episodic and simultaneous illitization in oil bearing brent group and fulmar formation sandstones from the northern and southern north sea based on illite k ar dating size fractions 0 4 and 0 4 1 0 m of brent group sandstones from the northern north sea contain mostly illite smectite mixed layers with kaolinite whereas the same size fractions of fulmar formation sandstones from the south central north sea consist of illite smectite mixed layers with minor chlorite transmission electron microscope observations show elongated illite laths or agglomerates consisting of small laths when larger individual laths are lacking the k ar data of the fractions less than 0 4 m of brent group samples plot on two arrays in a 40ar 36ar vs 40k 36ar diagram that have isochron characteristics with ages of 76 5 4 2 and 40 0 1 5 ma and initial 40ar 36ar ratios of 253 16 and 301 18 respectively for the fulmar formation samples the data points of the fractions less than 0 2 and less than 0 4 m also fit two isochrons with ages of 76 6 1 4 and 47 9 0 5 ma and initial 40ar 36ar ratios of 359 52 and 304 2 respectively some of the coarser 0 4 1 0 m fractions also plot on the two isochrons but most plot above indicating the presence of detrital components more than 0 4 m the almost identical ages obtained from illite type crystals of sandstones with different deposition ages that are located about 600 km 373 mi apart record two simultaneous illitization episodes these events were not induced by local burial conditions but are related to episodic pressure and or temperature increases in the studied reservoirs probably induced by hydrocarbon injection this interpretation is indirectly supported by notably different k ar illite ages from cores of a nearby reservoir at hydrostatic pressure illite is not as well crystallized as expected for potential crystallization temperatures above 160c measured by fluid inclusion determinations in both the northern and south central north sea the two illite generations remain unaffected after crystallization despite continued burial suggesting notably higher crystallization temperatures than those estimated from geothermal gradients no recent illite crystallization or alteration is recorded in the k ar ages despite a dramatic regional acceleration of the subsidence in the southern north sea

diagenesis in north sea hpht clastic reservoirs consequences for porosity and overpressure prediction upper jurassic clastic reservoirs of the fulmar formation in the central north sea possess anomalously high porosities for their present day depth of burial reservoirs with the highest overpressures have the highest porosities possess less macroquartz cement and have significant secondary porosity quartz cementation in hpht high pressure high temperature reservoirs has been inhibited by a combination of factors high overpressure limited fluid movement presence of early grain coating cements high pore fluid salinity and possibly petroleum migration secondary porosity has contributed to reservoir quality with an average of 4vol extra porosity created quantitative prediction of porosity would require an improved depositional model for the fulmar accurate thermal and pressure modelling and detailed knowledge of field filling and leakage histories theoretical calculations indicate that diagenetic reactions occurring in the fulmar formation smectite illitisation and quartz cementation did not generate significant overpressure because seal permeabilities were too high and the rate of volume increase associated with the reactions too small therefore diagenetic reactions can effectively be ignored when modelling overpressure generation in the central north sea although cementation will affect rock permeability and rates of fluid dissipation

exceptional reservoir quality in hpht reservoir settings examples from the skagerrak formation of the heron cluster north sea uk abstract as the exploration of hydrocarbons moves into more complex and deeper basinal settings the need to understand the effect of high temperatures and high pressures on reservoir quality and rock properties becomes even more important the fluvial channel sandstone reservoirs of the skagerrak formation in the central north sea exhibit anomalously high porosities and permeabilities considering their depth of burial 3500 m below sea floor despite the complex depositional setting diagenetic history high overpressure and temperatures encountered in the skagerrak formation hydrocarbons are currently being produced the skagerrak formation reservoirs used in this study have encountered overpressures of 40 mpa and temperatures up to 185 c at present day maximum burial to identify the role played by the high pressure and high temperature encountered in the reservoir sandstones a multidisciplinary approach involving petrographic fluid inclusion and burial history modelling studies has been adopted our interpretation of the results is that the generation of shallow overpressure in these fields limited mechanical compaction and also played an important role in minimizing pressure solution in the chemical compaction regime as evidenced by reduced quartz cementation fluid inclusions found in the quartz overgrowths indicate late stage development with temperatures of formation in the range 130 to 170 c coincident with late stage deeper burial hydrocarbon emplacement occurred after quartz cementation and has had little to no effect on porosity preservation the formation of well developed authigenic chlorite 70 surface coating and to a lesser extent illite clay coats with burial had a positive effect on porosity preservation even though permeability was marginally reduced in the illite rich sandstones a schematic porosity and quartz cement evolution model has been developed which allows for pre drill prediction of reservoir quality in the heron cluster and provide valuable insights for other complex high pressure high temperature reservoirs

ankerite cementation in deeply buried jurassic sandstone reservoirs of the central north sea abstract upper jurassic fulmar formation shelf sandstones of the high temperature high pressure franklin and elgin fields south central graben north sea contain abundant disseminated and concretionary ankerite in contrast most jurassic north sea reservoirs contain only minor amounts of dispersed ankerite disseminated ankerite cement in the franklin and elgin fields has a fairly uniform isotopic composition 18o 10 to 12 5o oo pdb 13c 3 to 5o oo pdb ankerite concretions have 18o values similar to disseminated cements but a wider range of 13c values 1 to 5 5o oo pdb they also have highly variable intergranular volumes which together with the 13c data are interpreted as a combination of pore filling cementation and in situ replacement of comminuted bioclastic debris by ankerite fluid inclusion 18o and paragenetic evidence suggests that ankerite formed during deep burial c 3 5 to 4 5 km 140 170 c after the onset of overpressuring but before hydrocarbon emplacement in the reservoirs the regionally consistent 18o data suggest that ankerite formed via a temperature influenced mechanism and the relatively uniform 13c cement value indicates that organic matter and marine bioclastic carbonate contributed to the dissolved carbon reservoir in constant proportion this can be explained by calcite dissolution in response to ph decrease during thermal breakdown of organic acids such acids were derived from adjacent mudrocks undergoing hydrocarbon maturation and clay mineral transformations and are likely to have been transported in pore fluids with mg2 and fe2 the presence of these cations in solution upon thermal decarboxylation is inferred to have stabilized ankerite at the expense of calcite a relative paucity of ankerite in other fulmar formation reservoirs may reflect different sedimentological compositions less bioclastic debris and or lower burial temperatures less advanced decarboxylation

mass transfer and problems of secondary porosity creation in deeply buried hydrocarbon reservoirs abstract secondary porosity created by the post depositional leaching of detrital grains and cements can be of great economic importance the term secondary porosity is only descriptive and does not give any information on whether the porosity of the rock has been enhanced in reality there are two end member volumetric outcomes to the leaching process firstly the mineral can be completely dissolved and its components transported out of the system in such an instance the porosity of the rock will be enhanced secondly it is possible that a mineral will decompose and by product minerals will be precipitated which will occupy approximately the same volume as the original mineral the latter case is more likely where the pore waters are close to equilibrium with the majority of the minerals of the rock this will occur at elevated temperatures and or where the pore waters have been in contact with the rock for a long time these conditions are commonly met during intermediate to deep burial many metastable minerals such as high temperature feldspars probably decompose in this way a consequence of the decomposition of such metastable minerals is that primary porosity will be swapped for secondary and that older and more deeply buried rocks will contain a higher proportion of secondary porosity the mass transport equation may be used to examine the physico chemical environment where enhanced porosities most likely occur during leaching by using a simple non equilibrium mass transport model based on the silica system it is possible to demonstrate the relative importance of flow rate reaction rates specific surfaces and temperature in controlling the areal extent of leaching wide leaching zones are related to high flow rates which in turn promote a high throughput of undersaturated fluid and dispersion leaching over a wide area is also favoured by low reaction rates which will occur at low temperatures although it is true to say that less of a given silicate mineral can be dissolved by an undersaturated pore water at low temperature than at high temperature it is probable that pore fluids from deeply buried sediments are more likely to be close to equilibrium with a given mineral than for instance meteoric water thus the most likely environment for the production of widespread enhanced porosity is in the shallow subsurface the lower flow rates and higher temperatures that are encountered during deep burial diagenesis will result in far narrower more intense leaching zones as a result providing sufficient reactive mineral is present any undersaturated pore water will approach equilibrium in a short distance in order to transport undersaturated pore waters appreciable distances in the deep subsurface it is necessary to minimize their interaction with the host rock this could occur if the pore fluid were to move through a fracture system where it would interact with a far smaller surface area of reactive mineral than if it moved through the pore network

hydrocarbon composition of authigenic inclusions application to elucidation of petroleum reservoir filling history abstract geochemical analysis of petroleum inclusions trapped in authigenic feldspar and quartz in the ula formation in the north sea ula oil field revealed a petroleum of markedly different composition than the oil presently in the reservoir using microthermometry and the burial history as a dating tools it is concluded that the petroleum in the k feldspar inclusions was present in the more porous and permeable parts of the ula formation as early as 45 75 my bp when the field was at a depth of about 1 0 1 5 km as compared with the current depth of 3 4 km this early petroleum which was trapped as inclusions in authigenic k feldspar shows a distinctly different distribution of tricyclic terpanes and pentacyclic triterpanes from that of the current petroleum charge in the ula formation which was derived from the mandal formation source rock in late neogene time molecular parameters show that the oil in the k feldspar inclusions is significantly less mature than the crude oil in the present reservoir the approximate 90 c temperature increase occurring after entrapment of the early petroleum in kfeldspar the field is currently at 143 c appears not to have reset the low maturity signature of the oil in the k feldspar inclusions this could suggest that the temperature in the inclusions is too low for isomerization selective thermal degradation to occur lack of catalysts or that there are other controls on the ratio of some of these parameters still parameters like the ratio of c21 to c28 triaromatic steroids and those based on dimethyl and trimethyl naphthalenes are comparatively similar in both the inclusions and in the reservoir crude the oil inclusions in authigenic quartz and albite formed from about 10 my bp burial depth 2 5 km until the present burial depth 3 4 km are interpreted as representing a palaeo petroleum charge having a composition intermediate between the oil found in k feldspar inclusions and the oil charge in the present reservoir our data suggest strongly that the petroleum trapped in the inclusions in quartz and plagioclase represents progressive dilution of the early reservoired oil now found only in k feldspar inclusions with oil from the mandal formation analysis of solution gas in these inclusions suggests that the first petroleum associated gas to arrive at the trap from the progressively maturing mandal formation had a much wetter composition than the current reservoir solution gas to explain the present comparatively drier and isotopically lighter reservoir solution gas composition the inference is made that there has been a recent during formation of k feldspar inclusions 45 75 my bp maturation of the upper jurassic mandal formation the main source rock in this region was limited possibly to the graben axis this suggests that the petroleum found in the k feldspar inclusions was most likely derived from a deeper post caledonian palaeozoic formation

sandstone diagenesis and reservoir quality prediction models myths and reality models and concepts of sandstone diagenesis developed over the past two decades are currently employed with variable success to predict reservoir quality in hydrocarbon exploration not all of these are equally supported by quantitative data observations and rigorous hypothesis testing simple plots of sandstone porosity versus extrinsic parameters such as current subsurface depth or temperature are commonly extrapolated but rarely yield accurate predictions for lithified sandstones calibrated numerical models that simulate compaction and quartz cementation when linked to basin models have proven successful in predicting sandstone porosity and permeability where sufficient analog information regarding sandstone texture composition and quartz surface area is available analysis of global regional and local data sets indicates the following regarding contemporary diagenetic models used to predict reservoir quality 1 the effectiveness of grain coatings on quartz grains e g chlorite microquartz as an inhibitor of quartz cementation is supported by abundant empirical data and recent experimental results 2 vertical effective stress although a fundamental factor in compaction cannot be used alone as an accurate predictor of porosity for lithified sandstones 3 secondary porosity related to dissolution of framework grains and or cements is most commonly volumetrically minor 2 exceptions are rare and not easily predicted with current models 4 the hypothesis and widely held belief that hydrocarbon pore fluids suppress porosity loss due to quartz cementation is not supported by detailed data and does not represent a viable predictive model 5 heat flow perturbations associated with allochthonous salt bodies can result in suppressed thermal exposure thereby slowing the rate of quartz cementation in some subsalt sands

chemical models of vitrinite reflectance evolution a global kinetic model of vitrinite reflectance evolution is developed to illustrate a new approach to vitrinite reflectance modelling the method utilizes quantitative pyrolysisgas chromatographic data from isolated vitrinite kerogens to derive the concentrations of structurally specific moieties alkylphenol precursors in vitrinite kerogens as a function of rank this data set is used to calibrate a gaussian distributed activation energy model for phenol precursor loss from vitrinites using published burial histories phenol yield is then correlated with vitrinite reflectance the predictive vitrinite reflectance model based on this phenol system is tested against a well from the us gulf coast area the results being compared with a lopatin model of the same well this type of vitrinite reflectance model is numerically compatible with current generation kinetic models of hydrocarbon production from source rock kerogens and has a better heating rate independence than lopatin s method

wind erodibility of soils at fort irwin california mojave desert usa before and after trampling disturbance implications for land management recently disturbed and control i e less recently disturbed soils in the mojave desert were compared for their vulnerability to wind erosion using a wind tunnel before and after being experimentally trampled before trampling control sites had greater cyanobacterial biomass soil surface stability threshold friction velocities tfv i e the wind speed required to move soil particles and sediment yield than sites that had been more recently disturbed by military manoeuvres after trampling all sites showed a large drop in tfvs and a concomitant increase in sediment yield simple correlation analyses showed that the decline in tfvs and the rise in sediment yield were significantly related to cyanobacterial biomass as indicated by soil chlorophyll a however chlorophyll a amounts were very low compared to chlorophyll a amounts found at cooler desert sites where chlorophyll a is often the most important factor in determining tfv and sediment yield multiple regression analyses showed that other factors at fort irwin were more important than cyanobacterial biomass in determining the overall site susceptibility to wind erosion these factors included soil texture especially the fine medium and coarse sand fractions rock cover and the inherent stability of the soil as indicated by subsurface soil stability tests thus our results indicate that there is a threshold of biomass below which cyanobacterial crusts are not the dominant factor in soil vulnerability to wind erosion most undisturbed soil surfaces in the mojave desert region produce very little sediment but even moderate disturbance increases soil loss from these sites because current weathering rates and dust inputs are very low soil formation rates are low as well therefore soil loss in this region is likely to have long term effects published in 2006 by john wiley sons ltd

massive recrystallization of low mg calcite at high temperatures in hydrocarbon source rocks implications for organic acids as factors in diagenesis we document massive recrystallization of low mg calcite lime mudstone source rocks at moderately high temperatures in the smackover formation of mississippi this process was driven by organic acids produced during kerogen maturation and holds the implications for studies of primary migration of hydrocarbons secondary porosity generation in reservoirs and global environmental change this interpretation is based on contrasting the following petrographic and bulk rock geochemical characteristics of organic poor vs organic rich lime mudstones

influence of multistage oil emplacement on carbonate cementation in the eocene shahejie formation dongying sag bohai bay basin china abstract the influence of multi stage oil emplacement on carbonate cementation was investigated on the basis of determining the sequence of oil emplacement and carbonate cementation in the clastic reservoirs of the upper part of the fourth member of the eocene shahejie formation es4s in the yanxie 229 area dongying sag the sequence of multistage oil emplacement and carbonate cementation was determined by projecting the homogenization temperatures ths of two phase aqueous inclusions on a burial and thermal history plot the ratio of the number of quartz grains with yellow fluorescent oil inclusions therein to the total number of quartz grains ygoi and the ratio of the number of quartz grains with blue fluorescent oil inclusions therein to the total number of quartz grains bgoi were calculated to analyze the paleo oil saturation of each stage of oil emplacement the ygoi and bgoi values reflect the relative level of the paleo oil saturation of the first stage yellow fluorescent fsyf oil and the second stage blue fluorescent ssbf oil respectively a large number of primary pores were preserved in the oil layers whereas only a small number of dissolution pores and micropores were developed in the oil bearing water layers and dry layers calcite dolomite ferrocalcite and ankerite cementation and two stage oil emplacement occurred in the es4s reservoirs the fsyf oil emplacement occurred earlier than the ankerite cementation and ferrocalcite cementation but later than the calcite cementation and dolomite cementation the ssbf oil emplacement occurred later than all phases of carbonate cementation the dolomite cementation was not influenced by the fsyf oil emplacement because it occurred earlier than the fsyf oil emplacement the ankerite cementation was dominantly inhibited by the fsyf oil emplacement in the oil layers and the degree of inhibition increased with the increase in paleo oil saturation of the fsyf oil the ssbf oil emplacement further increased the paleo oil saturation in the oil layers which enhanced the inhibition of the ankerite cementation and even stopped it the ankerite cementation in the dry layers and oil bearing water layers was slowed down or stopped due to the lack of pores and the low transportation rate of external material which was caused by the intense ankerite cementation prior to the ssbf oil emplacement

hydrocarbon filling and leakage history of a deep geopressured sandstone fulmar formation united kingdom north sea diagenetic minerals in a water filled borehole from the jurassic fulmar formation united kingdom central north sea record two phases of hydrocarbon filling and emptying initial oil charge was during the late cretaceous at shallow burial depths of about 1 5 km 0 9 mi as we consider that hydrocarbon has preserved porosity during burial this has implications for the understanding of the porosity evolution of the fulmar formation which in other locations is an important hydrocarbon reservoir the early oil charge as recorded by illite k ar ages progressively filled the structure from 84 to 59 ma and possibly precipitated bitumen because of biodegradation the first oil predated many of the burial diagenetic reactions within the sandstone after leak off at ca 60 ma diagenetic reactions continued in an open geochemical system with possible import of co2 products of these reactions include ankerite and quartz overgrowth cements hydrocarbon staining postdates these phases and provides evidence of a hydrocarbon charge probably gas condensate the second hydrocarbon charge also leaked off and the sandstone is now water bearing previous work on the fulmar formation has incorrectly placed all the diagenetic reactions as predating the first arrival of hydrocarbon the present day pore fluids are high salinity high 18o fluids derived from the underlying permian zechstein evaporates these fluids entered the reservoir during a phase of overpressure release that caused fracturing of the framework quartz grains possibly coincident with the second phase of hydrocarbon leak off at 2 5 ma mark wilkinson studied sandstone hosted calcite concretions for his ph d and has subsequently tried to take a more holistic view of the diagenesis of sedimentary basins his career has been funded by a variety of oil companies although he is currently involved in geological disposal of some of the co2 that he was partly responsible for producing stuart haszeldine is a professor at edinburgh university he received his ph d on clastic and coal sedimentology in 1981 from the university of strathclyde after an attempted reeducation in the oil industry he has collaborated with many companies to research on hydrocarbon reservoir quality radioactive waste disposal and coal resources he currently heads the united kingdom s largest group investigating carbon dioxide sequestration tony fallick was educated at the university of glasgow scotland in physics b sc and chemistry ph d he did research at mcmaster university ontario canada and cambridge england before moving in 1980 to the scottish universities research and reactor center where he is currently the director

oil charge preserves exceptional porosity in deeply buried overpressured sandstones central north sea uk abstract exceptionally high porosity sandstones are reported from oil provinces worldwide yet the mechanism of porosity preservation remains controversial we present strong evidence that the exceptional porosities in the jurassic fulmar formation within the north sea are the result of early oil charging preventing cementation and chemical compaction in sands with relatively low detrital clay contents although overpressure and grain coatings do have some effect the most dramatic evidence that reservoir quality is related to early oil charge is the spatial distribution of the porosity the maximum porosity of the fulmar formation is systematically highest immediately below the top seal and decreases below this despite uniform sedimentology hence porosity is preferentially preserved where oil first accumulated in these unusually homogeneous overpressured reservoir sandstones at the top oil charge prevents or significantly retards chemical compaction and cementation of a reservoir preserving porosity there is independent evidence for early oil charging of reservoirs within the central graben from fluid inclusion studies and k ar age dates of authigenic illite micro quartz grain coatings which have been proposed to preserve porosity during burial are associated with only 5 extra porosity in sands that exceed the worldwide average sandstone porosity by almost 20

porosity depth trends in reservoir sandstones theoretical models related to jurassic sandstones offshore norway abstract theoretical models of compaction and porosity modification in sandstones during burial are discussed theory predicts that mechanical compaction is the dominant porosity reducing mechanism during burial from 0 to 2 5 3 km porosity depth trends are at this stage mainly controlled by the stability of the grain framework and the effective stress time and temperature also influence the porosity depth gradients at shallow to moderate depths but are probably less important than effective stress and framework grain composition at greater burial depths below 2 5 3 km chemical compaction including pressure solution can explain most of the porosity reduction in well sorted clean arenites theoretical modelling of the intergranular pressure solution suggests that sandstones may lose porosity rapidly by this mechanism below 2 5 3 km and that factors capable of retarding pressure solution and quartz cementation are required to maintain prospective porosities below 4 km this theory and comparisons with empirical data from jurassic sandstones from offshore norway indicate that high porosity is favoured by 1 overpressures approaching the lithostatic pressure gradient 2 a short residence time below 3 km burial 3 coatings on grains of clay or microcrystalline quartz which prevent quartz dissolution and precipitation and 4 a high content of feldspar and early diagenetic secondary pores early hydrocarbon emplacement may retard quartz cementation and preserve porosity but chemical compaction probably continues after oil emplacement except at very high hydrocarbon saturations

quartz veins record vertical flow at a graben edge fulmar oil field central north sea the fulmar oil field lies at the faulted western margin of the central graben in the central north sea chalcedony and quartz and later dolomite occur as diagenetic cements in veins within the upper jurassic fulmar formation quartz in one particular vein contains primary aqueous fluid inclusions with homogenization temperatures ranging from 85 degrees to 140 degrees c with a mean of 117 degrees c the measured d18o of the authigenic quartz ranges from 25 1 to 27 1o oo smow the d18o of the water from which most of the quartz precipitated is calculated to have been around 8o oo which is significantly more positive than the measured present day formation water d18o of 4o oo quartz and chalcedony cements in the vein are postdated texturally by dolomite that has a d18o of 23 5o oo smow high salinities are recorded in the fluid inclusions in the quartz the majority ranging from 15 to 19 wt nacl equivalent also the quartz contains inclusions of anhydrite high salinity aqueous inclusions combined with anhydrite inclusions in the quartz and the 18o enriched calculated water composition suggest that the quartz precipitated from an evaporite influenced fluid the same fluid contributed mg for dolomite precipitation more deeply buried sections including the permian zechstein evaporites underlying the fulmar field are the most obvious source of an 18o enriched saline fluid mineralization of the vein resulted from cross formational flow of warm fluids from depth through the zechstein and into the jurassic succession over vertical distances of 600 2000 m fluids most probably migrated up faults and fractures during episodes of salt movement or overpressure release

reservoir quality and rock properties modeling triassic and jurassic sandstones greater shearwater area uk central north sea abstract the complex burial and diagenetic histories of the jurassic fulmar and triassic skagerrak sandstones in the uk central north sea present significant challenges with regard to reservoir quality and rock property prediction commercial reservoir quality is retained despite deep burial and associated high temperatures and pressures shallow marine fulmar sands are normally compacted mean igv 26 3 yet have porosities of 21 33 porosity was preserved through inhibition of quartz cementation by clay and microquartz coatings and enhanced by dissolution of framework grains 5 skagerrak fluvial sands are more compacted mean igv 23 2 exhibit minor feldspar dissolution we modeled reservoir quality evolution using the forward diagenetic model touchstone which simulates porosity loss due to compaction and quartz cementation quantitative petrographic analyses and burial history data were used to calibrate touchstone model parameters the results were applied to deeper prospects for pre drill prediction of porosity and permeability in parallel petrophysical data were used to characterize the elastic properties of the sandstones to provide a basis for quantitative seismic forward modeling experimental data and core calibrated petrophysical results reflecting variable in situ fluids and saturations were used to build an elastic properties model the model is robust and was used to generate fluid filled sandstone properties incorporating touchstone results for prospect specific seismic attribute modeling well results from exploration wells are in good agreement with pre drill touchstone and elastic properties model predictions

the effects of oil emplacement on diagenetic processes examples from the fulmar reservoir sandstones central north sea geologic note 1 sedimentary petrography and petroleum geochemistry studies have been done on fulmar formation upper jurassic cores from fulmar field central north sea the extent of quartz overgrowths and albitization of k feldspars is lower in oil saturated zones compared with water saturated zones however microthermometric studies of aqueous fluid inclusions in quartz overgrowths show similar temperature ranges for both oil and water saturated zones 85 125 degrees c with maximum recorded temperatures close to the current reservoir temperature about 130 degrees c this suggests that quartz overgrowth development in the oil saturated zones continued after oil emplacement but was retarded compared with development in the water saturated zones the quartz cement therefore mus have been essentially locally derived by pressure solution albitization of k feldspars is significantly less about 50 in the oil saturated zones compared with albitization in the water saturated zones because albitization of k feldspars depends on a supply of sodium and removal of potassium this process is more likely to be terminated by oil emplacement the observed difference in the albitization has been used to suggest that the fulmar reservoir probably was filled during the neogene this age estimate agrees broadly with timing determined independently by modeling of oil generation in adjacent source basins the fulmar sandstones contain little or no kaolinite and show no evidence of substantial leaching of feldspars or carbonates this is probably because the fulmar sandstones being partly turbiditic offshore sand deposits were not exposed to meteoric water their close proximity to the marginally mature to mature kimmeridge clay formation seems to have little effect in terms of developing secondary porosity suggesting that the role of organic acids or co 2 generated in these source rocks is of minor importance

pore fluid evolution in the kimmeridge clay formation of the uk outer moray firth implications for sandstone diagenesis abstract carbonate concretions in upper jurassic kimmeridge clay formation from three overpressured wells provide a detailed record of pore fluid evolution in the outer moray firth northern central graben the concretions contain multiple generations of septarian cements with morphologies ranging from simple cracks to complex fractures discrepancy exists between diagenetic studies which indicate these concretions formed during the initial 1 1 5 km of burial and palaeotemperature predictions based on a thermal history calibrated from the vitrinite reflectance kinetic model which indicate formation at much greater depths of between 1 5 and 2 5 km modelling undertaken for this study indicate that the concretions formed during the initial stages of burial under high heat flows fitting the early diagenetic model for their formation these conclusions have important implications for understanding the cementation of adjacent sandstone reservoirs where cement sequences have similar mineralogy and isotopic compositions with precipitation in the same temperature range from mudrock derived pore fluids early cementation of these sandstones is implied by analogy with the mudrock concretions

deep burial dissolution of k feldspars in a fluvial sandstone pentland formation uk central north sea the pentland formation of the uk central north sea is a quartzite in a sequence of more arkosic sandstones provenance sensitive heavy mineral indices are interpreted to indicate that it was probably derived by erosion of arkosic sediments which also supplied the arkosic marine sandstones of the contemporaneous fulmar formation a lack of apatite within the pentland formation suggests that the feldspars could have dissolved during exposure to groundwaters during very shallow burial however petrographic and geochemical evidence supports survival of at least some of the feldspar to burial depths in excess of at least 2 km when the sandstones had been stabilized against compaction by quartz cements or overpressure abundant kaolin which has not reacted to form illite is preserved at burial depths in excess of 4000 m suggesting an unusual lack of available potassium there is a strong contrast in diagenetic pathways with the overlying marine fulmar formation which most probably had the same initial composition but in which kaolin is absent and fibrous illite is present it is suggested that a lower concentration of potassium and aluminium within the pentland formation compared with the probable source and the comparably sourced fulmar formation represents export of k from the sands on a metre scale

vertical effective stress as a control on quartz cementation in sandstones abstract temperature controlled precipitation kinetics has become the overwhelmingly dominant hypothesis for the control of quartz cementation in sandstones here we integrate quantitative petrographic data high spatial resolution oxygen isotope analyses of quartz cement basin modelling and a kinetic model for quartz precipitation to suggest that the supply of silica from stress sensitive intergranular pressure dissolution at grain contacts is in fact a key control on quartz cementation in sandstones we present data from highly overpressured sandstones in which despite the current burial temperature of 190 c quartz cement occurs in low amounts 4 6 1 2 of bulk volume in situ oxygen isotope data across quartz overgrowths suggest that cementation occurred over 100 ma and a temperature range of 80 150 c during which time high fluid overpressures resulted in consistently low vertical effective stress we argue that the very low amounts of quartz cement can only be explained by the low vertical effective stress which occurred throughout the burial history and which restricted silica supply as a result of a low rate of intergranular pressure dissolution at grain contacts

diagenetic differences caused by the charging of natural gases with various compositions a case study on the lower zhuhai formation clastic reservoirs in the wc a sag the pearl river mouth basin abstract only a limited number of comparative studies have explored the diagenetic differences of reservoirs caused by the charging of natural gases with different compositions during the migration and accumulation of oil and gas in this study we quantitatively assessed the diagenetic variations of the lower zhuhai formation reservoirs in the wc a sag the pearl river mouth basin as a result of natural gas charging with different compositions the employed methods included electron microscopy scanning electron microscopy cathode luminescence x ray diffraction chemical composition analysis of formation water stable isotope analysis and fluid inclusion determination the results indicated that 1 in the lower zhuhai formation reservoir that are near the large fractures the late charging of co2 rich thermal fluid promoted the dissolution of minerals and changed the porosity permeability ph value of fluid and the contents of ions such as k na mg2 of the reservoirs these changes in turn indirectly affected the content form and distribution of clay minerals in the reservoirs in addition the secondary enlargement of quartz was enhanced 2 the charging of co2 rich thermal fluid caused strong dissolution and produced high contents of cements the dissolution mainly occurred in a half open system with strong fluid activity consequently the significantly decreasing of permeability was hindered and middle porosity middle permeability reservoirs were generated the reservoirs that are far from the large fractures however were mainly early charged by the hydrocarbon rich fluid the dissolution was weak and the generated cements were limited in addition the dissolution products could not be discharged in such closed diagenetic systems as a result the permeability of the reservoir was declined sharply and resulted in low porosity low permeability reservoirs

diagenetic signatures of stratal surfaces in the upper jurassic fulmar formation central north sea ukcs sedimentation rate and changes in relative sea level affect early diagenetic cementation along key stratal surfaces within the deeply buried upper jurassic fulmar formation south central graben ukcs as a result of the bioturbated nature of the shallow marine strata variations in ichnofabrics and cross cutting relationships between trace fossils are important in identifying and correlating stratal surfaces two transgressive and two regressive surfaces sequence boundaries were studied using petrographical mineralogical and geochemical techniques cements including early diagenetic pyrite framboids grain rimming apatite microcrystalline dolomite and ankerite occur at above and immediately below both transgressive and forced regressive surfaces sequence boundaries breaks or subdued rates of sedimentation associated with both types of surfaces meant that the sediment package resided within early diagenetic zones for prolonged periods of time enhancing diagenetic reactions at and below the surfaces the distribution of ankerite despite being a deep burial cement was primarily controlled by concentrations of bioclasts that are particularly abundant at transgressive surfaces the diagenetic character of the forced regressive surfaces sequence boundaries is more complex than that of the transgressive surfaces both in terms of mineralogy and paragenesis this is attributed to the superimposition of diagenetic assemblages as a result of erosion re sedimentation and sediment by passing although the diagenetic signature of the surfaces has been modified by dissolution replacement reactions during deep burial early diagenetic signatures can still be distinguished

factors controlling threshold friction velocity in semiarid and arid areas of the united states a physical model was developed to explain threshold friction velocities u t for particles of the size 60 120 m lying on a rough surface in loose soils for semiarid and arid parts of the united states the model corrected for the effect of momentum absorption by the nonerodible roughness for loose or disturbed soils the most important parameter that controls u t is the aerodynamic roughness height z0 for physical crusts damaged by wind the size of erodible crust pieces is important along with the roughness the presence of cyanobacteriallichen soil crusts roughens the surface and the biological fibrous growth aggregates soil particles only undisturbed sandy soils and disturbed soils of all types would be expected to be erodible in normal wind storms therefore disturbance of soils by both cattle and humans is very important in predicting wind erosion as confirmed by our measurements

diagenetic albitization of detrital k feldspars in jurassic lower cretaceous and tertiary clastic reservoir rocks from offshore norway ii formation water chemistry and kinetic considerations abstract formation waters from clastic reservoirs offshore norway are saturated or supersaturated with respect to albite deeper than 2 5 km and support the view of saigal et al 1988 that albitization of k feldspar is presently active in these sandstones thermodynamic and mass constraints show that the albitization process is more sensitive to potassium removal than sodium supply the temperature of albitization commencement coincides more or less with the estimated onset temperature of illitization of smectite 70 80 c interbedded shales seem to be the major potassium sink and source of sodium kinetic considerations indicate that at temperatures below 145 c the dissolution of k feldspar is always faster than albite growth combined with replacement constraints the kinetic data also explain the observed coarsening of growing albite crystals with increasing burial temperature

dust transport and the question of desert loess formation although contemporary dust storms are frequent in arid and semi arid areas desert loess deposits are poorly developed much of the world s loess occurs in mid latitude areas which experienced glaciation during the pleistocene ocean core evidence indicates that dust transport from sub tropical deserts increased during cold stages of the pleistocene but loess formed only on certain desert margins for reasons which have not been fully explained this paper re examines the mechanisms of dust transport and deposition and the circumstances leading to the accumulation of thick loess typical loess is composed mainly of medium silt grains which are transported in short term suspension a few metres above the ground significant thicknesses of loess form only when dust is trapped within a limited area often relatively close to the source dust particles finer than 20 m are transported mainly in long term suspension over a greater height range and may be widely dispersed the availability of silt and the frequency magnitude and direction of dust transporting winds are important factors governing the potential for loess formation but the existence of a suitable dust trap is particularly important traps may be formed by topographic obstacles areas of moist ground or vegetated surfaces vegetation adjacent to glacial outwash plains and braided meltwater streams trapped dust in mid latitudes during the pleistocene dust blown during glacial periods from certain deserts notably in sinai soviet central asia and china accumulated as loess in neighbouring semi arid regions on the margins of other deserts loess formation was inhibited partly by the absence of vegetation traps during most of the holocene net dust deposition rates in all desert marginal areas have been too low for significant loess accumulation this is mainly due to a reduction in silt availability and a tendency towards landscape stability reported dust storm frequencies during the past 50 years over estimate the longer term holocene dust flux due to the effects of human activities much modern dust owes its origin to erosion of cultivated soils in semi arid areas and is finer than typical loess

disseminated jigsaw piece dolomite in upper jurassic shelf sandstones central north sea an example of cement growth during bioturbation unusual textural and chemical characteristics of disseminated dolomite in upper jurassic shelf sediments of the north sea have provided the basis for a proposed new interpretation of early diagenetic dolomite authigenesis in highly bioturbated marine sandstones the dolomite is present throughout the franklin sandstone formation of the franklin and elgin fields as discrete non ferroan generally unzoned subhedral to highly anhedral jigsaw piece crystals these are of a similar size to the detrital silicate grains and typically account for 5 of the rock volume the dolomite crystals are never seen to form polycrystalline aggregates or concretions or ever to envelop the adjacent silicate grains they are uniformly dispersed throughout the sandstones irrespective of detrital grain size or clay content dolomite authigenesis predated all the other significant diagenetic events visible in thin section the dolomite is overgrown by late diagenetic ankerite and bulk samples display stable isotope compositions that lie on a mixing trend between these components extrapolation of this trend suggests that the dolomite has near marine delta18o values and low positive delta13c values the unusual textural and chemical characteristics of this dolomite can all be reconciled if it formed in the near surface zone of active bioturbation sea water provided a plentiful reservoir of mg and a pore fluid of regionally consistent delta18o labile bioclastic debris e g aragonite mg calcite supplied isotopically positive carbon to the pore fluids during shallow burial dissolution such dissolution took place in response to the ambient calcite sea conditions but may have been catalysed by organic matter oxidation reactions bioturbation not only ensured that the dissolving carbonate was dispersed throughout the sandstones but also prohibited coalescence of the dolomite crystals and consequent cementation of the grain framework continued exchange of mg2 and ca2 with the sea water reservoir maintained a sufficient mg ca ratio for dolomite rather than calcite to form irregular crystal shapes resulted from dissolution of both the dolomite and the enclosed fine calcitic shell debris before ankerite precipitation during deep burial diagenesis

depositional and diagenetic controls on deeply buried eocene sublacustrine fan reservoirs in the dongying depression bohai bay basin china abstract this paper investigates the reservoir potential of deeply buried eocene sublacustrine fan sandstones in the bohai bay basin china by evaluating the link between depositional lithofacies that controlled primary sediment compositions and diagenetic processes that involved dissolution precipitation and transformation of minerals this petrographic mineralogical and geochemical study recognizes a complex diagenetic history which reflects both the depositional and burial history of the sandstones eogenetic alterations of the sandstones include 1 mechanical compaction and 2 partial to extensive non ferroan carbonate and gypsum cementation typical mesogenetic alterations include 1 dissolution of feldspar non ferroan carbonate cements gypsum and anhydrite 2 precipitation of quartz kaolinite and ferroan carbonate cements 3 transformation of smectite and kaolinite to illite and conversion of gypsum to anhydrite this study demonstrates that 1 depositional lithofacies critically influenced diagenesis which resulted in good reservoir quality of the better sorted middle fan but poor reservoir quality in the inner and outer fan lithofacies 2 formation of secondary porosity was spatially associated with other mineral reactions that caused precipitation of cements within sandstone reservoirs and did not greatly enhance reservoir quality and 3 oil emplacement during early mesodiagenesis temperatures 70 c protected reservoirs from cementation and compaction

fluid inclusions and pvtx modelling examples from the garn formation in well 6507 2 2 haltenbanken mid norway abstract fluid inclusions study and multi techniques quantitative analyses have been coupled to reconstitute pvtx conditions of aqueous and petroleum fluid entrapment of a local haltenbanken area well 6507 2 2 from the garn formation unusual low temperature behaviour showing the liquid portion separating into two liquids has been described this behaviour is typical of a gas condensate volatile mixture with high methane content and heavy alkanes the variable liquid vapour ratio compositions and morphologies of all petroleum inclusions can be interpreted as a result of a combination of heterogeneous trapping and necking down thus all petroleum inclusions can be related to a single petroleum source three main episodes of fluid entrapment can be distinguished the first one corresponds to the water trapped within detrital quartz microfractures at around 50 c and 50 bar the second to the petroleum fluid arrival in the biphase field of a critical system around 100 120 c and 190 230 bar the third one to the main trapping of present day petroleum inclusions during the recrystallisation of quartz cements around 110 160 c and 230 280 bar within the last few million years of a short and relatively rapid burial

peristeritic plagioclase in north sea hydrocarbon reservoir rocks implications for diagenesis provenance and stratigraphic correlation reservoir sandstones from well 30 16 7 in the upper jurassic humber group uk north sea contain plagioclase feldspars with a range of chemical compositions microtextures and origins the chemical composition of feldspar grains in samples from three different depths in the core ranges from 0 to 18 mol an two of the samples have a well defined gap in the compositional range between an 4 and an 9 this gap reflects the absence of plagioclase feldspars with those compositions in the ultimate source area of the humber group sandstones examination of an bearing feldspars on either side of the gap by tem shows that they have lamellar and or tweed exsolution microtextures that are characteristic of plagioclase feldspars having bulk compositions within the peristerite miscibility gap this is the first time that peristerites have been found in a sedimentary rock and they suggest a low grade metamorphic provenance for the plagioclase population compositional gaps have been commonly observed in ab rich plagioclase feldspars from metamorphic rocks although the precise reasons why feldspars do not form within the gap remain poorly understood in addition to the an bearing peristerites 30 16 7 contains a varied suite of an poor feldspars the microtexture of some of these grains is consistent with a low grade metamorphic provenance whereas others are inferred to be diagenetic in origin and to have formed by albitization of detrital alkali or plagioclase feldspar stratigraphic differences in the presence and size of the compositional gap within the humber group suggests that plagioclase feldspars may be a powerful new tool for high resolution stratigraphic correlation and may also be very sensitive indicators of changes in provenance

secondary porosity creation of enhanced porosities in the subsurface from the dissolution of carbonate cements as a result of cooling formation waters abstract theoretical calculations indicate that the cooling of pore waters initially in equilibrium with a carbonate mineral can result in the water becoming undersaturated with respect to that mineral if the formation water has an initial ph below 6 9 then cooling will result in a water with a considerable potential to leach calcite the temperature of formation waters may be forced to decrease where faults act as major vertical migration pathways for waters escaping from a compacting basin where faults cease to act as permeable pathways the escaping formation water may be forced through an aquifer in a partly carbonate cemented aquifer the cooled formation waters are capable of dissolving the carbonate minerals and creating porosities that are higher than would otherwise be expected on the basis of burial depth

archean oil evidence for extensive hydrocarbon generation and migration 2 5 3 5 ga archean sedimentary rocks from the pilbara craton australia contain evidence for petroleum generation and migration in the form of bitumen nodules produced by radiogenic immobilization of fluid hydrocarbons around detrital uraninite thorite and monazite grains the nodules are preserved in sandstones at several stratigraphic levels in the fortescue group 2 75 ga and lalla rookh formation 3 0 ga both nonmarine successions and in deltaic sediments of the mosquito creek formation 3 25 ga the most ancient evidence comes from the warrawoona group 3 46 ga where hydrocarbon droplets were apparently formed in situ from kerogenous sediments by flash maturation during early hydrothermal silicification bituminous relics of petroleum are also commonly preserved in shallow marine sandstones of the black reef formation 2 59 ga and the witwatersrand supergroup 2 85 ga from the kaapvaal craton south africa along with subeconomic methane accumulations in all cases the petroleum was apparently sourced from archean shales generated during the archean and migrated before the late archean or early early proterozoic metamorphism occluded fluid pathways given this widespread and abundant evidence for hydrocarbon generation and migration in archean depositional basins it seems that primordial bacterial biomass producing labile type i kerogen was often buried in sufficient quantities to successfully generate and expel petroleum depositional basins on ancient cratons clearly contained permeable rocks amenable to the migration and probably to the accumulation of petroleum thus the main factors precluding the discovery of economically exploitable hydrocarbon accumulations in archean basins are the subsequent destructive effects of deformation and metamorphism which causes trap breaching imperfect sealing or thermal obliteration however there are ancient stable cratons where such disruption may not have occurred and so petroleum explorers may wish to reassess the possibility of finding valuable hydrocarbon resources in archean rocks

anomalously high porosity and permeability in deeply buried sandstone reservoirs origin and predictability porosity and permeability generally decrease with increasing depth thermal exposure and effective pressure however a significant number of deep 4 km approximately 13 000 ft sandstone reservoirs worldwide are characterized by anomalously high porosity and permeability anomalous porosity and permeability can be defined as being statistically higher than the porosity and permeability values occurring in typical sandstone reservoirs of a given lithology composition and texture age and burial temperature history in sandstones containing anomalously high porosities such porosities exceed the maximum porosity of the typical sandstone subpopulation major causes of anomalous porosity and permeability were identified decades ago however quantification of the effect of processes responsible for anomalous porosity and permeability and the assessment of the predictability of anomalous porosity and permeability occurrence in subsurface sandstones have rarely been addressed in published literature the focus of this article is on quantification and predictability of three major causes of anomalously high porosity 1 grain coats and grain rims 2 early emplacement of hydrocarbons and 3 shallow development of fluid overpressure grain coats and grain rims retard quartz cementation and concomitant porosity and permeability reduction by inhibiting precipitation of quartz overgrowths on detrital quartz grains currently prediction of anomalous porosity associated with grain coats and grain rims is dependent on the availability of empirical data sets in the absence of adequate empirical data sedimentologic and diagenetic models can be helpful in assessing risk due to reservoir quality such models provide a means to evaluate the effect of geologic constraints on coating occurrence and coating completeness required to preserve economically viable porosity and permeability begin page 302 in a given play or prospect these constraints include thermal history and sandstone grain size and composition the overall effect of hydrocarbon emplacement on reservoir quality is controversial it appears that at least some cements quartz and illite may continue to precipitate following emplacement of hydrocarbons into the reservoir our work indicates that integration of basin modeling with reservoir quality modeling can be used to quantify prior to drilling the potential impact of hydrocarbon emplacement on porosity and permeability the best case scenario for significant reservoir quality preservation due to fluid overpressure development is in rapidly deposited tertiary or quaternary sandstones our models suggest that significant porosity can be preserved in sandstones that have experienced continuous high fluid overpressures from shallow burial depths the models also indicate that the potential for porosity preservation is greatest in ductile grain rich sandstones because compaction tends to be the dominant control on reservoir quality in such rocks the case for significant porosity preservation associated with fluid overpressures in pre tertiary basins however is more problematic because of the complexities in the history of fluid overpressure and the greater significance of quartz cementation as a potential mechanism of porosity loss

relationship between reservoir diagenetic evolution and petroleum emplacement in the ula field north sea abstract the diagenetic and petroleum filling histories of the ula field have been studied by analysing aqueous and petroleum inclusions occurring in authigenic cements this study shows that diagenesis continues actively after the arrival of petroleum in the sandstones although the reaction rates and petroleum saturation remain obscure microthermometric measurements of fluid inclusions in authigenic quartz suggest an onset of extensive quartz cementation at temperatures around 110 c i e 2 5 km and that this cementation has continued to the present day synchronously with quartz cementation authigenic albite was formed as inter and intragranular cements large amounts of petroleum inclusions locally occur in the albite and quartz cements these inclusions were trapped in the cements as the main charge of petroleum arrived at the structure diffusion through thin water layers between the petroleum and the mineral surface would probably have dominated mass transfer of solutes for the precipitation of authigenic quartz and albite it is evident from analysis of the petroleum inclusions in early formed k feldspar overgrowths that some secondary migrated petroleum probably arrived in the reservoir at a shallow burial depth before the rapid subsidence in the last 25 my compared with petroleum in the free porosity of the ula formation the petroleum in the k feldspar inclusions is evidently sourced from a different source facies and most maturity parameters testify to the latter being less mature later trapped petroleum inclusions in quartz and albite have characteristics found both in k feldspar and in the ula formation dst oil and thus is likely to reflect the progressive change in the ula field petroleum charge which occurred during the time period of quartz diagenesis

microtextures of authigenic or rich feldspar in the upper jurassic humber group uk north sea detrital alkali feldspars currently at burial depths of 3 2 3 5 km in the upper jurassic humber group of the fulmar oilfield uk north sea are overgrown and have been partially replaced by authigenic or rich feldspar intracrystal microtextures suggest several different provenances for the detrital grains the overgrowths are uniformly non cathodoluminescent and have occasional celsian rich zones transmission electron microscopy shows that they are composed of a microporous mosaic of sub m to m sized sub grains associated with barite illite and pyrite the subgrains are somewhat rounded but have an approximate 110 adularia habit and display a faint modulated microtexture on the nanometre scale they have triclinic symmetry but the lattice angles depart only slightly from monoclinic symmetry these features are characteristic of k feldspar precipitated relatively rapidly and at low temperature authigenic or rich feldspar has also partially replaced microcline and perthitic albite within the detrital grains often at a suboptical scale although like diagenetic albitization replacement by k feldspar is probably a very common diagenetic reaction it has rarely been reported owing to difficulties in imaging the diagnostic textures with the scanning electron microscopy techniques used by most workers the permeability of the subgrain microtexture may significantly hinder the use of feldspar overgrowths for k ar and 4 0 ar 3 9 ar dating of diagenesis and the existence of suboptical replacive authigenic k feldspar within detrital grains may significantly modify the apparent ar ages of detrital grains similar subgrain microtextures in optically featureless quartz overgrowths are also illustrated

the influence of multiple stage oil emplacement on deeply buried marine sandstone diagenesis a case study on the devonian donghe sandstones tabei uplift tarim basin nw china abstract understanding the oil emplacement influence on diagenesis is one of the most key factors of reservoir quality prediction the devonian donghe sandstones are deeply buried to 5700 6000 m and they are important exploration target in the tarim basin additional the reservoirs experienced multi phase hydrocarbon accumulation and destruction the donghe sandstones contain various diagenetic minerals which are mainly composed of calcite cement a small amount of quartz overgrowth and pyrite trace ankerite and ferrocalctie the current study investigates the influence of multiple stage oil emplacement on the diagenesis with particular attention to calcite and quartz cementation of marine clastic reservoir interval of the donghe sandstones in the tarim basin the samples in the oil leg and water leg have been collected and a series of studies were performed including thin section petrography scanning electron microscopy sem carbon and oxygen isotopes and cathodoluminescence cl analysis the 13cv pdb values of calcite cement from the oil leg and water leg between 4 73 and 1 21 and 3 81 to 0 86 respectively the 18ov pdb values of calcite cement from the oil leg and water leg range from 15 59 to 6 61 and 14 97 to 7 76 respectively both of them display no obviously different in the oil leg and water leg all of the 13cv pdb values in the whole interval of the donghe sandstones show lightly depletion was explained mixing from marine carbon and carbon produced from bacterial sulfate reduction bsr that happens during petroleum degradation associated with sulfate reduced by hydrocarbons bacterially two dull luminescence zones trace fe and mn content and high fe accompanied by low mn content and orange red and bright orange yellow luminescence zones in oil leg indicate four generations of calcite occurred whereas two generations of calcite occurs in water leg which are dull and orange red luminescence zones two origins of calcite cement were clearly identified marine biogenic carbonates and hydrocarbon related from palaeo reservoir bsr during uplift the first generation of calcite cement originated from biogenic carbonate the second and third generations of calcite cement were mixing from predate carbonate cement dissolved and re precipitated and calcite precipitated by bsr directly some quartz grains and k feldspars were dissolved by residual organic acids that remained in the irreducible water and following replaced by the fourth generation of calcite cement some calcite cement in the oil leg precipitated in a temperature higher than the homogeneous temperature of fluid inclusion which indicates oil emplacement has no influence on calcite cementation the quartz overgrowth postdate the first hydrocarbon accumulation and predate the second hydrocarbon accumulation which originated from quartz grains dissolved by organic acids that produced from bsr and re precipitated at a low temperature the current study can promote the understanding of the influence of multiple stage hydrocarbon accumulation on diagenesis and it can also inspire the diagenesis process of other paleozoic hydrocarbon reservoirs in the tarim basin

influence of hydrocarbon injection on the compaction by pressure solution of a carbonate rock an experimental study under triaxial stresses abstract the preservation of good petrophysical properties high porosity high permeability at great depth in carbonate rocks may lead to the existence of a deeply buried reservoir dbr a target of interest for the oil industry one of the key processes controlling diagenesis of the burial environment is pressure solution creep psc an efficient compaction process responsible for the evolution of porosity and permeability in many carbonate reservoirs in this experimental study we examine the effect of i the presence of oil in the pore space and ii its timing of injection on the psc process and the petrophysical properties of a carbonate rock the experiments were performed using a flow through high pressure cell allowing the simulation of the pressure stresses and temperature conditions of a dbr multi disciplinary data mechanical chemical petrographical and petrophysical demonstrate that without oil in the pore space the main diagenetic process is the psc a process reducing by three the initial porosity but having no influence on intrinsic water permeability an early injection of oil prior to water circulation causes the inhibition of psc by the coating of the grains leading to the preservation of porosity conversely a late injection of oil does not preserve initial porosity the dataset obtained from these experiments show the importance of the timing of oil charging in a reservoir in the preservation of initial porosity at great depth by the inhibition of psc however the coating of grains by hydrocarbons may also inhibit further diagenetic processes leading to a creation of secondary porosity at depth

fluid circulation and deformational gradient in north pyrenean flyschs example from the saint jean de luz basin france abstract the relationships between fluid circulation and deformation are one of the issues of the sedimentary basin study in the cretaceous flysch of the saint jean de luz basin the evolution of folds geometry and the increasing volume of calcite filled fractures and veins evidence a northward deformational gradient along the french basque coast a combined approach is proposed to assess the corresponding physical and chemical conditions the microthermometric study of fluid inclusions in calcite sampled in different generations of veins and fractures and the x ray diffraction analysis of clay minerals from adjacent marl layers salinity of the trapped h 2 o cacl 2 nacl fluids increases with depth in the series in good agreement with salinity gradients reported in sedimentary basins dispersion of the data also increases with depth from 0 3 to 1 3 wt nacl eq in the shallowest formation haizabia flysch to 9 1 to 23 0 wt nacl eq in the deepest formation guethary flysch minimal trapping temperatures of the fluids in the haizabia and socoa flyschs 79 and 102 c respectively are consistent with the temperatures estimated from the depth of burial which did not exceed 5 km in good agreement with the stability of the smectite illite kaolinite assemblage found in marls in addition the kaolinite proportion significantly decreases with depth in the series as a potential consequence of climate changes and diagenetic transformations whereas the increasing dispersion of illite crystallinity data might indicate fluid mineral interactions we propose a synthetic model of fluid circulation in the folded series that involves the mixing of mainly horizontal fluid circulation potentially meteoric with an upward flow of high salinity fluid throughout the deepest formations potentially related to underlying evaporite rich layers the northward deformational gradient as exposed along the french basque coast is likely to be responsible for such a vertical circulation by increasing the volume of fracture particularly cross cutting fractures in the deeply buried formations

the effect of oil saturation on the mechanism of compaction in granular materials higher oil saturations lead to more grain fracturing and less pressure solution abstract whether addition of oil to sandstones slows stops or has no influence upon pressure solution and quartz cementation has long been disputed despite having major implications for petroleum exploration and appraisal strategies to elucidate the effect of addition of oil to compaction pressure solution and cementation processes this study utilizes an experimental approach simulating isochemical volumetric compaction using granular halite in the presence of variable brine oil mixtures each experiment at 500 kpa effective stress lasted 48 hours and involved repeated measurements of volumetric strain the lithified products of experiments were examined using sem techniques after 30 hours approximately 4 volumetric strain occurs when the brine oil ratio is 100 0 pure brine fractures are rare in samples from experiments undertaken with pure brine so that pressure solution and concomitant cementation must be the compaction mechanism when the brine oil ratio is decreased from 100 0 to 40 60 the volumetric strain remains about 4 but there are more fractures as quantified from sem image analysis these observations suggest that although pressure solution has occurred some of the volumetric strain is the result of fracturing implying that pressure solution has been less effective in the presence of some oil than it is in the presence of pure brine when the brine oil ratio is decreased further from 40 60 to 5 95 the volumetric strain increases up to 7 3 sem image analysis reveals that the strain is predominantly due to grain fracturing this change from pressure solution to grain fracturing is likely due to heterogeneous pressure solution resulting from heterogeneous distribution of stresses brine coated grain contacts undergo pressure solution but oil coated grain contacts experience increasing stress up to the point of failure as the surrounding grain pack compacts when the brine oil ratio is decreased to less than 5 95 volumetric strain decreased to about 1 with few induced fractures and little pressure solution compaction is negligible when the brine oil ratio is 0 100 only oil in the pores and fractures are negligible halite is more water soluble and more hygroscopic than quartz so that water will cling to halite surfaces more tenaciously than to quartz since even halite has its mechanism of volumetric strain affected when the brine oil ratio is less than 40 60 quartz pressure solution and subsequent cementation is even more likely to be affected by oil filling this experimental study suggests that the mechanism and extent of compaction and subsequent cementation in sandstones is strongly affected by the addition of oil

geochemical characteristics and implications of hydrocarbons in reservoir rocks of junggar basin china abstract a practical sequential extraction approach has been applied to two core samples collected from oil zones of triassic sandstones in junggar basin northwest china five bitumen fractions were obtained from each sample using this approach these fractions are free oil components in inter granular pores bitumen 1 adsorbed oil components on grain surfaces and liberated by hcl treatment bitumen 2 oil bearing fluid inclusions hosted by grains bitumen 3 and adsorbed oil components on clay mineral surfaces and liberated by hcl and hcl hf treatments bitumens 4 and 5 striking differences have been observed both in bulk compositions and in aliphatic biomarker distributions e g terpanes and steranes among the five bitumen fractions this phenomenon is ascribed to the indigenous compositional variations and differential maturation behaviors between adsorbed oil components in a network of polar compounds resins and asphaltenes and free oil components in even a same oil charge in addition to fractional adsorption effects and compositional changes of the oil charges in reservoir rocks during filling process the molecular compositions and homogenization temperatures of oil bearing fluid inclusions in these two samples indicate that trapping of these types of inclusions apparently ceased in sample a while continued in sample b after the initial oil charge suggesting that the relative content of polar components in the initial oil charge compared to the later oil charges plays a dominant role controlling the trapping of oil bearing fluid inclusions in an oil saturated zone sequential extraction studies are important and useful for reservoir geochemistry especially in the cases where only limited samples of oil and reservoir rocks are available and or where compositional heterogeneity of free oils has disappeared in a reservoir karlsen d a nedkvitne t larter s r bjorlykke k 1993 hydrocarbon composition of authigenic inclusions application to elucidation of petroleum reservoir filling history geochim cosmochim acta 57 3641 3659

vertical effective stress and temperature as controls of quartz cementation in sandstones evidence from north sea fulmar and gulf of mexico wilcox sandstones abstract we present quantitative petrographic data high spatial resolution oxygen isotope analyses of quartz cement basin modelling and a kinetic model for quartz precipitation for two paleocene eocene wilcox group sandstones from texas and two jurassic fulmar formation sandstones from the central north sea at each location one sandstone has been buried to ca 145 c and one to ca 185 c a key difference between the wilcox and fulmar burial histories is that the wilcox sandstones are currently at higher vertical effective stresses and from basin modelling studies have been subjected to generally higher vertical effective stresses through their burial history the amounts of quartz cement in the wilcox sandstones are between 12 and 18 and between 2 and 6 in the fulmar sandstones high spatial resolution oxygen isotope data obtained from the quartz cements suggest temperature ranges for quartz precipitation from 60 to 80 c to values approaching maximum burial temperature factors such as grain coatings or the timing of petroleum emplacement cannot explain the differences in the amounts of quartz cement petrographic data show that most of the silica for quartz cement can be derived from intergranular pressure dissolution although the sample set is small we interpret the results to suggest that the differences in quartz cementation in fulmar and wilcox sandstones can be explained better by differences in their vertical effective stress history than their temperature history in this case the supply of silica rather than the precipitation of quartz becomes an important control on the rate and extent of cementation

integrated stratigraphy of the upper neoproterozoic succession in yunnan province of south china re evaluation of global correlation and carbon cycle abstract this study provides a chemostratigraphic and sequence stratigraphic framework of a relatively unfossiliferous sub cambrian marine sedimentary succession in yunnan province of south china our results allow its global stratigraphic correlation and provide an improved understanding of marine carbon cycle during the late neoproterozoic the succession has a basal glacial diamictite nantuo formation and comprises two large scale depositional cycles towards the precambrian cambrian pc c boundary each of which changes from siliciclastic to carbonate sediments with environments ranging from shallow subtidal to evaporitic supratidal in the absence of biostratigraphic control attribution of the tillite has remained equivocal between the 710 ma sturtian and 635 ma marinoan glaciations a compilation of known paleomagnetic and other geochronologic constraints suggests that the nantuo glaciation in south china was at least diachronous in which the nantuo tillite in yunnan belongs to the older glaciation stratigraphic patterns of the diagenetically screened 13c variation and depositional sequences during the carbonate dominated interval show a striking similarity with those of the upper cryogenian and entire terminal proterozoic carbonates at widely separated other localities rather than the terminal proterozoic alone supporting the view that deposition of the nantuo tillite in yunnan reflects the sturtian rather than marinoan glaciation further considerations of the 13c results provide three important implications for the late neoproterozoic marine carbon cycle first comparison of different 13c profiles suggests considerable variability in the absolute values of carbonate on both intra and inter basinal scales which differs profoundly from their relative homogeneity in marine phanerozoic counterparts and may therefore reflect lateral isotopic heterogeneity in the oceanic carbonate reservoir during the late neoproterozoic second an empirical correlation between the 13c and relative sea level changes was applied to evaluate hypothesized explanations for 13c temporal variation in surface sea yielding short term inverse and long term positive relationships the short term trend marks periods of the negative 13c excursions and is compatible to most of the existing arguments for the 13c events on the other hand mechanisms causing the long term trend remain enigmatic in the absence of comparable data from other localities our observation however suggests that at least two different mechanisms operated marine carbon cycle during the late neoproterozoic third the negative 13c anomaly characterizing the period following the marinoan glaciation worldwide was not coincident between kerogen and carbonate but occurs as a lagged drop in yunnan where a 7 depletion in kerogen preceded an 3 depletion in carbonate their lagged trends likely reflect different sources as kerogen and carbonate signals are considered to monitor surface and deeper waters respectively the negative 13c event would therefore have occurred in surface water followed by a smaller 13c depletion through the entire water column due to vertical mixing between surface and deeper waters at the studied site this mechanism supports the proposed hypothesis that injection of the 13c depleted atmospheric co2 into the surface sea deriving ultimately from quantitative dissociation of methane hydrate in terrestrial permafrost during the post glacial warming contributed to the negative 13c anomaly

the effect of oil emplacement on quartz cementation in a deeply buried sandstone reservoir quartz is an important porosity occluding cement in sandstone reservoirs that have been subjected to elevated temperature 80 c to 100 c for a substantial period of time the effect of oil emplacement on quartz cementation in reservoir sandstones is controversial some studies have concluded that early oil emplacement can inhibit quartz cementation leading to the preservation of porosity whereas other studies have concluded that quartz cementation appears largely unaffected by oil emplacement here we have studied shallow marine upper jurassic sandstones from ula field norwegian north sea with reservoir temperatures of approximately 150 c to determine whether oil emplacement had a significant impact on diagenesis with particular attention to quartz cementation following sedimentological description of cores samples above and below the oil water contact have been collected adjacent to core analysis plug points these samples then underwent a series of studies including petrographic point counting with a transmitted light microscope scanning electron microscopy sem backscattered electron microscopy sem cathodoluminescence microscopy and fluid inclusion studies these data were integrated with routine core analysis and petrophysical log data density and resistivity log data have been used to determine the precise oil saturation of each sample studied the distributions of all potential controls on porosity and permeability such as grain size sorting matrix clay content and degree of bioturbation the presence of grain coatings and dolomite cement and the amount of quartz cement have been assessed the presence of primary oil inclusions within quartz cement shows that oil ingress into the ula reservoir commenced prior to the onset of quartz cementation very fine grained matrix rich bioturbated microquartz cemented sandstones have uniformly low quartz cement contents irrespective of oil saturation medium grained graded matrix poor microquartz poor sandstones have quartz cement ranging from 1 to greater than 17 associated with core porosities of approximately 22 and 7 respectively higher oil saturations equate to higher porosities and permeabilities in the medium grained graded matrix poor microquartz poor sandstones which cannot be explained by any control other than the amount of quartz cement as a function of pore fluid type oil emplacement therefore appears to have inhibited quartz cementation at high oil saturations and can be viewed as an important control on reservoir quality the significance of this study is that the presence of oil in a sandstone reservoir at the time that quartz cement was growing can have a considerable impact on reservoir quality models that seek to predict quartz cement and reservoir quality in sandstones need to account for the timing of oil emplacement compared with other diagenetic processes

scaled physical model of secondary oil migration discussion thomas and clouse have presented results of a detailed laboratory model of secondary oil migration using a sand pack and then used scaling group theory to extrapolate their observations to field scale source rock carrier bed reservoir systems they conclude that for water wet homogeneous systems the charging of traps is not controlled by secondary migration rates or losses but by the rate of primary migration from the source rock although this conclusion is compatible with the observation that estimates of field charging times are broadly similar to estimates of the time interval for a source rock to mature and expel oil we feel that several of their arguments about real secondary migration processes are misleading and require further discussion

diagenetic albitization of detrital k feldspar in jurassic lower cretaceous and tertiary clastic reservoir rocks from offshore norway i textures and origin abstract the origin of albitized grains in sandstones has been the topic of considerable discussion in recent years some workers believe that the albitization of feldspar can occur in situ during burial diagenesis but others argue that albitization is a feature inherited from source clastic grains our petrological and chemical studies of subsurface samples of the jurassic lower cretaceous and tertiary reservoir sandstones from offshore norway show that detrital grains of potassium feldspar have been albitized during burial diagenesis criteria suggesting that the albitization has occurred during burial diagenesis include 1 euhedral habit of albite crystals with sharp edges and comers and markedly smooth crystal faces 2 generally untwinned albitized grains mostly riddled with minute br wnish inclusions 3 lack of cathodoluminescence in albite 4 homogeneous and pure albite composition 99 ab 5 absence of albitized grains in carbonate cemented zones and 6 increase in the percentage and degree partial to complete of albitized k feldspar with depth under sem albitized grains display two types of fabrics type i albitized grains are composed of numerous tiny albite crystals 1 30 m growing along cleavage planes in parent grains and they have abundant dissolution porosity while type ii albitized grains show blocky albite crystals 5 80 m forming pseudomorphs of detrital k feldspar and they lack any dissolution porosity type i albitized grains are found commonly at shallower depths between 2 2 and 3 km and temperatures varying from 65 to 90 c whereas type ii albitized grains dominate at greater depths 3 5 km and at temperatures 90 c albitization proceeds by a dissolution precipitation mechanism at shallower depth dissolution of the parent k feldspar is more rapid than the precipitation of albite and rates of albite precipitation increase with depth favoring pseudomorphic replacement of k feldspars other factors that influence albitization processes are ana ak permeability reduction and surface dissolution kinetics of the parent grain

geochemical comparison of oil bearing fluid inclusions and produced oil from the toro sandstone papua new guinea abstract a detailed study was carried out to compare the geochemistry of the crude oil in the lower cretaceous toro sandstone reservoir in the iagifu 7x well with oil trapped in fluid inclusions in this sandstone in the iagifu 7x and p nyang 2x wells in the papuan fold belt the drill stem test dst oils and the oils trapped in fluid inclusions show different source and maturity dependent geochemical parameters biomarkers suggest that the dst oils at iagifu 7x were derived from clay rich mixed marine terrestrial source rocks deposited under oxic conditions probably middle to late jurassic mudstones the fluid inclusion oils were derived from a less terrestrially influenced marine source rock deposited under less oxic conditions the fluid inclusion oils contain 1 2 7 trimethylnaphthalene and oleanane biomarkers indicative of angiosperm input and a cretaceous or younger source rock these oils are more mature calculated reflectance rc 1 02 than the reservoired oil at iagifu 7x rc 0 84 and condensate at p nyang 2x rc 0 84 source and thermal maturity parameters for all three fluid inclusion samples are similar suggesting that they were derived from similar source rock facies furthermore samples with predominantly blue vs white fluorescing oil inclusions have no discernible geochemical maturity difference suggesting that the use of fluorescence colours as a qualitative guide to the maturity of the trapped oil is not valid in this case fluid inclusion oils were trapped prior to charge of the reservoir with oil at the iagifu 7x and gas condensate at the p nyang 2x wells the fluid inclusion oils therefore represent an early oil charge from a deeply buried probably cretaceous source rock which started migrating into the toro sandstone during the miocene at iagifu this early oil was probably diluted by a larger volume of jurassic oil generated at the end of the miocene the absence of fluid inclusions with compositions like the currently reservoired oil is interpreted to be due to oil charging an already oil saturated zone when diagenesis is likely to have been inhibited at the p nyang well the transition from oil as represented by the oil inclusions to gas condensate was most likely caused by gas displacement

aeolian dust in colorado plateau soils nutrient inputs and recent change in source aeolian dust windblown silt and clay is an important component in arid land ecosystems because it may contribute to soil formation and furnish essential nutrients few geologic surfaces however have been characterized with respect to dust accumulation history and resultant nutrient enrichment we have developed a combination of methods to identify the presence of aeolian dust in arid regions and to evaluate the roles of this dust in ecosystem processes unconsolidated sandy sediment on isolated surfaces in the canyonlands region of the colorado plateau differs greatly in mineralogical and chemical composition from associated bedrock mainly aeolian sandstone detrital magnetite in the surficial deposits produces moderately high values of magnetic susceptibility but magnetite is absent in nearby bedrock a component of the surficial deposits must be aeolian to account for the abundance of magnetite which formed originally in far distant igneous rocks particle size analysis suggests that the aeolian dust component is typically as much as 20 30 dust inputs have enriched the sediments in many elements including p mg na k and mo as well as ca at sites where bedrock lacks calcite cement soil surface biologic crusts are effective dust traps that apparently record a change in dust sources over the past several decades some of the recently fallen dust may result from human disturbance of land surfaces that are far from the canyonlands such as the mojave desert some land use practices in the study area have the potential to deplete soil fertility by means of wind erosion removal of aeolian silt

influence of cryptogamic crust disturbance to wind erosion on sand and loam rangeland soils a portable field wind tunnel was used to assess the sediment flux rates of loam and sand textured soils in the mallee region of southeastern australia three levels of crust disturbance nil moderate and severe simulating stock trampling were investigated the results demonstrated the importance of cryptogamic crusts in binding the soil surface and providing roughness after the soil was moderately disturbed on the loamy soil the crust helped maintain sediment flux rates below the erosion control target to 5 g m 1 s 1 for a 65 km h 1 wind measured at 10 m height once the crust was severely disturbed sediment fluxes increased to 1 6 times the erosion target on the sandy soil even with no crust disturbance the sediment flux was 1 6 times the erosion control target disturbing the crust increased sediment fluxes to a maximum of 6 7 times the erosion control target removal of the crust also decreased the threshold wind velocity that resulted in an increase to the ris k of erosion from 5 per cent to 20 per cent 1998 john wiley sons ltd