a review of techniques used to determine geological and thermal history in the southern north sea abstract the southern north sea has been a prolific source of gas to the uk economy over the last 30 years as many of the producing fields are now becoming depleted attention has switched to exploration in more marginal areas of the basin in order to evaluate suitable structures in these areas an understanding of the complex structural and thermal history of the southern north sea basin is required particular emphasis must be placed upon the prediction of source rock maturity the timing of generation and migration of hydrocarbons and the thermal controls exerted on potential reservoir sand units over large parts of the basin the carboniferous source rocks are not at their maximum depth of burial at present day a variety of techniques utilizing sonic velocity vitrinite reflectance and apatite fission track analysis have been used to quantify the magnitude of uplift that the carboniferous section has undergone through geological time with the exception of apatite fission track analysis these techniques provide only an estimate of net uplift i e the amount of uplift relative to maximum depth of burial or maximum temperature they record neither the timing and effects of individual uplift events nor the timing of maximum burial prior to net uplift in order to accurately reconstruct the burial and thermal history of the basin these techniques must be integrated with a detailed knowledge of the geological history of the southern north sea the use of these techniques in isolation together with a lack of appreciation of the limitations of the data has in the past resulted in spurious results leading to criticism and mistrust of the techniques this paper attempts to show that when correctly applied and in appropriate geological circumstances each analytical technique can provide useful data for burial and thermal history reconstruction examples are shown to highlight the effects of incomplete data interpretation and the impact of inaccurate determination of present day temperatures particular attention is paid to determining the burial and thermal history of the economically important carboniferous source and permian reservoir units and the interpretation of uplift data is restricted to the post zechstein section

discrimination of bitumen sources in precambrian and lower palaeozoic rocks southern u k by gas chromatography mass spectrometry abstract two distinct sources for bitumen shows in lower palaeozoic and precambrian rocks in the welsh borderland can be successfully discriminated using gas chromatography mass spectrometry bitumens are widespread in the region in rocks from late precambrian longmyndian to silurian age previous geological studies have suggested that some bitumens originated from pre devonian source rocks while others migrated downwards laterally from carboniferous rocks this possibility has now been confirmed by organic geochemical studies which have also clarified the origin of some bitumens whose affinities had not been clear those bitumens concluded to have migrated from the carboniferous have also been correlated with bitumens within carboniferous rocks further east they are characterized by relatively high levels of hopanes c 27 c 29 to c 35 max c 29 or c 30 the bitumens with an assumed pre devonian source contain n alkylcyclohexanes range c 15 to c 32 max c 18 or c 19 which are absent in the other bitumens and they can be matched with a longmyndian black shale although detailed correlations are limited by the very high maturity of the samples the pre devonian bitumens also show several affinities with the distinctive ordovician oils

convection driven melting near the grounding lines of ice shelves and tidewater glaciers subglacial meltwater draining along the bed of fast flowing marine terminating glaciers emerges at the groundingline wheretheiceeithergoesafloattoformaniceshelforterminates inacalvingface theinputof freshwater to the ocean provides a source of buoyancy and drives convective motion alongside the ice ocean interface this process is modeled using the theory of buoyant plumes that has previously been applied to the study of the larger scale circulation beneath ice shelves the plume grows through entrainment of ocean waters and the heat brought into the plume as a result drives melting at the ice ocean interface the equations are nondimensionalized by using scales appropriate for the region where the subglacial drainage rather than the subsequent addition of meltwater supplies the majority of the buoyancy forcing it is found that the melt rate within this region can be approximated reasonably well by a function that is linear in ocean temperature has a cube root dependence on the flux of subglacial meltwater and has a complex dependency on the slope of the ice ocean interface the model is used to investigate variability in melting induced by changes in both ocean temperature and subglacial discharge for a number of realistic examples of ice shelves and tidewater glaciers the results show how warming ocean waters and increasing subglacial drainage both generate increases in melting near the grounding line

paleosalinity determination in ancient epicontinental seas a case study of the t oae in the cleveland basin uk abstract the early jurassic toarcian oceanic anoxic event t oae was marked by deposition of organic rich sediments and a pronounced negative carbon isotope excursion across the northwest european sea nwes representing a signal that was stronger regionally than globally various models for the t oae have been proposed some focusing on general atmospheric oceanic controls to reflect its global character and others emphasizing local or regional factors to account for its strong expression in the nwes in this study we evaluate watermass dynamics of the early jurassic cleveland basin uk during the t oae using a combination of paleosalinity proxies b ga sr ba and s toc these proxies suggest that brackish conditions existed prior to onset of the t oae with a shift to nearly freshwater conditions during the t oae reflecting a large increase in terrestrial runoff due to a stronger hydrological cycle possibly combined with shallowing of the marginal sill of the cleveland basin through sea level fall following the t oae basinal salinity conditions first returned to brackish and then gradually shifted to fully marine this hydrographic history accounts for low mo concentrations in t oae sediments of the cleveland basin demonstrating control by watermass restriction and local rather than global aqueous mo drawdown these findings demonstrate the importance of local hydrographic controls on the sedimentary expression of the t oae the more general significance of this study is that it highlights the need for application of paired salinity and redox proxies in order to correctly interpret paleoenvironmental variations in marginal marine basins

geographical variability of the first baroclinic rossby radius of deformation global 1 83 18 climatologies of the first baroclinic gravity wave phase speed c1 and the rossby radius of deformation l1 are computed from climatological average temperature and salinity profiles these new atlases are compared with previously published 5 83 58 coarse resolution maps of l1 for the northern hemisphere and the south atlantic and with a 1 83 18 fine resolution map of c1 for the tropical pacific it is concluded that the methods used in these earlier estimates yield values that are biased systematically low by 5 15 owing to seemingly minor computational errors geographical variations in the new high resolution maps of c1 and l1 are discussed in terms of a wkb approximation that elucidates the effects of earth rotation stratification and water depth on these quantities it is shown that the effects of temporal variations of the stratification can be neglected in the estimation of c1 and l1 at any particular location in the world ocean this is rationalized from consideration of the wkb approximation

a new and working petroleum source rock on the uk continental shelf upper permian offshore yorkshire we report on the discovery of oil from the boulby mine and its likely productive source rock from yorkshire in ne england located to the west 30 km of the newly licenced petroleum exploration areas in the vicinity of the mid north sea high oil samples from the mine have likely been generated from zechstein group kirkham abbey formation kaf sapropelic carbonate rock as indicated by aliphatic and aromatic hydrocarbon biomarkers other potential source rocks of carboniferous westphalian namurian visean coals and mudrocks and jurassic the jet rock bituminous shales kimmeridge clay formation age are ruled out on the basis of organic geochemical data boulby oil was generated in the peak to late oil window and it is characterised by the high abundance of c32 and c34 homohopanes slight even over odd predominance eop of c20 25 n alkanes indicating restricted carbonate evaporite depositional conditions and c29 ethyl diacholestane 20 s likely implying a clay rich source rock the structural framework and tectonic history of the permian strata reveal the presence of several fault systems which served as conduits for migrating petroleum similar zechstein sourced oil is known from poland and germany but the occurrence at boulby is the first positive identification of oil derived from zechstein source rock in the north sea area the boulby oil is reservoired in zechstein 3 z3 brotherton formation dolomite and sealed by z3 evaporite rocks the proven oil occurrence at boulby has significant implications in terms of reducing the risk of a lack of oil mature source rock for acreage offered in the neighbouring north sea during the uk s 30th and 31st licencing rounds

attenuation of nitrate in aquitard sediments of southern ontario to examine nitrate persistence detailed geochemical profiling using core squeezed water and piezometer samples was carried out at five sites in southern ontario where groundwater is moving downward in silt rich aquitard sediments at rates of 16 to more than 20 cm year 1 elevated levels of no3 n 5 50 mg 1 1 that occur in the shallow groundwater as a result of agricultural activity were found to be consistently attenuated generally to very low levels 0 05 mg 1 1 n at the redoxcline the horizon marking the boundary between the surficial weathered brown sediments and the underlying unweathered grey sediments tritium dating suggests that groundwater at the redoxcline depths 3 5 m was recharged between 1970 and 1980 thus the n03 depletion appears to result from biodegradation reactions since no major landuse changes have occurred during this period the close association of the nitrate depletion zones with the redoxcline where in particular sediment sulphur contents increase abruptly and where also porewater so42 levels increase suggests that the dominant attenuation reaction is autotrophic denitrification using reduced sulphur compounds present in the unweathered sediment as the electron donor mass balance calculations suggest that the increase in the downward rate of migration of the redoxcline owing to added sulphur consumption from no3 contamination is only about 1 mm year 1 at these sites review of the literature indicates that most silt and clay rich sediments have s contents in the same range or higher than those investigated here thus in most cases where aquifers are overlain by several metres or more of unweathered confining sediments it is likely that a high degree of protection is afforded from surficial no3 contamination

sedimentary organic matter sources and depositional environment in the yegua formation brazos county texas abstract the complex depositional environment of the eocene yegua formation brazos county texas can be better understood by integrating organic matter om geochemistry with stratigraphy yegua sediments represent parasequences separated by exposure surfaces organic petrography and geochemistry biomarkers c n ratios and carbon isotopes indicate the presence of both terrestrial and marine om in transgressive sediments in contrast regressive sediments contain only terrestrial om these differences relate to contrasting om sources and depositional styles on the shelf om in the sediments is immature and the potential for generating hydrocarbons is poor the study suggests that organic geochemical data can help in distinguishing transgressive and regressive environments in sedimentary formations

palaeogeography of radiolarite and organic rich deposits in mesozoic tethys siliceous and marine organic rich deposits are sometimes associated sometimes separate in space and time however both are generally accepted to be the result of high planktonic productivity among the siliceous marine deposits the phtanite family facies is distinguished from the radiolarite family facies by several characteristics they contain organic material and as a result are blackish vs red green for radiolarite facies their time of deposition corresponds with strong faunal modifications and they are deposited generally in shallower environments a palaeogeographic analysis of locations of tethyan biosiliceous and marine organic rich rocks both resulting from a high planktonic palaeoproductivity for three mesozoic high sea level intervals toarcian kimmeridgian and cenomanian show a during jurassic times these tethyan deposits were dissociated the siliceous deposits being closer to open ocean waters than the organic rich ones this is a common disposition in modern upwelling systems and suggests a common process b during cretaceous times these tethyan deposits were often associated i e both occur at the same site and are probably the result of a different process from that in the jurassic

equilibrium structure and dynamics of the california current system this paper addresses the structure and dynamical mechanisms of regional and mesoscale physical variability in the subtropical northeast pacific ocean using the regional oceanic modeling system roms the model is configured with a u s west coast domain that spans the california current system ccs with a mesoscale horizontal resolution up to as fine as 3 5 km its mean seasonal forcing is by momentum heat and water fluxes at the surface and adaptive nudging to gyre scale fields at the open water boundaries its equilibrium solutions show realistic mean and seasonal states and vigorous mesoscale eddies fronts and filaments the level of eddy kinetic energy eke in the model is comparable to drifter and altimeter estimates in the solutions with sufficiently fine resolution because the model lacks nonseasonal transient forcing the authors conclude that the dominant mesoscale variability in the ccs is intrinsic rather than transiently forced the primary eddy generation mechanism is the baroclinic instability of upwelling alongshore currents there is progressive movement of meanseasonal currents and eddy energy offshore and downward into the oceanic interior in an annually recurrent cycle the associated offshore eddy heat fluxes provide the principal balance against nearshore cooling by mean ekman transport and upwelling the currents are highly nonuniform along the coast with important influences by capes and ridges in both maintaining mean standing eddies and launching transient filaments and fronts

thermal history reconstruction using apatite fission track analysis and vitrinite reflectance a case study from the uk east midlands and southern north sea apatite fission track analysis afta is a method of obtaining ther mal history information in sediments in sections that have been hotter in the past afta provides estimates of maximum palaeotemperatures and the time at which a sedimentary section began cooling from maximum palaeotempera tures it is useful for studying the thermal history of sequences containing source rocks and can provide information critical to the understanding of the timing of oil generation integration of afta and vitrinite reflectance vr data allows a coherent thermal history framework to be established and a formal method of analysing palaeogeothermal gradients has been developed by combining afta and vr data this approach allows definition of the major facets of thermal history based on directly measurable parameters rather than on assumed values of palaeo heatflow and provides rigorous estimates of the amount of uplift and erosion the case study demonstrates the application of this approach to the east midlands shelf region here afta data have revealed an episode of maximum palaeotemperatures prior to cooling commencing at around 60 ma combined afta and vr data show that this cooling was due to kilometre scale early ter tiary uplift and erosion under near normal geothermal gradients our estimates of uplift and erosion are significantly greater than those of pervious workers and show that uplift and erosion was not restricted to the recognized axes of inversion but was regional in extent this has implications for the evolution of source rock maturity in this region and in other basins in around the uk where afta has indicated early tertiary heating and where the extent and amount of tertiary uplift and erosion may have also been underestimated by previous workers apatite fission track analysis afta tm is a method of obtaining thermal history information in sediments and is particularly useful for studying the thermal history of sequences containing hydrocarbon source rocks as well as providing estimates of maximum palaeotemperatures afta provides a direct estimate of the time at which a sedimentary section began cooling from its maximum palaeotemperature a key advantage over traditional geochemical methods in many situations this knowledge is critical to the understanding of the timimg of oil generation and migration in relation to trap formation information on the timing of maximum palaeotempera ture can also be of significant value in the interpretation of vitrinite reflectance vr data and combination of afta and vr data allows a consistent integrated thermal history interpretation the afta technique is applied most usefully in sedimentary sections which have been hotter in the past than they are at the present day it is therefore applicable to large regions of britain and the uk continental shelf that have been subject to deeper burial in the past or affected by other sources of heating the basis of the technique has

geological controls on the formation and distribution of gas in british coal measure strata abstract the methane ethane higher alkanes and nitrogen found in british coal seams are residual products of coalification occasionally complemented by petroleum derived hydrocarbons carbon dioxide a minor constituent of coal seam gas may have been introduced during the phase of cleat mineralisation following uplift helium and argon are considered to be radiogenic in origin variations in seam methane content with depth geographic location and rank can be interpreted in terms of the coalification and erosion history of a coalfield however the limitations inherent in an analysis in which coal rank is represented by a single parameter must be recognised there are indications that the methane content of a seam is not likely to exceed the methane sorption capacity of the coal determined at a temperature commensurate with that obtaining at the maximum burial depth research in the east midlands coalfield suggests that structurally controlled migration of coalification gas was contemporaneous with folding during the permo carboniferous erosion period gas was lost from outcropping seams subsequently the bulk permeability of the seams must have been reduced perhaps as a result of cleat mineralisation because no re distribution of methane appears to have occurred in response to later tertiary deformation and gas contents of seams at incrop below the base of the permo triassic have remained zero data on the effects of faulting on seam gas contents are few but the expectation is that faults of appropriate age and orientation whilst inhibiting in seam migration may in some instances have aided cross measures movement of gas elevated seam ethane and propane contents are frequently encountered in seams disturbed by oil bearing faults

in situ rates of sulfate reduction in an aquifer romo denmark and implications for the reactivity of organic matter estimates of rates of organic matter degradation by bacterial sulfate reduction in aquifers are few and all are obtained by indirect means here we present the first direct radiotracer measurements of in situ rates of sulfate reduction for an aquifer romo denmark sulfate reduction occurs in distinct reaction zones within the aquifer at rates that vary greatly over short lateral distances in situ rates of sulfate reduction are significantly higher than previous indirect estimates in contrast rates of sulfate reduction in aquifers are orders of magnitude lower than in marine and limmic environments emphasizing the low reactivity of natural organic matter in aquifers

regional velocity depth anomalies north sea chalk a record of overpressure and neogene uplift and erosion a normal velocity depth trend for the upper cretaceous danian chalk group is determined by identifying interval velocity data that represent maximum burial in areas unaffected by overpressuring these data are derived from 845 wells throughout the north sea basin data from pelagic carbonate deposits on a stable plateau constrain the trend for shallow depths positive velocity anomalies relative to the trend are mapped along the western and eastern margins of the north sea basin and reflect regional neogene uplift and erosion of up to 1 km along the present day limit of the chalk a hiatus at the base of the quaternary increases in magnitude away from the basin center where a complete cenozoic succession is found this hiatus is consistent in size with the missing section estimated from chalk velocities when allowance is made for the quaternary reburial of the chalk negative velocity anomalies in the central and southern parts of the basin outline an area within which overpressures in the chalk exceed 10 mpa equivalent to a burial anomaly greater than 1 km relative to the normal trend the chalk pressure system is primarily dependent on overburden properties because retention of overpressure generated by the load of the upper overburden depends on the thickness and sealing quality of the lower overburden therefore the chalk is considered to represent a regional aquitard and the hydrodynamic model of long distance migration within the chalk is rejected the neogene uplift and erosion of the margins of the north sea basin and the rapid late cenozoic subsidence of its center fit into a pattern of late cenozoic vertical movements around the north atlantic

geochemical variability induced by entrainment sorting quantified signals for provenance analysis abstract a substantial part of the mineralogical and chemical heterogeneity of clay poor sands at the bed scale results from tractive redistribution of the grains in the bedload since coarse and light particles are more easily entrained than denser and smaller heavy minerals the latter commonly concentrate in grain scale lags that alternate with sand depleted in these minerals when sampled and analysed at the centimetre scale this primary heterogeneity translates into nice geochemical covariations between the minor trace elements that concentrate in some heavy minerals zr ti p cr v y ce among others regressing these variations allow us to derive two chemical fingerprints of the sand one records the proportions between some heavy minerals the other the average composition of framework grains i e a sand composition corrected from the contribution of heavy minerals both relate to provenance and allow different sand beds to be compared in appropriate spider diagrams the construction of these diagrams their potential use and their limitations are discussed using two contrasting examples continental channel sandstones from the ravenscar group middle jurassic yorkshire uk illustrate how these geochemical fingerprints vary from the local to the group scales and how robust they are against diagenetic overprint at ravenscar the zr vs ti v cr covariations record the presence of ti oxides and magnetite together with zircon and of volcanic lithoclasts at the deposition stage despite a high degree of diagenetic alteration overbank sandstones lateral to channels from the deep marine gres d annot formation eocene oligocene alps illustrates how apatite and monazite delivered by a dominantly granitic source generate covariations between zr and p y ce and th contents with characteristically steep slopes

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

the petrology and diagenesis of middle jurassic clastic sediments ravenscar group yorkshire the petrology of non marine clastic sediments from the middle jurassic ravenscar group was investigated to identify and to assess the significance of possible controls on diagenesis diagenetic modifications took place in three broad regimes within the depositional groundwater eogenesis during burial mesogenesis and during uplift and erosion telogenesis eogenesis involved the initial interaction of the original sedimentary assemblage with its depositional pore waters eogenetic processes were influenced strongly by bacterial degradation of organic matter present in the finer grained sediments this bacterial activity caused a lowering of pore water ph and subsequently reduced eh feldspar dissolution and alteration of muscovite to kaolinite occurred quartz overgrowths and vermiform kaolinite precipitated in sands with oxygenated and mildly acidic pore waters whereas chlorite and quartz overgrowths formed in sands with anoxic and neutral pore waters this caused complete reduction of porosity in both finer grained and texturally less mature floodplain facies sandstones channel facies sandstones were cemented only with a thin veneer of quartz overgrowths creating a rigid but still porous quartzose framework bacterial ferric iron reduction throughout the water table subsequently established more ubiquitous conditions raised ph and caused rhombic siderite cementation within the remaining porosity mesogenetic modifications of the mineralogy formed at the surface occurred when pore water composition changed during burial bicarbonate saturated solutions migrated through the surviving porosity precipitating replacive poikilotopic ferroan calcite carbonate dissolution occurred subsequently carbonate dissolution was possibly caused by the same solutions which then precipitated dickite and further authigenic quartz telogenetic modifications took place during uplift and exposure groundwater reactions and subaerial exposure probably caused further carbonate and feldspar dissolution as well as alteration of chlorite to interstratified chlorite vermiculite it is concluded that the diagenesis of these non marine clastic sediments reflects various factors the complex interrelationship of which precludes the identification of any single factor as wholly controlling their diagenesis eogenetic modifications involved the interaction of the sediments and their pore waters as a result of the metastability of the mineral assemblage established at this time these modifications can still be recognized despite the sediments subsequent history

reactivity of organic matter in aquifer sediments geological and geochemical controls reduction rates in aquifers are commonly carbon limited but little is known about the molecular composition and degradability of sedimentary organic matter som in aquifer sediments the composition source and degradation status of som in aquifer sediments of fluvio glacial pleistocene and shallow marine pliocene origin were determined using flash pyrolysis gas chromatography mass spectrometry incubation experiments 106 d were used to assess the reactivity of som towards molecular oxygen a dominance of lignin derived components and long chain odd over even predominant alkanes indicate that terrestrial higher land plants were the main source of som even in the shallow marine sediments while bacterial lipid derived hopanoids and iso and anteiso c15 and c17 fatty acids indicate a minor contribution of microbial biomass no compositional difference was observed between som present in the fine 63 m and coarse fraction 63 2000 m a significant part of som was not present as low molecular weight compounds but was macromolecularly bound for the fluvio glacial sediments a relatively higher abundance of resistant macro molecular compounds was in agreement with stronger signs of aerobic lignin alkane and hopanoid oxidation the more degraded status of som in the fluvio glacial sediments was consistent with their significantly lower som mineralization 2 6 during incubation as compared with the shallow marine sediments 9 14 the reactivity towards oxygen of som was controlled by the extent of past aerobic oxidation not the age of som but the extent of oxygen exposure during syn and postdepositional conditions seems most important in affecting the degradation status of som in aquifer sediments and thus their ability to reduce oxidants copyright 2004 elsevier ltd

spatial and temporal variability of global ocean mixing inferred from argo profiles 1 the influence of turbulent ocean mixing transcends its inherently small scales to affect large scale ocean processes including water mass transformation stratification maintenance and the overturning circulation however the distribution of ocean mixing is not well described by sparse ship based observations since this mixing is both spatially patchy and temporally intermittent we use strain information from argo float profiles in the upper 2 000 m of the ocean to generate over 400 000 estimates of the energy dissipation rate indicative of ocean mixing these estimates rely on numerous assumptions and do not take the place of direct measurement methods temporally averaged estimates reveal clear spatial patterns in the parameterized dissipation rate and diffusivity distribution across all the oceans they corroborate previous observations linking elevated dissipation rates to regions of rough topography we also observe heightened estimated dissipation rates in areas of high eddy kinetic energy as well as heightened diffusivity in high latitudes where stratification is weak the seasonal dependence of mixing is observed in the northwest pacific suggesting a wind forced response in the upper ocean

thermal and tectonic history of the east midlands shelf onshore uk and surrounding regions assessed by apatite fission track analysis apatite fission track analysis has been used to study the thermal and tectonic history of the east midlands shelf and surrounding areas of onshore uk apatites from outcrops of palaeozoic crystalline basement give fission track ages between 300 29 and 420 31 ma which are the oldest yet reported in the uk region and reflect the relative stability of this cratonic region mean track lengths in these samples are between 12 and 13 m namurian and triassic sandstones from outcrop give apatite fission track ages between c 100 and c 390 ma with mean track lengths generally between 10 and 12 5 m distributions of single grain ages and track lengths suggest that all samples have been annealed implying exposure to higher temperatures since deposition with maximum palaeotemperatures reached some time in the 50 80 ma range prior to the onset of cooling samples from five exploration wells show consistent patterns in which apatite fission track ages decrease sharply from around or just less than the stratigraphic age at near surface to values in the 50 80 ma range at depths of c 1 2 km present temperatures of c 40 70 c showing that the samples studied have all been exposed to higher temperatures single grain ages and track length data suggest that maximum palaeotemperatures were attained at or before c 60 ma subsequent cooling was protracted and the data are compatible with two phases in the early and mid tertiary palaeotemperature analysis suggests that palaeo geothermal gradients were similar to present values and that elevated palaeotemperatures were due to greater depth of burial with cooling due to subsequent uplift and erosion estimates of uplift and erosion vary from c 1 1 5 km on the east coast around the humber estuary to c 2 2 km in the east midlands and southern pennines compilation of palaeotemperature estimates from all samples analysed from the uk onshore region shows that mesozoic burial and tertiary uplift has affected a wide area and that uplift was due to broad scale regional warping with recognized inversion axes being areas where uplift and erosion were particularly pronounced or where younger sediments and or inverted basinal structures allow the identification of uplift and erosion the recognition of late cretaceous early tertiary unconformities across wide areas of northwest europe both onshore and offshore suggests that the regional uplift and erosion may have been far more widespread than previously recognized this has important implications for hydrocarbon exploration in northwest europe

upper carboniferous source rocks north and south of the variscan front nw and central europe abstract some topical aspects of upper carboniferous source rocks within the three main tectonic settings in nw and central europe are described these are likely to offer an effective tool for reduction of risk in future exploration in the foreland the main geological factors responsible for the nitrogen content of gas accumulations and for the isotopic composition of methane are outlined it will probably be possible to predict the molecular and isotopic composition of gas accumulations in the future in addition the application of the dynamics of diffusion processes to the loss of gas accumulations over geological time offers the possibility of predicting hydrocarbon fill in some areas of the basin in the variscan fold belt the model of thin skin tectonics suggests that the lowermost parautochthonous low rank thrust sheets are more likely to be source rocks than are allochthonous high rank thrust sheets additionally young subsidence is required in order to meet the conditions of gas dynamics the possible source rock potential of the permo carboniferous fills of the intramontane grabens south of the variscan front are reported here for the first time some promising discoveries below the swiss and german molasse basin as well as in the eastern paris basin mean that the intramontane grabens are now an interesting exploration objective

surface area control of organic carbon accumulation in continental shelf sediments the relationship between organic carbon oc and grain size found in most continental shelf sediments is here reinterpreted in terms of the surface area of the sediments cores from many north american shelf environments show downcore decreases in oc to similar refractory background concentrations if expressed relative to the surface area of the sediments this consistent concentration is 0 86 mg oc m 2 which is equivalent in concentration to a monolayer of organic matter coating all mineral surfaces a more global collection of sediment water interface samples show that this relationship is even more extensive with exceptions occurring in areas of very high riverine sediment input organic pollution or low oxygen water columns density separations indicate that organic matter is largely adsorbed to mineral grains the microtopography of surfaces was examined with n2 sorption and most surface area was found to be inside pores of 10 nm width these data lead to a hypothesis that organic matter is protected by its location inside pores too small to allow functioning of the hydrolytic enzymes necessary for organic matter decay such protection would likely work in concert with other protection mechanisms such as humification this consistent surface area correlation with oc concentration may explain control of spatial and temporal variations in oc burial rates by sedimentation rates the pore protection hypothesis provides a causal mechanism for this observed control