Combining sensitivities and prior information for covariance localization in the ensemble Kalman filter for petroleum reservoir applications

Sampling errors can severely degrade the reliability of estimates of conditional means and uncertainty quantification obtained by the application of the ensemble Kalman filter (EnKF) for data assimilation. A standard recommendation for reducing the spurious correlations and loss of variance due to sampling errors is to use covariance localization. In distance-based localization, the prior (forecast) covariance matrix at each data assimilation step is replaced with the Schur product of a correlation matrix with compact support and the forecast covariance matrix. The most important decision to be made in this localization procedure is the choice of the critical length(s) used to generate this correlation matrix. Here, we give a simple argument that the appropriate choice of critical length(s) should be based both on the underlying principal correlation length(s) of the geological model and the range of the sensitivity matrices. Based on this result, we implement a procedure for covariance localization and demonstrate with a set of distinctive reservoir history-matching examples that this procedure yields improved results over the standard EnKF implementation and over covariance localization with other choices of critical length.     

Fast ion conduction character and ionic phase-transitions in disordered crystals: the complex case of the minerals of the pearceite–polybasite group

The minerals of the pearceite–polybasite group, general formula (Ag,Cu)₁₆ M ₂S₁₁ with M = Sb, As, have been recently structurally characterized. On the whole, all the structures can be described as a regular succession of two module layers stacked along the c axis: a first module layer (labeled A), with general composition [(Ag,Cu)₆(As,Sb)₂S₇]²⁻, and a second module layer (labeled B), with general composition [Ag₉CuS₄]²⁺. In detail, in the B layer of the pearceite structure silver cations are found in various sites corresponding to the most pronounced probability density function locations of diffusion-like paths. We have shown for the first time that the observed structural disorder in the B layer is strongly related to the fast ion conduction character exhibited by these minerals. This paper reports an integrated XREF, DSC, CIS and EPMA study on all the members of the pearceite–polybasite group. DSC and conductivity measurements pointed out that the 222 members show ionic-transitions at 340 K (arsenpolybasite-222) and 350 K (polybasite-222), whereas the 221 members have transitions at lower temperature, that is, 310–330 K (arsenpolybasite-221) and 335 K (polybasite-221). For the 111 members (pearceite and antimonpearceite), the transition occurs below room temperature at 273 K. In situ single-crystal X-ray diffraction experiments showed that these minerals present the same high temperature structure and are observed at room temperature either in their high temperature (HT) fast ion conductivity form or in one of the low temperature (LT) fully ordered (222), partially ordered (221) or still disordered (111) forms, with transition temperatures slightly above or below room temperature. The pearceite–polybasite group of minerals can be considered as a homogeneous series with the same aristotype, fast ion conducting form at high temperature. Depending upon the Cu content, an ordering occurs with transition temperatures related to that content: the lower the Cu content, the higher the transition temperature from the fast ion conducting HT form to the non-ion conducting form.Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Late Pleistocene palaeolake in the interior of Oman: a potential key area for the dispersal of anatomically modern humans out‐of‐Africa?

Relict fluvial and lacustrine deposits in the interior of Oman near Saiwan consist of waterlain breccias with pebble imbrications and current ripples, covered by algal laminites containing calcified reeds and charcoal pieces. Geomorphological evidence suggests that the palaeolake covered a maximum surface of 1400 km² and had a maximum depth of about 25 m. Optically stimulated luminescence (OSL) and thermally transferred OSL dating indicate that this palaeolake existed sometime between 132 and 104 ka. The rich archaeological evidence in the area suggests substantial human occupation, possibly at the time when Palaeolake Saiwan existed. Interestingly, the lithic traditions of the archaeological material show no clear relation to coeval findings from neighbouring areas, putting a question mark on the origin of the culture found at Saiwan. This finding raises questions concerning the origin of the population inhabiting Saiwan, as the site lies along one possible route for the dispersal of anatomically modern humans out‐of‐Africa. Copyright © 2011 John Wiley & Sons, Ltd.     

The Exploitation of Plant Resources by Early Homo sapiens: The Phytolith Record from Pinnacle Point 13B Cave,South Africa

The capacity to detect remnants of fire usage in archaeological sites, along with a better understanding of its use by prehistoric populations, can help us to shed light on hominin cognition, social organization, and technology. The application of phytolith studies to understand fire and its use can be widely applicable since plants typically contain phytoliths that are identifiable to different taxonomic levels. Pinnacle Point 13B (PP13B) is one of several South African Middle Stone Age (MSA) sites that shows an abundant presence of lenses of burnt material and intact hearths. Phytolith analyses at PP13B have been used to identify the types of plants used as fuel in the hearths. The phytoliths overall show high levels of alteration, and varying alteration is used to identify areas with higher alkaline conditions related to dripping water. In some areas of the site, there is excellent preservation of multicellular structures from the epidermal leaves of dicotyledonous plants, suggesting in situ structures with practically no chemical or post‐depositional alteration. A notable pattern is the abundance of dicotyledonous leaves from the rear of the cave, which might indicate specific leaf‐fuels for the fires, short‐term fire activities, or other actions such as cooking.     

D–H evidence for the timing of kaolinization in Northeast Bavaria, Germany

The δD_SMOW values of sedimentary kaolins from the western border of the Bohemian Massif, northeast Bavaria, that did not suffer a deep burial (less than 1000 m) nor a hydrothermal overprint, change systematically from Late Triassic (−50‰) to Mid-Jurassic and Late Cretaceous (−56‰ to −66‰) to Upper Oligocene–Mid-Miocene (−77‰ to −90‰). All analyzed clays are far from hydrogen isotope equilibrium with present-day meteoric waters. Combined oxygen and hydrogen isotope data of selected samples indicate low temperatures of formation (<30°C) and no evidence for preferential D/H exchange with younger waters. The hydrogen isotopic evolution of kaolins is interpreted as reflecting a systematic isotopic change of paleo-meteoric waters in that region. This can be related mainly to the northward drift of stable Europe after the break-up of Gondwana. Increasing continentality, surface uplift and global cooling are additional factors responsible for decreasing δD_SMOW values since the Mid-Cretaceous.

Kaolinite hydrogen isotope ratios of two large residual economic deposits (Tirschenreuth: δD_SMOW=−80‰ to −76‰; Hirschau–Schaittenbach: δD_SMOW=−70‰ to −63‰) can be used in combination with additional geological evidence to constrain the timing of weathering in these areas. A late Early Cretaceous kaolinization age is suggested for the Early Triassic sandstone-hosted deposits near Hirschau–Schnaittenbach, whereas a Late Oligocene to Mid-Miocene age is indicated for the Carboniferous granite-hosted Tirschenreuth deposits.     

CLIMATOLOGY OF STATEWIDE EXTREME TEMPERATURES DURING WINTER MONTHS IN THE COTERMINOUS UNITED STATES

A 20-year (1961-1980) record of statewide extreme maximum and minimum temperatures for the winter season is examined for the coterminous United States. Extreme maxima exhibit a zonal pattern, modified somewhat by terrain features in the west. Extreme minima are more dramatically skewed from a zonal orientation, because of the effects of elevation and proximity to oceans. Interannual variability in the record of extreme temperatures appears to be dictated by proximity to air-mass source regions (e.g., low variability of minima in the Upper Midwest, low variability of maxima in the Deep South) and sensitivity to the position and amplitude of mid-tropospheric circulation and snowpack dynamics (e.g., high variability of minima in the Ohio Valley). Strong, spatially coherent patterns of correlation between statewide extremes and amplitude of midtropospheric flow are evident (negative correlation with enhanced troughing in the east; positive correlation with enhanced ridging in the west). Lag correlations with ENSO indices suggest a relatively weak linkage between antecedent conditions in the Pacific and the degree of extremeness of maximum (but not minimum) temperatures in the eastern United States. [Key words: extreme maximum temperatures, extreme minimum temperatures, climatic variability, meridionality indices, El Nino-Southern Oscillation, snowpack.]     

Monsoon triggered formation of Quaternary alluvial megafans in the interior of Oman

A vast bajada consisting of coalescing low-gradient (< 0.3°) alluvial fans exceeding 100 km in length formed along the southwestern margin of the Oman Mountains. It comprises an old fan sequence of inferred Miocene to Pliocene age termed Barzaman Formation, diagenetically highly altered to dolomitic clays, and a thin veneer of weakly cemented Quaternary gravels. A combination of remote sensing, lithological analyses and luminescence dating is used to interpret the complex aggradation history of the Quaternary alluvial fans from the interior of Oman in the context of independent regional climate records. From satellite imagery and clast analysis four fans can be discerned in the study area. While two early periods of fan formation are tentatively correlated to the Miocene–Pliocene and the Early Pleistocene, luminescence dating allows the distinction of five phases of fan aggradation during the Middle–Late Pleistocene. These phases are correlated with pluvial periods from Marine Isotope Stage (MIS) 11 through 3, when southern Arabia was affected by monsoonal precipitation. It is concluded that the aggradation of the alluvial fans was triggered by the interplay of increased sediment production during arid periods and high rainfall with enhanced erosion of hillslopes and transport rates during strong monsoon phases. However, the lack of fine-grained sediments, bioturbation and organic material implies that although the Quaternary fans are sourced by monsoonal rains they formed in a semi-arid environment. Thus, it appears that, in contrast to the Oman Mountains, the interior was not directly affected by monsoonal precipitation.     

Participation in Field Learning and Teaching Opportunities: Avenues to Research and Publication

Field experiences continue to be a hallmark of a geographer's education and, for that matter, reeducation, as we all strive to remain current in the real world. Academic geographers beginning their ascent towards tenure and promotion might consider augmenting their portfolios with materials emerging from field teaching and learning activities.     

Changes in hydrodynamic process dominance (wave,tide or river) in foreland sequences: The subalpine Miocene Molasse revisited (France)

A comprehensive sedimentological study was undertaken in the Miocene of the subalpine massifs and southern Jura (France) with the aim to constrain the evolution of process changes in third-order sequences of peripheral foreland basins during the overfilled phase (i.e. sediment supply higher than accommodation space). Fieldwork analyses based on 35 sedimentological sections allowed the identification of four depositional models: wave dominated, mixed wave-tide, river to tide and river dominated. The sections were dated using chemostratigraphy (i.e. marine strontium isotopic ratios), revealing three-third-order sequences between the Upper Aquitanian and the Langhian. Chronostratigraphical and sedimentological results document prominent and recurrent changes in depositional models along third-order sequences: (i) in the earliest stage of the transgression, mixed-energy coastal environments influenced by the local coastal morphology prevailed (in palaeo-highs or incised valleys); (ii) during the course of the transgression, Gilbert delta deposits suggest a prominent steepening linked to a tectonic uplift in the proximal depozone (between the tectonically active frontal part of the orogenic wedge and the proximal foredeep). Instead, in the distal depozone (between the proximal foredeep and the proximal border of the flexural uplifted forebulge), deposits were characterized either by wave-dominated or mixed wave-tide environments and are likely eustatically-driven; (iii) during the maximum flooding stage, water depth remained shallow below the storm-weather wave base; and (iv) during the regression, the proximal depozone is characterized by the progradation of gravel-rich fan deltas. In the distal depozone, mixed wave-tide systems preceded the development of river to tidal depositional environments. These results were integrated and compared with facies models from other basin analogues worldwide. A model tackling the evolution of process changes within third-order sequences (of the overfilled phase) of foreland basins is proposed, thereby improving sequence stratigraphic predictions in foreland basins.