isolation of diagenetic overgrowths on quartz sand grains for oxygen isotopic analysis abstract diagenetic overgrowths on detrital quartz sand grains in sandstones can be isolated for direct measurement of their oxygen isotope ratios interfacial materials between detrital cores and diagenetic overgrowths are dissolved by etching with dilute hf the weakened interface breaks when grains are agitated ultrasonically subsequent particle size separation is an effective means of concentrating the diagenetic component

geothermometry and geochemistry of aqueous and oil bearing fluid inclusions from fateh field dubai abstract the production of oil from fateh field dubai is principally from the cretaceous mishrif formation porosity in the reservoir facies is partly occluded by late coarse grained calcite cement this cement contains both aqueous and oil bearing fluid inclusions fluid inclusion geothermometry was used to determine the temperature of that cement precipitation and calculate its timing relative to basin burial history these data were related to source rock maturation and from that the timing of oil migration was inferred to be oligocene to mid miocene which is geologically contemporaneous with early petroleum generation r 0 0 5 0 6 in the khatiyah source rock the fluid inclusions were analyzed by a thermal decrepitation gas chromatography technique benzene and low molecular weight paraffins were present and a series of normal alkenes was tentatively identified in the aqueous inclusions fresh oil inclusions consisted of c 1 n c 25 compounds whose light hydrocarbon composition was consistent with derivation from an early mature source rock altered petroleum residues detected in many mishrif samples are contained either in inclusions or along crystal cleavages these materials are the subject of an ongoing geochemical study of fateh field

segmentation and differential post rift uplift at the angola margin as recorded by the transform rifted benguela and oblique to orthogonal rifted kwanza basins abstract we analyse tectonic and sedimentary field and subsurface data for the angola onshore margin together with free air gravity anomaly data for the offshore margin this enables us to characterize the mode of syn rift tectonism inherited from the precambrian and its impact on the segmentation of the angola margin we illustrate that segmentation by the progressive transition from the benguela transform rifted margin segment to the oblique rifted south kwanza and orthogonal rifted north kwanza margin segments the spatial variation in the intensity of post rift uplift is demonstrated by the study of a set of geomorphic markers detected in the post rift succession of the coastal benguela and kwanza basins upper cretaceous to cenozoic uplifted palaeodeltas erosional unconformities palaeovalleys quaternary marine terraces and perched gilbert deltas the onshore benguela transform margin has a distinctive mainly progradational stratigraphic architecture with long term sedimentary gaps and high elevation marine terraces resulting from moderate upper cretaceous cenozoic to major quaternary uplifting i e 775 1775 mm ky or m ma by contrast repeated synchronous episodes of minor cenozoic to quaternary uplift occurred along the orthogonal rifted north kwanza segment with its cenozoic aggradational architecture short term sedimentary gaps and low elevation pleistocene terraces margin style likewise governs spatial variations in the volume of offshore sediment dispersed in the associated deep sea fans along the low lying north kwanza margin sedimentation of the broad cenozoic to pleistocene kwanza submarine fan was probably governed by the width of the kwanza interior palaeodrainage basin combined with the wet tropical neogene climate along the high rising benguela margin the small size of the benguela deep sea fan is related to the interplay between moderate continental sediment dispersal from long lived small catchments and a warm very arid neogene climate however the driving forces behind the epeirogenic post rift uplift of the angola coastal bulge remain a matter of speculation

an improved method for reconstructing the stratigraphy and bathymetry of continental margins application to the cenozoic tectonic and sedimentary history of the congo margin a number of techniques such as backstripping and forward modeling have been previously used to reconstruct the tectonic sedimentary and thermal history at passive margins still these techniques are generally inaccurate in places such as the west african margin where the stratigraphic record is disrupted by continuous faulting of the sediment pile caused by salt tectonics and by erosional unconformities the ability to reconstruct the stratigraphy salt thickness and water depth along profiles of these types of margins is important for determining the factors controlling the geological history of the margin and the evolution of hydrocarbon systems to reconstruct the stratigraphy paleowater depths and salt flowage on a profile across the congo margin we combined backstripping and palinspastic reconstruction this enabled us to restore the faulting and salt flowage and simultaneously to take into account the isostatic response of the lithosphere to sediment loading and unloading this method allowed us to estimate the paleowater depth as well as the stratigraphy and the geometry of the sedimentary section on the profile through time the determination of the regional isostatic response of the lithosphere is performed with the use of a new model to estimate the flexural rigidity of continental lithosphere to demonstrate this method we perform the cenozoic reconstruction of the congo margin the results yield a coherent geological history of the margin from the eocene to the present in the eocene the margin was a carbonate ramp with a deep seated shelf break 600 m during the eocene oligocene transition intermediate water depth oceanic currents triggered submarine erosion of the eocene slope and shelf from the miocene to the present we observe a large increase in the input of terrigenous clastic sediments as a result the shelf prograded 40 km across the eocene ramp as the clastic sediments prograded across the margin into the deep basin faulting of the sediment pile and salt withdrawal caused the formation of grabens in places where salt swells had previously formed finally the reconstruction suggests that the congo margin was uplifted by at least 330 m between the late oligocene and the middle miocene

diagenesis of the lower cretaceous arkoses of the angola margin petrologic k ar dating and 18 o 16 o evidence abstract the lower cretaceous arkoses of the angola offshore basin have been extensively diagenetically altered the results of a multidisciplinary study which included petrography k ar dating and 18o 16o and 13c 12c ratio measurements combined with fluid inclusion data provide information about the timing and temperatures of formation of the diagenetic cements as well as the nature and origin of the diagenetic fluids during burial k feldspar overgrowths formed at around 100 ma followed by discrete development of quartz overgrowths locally both are preceded by chlorite rim cement then chronologically dolomite calcite and clay cements precipitated the microcrystalline clays are an imii ordered illite smectite of eocene age 50 ma and or a pure illite of miocene age 16 ma both are commonly mixed with a fe rich chlorite fluid inclusion data walgenwitz and pagel 1984 walgenwitz et al 1985 1989 from the k feldspar quartz and dolomite cements indicate that diagenetic fluids were high temperature 110 165 c brines 24 wt eq nacl estimated temperatures of formation of the diagenetic clays based on k ar ages and burial history curves are 66 125 c for the imii illite smectite and 140 195 c for the illite these data suggest the existence of high geothermal gradients during the cretaceous and a diminution of the geothermal gradient from cretaceous to present the 18o 16o ratios indicate that diagenetic fluids were enriched in 18o with 18o values between 1 7 and 9 5 per rail for the imii illite smectite and between 5 6 and 10 5 per mil for the k feldspar dolomite and illite cements such values preclude any significant direct contribution of meteoric water these 18o rich brines probably derived from the aptian evaporites overlying the sediments studied

geochemical characterization of selected western canada oils by c5 c8 compound specific isotope correlation csic abstract forty two oils condensates from the western canada sedimentary basin wcsb ranging in reservoir age from devonian to cretaceous are compared using the stable carbon isotopic ratio of individual hydrocarbons in the gasoline range fraction c 5 c 8 of these oils continuous flow isotope ratio mass spectrometry cf irms using a modified solid dilution purge and trap inletting system can provide reproducible isotopic ratios for the ca 26 dominant n iso and cyclo alkanes and aromatics in the gasoline range the resulting isotopic signatures or gasoline isotopograms for oils condensates provide a diagnostic geochemical technique for oil oil and oil source correlations in many instances the non straight chained hydrocarbons are more diagnostic than n alkanes hierarchical cluster analysis provides a non subjective approach to the treatment of individual isotopic information this study confirms that oils from similar or the same sources produce isotopic signatures that are highly correlated clear carbon isotopic relationships between individual hydrocarbon compounds in the oils suggest a steady state kinetic relationship for the generation of oil and gas as opposed to a thermodynamic equilibrium relationship this mechanism explains the consistent shift in isotopic ratios between similar compounds such as the methylalkanes and dimethylalkanes

authigenic potassium feldspar a tracer for the timing of palaeofluid flow in carbonate rocks northern calcareous alps austria

tectonic history along the south gabon basin anomalous early post rift subsidence an integrated study of the south gabon margin south atlantic based on reflection seismic and well data has been performed to quantify tectonic activity a regional profile crossing the entire basin together with subsidence analysis highlights important aspects of the post rift history the most striking event in the margin evolution appears to be the anomalous extra subsidence during the early post rift period characterized by high sedimentation rates equivalent to one third of the syn rift subsidence although the presence of evaporite layers restricts knowledge of the underlying structures essentially composed of pre rift and syn rift sequences the outcome of this post rift tectonic study has strong implications for the rifting history the early post rift subsidence patterns can be related to a high thermal anomaly during the early rifting thermal state of the lithosphere these findings are highly relevant for petroleum system studies and have implications for hydrocarbon generation

thermo chronological approach to reservoir diagenesis in the offshore angola basin a fluid inclusion 40 ar 39 ar and k ar investigation 1 microthermometric analyses of fluid inclusions and geochronological surveys of authigenic silicates in neocomian sandstones and albian dolomitic reservoirs from the offshore angola basin have allowed determination of the age and the physical chemical conditions for fluid migration in a late diagenetic stage the 40 ar 39 ar laser probe dating of adularia overgrowths from the albian reservoirs shows that the trapping of primary aqueous and oil inclusions occurred during the early miocene 25 6 or 3 2 ma the k ar age of authigenic illites from the neocomian sandstones is slightly younger 19 or 1 ma and the crystallization of regular illite smectite mixed layers is related to a previous eocene to paleocene diagenetic stage 51 58 ma there appears to have bee both a regional and vertical variation in the salinity of aqueous fluids during the miocene the fluid inclusion trapping temperatures were estimated by correcting the homogenization temperatures of aqueous fluids assuming that pressures during the miocene were close to the present reservoir pressures and by intersecting the isochores of aqueous and oil inclusions assuming they were trapped at strictly similar pressure and temperature conditions the fluid inclusion temperatures are consistent with those derived from the tetrahedral aluminum content aliv of authigenic chlorites 168 206 degrees c in the albian reservoirs at depths between 2350 and 3580 m and 175 237 degrees c in the neocomian sandstones at depths between 2770 and 4470 m in both reservoir formations miocene tempe atures were higher than the present ones and related either to deeper burial or higher geothermal gradient

separation kar dating and 18o16o ratio measurements of diagenetic k feldspar overgrowths an example from the lower cretaceous arkoses of the angola margin abstract k feldspar overgrowths adularia on detrital k feldspar were removed from the lower cretaceous 130 115 ma arkoses of angola and concentrated for oxygen isotopic analysis and k ar dating the concentration technique consisted of etching with dilute h2sif6 followed by ultrasonic agitation and particle size separation the diagenetic k feldspar content of each of fourteen subfractions as estimated by cathodoluminescence was between 45 and 92 percent the rest being mostly detrital microcline and in some cases small amounts of quartz and or albite extrapolation of 40 ar 40 k and 18 o 16 o ratios indicated a k ar age of the adularia of 98 16 ma and a 18o of 20 2 0 8 per mil smow the age is consistent with petrographic observations which suggest that precipitation of adularia occurred relatively early in the diagenetic history the k ar age together with the 18o value burial history and fluid inclusion data indicate that adularia formed at relatively shallow depth

maximum diagenetic temperature in a petroleum source rock from venezuela by fluid inclusion geothermometry abstract the maximum diagenetic temperature in the la luna petroleum source rock from a well in venezuela was determined by fluid inclusion geothermometry the result of 145 160 c is in good agreement with the formation temperature deduced from well logs being 157 c the results further indicate that quartz vein fillings were formed 4 ma ago at t 125 130 c at the onset of liquid hydrocarbon expulsion from the source rocks

vertical evolution of fluid venting structures in relation to gas flux in the neogene quaternary of the lower congo basin offshore angola abstract the processes preceding the genesis of pockmarks and their subsequent evolution has been rarely studied since pockmarks were discovered in 1970 a high resolution 3 d seismic survey from the continental slope of the lower congo basin allows us to study in detail the structures that developed before the formation of pockmarks as well as their subsequent geometric evolution this paper aims at reporting new type of fluid related structures that relate to pockmarks the objective of this research is to understand 1 the evolution of these structures and their genetic relationship with pockmarks and 2 how their morphologies may be influenced by changes in venting intensity over time pockmark and chimney structures in this survey are hosted within late miocene to quaternary hemipelagites above clastic reservoirs in a petroliferous basin they are all associated with positive high amplitude anomalies phaas on seismic data phaas are interpreted from their dramatically higher acoustic impedance relative to the background sedimentary values of the host units and by their association with seismic pull up effects from their planform and cross sectional geometry phaas are primarily interpreted as being due to the development of methane related carbonates but they may be associated with gas hydrates in one or two examples methanogenic carbonates form either by authigenic accumulation or by anaerobic methane oxidation amo once upwardly migrating methane enters the sulfate methane transition zone smtz typically within the first few tens of meters below the seabed therefore vertical accumulations of phaas are interpreted to reflect the temporal evolution of methane flux migrating upward to reach the paleo seabed or smtz timing of local scale fluid leakage can therefore be diagnosed by analyzing phaas at seep locations that occur in association with pockmark chimney structures a conceptual model for the evolution of different fluid venting structures is proposed here and is based on the analysis of venting structures that we expressed by 3 d seismic reflections in this model the main features are linked into a process based sequence culminating in the infill and termination of the pockmarks as fluid venting sites our model summarizes different rhythms of venting which are expressed by different types of phaas along with venting structures linear phaas and conduits inferred slow seeps they can be succeeded by sub circular phaas and shallow depressions which inferred slow to moderate seeps then the seep can become either a fast vent and terminated into a pockmark crater or it can become less active and be sealed afterwards subsurface sediment fluidizations and remobilizations are likely to happen in the last case and are probably caused by hydrate dissociations

perturbation of polygonal fault propagation by buried pockmarks and the implications for the development of polygonal fault systems abstract evidence is presented that constrains the earliest part of the growth history of a polygonal fault tier using high resolution 3d seismic data from the west african margin we document the interaction between a shallow buried wedge shaped polygonal fault tier and pockmarks located at the base of the fault tier strongly curved patterns of perturbed polygonal faults are mapped in the shallowest portions of a tier overlying a series of pockmark craters these perturbed patterns differ markedly from the background polygonal pattern developed away from areas of pockmark influence perturbation is attributed to local stress effects relating to residual topography of the underlying pockmarks by analysing the thickness variations of mapped layers within the polygonally faulted tier we demonstrate that any residual topography was most likely healed after approximately one third of the total tier thickness was deposited hence the nucleation of the polygonal faults into their perturbed propagation trajectories must have been established within the first 100 150 m of burial from these observations of early nucleation in the basal third of a tier we propose a new model of polygonal fault growth in which individual faults nucleate near the base of a tier at an early stage of burial and then propagate preferentially upwards with basal tips restricted by an underlying mechanically competent unit the base of the tier continued growth then shifts the locus of displacement such that final displacement distributions are more axio symmetrically disposed with respect to the base and top of the tier

hump shaped 40ar 39ar age spectra in k feldspar and evidence for cretaceous authigenesis in the fountain formation near eldorado springs colorado the fountain formation near eldorado springs co usa shows evidence of alteration by hydrothermal fluids that precipitated authigenic potassium feldspar adularia as rims on detrital feldspars and as interstitial cement deposits detailed 40ar 39ar step heating experiments of samples from the fountain formation reveal age spectra having a characteristic hump at laboratory temperatures between 900 and 1080 c the humps appear to be related to the presence of adularia laser analyses of small grains of adularia indicate that the hump can be physically dissected unlike age gradients found in igneous feldspars such behavior is consistent with the mixing of gas from two or more generations of k feldspar each having unique diffusion properties the feldspars studied here indicate that two pulses of 150 c hydrothermal fluids migrated through the fountain formation at 135 and 94 ma prior to the main phase of laramide tectonic activity in the region the limited occurrence of authigenic cements suggests that reactivation of the underlying precambrian idaho springs ralston creek shear zone was significant enough to heat and mobilize large quantities of meteoric fluids in addition these data also suggest that cretaceous movements along the transcontinental arch correlated with stratigraphic events began approximately 40 million years earlier than previously thought

contraction induced by block rotation above salt angolan margin gravity spreading above salt at passive margins is the major mode of deformation of post salt sediments whereas this process generally creates a structural zoning extensional upslope and contractional downslope discrepancies can however arise for example evidence of contractional deformation occurs in the extensional domain of the angolan margin to the south of the congo delta fan slope parallel seismic lines show grabens rollover and extensional diapirs conversely strike parallel seismic lines present inversion of early grabens apparently related to a regional scale decrease in sedimentary thickness away from the congo delta as the spreading rate and the characteristic spacing of structures are direct functions of sedimentary loading one can expect structural changes along strike due to sedimentary thickness variations this hypothesis was tested using spreading type experiments of brittle ductile models lying on top of an inclined rigid substratum the experiments simulate the progradation of a synkinematic sedimentary cover above salt with a lateral variation of sedimentation rate the models show that the spreading rate was higher in the thicker part early grabens initiated perpendicular to the slope direction where sedimentation rate was high they kept their orientation during spreading and formed purely extensional synsedimentary structures grabens rollovers and diapirs where sedimentation rate was low blocks separated by grabens rotated in a domino type fashion but this domain continued to extend in a slope parallel direction strike slip between blocks was entirely localised within the early grabens which inverted and formed anticlines structures obtained in experiments are directly comparable to those in seismic lines of the angolan margin in both the angolan margin examples and the laboratory experiments block rotation is interpreted as slope parallel strike slip shear zones due to lateral variations in spreading rate

fluid history during deep burial and exhumation of oil bearing volcanics hercynian belt of southern brittany france in southern brittany france the porphyroid nappe a variscan tectonic unit made of volcanic pyroclastic deposits that was involved in a low temperature high pressure prograde metamorphic path followed by exhumation and extension displays evidence for very large and distributed strains achieved through dissolution crystallization processes such distributed strains at low temperature are rather unusual in felsic lithologies recent studies have documented that these rocks deformed within a system closed to fluids this paper presents results of detailed analyses of fluid inclusions and stable isotopes within the deformed metavolcanics and their undeformed counterparts in order to document the fluid history of the tectonic unit results show that its initial thermal evolution likely occurring in a context of basin inversion involved important open system alteration with k metasomatism adularization oil impregnation and hydration during the subsequent tectonic evolution the fluids maintained a constant composition in the c h o n system within the porphyroid nappe and its para autochthon being water rich and containing hydrocarbon components likely related to oil pyrolysis oxidation atoll like fluid inclusion morphology and rare undisturbed fluid inclusions with steep isochore slopes are consistent 750 mpa with pressures estimated for the peak of metamorphism using mineral solid solution thermodynamics fluid inclusions indicate a sub isothermal retrograde path between 420 and 300 c followed by a high thermal gradient at the end of the retrograde path within the underlying para autochthonous unit only the late high temperature gradient is recorded by fluid inclusions the sub isothermal low grade path suggests fast exhumation of the unit without thermal relaxation the high geotherm observed at the end of the p t path is consistent with the post thickening extensional collapse and associated intrusion of granites documented in the area hydration and heterogeneous oil water distribution achieved in the early metasomatic evolution of the porous pyroclastic deposits are responsible for subsequent fluid confining in the porphyroid nappe and for its strongly distributed strain pattern

dynamic interactions between the gulf of guinea passive margin and the congo river drainage basin 2 isostasy and uplift 1 the gulf of guinea continental margin is paralleled by a coastal relief that isolates the congo drainage basin and can play a subsequent role in interactions between margin deposits and continental denudation migration of depocenters from the ogooe and kwanza rivers to the congo fan in the oligocene low present day sediment delivery or pleistocene submarine channels instabilities are possible evidences for this interaction in order to test this hypothesis we examine successively margin isostasy regional uplift erosional unloading and flexural bulge created by sediment loads satellite free air gravity and two dimensional thermomechanical modeling including depth of necking are used to determine the structure of continental margin and strength conditions for two selected sections across south gabon and congo fan both verify similar conditions for extension geometry depth of necking 10 20 km and strength equivalent elastic thickness of 20 30 km differences in observed gravity are explained mainly by differences in sediment volumes regional uplift during miocene of at least 450 m is attributed for both continent and margin as evidenced by the elevated base level of the congo drainage basin and the uplift of shelf break and coastal plains topography corrected for regional uplift valley erosion below relics surfaces and corresponding erosion unloading has been determined as a proxy of oligocene topography it shows a typical rift shoulder topography isolating congo drainage basin from margin in such conditions small amplitude flexural uplift related to sediment loads can possibly have negative feedbacks for both oligocene and present time

geochemistry and origin of sour gas accumulations in the northeastern sichuan basin sw china significant natural gas reserves have recently been discovered in the lower triassic oolitic reservoirs from northeastern sichuan basin sw china in the wake of the december 2003 sour gas well blow out this study presents an overview on the petroleum geology and geochemistry of the sour gas accumulations in the study area two types of natural gas accumulations were identified in the lower triassic oolitic reservoirs both containing highly mature thermogenic gases with their hydrocarbon source rocks in upper permian strata natural gases from the area south of the ancient kaijiang liangping seaway are generally sweet gases formed as the result of thermal maturation whereas those discovered from north of the seaway are products of both thermal maturation and thermochemical sulfate reduction of early accumulated oils in the feixianguan formation reservoirs the proposed origins of the gases are supported by their chemical and stable carbon isotope compositions as well as the presence or absence of pyrobitumens in the reservoir the distribution of gas accumulations is controlled predominantly by the combination of lithologic and structural factors the regional variation in the concentrations of h2s in the gases appears related to the presence and thickness of anhydrite bearing evaporitic rocks interbedded or intercalated with the oolitic reservoirs

thermochemical sulphate reduction in cambro ordovician carbonates in central tarim h2s and co2 are found in elevated concentrations in palaeozoic reservoirs in the tarim basin in china we have carried out analyses on gas petroleum mineral cement and bulk rock compositions and isotope ratios together with petrography and fluid inclusion to assess the origin of the h2s a deep crustal e g volcanic origin of the h2s and co2 is unlikely since the inert gases n2 and he have isotope ratios totally uncharacteristic of this source organic sources are also unlikely since the source rock has low a sulphur content and the sulphur isotope ratio of the petroleum correlates positively with the sulphur content the opposite of what would be anticipated from petroleum derived h2s bacterial sulphate reduction is unlikely because temperatures are too high for bacteria to have survived thermochemical sulphate reduction of petroleum fluids by anhydrite in lower ordovician and cambrian carbonate reservoirs is the most likely source of both the h2s and the co2 causing isotopically characteristic pyrite co2 gas and calcite cement h2s and possibly co2 migrated into silurian sandstone reservoirs by cross formational flow the h2s with the same sulphur isotope ratio as ordovician anhydrite was partially lost from the fluid phase by extensive growth of late diagenetic pyrite similarly the co2 was partially lost from the fluid phase by precipitation of late diagenetic calcite the h2s that resulted from tsr underwent reaction with the remaining petroleum resulting in locally elevated organic sulphur concentrations in the petroleum and the progressive adoption of the ordovician anhydrite sulphur isotope ratio

egyptian tethyan margin in the mesozoic evolution of a mixed carbonate siliciclastic shelf edge from western desert to sinai abstract during the mesozoic the egyptian margin was an oblique margin with nw se oriented transform fault system in the eastern mediterranean basin mesozoic margins were characterized by mixed carbonate siliciclastics platforms where subsidence and eustasy were the main parameters controlling the facies distribution and geometries of the platform to basin transition geometries and facies on the platform slope basin profile are today well constrained on the levant side but are still poorly known on the egyptian side geometries and stratigraphic architecture of the egyptian margin have been mapped thanks to a regional seismic and well data base provided by a joint industrial academic group upmc total this study brings new understandings on the seismo stratigraphic architecture of the platform slope basin system in a key area from western desert to nile delta and the levant margin mapping of the top jurassic and top cretaceous horizons shows the seismic geomorphology of the margin and the hinge line from western desert to sinai during the jurassic carbonate platforms were prograding with a distal thickening of the external platform with gentle slope profiles paleo valleys and embayments since the cretaceous the margin shows an aggrading and retrograding mixed carbonate siliciclastic platform succession with abrupt nw se trending paleo shelf edges affected by large gravity slide scars and distally steepened segments this structure of the platform edges is strongly controlled by the inherited tethyan transform fault directions along the hinge line embayments are interpreted as megaslides the basin filling is characterized by an alternation of chaotic seismic facies and high amplitude reflectors onlapping the paleoslopes mass transport complex mtc deposits are characterized by the mobilization of thick sedimentary sections up to 3500 m in thickness as a mixed amalgamation of debris flows internally preserved blocks and or compressively deformed distal allochthonous masses transported materials have been sourced from the dismantling of the mesozoic mixed carbonate siliciclastic platform and they can spread down slope over areas as large as 70 000 km 2 based on stratigraphic correlations with global sea level changes platform instability could have been triggered by the gravitational collapse of the carbonate siliciclastic platform under its own weight after successive subaerial exposures periods which were able to enable karstification processes

application of the coherent to incoherent intensity ratio to estimation of ocean surface roughness from high frequency shallow water propagation measurements for acoustic propagation through a shallow ocean channel or waveguide the coherence between different transmissions is controlled primarily by the roughness of the ocean surface and to a lesser degree by fluctuations in the volume in this study the coherent to incoherent intensity ratio ctir is defined as a way to quantify the coherence between multipath transmissions and ocean surface rms wave height and wind speed a theory that connects the ctir and the coherent surface reflection coefficient is developed using both kirchhoff and small slope approximations as rough surface scattering models the ctirs have been evaluated over a period of several days using broad band experimental results from shallow water deployment of source and receiver arrays that span most of the water column estimates of wind speed and rms wave height obtained using these ctir calculations are compared with environmental measurements to demonstrate the validity of the theory

the long term evolution of the congo deep sea fan a basin wide view of the interaction between a giant submarine fan and a mature passive margin zaiango project abstract we have integrated the relatively unknown distal domains of the lower congo basin where the main depocenters of the congo submarine fan are located with the better constrained successions on the shelf and upper slope through the analysis of thousands of km of 2d seismic reflection profiles off shore the congo angola passive margin the basin architecture is depicted by two ca 800 km long regional cross sections through the northern congo and southern angola margin a large unit deposited basinward of the aptian salt limit is likely to be the abyssal plain equivalent of the upper cretaceous carbonate shelf that characterized the first post rift deposits in west equatorial african margins a latest turonian shelf deepening event is recorded in the abyssal plain as a long period coniacian eocene of condensed sedimentation and basin starvation the onset of the giant tertiary congo deep sea fan in early oligocene following this event reactivates the abyssal plain as the main depocenter of the basin the time space partitioning of sedimentation within the deep sea fan results from the interplay among increasing sediment supply margin uplift rise of the angola salt ridge and canyon incision throughout the neogene oligocene early miocene turbidite sedimentation occurs mainly in nw se grabens and ponded inter diapir basins on the southern margin angola seaward tilting of the margin and downslope salt withdrawal activates the up building of the angola escarpment which leads to a northward congo shift of the transfer zones during late miocene around the miocene pliocene boundary the incision of the congo submarine canyon confines the turbidite flows and drives a general basinward progradation of the submarine fan into the abyssal plain the slope deposition is dominated by fine grained hemipelagic deposits ever since results from this work contribute to better understand the signature in the ultra deep deposits of processes acting on the continental margin as well as the basin wide sediment redistribution in areas of high river input

origin of reservoir fractures in little knife field north dakota thin vertical planar fractures observed in the mission canyon formation at the little knife field are naturally occurring and appear to be extension fractures the predominant east west trend of the fractures measured in oriented core from six wells parallels the contemporary maximum horizontal compressive stress in the williston basin the fractures occur only in carbonate units but within the carbonates their occurrence is not lithology dependent fracture density measured in cores of the reservoir carbonates averages 1 ft 3 m of fracture per 2 3 ft 7 m of core the formation and mineralization of reservoir fractures were the most recent diagenetic events in the mission canyon formation at little knife study of aqueous and hydrocarbon fluid inclusions associated with the fractures reveals 1 fractures formed after the strata were buried to at least their present depth of 9 800 ft 2 987 m which indicates their age is post mesozoic 2 the pore fluid pressure gradient was normal hydrostatic immediately after if not during fracture system development 3 formation water salinity has remained fairly constant since fracture initiation 4 migration of hydrocarbons into the reservoir probably preceded fracture genesis and 5 methane concentration may have decreased since fracture initiation

carbon isotope fractionation during methane dominated tsr in east sichuan basin gasfields china a review thermochemical sulfate reduction is considered to result in h2s 10 and high dryness coefficient values in the lower triassic and upper permian carbonate reservoirs in the ne sichuan basin the gases pro duced from gasewater transition and water intervals were measured to have h2s higher than 30 and are calculated to have significantly higher h2s and co2 contents than a gas produced from a gas interval and thus are not used to reflect tsr extents methane and ethane were shown to shift positively in carbon isotopes as a result of tsr however the fractionation has not quantitatively described a loga rithmic relationship is found to give the best fit for methane d 13 c 1 and 1 h2s

scales of transient convective support beneath africa beneath sub equatorial africa there is convincing evidence for a gigantic slow velocity anomaly within the lower mantle that is inferred to be a thermochemical superplume here we define the spatial and temporal pattern of uplift across the northern edge of this putative superplume by analyzing stratal geometries and stacking velocities of neogene sedimentary rocks these marine deposits are imaged on a dense grid of seismic reflection profiles that straddle the southwest african coastal shelf inverse modeling of stacking velocity profiles demonstrates that there have been at least two significant phases of uplift a post pliocene uplift event increases from 0 to 500 m elevation over a distance of 1000 km in close agreement with geophysical estimates of the present day variation of dynamic topography an earlier phase of uplift occurred in middle oligocene time ca 30 35 ma ago we propose a two stage development of epeirogeny in which a middle oligocene phase of uplift may be related to initiation and growth of the african superplume post pliocene denudation records rapid 100 250 m ma vertical motions generated by upper mantle convection which vary on length scales of 500 1000 km our results support the existence of a convective circulation system that has operated on different temporal and spatial scales beneath the african plate

the influence of thermochemical sulphate reduction on hydrocarbon composition in nisku reservoirs brazeau river area alberta canada abstract the upper devonian nisku formation reservoirs of the brazeau river area of west central alberta canada produce oil and sweet and sour gas condensate generally oil pools are located updip in the study area and sour 6 31 h2s gas condensate downdip h2s in the study area is formed by thermochemical sulphate reduction tsr all liquid hydrocarbons probably have one source with the duvernay formation being the most likely candidate assessing the relative maturity of the oils and condensates is difficult because of the wide variation in thermal maturity and the effects of tsr but the ratio of pristane n heptadecane does appear to decrease with increasing maturity for this sample set with increasing tsr the following changes were noted decrease in the saturate aromatic hydrocarbon ratio increase in the relative abundance of organo sulphur compounds e g benzothiophenes 34s values of liquid hydrocarbon samples approached the values for anhydrite of the nisku formation in the study area and an increase in 13c of the saturate fraction h2s concentrations in hydrodynamically isolated pools provide a good estimate of the extent of tsr in these reservoirs however other pools have anomalously high h2s concentrations for their depth suggesting that the h2s was generated at greater depths and migrated updip into these pools

fluid inclusion petrography and tectonic burial history of the al ali no 2 well evidence for the timing of diagenesis and oil migration northern oman foredeep cores cut in shallow marine lower and middle cretaceous carbonates in the al ali no 2 well umm al qaiwain united arab emirates display a five stage sequence of calcite cementation fracturing and stylolitization these stages are correlated with the tectonic history of the northern oman orogeny occurrence of fluorescent hydrocarbon fluid inclusions is restricted to stage 2 and stage 3 fracture filling calcites that are crosscut by stylolites stylolite formation during rapid burial beneath abyssal campanian foredeep sediments restricts hydrocarbon migration to post mid cenomanian and pre mid campanian time

ar ar dating of authigenic k feldspar quantitative modelling of radiogenic argon loss through subgrain boundary networks we have analysed two distinct generations of authigenic k feldspar in fucoid bed sandstones from an t sron and skiag bridge nw highlands scotland which have experienced post growth heating to levels in excess of the predicted ar closure temperature authigenic k feldspars show microtextural similarities to patch perthites that is subgrains separated by dislocation rich boundary networks that potentially act as fast diffusion pathways for radiogenic argon the two generations of authigenic k feldspar in the fucoid bed sandstones can be distinguished by different microtextural zones bulk mineral compositions fluid inclusion populations and inferred temperatures and chemistries of parent fluids ar ar age data obtained using high resolution ultraviolet laser ablation show that the first cementing generation is ordovician and the second cementing generation is silurian modelling of ar diffusion using subgrain size as the effective diffusion dimension and a simplified tectono thermal thrust model assuming transient heating of the fucoid beds is inconsistent with observed data removal of heat from the thrust zone through rapid flushing of heated fluids rather than transient heating can be invoked to explain the observed ar ar ages for both generations of cement alternatively ar diffusion modelling using overgrowth thickness as the effective diffusion dimension instead of subgrain size also yields models that are consistent with both the fucoid bed palaeothermal maxima and determined ar ar age ages for the two generations of k feldspar cement based on this alternate explanation we propose a theoretical microtextural model that highlights fundamental differences between the microtextures of deuterically formed patch perthites and authigenic k feldspars explaining the apparent robustness of authigenic k feldspar with respect to ar retention

dynamic interactions between the gulf of guinea passive margin and the congo river drainage basin 1 morphology and mass balance 1 a sediment budget between the congo river drainage basin and the western african margin in the gulf of guinea is proposed on the basis of published and unpublished offshore tertiary isopach maps and onshore digital elevation analysis the overall denudation of that area may be as high as 3 5 106 km3 with a maximum of 10 coming from mesozoic and cenozoic covers the southern part of the congo river basin related to uplift of the south african and kalahari shields appears as the most immature from the morphologic standpoint but provides one third of the present day sediment production it represents a maximum denudation of 150 000 km3 on the easternmost part east african rift drainage basins show a more mature relief and a maximum denudation of 270 000 km3 which can increase to 570 000 km3 if the congo drainage basin had extended up to the east branch of the rift these values are confidently established from the existence of remnant geological surfaces and can explain the volume of tertiary sediment in the gulf of guinea during upper cretaceous the most important accumulations of sediments correspond to the ogooe and kwanza fans which shows that the organization of continental drainage was different as was potential source of sediments in the northern part of the present day congo river drainage basin there was a compressional episode during santonian that may have caused significant relief and erosion along a benue chad axis

chemical and carbon isotopic fractionations of gaseous hydrocarbons during abiogenic oxidation abstract the variations of chemical and carbon isotopic compositions in gaseous hydrocarbons upon abiogenic oxidation by sulfates hematite and both in laboratory studies were clearly demonstrated through a unique two step experimental approach in the first step a large number of small gold capsules containing gaseous hydrocarbons were prepared by kerogen pyrolysis experiments which were conducted under the same condition 450 c and 50 mpa for 72 h using the same kerogen the analytical results demonstrated that the chemical and carbon isotopic compositions of gases were almost identical among the capsules in the second step each of these gaseous hydrocarbon bearing capsules along with mineral oxidant hematite or magnesium sulfate heptahydrate or both and deionized water 15 of the amount of the oxidant was placed into a large gold capsule after welding sealing the small gaseous hydrocarbon bearing capsule was forced to leak while the large capsule remained undamaged by compressing the large capsule from the outside at the position where this small capsule was located then these large capsules containing gas and mineral reactants were heated isothermally at 350 c and 50 mpa for 72 144 216 and 288 h respectively the results of the oxidation experiments with increasing heating time can be outlined as follows 1 the amount of methane remained almost unchanged in the experiments using hematite and the mixed oxidants hematite mgso4 while it increased substantially in the experiment using mgso4 after 72 h indicating methane was one of the final products of c2 oxidation 2 the amounts of c2 c5 hydrocarbons decreased consistently and more rapidly in the experiment using mgso4 than that using hematite and the mixed oxidants 3 the oxidation rates of gas hydrocarbons increased with increasing carbon number of hydrocarbons 4 the oxidation rates of i butane and i pentane were substantially higher than those of the corresponding n butane and n pentane 5 the amount of h2s increased substantially in the experiment using mgso4 whereas it was below the detection level in the experiments using hematite and the mixed oxidants 6 the 13c values of c1 c5 hydrocarbons became less negative and the isotopic fractionation extent increased with increasing carbon number of hydrocarbons and oxidation extent 7 carbon isotopic fractionation factor k12 k13 decreased with increasing oxidation rate of gas hydrocarbons and 8 the rate of thermochemical sulfate reduction tsr was strongly dependent upon the presence and concentrations of h2s

reconnaissance study of the ancient zaire congo deep sea fan zaiango project analysis of more than 19 000 km of multichannel seismic reflection data from the zaiango project covering the zaire deep sea fan allowed us to identify the oceanic crust and five seismostratigraphic units on the basin floor and abyssal plain in the absence of a direct stratigraphic tie the time frame is provided by long distance correlations a stratigraphic model for the evolution of the zaire deep sea fan is proposed a widespread major regional unconformity representing probably the eocene oligocene transition marks a drastic change of sedimentation pattern in the basin floor a pelagic aggradational sequence albian eocene overlying the oceanic crust gives way to an onlapping prograding sequence of turbidite deposits oligocene recent this change marks the onset of the ancient zaire deep sea fan triggered by the climatic change related to the greenhouse icehouse shift at the eocene oligocene transition which was responsible for a drastic increase in continental erosion and terrigenous sedimentary supply to the margin the volume of the fan is at least 0 7 mkm 3 much broader and thicker than formerly assumed a short lived episode of rapid facies progradation

a chlorite solid solution geothermometer the los azufres mexico geothermal system chlorite constitutes a major hydrothermal alteration product of metamorphism of andesites in the active geothermal system of los azufres mexico electron microprobe analyses performed on a set of crystals from each sample show wide variations in composition correlation coefficients among chemical constituents were calculated it is shown that the tetrahedral charge is positively correlated with the octahedral vacancy and negatively with the iron content and there is almost no correlation with the octahedral aluminium and magnesium content a procedure is proposed to select end members and substitution vectors and to give a general formula for these chlorites

methane dominated thermochemical sulphate reduction in the triassic feixianguan formation east sichuan basin china towards prediction of fatal h2s concentrations abstract new sour pools have recently found in the lower triassic feixianguan fm carbonate reservoirs in the east sichuan basin in china with h 2 s up to 17 4 by volume a recent blowout from a well drilled into this formation killed hundreds of people as a result of the percentage concentrations of h 2 s in order to assess the origin of fatal h 2 s as well as the cause of petroleum alteration h 2 s concentrations and the isotopes 34 s and 13 c have been collected and measured in gas samples from reservoirs anhydrite pyrite and elemental sulphur 34 s values have been measured for comparison the high concentrations of h 2 s gas are found to occur at depths 3000 m temperature now at 100 c in evaporated platform facies oolitic dolomite or limestone that contains anhydrite nodule occurrence within the reservoirs where h 2 s concentrations are greater than 10 its 34 s values lie between 12 0 and 13 2 cdt this is within the range of anhydrite 34 s values found within the feixianguan fm 11 0 to 21 7 average 15 5 3 5 cdt thus h 2 s must have been generated by thermochemical sulphate reduction tsr locally within the reservoirs burial history analysis and fluid inclusion data reveal that the temperature at which tsr occurred was greater than about 130 140 c suggesting that the present depth temperature minimum is an artifact of post tsr uplift both methane and ethane were actively involved in tsr since the petroleum became almost totally dry no alkanes except methane and methane 13 c values become significantly heavier as tsr proceeded methane 13 c difference thus reflects the extent of tsr while it is tempting to use a present day depth control 3000 m to predict the distribution of h 2 s in the feixianguan fm this is an invalid approach since tsr occurred when the formation was buried some 1000 2000 m deeper than it is at present the likelihood of differential uplift across the basin means that it is important to develop a basinal understanding of the thermal history of the feixianguan fm so that it is possible to determine which parts of the basin have been hotter than 130 140 c

the great west african tertiary coastal uplift fact or fiction a perspective from the angolan divergent margin 1 we explore exhumation in the coastal kwanza basin by combining analyses of tertiary hiatuses and apatite fission tracks planktonic biozones show five major hiatuses in the oligo miocene and plio pleistocene between gaps oligo miocene strata accumulated under marine conditions a marine setting refutes the idea of a massively raised coastal plateau in the mid tertiary marine conditions continued until 5 ma fission track data suggest three thermal events 150 ma during rifting and volcanism 100 70 ma during shortening and volcanism and 20 10 ma during exhumation tertiary uplift was spatially highly variable for the kwanza basin we infer that tertiary uplift on the west african margin is indeed a fact but that estimates of uplift timing and size are unreliable when extrapolated to adjoining areas massive uplift 2000 4000 m of the precambrian craton had little structural effect in the outer basin instead minor uplifts on the shelf drove late tertiary deformation on the slope

testing clumped isotopes as a reservoir characterization tool a comparison with fluid inclusions in a dolomitized sedimentary carbonate reservoir buried to 2 4 km constraining basin thermal history is a key part of reservoir characterisation in carbonate rocks conventional palaeothermometric approaches cannot always be used fluid inclusions may be reset or not present while 18o palaeothermometry requires an assumption on the parent fluid composition the clumped isotope palaeothermometer however is a promising technique for constraining the thermal history of basins in this study we test if clumped isotopes record temperatures of recrystallisation in deeply buried dolomitic reservoirs through comparison with fluid inclusion data the studied reservoir is the cretaceous pinda formation offshore angola a deeply buried dolomitised sedimentary carbonate hydrocarbon reservoir it provides an ideal test case as samples from industry wells are available over a relatively wide burial depth range of 2000 4000 metres below sea floor mbsf and the constituent dolomites are relatively homogeneous across this depth range fluid inclusion homogenisation temperatures for the pinda formation record a range of temperatures from 110 170 c increasing with depth these closely match present day ambient well temperatures indicating recent resetting of the fluid inclusions clumped isotopes however record temperatures significantly 20 60 c below fluid inclusion and well temperatures for the seven samples analysed the deepest five samples 2800 3700 mbsf record clumped isotope temperatures around 100 120 c interpreted to represent a deep burial recrystallisation event responsible for a massive re dolomitisation of the reservoir the lower clumped isotope temperatures 65 and 82 c of the shallower 2055 and 2740 mbsf samples are interpreted to represent physical mixing of two dolomite generations due to incomplete burial recrystallisation of an early shallow dolomite determination of temperature through clumped isotopes allows calculation of the parent fluid 18o values in the five deepest samples the fluid 18o values of 3 7 6 5 cluster around the modern day porewater composition 5 suggesting burial dolomitisation occurred in the presence of evolved brine mineral 18o values of 4 5 to 7 are lower than pristine cretaceous marine dolomite and are in accordance with burial recrystallisation clumped isotopes are therefore interpreted to record temperatures corresponding to open system burial recrystallisation events this study shows that clumped isotopes are a valuable tool in characterising the thermal history of deeply buried 2000 m carbonate hydrocarbon reservoirs

structural segmentation inversion and salt tectonics on a passive margin evolution of the inner kwanza basin angola the kwanza basin angola is divided into the inner and outer kwanza salt basins separated by a chain of synrift basement highs on which aptian 112 122 ma salt is thin or absent north to northwest trending basement structures in the inner kwanza basin have repeatedly been reactivated since neocomian 144 127 ma rifting reactivation formed three northwest striking fold and thrust belts near basement uplifts the thrust belts are bounded by northeast striking rift related transfer fault zones that were apparently reactivated during subsequent shortening three episodes of postrift basement involved shortening are documented in the inner kwanza basin 1 albian early cenomanian 112 96 ma 2 senonian 89 65 ma and 3 oligocene holocene 34 0 ma we relate the albian early cenomanian event to ridge push the senonian event to global plate reorganization and oligocene holocene shortening to uplift of the african superswell structural segmentation of the inner kwanza basin controlled the evolution of salt structures adjacent to basement uplifts diapirs were initiated as buckle folds some anticlines were unroofed by erosion and evolved into passive salt walls elsewhere broad salt walls were triggered by either detached extension or basement block uplift these walls grew until they exhausted their supply of salt thereafter dissolution rates exceeded rates of salt inflow so the walls began to subside withdrawal of salt from the walls produced the elongate sedimentary troughs for which the basin is famous trough fill ranges in age from cenomanian to pliocene and this age varies greatly from trough to trough and along strike within troughs

lithostratigraphic geophysical and morpho tectonic studies of the south cameroon shelf abstract a geological and structural sketch of the southern cameroon shelf was drawn on the basis of a high resolution seismic survey dredgings and kullenberg core sampling of the outcropping rocks at sea bed palynological and mineralogical data x ray diffractometry supported by occasional foraminiferal evidence revealed four main lithostratigraphic sets resting on a precambrian basement upper cretaceous albian to senonian paleocene lower eocene miocene pliocene and pleistocene three successive deformational types characterise the structure of this margin 1 n s trending fractures parallel to the onshore precambrian structural directions formed during the rifting period and crosscut by n60 e trending cross faults induced by the transform movement of the south atlantic 2 compressive structures governed by local stresses during the eocene transcurrent reactivation along the n60 e transform faults excluding a salt controlled deformation 3 a set of approximately n s faults affecting the unfolded sequence of the miocene pliocene or the quaternary and indicative of distensive conditions the geodynamic evolution is placed in the general framework of the central and south atlantic rifting and passive margin history interestingly the age of the compressive deformation varies from late albian to mid eocene depending on the regional plate motion events which are generated along the narrow fracture zones extending as far as or even into the continental domain

39ar 40ar laser probe dating of terrestrial rocks we have shown that for metamorphosed granites as well as for metamorphosed sediments it is possible to obtain a reliable k ar age from a sample as small as 0 2 g samples with ages as young as 40 m y are easily studied there is no reason to believe that the method could not be used for samples considerably younger the main application of the laser will probably be for cases where the same mineral species has been formed during two distinct metamorphic events in the same rock in this case it is difficult if not impossible to make a mineral separate suitable for a 39 ar 40 ar study we have shown that in the case of an old granite the biotite in one sample distinctly shows thermal events at 1600 m y whileits primary cooling age is 2100 m y the thermal release study yields a good plateau age roughly an average age of the two thermal events an other interest of this method is demonstrated by argon dating of two different kinds of glaucophane only one of theseshowing argon in excess

constraining uplift and denudation of west african continental margin by inversion of stacking velocity data an inverse model has been developed to determine the magnitude of denudation at seabed and subsurface unconformities by using root mean square rms stacking velocity data derived from processing a set of seismic reflection profiles this approach provides superior spatial coverage in comparison to other methods such as vitrinite reflectance apatite fission track and sonic velocity modeling which are restricted to borehole locations the model assumes exponential porosity decay with depth and a standard velocity porosity relationship in order to compute a synthetic rms velocity profile denudation values at two levels in the stratigraphy are then adjusted until the fit between the model and the data is optimized successful modeling is dependent upon independent estimates of the initial porosity of sediment since significant trade off occurs between initial porosity and denudation application to the west african shelf shows that 0 5 1 km of denudation occurred along the entire margin probably during late neogene times the amount of denudation decreases oceanward and was probably triggered by regional tilting associated with initiation and or regeneration of continent wide mantle convective upwelling which is thought to have affected much of subequatorial africa a subsurface oligocene unconformity represents as much as 2 5 km of denudation and was probably produced by initiation of an oceanic current