hydrocarbon generation and expulsion characteristics of eocene source rocks in the huilu area northern pearl river mouth basin south china sea implications for tight oil potential abstract in recent years new oil reservoirs have been discovered in the eocene tight sandstone of the huilu area northern part of the pearl river mouth basin south china sea indicating good prospects for tight oil exploration in the area exploration has shown that the huilu area contains two main sets of source rocks the eocene wenchang e2w and enping e2e formations to satisfy the requirements for further exploration in the huilu area particularly for tight oil in eocene sand reservoirs it is necessary to re examine and analyze the hydrocarbon generation and expulsion characteristics based on mass balance this study investigated the hydrocarbon generation and expulsion characteristics as well as the tight oil resource potential using geological and geochemical data and a modified conceptual model for generation and expulsion the results show that the threshold and peak expulsion of the e2w source rocks are at 0 6 vitrinite reflectance and 0 9 vitrinite reflectance respectively there were five hydrocarbon expulsion centers located in the western eastern and northern huizhou sag and the southern and northern lufeng sag the hydrocarbon yields attributed to e2w source rocks are 2 4 1011 tons and 1 6 1011 tons respectively with an expulsion efficiency of 65 the e2e source rock threshold and peak expulsion are at 0 65 vitrinite reflectance and 0 93 vitrinite reflectance respectively with hydrocarbon expulsion centers located in the centers of the huizhou and lufeng sags the yields attributed to e2e source rocks are 1 1 1011 tons and 0 2 1011 tons respectively with an expulsion efficiency of 20 using an accumulation coefficient of 7 13 the eocene tight reservoirs could contain approximately 1 3 1010 tons to 2 3 1010 tons with an average of 1 8 1010 tons of in place tight oil resources highest recoverable coefficient can reach 17 18 indicating that there is significant tight oil potential in the eocene strata of the huilu area

dating crystalline groundmass separates of altered cretaceous seamount basalts by the 40ar 39ar incremental heating technique abstract alteration of submarine basalts compromises geochronology using conventional k ar and 40 ar 39 ar techniques to help overcome these problems we re evaluated the potential of groundmass dating techniques incremental heating on acid leached groundmass samples following an overnight bakeout at 200 c and using high resolution heating schedules eliminated most of the low temperature alteration effects in the submarine basalts studied more than 75 of the groundmass analyses n 32 display accurate age plateaus consisting of 30 70 of the total amount of 39 ar k released more than 50 of the analyses have plateau ages concordant with their total fusion ages implying minor or proportional loss of radiogenic 40 ar and 39 ar k overall we could show a high degree of coherence between ages of groundmass separates and comagmatic phenocrysts this suggests that the dating of aphyric basalts which previously has proven problematic can be accomplished with increasing confidence as well adding these rock types to the list of datable submarine basalts significantly enhances our ability to understand the eruptive history of linear submarine volcanic chains

geochemistry origin and accumulation of petroleum in the eocene wenchang formation reservoirs in pearl river mouth basin south china sea a case study of hz25 7 oil field abstract the pearl river mouth basin in the south china sea has accumulated 2 km of eocene sediments in its deep basin and has become the exploration focus due to the recent discoveries of the hz25 7 oil field in the eocene wenchang e 2 w formation in this study the geochemical characteristics of potential source rocks and petroleum in the hz25 7 oil field are investigated and the possible origins and accumulation models developed the analytical results reveal two sets of potential source rocks e 2 w and enping e 2 e formations developed in the study area the semi deep to deep lacustrine e 2 w source rocks are characterized by relatively low c 29 steranes low c 19 c 23 tricyclic terpane 24 tetracyclic terpane c 30 hopane trans trans trans bicadinane t c 30 hopane most 30 4 methyl sterane c 29 sterane 0 2 ratios in contrast the shallow lacustrine and deltaic swamp plain e 2 e source rocks are characterized by relatively high c 29 steranes high c 19 c 23 tricyclic terpane 0 6 high c 24 tetracyclic terpane c 30 hopane 0 1 variable yet overall high t c 30 hopane and low c 30 4 methyl sterane c 29 sterane 19 c 23 tricyclic terpane ratios mean value 0 39 low c 24 tetracyclic terpane c 30 hopane ratios mean value 0 07 high c 30 4 methyl sterane c 29 sterane ratios mean value 1 14 and relatively high c 27 regular sterane content of petroleum in the hz25 7 oil field indicate that the petroleum most likely originated from the e 2 w formation mudstone in the huizhou depression one stage of continuous charging is identified in the hz25 7 oil field oil injection is from 16 ma to present and peak filling occurs after 12 ma thin sandstone beds with relatively good connectivity and physical properties porosity and permeability in the e 2 w formation are favorable conduits for the lateral migration of petroleum this petroleum accumulation pattern implies that the e 2 w formation on the western and southern margins of the huizhou depression are favorable for petroleum accumulation because they are located in a migration pathway thus exploration should focus in these areas in the future

40ar 39ar illite dating of late caledonian acadian metamorphism and cooling of k bentonites and slates from the welsh basin u k the vacuum encapsulation laser 40ar39ar technique allows extremely small 10 6 g samples of fine grained materials such as diagenetic clays to be dated here we show that the method can be extended to higher grade clay minerals the integration of transmission electron microscopic tem characterization with 40ar39ar dating of vacuum encapsulated samples permits the resolution of the timing of metamorphic growth cooling from the time of diagenesis we have applied this technique to well characterized lower paleozoic slates and k bentonites from the welsh basin which span the transition from anchizonal to epizonal grade which had been previously studied using rbsr and smnd dating tem observations of epizonal k bentonites and slate showed that illite in these samples is of 2m1 polytype of muscovite like composition and oriented parallel to cleavage suggesting that they are of metamorphic origin total gas ages equivalent to conventional kar ages for encapsulated epizonal k bentonites and slate 340 408 ma are considerably variable the ar retention ages calculated from 39ar and 40ar atoms retained in the sample after irradiation are more consistent 383 411 ma the 39ar recoil losses are minor for illites from whole rock samples of epizonal k bentonites but very significant for clay separates of epizonal slate plateaus in age spectra were observed in epizonal k bentonites and slate the plateau ages 414 421 ma and retention ages 383 411 ma can be correlated with the onset of acadian metamorphism and culmination of uplift and inversion of the welsh basin respectively these ages are significantly younger than the 450 ma ages previously reported for diagenetic clays using the same method suggesting that diagenetic history has been lost in these epizonal k bentonites and slate tem observations of anchizonal slates showed that there are two modes of illite the first mode is similar to that observed in epizonal samples suggesting a metamorphic origin the second mode consists of the 1md polytype has typical diagenetic illite composition and is oriented parallel to bedding suggesting a diagenetic origin total gas ages for encapsulated anchizonal slates vary considerably 361 422 ma the retention ages are more consistent 413 432 ma than the total gas ages the 39ar recoil losses are more significant than those for epizonal k bentonites and slate plateaus in age spectra are generally not observed however the consistent retention ages for the anchizonal slates correspond to the plateau ages for the epizonal samples and are inferred to represent the onset of acadian metamorphism these data when combined with our previously published results for diagenetic shales suggest that thermal conditions near the boundary of anchizonal and epizonal grades are necessary to completely reset ar systems in shales and slates

determination of crustal fluid residence times using nucleogenic 39ar abstract argon 39 is a radioactive isotope of ar with a half life of 269 years there are two natural sources of 39 ar on earth cosmogenic production in the upper atmosphere and nucleogenic production in rocks cosmogenic 39 ar has been recognized as an ideal tracer for groundwater residence times and oceanic circulation nucleogenic 39 ar has generally been considered problematic for groundwater studies in this work we developed simple models for the evolution of 39 ar 40 ar ratios where 40 ar denotes radiogenic 40 ar produced in the model system in rocks and fluids in rocks both 39 ar and 40 ar are produced and a fraction of the 39 ar and 40 ar is released into the mobile fluid phase in both reservoirs 39 ar reaches production or supply decay equilibrium whereas 40 ar accumulates with time thereby the 39 ar 40 ar ratio in rocks and fluids generally decreases with time our models suggest that the co evolution of these two isotopes in rocks and fluids provides a new chronometer of fluid migration and reaction in the crust this chronometer is relevant to a much wider time range than that limited by radioactive decay of cosmogenic 39 ar i e 1400 years at the current detection limit of 3 modern atmospheric value the 39 ar 40 ar chronometer is applicable to estimating residence times of very old groundwater as well as for studying fluid rock interactions and fluid migration rates such as those occurring during co 2 sequestration and the formation of hydrocarbon reservoirs

rb sr and k ar dating of clay diagenesis in jurassic sandstone oil reservoir north sea rb sr and k ar isotopic determinations were combined with scanning electron microscopy sem observations and x ray diffractometry xrd controls to date the diagenetic formation of clay minerals from oil bearing sandstones of the middle jurassic brent sandstone in the north sea the separated clay fractions often contain detrital components especially k feldspars which are reduced to sizes smaller than 0 4 m either by intense in situ alteration or by destructive sample preparation direct rb sr and k ar isotopic dating of diagenetic illite therefore was not possible isotopic data obtained on mixtures of detrital and newly formed components showed however that illite formed about 40 to 45 ma this value also was obtained directly on clay fractions separated by a gentle method of disaggregation illitization is related to emplacement of hydrocarbons and associated waters into the formation aqueous and hydrocarbon fluids simultaneously trapped in quartz overgrowth inclusions indicate late silicification contemporaneous with this fluid migration minimum trapping temperatures of at least 110 c for these fluid inclusions are too high for the present burial depth hot fluids are assumed to have originated in deep source rocks beneath the brent reservoirs and migrated upward during the middle eocene

first successful 40ar 39ar dating of glauconies argon recoil in single grains of cryptocrystalline material potassium argon dating of glaucony one of the few widely occurring authigenic materials in the sedimentary record has provided a large percentage of the dates used for the calibration of the geologic time scale this material has been considered unsuitable for 40 ar 39 ar dating because of the large losses of neutron induced 39 ar during or subsequent to irradiation of samples we have measured recoiled 39 ar and 37 ar from single grains of glaucony 100 350 m in diameter irradiations were done both in air and in vacuo by using a microampoule technique of encapsulation the observed in vacuo behavior of recoiled atoms can be readily understood by a simple model of their interaction with the network of very fine laminae composing the grains when the microampoule technique is used the integrated 40 ar 39 ar ages and their associated precision compare very favorably with conventional k ar results because glauconies are inherently heterogeneous populations owing to their complex evolution it is anticipated that single grain 40 ar 39 ar dating may lead to significant refinements in the geologic time scale especially for the cenozoic

noble gas geochemistry in thermal springs abstract the composition of noble gases in both gas and water samples collected from horseshoe spring yellowstone national park was found to be depth dependent the deeper the sample collection within the spring the greater the enrichment in kr xe radiogenic 4he and 40ar and the greater the depletion in ne relative to 36ar the compositional variations are consistent with multi component mixing the dominant component consists of dissolved atmospheric gases acquired by the pool at the surface in contact with air this component is mixed in varying degree with two other components one each for gas and water entering the bottom of the pool the two bottom components are not in equilibrium in horseshoe spring the bubbles entering at the bottom strip the atmospheric derived pool gases from the surrounding water while en route to the surface if the original bottom bubbles are noble gas poor as in the case of horseshoe the acquired pool gases can then quickly obliterate the original bubble composition these results are used to demonstrate that yellowstone spring surface gas samples and perhaps similarly sampled thermal springs from other hydrothermal systems have gas abundances that depend more on spring morphology than processes occurring deeper in the hydrothermal system

subterranean production of neutrons 39 ar and 21 ne rates and uncertainties abstract accurate understanding of the subsurface production rate of the radionuclide 39 ar is necessary for argon dating techniques and noble gas geochemistry of the shallow and the deep earth and is also of interest to the wimp dark matter experimental particle physics community our new calculations of subsurface production of neutrons 21 ne and 39 ar take advantage of the state of the art reliable tools of nuclear physics to obtain reaction cross sections and spectra talys and to evaluate neutron propagation in rock mcnp6 we discuss our method and results in relation to previous studies and show the relative importance of various neutron 21 ne and 39 ar nucleogenic production channels uncertainty in nuclear reaction cross sections which is the major contributor to overall calculation uncertainty is estimated from variability in existing experimental and library data depending on selected rock composition on the order of 10 7 10 10 particles are produced in one kilogram of rock per year order of 1 10 3 kg 1 s 1 the number of produced neutrons is lower by 6 orders of magnitude 21 ne production rate drops by an additional factor of 15 20 and another one order of magnitude or more is dropped in production of 39 ar our calculation yields a nucleogenic 21 ne 4 he production ratio of 4 6 0 6 10 8 in continental crust and 4 2 0 5 10 8 in oceanic crust and depleted mantle calculated 39 ar production rates span a great range from 29 9 atoms kg rock 1 yr 1 in the k th u enriched upper continental crust to 2 6 0 8 10 4 atoms kg rock 1 yr 1 in depleted upper mantle nucleogenic 39 ar production exceeds the cosmogenic production below 700 m depth and thus affects radiometric ages of groundwater the 39 ar chronometer which fills in a gap between 3 h and 14 c is particularly important given the need to tap deep reservoirs of ancient drinking water

the temporal and spatial distribution of volcanism in the south china sea region cenozoic magmatism in the south china sea the associated continental margins coastal south china and indochina is reviewed volcanism contemporaneous with rifting and sea floor spreading in the south china sea was very weak in the margins and adjacent areas there was only very limited magmatic activity most probably after the cessation of sea floor spreading the south china sea has non volcanic margins large basement relief caused by extension formed major basins and a continental oceanic transitional zone in the northern margin reflects high crustal rigidity during rifting and drifting the paucity of a magma supply and presence of rigid fault blocks are unfavorable to the mantle flow model previously proposed to account for the opening of the south china sea which was supposedly linked to the escape of indochina induced by indo eurasia collision c 2005 published by elsevier ltd

the age of illite cement growth village fields area southern north sea evidence from k ar ages and 18o 16o ratios in this paper we describe k ar radiometric ages and oxygen isotope ratio measurements of illite cements from eolian sandstone samples of the permian lower leman sandstone formation rotliegendes group the samples come from a large about 100 100 km part of the southern north sea basin village fields area and from a range of burial depths 2 8 3 4 km subsea bed mean illite ages in the gas fields of ravenspurn north 164 ma 20 1 2 sgr n 11 hyde 161 ma 12 3 n 7 cleeton 155ma 20 2 n 3 and hoton 152 ma 14 0 n 14 fields and in two wells located in block 47 5a 162 ma 16 1 n 4 are all in the middle to late jurassic apart from two early cretaceous ages considered to represent unreliable analyses the m an for all ages is 158 ma 18 6 2 sgr n 39 we largely avoided contamination with old detrital k bearing minerals by careful sample selection and monitoring to a first approximation the assumptions of k ar dating appear to be fulfilled so the measured ages are likely to date illite cement precipitation oxygen isotope ratios measured on the dated illite samples show small but significant variations among fields and a weak correlation with depth during the middle jurassic when illite grew both k ar dating and oxygen isotope ratios are compatible with a regional middle to late jurassic phase of illite growth during which the mineral precipitated over a large area and over a range of temperatures governed by the depth of the formation at that time the age of illite cementa ion coincides with rifting which ultimately may have caused illite growth by driving pore water flow

direct dating of carbonate remagnetization by 40ar 39ar analysis of the smectite illite transformation abstract the occurrence of remagnetizations in carbonate rocks is a longstanding issue the result of a cryptic process that affects rocks in both fold thrust belts and undisturbed continental interiors this paper examines the clay mineralogy of insoluble material from remagnetized carbonates taken from the cantabrian asturian belt northern spain detailed x ray diffraction patterns indicate that mixed layer clays from these rocks are mostly illitic with an increasing proportion of authigenic illite polytype 1m d in finer grain size fractions the 40 ar 39 ar age data from the different grain size fractions coupled with quantitative polytype modeling indicates an authigenic age that is coeval with the established late paleozoic remagnetization age of these rocks we propose that the transformation of fe rich smectite to fe poor illite created a population of authigenic magnetite that is responsible for the remagnetization of these rocks the irreversible kinetic dependence of clay mineral transformations appears to account for the irreproducibility and unpredictability of carbonate remagnetization and may explain such vexing phenomena as synfolding magnetizations this technique provides an independent assessment of the presence and timing of rock remagnetization with numerous potential applications in future studies of carbonate sequences

multi stage zircon and monazite growth revealed by depth profiling and in situ u pb geochronology resolving the paleoproterozoic tectonics of the trans hudson orogen on southeastern baffin island canada abstract understanding the growth and recrystallization of zircon and monazite during regional metamorphism is important for identifying the nature and timescales of tectonic processes in continental crust we present extensive in situ u pb monazite geochronology and u pb zircon depth profiling analyses from the mid crust exposed on southeastern baffin island canada in the internal zone of the paleoproterozoic trans hudson orogen in situ la icp ms u pb monazite dating on ten metasedimentary rocks totalling 300 analyses from different paleodepths reveals a dominant ca 1860 1820 ma monazite population that records regional mid amphibolite to granulite facies metamorphism with coeval fabric development subordinate monazite with ages between ca 1800 1750 ma are interpreted to have formed through dissolution reprecipitation the sims u pb depth profiling technique allowed dating of ca 1861 1833 ma zircon rims and less abundant ca 1800 1764 ma zircon rims that would be unresolvable using conventional spot dating trace element depth profiling via la icp ms shows that zircon rims typically have lower th u values higher hf contents and different ree compositions than zircon interiors zircon rim compositions are highly variable with no apparent link to u pb age and are interpreted as the combined result of metamorphic zircon growth and dissolution reprecipitation our data demonstrate that the timing of peak metamorphism is recorded by both monazite and zircon and that they are both sensitive to u pb age resetting at lower temperatures during late orogenic tectonism regional mid amphibolite to granulite facies metamorphism and east directed crustal shortening are bracketed between ca 1860 1820 ma these events are interpreted to result from the accretion of cratonic blocks in the internal zone of the trans hudson orogen the integrated geochronological approach on regionally distributed samples enabled us to document ca 1800 1750 ma monazite domains and zircon rims which we interpret to result from the terminal collision with the lower plate superior craton followed by late to post orogenic tectonothermal activity that continued for at least 70 myr after thermal peak this prolonged history is characteristic of the northeastern trans hudson orogen and implies that orogenic collapse may not be a hallmark of all hot thickened orogens

mechanisms of argon retention in clays revealed by laser 40ar 39ar dating a method for dating clays is important for studies of weathering diagenesis hydrocarbon migration and the formation of major metalliferous deposits however many attempts have produced imprecise or inaccurate results data from shales show that contrary to expectations the 40ar 39ar dating technique can be successfully used to determine the diagenetic age of ancient sediments because 39ar losses during irradiation are controlled by release from low retentivity sites in illite equivalent to those that have lost radiogenic 40ar in nature rather than by direct recoil as is generally assumed

k ar dating of time of gas emplacement in rotliegendes sandstone netherlands the time of emplacement of gas into each of two reservoirs in the rotliegendes sandstone one in the groningen field and one in the broad fourteens basin was estimated from potassium argon k ar ages of diagenetic illitic clays and careful petrographic observations of the rocks in which the clays occurred in each reservoir the intensity of diagenetic alteration increased downward within a relatively thin 15 to 175 m or 50 to 580 ft interval apparently reflecting a longer period of alteration for deeper samples this is expected in cases where a gradually accumulating gas column expels pore water from progressively deeper parts of the reservoir rock illite is a common diagenetic phase in the rotliegendes because detrital contamination in these rocks has been infer ed to be minimal k ar ages of very fine grained illite should indicate the time the illite forming process ceases in each case reported here the age of the deeper sample is younger than that of the shallower we interpret the age of the finest illite fraction in the gas zone of each reservoir to indicate the time at which illite formation ceased as the result of gas emplacement and pore fluid displacement in the rock our results indicate that gas emplacement in the groningen field commenced prior to 150 ma and that the growth of diagenetic illite continued in the present day water zone from that time until at least 120 ma

age of metamorphism in the lesser himalaya and the main central thrust zone garhwal india results of illite crystallinity 40 ar 39 ar fusion and k ar studies

comparative 40ar 39ar and k ar dating of illite type clay minerals a tentative explanation for age identities and differences abstract the 40 k 40 ar k ar and 40 ar 39 ar dating methods are applied here to the same very small micrometric illite type particles that crystallized under low temperature 5 9 ma the same size fractions dated by the 40 ar 39 ar method give total gas ages ranging from 173 3 2 0 to 228 8 1 6 ma nearly all 40 ar 39 ar total gas ages are slightly older which cannot be explained by the recoil effect only the impact of which being amplified by the inhomogeneous shape of the clay minerals and their crystallographic characteristics with varied crystallinity indices and a particle width about 10 times large than thickness the 40 ar 39 ar data outline some advantages such as the plateaus obtained by incremental step heating of the various size fractions even if not translatable straight as ages of the illite populations they allow identification of two generations of authigenic illite that formed at about 200 and 175 ma and one detrital generation however 40 ar 39 ar dating of clay minerals remains challenging as technical factors such as the non standardized encapsulation may have potential unexpected effects both dating methods have their limitations 1 k ar dating requires relatively large samples ca 10 20 mg incurring potential sample homogeneity problems with two aliquots required for k and ar analysis for an age determination also inducing a higher analytical uncertainty 2 an identified drawback of 40 ar 39 ar dating is ar recoil and therefore potential loss that occurs during neutronic creation of 39 ar from 39 k mostly in the finer mineral particles if all the recoiled 39 ar is redistributed into adjacent grains minerals the final 40 ar 39 ar age of the analyzed size fraction remains theoretically identical but it is not systematic in clay type material the finest grain sizes e g 39 ar recoil and are therefore only datable by the 40 ar 39 ar method using an encapsulation technique that still needs to be technically evaluated in this study 39 ar recoil during irradiation was quantified by encapsulation it ranged from about 20 of the total 39 ar for the 39 ar recoil confirms that grain size increasing minimizes the recoil effect but how it proceeds has still to be explained despite recent developments in 40 ar 39 ar dating the conventional k ar method is still a valuable tool for clay dating due to a convenient and straightforward use supported by a standardized and well controlled technical approach the present comparison of the two ar dating methods as applied to clay material shows that neither method is presently outdated and that they are even of reciprocal use both methods have distinct application fields in clay geochronology and complementary application fields in clay crystallography

40ar39ar dating of very fine grained samples an encapsulated vial procedure to overcome the problem of 39ar recoil loss 40ar39ar dating of very fine grained materials is compromised by the loss of 39ar by recoil and also sometimes other ar isotopes during irradiation an encapsulated sample 40ar39ar procedure is described which overcomes this problem and which produces a k ar equivalent date measurements indicate that the procedure yields reliable and reproducible results it offers the potential for dating small amounts of material of many types for example very fine grained authigenic sheet silicates from rock cores which may suffer ar loss during irradiation

sphalerite 40ar 39ar progressive crushing and stepwise heating techniques abstract it is difficult to determine the mineralization ages of sulfide deposits because no suitable minerals can be selected for the traditional isotopic dating methods the 40ar 39ar method rarely applies well to sulfide minerals because of their trace potassium content and strong radioactivity after irradiation this preliminary study investigates the possibility of dating the hydrothermal sphalerites by 40ar 39ar using crushing and heating techniques the 40ar 39ar crushing experiments of two sphalerite samples from the fankou lead zinc deposit yield well defined and concordant isochron ages of 265 8 1 0 1 ma and 269 7 6 2 ma corresponding to initial 40ar 36ar ratios of 290 7 2 4 and 294 6 8 1 respectively the crushed powders of the later sphalerite yields with stepwise heating a flat age spectrum with 40ar 39ar plateau age of 267 2 2 5 ma which define a good isochron with an age of 271 3 5 4 ma and initial 40ar 36ar ratio of 294 0 1 7 this is the first report to distinguish secondary and primary fluid inclusions within sphalerite to obtain their ages using the crushing technique and to obtain good concordant ages both of fluid inclusions by crushing and heating of k containing minerals we expect progressive crushing and stepwise heating techniques will become an effective approach to 40ar 39ar dating of the mineralization ages of hydrothermal sulfide deposits

laser 40ar39ar dating of microgram size illite samples and implications for thin section dating laser 40ar 39ar ages were obtained from a series of replicate illite samples ranging in mass from 14 to 77 pg using vacuum encapsulated irradiation and the results were compared with earlier analyses of the same material doug et al 1995 this test confirmed that measured argon ages in particular argon retention ages are independent of sample size since the argon retention age does not require the 39ar recoil gas fraction it is clear that retention ages can be measured on illite within thin sections laser spot ages were obtained from thin sections of the same welsh basin shale analyzed as illite separates early results on untreated thin sections yielded a wide range of ages some much younger than those measured previously however apparent age was negatively correlated with measured clk ratios suggesting the presence of a young water soluble component which had been removed during mineral separation prolonged water rinsing of the thin sections reduced or virtually eliminated the young phase yielding age results consistent with earlier work on illite separates this indicates that dating of clays in thin section has the potential of being a powerful tool in unraveling complex histories of diagenesis and metamorphism

direct radiometric dating of hydrocarbon deposits using rhenium osmium isotopes rhenium osmium re os data from migrated hydrocarbons establish the timing of petroleum emplacement for the giant oil sand deposits of alberta canada at 112 5 3 million years ago this date does not support models that invoke oil generation and migration for these deposits in the late cretaceous most re os data from a variety of deposits within the giant hydrocarbon system show similar characteristics supporting the notion of a single source for these hydrocarbons the re os data disqualify cretaceous rocks as the primary hydrocarbon source but suggest an origin from older source rocks this approach should be applicable to dating oil deposits worldwide

high precision 40ar 39ar age of the gas emplacement into the songliao basin the problem of determining an exact isotopic age of hydrocarbon emplacement is complex because minerals suitable for dating with common isotopic methods are often lacking in the sedimentary domain however the igneous quartz from the cretaceous volcanic rocks that host the gas reservoir in the songliao basin northeastern china contains abundant secondary fluid inclusions with high concentrations of k and high partial pressures of methane trapped during gas emplacement quartz with abundant k rich fluid inclusions provides an excellent closed system well suited for 40 ar 39 ar dating three igneous quartz samples were measured by stepwise crushing to release the inclusion based argon gas all three samples yielded well defined isochrons with ages in close agreement precisely constraining the gas emplacement at 42 4 0 5 ma 2 below the daqing oil field in the songliao basin extending possible gas reservoirs from the upper cretaceous to the middle eocene

evaluation of bitumen as a 187re 187os geochronometer for hydrocarbon maturation and migration a test case from the polaris mvt deposit canada abstract re os analyses of bitumen from the polaris mississippi valley type zn pb deposit demonstrate the potential utility of the re os system for establishing the absolute timing of hydrocarbon migration and generation the polaris deposit is hosted by carbonate rocks of the upper ordovician thumb mountain formation and sulphide mineralization during late devonian time is constrained by a rb sr sphalerite date of 366 15 ma and a paleomagnetic date bitumen occurs in open space fillings and coatings and is interpreted to have formed during the main and late stages of sulphide mineralization samples of bitumen were taken from a variety of locations from the main deposit at polaris re and os contents are 3 78 ppb and 55 1155 ppt respectively the 187re 188os ratios are very high ranging from 300 to 1800 and the os isotopic composition is moderately to highly radiogenic with 187os 188os ratios ranging from 3 3 to 13 7 calculated 187os 188os ratios at 366 ma are variable but an obvious grouping is defined by 11 of 16 analyses on a re os isochron diagram and these data yield a re os date of 374 9 ma mswd 12 model 3 in agreement with rb sr sphalerite and paleomagnetic dates for mineralization at polaris the combined dates suggest that the migration of fluids responsible for both pb zn mineralization and bitumen formation at polaris was broadly contemporaneous and was coeval with the late devonian ellesmerian orogeny though considerable further research is required to understand the re os isotopic systematics of migrated hydrocarbon systems bitumen re os geochronology may provide the possibility of establishing a more comprehensive understanding of mvt deposits sedimentary basin and petroleum systems

radioisotope dynamics the origin and fate of nuclides in groundwater the use of radionuclides as clocks for groundwater dating and as probes to investigate the geometry and spatial extent of the contact area between rocks and water is reviewed subsurface production rates for 222 rn 37 ar 55 kr 39 ar 36 cl 3 he 4 he and 40 ar in various rock types are listed measured rn fluxes from the surface of sandstone grains and from pieces of granite point to scale dependent diffusion coefficients the temporal evolution of subsurface produced 222 rn 37 ar 85 kr and 39 ar activities in groundwaters yields radionuclide escape factors between 0 1 and 9 for the stripa granite sweden and between 1 and 4 for the milk river sandstone canada the combination of 3 h 85 kr 39 ar 14 c 36 cl 4 he in the uk triassic sandstone aquifer allows groundwater dating up to 40 000 a very old groundwaters can be studied using cl 36 cl and 4 he evolution as demonstrated in the milk river aquifer in canada

illite k ar dating and crystal growth processes in diagenetic environments a critical review k ar dating of illitic minerals is commonly used in studies of diagenetic series applied to oil prospecting in spite of a great number of specialized papers some problems remain unresolved these are mostly due to a misunderstanding of the argon accumulation process during illitization criteria for identifying detrital authigenic mineral mixtures crystal ripening fast precipitation or continuous nucleation growth processes are discussed using k ar data available in the literature using different parameters such as age agek ar agestrati cryst diagenetic agek ar agestrati or frac agek arfraction agek arfinest it is shown that the k ar age significance depends on the illite nucleation growth processes a diagenetic age is obtained when these processes are rapid the k2o accumulation period is shorter than 2 if lower than this value the k ar ratio depends on the proportions of new and old particles respectively which are controlled by the relative rates of nucleation crystal growth and ripening

generation and evolution of hydrothermal fluids at yellowstone insights from the heart lake geyser basin 1 we sampled fumaroles and hot springs from the heart lake geyser basin hlgb measured water and gas discharge and estimated heat and mass flux from this geothermal area in 2009 the combined data set reveals that diverse fluids share an origin by mixing of deep solute rich parent water with dilute heated meteoric water accompanied by subsequent boiling a variety of chemical and isotopic geothermometers are consistent with a parent water that equilibrates with rocks at 205 c 10 c and then undergoes 21 2 adiabatic boiling measured diffuse co2 flux and fumarole compositions are consistent with an initial dissolved co2 concentration of 21 7 mmol upon arrival at the caldera boundary and prior to southeast flow boiling and discharge along the witch creek drainage the calculated advective flow from the basin is 78 16 l s 1of parent thermal water corresponding to 68 14 mw or 1 of the estimated thermal flux from yellowstone helium and carbon isotopes reveal minor addition of locally derived crustal biogenic and meteoric gases as this fluid boils and degasses reducing the he isotope ratio rc ra from 2 91 to 1 09 the hlgb is one of the few thermal areas at yellowstone that approaches a closed system where a series of progressively boiled waters can be sampled along with related steam and noncondensable gas at other yellowstone locations steam and gas are found without associated neutral cl waters e g hot spring basin or cl rich waters emerge without significant associated steam and gas upper geyser basin

pb pb dating of hydrocarbon migration into a bitumen bearing ore deposit north wales previous attempts at u pb dating of uraniferous bitumens have had limited significance because of radioelement migration pb pb dating which can be undertaken regardless of recent lead migration has been successfully applied to uraniferous solidified bitumen from the ty gwyn copper deposit north wales analyses of five bitumen samples with variable mixtures of radiogenic and common lead yield a sup 207 pb sup 206 pb age of 248 plus minus 21 ma early triassic this age is interpreted as the date of hydrocarbon migration into the deposit and is reasonably consistent with the timing of hydrocarbon generation calculated from the regional burial history the pb pb dating method could be applied to date uraniferous bitumens representing hydrocarbon migration in diverse geologic environments

evaluation and models of cenozoic sedimentation in the south china sea abstract the south china sea is a marginal basin with the most complex geological structures in the west pacific region and is characterized by the extensive distribution large thickness diversity and complicated evolution of the cenozoic sediments the author believes that studies of the sedimentary features and evolution of the cenozoic are fundamental to understanding the cenozoic evolution of the south china sea and that the definition of sedimentation models will be helpful in inferring the basin development at early stages thus defining the basin patterns and guiding hydrocarbon exploration this article based on data obtained from hydrocarbon exploration in the south china sea together with related references analyses the characteristics of cenozoic sediments in the south china sea and infers their evolution and models of formation the cenozoic in the south china sea is divided into eight sedimentary provinces a brief introduction is given to changes in sedimentary environments of the south china sea during the eocene late oligocene early miocene middle late miocene and pliocene quaternary the line linking taiwan and natuna as this article points out was the sea land boundary of the south china sea in the early cenozoic the transgression cycle is dominant west of this line while regression prevails to the east during the cenozoic five different patterns of sedimentation existed in the south china sea offshore lake shallow sea deep sea transgression regression and platform the former two belong to the lower cycle whilst the latter three to the upper cycle the time and spatial variations in the sedimentary sequence of these patterns is also discussed

a crustal structure profile across the northern continental margin of the south china sea an ocean bottom seismometer obs geophysical experiment was carried out across the northern continental margin of the south china sea two dimension kinematic ray tracing was used to construct synthetic records and to define the geological model along the obs profile according to the calculation and interpretation of refracted p wave phases the crustal thickness decreases from 22 km obs 1 in littoral area to 8 km obs 15 in deep sea the lower slope structured by volcanism changes sharply in moho depth from 22 to 9 km the volcanoes over the lower slope were markers of ocean continental transition zone a high velocity layer hvl with an average thickness of 4 km was found in the base of the lower crust over the shelf and slope the south china sea was originated by the passive rifting of south china continent and seafloor spreading however geological and geophysical data collected in the northern margin of the south china sea show a small amount of magma intruded into or erupted onto the crust during pull apart and breakup further analysis shows that the hvz in the base of the thinned continental crust was related to magmatism that occurred after the cessation of seafloor spreading of the south china sea magmatism during the crustal extension and breakup is very weak therefore the northern continental margin of the scs is classified as non volcanic passive margin the southern margin of the south china sea has a crust with a steady thickness of 18 24 km which is different from the nor them margin of the south china sea with a gradually thinned crust they constitute an asymmetrically conjugated pair of passive margins c 2001 elsevier science b v all rights reserved

40ar 39ar dating of oil generation and migration at complex continental margins oil bearing fluid inclusions trapped within authigenic k feldspar in late jurassic reservoirs throughout the uk atlantic margin are evidence for an early oil migration event from late jurassic source rocks in the faeroe shetland basin previous compound specific isotope studies confirm the hydrocarbon provenance temperature composition time data show that the late jurassic source rocks were oil mature 113 3 5 ma 2 this new 40ar 39ar authigenic k feldspar age shows that the late jurassic source rocks were oil mature 30 40 m y prior to current estimates

coupled variations in helium isotopes and fluid chemistry shoshone geyser basin yellowstone national park abstract early studies of 3he 4he variations in geothermal systems have generally attributed these fluctuations to either differences in the source of the magmatic 3he rich helium or to local differences in the deep flux of magmatic 3he rich helium kennedy et al 1987 however show that near surface processes such as boiling and dilution may also drastically affect 3 he 4 he ratios of geothermal vapors helium isotope ratios were determined for several hot springs at shoshone geyser basin of yellowstone national park for this study along with other noble gas data stable isotope data and water and gas chemistry data for each spring were also compiled the water chemistry indicates that there is one deep hot thermal water in the area which is mixing with dilute meteoric water that has entered the system at depth spring hco3 concentrations correlate with 3 he 4 he values as in nearby lower geyser basin this correlation is attributed to variable amounts of deep dilution of thermal waters with a relatively cool water that inhibits boiling at depth thus preventing the loss of co2 and therefore hco3 and magmatic he in the most diluted samples oxygen and hydrogen isotope data also support a boiling and dilution model but to produce the observed fractionations the boiling event would have to be extensive with steam loss at the surface whereas the boiling that affected the helium isotope ratios was probably a small scale event with steam loss at depth it is possible that deep boiling occurred in the basin and that small amounts of steam escaped along fractures at about 500 m below the surface while all subsequently produced steam was lost near or at the surface

cooling rates and the depth of detachment faulting at oceanic core complexes evidence from zircon pb u and u th he ages oceanic detachment faulting represents a distinct mode of seafloor spreading at slow spreading mid ocean ridges but many questions persist about the thermal evolution and depth of faulting we present new pb u and u th he zircon ages and combine them with magnetic anomaly ages to define the cooling histories of gabbroic crust exposed by oceanic detachment faults at three sites along the mid atlantic ridge ocean drilling program odp holes 1270d and 1275d near the 15 20 n transform and atlantis massif at 30 n closure temperatures for the pb u 800 c 850 c and u th he 210 c isotopic systems in zircon bracket acquisition of magnetic remanence collectively providing a temperature time history during faulting results indicate cooling to 200 c in 0 3 0 5 myr after zircon crystallization recording time averaged cooling rates of 1000 c 2000 c myr assuming the footwalls were denuded along single continuous faults differences in pb u and u th he zircon ages together with independently determined slip rates allow the distance between the 850 c and 200 c isotherms along the fault plane to be estimated calculated distances are 8 4 4 2 km and 5 0 2 1 km from holes 1275d and 1270d and 8 4 1 4 km at atlantis massif estimating an initial subsurface fault dip of 50 and a depth of 1 5 km to the 200 c isotherm leads to the prediction that the 850 c isotherm lies 5 7 km below seafloor at the time of faulting these depth estimates for active fault systems are consistent with depths of microseismicity observed beneath the hypothesized detachment fault at the tag hydrothermal field and high temperature fault rocks recovered from many oceanic detachment faults

thermal 40ar 39ar separation of diagenetic from detrital illitic clays in gulf coast shales the 40ar 39ar age spectra for illite rich mixed layered illite smectite i s from texas gulf coast bentonites contain only a diagenetic component and are characteristically staircase shaped this spectrum shape contrasts with patterns seen for anchizonal and epizonal grade bentonites that are flatter this may be due to the structural and compositional heterogeneity mixed layering typical of low grade clays especially with respect to k on the other hand shales from depths exceeding those of the smectite to illite transformation have both detrital and diagenetic components although diagenetic illite rich i s dominates shale fine fraction detrital mica is observed even in the 0 2 m size fraction age spectra of shales consist of two portions 1 a staircase shaped low temperature section corresponding to degassing of the diagenetic illite rich i s component and 2 a peak in the high temperature section corresponding to degassing of well crystallized detrital mica the data obtained from 40ar 39ar stepwise analysis of these mixtures of diagenetic and detrital components permit at least partial resolution of the diagenetic from the detrital ages we have developed techniques for quantifying the separate diagenetic and detrital components based on subtraction of the detrital contribution from the age spectrum of mixture giving rise to an age corrected for the detrital contamination the resulting corrected 40ar 39ar ages are closer to the diagenetic ages this is in sharp contrast to the old k ar ages of the same samples although still in the developmental state these techniques appear to represent a solution to the problem of contamination of diagenetic illitic clays by detrital illitic phyllosilicates

xrd based 40ar 39ar age correction for fine grained illite with application to folded carbonates in the monterrey salient northern mexico abstract due to their minute size 40ar 39ar analysis of illite faces significant analytical challenges including mineral characterization and especially effects of grain size and crystallography on 39ar recoil quantifying the effects of 39ar recoil requires the use of sample vacuum encapsulation during irradiation which permits the measurement of the fraction of recoiled 39ar as well as the 39ar and 40ar retained within illite crystals that are released during step heating total gas ages tga are calculated by using both recoiled and retained argon which is functionally equivalent to k ar ages while retention ages ra only involve retained ar in the crystal natural applications have shown that tga fits stratigraphic constraints of geological processes when the average illite crystallite thickness ict is smaller than 10 nm and that ra matches these constraints for icts larger than 50 nm we propose a new age correction method that takes into account the average ict and corresponding recoiled 39ar for a sample with x ray corrected ages xca lying between total gas and retention ages depending on ict this correction is particularly useful in samples containing authigenic illite formed in the anchizone with typical ict values between 10 and 50 nm in three samples containing authigenic illite from cretaceous carbonates in the monterrey salient in northern mexico there is a range in tgas among the different size fractions of 46 49 36 43 and 40 52 ma while ras range from 54 64 47 52 and 53 54 ma respectively xca calculations produce tighter age ranges for these samples of 52 5 56 45 5 48 5 and 49 52 5 ma respectively in an apparent age vs ict or 2m 1illite plot authigenic illite grains show a slope that is in general slightly positive for tga slightly negative for ra but close to zero for xca with thinner crystallites showing more dispersion than thicker ones in order to test if dispersion is due to a different formation history or the result of retention capability degassing spectra were modeled for site xca averages and overall xca average modeling shows that local site age average best match the measured spectra instead of a global average age indicating that illite growth reflects local deformation and is not the result of regional metamorphism modeling also shows that ar degassing spectra are very sensitive to grain size such that age interpretation based on ar plateaus is meaningless for most fine grained clays

noble gas radionuclides in yellowstone geothermal gas emissions a reconnaissance abstract a reconnaissance investigation of noble gas radionuclides 39 ar 81 kr and 85 kr in gas emissions from several geothermal features at yellowstone national park was performed to explore tracer applications of these nuclides in an active hydrothermal system prior studies of the yellowstone system using stable noble gas isotopes show that the thermal fluids contain a mixture of atmospheric mantle and crustal components noble gas radionuclide measurements provide complementary chronometric information regarding subsurface residence times of thermal fluids from decay of 81 kr and 85 kr as well as in situ production and release of nucleogenic 39 ar and radiogenic 40 ar in the reservoir rock argon 39 isotopic abundances in air corrected samples exceed those of atmospheric ar by 705 to 1217 indicating substantial ar contribution to thermal fluids by fluid rock interaction upper limits on deep thermal fluid mean residence times estimated from 39 ar 40 ar ratios are about 100 kyr for features in the gibbon and norris geyser basin areas and is about 30 kyr in lower geyser basin with the key assumption that the fluid acquires its crustal component of ar in quaternary volcanic rock of the yellowstone caldera input of crustal ar from older aquifer rocks would reduce these apparent mean residence times 81 kr isotopic abundances in the gas samples yield upper limits on residence time that are consistent with those obtained from 39 ar 40 ar ratios

dating petroleum emplacement by illite 40ar 39ar laser stepwise heating the timing of petroleum emplacement is crucial in understanding petroleum forming processes and predicting new prospecting regions this study investigates the possibility of determining the age of petroleum emplacement by an illite 40ar 39ar laser stepwise heating technique a specially designed apparatus has been used to clean up the organic gases in samples contaminated with residues from oil or gas fields resulting in credible 40ar 39ar data the sandstone samples are from the zhuhai and zhujiang formations in the huizhou sag in the pearl river mouth basin south china sea illite 40ar 39ar laser stepwise heating consistently yields gradually rising age spectra at the low temperature steps until reaching age plateaus at high temperature steps the first step apparent ages of the six samples from the zhujiang formation give a weighted mean age of 11 4 0 5 ma 1 in agreement with an age of 12 1 1 1 ma in the first three steps of the sample from the zhuhai formation these dates are interpreted as being caused by the formation of fine authigenic illite and therefore indicative of the maximum age of petroleum emplacement in the huizhou sag because illite is commonly one of the latest forming minerals prior to hydrocarbon accumulation the plateau ages ranging from 58 to 276 ma in high laser output steps are interpreted as being caused by variable contributions from detrital k feldspar in the sandstones the older total ages based on the illite 40ar 39ar stepwise heating results are equal to the k ar ages and are not usable in delineating petroleum emplacement because they represent mixtures with variable contributions from authigenic illite and detrital k feldspar the 40ar 39ar stepwise heating technique thus can be applied to resolve important petroleum emplacement age information from illite samples that otherwise cannot be provided by traditional k ar methods

dating petroleum emplacement by illite 40ar 39ar laser stepwise heating discussion isotopic dating of diagenetic processes in relation to hydrocarbon charge in buried sedimentary sequences is of importance to the oil industry yun et al recently published in the aapg bulletin v 94 no 6 p 759 771 on that topic a study entitled dating petroleum emplacement by illite 40ar 39ar laser stepwise heating technique this study raises questions about basic aspects of 40ar 39ar dating fine grained authigenic clay minerals describes flawed methodology and ignores important previous work as isotopic dating of diagenetic processes may interest readers of the aapg bulletin that are not necessarily specialists in either isotopic geochronology or clay mineralogy it appeared necessary to comment on both the technical aspects and the scientific interpretations of this study we discuss especially the fact that the authors discarded 39ar recoil followed by an evaluation of its impact on the results and the interpretations

introduction to the special issue on frontiers in gas geochemistry abstract the study of the geochemistry of gases pervades the earth and environmental sciences this is due in no small measure to the well established thermodynamic properties of gases which allow their application to a variety of processes occurring over a wide spectrum of natural conditions in this respect both major and associated minor gases have been proven useful indeed the trace gases have been particularly important given their role as sensitive geochemical tracers examples where gas geochemistry places key constraints on geochemical processes include the degassing history of the solid earth to form the atmosphere and oceans the origin and migration characteristics of hydrocarbon deposits the scale of climate variability the p t characteristics of geothermal reservoirs and the dynamics of the earthquake cycle and volcanic activity to name but a few this volume continues this rich tradition with an eclectic selection of papers aimed at exploring and exploiting gas geochemistry over a myriad set of research themes

paleozoic ages and excess 40ar in garnets from the bixiling eclogite in dabieshan china new insights from 40ar 39ar dating by stepwise crushing abstract the 40 ar 39 ar stepwise crushing technique is applied for the first time to date garnet from ultra high pressure metamorphic uhpm eclogites three garnet samples from the bixiling eclogites analyzed by 40 ar 39 ar stepwise crushing yield regular predictable age spectra and a clear separation between excess 40 ar and concordant plateau and isochron ages all three age spectra begin with high apparent ages followed by step by step decreasing ages and finally age plateaux with apparent ages in the range from 427 20 to 444 10 ma the data points constituting the age plateaux yield excellent isochrons with radiogenic intercept ages ranging from 448 34 to 459 58 ma corresponding to initial 40 ar 36 ar ratios from 292 1 4 5 to 294 5 6 7 statistically indistinguishable from the modern air the high initial ages are interpreted to derive from secondary fluid inclusions containing excess 40 ar whereas the plateau ages are attributed to gas from small primary fluid inclusions without significant excess 40 ar the plateau ages are interpreted to approximate the time of garnet growth during initial uhpm metamorphism phengite analyzed by laser stepwise heating yielded a complicated two saddle age spectrum with a scattered isochron corresponding to age of 463 116 ma and initial 40 ar 36 ar ratio of 1843 1740 indicative of the presence of extraneous 40 ar within phengite these concordant isochron ages measured on minerals diagnostic of eclogite grade metamorphism strongly suggest that dabie uhpm eclogites were first formed in the early paleozoic during the same event that caused the qinling northern qaidam basin altyn tagh eclogites