locating and quantifying geological uncertainty in three dimensional models analysis of the gippsland basin southeastern australia abstract geological three dimensional 3d models are constructed to reliably represent a given geological target the reliability of a model is heavily dependent on the input data and is sensitive to uncertainty this study examines the uncertainty introduced by geological orientation data by producing a suite of implicit 3d models generated from orientation measurements subjected to uncertainty simulations the resulting uncertainty associated with different regions of the geological model can be located quantified and visualised providing a useful method to assess model reliability the method is tested on a natural geological setting in the gippsland basin southeastern australia where modelled geological surfaces are assessed for uncertainty the concept of stratigraphic variability is introduced and analysis of the input data is performed using two uncertainty visualisation methods uncertainty visualisation through stratigraphic variability is designed to convey the complex concept of 3d model uncertainty to the geoscientist in an effective manner uncertainty analysis determined that additional seismic information provides an effective means of constraining modelled geology and reducing uncertainty in regions proximal to the seismic sections improvements to the reliability of high uncertainty regions achieved using information gathered from uncertainty visualisations are quantified in a comparative case study uncertainty in specific model locations is identified and attributed to possible disagreements between seismic and isopach data further improvements to and additional sources of data for the model are proposed based on this information finally a method of introducing stratigraphic variability values as geological constraints for geophysical inversion is presented

hydrocarbon evolution of the over mature sinian dengying reservoir of the neoproterozoic sichuan basin china insights from re os geochronology abstract rhenium osmium re os geochronology is a robust technique for constraining the accurate timing of hydrocarbon evolution but its effectiveness in over mature petroleum systems has not been well constrained here we conduct a case study in the neoproterozoic sichuan basin china i e the sinian dengying sequence 635 542 ma re os isotopic data from this highly mature hydrocarbon reservoir yield a re os isochron ages of 414 44 ma with an initial 187os 188os osi of 1 36 0 19 for bitumen and a 78 37 ma age with an osi of 3 21 0 17 for pyrobitumen coupled with thermal evolution of possible source rocks and fluid inclusion homogenization temperatures the 414 ma age marks the timing of oil generation and the 78 ma age indicates the timing of gas generation associated with the thermal cracking of oil and the formation of pyrobitumen in the reservoir a comparison of the osi of the 414 ma bitumen and that of possible source rocks at the time of bitumen formation reveal that it was primarily sourced from shale of the lower cambrian qiongzhusi formation the results of this study provide new insights to better understand the over mature oil gas system in the sinian sichuan basin and show that dengying formation reservoirs in the depocenter areas of the basin are potentially favorable for oil and gas exploration the results further highlight the utility of the re os geochronometer to constrain the accurate age and source of hydrocarbon evolution

three dimensional implicit stratigraphic model building from remote sensing data on tetrahedral meshes theory and application to a regional model of la popa basin ne mexico remote sensing data provide significant information to constrain the geometry of geological structures at depth however the use of intraformational geomorphologic features such as flatirons and incised valleys often calls for tedious user interaction during 3 d model building we propose a new method to generate 3 d models of stratigraphic formations based primarily on remote sensing images and digital elevation models this method is based on interpretations of the main relief markers and interpolation of a stratigraphic property on a tetahedral mesh covering the domain of study the tetrahedral mesh provides a convenient way to integrate available data during the interpolation while accounting for discontinuities such as faults interpretive expert input may be provided through constrained interactive editing on arbitrary cross sections and additional surface or subsurface data may also be integrated in the modeling we demonstrate this global workflow on a structurally complex basin in the sierra madre oriental northeastern mexico

recognition of an infracambrian source rock based on biomarkers in the baghewala 1 oil india heavy sulfur rich oil produced from the infracambrian 540 640 ma jodhpur formation in the baghewala 1 well represents a new exploration play in the bikaner nagaur basin in india and the punjab basin in pakistan the baghewala 1 oil is nonbiodegraded and thermal maturation dependent biomarker ratios indicate generation from the source rock within the early oil window age diagnostic and source dependent biomarkers indicate that the oil originated from algal and bacterial organic matter with no higher plant input in an infracambrian carbonate rich source rock deposited under anoxic marine conditions these characteristics support a local origin of the baghewala 1 oil from organic rich laminated dolomites in the infracambrian bilara formation significant amounts of petroleum could originate from equivalents of the proposed bilara source rock in the punjab basin pakistan where the precambrian to lower paleozoic section is thicker and more deeply buried than in india deeper burial of the source rock in the punjab basin than in the bikaner nagaur basin could generate more mature equivalents of the baghewala 1 oil the baghewala 1 oil is geochemically similar to another heavy oil from the infracambrian salt range series in the nearby karampur 1 well in pakistan and to oils derived from carbonate evaporite facies of the infracambrian huqf group about 2000 km 1243 mi to the southwest in the eastern flank province of southern oman these findings are consistent with published evidence that subsiding rift basins in northwest india pakistan and southern oman were in close proximity during the infracambrian along the middle eastern edge of gondwanaland

the western desert versus nile delta a comparative molecular biomarker study abstract the western desert and the nile delta are prolific petroleum producing basins the correlation and matching between their petroleum systems have been briefly studied overall geochemical and biological marker analyses of twelve crude oil and condensate samples taken from seven oil gasfields located in the nile delta and the western desert basins of egypt were effective in identifying and geographically defining two major oil families related by age and source rock depositional environment these samples were classified respectively as late cretaceous tertiary and jurassic early cretaceous the source rocks giving rise to the late cretaceous tertiary oils are associated with abundant terrestrial organic matter the source rocks giving rise to jurassic early cretaceous oil are presumably generated from a mixed terrestrial marine source meanwhile the western desert oil samples show prominent c 30 steranes elevated c 27 monoaromatic steroids and a distinct homohopane distribution suggesting this oil type is derived from a more marine influenced source rock than terrigenous sourced oil in the nile delta the most crucial geochemical fingerprinting biomarkers that have been used to differentiate between nile delta and the western desert oil are oleananes highly branched isoprenoids hbi sterane isomerization 24 norcholestanes aromatic steroid hydrocarbons and c 30 24 n propylcholestanes

24 norcholestanes as age sensitive molecular fossils biomarkers molecular fossils are organic compounds in holocene to precambrian sedimentary deposits that can be related to specific chemical compounds produced in the biosphere we demonstrate here that 24 norcholestane biomarkers i e c26 steranes saturated hydrocarbons having a steroid skeleton can be useful to constrain the age and paleolatitude of geologic samples the biological precursors of 24 norcholestanes remain unclear but samples from more than 100 basins provide evidence that 24 norcholestanes may be derived directly or indirectly from diatoms ratios of 24 norcholestanes to 27 norcholestanes show an initial increase above background in jurassic oils but they increase dramatically in cretaceous oils coincident with diatom evolution the highest ratios are found in oils and rock extracts from oligocene or younger marine siliceous source rocks in which the sources were deposited at paleolatitudes 30 n

relationship between petroleum composition and depositional environment of petroleum source rocks crude oils of nonmarine source can be distinguished from those of marine shale source and from oils originating in marine carbonate sequences by using a battery of geochemical parameters as demonstrated with a sample suite of nearly 40 oils a novel parameter based on the presence of c30 steranes in the oil was found to be a definitive indication of a contribution to the source from marine derived organic matter a second novel parameter based on monoaromatized steroid distributions was effective in helping to distinguish nonmarine from marine crudes and can be used to gauge relative amounts of higher plant input to oils within a given basin sterane distributions were similarly useful for detecting higher plant input but were less effective than monoaromatize steroid distributions for making marine versus nonmarine distinctions concentrations of high molecular weight paraffin can also be effective nonmarine indicators but are influenced by maturation and biodegradation processes certain algal derived nonmarine oils may show little high molecular weight paraffin response oils from carbonate sources with a few exceptions can be distinguished by having low pristane phytane ratios low carbon preference indexes and high sulfur contents gammacerane indexes and carbon isotope ratios of the whole crude are not effective in distinguishing these types of environmental differences on a global basis

structural data constraints for implicit modeling of folds a recent method for modeling folds uses a fold frame with coordinates based on the structural geology of folds fold axis direction fold axial surface and extension direction the fold geometry can be characterised by rotating the fold frame by the pitch of the fold axis in the axial surface and the angle between the folded foliation and the axial surface these rotation angles can be expressed as 1d functions of the fold frame coordinates in this contribution we present methods for extracting and automatically modeling the fold geometries from structural data the fold rotation angles used for characterising the fold geometry can be calculated locally from structural observations the fold rotation angles incorporate the structural geology of the fold and allow for individual structural measurements to be viewed in the context of the folded structure to filter out the effects of later folding the fold rotation angles are plotted against the coordinates of the fold frame using these plots the geometry of the folds can be interpolated directly from structural data where we use a combination of radial basis function and harmonic analysis to interpolate and extrapolate the fold geometry this contribution addresses a major limitation in existing methods where the fold geometry was not constrained from structural data we present two case studies a proof of concept synthetic model of a non cylindrical fold and an outcrop of an asymmetrical fold within the lachlan fold belt at cape conran victoria australia

source rock evaluation and nature of hydrocarbons in the khalda concession shushan basin egypt s western desert abstract organic geochemical studies were undertaken on 241 jurassic cretaceous ditch cuttings samples and 9 gases and 10 crude oils from some productive mesozoic pay zone intervals in the khalda concession shushan basin northern western desert egypt we aim to investigate source rock characteristics origin of gases nature of oils and their inferred source rocks and to establish genetic relationships the middle jurassic khatatba formation and older rocks contain organic rich mature coaly and carbonaceous shales which are the main source for hydrocarbons in egypt s western desert including the shushan basin which generally contains kerogen type iii with some contribution of type iii ii kerogen molecular and isotopic composition indicates that the khalda gases are of thermogenic origin a mature 0 6 2 0 ro source rock with terrigenous organic matter kerogen type iii was identified as a precursor of wet thermogenic gases in the shushan basin geochemical results indicate that the oil samples were derived from a source rock deposited mainly under oxic conditions and containing terrigenous organic matter biomarker data indicate that paraffinic oils and condensates from the shushan basin were derived from mature source rocks within the oil window the light hydrocarbon data suggests that the analyzed oils can be classified into two groups as a result of evaporative fractionation and maturation this signal of a secondary reservoir transformation which is not revealed by biomarker analysis emphasizes the importance of integrating biomarker and light hydrocarbon data to understand variations in the chemical composition and origin of oils

hydrocarbon source rock potential in the southwestern gulf of suez graben insights from organic geochemistry and palynofacies studies on well samples from the ras el bahar oilfield abstract palynological and biomarker characteristics of organic facies recovered from cretaceous miocene well samples in the ras el bahar oilfield southwest gulf of suez and their correlation with lithologies environments of deposition and thermal maturity have provided a sound basis for determining their source potential for hydrocarbons in addition to palynofacies analysis toc rock eval pyrolysis kerogen concentrates bitumen extraction carbon isotopes and saturated and aromatic biomarkers enable qualitative and quantitative assessments of sedimentary organic matter to be made the results obtained from rock eval pyrolysis and molecular biomarker data indicate that most of the samples come from horizons that have fair to good hydrocarbon generation potential in the study area the upper cretaceous paleocene lower eocene samples contain mostly type ii to type iii organic matter with the capability of generating oil and gas the sediments concerned accumulated in dysoxic anoxic marine environments by contrast the miocene rocks yielded mainly type iii and type ii iii organic matter with mainly gas generating potential these rocks reflect deposition in a marine environment into which there was significant terrigenous input three palynofacies types have been recognized the first a consists of type iii gas prone kerogen and is typical of the early middle miocene belayim kareem and upper rudeis formations the second b has mixed oil and gas features and characterizes the remainder of the rudeis formation the third association c is dominated by amorphous organic matter classified as borderline type ii oil prone kerogen and is typical of the matulla turonian santonian and wata turonian formations rock eval t max pi hopane and sterane biomarkers consistently indicate an immature to early mature stage of thermal maturity for the whole of the studied succession

geochemical correlation of windalia oil and extracts of winning group cretaceous potential source rocks barrow subbasin western australia windalia oil is a commercial accumulation in a cretaceous reservoir at barrow island western australia a chemical correlation study has been made between this oil and hydrocarbons extracted from 37 cretaceous sedimentary rock samples from different stratigraphic levels in seven wells on and around barrow island seven established correlation parameters and one new parameter have been applied to establish oil source rock relations the new parameter is the ratio of pristane heptadecane to phytane octadecane the influence of maturation on several correlation parameters has been investigated by pyrolyzing source rocks at 300 c for varying times the changes observed in these experiments have been used to infer the effects which maturation processes have on the co relation parameters to assess the overall degree of correlation between source rock extracts and oil each method has been assigned a weighting which reflects its power as a correlation technique an overall correlation score was obtained for each potential source rock by totaling the contributions from the eight individual methods it appears likely that the windalia oil had its source within the winning group of sedimentary rocks the most probable source formations being the middle shale of the windalia sandstone member the muderong shale and the windalia radiolarite several mechanisms of formation of windalia oil from these source beds are possible

inversion of geological knowledge for fold geometry abstract the process of building three dimensional 3d geological models can be framed as an inverse problem where a model describing the 3d distribution of rock units is non uniquely derived from geological observations the inverse problem theory provides a powerful framework for inferring these parameters from all geological observations in a similar way to how a geologist can iteratively update their structural interpretation while mapping existing geological knowledge is usually indirectly incorporated into 3d models using the geologist s non unique interpretation as form lines cross sections and level maps these approaches treat constraints derived from geological knowledge in the same way as direct observations diluting and confusing both information provided by geological knowledge and hard data resulting in significant subjectivity we present a geological inversion using bayesian inference where geological knowledge can be incorporated directly into the interpolation scheme with likelihood functions and informative prior distributions we demonstrate these approaches on a series of synthetic fold shapes as a proof of concept and a case study from the proterozoic davenport province in the northern territory australia the combined inversion of geological data and knowledge significantly reduces the uncertainty in possible fold geometries where data is sparse or highly ambiguous this could be used by geologists while mapping to propagate information about uncertainties throughout the mapping model building process and would allow for different structural interpretations to be rapidly tested for targeted data collection

unified structural representation of the southern california crust and upper mantle we present a new 3d description of crust and upper mantle velocity structure in southern california implemented as a unified structural representation usr the usr is comprised of detailed basin velocity descriptions that are based on tens of thousands of direct velocity vp vs measurements and incorporates the locations and displacement of major fault zones that influence basin structure these basin descriptions were used to developed tomographic models of crust and upper mantle velocity and density structure which were subsequently iterated and improved using 3d waveform adjoint tomography a geotechnical layer gtl based on vs30 measurements and consistent with the underlying velocity descriptions was also developed as an optional model component the resulting model provides a detailed description of the structure of the southern california crust and upper mantle that reflects the complex tectonic history of the region the crust thickens eastward as moho depth varies from 10 to 40 km reflecting the transition from oceanic to continental crust deep sedimentary basins and underlying areas of thin crust reflect neogene extensional tectonics overprinted by transpressional deformation and rapid sediment deposition since the late pliocene to illustrate the impact of this complex structure on strong ground motion forecasting we simulate rupture of a proposed m 7 9 earthquake source in the western transverse ranges the results show distinct basin amplification and focusing of energy that reflects crustal structure described by the usr that is not captured by simpler velocity descriptions we anticipate that the usr will be useful for a broad range of simulation and modeling efforts including strong ground motion forecasting dynamic rupture simulations and fault system modeling the usr is available through the southern california earthquake center scec website http www scec org

chemostratigraphic reconstruction of biofacies molecular evidence linking cyst forming dinoflagellates with pre triassic ancestors new data from numerous detailed mass spectrometric studies have detected triaromatic dinosteroids in precambrian to cenozoic rock samples triaromatic dinosteroids are organic geochemicals derived from dinosterols compounds known in modern organisms to be the nearly exclusive widely occurring products of dinoflagellates we observed the ubiquitous occurrence of these dinosteroids in 49 late triassic through cretaceous marine source rocks and the absence of them in 13 permian carboniferous source rocks synergistic with the dinoflagellate cyst record however finding dinosteroids in lower paleozoic and precambrian strata presents challenging results for molecular paleontologists evolutionary biologists palynologists and especially for those concerned with the food web at various times of biological crisis other than the few species known as parasites and symbionts many other dinoflagellate species are important as primary producers the presence of precambrian to devonian triaromatic dinosteroids gives chemostratigraphic evidence of dinoflagellates or other organisms with similar chemosynthetic capabilities in rocks significantly older than the oldest undisputed dinoflagellate fossils dinoflagellate cysts from the middle triassic 240 ma and older than the putative silurian 420 ma dinocyst arpylorus antiquus calandra sargent from tunisia this systematic chemostratigraphic approach can shed light not only on lineages of dinoflagellates and their precursors but potentially on many other lineages especially bacteria algae plants and possibly some metazoans

paleoenvironmental reconstruction and hydrocarbon potentials of upper cretaceous sediments in the anambra basin southeastern nigeria abstract palynological organic petrographic and organic geochemical analyses of the campanian maastrichtian sediments in akukwa 2 well were carried out to infer their paleoenvironments origin of the organic matter and hydrocarbon generation potentials the toc values of the analysed sediments range from 0 27 3 02 wt while the s2 pyrolysis yield range from 0 55 to 3 35 mg hc g rock this indicates that the nkporo and mamu sediments possess fair source generative potential the samples contain type iii ii and type iii kerogen as shown by the present day hi values between 58 and 292 mg hc g toc and pyrolysis gc data the organic matter within the sediments is also likely to generate mainly gas this is in agreement with the petrographic observations which revealed that the analysed shale samples contain abundant vitrinite macerals apart from bituminite alginite cutinite and resinite also the sediments are immature to early mature in terms of hydrocarbon generation as indicated by vitrinite reflectance biomarker maturity and pyrolysis tmax data biomarker distribution ratios palynomorphs assemblage and organic petrographic observations further point out that the organic materials within the sediments were of mixed aquatic and terrigenous origin and were deposited under suboxic paleodepositional conditions based on sedimentological palynological and biomarker characteristics the environment of deposition of the analysed sediments was inferred to be a relatively quiet shallow marine with fluvial incursion most especially at the upper part of the intervals studied and consequently it is a delta associated depositional environment with a fluviatile influence the sediments are therefore suggested to be deposited in a paleogeographic setting close to vegetation source

geochemistry of crude oils from eastern indonesia high resolution geochemistry shows genetic relationships among 27 crude oils from eastern indonesia and suggests the ages and paleoenvironments of their source rocks oils inferred to originate from tertiary and triassic jurassic source rocks in the study occur north and south of 2is latitude respectively twenty oils mainly from irian jaya and sulawesi originated from tertiary marine marlstone source rocks that contained type ii iii kerogen deposited under suboxic conditions probably the upper miocene klasafet formation these low sulfur oils show high oleanane c26 24 nordia cholestane and pristane phytane ratios and 13c rich carbon isotope compositions high oleanane and 24 nordiacholestane ratios are diagnostic of tertiary oils and source rocks these oils account for about 16 of the estimated ultimate recoverable reserves in eastern indonesia

regional dome evolution and its control on ore grade distribution insights from 3d implicit modelling of the navachab gold deposit namibia abstract we introduce a novel approach to analyse and assess the structural framework of ore deposits that fully integrates 3d implicit modelling in data rich environments with field observations we apply this approach to the early palaeozoic navachab gold deposit which is located in the damara orogenic belt namibia compared to traditional modelling methods 3d implicit modelling reduces user based modelling bias by generating open or closed surfaces from geochemical lithological or structural data without manual digitisation and linkage of sections or level plans instead a mathematically defined spatial interpolation is used to generate 3d models that show trends and patterns that are embedded in large drillhole datasets in our 3d implicit model of the navachab gold deposit distinctive high grade mineralisation trends were identified and directly related to structures observed in the field the 3d implicit model and field data suggest that auriferous semi massive sulphide ore shoots formed near the inflection line of the steep limb of a regional scale dome where shear strain reached peak values during fold amplification this setting generated efficient conduits and traps for hydrothermal fluids and associated mineralisation that led to the formation of the main ore shoots in the deposit both bedding parallel and highly discordant sets of auriferous quartz sulphide veins are interpreted to have formed during the later lock up stage of the regional scale dome additionally pegmatite dykes crosscut and remobilise gold mineralisation at the deposit scale and appear to be related to a younger joint set we propose that kilometre scale active folding is an important deformation mechanism that influences the spatial distribution and orientation of mineralisation in ore deposits by forming structures traps and pathways for fluids at different preferred sites and orientations we also propose that areas that experience high shear strain located along the inflection lines of folds can act as preferred sites for syn deformational hydrothermal mineralisation and should be targeted for regional scale exploration in fold and thrust belts our research also suggests that examination of existing drillhole datasets using 3d implicit modelling is a powerful tool for spatial analysis of mineralisation patterns when combined with fieldwork this approach has the potential to improve structural understanding of a variety of ore deposits

towards an integrated inversion of geoscientific data what price of geology abstract the inversion of multiple geoscientific datasets to define structures in hard rock terrains has for the most part been limited to geophysical methods supported by rock property constraints in this paper we expand the range of constraints available to geological modellers by defining a series of misfit functions that are used to define the quality of a given geological model with respect to the available geological data these misfit functions encompass both spatial and temporal observations and include primary data such as the location of an outcrop or bore hole location of a given rock type the orientation of its different generations of foliations as well as secondary geologic observations such as age relationships when we combine geologic conditions with geophysical misfit functions which already exist for many geophysical measures we have the potential to better constrain two and 3d models of the earth as a first test a subset of these misfit functions are applied to a set of synthetic three dimensional volumetric models built with an implicit surface modelling scheme by perturbing the input structural data within a narrow range we can simulate a range of significantly different three dimensional models for which we can then apply both geological and geophysical misfit functions several joint geological geophysical inversion schemes may be developed based on this methodology with the ultimate goal being an inversion scheme that simultaneously minimizes both geological and geophysical misfit

defining folds on three dimensional surfaces we present a new method for the structural analysis of folded surfaces the method requires a generalized definition of a fold as a patch consisting of contiguous points with gaussian curvature and mean curvature of consistent sign four classes of folds result from this definition antiforms and synforms each of synclastic and anticlastic forms this new analysis is illustrated with reference to several examples

implicit modeling of folds and overprinting deformation abstract three dimensional structural modeling is gaining importance for a broad range of quantitative geoscientific applications however existing approaches are still limited by the type of structural data they are able to use and by their lack of structural meaning most techniques heavily rely on spatial data for modeling folded layers but are unable to completely use cleavage and lineation information for constraining the shape of modeled folds this lack of structural control is generally compensated by expert knowledge introduced in the form of additional interpretive data such as cross sections and maps with this approach folds are explicitly designed by the user instead of being derived from data this makes the resulting structures subjective and deterministic this paper introduces a numerical framework for modeling folds and associated foliations from typical field data in this framework a parametric description of fold geometry is incorporated into the interpolation algorithm this way the folded geometry is implicitly derived from observed data while being controlled through structural parameters such as fold wavelength amplitude and tightness a fold coordinate system is used to support the numerical description of fold geometry and to modify the behavior of classical structural interpolators this fold frame is constructed from fold related structural elements such as axial foliations intersection lineations and vergence poly deformed terranes are progressively modeled by successively modeling each folding event going backward through time the proposed framework introduces a new modeling paradigm which enables the building of three dimensional geological models of complex poly deformed terranes it follows a process based on the structural geologist approach and is able to produce geomodels that honor both structural data and geological knowledge

book review the techniques of modern structural geology volume 2 folds and fractures john g ramsay and martin i huber academic press london 1987 xi 309 398 pp price stg17 50 us 34 50 paperback

egypt far western desert basins petroleum charge system as defined by oil chemistry and unmixing analysis abstract source rock analysis and maturity modeling in egypt s far western desert basins indicates that several jurassic and cretaceous petroleum source rocks could be mature and expelling hydrocarbons over much of the area highly variable oil characteristics suggest that more than one charge system may be present reservoir crude oil chemistry indicates that the fluids originated from a single source facies generated over a wide range of organic maturity secondary alteration mechanisms include water washing biodegradation and evaporative fractionation crude oils in the upper reservoirs tend to be most affected by biodegradation and water washing condensates and light crude oils in the deepest reservoirs appear to be the residual phase from evaporative fractionation the broad variation in fluid properties and chromatogram character can be attributed to a multi end member mixing system early mature waxy oil later high maturity volatile oil low level biodegradation migrated phase evaporative fractionation and residual phase evaporative fractionation a statistical unmixing algorithm was used to determine end member compositions and relative mixing proportions for each crude oil sample remigration and mixing of the products from multi phase generation and alteration results in complex reservoir fluid compositions such that no single reservoired oil represents a compositional end member rather the oil samples are mixtures of various amounts of multiple end members mixing proportions show regional trends and variations in different reservoirs within a single field the crude oil geochemical data and unmixing analysis support a complex charge history from a single source rock rather than mixing of oils from two source rocks

structural geometry of the source region for the 2013 mw 6 6 lushan earthquake implication for earthquake hazard assessment along the longmen shan abstract the 2013 mw 6 6 lushan earthquake occurred in the longmen shan fold and thrust belt sichuan province china near the five year anniversary of the devastating 2008 mw 7 8 wenchuan earthquake to define the fault that generated the 2013 earthquake and its relationship with the beichuan fault which ruptured in the wenchuan earthquake we construct several cross sections and a 3d structural model the sections and models reveal that the main shock of the lushan earthquake occurred on a portion of the range front blind thrust rfbt and that the structural geometry of this fault varies along strike the lushan main shock occurred at a location along the strike of the fault where the geologic shortening and total fault slip are greatest a lateral ramp of the rfbt appears to coincide with the northern limit of aftershocks from the lushan earthquake leading to a 75 km seismic gap between the wenchuan earthquake and the 2013 earthquake sequence although both the wenchuan and lushan earthquakes occurred within the longmen shan fold and thrust belt different faults generated the two events based on this structural characterization and analysis of the aftershocks of the wenchuan and lushan earthquakes we suggest that the lushan earthquake may have been triggered by the 2008 rupture but is best considered as an independent event rather than an aftershock of the wenchuan earthquake the rfbt that generated the lushan earthquake is linked to a detachment that extends into the sichuan basin along the triassic evaporite layer the coulomb stress change simulation suggests that other faults linked to this detachment may have been loaded by the 2008 and 2013 earthquake posing the risk of future earthquakes along the longmen shan and in the densely populated sichuan basin

impacts of source input and secondary alteration on the extended tricyclic terpane ratio a case study from palaeozoic sourced oils and condensates in the tarim basin nw china abstract a suite of oils and condensates from cambrian ordovician sources in the cratonic region of the tarim basin nw china has been characterized by gas chromatography mass spectrometry with a focus on factors affecting the extended tricyclic terpane ratio etr c28 c29 tt ts these oils and condensates show etr values ranging from 0 3 to 12 4 suggesting variable source contributions deposited under different environmental conditions coupled with tri and tetracyclic terpane distributions etr can be used as an organofacies discriminator most of the samples have high etr values 2 0 a low relative abundance of the c19 tricyclic terpane c19tt and the c24 tetracyclic terpane c24tet characteristic of a clastic marine depositional environment possibly with upwelling conditions a specific organofacies for a few samples is characterized by low etr values 0 8 and c24tet c26tt 1 0 ratios reflecting deposition under dysoxic conditions however various other alteration processes such as thermal maturity thermochemical sulfate reduction tsr biodegradation and in reservoir mixing probably have a significant impact on this parameter which makes understanding the meaning of the etr complicated the effect of thermal maturity on etr is minimal in the oil generation window but the etr values decrease dramatically at the post oil generation stage tsr seems to preferentially lead to the loss of the c28tt and c29tt before the 18 h 22 29 30 trisnorneohopane ts and thus the etr value decreases as tsr progresses while the dibenzothiophene phenanthrene ratio can help to differentiate tsr alteration from thermal cracking processes the etr is no longer a valid geochemical parameter when these two processes occur etr also increases with biodegradation the c28 and c29tt are more resistant to biodegradation than ts supporting evidence can be derived from a positive correlation between etr and c29 25 norhopane to c30 hopane ratio in reservoir mixing is another important alteration process which normally results in lower etr values after a fresh charge is mixed with a biodegraded charge in a reservoir evaporative fractionation may also have an impact on etr values some unusually high etr values might be attributed to evaporative fractionation the etr values from our case study suggests that the etr parameter is much more complicated than just being an age diagnostic parameter maturation and secondary alteration processes need to be fully accounted for before the etr can be properly applied to interpretations of depositional environment conditions such as upwelling

provenance and paleoenvironment of organic matter within the fika sediments in chad bornu basin northeastern nigeria an integrated organic geochemical and palynofacies approach abstract the upper cretaceous fika sediments in the chad bornu basin were characterised to define paleoenvironment and source of the organic matter using an integration of organic geochemical analyses and palynofacies observations the present day toc contents of the fika sediments are in the range of 0 41 2 37 wt while the hydrogen index hi values are largely below 200 mg hc g toc suggesting that the sediments generally have fair to good generative potential and contain mainly type iii and type iv kerogens this is supported by palynofacies observations whereby the kerogen is characterised by large amounts aom and phytoclasts the fika sediments have vitrinite reflectance and t max values in the range of 0 70 1 34 and 429 465 c respectively indicating early mature to late mature stage of oil window the level of organic maturation in fika sediments was further supported by the several biomarker maturity indicators biomarker distributions and palynofacies data provide evidence for a mixed aquatic algae and terrigenous organic matter input deposited in a marginally marine moderate salinity proximal setting with a strong influx of terrestrial plant under environmental conditions that were mostly suboxic compound specific isotopic analysis of n alkanes fraction were also used for the paleoenvironmental inferences the flat portion pattern of the n alkane profile between n c 21 and n c 26 in fika samples is an indication of marine environment and it shows that fika sediments consist of both terrestrial and marine organic matter deposited in fluvial deltaic to shallow marine settings

application of biological markers in the recognition of the geochemical characteristics of some crude oils from abu gharadig basin north western desert egypt abstract the objective of this study is to establish the relationship between the hydrocarbons recovered from the abu gharadig basin and middle jurassic khatatba formation as well as the previously established upper cretaceous abu roash f member a suite of fourteen crude oil and condensate samples from the mesozoic reservoirs jurassic cretaceous of the abu gharadig basin western desert egypt was analyzed and geochemically compared with extracts from source rocks of the middle jurassic khatatba formation and the upper cretaceous abu roash f member based on molecular indicators of organic source input depositional environment and age diagnostic biomarkers at least two petroleum systems operate in the abu gharadig basin those derived from late cretaceous marine shales and jurassic early cretaceous non marine organic rich shales as source rocks and possibly a third system appears to be a mixture of them both the correlation of some abu gharadig oils to a late cretaceous source establishes a petroleum system involving these marine cretaceous source rocks

structural control of greenstone hosted gold mineralization by transcurrent shearing a new interpretation of the kalgoorlie mining district western australia abstract the kalgoorlie mining district encloses the golden mile and mt charlotte mining areas and has produced 1200 metric tons of gold it is classed as one of the largest archaean gold deposits in the world the district is located in the norseman wiluna greenstone belt of the yilgarn craton western australia this greenstone belt is characterized by nnw trending transcurrent shear zones some of which are traceable over more than 500 km strike length mineralization in the district is controlled by second third and fourth order structures located between principal shear zones of regional extent an early deformation phase of regional folding d1 associated with prograde regional metamorphism of low grade preceded two phases of wrenching the geometry of mineralized shear zones in both mining areas as well as field and microstructural studies allow us to distinguish an early phase of transpressional sinistral shearing d2 from a later phase of transcurrent dextral shearing d3 the earlier sinistral shear system in the golden mile area was in part reactivated but its gross original geometry was preserved these observations are consistent with clay model experiments carried out by the authors hydrothermal carbonate sericite alteration and gold mineralization commenced during the sinistral shear event and continued into the dextral event mineralization within individual shear zones forms tabular orebodies whereas pipe shaped ore shoots occur at the intersection of shear zones the ore shoots contain high grade telluride mineralization locally relative timing relationships show that mineralization postdates the peak of regional metamorphism hydrothermal activity in the district was preceded by the intrusion of calc alkaline porphyry dykes of granodioritic to tonalitic composition and accompanied by the intrusion of lamprophyre dykes

the occurence of unusual c27 and c29 sterane predominances in two types of oman crude oil abstract two types of oman crude oils reveal unusual sterane distributions type a which is the more common 74 examples is characterised by a predominance of c 29 iso and normal steranes and generally none or only very low relative concentrations of rearranged steranes the triterpanes are characterised by the predominance of the c 29 17 h 21 h norhopane over the c 30 17 h 21 h hopane and non predominant c 20 c 30 tricyclic terpanes the c 29 steranes of this type of crude were not derived from the c 29 sterols of land plant origin frequently proposed as the source of c 29 steranes since there is good geological evidence that these crudes were generated from a pre cambrian source rock a geological period when land plants did not exist the type b crude oil 11 known examples is characterised by a strong predominance of c 27 iso normal and rearranged steranes relatively lower concentrations of 17 h 21 h hopanes and relatively high concentrations of c 20 c 30 tricyclic terpanes the remarkably different biomarker characteristics of these crude oils imply that the organisms active in the depositional environment of the respective source rocks were significantly different

synchronous development of type 2 and type 3 fold interference patterns evidence for recumbent sheath folds in the allendale area broken hill nsw australia abstract synchronous development of type 2 and type 3 fold interference patterns occur in the poly deformed broken hill inlier the interference patterns have resulted from the superposition of recumbent f2 folds and n s oriented upright f3 folds the synchronous development of type 2 and type 3 fold interference patterns is attributed to variation in the hinge of f2 folds by as much as 90 suggesting the development of a regional scale sheath like fold geometry during d2 overprinting relationships along the eastern limb of the pap synform suggests that it formed part of a recumbent fold hinge that was flattened during horizontal crustal shortening this has resulted in the development of a modified type 3 fold interference pattern the results of this study show that the type 2 and type 3 fold interference patterns can develop during the same deformation event in an evolving orogen

effects of source thermal maturity and biodegradation on the distribution and isomerization of homohopanes in petroleum abstract the distributions of c31 c35 17 21 h homohopanes in marine petroleums can be used to describe redox conditions in the source rock depositional environment for example high c35 homohopane indices c35 c31 c35 are typical of petroleums generated from source rocks deposited under anoxic marine conditions where the c35 backbone of the precursor bacteriohopanetetrol is preferentially preserved homohopane distributions have been used to distinguish petroleums from different organic facies of the same source rock homohopane distributions are altered by secondary processes for example the c35 homohopane index decreases with increasing api gravity and thermal maturity of related oils from the monterey formation offshore california similar decreases in the homohopane index are observed for expelled oils from monterey source rock during hydrous pyrolysis biodegradation can result in selective loss of high or low molecular weight homologs apparently depending on the bacterial population in the reservoir bacterial demethylation of homohopanes appears necessary to explain 25 norhopanes in at least one biodegraded oil hydrous pyrolysis studies of the phosphatic and siliceous members in the monterey formation show that identical heating conditions result in different homohopane c 22 epimer ratios furthermore a reversal in the trend of the epimer ratio at high experimental temperatures is observed for the phosphatic but not for the siliceous lithology this could be caused by the combined effects of isomerization and differential thermal destruction of epimers at high temperatures possibly mediated by rock mineralogy

complex fold patterns developed by progressive deformation abstract complex folding patterns include folds with curved hinges and or curved axial surfaces in simple shear zones folds commonly nucleate with hinges at a high angle to the shear direction and progressively rotate towards parallelism with the shear extension direction giving rise to sheath folds axial surfaces also change in attitude with increasing strain becoming parallel to the shear plane however when folds do nucleate with hinges along the extension direction but oblique to a shear component they are not subjected to hinge rotation but can experience a variable amount and sense of rotation of axial surfaces developing into polyclinal folds foliated quartzite beds from the cap de creus massif in the eastern pyrenees exhibit complex fold patterns produced during a single transpressional event while folds in the embedding schists are harmonic and asymmetric folds in the quartzite are strongly disharmonic and polyclinal displaying large axial surface rotations a plausible interpretation is that folds nucleate with hinges closely parallel to the extension direction developing thickened short limbs that rotate synthetically with regard to the shear component this may lead to strain partitioning with localized antithetic rotations enhanced by progressive softening of the quartzite thus fold interference patterns are not necessarily linked to polyphase tectonics but they may result from single phase progressive deformation in the here described cases unfolding during progressive deformation is highly improbable

source rock potential and depositional environment of upper cretaceous sedimentary rocks abu gharadig basin western desert egypt an integrated palynological organic and inorganic geochemical study abstract the abu gharadig basin in the northern part of the western desert egypt is among the most important petroleum provinces in egypt here cenomanian to santonian rocks of bahariya formation and abu roash a g members from the gpt 3 well were investigated for their depositional environment kerogen quality petroleum generation potential and thermal maturity by geochemical petrological and palynological methods the sediments of the bahariya and abu roash formations excluding the abu roash f member represent variable shallow marine environments with poor preservation of marine organic matter whereas there is an excellent preservation in the abu roash f member the basal part of this member is characterized by anoxic carbonate rich depositional conditions high toc values up to 7 high hi up to 700 mghc gtoc and liptinite dominated organic matter depletion of iron leads to sulfur incorporation into organic matter which is reflected in high thiophene benzene ratios above sediments are enriched in terrestrial elements fe ti as well as k and mn this pattern and the occurrence of the fresh brackish water algae botryococcus suggests a regression phase during deposition the upper part of the abu roash f member is again characterized by fully marine suboxic conditions and a lower thiophene benzene ratio the source rock section demonstrates lower thermal maturity compared to the above and below sections based on all microscopic pyrolysis and biomarker data this indicates thermal maturity retardation suppression most likely due to a different organic facies and early diagenetic transformation of kerogen as indicated by the geochemical and palynological data the source rock heterogeneity indicates a relatively shallow marine environment where the changes in bottom water conditions are very sensitive to sea level fluctuations residual oil characterization reveals two reservoir compartments which are in the abu roash d and c members the abu roash f source rocks are compared with time equivalent sediments in the tarfaya basin in morocco which represent more distal settings the more proximal group a is characterized by s fe ratios between 0 57 and 0 67 indicating suboxic conditions it has toc and hi values not exceeding 4 and 600 mghc gtoc respectively at variable caco3 contents the more distal tarfaya sediments group b have higher toc and hi as well as high organic sulfur contents group a is higher in terrigenous element concentrations the tmax of immature source rocks ranges from 410 to 415 c in organic sulfur rich samples and from 420 to 430 c in organic sulfur poor samples revealing an important impact of organic facies on maturity parameters furthermore there is an inverse relationship between s fe and al2o3 toc which might become an important tool in paleoenvironmental reconstructions as well as petroleum source rock studies

progress in the study and modelling of similar fold interferences abstract similar fold interferences are examined in two different ways 1 1 we show the importance of taking into account the original position of the stratification with respect to the orientations of two interfering fold phases basin and dome cases correspond to particular orientations of the interfering phases with respect to each other in all other cases coaxiality may be achieved for particular positions of the original stratification 2 2 we have achieved a computer aided modelling which allows the visualization by means of sections and block diagrams of any two or three similar fold interferences the program allows us to define the stratification orientations of the various phases and their profiles sinusoidal or drawn by hand and orientations and dimensions of the selected section or block diagram faults may also be represented this modelling method is a powerful research tool that we hope to apply in future to detailed studies of fold interferences to achieve quantitative field models of multiply folded and faulted terranes