Showing papers by "Alastair Ruffell published in 2004"

Use of Variography in Permeability Characterization of Visually Homogeneous Sandstone Reservoirs with Examples from Outcrop Studies

Abstract: Sandstones of different ages provide economically significant oil, gas, and water reservoirs. In sandstones where heterogeneities are not visually obvious, it is particularly difficult to predict the location of permeability barriers and the scale at which high and low permeability zones occur, yet this is critical in providing information on hydrocarbon reservoir performance. This study uses variogram analysis to investigate spatial variation in permeability in visually homogeneous reservoir sandstone successions. Air permeability measurements were taken using unsteady state probe permeametry following regular grid schemes with centimeter spacing. Spatial variation in permeability was characterized using omnidirectional and directional variograms. This study combines variography with geological interpretation to assess the degree of heterogeneity of permeability in visually homogeneous sandstone successions. Variography indicates spatial dependence and short-range variation at 1 cm grid spacings that is not apparent at a larger 5 cm grid spacing in the visually homogeneous sandstones studied. The range of the models fitted to the variograms provide a potentially important index of spatial variability in permeability for different depositional settings including aeolian, fluvial, shallow marine, and marine/mass- flow turbidite.

Heterogeneous mixed-layer clays from the Cretaceous greensand, Isle of Wight, southern England

Abstract: The sea-cliffs of the Isle of Wight were deposited during a period of overall sea-level rise starting in the Barremian (Lower Cretaceous) and continuing into the Aptian and Albian. They consist of fluvial, coastal and lagoonal sediments including greensands and clays. Numerous episodes of erosion, deposition and faunal colonization reflect condensation and abandonment of surfaces with firmgrounds and hardgrounds. This study focused mainly on shallow marine cycles where variations in clay mineralogy would not be expected, because overall system composition, sediment source, and thermal history are similar for all the samples in the studied section. Instead we found a wide variety of clay assemblages even in single samples within a 200 m interval. In this interval, distinct clay mineral assemblages were found and can be described as consisting of Al-rich, Fe-richand intermediate Fe and Al compositions withrespect to 2:1 and 1:1 layers in mixed-layer arrangements. Nearly pure glauconite-nontronite clays exist in the <2 µm fraction only when the bulk rock is free of K- and plagioclase feldspar. Conditions favorable to glauconite-nontronite formation are interpreted to result from a hiatus in volcanoclastic sedimentation, thus providing a stable substrate for glauconitization. The Fe-bearing mixed-layer clay assemblages consist of glauconite, nontronite and berthierine-like layers in various proportions withseveral mixed-layer clays often coexisting in the same sample. In different samples, Al-richand Fe-Mg-richmixed-layer clays are similar in their content and distribution of 1:1 and 2:1 layers. This suggests that the original clay assemblages were similar and later diagenesis affected certain horizons resulting in substitution of Al by Fe + Mg while preserving the original layer structure and arrangement. Structural formulae for the berthierine-like phase and berthierine-like layers in these mixed-layer clays show their layer cation composition is intermediate between odinite and standard berthierine. The total sum of octahedral cations varies from 5.26 to 5.55 whereas the amount of Fe2+ cations varies from 2.12 to 2.22 per O10(OH)8. A feature of the berthierine-like phase as well as of berthierine-like layers is that they are di-trioctahedral and Fe2+ and Fe3+ are the prevalent cations. Moreover, in these berthierine-like components, the amount of Fe2+ is greater than that of Mg (in contrast to odinite) and Fe3+ cations prevail over Al (in contrast to berthierine). The presence of authigenic ferrous Fe clays and the relationship between glauconite-nontronite and bulk mineralogy has implications for sedimentological processes and geochemical conditions during and shortly after deposition.

Ground-penetrating radar facies as an aid to sequence stratigraphic analysis: application to the Archaeology of Clonmacnoise Castle, Ireland

Abstract: This paper describes the advantages of interpreting ground-penetrating radar (GPR) facies in a sequence stratigraphic context. These data have been collected adjacent to the River Shannon at Clonmacnoise Castle in central Ireland and demonstrate that stratigraphy is not necessarily made up of horizontal isochronous surfaces; some reflective surfaces are more important than others in interpretation and that two-dimensional sections must be interpreted with caution. Radar facies analysis and thus sequence stratigraphy can be used to both interpret the sedimentary record in terms of palaeoenvironments as well as identify important (key) surfaces. Bright, continuous surfaces are often found at horizons of palaeoenvironmental change and have distinctive reflector shapes and terminations (or stratal patterns) above and below. Such stratal patterns define and reflect linked fluvial and human environments that have developed and been preserved through the creation of accommodation space. Key surfaces have been identified when sedimentation was altered, thus reflecting a natural or anthropogenic palaeoenvironmental change. In anthropogenically modified or created successions, such as those seen at Clonmacnoise, key surfaces have produced changes in dielectric constants in the sediments that are seen as radar reflections. Natural activity includes lateral migration of point-bars and fluvial onlap. Anthropogenic activity has created accommodation space (the moat), a major sequence boundary and subdwelling surface disruption of reflectors. A pre-moat fault can be observed on GPR data that is along strike of a surface scarp to the southwest of Clonmacnoise Castle. The results demonstrate the enhanced interpretive and predictive use of radar facies in sequence stratigraphy. Copyright © 2004 John Wiley & Sons, Ltd.

Distinguishing faults from flooding surfaces on spectral gamma-ray logs

Abstract: Elevated gamma-ray emission from discrete beds in sedimentary deposits may conventionally be interpreted as representing flooding surfaces or transgressive beds. Potassium (K), uranium (U), or (less commonly) thorium (Th) concentrations in outcrop or borehole successions cause such elevated gamma-ray emission and may be linked to the presence of specific mineral hosts. Furthermore, specific Th/K and Th/U ratios occur at correlated stratigraphic surfaces and form part of a pattern that reflects a sequence-stratigraphic hierarchy. Spectral gamma-ray logs from uncored boreholes or weathered cliffsides and acquired without full petrographic descriptions can intersect low-angle faults such as thrusts. Our study demonstrates that bedding-parallel faults can be mistaken for flooding surfaces. We document the spectral gamma-ray response through a range of visually obvious and cryptic faults that may serve as proxy examples for other areas. Finally, we derive a preliminary generic model for the origin of spectral gamma-ray variations in faulted sandstones, limestones, and metamorphic successions. This shows why fluid-rock interactions along bedding-parallel zones of deformation generate elevated K and U and depressed Th/K and Th/U. Our observations may aid subsurface studies of the complex stratigraphy below thrusts.

Variscan structure of the inner Bristol Channel, UK

Abstract: Major Variscan structures under the Bristol Channel are concealed beneath a Mesozoic and Quaternary cover. Mapping of reflectors on offshore seismic sections reveals two major WNW–ESE-striking thrusts within the underlying Palaeozoic sequence. The more southerly, with a moderate SSW dip, has previously been identified as the Bristol Channel Thrust. Another thrust, the previously undescribed Gravel Margin Thrust, has been identified in the eastern part of the inner Bristol Channel and lies in the footwall of the Bristol Channel Thrust. It has a similar strike but steeper dip and extends to anticipated Precambrian basement depths. The Bristol Channel and Gravel Margin Thrusts together juxtaposed the contrasting Palaeozoic successions of South Wales and SW England before Mesozoic reactivation. Reflection packages identified on the offshore seismic sections are tentatively correlated with specific onshore Palaeozoic sequences. Interpretation of the offshore seismic sections in conjunction with geological mapping along the north Devon coast shows that the Bristol Channel and Gravel Margin Thrusts have orientations and geometries typical of several mesoscale thrusts observed and measured at Foreland Point, north Devon. Along-strike structural changes beneath the inner Bristol Channel seen on seismic sections provide evidence, additional to earlier refraction and structural studies, for the offshore continuation of the NW–SE-trending Watchet–Cothelstone–Hatch Fault and its linkage with the previously described Central Bristol Channel Fault Zone. The Bristol Channel and Gravel Margin Thrusts are interpreted to be offset from the Cannington Park Thrust in north Somerset by a 14–16 km Variscan dextral strike-slip displacement across the Watchet–Cothelstone–Hatch Fault. The Bristol Channel Gravel Margin Thrust and the Cannington Park Thrust are interpreted as syngenetic structures linked by their lateral ramp, the Watchet–Cothelstone–Hatch Fault. Using all the above information, the overall structure of the Variscides of the inner Bristol Channel is re-evaluated and a new tectonic model is presented. The structures discussed in this paper, at a scale portrayed by commercial seismic data and published geological maps, may represent merely the tips of linked thrusts that have previously been mapped down to mid-crustal depths on BIRPS seismic sections from the South West Approaches Traverse. We show that enough data now exist to resolve outstanding problems of the concealed Variscan structure in the inner Bristol Channel, providing local structural detail that can be incorporated into the wider tectonic framework of NW Europe.