by a wide variety of depositional processes and controls which reflect the upstream supply of sediment and water, the characteristics of the receiving basin, relative sea level, tectonic setting, and the internal dynamics of depositional systems. Consequently, they exhibit much variability in their stratigraphic architecture and sedimentological heterogeneity, which translates into complex patterns of reservoir distribution and reservoir performances that are challenging to predict, optimize and manage. This Special Publication presents new research and developments in established approaches to exploration and production of paralic reservoirs. It arises from the conference and associated core workshop titled ‘Sedimentology of Paralic Reservoirs: Recent Advances and their Applications’, which was organized by the Petroleum Group of the Geological Society of London and held in London from 18 to 21 May 2015.