hazard investigation of the portillo rock avalanche site central andes chile using an integrated field mapping and numerical modelling approach abstract this paper reports a detailed rock slope hazard investigation of the portillo rock avalanche site located in the rugged mountains of the andean cordillera of central chile the site is important as it lies along the international santiago mendoza highway corridor connecting chile and argentina and a major ski resort is located on its deposits a number of large lobate shaped diamicton deposits were mapped and dated by cosmogenic nuclides 36 cl with the results showing that they correspond to two significant prehistoric rockslide events an integrated field mapping and numerical modelling investigation was subsequently carried out to assess the threat posed to the area by further rocksliding activity distinct element modelling was used to back analyze the failure mechanism and identify the geological model that best reproduced the portillo rock avalanche failure surface results show that a stress controlled failure at the toe of the slope followed by sliding along volcaniclastic bedding was the likely failure mechanism a 3 d dynamic runout analysis was carried out to back analyze which combinations of rheologies material properties and rockslide sequencing were best able to reproduce the current distribution of rockslide deposits results indicate that two separate sliding events originating from different sources had occurred with each involving different combinations of frictional and voellmy rheologies depending on the level of entrainment that occurred along each travel path insights gained from the back analysis were then used to carry out a forward analysis to assess the potential for a recurring major rockslide under several different triggering scenarios results suggest that a low probability m 7 8 earthquake would be required to trigger another rockslide from the original source area the rock slope was otherwise found to be stable even following high precipitation events nevertheless runout simulations for the estimated large magnitude earthquake triggered rockslide volume showed that for both a highly frictional and non saturated path i e dry season and a snow covered path winter the leading edge of the flow would override part of the international santiago mendoza corridor with the debris coming to rest in a flat lying area in the upper part of the valley overall the results from this integrated hazard assessment suggest that the hazard level is low

developments in the characterization of complex rock slope deformation and failure using numerical modelling techniques recent advances in the characterization of complex rock slope deformation and failure using numerical techniques have demonstrated significant potential for furthering our understanding of both the mechanisms processes involved and the associated risk this paper illustrates how rock slope analyses may be undertaken using three levels of sophistication level i analyses include the conventional application of kinematic and limit equilibrium techniques with modifications to include probabilistic techniques coupling of groundwater simulations and simplistic treatment of intact fracture and plastic yield such analyses are primarily suited to simple translational failures involving release on smooth basal rear and lateral surfaces where the principle active damage mechanisms are progressive failure and or asperity breakdown level ii analyses involve the use of continuum and discontinuum numerical methods in addition to simple translation level ii techniques can be applied to complex translational rock slope deformations where step path failure necessitates degradation and failure of intact rock bridges along basal rear and lateral release surfaces active damage processes in this case comprise not only strength degradation along the release surface e g asperity breakdown but also a significant component of brittle intact rock fracture level iii analyses involve the use of hybrid continuum discontinuum codes with fracture simulation capabilities these codes are applicable to a wide spectrum of rock slope failure modes but are particularly well suited to complex translation rotational instabilities where failure requires internal yielding brittle fracturing and shearing in addition to strength degradation along release surfaces through a series of rock slope analyses the application of varied numerical methods are discussed particular emphasis is given to state of the art developments and potential use of level iii hybrid techniques d 2005 elsevier b v all rights reserved

numerical analysis of initiation and progressive failure in natural rock slopes the 1991 randa rockslide the 1991 randa rockslide in the swiss alps involved several complex mechanisms relating to geological mechanical and hydrological processes for which no clear trigger can be asserted this paper investigates the concept of progressive failure and the numerical modelling of rock mass strength degradation in natural rock slopes using the randa rockslide as a working example results from continuum i e finite element modelling are presented to illustrate a hypothesis suggesting that initiation of a progressive rock mass degradation process ultimately leading to failure began following deglaciation of the valley below discontinuum distinct element modelling is then applied to investigate the underlying mechanisms contributing to the episodic nature of the rockslide finally the use of a hybrid method that combines both continuum and discontinuum techniques to model fracture propagation are discussed in the context of modelling progressive slide surface development linking initiation and degradation to eventual catastrophic failure

a model for the analysis of rapid landslide motion across three dimensional terrain a new numerical model for the dynamic analysis of rapid flow slides debris flows and avalanches has been developed the model is an extension of an earlier algorithm and is implemented using a nu

mechanics of the earthquake induced hongshiyan landslide in the 2014 mw 6 2 ludian earthquake yunnan china abstract the hongshiyan landslide the largest landslide volume 12 24 mm3 triggered by 2014 mw 6 2 ludian earthquake blocked the niulan river forming a rockslide dam the failure potential of the dam created an elevated risk to population and infrastructure downstream we provide insight into the failure mechanics of the hongshiyan landslide using data obtained by means of remote sensing techniques and traditional in situ surveys geological data obtained by these methods was then used for kinematic and numerical analyses our study shows that failure of the hongshiyan landslide involved a high rock slope with an average slope of 55 and aspect of 185 ssw in which the geological setting consists of an upper strong limestone slab and an underlying weak silty mudstone the geological setting allowed the landslide to develop as follows i the silty mudstone due to its poor mechanical properties was subject to ductile deformation under the compressive loading from the overlying strong limestone slabs ii a large number of release fractures in the limestone developed as a consequence of pre failure progressive deformation in the underlying weak mudstone iii the oxidation and solution along steep joint walls in the upper limestone indicates that the joints are pathways that allow water ingress into the mudstone further promoting the degradation of shear strength in this layer distinct element modelling udec was used to back analyse the failure mechanism and develop the geological model that best reproduced the hongshiyan failure geometry results show that the failure surface consisted of two elements the upper steeply dipping release surface formed along the slope parallel steep cross joints defining a jointed strong limestone slab and the toe of the failure formed a curved failure surface through the ductile weak silty mudstone underlying the limestone slab independent of any discontinuities within it the pre failure observations and numerical analysis suggest that the hongshiyan slope was in a state of marginal static stability prior to the earthquake as a result of pre failure progressive deformation controlled by the shear strength of the underlying mudstone overall the marginal pre earthquake stability of the hongshiyan slope coupled with the strongest seismic loading which the hongshiyan slope has been historically subjected to are the reasons why this moderate earthquake induced a large scale catastrophic rock slope failure

the use of ballistic trajectory and granular flow models in predicting rockfall propagation the term rockfall is often used ambiguously to describe various mass movement processes here we propose more precise terminology based on the physical nature of the moving mass differentiating between two distinct types of rockfall fragmental rockfall and rock mass fall for both rockfall types the current knowledge of the mechanisms controlling propagation of the mass movement are described showing how these mechanisms can be simulated with different modelling approaches however we point out that almost no development has been realized concerning dynamic behaviour of the transitional processes between these two end member rockfall types some simplified means of dealing with these complications are suggested but we emphasize that a considerable amount of fundamental methodological development remains necessary

lower tertiary and upper cretaceous source rocks in louisiana and mississippi implications to gulf of mexico crude oil the lower tertiary sparta formation wilcox group and the midway group in southern louisiana include organic rich source facies that generate crude oil at relatively high thermal maturities the wilcox group is an important source of wilcox crude oil and regional kerogen variations explain two crude oil subfamilies wilcox crude oils in downdip areas of southern louisiana migrated short distances but long range lateral migration about 150 km best explains wilcox crude oils far updip from mature source rocks crude oils in oligocene and younger reservoirs in southern louisiana migrated vertically from deep lower tertiary source rocks some crude oils in upper cretaceous tuscaloosa reservoirs were emplaced by long range lateral migration from tuscaloosa source rocks g ven little evidence of upper tertiary source rocks and the overmaturity problems of mesozoic source rocks most crude oils in upper tertiary and pleistocene reservoirs of the gulf continental shelf are best explained by vertical migration from deep lower tertiary source rocks even so it is simplistic to assume an exclusive lower tertiary origin many tertiary and pleistocene crude oils of this study probably include an overprint of high maturity hydrocarbons from mesozoic sources

geochemical and carbon isotopic studies of crude oil destruction bitumen precipitation and sulfate reduction in the deep smackover formation abstract crude oil generated by the lower smackover source facies migrated to upper smackover reservoirs where slow thermal cracking of crude oil resulted in the formation of gas condensate and late solid bitumen ultimately only pyrobitumen methane and nonhydrocarbon gases including hydrogen sulfide persist in the deepest smackover reservoirs the carbon isotopic compositions of crude oils became heavier during crude oil destruction the carbon isotopic compositions of asphaltenes nso compounds and saturated hydrocarbons in late solid bitumen and the lower smackover source facies became isotopically lighter during crude oil destruction it is suggested that some isotopically light components from crude oils were incorporated in late solid bitumen by reactions involving thermochemical sulfate reduction thermochemical sulfate reduction and crude oil destruction occurred over a long span of geologic time at temperatures in the 120 150 c range

gas hydrate and crude oil from the mississippi fan foldbelt downdip gulf of mexico salt basin significance to petroleum system abstract structure ii gas hydrate methane ethane hydrate and crude oil occur on the sea floor at 1920 1930 m water depth in atwater valley at block 425 near the juncture of the lower slope of the gulf of mexico salt basin and the abyssal plain the site is in the eastern mississippi fan foldbelt mff a distinct structural province at the downdip limit of the gulf of mexico salt basin the presence of thermogenic hydrocarbons confirms an active petroleum system in the deep eastern mff the hydrate bound c 2 c 5 hydrocarbon gases of the mff are isotopically distinct when compared to other gases from the upper and middle slope being strongly depleted in 13 c the biomarkers m z 191 and 217 of oil inclusions from at 425 gas hydrate are also distinct when compared to oils of the upper gulf slope and the smackover trend biomarkers of at 425 oil are consistent with a marine source rock deposited in an area of strong siliciclastic influx with possible higher plant organic matter the presence of a shale or mudstone source rock at the downdip limit of the gulf raises new questions as to paleogeography during source rock deposition emergent highlands immediately to the south and east of the downdip limit of the gulf of mexico salt basin during the mesozoic may explain the occurrence of the shale or mudstone source rock that gave rise to the gas and oil at at 425

the origins of petroleum in the northern gulf of mexico abstract the distribution and chemistry of oils in the northern gulf of mexico and the adjacent onshore can be explained by multiple sources fades variations maturation and post generation alteration genetic families include jurassic smackover flexure trend upper cretaceous and paleogene oils smackover oils have high sulfur contents pristane phytane ratios c 35 c 34 hopane ratios 1 0 and c 30 c 29 hopane ratios usuall 1 0 flexure trend oils are similar and contain abundant extended hopanes high sulfur contents and v ni v 0 5 these oils are inferred to be sourced in upper jurassic lower cretaceous strata upper cretaceous oils contain a distinctive suite of tricyclic and nonhopanoid triterpanes oleanane is low or absent paleogene oils can be recognized by the presence of 18 h oleanane and a lack of extended hopanes these oils occur from south texas to southern and offshore louisiana two subfamilies can be recognized based on the relative abundance of triterpanes and steranes these oils have a presumed paleogene source mixing of oil types is quite prevalent at the geographic boundaries of oil types the initial control on the distribution of oil and gas in the northern gulf of mexico is provided by the areal extent of source rocks secondary control is due to cenozoic deposition which provides the thermal stress to generate as well as destroy oil salt tectonics provide conduits for migrating fluids to escape the zone of thermal destruction mesozoic source strata again become important in the deep gulf of mexico where cenozoic sediments thin

pathways of migration of oil and gas in the south mississippi salt basin abstract the south mississippi salt basin is one of three interior basins characterized by structures formed by movement of the late jurassic louann salt an analysis of pathways of migration within the basin has revealed that it is possible to explain why hydrocarbons have accumulated in some structures yet are absent from others that would appear to be favorable of the more than 840 fields within the basin 74 including the largest known accumulations have hydrocarbons stacked in reservoirs in more than one formation these stacked reservoirs result from vertical migration brought about by faulting more than 750 fields have hydrocarbons confined to one formation in traps associated with four distinct trends of production that decrease in age systematical iy from the margin of the basin into the interior the hydrocarbons in these trends have accumulated by intrastratal migration without the agency of faulting from a nearby source in the same unit as the reservoir on the northwest side of the basin migration between units brought into contact along unconformities has resulted in ten fields vertical migration brought about by faulting around shallow salt diapirs has allowed hydrocarbons to escape so that only 5 of 56 such structures have ever produced oil or gas these conclusions derived from geological deductions are supported by preliminary geochemical data but more extensive and detailed geochemical analyses of oils from the various sources are under way

illite polytype distribution as an inorganic indicator of thermal maturity in the smackover formation of the manila embayment southwest alabama abstract illite polytype distribution is compared to pyrolysis tmax values in the smackover formation in the manila embayment southwest alabama to determine its potential as an indicator of thermal maturity the percent 2m illite polytype 2m 1m 2m increases from 20 to 80 over the formation temperature interval 100 165 c and as tmax values increase from 432 to 461 c illite polytype distribution is considered a good qualitative indicator of thermal maturity in the smackover formation of the study area the correlation between the percentage of 2m illite and temperature and thermal maturity in the smackover formation is believed to be primarily the result of the diagenetic transition from the 1m to the 2m polytype depositional effects are manifested in four samples as anomalously high 2m polytype percentages this variation is interpreted to be the result of locally high concentrations of detrital 2m muscovite because original sediment composition and illite chemistry may vary significantly both locally and regionally illite polytype distribution is a useful thermal maturity indicator only within a particular basin and when calibrated with other thermal maturity indicators

tectonic geomorphology in the shanxi graben system northern china abstract the shanxi graben system is composed of a series of normal fault controlled basins which are arranged in an s shaped curve over a distance of about 1200 km active faults mainly trending nne ssw ne sw or ene wsw caused blocks to become rotated and tilted and river courses and terraces to become deformed volcanoes in the datong basin indicate that the formation of the shanxi graben system has a relationship to upper mantle activity and that the tectonic activity was intense during the middle pleistocene arrangement and movement of the faults indicates there has been right lateral shear in this area which may have resulted from the northward push of the qinghai tibet plateau

determination of source ages and migration patterns of brines from the u s gulf coast basin using 129i abstract the long lived cosmogenic and fissiogenic isotope 129i has been applied to the study of source ages and migration patterns of oil field brines from the u s gulf coast basin 129 i i ratios were measured in sixty samples of saline formation water from depths of 660 4503 m in several oil fields in southeast texas south central louisiana and offshore louisiana comparisons between measured ratios and the decay curve for surface hydrospheric 129i result in minimum source ages much older than present host formation ages indicating vertical migration of brine from older deeper sources conservative corrections for a fissiogenic component due to spontaneous fission of 238u in the subsurface result in the following median source ages iberia field 55 ma port barre field 53 ma and eugene island blocks 316 and 330 55 ma pointing to wilcox group shales as source formations mesozoic sources cannot be ruled out due to the uncertainties in estimating the fissiogenic component some of the ratios measured in texas gulf coast brines show evidence of a greater fissiogenic component which indicates that the brines have resided in formations with locally high u concentrations

framework of hydrocarbon generation and destruction in eastern smackover trend laminated lime mudstones of the lower member of the jurassic smackover formation are significant source rocks for crude oil across mississippi alabama and florida the source facies was deposited in an anoxic and perhaps hypersaline environment that preserved algal derived kerogen the distribution of kerogen along laminations of depositional origin and along stylolites of diagenetic origin resulted in efficient expulsion of crude oil with increasing thermal maturity crude oil initially emplaced in reservoirs was cracked to yield gas condensate and then methane rich in nonhydrocarbon gases such as hydrogen sulfide carbon dioxide and nitrogen early destruction of methane was driven by thermochemical sulfate reduction the thermal maturity framework of the smackover rend explains the distribution of hydrocarbon discoveries and suggests areas previously overlooked by exploration

migration of crude oil from the smackover source rock to jurassic and cretaceous reservoirs of the northern gulf rim abstract migration of distinctive crude oils from the upper jurassic smackover source rock across the northern gulf rim began at various times during the cretaceous regional source rock differences resulted in two compositional subfamilies of smackover crude oils type i crude oils in upper smackover reservoirs of east texas arkansas louisiana and mississippi reflect an origin from algal derived kerogen deposited in an anoxic environment type ii crude oils in upper smackover reservoirs of alabama and florida reflected an origin from kerogen preserved in a less anoxic environment with a terrestrial component comparison of molecular and carbon isotopic compositions of crude oils suggests that vertical migration of type i smackover crude oils charged many overlying upper jurassic lower cretaceous and upper cretaceous reservoirs some lateral migration to cretaceous reservoirs north of the updip limit of the smackover formation also occurred both type i smackover crude oils and geochemically dissimilar cretaceous sourced oils are found in upper cretaceous tuscaloosa and woodbine reservoirs because of impermeable barriers to migration especially in alabama vertical migration of type ii crude oils was less significant

the framework of hydrocarbon generation and migration gulf of mexico continental slope abstract the occurrence of large volumes of crude oil and thermogenic gas in plio pleistocene reservoirs and holocene seeps of the gulf of mexico slope argues that the process of hydrocarbon generation and migration continues at the present time calculated thermal maturity models based on deep seismic stratigraphy indicate that cretaceous and possibly early tertiary sediments are presently within the thermal maturity range for generation and expulsion of crude oil the cretaceous and early tertiary sediments are suggested as possible source rocks because younger sediments are thermally immature for crude oil generation and older sediments are thermally overmature for crude oil preservation the calculated thermal maturity profile for the slope indicates that some generation of crude oil could occur as shallow as 6 km and that liquid hydrocarbons could be preserved as deep as 9 km thermogenic gas is stable at even greater depths migration with a strong vertical component must be invoked to explain thermogenic hydrocarbons in shallow reservoirs and seeps of the slope the great depth at which liquid hydrocarbons and dry gas can be preserved in reservoirs of the gulf of mexico salt basin indicates that huge volumes of prospective section remain essentially unexplored

distribution of hydrocarbon source potential in the jurassic smackover formation abstract there are regionally consistent differences in source potential between the upper and lower members of the smackover formation measurements of total organic carbon and visual kerogen assessments indicate that the lower smackover has source potential for crude oil from texas to florida source potential is largely restricted to the laminated lime mudstone facies of the lower smackover where deposition occurred in a subtidal environment that was anoxic and probably hypersaline algal derived amorphous kerogen is concentrated in laminations of depositional origin carbonate diagenesis during burial enhanced source potential by further concentrating kerogen along stylolites we suggest that the efficiency of hydrocarbon expulsion has been enhanced by depositional and diagenetic factors that concentrate source rock kerogen along conduits for fluid movement the upper smackover serves mainly as a reservoir rock and other jurassic formations lack source potential

tectonothermal history of the basement rocks in the western zone of the north china craton and its tectonic implications the basement of the north china craton can be divided into the eastern central and western zones based on lithological structural metamorphic and geochronological data the western zone comprises two different petrotectonic units archaean tonalitic trondhjemitic granodioritic ttg grey gneisses and metamorphic mafic rocks and palaeoproterozoic khondalite series the former is characterized by isobaric cooling ibc type anticlockwise p t paths in the north northwestern part of the zone and near isothermal decompression itd type clockwise p t paths in the eastern part adjacent to the central zone on the other hand the tectonothermal evolution of palaeoproterozoic khondalite series rocks is characterized exclusively by nearly isothermal decompression following the peak of metamorphism and then cooling defining clockwise p t paths the archaean ttg gneisses and associated mafic rocks with anticlockwise metamorphic p t paths reflects an origin related to underplating and intrusion of mantle derived magmas which may be derived from mantle plumes they represent a late archaean continental block in the western part of the north china craton the palaeoproterozoic khondalite series rocks represent passive continental margin deposits they were metamorphosed and deformed in the late palaeoproterozoic during the amalgamation of the western continental block with another continental block in the east part of the north china craton the itd type clockwise p t t paths of the palaeoproterozoic khondalite series rocks record the tectonothermal histories of the collision of the western and eastern continental blocks which resulted in the final assembly of the north china craton at c 1800 ma

integrated back and forward analysis of rock slope stability and rockslide runout at afternoon creek washington hazard assessments involving large rock slopes are often problematic given the influence of geology on failure kinematics and the subsequent influence of the failure kinematics on the rockslide ru

geochemical studies of crude oil generation migration and destruction in the mississippi salt basin abstract the main source for crude oil in the mississippi salt basin is the laminated lime mudstone facies of the lower smackover crude oil generation and migration commenced at a level of thermal maturity equivalent to about 0 55 vitrinite reflectance short range lateral migration of crude oil was focused to upper smackover and norphlet reservoirs but vertical migration also charged some overlying cotton valley rodessa lower tuscaloosa and eutaw reservoirs following migration from the lower smackover thermal maturity history of reservoir rocks controls the preservation of crude oil gas condensate and methane slow thermal cracking of crude oil occurred in deep upper smackover reservoirs resulting in formation of gas condensate and precipitation of solid bitumen the maximum thermal maturity for preservation of condensate is equivalent to about 1 3 vitrinite reflectance only methane pyrobitumen and nonhydrocarbon gases including hydrogen sulfide persist at higher levels of thermal maturity early destruction of methane in deep upper smackover reservoirs near the wiggins arch is driven by thermochemical sulfate reduction lesser availability of sulfate in norphlet reservoirs in lower mobile bay could account for methane preservation at higher levels of thermal maturity one basic geochemical strategy for further exploration of the mississippi salt basin is to focus exploration effort on traps with reservoirs in the thermal maturity window for hydrocarbon preservation another strategy is to avoid drilling traps with overmature reservoir rocks

a model for the runout analysis of rapid flow slides debris flows and avalanches runout analyses are used for risk assessment and design of remedial measures against rapid landslides such as debris flows debris avalanches rockslide avalanches large scale liquefaction failure

high pressure granulites in the sanggan area north china craton metamorphic evolution p t paths and geotectonic significance high pressure basic granulites are widely distributed as enclaves and sheet like blocks in the huaian ttg gneiss terrane in the sanggan area of the central zone of the north china craton four stages of the metamorphic history have been recognised in mineral assemblages based on inclusion exsolution and reaction textures integrated with garnet zonation patterns as revealed by compositional maps and compositional profiles the p t conditions for each metamorphic stage were obtained using thermodynamically and experimentally calibrated geothermobarometers the low ca core of growth zoned garnet along with inclusion minerals defines a prograde assemblage m1 of garnet clinopyroxene plagioclase quartz yielding 700 c and 10 kbar the peak of metamorphism at about 750 870 c and 11 14 5 kbar m2 is defined by high ca domains in garnet interiors and inclusion minerals of clinopyroxene plagioclase and quartz kelyphites or coronas of orthopyroxene plagioclase magnetite around garnet porphyroblasts indicate garnet breakdown reactions m3 at conditions around 770 830 c and 8 5 10 5 kbar garnet exsolution lamellae in clinopyroxene and kelyphites of amphibole plagioclase around garnet formed during the cooling process at about 500 650 c and 5 5 8 kbar m4 these results help define a sequential p t path containing prograde near isothermal decompression itd and near isobaric cooling ibc stages the clockwise hybrid itd and ibc p t paths of the hp granulites in the sanggan area imply a model of thickening followed by extension in a collisional environment furthermore the relatively high pressures 6 14 5 kbar of the four metamorphic stages and the geometry of the p t paths suggest that the hp granulites together with their host huaian ttg gneisses represent the lower plate in a crust thickened during collision the corresponding upper plate might be the tectonically overlying khondalite series which was subjected to medium to low pressure mp lp 7 4 kbar granulite facies metamorphism with a clockwise p t path including an itd segment both the hp and the mp lp granulite facies events occurred contemporaneously at c 1 90 1 85 ga in a collisional environment created by the assembly process of the north china craton

the ratios of dibenzothiophene to phenanthrene and pristane to phytane as indicators of depositional environment and lithology of petroleum source rocks abstract the ratio of dibenzothiophene to phenanthrene and the ratio of pristane to phytane when coupled together provide a novel and convenient way to infer crude oil source rock depositional environments and lithologies such knowledge can significantly assist in identifying the source formation s in a basin thereby providing valuable guidance for further exploration the ability to infer this information from analysis of a crude oil is especially valuable as frequently the earliest samples in a new area may be shows and or drill stem test samples from exploratory wells which are characteristically drilled on structural highs stratigraphically remote from the source formation s a cross plot of dibenzothiophene phenanthrene versus the pristane phytane ratios measured on seventy five crude oils from forty one known source rocks ranging in age from ordovician to miocene consistently classified the oils into the following environment lithology groups marine carbonate marine carbonate mixed and lacustrine sulfate rich lacustrine sulfate poor marine and lacustrine shale and fluvial deltaic carbonaceous shale and coal the dibenzothiophene phenanthrene ratio alone is an excellent indicator of source rock lithology with carbonates having ratios 1 and shales having ratios the dibenzothiophene to phenanthrene and the pristane to phytane ratios can also be used to classify source rock paleodepositional environments the classification scheme is based on the premise that these ratios reflect the different eh ph regimes resulting from the significant microbiological and chemical processes occurring during deposition and early diagenesis of sediments the dibenzothiophene phenanthrene ratio assesses the availability of reduced sulfur for incorporation into organic matter and the pristane phytane ratio assesses the redox conditions within the depositional environment interpretation of these ratios has been aided by quantitative biomarker analysis and by carbon isotope data for pristane and phytane obtained by gas chromatography isotope ratio mass spectrometry

p t t paths and tectonic history of an early precambrian granulite facies terrane jining district south east inner mongolia china abstract the widespread khondalite series of south east inner mongolia consists largely of biotite sillimanite garnet gneiss and quartzo feldspathic gneiss with some marble and mafic granulite layers it has experienced two metamorphic events at c 2500 and 1900 2000 ma a pre peak stage of the first metamorphism at t 600 700 c and p 6 7 kbar is recognized by the relict amphibolite facies assemblage ky grt bt pl qtz and protected inclusions of biotite hornblende sodic plagioclase and quartz in garnet or orthopyroxene the peak stage with t c 800 50 c and p 8 10 kbar is characterized by the widespread granulite facies assemblages sil grt bt kfs pl qtz in gneiss and opx cpx pl hbl grt in granulite the p t t path suggests that the supracrustal sequence was buried in the lower crust by tectonic thickening during d1 d2 the beginning of the second metamorphism is characterized by further temperature rise to 700 c or more at lower pressure this stage is manifested by the appearance of cordierite after garnet fibrolite sil2 after biotite in gneiss and transformation of hbl1 into opx2 and cpx2 in granulite coronas of symplectitic opx2 pl2 surrounding grt1 and cpx1 in mafic granulite are interpreted as products of near isothermal decompression the p t t path may be related tectonically to waning extension of the crust by the end of the early proterozoic

some optical characteristics of solid bitumen in visual kerogen preparations solid bitumen probably derived from liquid hydrocarbon can be identified in visual kerogen strewn slides or palynology slides this kind of material may have been previously identified as amorphous or structured kerogen organic matter in most petroleum source rocks is too dispersed to be studied in situ therefore it is isolated from the mineral matrix and concentrated to make strewn slides for optical study both insoluble and soluble bitumen may be observed in visual kerogen slides when no solvent extraction is used during sample preparation examination of a series of samples prepared both as thin sections and visual kerogen strewn slides led to the observation of solid bitumen in the strewn slides when an organic petrographer sees solid bitumen particles in visual kerogen slides they may be classified as either woody herbaceous coaly or most often amorphous kerogen types thompson and dembicki 1986 describe four kinds of amorphous kerogens optically and relate them to source rock hydrocarbon generating potential a fifth kind of amorphous kerogen is documented in this paper this material appears through the microscope as thin smooth brown flakes in transmitted light with a white to grey color in reflected light some of the particles appear to have the structure of the space between mineral grains where they form at this time it is not possible to identify the exact mode of origin of this material such as thermal alteration of oil gas deasphalting of oils or water washing and biodegradation of oils however the homogenous flowed appearance implies a liquid hydrocarbon origin therefore it is suggested that this kind of kerogen be called solid bitumen the petrographic documentation of solid bitumen in visual kerogen strewn slides can be very meaningful to organic geochemists and geologists because it may indicate that liquid hydrocarbons have been generated in or migrated through the rock interval of interest further study is needed to optically characterize the various stages of maturation of solid bitumen in visual kerogen preparations

hydrocarbon generation and migration routes in the east texas basin the east texas basin is a prolific mature hydrocarbon province producing oil and gas from several reservoirs and a variety of trap types many of the liquid hydrocarbons discovered in the basin are trapped in structures related to movement of the underlying louann salt by determining the structural evolution of the basin we constructed a framework to model the generation of hydrocarbons in the basin geochemical data indicate three major oil types jurassic oil lower cretaceous oil and upper cretaceous oil the jurassic source is mature throughout the basin and began to expel oil at approximately 88 ma the distribution of jurassic oil in cretaceous reservoirs shows that vertical migration routes predominated prospective lower cretaceous source rocks are only matur in the deep central portion of the basin where expulsion began about 47 ma distribution of this oil type suggests that lower cretaceous source rocks occur only in localized areas of the east texas basin organic rich upper cretaceous shales are immature in the main part of the basin but are mature south of the angelina caldwell flexure where they reached peak generation at approximately 20 ma long distance lateral migration routes are necessary to explain the distribution of this oil type migration routes to the giant east texas field may be 100 km or more modeling of this basin suggests an exploration approach in mature basins of defining migration pathways and seeking traps astride them traps in this position have a better probability of being filled and all else being equal are likely to be better fields than traps located away from the major migration routes

the petrology of two distinct granulite types in the hengshan mts china and tectonic implications abstract the archean to proterozoic hengshan complex north china craton comprises tonalitic and granodioritic gneisses with subordinate mafic lenses pegmatites and granites amphibolite facies assemblages predominate although granulite facies relics are widespread and greenschist facies retrogression occurs in km wide shear zones mafic lenses locally abundant occur as strongly deformed amphibolite hornblende plagioclase boudins or sheets in contrast to previously published models we find two series of mafic rocks with distinctly different granulite facies evolutions in the north of the complex relict high pressure mafic granulites are garnet clinopyroxene bearing rocks with a secondary development of orthopyroxene around both garnet kelyphites and clinopyroxene coronas south of the newly defined central e w trending zhujiafang shear zone numerous mafic boudins and less deformed dykes exhibit a macroscopically visible magmatic texture with coronitic growth of metamorphic garnet full of quartz inclusions between the magmatic plagioclase and pyroxene domains additional orthopyroxene after magmatic augite and sodic rims to magmatic plagioclase clearly indicate medium pressure granulite facies metamorphism these findings suggest tectonic juxtaposition in this area of three different structural levels of the same proterozoic imprinted crust high pressure granulite grade in the northern hengshan medium pressure granulite grade in the southern hengshan and amphibolite to greenschist facies grade in the wutaishan to the se

petrochemistry and sr pb and nd isotopic geochemistry of early precambrian rocks wutaishan and taihangshan areas china abstract in the wutaishan and taihangshan region china the lowermost high grade metamorphic complex of grey gneiss and amphibolite the fuping complex gives a 2 2 0 2 ga pb pb isochron age with a first stage growth 7 73 and a 2 37 0 07 ga sm nd isochron age with 143nd 144nd 0 0 50963 0 00005 or nd t 1 50 nd depleted mantle model dates tdm of amhibolites 2 48 to 2 60 ga those of gneisses are 2 43 to 2 46 ga the unconformably overlying medium to low grade metavolcanic complex the wutai complex gives a 2 0 0 1 ga rb sr isochron age with 87 86sr 0 0 7025 2 a 2 27 0 02 ga pb pb isochron date with afirst stage growth 7 73 and a 2 26 0 06 ga sm nd isochron first stage 143nd 144nd 0 0 5097 2 or nd t 1 08 individual sample nd depleted mantle model date are 2 5 ga metavolcanic samples from low grade metasedimentary rocks of the hutuo group all lie close to a 2 3 ga reference line on a rb sr isochron plot have nd tdm 2 32 2 34 and 2 62 ga from previous geological work published u pb zircon dating and ourisotopic and geochemical results we infer that the fuping rocks formed 2 6 ga ago in an environment like a modern island arc the wutai complex formed by 2 5 ga ago mostly formed in a tectonic setting similar to that of the fuping complex with the exception that one volcanic cycle formed in an environment like a modern mor and one unit formed in an environment transitional between modern within plate and plate margin settings the hutuo group formed 2 4 ga ago in a within plate tectonic environment there is no evidence for continental crust before 2 6 ga in the wutaishan and taihangshan region this contradicts previous assignment of the fuping complex to 2 8 ga ol older the fuping complex is not an early nucleus of the sino korean craton the new data also confirm that the wutai complex is older than its original assignment as early proterozoic the sm nd systems for metabasaltic rocks in the region either in amphibolite facies or in greenschist facies are all significantly disturbed in contrast with the undisturbed sm nd reported in previous studies of the sino korea craton this study provides further evidence that the archean igneous rocks in china formed at different times from heterogeneous and depleted mantle sources

origin of crude oil in the wilcox trend of louisiana and mississippi evidence of long range migration abstract geochemical characterization of crude oils from wilcox reservoirs in central louisiana and southwest mississippi suggests that they represent a single crude oil family that is distinct when compared to crude oils in deeper tuscaloosa and smackover reservoirs this observation is consistent with geologic constraints that suggest an origin of crude oil from within the wilcox group itself although shales of the shallow wilcox group in central louisiana and southwest mississippi contain gas prone kerogen and are thermally immature a more oil prone source facies is present in marine shales of the deep wilcox group in southcentral louisiana thermal maturity measurements based on pyrolysis suggest a broad area of effective wilcox source rock in southcentral louisiana migration distances from source to reservoir rocks of the downdip wilcox trend of southcentral louisiana appear to be relatively short however long range updip migration sometimes greater than 100 km from deeply buried wilcox source facies provides the best explanation for emplacement of crude oil in the shallow wilcox trend of central louisiana and southwest mississippi

evolution of salt structures east texas diapir province part 2 patterns and rates of halokinesis the effects of salt mobilization on aptian and younger post 112 ma strata in the east texas basin can be used to illustrate patterns of dome growth through time and to estimate rates and amounts of salt movement pre aptian domes grew in three areas around the margin of the diapir province apparently in pre aptian depocenters maximum dome growth along the basin axis coincided with maximum regional sedimentation there during the middle cretaceous aptian albian and cenomanian in the late cretaceous the sites of maximum diapirism migrated to the periphery of the diapir province diapirism began after pillows were erosionally breached leading to salt extrusion and formation of peripheral sinks the duration of pillow and diapir stages of growth was subequal ranging from 10 to 30 ma post diapiric stage of growth continued for more than 112 ma in some cases diapirs grew fastest in the early cretaceous when peak growth rates ranged from 150 to 530 m ma 490 to 1 740 ft ma declining in the early tertiary to 10 to 60 m ma 30 to 200 ft ma assuming steady state conditions over periods of 1 to 17 ma strain rates for the rise of the east texas diapirs averaged 6 7 10 16 sec peak gross rate of growth averaged 2 3 10 15 sec similar to slow orogenic rates the evolution of east texas salt domes essentially ended in the early tertiary with uplift rates less than 30 m ma 100 ft ma

biomarkers and carbon isotopes of oils in the jurassic smackover trend of the gulf coast states u s a abstract the geochemistry of 25 oils from 23 fields in alabama and the mississippi and louisiana arkansas jurassic salt basins was evaluated results show that the oils were generated by a carbonate source rock which was deposited under highly anoxic conditions and which contains mainly marine derived organic matter the mississippi and louisiana arkansas oils are geochemically similar indicating similar depositional environments for the source although the alabama oils were also derived from a carbonate source rock they are dissimilar to the mississippi and louisiana arkansas oils terpane biomarkers suggest that in addition to marine derived organic matter the source for the alabama oils had an organic input from a more near shore paralic environment i e with a component of terrestrially derived kerogen within each area the oils are similar therefore the norphlet and upper smackover oils in alabama share a common source and the upper smackover cotton valley and some of the lower tuscaloosa oils where production is from faulted structures in mississippi also share a common source maturities of the oils in the three areas vary from low in the updip mississippi salt basin high in the louisiana arkansas salt basin to very high in portions of alabama based on the maturity of oils in mississippi oil generation and migration commenced during the cretaceous when the source was at modest levels of thermal maturity oils migrated relatively short distances into nearby reservoir rocks some oils reached high maturities in the reservoirs resulting in abundant late forming bitumen and pyrobitumen deposition in pore spaces