pore characterization of isolated organic matter from high matured gas shale reservoir abstract knowledge of pore structure of isolated organic matter om provides guidance for better understanding the origin distribution and characterization of pore system in gas shale reservoir and can be used to better comprehend the storage and transport mechanism of natural gas in shale reservoir pore structure characteristics of isolated organic matters and its corresponding bulk shales were investigated for five high matured marine shale samples from the lower silurian longmaxi formation in south china using x ray diffraction xrd total organic carbon content toc tests field emission scanning electron microscope fe sem observation and ultra low pressure nitrogen gas physisorption the results indicate that om hosted pores are abundant in bulk shale samples under fe sem observation the bet surface area and the bjh pore volume of isolated om are approximately three times greater than that of bulk shale samples pore size distribution psd of isolated om from five shale samples show a consistent variation pattern additionally with increasing toc content the psd curves of isolated om gradually grow down as result of the ductility of om bimodal psd in surface area and unimodal psd in pore volume exist in both isolated om and bulk shale samples indicating surface area is mainly associated with micropores and fine mesopores

fault interaction may generate multiple slip vectors on a single fault surface slip vectors of multiple orientations on a single fault surface may be explained by the interaction of crustal scale faults in response to nearby earthquakes field observations of at least five sets of striae on a single planar fault surface can be reproduced in numerical experiments these experimental results suggest that slip vectors having variable orientations may reflect a highly transient local stress state in an otherwise constant regional stress field our results suggest that stress inversion techniques should be applied with caution to areas with complex regional scale fault patterns

depositional environment and oil generation in ordovician source rocks from southwestern ontario canada organic geochemical and petrological approach the ordovician trenton group sherman fall and cobourg formations and the lindsay collingwood member and blue mountain formations of southwestern ontario were examined using rock eval pyrolysis gas chromatography gas chromatography mass spectrometry and incident light microscopy to evaluate their paleodepositional environments thermal maturities and source rock potential all units contain sufficient amount of oil prone type ii predominantly marine organic matter to be considered as petroleum source rocks unstructured bituminite with varying proportions of unicellular alginite are the dominant dispersed organic matter macerals the bituminite typically occurs in massive to laminated granular or patchy populations that commonly show microtextural relationships persistent inclusions of leiosphaeridia telalganite demonstrate that planktonic algal debris was a primary organic substrate for blooming microbes disseminated coccoidal gloeocapsomorpha prisca is found in minor amounts usually in association with common to abundant acritarchs zooclasts chitinozoa graptolites scolecodonts and solid bitumen also are present as maceral inclusions within the bituminite network the biomarker distributions for all of the studied units are those expected for marine organic matter deposited in a clastic dominated environment the extracts are characterized by a smooth n alkane profile with low abundance of c20 members typical for marine derived organic matter pristane phytane ratios range from 0 97 to 1 72 indicating dysoxic conditions during deposition smooth c31 c35 homohopane profiles ts tm ratios typically above 1 0 and a higher concentration of diasteranes relative to regular steranes all appear to indicate the clay bearing character of these rocks the predominance of c30 hopane over c29 regular sterane is interpreted to reflect a primary microbial input and extensive reworking of the organic matter optical reflectance fluorescence and geochemical tmax biomarker data thermal maturity parameters indicate that the trenton and blue mountain strata are within the zone of prolific oil generation throughout the whole area of study the collingwood shales are mature with respect to petroleum generation in the eastern part toronto area and only marginally mature in the northern part georgian bay area of the study area in general the biomarker composition of the extracts from all examined units is compatible with that of the oils found in the trenton reservoirs of southwestern ontario however geochemical and geological evidence suggests that the organic rich shaly laminae within the trenton group are the principal source of these oils accumulation of organic carbon in the ordovician sediments of southern ontario is suggested to derive from low energy normal marine environments grading from shallow shelf into deep shelf and open basinal settings the nutrient availability and consequently higher bioproductivity more intense consumption of oxygen and progressing anoxia controlled by a low latitude location diminished water circulation stratification of the water column and a depressed pycnocline resulted in high preservation rates the amorphous nature of kerogen reflects significant microbial interaction at the water sediment interface and within the sediments where reducing conditions must have periodically predominated

sulfur carbon ratios and extractable organic matter of the middle proterozoic nonesuch formation north american midcontinent rift abstract the nonesuch formation ca 1 1 ga host to the white pine copper deposit is a generally organic rich unit of relatively wide extent selected strata away from mineralization were analyzed for their organic carbon and total sulfur contents and for the amount and composition of extractable organic matter sulfur to carbon ratios are covariant but the ratios of total sulfur to organic carbon are generally higher than those found for marine phanerozoic black shales deposited under oxic bottom water conditions the sulfur to carbon ratios and the intimate association fo framboidal pyrite and organic matter are suggested to be the result of bacterial sulfate reduction in the nonesuch sediments activity of sulfate reducing bacteria requires a supply of sulfate in the water column and implies that the depositional environment was not a low sulfate aqueous environment a marine embayment dominated more by sea water than riverine input is suggested as the environment of deposition based on the correlation of total sulfur and organic carbon the relatively high total sulfur content and on the absence of sedimentologic and geochemical criteria for saline lacustrine environments deposition of highly reactive organic matter in low concentrations coupled with high concentrations of reactive iron and the absence of bioturbation resulted in unusually efficient pyrite formation extractable organic matter in unmineralized nonesuch strata is interpreted to be autochthonous in origin homologous series of n alkanes and cyclohexyl alkanes regular isoprenoids and monomethyl branched alkanes were identified by gas chromatography ratios of pristane phytane pristane n c17 and phytane n c18 suggest relatively low thermal maturity for the overall unit with a slight increase in the level of thermal maturity from michigan westward to wisconsin the aliphatic character and high extractability of organic matter indicate that the petroleum potential of the nonesuch formation will be good to excellent if the organic rich facies thicken down dip under lake superior or into the southern extension of the rift

demonstration of two pulses of paleogene deformation in the andes of peru abstract new radiometric ages of about 25 m y on volcanic materials in a marine intercalation within clastic continental strata of the upper moquegua formation near caraveli southern peru together with an age of 25 3 0 4 m y obtained by tosdal et al from a locality about 300 km to the ese show that the formation contains strata of late oligocene as well as miocene age and demonstrate that the coastal region was at a low elevation during latest oligocene time because the unconformities between the upper moquegua formation and the underlying lower moquegua formation and between the lower moquegua formation and underlying paleocene rocks cannot both represent the same tectonic event two discrete paleogene events must be present in the andes of peru although the exact timing of these events is uncertain the unconformities are likely to be of paleocene and middle eocene age or possibly of middle eocene and oligocene age

burial of organic carbon and pyrite sulfur in sediments over phanerozoic time a new theory in present day marine sediments almost all of which are deposited in normal oxygenated seawater rates of burial of organic carbon c and pyrite sulfur s correlate positively and bear a constant ratio to one another c s 3 on a weight basis by contrast calculations based on the isotopic model of garrels and lerman 1981 indicate that at various times during the phanerozoic the worldwide burial ratio must have been considerably different than the present day value this ratio change is caused by the requirement that increases in the worldwide mass of organic carbon must be accompanied by equivalent decreases in the mass of sedimentary pyrite sulfur in order to maintain a roughly constant level of o2 in the atmosphere such apparently contradictory behavior can be explained if the locus of major organic carbon burial has shifted over time from normal marine environments as at present to non marine freshwater or to euxinic environments in the geologic past a shift to predominantly freshwater burial can help explain predicted high c s ratios in permo carboniferous sediments and a shift to euxinic environments can help explain predicted low c s ratios during the early paleozoic it is demonstrated that the three environments today exhibit distinguishably different average c s ratios

sedimentology and organic properties of lower tertiary lacustrine source rocks lunpola basin central tibetan plateau implications for hydrocarbon potential abstract the tertiary lacustrine niubao formation is well exposed in the middle western part of the lunpola basin central tibetan plateau where crude oil was once discovered during drilling campaigns sedimentological organic petrographic and geochemical work was performed on the upper and middle members of this formation in order to investigate its hydrocarbon source rock potential sedimentological analyses indicate that both members were deposited in relatively deep lake conditions away from the influence of big stream system with the upper niubao formation unf having higher sand content the two members contain abundant high quality type i kerogen derived mainly from plankton algae and bacteria compared with samples of the middle niubao formation mnf unf samples also contain more type ii and type iii kerogens of terrestrial origin biomarkers of source rock extracts suggest that a long standing anoxic stratified and saline ancient lake occupid the basin during deposition of the two members which was conducive to the preservation of organic matter however higher gammacerane content and the presence of carotane in mnf samples imply a more saline lake condition rock eval t max and biomarker parameters reveal marginal maturity and immaturity for samples from mnf and unf respectively therefore mnf has better source rocks of higher maturity might be the main hydrocarbon generating member of the basin similar diagnostic biomarkers and biomarker parameters reported in the crude oil also imply its affiliation with the source rocks of mnf

influence of paleoenvironmental factors on preservation of organic matter in middle cretaceous greenhorn formation pueblo colorado sedimentological and geochemical studies of the upper cretaceous hartland shale and overlying bridge creek limestone members of the greenhorn formation reveal close associations between abundance of current induced sedimentary structures extent of bioturbation and type of preserved organic matter abundant hydrogen rich organic matter 4 5 wt and low sulfur to organic carbon ratios are characteristic of the laminated facies which lack current induced sedimentary structures sparse hydrogen poor organic matter 0 1 0 5 wt and relatively high sulfur to organic carbon ratios are characteristic of the bioturbated facies which contain numerous sedimentary structures indicative of currents the concentration of oxygen in the benthic environment and the degree of bio urbation of the sediment apparently were determined principally by the rate that oxygen was supplied advectively by currents rather than by the rate of oxygen consumption by decomposition thus paleoclimatic and paleo oceanographic factors that influenced mixing and current strength in the water column profoundly affected the amount and type of organic matter preserved in these units it is proposed that episodes of high river discharge lead to density stratification of the water column and in turn to quiescent oxygen depleted bottom water and high preservation of organic matter thus accounting for the intermittent widespread deposition of organic rich strata during maximum transgression and depth of the greenhorn sea the lateral association of voluminous deltaic deposits with dee water black shales in other elongate or restricted marine basins may have causes similar to those envisioned for the hartland shale and bridge creek limestone members

geochemical character and origin of oils in ordovician reservoir rock illinois and indiana usa twenty three oils produced from reservoirs within the ordovician galena group trenton equivalent and one oil from the mississippian ste genevieve limestone in the illinois and indiana portions of the illinois basin are characterized two end member oil groups 1 and 2 and one intermediate group 1a are identified using conventional carbon isotopic analysis of whole and fractionated oils gas chromatography gc of saturated hydrocarbon fractions isotope ratio monitoring gas chromatography mass spectrometry irm gc ms of n alkanes ranging from c15 to c25 and gas chromatography mass spectrometry gc ms of the aromatic hydrocarbon fractions group 1 is characterized by high odd carbon predominance in mid chain n alkanes c15 c19 low abundance of c20 n alkanes and an absence of pristane and phytane group 1a is characterized by slightly lower odd carbon predominance of mid chain n alkanes greater abundance of c20 n alkanes compared to group 1 and no pristane and phytane the carbon isotopic composition of n alkanes and the distribution of n alkylarenes for groups 1 and 1a indicate that these oils were generated from strata containing abundant gloeocapsomorpha prisca in contrast group 2 is characterized by low odd carbon predominance in mid chain n alkanes moderate abundance of c20 n alkanes and the presence of pristane and phytane the mississippia reservoired oil is characterized by low odd carbon predominance in mid chain n alkanes and greater abundances of c20 n alkanes pristane and phytane compared to group 2 distributions of isoprenoidal alkylarenes aryl isoprenoids for group 2 and the mississippian reservoired oil are similar suggesting that aryl isoprenoids fail to provide a diagnostic characteristic that can be used to precisely correlate oils and source rocks conventional correlations of oil to source rock based on carbon isotopic type curves and hopane m z 191 and sterane m z 217 distributions are of limited use in distinguishing ordovician reservoired oil groups and determining their origin oil to source rock correlations using the distribution and carbon isotopic composition of n alkanes and the m z 133 chromatograms of n alkylarenes show that groups 1 and 1a originated from strata of the upper ordovician galena group group 2 either originated solely from the upper ordovician maquoketa group or from a mixture of oils generated from the maquoketa group and the galena group the mississippian reservoired oil most likely originated from the devonian new albany group the use of gc irm gc ms and gc ms illustrates the value of integr ted molecular and isotopic approaches for correlating oil groups with source rocks

distribution and structure of hydrocarbons in four organic rich ordovician rocks abstract organic matter in four north american rocks of ordovician age has been analysed using geochemical and optical techniques including reflected light and fluorescence microscopy rock eval pyrolysis elemental analysis of isolated kerogens gas chromatography gc and gas chromatography mass spectrometry gc ms of hydrocarbons in the solvent extracted bitumens the samples analysed were guttenberg oil rock and black shales from the vinini womble and big fork formations in the united states the origin of some of the biomarkers found in the bitumen and the vitrinite particles observed in reflected light are discussed briefly with reference to possible algal and bacterial precursors the results indicate that the four samples are potentially excellent oil source rocks although the guttenberg oil rock is too immature to have generated a substantial quantity of hydrocarbons

formation and interpretation of dilatant echelon cracks the relative displacements of the walls of many veins joints and dikes demonstrate that these structures are dilatant cracks we infer that dilatant cracks propagate in a principal stress plane normal to the maximum tensile or least compressive stress arrays of echelon crack segments appear to emerge from the peripheries of some dilatant cracks breakdown of a parent crack into an echelon array may be initiated by a spatial or temporal rotation of the remote principal stresses about an axis parallel to the crack propagation direction near the parent crack tip a rotation of the local principal stresses is induced in the same sense but not necessarily through the same angle incipient echelon cracks form at the parent crack tip normal to the local maximum tensile stress further longitudinal growth along surfaces that twist about axes parallel to the propagation direction realigns each echelon crack into a remote principal stress plane the walls of these twisted cracks may be idealized as helicoidal surfaces an array of helicoidal cracks sweeps out less surface area than one parent crack twisting through the same angle thus many echelon cracks grow from a single parent because the work done in creating the array as measured by its surface area decreases as the number of cracks increases in cross sections perpendicular to the propagation direction echelon cracks grow laterally each crack overlapping its neighbors until the mechanical interaction of adjacent cracks limits this growth dilation of each crack pinches the tips of adjacent cracks into an asymmetrical form and introduces local stresses that can cause lateral growth along a curving sigmoidal path sigmoidal echelon cracks may link at tip to plane intersections leaving a step in the through going crack wall the geometry of dilatant echelon cracks may be used to infer spatial or temporal changes in the orientation of principal stresses in the earth

bridge structures as sense of displacement criteria in brittle fault zones abstract fault zones often occur as arrays of en echelon fractures there are two main types of en echelon patterns involving right or left stepping fractures in such arrays first generation fractures define compressive or tensile bridges the structural features of such natural bridges at any scale can be used as criteria to determine the sense of displacement small compressive bridges provide criteria based on the increase in linear density of solution seams related to the degree of asymmetry of the fault trace and to the sequence of formation of the different types of en echelon fractures the criteria deduced from large compressive bridges involve upward displacement of the material by folds and hill or mountain ranges asymmetry of the fault profile and relative chronology of the segments tensile bridges yield criteria based upon vein fillings asymmetry and relative chronolog in large scale structures some pull apart basins are shown to be bounded by normal faults parallel to but outside the strike slip segments from a comparison of analytical models with well known large scale fault zones a new criterion can be proposed in fault zones formed through coalescence of en echelon fractures the bridging segments are wide if facing leeward of the bulk movement sense and narrow if forward facing

biochemical evolutionary significance of ordovician oils and their sources abstract geochemical data from several ordovician oils and kerogens from distinct basins in canada united states estonia and australia show some unique compositional characteristics the oils analyzed by gas chromatography and gas chromatography mass spectrometry are characterized by an odd dominance in the c9 c19 molecular weight range of normal paraffins and an odd dominated homologous series of cyclohexyl substituted normal paraffins the oils contain an unusually small amount of c20 components and pristane and phytane are conspicuous by their virtual absence the compositions of the oils and kerogens correlate closely and display characteristics that have not been observed in younger sediments the predominant organism found in the source kerogens for these unique oils is gloeocapsamorpha prisca a primitive prokaryotic organism data from the analysis of gloeocapsamorpha rich kerogens and the association of this organism with ordovician source rocks indicate a genetic relationship exists between the organism and the unusual oil chemistry the chemistry of the oils and kerogens suggests that gloeocapsamorpha was either non photosynthetic or that it fixed light using a chlorophyll that did not contain a phytyl side chain e g a bacteriochlorophyll the widespread occurrence of oils and kerogens possessing such a uniform composition appears unique to the ordovician certainly to the early paleozoic this suggests that paleocological conditions of ordovician anoxic environments were very similar and g prisca was particularly well adapted to these conditions

fault relays bends and branch lines abstract branch lines between normal faults and their sub parallel splays mapped from 3d seismic reflection data show a range of forms from straight lines to closed loops the different geometries are interpreted as representing stages in the failure of relay zones and in the progressive replacement of fault tip lines with fault branch lines the geometries of these normal fault branch lines are similar to those for thrusts previously inferred from limited two dimensional data the orientation of the axis of a relay and its associated bends relative to a fault slip direction is identified as an important control on the structures developed within the relay neutral restraining and releasing bends can each occur on any fault type normal reverse and strike slip but data bias is a major factor in determining which bend geometry is most often observed with each fault type on normal faults the initial relay zone geometry controls the dominant branch line orientation and the same control is likely on branch lines associated with the other modes of faulting a review of the relay geometries and strains occurring with all three modes of faulting highlights the role of the orientation of the mechanical anisotropy of a bedded sequence relative to the orientations of fault surface and slip directions this relative orientation determines how the relay strain is accommodated and hence the degree of hard linkage and development of branch lines

comparison of michigan basin crude oils abstract michigan basin oils from the ordovician trenton silurian niagaran and devonian dundee formations have been geochemically compared by gc gc ms and carbon isotope mass spectrometry one oil from each formation was selected for detailed analysis which included measurement of individual n alkane 13 c values the ordovician and devonian oils are strikingly similar to one another yet clearly different from the silurian oil this pattern is unexpected because ordovician and devonian reservoirs are physically separated by the silurian strata from time temperature considerations the devonian oil probably was formed in older strata and has migrated to its present location our analyses suggest a common source for the devonian and ordovician oils

middle ordovician organic matter assemblages and their effect on ordovician derived oils two distinct middle ordovician oil prone organic matter assemblages occur in thermally immature to marginally mature petroleum source rocks from iowa the substantially more oil prone assemblage a generates the unique ordovician oil fingerprint which has been associated with the organic walled microfossil gloeocapsamorpha prisca the second assemblage b generates a more ordinary signature the two assemblages which are mixed or interbedded in many ordovician sediments of north america explain the variations in oils derived from ordovician source rocks these mixtures also aid in interpretation of the chemistry of g prisca core samples examined are from three wells in iowa penetrating the primarily carbonate sequence from the guttenberg member of the decorah formation down to the st peter sandstone microscopy rock eval pyrolysis and capillary gas chromatography on source rock bitumen extracts and nearby oils were used to evaluate the two assemblages solvent extracts from rocks containing predominantly assemblage a have the diagnostic ordovician gas chromatographic signatures with an odd preference in c11 c19 normal alkanes low amounts of heavier normal alkanes and virtual absence of isoprenoids including pristane and phytane although assemblage a occurs only in very thin beds it is potentially so oil generative that thicknesses in inches may be significant extracts from r cks containing predominantly assemblage b show reduced odd preference a full profile of normal alkanes and conspicuous occurrence of isoprenoids including pristane and phytane these samples from iowa are representative of middle ordovician source rocks found in most paleozoic basins of north america as well as baltic europe and australia therefore the conclusions drawn from these middle ordovician rock samples from iowa provide a basis to explain variations in ordovician derived oils found in many other areas

the initiation and growth of en echelon veins abstract numerical models are used to understand the evolution of mode i opening fractures from spatially random distributions in a brittle elastic material en echelon arrays commonly develop because mechanical fracture interaction promotes growth for this geometry this provides a new mechanism for en echelon vein formation in rock which is distinctly different than previously proposed mechanisms it is suggested that some macroscopic en echelon vein arrays may have served as zones of weakness that localized later shear zone development in a manner analogous to that observed by experimentalists examining micro cracking and subsequent shear rupture of rocks loaded under compression sigmoidally shaped veins and vein fillings are explicitly modeled showing that they can form in response to the mechanical interaction of neighboring fractures which redirects the propagation path numerical comparison of sigmoidal veins formed by brittle fracture and by ductile shear zones demonstrates some of the pitfalls of failing to correctly identify the mechanism of formation

sensitivity and inversion of borehole flexural dispersions for formation parameters propagation characteristics of borehole flexural waves are functions of five borehole fluid and formation parameters uncertainties in any of these model parameters may introduce errors in the estimate of formation shear wave speed other sources of error in the estimate may be caused by deviations from the assumed circular borehole cross section or heterogeneity in the material properties of formation in the vicinity of a borehole from the usually assumed homogeneous properties the influence of uncertainties in model parameters on borehole flexural dispersion has been calculated from a general perturbation model based on hamilton s principle a sensitivity analysis of the flexural dispersion to small variations in the model parameters shows that the formation shear speed has by far the most dominant influence in a slow formation in contrast the flexural dispersion in a fast formation is significantly influenced by three of the five model parameters the formation shear speed the borehole fluid compressional speed and the borehole diameter the frequency dependence of these sensitivity functions indicates that the inversion of flexural dispersion for formation shear speed is optimal in the range 2 4 khz for a borehole of diameter 25 4 cm the range of validity of the perturbation model has been estimated by comparing results of concentric annuli of different thicknesses and shear wave speeds different from that of the formation with those from exact numerical solutions from a modal search program generally the perturbation results for altered annuli of thicknesses up to 15 cm are accurate to within 1 per cent for a shear wave speed 10 per cent lower than that of the formation this difference between the perturbation and exact results increases to approximately 1 to 3 per cent for a shear wave speed 20 per cent lower than that of the formation

origin of illite in the lower paleozoic of the illinois basin evidence for brine migrations in the lower paleozoic of the illinois basin three illite polytypes are found 2m 1 of detrital origin and 1m d and 1m of diagenetic origin illite polytype quantification of detrital 2m 1 illite and diagenetic 1m d and 1m illite combined with k ar age dating allows extrapolation to apparent detrital and diagenetic illite ages kinetic modeling of smectite illitization combined with the calculated age of illitization can evaluate different origins of illite the diagenetic illite in the lower paleozoic of the illinois basin is interpreted not to have formed solely by burial diagenesis but mainly during multiple brine events the upper ordovician maquoketa group contains diagenetic illite dominantly 1m d with minor 1m with an extrapolated age of 360 m y 356 377 m y and formed from smectite at temperatures of 50 100 c this age falls within the span of dates for illite smectite i s in k bentonites from the upper mississippi valley and is interpreted to be a combined result of illitization by burial diagenesis and either a hydrothermal brine from the southern and deeper part of the basin or a k rich brine from the michigan basin upper mississippi valley area or forest city basin in ordovician and cambrian shale partings and sandstone older than the maquoketa group the diagenetic illite 1m d in shale and 1m in sandstone has an age of 300 m y and formed at temperatures 140 c this late paleozoic age falls within the range of illites from sandstone in the upper mississippi valley and k bentonites of the appalachian basin it coincides with the alleghany orogeny and is interpreted as having formed by gravity driven flow from the uplifted alleghanian ouachita orogenic belt that drove hot 140 c fluids through the illinois basin

geometry and development of relay ramps in normal fault systems relay ramps occur between normal fault segments that overstep in map view the geometry and evolution of exposure scale relay ramps are described from the somerset coast england and are compared with larger scale ramps from elsewhere relay ramps can be classified into four groups based on the degree of interaction and linkage between the overstepping segments these groups are interpreted as being evolutionary stages in stage 1 the segments do not interact stage 2 involves the reorientation of bedding between two interacting faults to produce a relay ramp in stage 3 connecting fractures start to break the relay ramp stage 4 is when the relay ramp is destroyed to produce a single fault that has an along strike bend these evolutionary stages can develop through ti e but they can also be seen spatially a branch line between normal faults or an along strike bend may represent a stage 4 relay with progressively earlier stages occurring updip or downdip characteristic variability in displacement distance profiles for fault segments and linked faults accompanies the interaction and linkage processes displacement transfer by relay ramps is accompanied by steep displacement gradients along fault segments at oversteps relay ramps often contribute to a minimum in total fault displacement at a linkage point

geochemical imprint of depositional conditions on organic matter in laminated bioturbated interbeds from fine grained marine sequences laminated organic rich shales are interbedded at a scale of centimeters to a few meters with bioturbated organic poor mudstones or limestones in some fine grained marine sequences we have analyzed the organic matter in pairs of laminated bioturbated interbeds from cretaceous and devonian rocks deposited in epicontinental and oceanic settings for the purpose of studying the influence of depositional and early diagenetic environment on the organic geochemical properties of marine shales results of these analyses indicate that for rocks that are still in a diagenetic stage of thermal alteration the relative abundance of biomarker compounds and specific biomarker indices can be useful indicators of depositional and early diagenetic conditions pristane phytane ratios are generally highest for laminated rocks from epicontinental basins and appear to reflect the input of isoprenoid precursors more than oxygenated versus anoxic depositional conditions the thermally immature laminated rocks are characterized by relatively high contents of 17 h 21 h hopanes hopenes sterenes and diasterenes and by strong predominance of the 22r over 22s homohopane isomers thermally immature bioturbated samples are characterized by absence of the hopanes by low contents of both saturated and unsaturated polycyclic hydrocarbons and by slight or no predominance of the 22r over 22s homohopane isomers there are less obvious compositional differences between the saturated hydrocarbons in the laminated and bioturbated units from the thermally mature sequences for both the thermally mature and immature laminated samples the degree of isomerization at the 22c position for hopanes and at the 20c position for steranes is generally consistent with the degree of thermal maturity interpreted from other properties of the organic matter the bioturbated samples however exhibit inconsistent and anomalously high degrees of isomerization for the homohopanes resulting either from reworking and oxidation of the primary organic matter or from the presence of older recycled organic matter

recurrent depletion of benthic oxygen with 4th order transgressive maxima in the cretaceous western interior seaway abstract a study was designed to investigate the relationship between depletion of benthic oxygen and 4th order sea level change within the cretaceous western interior seaway of north america three 4th order sea level change cycles were studied in strata belonging to the tropic shale and the tununk shale member of the mancos shale in utah these sedimentary deposits accumulated within prodeltaic environments in utah strata belonging to all three 4th order cycles exhibit evidence for depletion of benthic oxygen at times of peak 4th order transgression and development of condensed sections the evidence includes decreased intensities of bioturbation increased cr al ni al and zn al ratios and decreased mn fe ratios the data also suggest that depletion of benthic oxygen in the distal section preceeded that in the more proximal one and thus may reflect episodic expansion of an oxygen depleted water mass es from the pelagic to the prodeltaic environment of the seaway evidence for subsequent increases in benthic oxygen level with the onset of 4th order regressions is interpreted to reflect either aggradation of the seafloor above the oxygen poor water due to sediment accumulation or contraction of the oxygen depleted water mass from the prodeltaic part of the seaway or a combination of the two s organic c ratios of strata interpreted to have been deposited under conditions of depleted benthic oxygen are distinctly below 0 4 and thus were not useful for recognition of paleo oxygen levels in the prodeltaic environment this study highlights the importance of depletion of benthic oxygen as a mechanism for enhancing preservation of organic carbon in condensed sections on a hundred thousand year time scale our results underline the potential for source rock formation in similar prodeltaic deposits of other epicontinental seaways during periods of relatively high sea level and global depletion of benthic oxygen

late cainozoic rotation of the peruvian western cordillera and the uplift of the central andes abstract a palaeomagnetic study was carried out in the region of lima peru on 55 sites 564 cores of middle cretaceous to late miocene age distributed along three northeast southwest transverses which cross the western cordillera of the central andes over the entire studied area the results consistently yield westerly declinations corresponding to post cretaceous and post eocene counterclockwise rotations with respect to stable south america of 16 6 and 13 7 8 1 respectively when compared to the ill defined reference poles of this continent these new results are in good agreement with previous ones from the central peruvian andes and support the model recently proposed for the uplift of the andean cordillera

miocene volcanism and deformation in the western cordillera and high plateaus of south central peru new radiometric ages on tuffs from south central peru support the postulated flare up of volcanic activity during early miocene time in the region of huancavelica julcani and lircay lower miocene rocks lie on folded strata of pre cenozoic age the absence of units of eocene and early oligocene age indicates that this area remained positive after incaic deformation in eocene time conglomerate beds reflecting erosion attendant on the first pulse of late cenozoic quechuan compressive deformation do not appear in the stratigraphic record until after 21 5 m y b p in one section beds of coarse conglomerate are underlain by tuff dated at 18 3 0 6 m y and overlain by tuff dated at 17 3 0 2 m y at another locality beds of conglomerate conformably overlie tuff dated at 19 6 0 8 m y if we incorporate published data from other areas in central and southern peru it appears that quechuan deformation began in early miocene time between 19 5 and 17 m y ago a firm age for the end of the first pulse of quechuan deformation in this region is provided by an ash flow sheet dated at 12 to 12 5 m y that unconformably overlies strata of eocene to early miocene age

undulatory silver rich polymetallic veins of the castrovirreyna district central peru fault growth and mineralization in a perturbed local stress field strongly undulatory silver veins of the castrovirreyna district central peru formed in a dilatant anastamosing low displacement left lateral fault system during late miocene east west directed compression the steeply dipping veins have multiple orders of undulations as seen in map view and underground mines expose strong dip reversals kinematic indicators from the east northeast to west northwest striking veins record mainly left lateral slip and lesser oblique reverse slip these striation data considered along with vein intersection angles and map patterns indicate that the veins did not form as a simple conjugate fracture set the preferred interpretation is that the curved fracture growth and complex slip distributions developed because of local stress field modifications by the propagating faults and dilating vein segments during regional east west directed compression variably oriented fault striations compound displacement profiles and multiple scales of vein curvature are all characteristics of stress interaction between subparallel veins veins possessing these features may be expected to host oreshoots with a complex geometry that depends not only on local accommodation space produced at dilant jogs but also on the proximity to other veins and changing slip directions

petrology and hydrocarbon potential of microalgal and macroalgal dominated oil shales from the eocene huadian formation ne china abstract a series of oil shales of middle to late eocene age occur in the huadian area of jinlin province northeast china analytical results from 22 rock samples collected from an underground mine in the guanglangtou district in the huadian basin show that the total organic carbon toc contents of the oil shale layers are very high 10 6 wt to 39 6 wt with vitrinite reflectance ro values of 0 41 0 02 and rock eval hydrogen index hi values up to 887 biomarker analyses show a high proportion of hopenes and hopanes with stereochemistry indicative of low maturity detailed organic petrologic analysis demonstrates that the organic matter is dominated by lamalginite and telalginite the presence of botryococcus in the telalginite indicates that the depositional environment of the oil shale is lacustrine this is consistent with low si and s contents and high c s ratios fragments from a newly identified benthic macroalgal rhodophyte were found in some thin layers raising the possibility that the depositional environment of some oil shale layers may have been influenced by marine flooding since rhodophyta are uncommon in freshwater systems two oil shale samples with different algal compositions were selected for assessment of hydrocarbon generation potential sample hd 21 is dominated by phytoplanktic lamalginite with a high hi value of 869 mg hc g toc while sample hd 20 has a lower hi value 490 mg hc g toc and contains high contents of the benthic rhodophyte as well as some phytoplanktic lamalginite hi values and thermal simulation experiments showed that the phytoplankton dominated shale hd 21 has a much higher hydrocarbon generation potential than the rhodophyte dominated oil shale hd 20 even though the toc content of hd 20 was much higher indicating that the benthic macroalgal macerals have a much lower capacity to generate oil when thermally mature

dipole dispersion crossover and sonic logs in a limestone reservoir analyses of sonic logs in a horizontal well provide new information about mechanical properties of rocks made possible by recent developments in our understanding of acoustic wave propagation in prestressed formations most sections of this horizontal well exhibit azimuthal shear isotropy indicating isotropic stresses in the plane perpendicular to the well trajectory leading to stable wellbore conditions however two sections show dipole dispersion crossovers that confirm the presence of stress induced shear anisotropy caused by a difference between the maximum and minimum stresses in the plane perpendicular to the well trajectory the two dipole dispersions are obtained by processing the recorded waveforms by a modified matrix pencil algorithm the fast shear direction is estimated from alford rotation of the cross dipole waveforms one section of the well exhibits the fast shear direction parallel to the overburden stress as the maximum stress direction whereas the other section has the fast shear direction parallel to the horizontal stress that is larger than the overburden stress the cause of this change in the fast shear direction is believed to be the well s penetration into a 3 ft thick bed with lower porosity and permeability and significantly higher elastic stiffnesses than those in the other part of the homogeneous high permeability reservoir a stiff bed is likely to have greater stresses in its plane than perpendicular to it which would make the horizontal stresses greater than the vertical

depletion of 13c in cretaceous marine organic matter source diagenetic or environmental sigal geochemical studies of cretaceous strata rich in organic carbon oc from deep sea drilling project dsdp sites and several land sections reveal several consistent relationships among amount of oc hydrocarbon generating potential of kerogen measured by pyrolysis as the hydrogen index hi and the isotopic composition of the oc first there is a positive correlation between hi and oc in strata that contain more than about 1 oc second percent oc and hi often are negatively correlated with carbon isotopic composition 13c of kerogen the relationship between hi and oc indicates that as the amount of organic matter increases this organic matter tends to be more lipid rich reflecting the marine source of the organic matter cretaceous samples that contain predominantly marine organic matter tend to be isotopically lighter than those that contain predominantly terrestrial organic matter average 13c values for organic matter from most cretaceous sites are between 26 and 28 and values heavier than about 25 occur at very few sites most of the 13c values of miocene to holocene oc rich strata and modern marine plankton are between 16 to 23 values of 13c of modern terrestrial organic matter are mostly between 23 and 33 the depletion of terrestial oc in 13c relative to marine planktonic oc is the basis for numerous statements in the literature that isotopically light cretaceous organic matter is of terrestrial origin even though other organic geochemical and or optical indicators show that the organic matter is mainly of marine origin a difference of about 5 in 13c between modern and cretaceous oc rich marine strata suggests either that cretaceous marine planktonic organic matter had the same isotopic signature as modern marine plankton and that signature has been changed by diagenesis or that oc derived from cretaceous marine plankton was isotopically lighter by about 5 relative to modern plankton oc diagenesis does not produce a significant shift in 13c in miocene to holocene sediments and therefore probably did not produce the isotopically light cretaceous oc this means that cretaceous marine plankton must have had 13c values that were about 5 lighter than modern marine plankton and at least several per mil lighter than cretaceous terrestrial vegetation the reason for these lighter values however is not obvious it has been proposed that concentrations of co2 were higher during the middle cretaceous and this more available co2 may be responsible for the lighter 13c values of cretaceous marine organic matter

sedimentary and organic geochemical investigation of tertiary lacustrine oil shale in the central tibetan plateau palaeolimnological and palaeoclimatic significances article i nfo the zhuonai lake and lunpori oil shale located in the central tibetan plateau were deposited in a tertiary lacustrine environment in the hoh xil and lunpola basins respectively a combined investigation of sedimentary organic geochemistry and organic carbon isotope was performed to define source rock potential the palaeoenvironment and source of the organic matter three types of facies sequences are developed in the zhuonai lake section and oil shale mainly occurs in the oil shale marl sequence the oil shale silty claystone and oil shale marl facies sequences are developed in the lunpori section both oil shales are finely stratified especially for the lunpori oil shale with thickness of the laminae about 10 m which contains abundant pyrite crystals and fossilized fish bones both the zhuonai lake and lunpori oil shale contain high values of total organic matter toc s2 and hydrogen index hi indicating the oil shales have good oil source rock potential tmax values show an immature stage of organic matter for the zhuonai lake oil shale and thermally marginally mature for the lunpori oil shale because of difference in their burial history lunpori oil shale shows the characteristics of amorphous organic matter aom dominance odd over even predominance maximum n alkanes peak at nc25 or nc23 higher proportion of c29 sterane light 13 corg values 30 0 to 27 4 high concentrations of phytane pr ph ratio ranging from 0 02 to 0 18 high values of gammacerane index up to 25 26 and presence of carotane which are consistent with a reducing stratified and hypersaline palaeo lake with main contribution of algae and bacteria to the organic matter om oil shale samples from the lower and upper part of the zhuonai lake section are characterized by low h c ratios and high o c ratios amorphous humic om and vitrinite dominance odd over even predominance with long chain n alkanes dominance heavy organic carbon isotope and c29nc27 sterane distribution in agreement with a main contribution of land plant the heavy organic carbon isotope suggests the expanding of c4 land plant during the early miocene while the most samples show characteristics of aom dominance maximum peak at nc29 or nc31 and high c27 sterane concentration consistent with a main contribution from algae to the om the pr ph ratio cannot be applied as a measure for redox conditions in the section because of maturity effects biomarker analysis indicates that the salinity during the deposition of the zhuonai lake oil shale was not enhanced the oligocene lunpori oil shale and the early miocene zhuonai lake oil shale recorded the variation of palaeolimnological water chemistry the development history of the palaeo lakes from the oligocene saline to the early miocene fresh water reflects the variation of palaeoclimate from dry to humid in the central tibetan plateau

guidelines for evaluating petroleum source rock using programmed pyrolysis rock eval pyrolysis is used to rapidly evaluate the petroleum generative potential and thermal maturity of rocks accurate conclusions require programs every 30 60 ft 9 18 m understanding of interpretive pitfalls and supporting data such as visual kerogen vitrinite reflectance and elemental analyses the generative potential of coals is commonly overestimated by pyrolysis and is best determined by elemental analysis and organic petrography most coals show high s2 s3 5 and low hi values 300 mg hc g toc migrated oil and mud additives which alter rock eval data can sometimes be removed by special processing for immature rocks bimodal s2 peaks and pi values over 0 2 indicate contamination pyrolysis downgrades organic poor clay rich rocks which show lower hi and higher tmax values than isolated kerogen because of adsorption of pyrolyzate on the clays tmax values for small s2 peaks 0 2 mg hc g toc are unreliable tmax is affected by maturation organic matter type contamination and the mineral matrix s3 is sensitive to inorganic and adsorbed carbon dioxide and to instrumentation problems acidification of carbonate rich samples and proper maintenance improves s3 measurement constant sample weights 100 mg are recommended below a threshold weight tmax increases by up to 10 c and other parameters decrease organic rich samples which overload the detector can be diluted with carbonate detector linearity is determined by pyrolyzing various weights of an organic rich rock

hydrocarbon biomarkers from ordovician sediments and the fossil alga gloeocapsomorpha prisca zalessky 1917 numerous ordovician oils worldwide are known to show unusual and distinctive distributions of hydrocarbons which it has been suggested are derived from a unique benthonic mat forming non photosynthetic prokaryotic organism gloeocapsomorpha prisca zalessky 1917 which is the major contributor of organic matter organic matter rich sediments from the canning and amadeus basins of australia known to contain g prisca fossils and to have the characteristic predominance of odd carbon number c13 c19 n alkanes were investigated to determine other hydrocarbon distributions sediments from both basins contained abundant n alkylcyclohexanes with odd carbon number predominance c13 c19 and methyl n alkylcyclohexanes c14 c20 which in immature sediments showed an even carbon number predominance the isomer distribution of these latter compounds was determined by direct comparison with synthetic standards the sediments from both basins also contained very similar distributions of steranes and pentacyclic triterpanes and the derived kerogens had a characteristically light carbon isotope signature pyrolysis of a g prisca rich kerogen yielded a hydrocarbon mixture with a similar composition to the sediment extract except that there was a marked increase in the relative abundance of pristane phytane alkylcyclohexanes steranes and hopanes relative to n alkanes we argue on the basis of the geological geochemical and palynological data that g prisca was probably planktonic photosynthetic and very possibly eukaryotic and that the striking character of ordovician oils and sediments derive from bacterial and other diagenetic imprints superimposed on the primary signature of this organism

an isotopic study of biogeochemical relationships between carbonates and organic carbon in the greenhorn formation carbon isotopic compositions of total carbonate inoceramid carbonate micritic carbonate secondary cements total organic carbon and geoporphyrins have been measured in 76 different beds within a 17 m interval of a core through the greenhorn formation an interbedded limestone and calcareous shale unit of cretaceous age cenomanian turonian from the western interior seaway of north america results are considered in terms of variations in the processes of primary production which led to the biosynthesis of the molecular precursors of the geoporphyrins and in secondary processes those mediating the transformation of primary organic material into sedimentary total organic carbon it is shown that the porphyrin isotopic record reflects primary isotopic variations more closely than the toc isotopic record and that in these sediments toc is enriched in 13c relative to its primary precursor by 0 6 to 2 8 this enrichment is attributed to isotope effects within the consumer foodweb and is associated with respiratory heterotrophy variations in this secondary enrichment are correlated with variations in the isotopic composition of marine carbonate this correlation is attributed to effects of environmental changes on the marine foodweb these may have included increased atmospheric oxygen associated with the cenomanian turonian oceanic anoxic event the isotopic fractionation associated with fixation of carbon by primary producers is observed to have varied by 1 5 during the interval of deposition it is suggested that this change is due to a variation in the makeup of the community of primary producers and or to a decrease in the atmospheric abundance of co2 during the oceanic anoxic event

dolomitization and dolomite neomorphism trenton and black river limestones middle ordovician northern indiana u s a abstract the trenton and black river limestones are dolomitized extensively along the axis of the kankakee arch in indiana with the proportion of dolomite decreasing to the south and southeast of the arch planar and nonplanar dolomite replacement textures and rhombic type 1 and saddle type 2 void filling dolomite cements are present three stages of dolomitization involving different fluids are inferred on the basis of petrographic and geochemical characteristics of the dolomites nonferroan planar dolomite has relatively high 18o values 1 8 to 6 1o oo pdb and has 87sr 86sr ratios 0 70833 to 0 70856 that overlap those of middle ordovician seawater petrography geochemistry and the geometry of the dolomitized body suggest that the planar dolomite was formed in middle and late ordovician seawater during the deposition of the overlying maquoketa shale ferroan planar and nonplanar dolomite occurs in the upper few meters of the trenton limestone confined to areas underlain by planar dolomite this dolomite contains patches of nonferroan dolomite with cathodoluminescence cl characteristics similar to underlying planar dolomite ferroan dolomite has relatively low 18o values 5 1 to 7 3o oo pdb and has slightly radiogenic 87sr 86sr ratios 0 70915 to 0 70969 similar to those obtained for the overlying maquoketa shale these data indicate that ferroan dolomite formed by neomorphism of nonferroan planar dolomite as fluids were expelled from the overlying maquoketa shale during burial the absence of ferroan dolomite at the trenton maquoketa contact in areas where the earlier formed nonferroan planar dolomite also is absent indicates that the fluid expelled from the overlying shale did not contain enough mg2 to dolomitize limestone type 1 dolomite cement has isotopic compositions similar to those of the ferroan dolomite suggesting that it also formed from shale derived burial fluids cl growth zoning patterns in these cements suggest that diagenetic fluids moved stratigraphically downward and toward the southeast along the axis of the kankakee arch type 2 saddle dolomite cements precipitated late their low 18o values 6 0 to 7 0o oo pdb are similar to those of the type 1 dolomite cement however fluid inclusion data indicate that the saddle dolomite was precipitated from more saline basinal fluids and at higher temperatures 94 to 143 c than the type 1 cements 80 to 104 c a trend of decreasing fluid inclusion homogenization temperatures and salinities from the michigan basin to the axis of kankakee arch suggests that these fluids emerged from the michigan basin after precipitation of type 1 cement

modeling of low frequency stoneley wave propagation in an irregular borehole a fast modeling method is formulated for low frequency stoneley wave propagation in an irregular borehole this fast modeling method provides synthetic waveforms which include the effects of two borehole irregularities diameter changes washout and formation property changes the essential physics of the low frequency stoneley waves are captured with a simple 1 d model a mass balance boundary condition and a propagator matrix are used to express stoneley wave interactions with the borehole irregularities the accuracy of the proposed method was confirmed through comparison with existing finite difference and boundary integral modeling methods that yielded cross correlations greater than 0 98 comparison of synthetic records calculated for an actual borehole with field records showed qualitative agreement in the major reflections because of the washout zones but showed some disagreements in the reflections caused by the fractures since the synthetic records include only information relating to the borehole geometry and the elastic properties of formation the reflection caused by the fracture will appear only in the field record these results suggest the possibility of distinguishing stoneley wave reflections caused by fractures from those caused by borehole irregularities further the fast computational speed of this method over 300 times faster than either boundary integral or finite difference methods makes it quite suitable for field application

the importance of a small wall deformation in the three dimensional acoustic logging results summary a variety of seismic techniques use fluid filled boreholes to host detectors and seismic sources the complexity of the wavefield generated depends on the position of the receivers and the source frequency excited type of formation and type of source in the present paper the boundary element method is used to study the wavefield elicited by monopole or dipole sources within a fluid filled cylindrical cavity drilled through an unbounded elastic formation this model is used to assess the effects of the existence of a small localized wall deformation on the response such a deformation may occur through the mechanical action of the drill string rock failure adjacent to a drilled borehole plastic deformation and washing out of the borehole in soft or poorly consolidated rocks in addition the effect of a source receiver tool placed in an off centre and or tilted position inside the fluid filled borehole on the propagation of both axisymmetric and non axisymmetric wave modes are studied

lacustrine environmental evolution and implications on source rock deposition in the upper cretaceous paleocene of the south yellow sea basin offshore eastern china abstract lacustrine source rocks are well developed within the upper cretaceous paleocene in the south yellow sea basin sysb in offshore eastern china understanding the deposition of these source rocks could provide critical information for hydrocarbon exploration in the sysb as well as reveal the paleoenvironment across the cretaceous paleogene boundary in eastern china this study investigates the lacustrine environmental evolution during the upper cretaceous taizhou formation k2t to paleocene funing formation e1f within the sysb and its implications on source rock deposition based on core observation combined with organic geochemical indicators multiple maturity parameters vitrinite reflectance tmax and biomarker isomerization ratios reveal that the organic matter om has reached early oil window maturity the om of source rocks is dominated by kerogen type ii iii liptinite and vitrinite anoxic saline brackish deep to semi deep lacustrine environments prevailed during the deposition of k2t2 second member of the k2t e1f2 second member of the e1f and e1f4l lower unit of fourth member of the e1f the deposition of three intervals was influenced by marine incursion as evidenced by geochemical and paleontological results low to moderate toc contents avg 0 78 wt to 1 91 wt which corresponds to low to moderate terrestrial om input are observed through three intervals abundant freshwater together with high terrestrial om flowed into the lacustrine environments during the deposition of e1f4u upper unit of fourth member of the e1f leading to dysoxic oxic and freshwater conditions high terrestrial om input is probably the major controlling factor for relatively high toc contents avg 2 49 in the e1f4u an evolution from balanced fill to overfilled lake type is observed in the upper cretaceous paleocene of sysb consistent with an evolution from semi arid to humid paleoclimate conditions the e1f4u and e1f2 source rocks exhibit fair to good potential whereas poor to fair potential is present in the e1f4l and k2t2 source rocks compared with the paleogene source rocks in onshore eastern china the upper cretaceous paleocene source rocks in the sysb probably hold good potential for generating commercial quantity of oil although further study on hydrocarbon generation kinetics of source rocks and overall basin history are needed

simulation and interpretation of sonic waveforms in high angle and horizontal wells abstractwe have developed a new method to process and interpret sonic waveforms acquired in high angle ha and horizontal hz wells in the vicinity of layer boundaries numerical simulations are examined for ha hz fluid filled borehole models in which sonic tools with monopole and dipole sources operate across a horizontal bed boundary in hard and soft formations simulations are performed with a 3d time domain finite difference method assuming a sonic logging tool in which each receiver station consists of eight azimuthal receivers instead of conventional processing for monopole and dipole waveforms which takes the average and difference respectively of waveforms acquired with azimuthal receivers we analyze the waveforms acquired individually by each azimuthal receiver for dipole sources we refer to the dispersive signals recorded by individual receivers as pseudo flexural waves and we first process them with a weighted spectral semblance method to obtain the frequency dispersion curves we t

pgm facies variations for cu pge deposits in the coldwell alkaline complex ontario canada abstract accurate characterization of the platinum group mineral pgm assemblages for cu ni pge deposits are typically constrained by sample size and the difficulty of finding statistically significant numbers of grains which is expected given the low concentrations of platinum group elements 75 m can account for large fractions of total mass despite these limitations an accurate survey of pgm from different deposit types would have significant value towards developing deposit models and respective exploration strategies in this study we present results for a comprehensive evaluation of pgm at four copper pge occurrences hosted within separate but co genetic gabbro or troctolite intrusions in the coldwell alkaline complex and confirm that accurate surveys are possible with sufficient sample material and efficient pgm concentration methods the pgm concentration methods used include 1 hydroseparation of sieved size fractions of pulverized material and 2 panning of grain separates produced by electric pulse disaggregation of drill core specimens a favourable comparison of the results has verified the reliability of each method and added confidence that the pgm assemblages identified at three of the four locations are fully characterized precious metal mineral pmm assemblages are determined for the main zone and w horizon at the marathon deposit and the main zones at each of the geordie lake deposit and area 41 occurrence a total of 10 824 pmm grains pge and au ag and 68 mineral species including 16 unknown minerals were identified of which 768 grains and 31 species occur at the main zone 523 grains and 41 species at area 41 9485 grains and 43 species at w horizon and 56 grains and 12 species at geordie lake the pmm are grouped as follows pd ge pge s as pt fe alloy pd cu pb au pd ni s pd pt sn pt as pd as pd pt sb as pd pt bi te and au ag all of the deposits were found to contain similar proportions of pd pt sb as pd pt bi te and au ag minerals but the w horizon and area 41 are distinguished from the marathon main zone and geordie lake deposits by the presence of minerals in the pge s as pt fe alloy pd cu pb au and pd ge groups taken together the pmm assemblages for deposits in the coldwell exhibit a strong correlation to pge enrichment relative to the range for mantle cu pd values 1000 10 000 and there is no relationship between the abundances of pd pt bi te and pd pt sb as minerals that are commonly associated with hydrous phases and the intensity of hydrothermal alteration thus minerals found only at the w horizon and area 41 where significant pge upgrading has occurred including pt fe alloys rustenburgite marathonite palladogermanide unknown rh ni fe sulfide au pd cu alloy braggite coldwellite laurite zvyagintsevite laflammeite and unknown phases pd 5 as 2 pd 3 as pd 3 as pb bi might be considered as index minerals for pge enriched types of mineralization in the coldwell

cenozoic stratigraphy magmatic activity compressive deformation and uplift in northern peru integrated field study and radiometric dating in the llama bambamarca area of the western cordillera of northern peru have resulted in major revisions in the stratigraphic and structural relations and ages of thick sequences of tertiary volcanic rocks and constrain the timing of compressive deformation magmatic activity and uplift rocks of the upper part of the llama formation yield k ar mineral dates of 54 8 1 8 and 44 2 1 2 ma altered volcanic rocks composing the lower part of the formation in the western part of the area may be older eocene k ar dates have been reported for several granitic plutons in the region and other plutons of probable similar age intrude the llama formation and are overlain by upper eocene strata the pervasive alteration that affects much of the llama formation may be the result of large hydrothermal systems driven by a coeval and presumably cogenetic batholith the chota formation which is mostly volcaniclastic has yielded ages of about 50 and 44 ma the chota formation thus does not predate the llama formation but rather is an eastern distal facies equivalent of rhyolitic ash flow sheets and dacitic volcanic rocks of the llama formation the major unconformity at the base of the llama formation reflects deformation during late cretaceous peruvian tectonic phase and or paleocene incaic i time the time of incaic ii deformation is bracketed between the 44 2 1 2 ma date on the llama formation and the age of about 39 ma of a thick ash flow sheet that comprises the lower part of the unconformably overlying huambos formation dates on rocks postdating incaic ii tectonism in central peru suggest that deformation had ceased by about 41 ma incaic ii tectonism in northern and central peru appears to have been a short but intense compressive event that peaked about 43 m y ago the incaic ii event is coeval with formation of the hawaiian emperor bend and with a period of rapid convergence of the nazca and south american plates and therefore may reflect a major plate change in lithospheric plate movement patterns a largely volcaniclastic sequence of early miocene age shows that the early neogene pulse of volcanic activity recognized throughout the central andes is represented in northern peru beds of conglomerate within this sequence are probably the result of quechua i tectonism although the 23 2 1 5 ma date obtained on tuff from the unit appears somewhat older than the time of about 19 ma recognized for quechua i tectonism in central and southern peru units of partly welded and unaltered ash flow tuff that fill deeply incised paleovalleys have ages of about 8 2 and 11 4 ma showing that the western cordillera of northern peru was uplifted before late miocene time

geochemical indicators of depositional environment and source rock potential for the upper ordovician maquoketa group illinois basin two depositional cycles are recognized within the upper ordovician maquoketa group of the illinois basin in a core from the new jersey zinc 1 parrish well of fulton county illinois organic carbon corg total sulfur 13c content of the organic carbon dgr13corg hydrogen and oxygen indices hi and oi from rock eval pyrolysis and yields of extractable organic matter eom vary through the cycles dark brown to black laminated shales are present in the lower portion of each cycle and have high values of corg 1 0 3 0 hi 500 1000 mg hydrocarbon hc g total organic carbon toc and eom 500 2500 ppm and more negative dgr13corg values dgr13corg 30 to 30 5 mil gray to greenish gray bioturbated shales are present in the upper portion of each cycle and have low values of corg 1 0 hi 500 mg hc g toc and eom 500 ppm and more positive dgr13corg values 28 5 to 29 5 pmil compared to the laminated shales although thermally immature to marginally mature in this core the laminated shales represent potential source rocks for petroleum because they contain good to excellent quantities of oil prone organic matter and are more deeply buried in other areas of the basin kerogen elemental data and rock eval data suggest that the source of organic matter in the maquoketa was uniform with the notable exception of graptolite rich layers distributions of saturated hydrocarbons for maquoketa samples resemble those derived from amorphous organic matter the organic walled microfossil gloeocapsomorpha prisca was not identified in kerogens of the maquoketa shale and its absence is most likely due to an unfavorable depositional environment and or climate variations in bulk geochemical data and carbon isotopic compositions within the maquoketa group indicate substantial reworking and degradation of organic matter associated with bioturbation and oxygenated depositional conditions organic rich shale was deposited in an environment where circulation of the water column was sluggish and dissolved oxygen concentrations were low resulting in the preservation of organic matter organic poor shale was deposited in an environment where circulation of the water column was strong dissolved oxygen concentrations were high and burrowing organisms thoroughly reworked the sediment resulting in the degradation of organic matter a maximum difference in carbon isotopic ratios of 2 pmil between organic rich and organic poor units of the maquoketa roup suggests that heterotrophic reworking of organic matter was important in the depositional environment the carbon isotopic composition and h c ratio of kerogen for graptolite rich shale suggests that organic productivity may have played an intermittent role in the accumulation of organic matter in the maquoketa shale

theoretical analysis of cross joint geometries and their classification one commonly observed bedrock joint pattern consists of cross joints that terminate against an older set of longer systematic joints dyer 1988 showed that unique cross joint geometries may develop in response to a combination of remote stresses and local perturbations of the stress field in the vicinity of preexisting joints we expand upon dyer s analysis by providing the general solutions for cross joint geometry as a function of remote principal stress orientations and ratios specific cross joint geometries are determined by solving for the local cross joint angle as a function of distance from the preexisting joint on the basis of theoretical derivations cross joint geometries are grouped into five main categories curving parallel curving perpendicular quasi curving parallel quasi curving perpendicular and noncurving geometries furthermore by discussing the stress state in the vicinity of the preexisting joint a general expression for the dimension of compressive zone is derived and a more detailed classification of cross joint geometry is provided results from this study provide a method to estimate the relative magnitudes of remote principal stresses simply based on the observed cross joint geometry which may help constrain the tectonic history of a region in addition knowledge of precise cross joint curvature and intersection geometries may provide important insights into their connectivity and fluid flow characteristics

borehole wave propagation in three dimensions in this paper the three dimensional finite difference method is used to simulate borehole wave propagations in an isotropic as well as an anisotropic formation the finite difference results agree excellently with the analytic solutions of a point force source in the transversely isotropic medium the finite difference synthetics are also in very good agreement with the discrete wave number solutions for fluid filled borehole wave propagation the finite difference synthetics are compared with ultrasonic lab measurements in a scaled borehole model the borehole is drilled along the x axis in an orthorhombic phenolite solid both monopole and dipole logs agree well the observations of the shear wave splitting in the dipole logs are confirmed by the finite difference simulations the 3 d finite difference method is applied to the fluid filled borehole wave propagation in the tilted isotropic formation and in the orthorhombic phenolite formation in a borehole drilled along the z axis in a phenolite formati

a numerical simulation of the acoustic and elastic wavefields radiated by a source on a fluid filled borehole embedded in a layered medium the authors present a method of calculation to simulation the propagation of acoustic and elastic waves generated by a borehole source embedded in a layered medium the method is formulated as a boundary element technique where the green s functions are calculated by the discrete wavenumber method the restrictive assumptions are that the borehole is cylindrical and that its axis runs normal to the layer interfaces the physics of the method rely on huygens s principle that states that a diffracting boundary the borehole wall in the present case can be represented as a distribution of secondary sources the borehole is discretized into small cylindrical elements and each element is represented by three sources a volume source representing the wavefield diffracted in the fluid and two surface forces that give rise to the elastic wavefield radiated outside the borehole the strength of each source is obtained by solving the linear system of equations that describes the boundary conditions at the borehole wall the method is used to generate synthetic acoustic log s and to investigate the wavefield radiated into the formation the simulations considered display the stoneley wave reflections at the bed boundaries and show the importance of the diffraction that takes place wheremore the borehole wall intersects the layer interfaces less

dynamic fracturing field and experimental observations abstract we analyzed a system of complex joints in thick dolomite layers that are exposed within the western margins of the dead sea basin these joints display two dominant features tree like branching and a gradual increase of density that leads to local fragmentation the development of this joint system is investigated in laboratory experiments with samples of brittle ductile layered composites the samples were subjected to layer parallel extension and displayed three styles of fracturing planar fractures known from previous tests branching fractures and clustering fractures observed here for the first time in layered composites based on fracture morphology we deduced that the branching and clustering fractures in the experiments and the tree like closely spaced joints in the field propagated at dynamic high velocity growth rates it is proposed that the morphological features described here could be used as field criteria to recognize dynamic rates of rock fracturing

borehole flexural modes in anisotropic formations a perturbation method of solution is an efficient way of analyzing elastic wave propagation along a borehole in anisotropic formations the perturbation model allows us to calculate changes in the modal dispersion curves caused by the differences in elastic constants between the anisotropic formation of interest and a reference or unperturbed isotropic formation the equivalent isotropic constants in the reference formation are obtained from the appropriate compressional and shear wave velocities for the selected propagation and polarization directions of the flexural mode this choice of the unperturbed solution means that the required perturbation is minimal resulting in enhanced accuracy of the perturbed solution computational results are presented for the dispersion curves of borehole flexural waves in a transversely isotropic ti formation as a function of borehole deviation from the ti symmetry axis in addition radial distributions of displacement and stress fields associated with the flexura

time dependent changes in failure stress following thrust earthquakes two dimensional 2 d viscoelastic finite element models were used to calculate the time dependent changes in coulomb failure stresses following thrust earthquakes due to respective effects of relaxation of viscous lower crust or upper mantle and postseismic creep on the main fault or its downdip extension results suggest that thrust earthquakes cause a coseismic increase in coulomb stress along antithetic lobes normal to the slip plane following a quake creep processes that reduce stresses in a ductile lower crust or upper mantle are calculated to cause a transfer of stress to the upper crust under certain conditions transfer of stress may lead to a further buildup of high coulomb stress along the base of the upper crust potentially shortening the time to failure of other faults in the region the conditions under which an antithetic lobe of high coulomb stress are favored to expand at the base of the upper crust postseismically within a few decades include the following the lower crust or upper mantle has an effective viscosity not greater than 1019 pa s the thrust fault has a moderate dip angle 40 50 the brittle ductile transition is deep enough to provide a corridor at the base of the upper crust for expansion and the crust has a low apparent coefficient of friction 0 2 postseismic increases in coulomb stress within the upper crust may also be caused by aseismic creep on the fault stress changes due to this mechanism are maximized with a high apparent coefficient of friction analysis of experimentally determined non newtonian flow laws suggests that wet granitic quartz and feldspar aggregates may have a viscosity of the order of 1019 pa s the calculated rate of stress transfer from a viscous lower crust or upper mantle to the upper crust becomes faster with increasing values of the power law exponent and the presence of a regional compressive strain rate results of this 2 d analysis suggest a potentially important role of viscous flow in controlling time dependent postseismic stress changes that warrant further investigation using 3 d viscoelastic analysis

controls on deposition of aquatic and terrestrial organic matter in the lacustrine namling oiyug basin oligocene miocene southern tibet abstract the namling oiyug basin is the only cenozoic lake basin that has hydrocarbon potential in southern tibet china coal was once exploited in the basin from the early miocene mangxiang formation which also hosts oil prone source rock layers herein a combined investigation of sedimentological features and organic geochemistry is performed on a measured outcrop section of the mangxiang formation to study the lake basin types as well as the depositional history of aquatic and terrestrial organic matter om there are two periods in the lake basin evolution history the balanced fill and overfilled stages in the balanced fill basin stage oil prone laminated shale was deposited in a relatively deep lake environment with total organic carbon toc contents around 5 wt and hi values up to 316 mg hc g toc indicating hydrogen poor type ii kerogen with transition to type iii kerogen comparison between maceral composition and geochemical properties suggests that the relative low hi values are results of thermal maturity the high value of the gammacerane index indicates water stratification of the lake in this stage water and sediment input approximately equaled potential accommodation the lake was relatively stable and hosted large amounts of lacustrine algae and accumulated om with low terrestrial input in an anoxic environment after the onset of the thrusting the potential accommodation was decreased large volumes of clastic sediments also drained into the lake from the surrounding area resulting in an overfilled lake basin during this stage peat was deposited in freshwater environment with 30 wt toc low hi and type iii om the high pr ph 3 implies oxic conditions

volume loss and deformation around conjugate fractures comparison between a natural example and analogue experiments abstract in well stratified sedimentary rocks measurement of the bedding offsets along fractures gives a value for the relative displacement at each location to restore the undeformed geometry of the layers the offset along each fracture must be cancelled restoration of the geometry enables the location of places where shortening is concentrated to be established namely at the fracture intersection and at fracture tips slow displacement during deformation is demonstrated by the growth of fibres on the fault surfaces pressure solution inside the layers is responsible for the internal strain with associated volume loss this mechanism is compatible with low strain rates giving aseismic displacements along the fractures to model rock rheology dominated by pressure solution a viscous material paraffin wax has been employed in the analogue model volume loss in nature is represented by area changes during the analogue experiments in the experiments shortening is concentrated around the fractures in exactly the same geometrical positions as in nature when the amount of displacement is different on each of the two fractures one of the fractures is observed to offset its conjugate fracture when this is the case relative timing of displacement along conjugate fractures cannot be determined without ambiguity from the observation of fracture offsets

inferring paleostresses from natural fracture patterns a new method we introduce a method to infer the remote differential stress magnitude from the curvature of overlapping echelon fracture traces the curving paths of overlapping echelon cracks imply the predominance of local crack induced stresses over remote stresses during propagation nearly straight crack paths imply the controlling influence of a remote compressive crack parallel differential stress this method is used to interpret complex joint patterns mapped in sedimentary rock it is also applied to the problem of fracture pattern generation using computer models

comment on comparison of michigan basin crude oils by vogler et al abstract data is presented for michigan basin oils that indicate in addition to the three distinct oil types oils of mixed composition intermediate between the silurian and ordovician types also exist these mixed oils which occur in devonian detroit river reservoirs are identified primarily by their isoprenoid and normal paraffin compositions

organic geochemistry and oil source correlations paleozoic of ohio routine analytical methods and new kerogen kerogen pyrolyzate techniques used for detailed petroleum geochemical studies permit the correlation of oils to their respective source rocks in ohio in the paleozoic section four good to excellent source units were identified the mississippian sunbury shale the devonian ohio shale and olentangy shale and the shales of the ordovician point pleasant formation these marine shales are dominantly oil prone with maturation levels that span the immature to peak oil generation range no source rocks examined exceeded an ro of 1 0 1 1 or equivalent the reservoired oils in ohio are mostly supermature based on normal alkanes pristane n c17 to phytane n c18 ratios low asphaltene and sulfur contents and high paraffin indices three basic oil groups were identified using oil characterization procedures group 1 consists of cambrian ordovician and some silurian oils group 2 consists of some silurian devonian one mississippian and some pennsylvanian oils and group 3 consists of oil from the mississippian berea sandstone oil source rock correlation techniques permitted the assignment of probable source rocks to each oil group group 1 oil was probably derived from the point pleasant shales group 2 oil was derived from the ohio shale and olentangy shale and group 3 oil came from the sunbury shale because the supermature oils in ohio occur in reservoirs with maturities generally under 0 8 ro the oils would have had to migrate from deep within the appalachian basin migration could have followed fracture zones unconformities or extensive and permeable units such as the silurian clinton sands

the andean orogenic period and its major structures in central and northern peru the andean orogenic period began in the albian and consisted of at least 6 discrete compressional phases which are albian to late miocene in age deformation migrated progressively toward the amazonian foredeep the major andean structures in central and northern peru are related to the eocene incaic early miocene quechua 1 and late miocene quechua 3 phases the maranon thrust and fold belt mtfb which bounds the western cordillera to the ne rose in the eocene it was reactivated partly by the quechua 1 phase which also folded the altiplano farther e and possibly part of the eastern cordillera the sub andean thrust and fold belt stfb is related to the quechua 3 phase in both mtfb and stfb the imbricate thrusts merge at depth in large decollement faults gravity sliding does not play a dominant role for these faults have to be rooted westward in the pre andean basement the corresponding crustal shortening played a significant part in the creation of the sialic root of the andes

anoxia vs productivity what controls the formation of organic carbon rich sediments and sedimentary rocks discussion the prevailing explanation for the origin of organic rich sediments and rocks invokes deposition under conditions of anoxia however recent research suggests that high primary production and not water column anoxia provides the first order control on the accumulation of organic rich facies in the modern oceans oxygen minima do not appear to have any direct effect on carbon accumulation in continental margin or marginal sea environments sediments accumulating in the modern black sea the type euxinic basin are not particularly enriched in organic matter despite the presence of an anoxic water column although a sapropel containing extremely high carbon concentrations was deposited during the holocene at a time when the basin was oxic results of a recently published co pled ocean atmosphere model indicate that during the cretaceous thermohaline and surface circulation in the oceans was similar to or more intense than modern conditions despite the overall equable climate such conditions confound the idea that circulation in the cretaceous atlantic for example was punctuated by oceanic anoxic events brought about by more sluggish circulation sporadic temporal and spatial increases in primary production reflecting changes in the behavior and or state of the ocean atmosphere system constitute a more tenable explanation for the occurrence of modern and quaternary carbon rich sediments and cretaceous black shales consequently the fundamental control on the accumulation of carbon rich facies in the oceans and marginal seas is not the presence or absence of anoxia

pyrite and organic matter in phanerozoic normal marine shales data have been obtained from our own chemical analyses and from the literature for the concentrations of organic carbon c and pyrite sulfur s in over 600 samples of cambrian to recent normal marine shales normal marine refers to deposition in oxygenated bottom water as evidenced by the presence of benthic fossils and or indicators of bioturbation all samples were selected to minimize 1 1 loss of c and or s due to weathering at the outcrop by emphasizing the use of drill core material 2 2 analytical errors and non diagenetic effects by avoiding sediments low in c or s content such as sandstones 3 3 fe limitation of pyrite formation by avoiding limestones cherts and euxinic shales 4 4 metamorphic loss of c relative to s by avoiding rocks obviously subjected to even low grade metamorphism our results indicate that 1 there is generally a good positive linear correlation between organic c and pyrite s for normal marine shales of all ages and 2 the mean cs ratio for normal marine shales has varied over time devonian to tertiary shales exhibit mean cs weight ratios 1 8 0 5 somewhat lower than quaternary sediments meancs 2 8 this we believe is due to preferential c loss relative to s during biogenic methanogenesis plus diagenetic heating distinctly lower mean c s values for the cambrian and ordovician 0 5 0 1 cannot be explained solely in terms of diagenetic c loss and instead must represent low original ratios the low original ratios we suggest were due mainly to the absence of bacterially refractory organic matter added to the marine environment by rivers at that time because the major source of such material vascular land plants had not yet evolved this along with a possibly lower degree of bioturbation contributed to enhanced pyrite formation and preservation and thus lower early paleozoic cs ratios

possible late middle ordovician organic carbon isotope excursion evidence from ordovician oils and hydrocarbon source rocks mid continent and east central united states oils generated by middle ordovician rocks are found throughout the mid continent and east central regions of the united states gas chromatographic characteristics of these oils include a relatively high abundance of n alkanes with carbon numbers less than 20 a strong predominance of odd numbered n alkanes between c10 and c20 and relatively small amounts of branched and cyclic alkanes saturated and aromatic hydrocarbon fractions of 43 ordovician oils from the anadarko ardmore forest city illinois michigan salina sedgwick and williston basins and the iowa shelf demonstrate a wide range in carbon isotope composition dgr13csat 24 9 pmil to 33 9 pmil dgr13carom 24 3 pmil to 33 7 pmil saturated and aromatic hydrocarbons extracted from late middle ordovician shales 17 core samples show ranges in dgr13c similar to that of the oils the wide ranges in dgr13c for oils and rock extracts reflect a major positive excursion s 6 9 per pmil in organic matter dgr13c in late middle ordovician rocks this excursion has at least a regional significance in that it can be documented in sections 480 mi 770 km apart in south central kansas and eastern iowa the distance may be as much as 930 mi 1 500 km if the carbon isotope variations observed in michigan basin ordovician oils and in organic matter from late middle ordovician rocks in southwestern ontario are related to the same carbon isotope excursion organic matter dgr13c in core samples from south central kansas and eastern iowa is not directly related to variations in quantity or quality of organic matter or maceral compositi n the positive excursion in organic matter dgr13c is a possible result of increased organic matter productivity and or preservation the parallel shifts in organic and carbonate dgr13c in core samples from 1 e m greene well washington county iowa imply changes in the isotope composition of the ocean atmosphere carbon reservoir differences in the magnitude of the carbon isotope shifts between organic matter 8 8 pmil and carbonate 4 2 pmil in this core suggest a decrease either locally or regionally in available dissolved co2 possibly a result of high organic matter productivity and or limited circulation in the late middle ordovician seas

are large differential stresses required for straight fracture propagation paths abstract fracture growth geometries in linear elastic materials are sensitive to the ratio between the remote differential stress or t stress and the fracture driving stress two mechanically interacting fractures follow straighter paths for greater values of this ratio we demonstrate that the degree of curvature that develops between two mechanically interacting fractures is also controlled by such factors as sub critical fracture growth and fracture surface roughness sub critical fracture growth limits the development of fracture curvature by decreasing the fracture driving stress needed for propagation fracture surface roughness can affect the development of fracture curvature by limiting the magnitude of the shear displacement discontinuity that can be accommodated along the fracture thus propagation paths for two mechanically interacting fractures can be quite straight even in the presence of small to moderate differential stresses consequently the absence of fracture curvature does not necessarily imply a high differential stress state

kinematic indicators on active normal faults in western turkey quaternary normal fault zones in western turkey comprise multiple slip planes and zone parallel layers of fault breccia they also contain several little known kinematic indicators that are probably typical of many formed at shallow levels in extending terrains the recent exhumation by contractors of about 2000 m2 of slip planes in a sse dipping fault zone separating quaternary colluvium from bedrock carbonates at yavansu 7 km se of kusadasi permits an unusually complete inventory of the indicators to be compiled the most spectacular indicators are metre scale 69 w pitching corrugations in slip planes and recemented breccia sheets underlying them corrugations characterized by sinusoidal profiles normal to their long axes and less commonly culminations and depression along their axes possibly developed as a result of upwards propagating slip planes seeking undemanding pathways through heterogeneous fault precursor breccias that formed in advance of tip lines parallel to corrugation long axes are those of gutters flat floored sleep sided channels a few centimetres wide probably related to the abrasion of subslip plane breccia sheets centimetre scale tool tracks scored in the uppermost subslip plane breccia sheet by resistant colluvial clasts are irregular at their proximal ends but distally they swing into alignment with corrugation axes frictional wear striae centimetres long but only a few millimetres wide and pitching 78 w are superimposed on the other slip parallel lineations comb fractures nearly perpendicular to slip planes define an intersection lineation which is normal to corrugation axes fault plane solutions of earthquakes on sse dipping active faults in the west anatolian extensional province indicate that mainly normal combined with minor dextral slip is the dominant mode a conclusion in accord with the sense of slip inferred from the indicators exposed on the yavansu slip planes

oil generation in the michigan basin a biological marker and carbon isotope approach abstract palaeozoic strata in the michigan basin produced crude oils which despite their rather small volume and economic value are interesting because of the considerable age of the basin and because of the opportunity to study generation and migration of oils within a relatively simple but ancient geological setting based on n alkane profiles biological marker distributions and carbon isotope ratios the oils belong to three main families of different genetic origin and a few less important mixed types silurian oils from salina and niagara limestones have broad n alkane distributions and abundant isoprenoid hydrocarbons a strong phytane over pristane predominance and the lack of diasteranes indicate a carbonate source for these oils chemical maturity parameters show that they are more mature than the oils from the other main families from which they are also clearly distinguished by carbon isotope ratios of hydrocarbon fractions and single n alkanes oils found in the calcareous ordovician trenton formation contain n alkane cyclohexylalkane and alkyl phenanthrene distributions typical of immature oils many oils from the devonian dundee reservoirs are very similar in overall composition to the trenton oils devonian traverse oils are considered to be mainly from a devonian source with some contribution of ordovician type oil and are of intermediate maturity based on calculations using kinetic parameters of biological marker reactions considerably deeper subsidence of the devonian source rock in the past is implied

paleocene eocene potential source rocks in the avengco basin tibet organic geochemical characteristics and their implication for the paleoenvironment abstract the avengco basin is located in the western part of the tibetan plateau and is similar to the nima basin in the central part of the plateau and the lunpola basin in the eastern part in terms of sedimentary characteristics and tectonic settings which are well known to provide a good source rock potential however the organic geochemical characteristics of the paleocene eocene potential source rocks in the avengco basin have been under debate thirty four marl and mudstone outcrop samples of the niubao formation in the avengco basin were collected and subjected to the following analyses total organic carbon toc rock eval pyrolysis stable carbon isotopes of kerogen gas chromatography gc and gas chromatography mass spectrometry gc ms here we present the results indicating the organic matter of the upper niubao formation is mainly composed of type ii kerogen with a mixed source which is dominated by algae however the lower niubao formation has the less oil prone type ii iii kerogen and the sources of the organic matter are mainly terrestrial plants with less plankton in addition the samples are thermally immature to marginally mature the niubao formation was deposited in an anoxic oxic environment which was brackish with an imperceptible stratified water column the upper niubao formation has a medium to good hydrocarbon generating potential however the lower niubao formation has a zero to poor hydrocarbon generating potential

modes of noncircular fluid filled boreholes in elastic formations dispersion curves for4 the modes of noncircular fluid filled boreholes in homogeneous elastic formations are calculated with a boundary integral formulation results are displayed for the propagatory modes of several borehole shapes in both fast and slow formations any departure from circularity reduces the tube wave speed agreement with exact analytical results is obtained for elliptical boreholes while a simple approximate relation involving borehole perimeter and area is verified for arbitrary cross sections higher order surface modes such as the flexural and screw modes split into two distinct branches distinguished by their orientation with respect to the principal axes of the borehole cross section modal dispersion characteristics in elliptical boreholes may be approximately by scaled circular borehole results in fast formations the effective scaled radii are related to transverse dimensions of the borehole but in slow formations they are more closely related to local radii of curvature of the borehole for more complex cross sections with a single axis of symmetry dispersion of the flexural mode is well modeled by that in elliptical boreholes of similar dimensions while that of stoneley and screw modes is not in asymmetrical boreholes significant torsional and out of phase motion may occur

simulation of borehole sonic waveforms in dipping anisotropic and invaded formations a numerical simulation study has been made of borehole sonic measurements that examined shoulder bed anisotropy and mud filtrate invasion effects on frequency dispersion curves of flexural and stoneley waves numerical simulations were considered for a range of models for fast and slow formations computations are performed with a cartesian 3d finite difference time domain code simulations show that presence of transverse isotropy ti alters the dispersion of flexural and stoneley waves in slow formations the flexural wave becomes less dispersive when the shear modulus c44 governing wave propagation parallel to the ti symmetry axis is lower than the shear modulus c66 governing wave propagation normal to the ti symmetry axis conversely the flexural wave becomes more dispersive when c44 c66 dispersion decreases by as much as 30 at higher frequencies for the considered case where c44 c66 and decrease

multipole acoustic waveforms in nonaxisymmetric boreholes and formations using a 2 5 dimensional velocity stress finite difference vs fd technique waveforms are calculated for multipole excitation of azimuthally nonsymmetric boreholes and formations axial uniformity of the borehole cross section and formation is assumed the vs fd waveforms agree with previous transform methods for circular boreholes and the modes found for elliptical boreholes agree with those derived by a boundary integral technique borehole ellipticity substantially reduces monopole compressional headwave amplitudes in fast formations low frequency dipole waveforms in elliptical boreholes show marked sensitivity to source dipole orientation in fast formations but small sensitivity in slow formations waveforms from dipole receivers perpendicular to the source dipole may be comparable in amplitude to those recorded by parallel dipole receivers dipole waveforms from an exaggerated keyhole shaped borehole are quite similar to those from an elliptical borehole of similar aspect ratio in a slow formation finally the behavior of early compressional headwaves arising from a nearby faster bed parallel to the borehole axis is qualitatively similar to that for a point source above a simple planar interface

paleomagnetic evidence for rapid vertical axis rotation in the peruvian cordillera ca 8 ma paleomagnetic results from 31 neogene sites in the peruvian andes yield primary magnetizations as demonstrated by positive fold and reversal tests strata dated as 18 9 ma record a significant counterclockwise rotation 11 5 whereas unconformably overlying younger strata 7 6 ma are not rotated the age of rotation thus is between 9 and 7 ma a period that coincides with the widespread quechua 2 deformation phase moreover eight independent studies on 107 9 ma rocks from peru between 9 s and 15 s reveal similar and significant rotations 15 6 this suggests that the region rotated during a 2 m y period of deformation ca 8 ma when the andes underwent rapid uplift and important deformation commenced in the subandean zone

interpreting rock eval pyrolysis data using graphs of pyrolizable hydrocarbons vs total organic carbon kerogen the major organic component of sedimentary rocks is commonly analyzed by rock eval pyrolysis plotting the data on a graph of s 2 vs total organic carbon toc and determining the regression equation is the best method for determining the true average hydrogen index and measuring the adsorption of hydrocarbon by the rock matrix such a plot also indicates the type of kerogen present and avoids the problem of increasing hydrogen index with total organic carbon content with the s 2 vs toc diagram the organic component of different suites of samples may be compared and their petroleum generation potentials established as an example the diagrams are used to evaluate the sedimentary environments and petroleum potential of the paleogene evaporitic sediments of the mulhouse basin alsace france

neogene paleomagnetism and oroclinal bending of the central andes of bolivia geophysical and tectonic evidence suggests that the principal vertical axis bending of the bolivian orocline is a late cenozoic event related to subduction and andean uplift to further test this hypothesis paleomagnetic data are presented from miocene basins within the altiplano and cordillera oriental of bolivia located in the presumed southern limb of the orocline to factor out potential vgp data samples were collected from sedimentary sections spanning at least 0 5 m y new paleomagnetic data are presented for two sites i e gerdas 21 s latitude 16 15 ma and quehua 20 s latitude 13 7 ma in conjunction with previously published paleomagnetic results from middle to late tertiary rocks from bolivia and peru spanning some 16 of latitude 6 22 s a consistent pattern of oroclinal development is as follows significant counterclockwise rotation about 15 20 in the northern limb negligible rotation in the axis at the latitude of the arica deflection and significant clockwise rotation about 15 20 in the southern limb although these paleomagnetic data could represent independent mesoscale 102 to 104 km2 block rotations the preferred interpretation is that they confirm the hypothesis that a significant portion about 30 40 of the oroclinal bending occurred during the late cenozoic using the unit mean paleomagnetic declinations in the well dated sections minimum rates of oroclinal bending during the late cenozoic are calculated to range from 0 6 m y to 1 7 m y the remaining 15 20 of observed curvature of the orocline probably resulted from an earlier phase of andean deformation or may represent the shape of the ancestral andes

material balance assessment of the new albany chesterian petroleum system of the illinois basin the new albany chesterian petroleum system of the illinois basin is a well constrained system from which petroleum charges and losses were quantified through a material balance assessment this petroleum system has nearly 90 000 wells penetrating the chesterian section a single new albany shale source rock accounting for more than 99 of the produced oil well established stratigraphic and structural frameworks and accessible source rock samples at various maturity levels a hydrogen index hi map based on rock eval analyses of source rock samples of new albany shale defines the pod of active source rock and extent of oil generation based on a buoyancy drive model the system was divided into seven secondary migration catchments each catchment contains a part of the active pod of source rock from which it derives a petroleum charge and this charge is confined to carrier beds and reservoirs within these catchments as accountable petroleum petroleum losses or undiscovered petroleum a well constrained catchment with no apparent erosional or leakage losses is used to determine an actual petroleum charge from accountable petroleum and residual migration losses this actual petroleum charge is used to calibrate the other catchments in which erosional petroleum losses have occurred petroleum charges determined by laboratory pyrolysis are exaggerated relative to the actual petroleum charge rock eval charges are exaggerated by a factor of 4 14 and hydrous pyrolysis charges are exaggerated by a factor of 1 7 the actual petroleum charge provides a more meaningful material balance and more realistic estimates of petroleum losses and remaining undiscovered petroleum the total petroleum charge determined for the new albany chesterian system is 78 billion bbl of which 11 4 billion bbl occur as accountable in place petroleum 9 billion bbl occur as residual migration losses and 57 6 billion bbl occur as erosional losses of the erosional losses 40 billion bbl were lost from two catchments that have highly faulted and extensively eroded sections anomalies in the relationship between erosional losses and degree of erosion suggest there is potential for undiscovered petroleum in one of the catchments these results demonstrate that a material balance assessment of migration catchments provides a useful means to evaluate and rank areas within a petroleum system the article provides methodologies for obtaining more realistic petroleum charges and losses that can be applied to less data rich petroleum systems

the regional intraplate stress field in south america a compilation of lithospheric stress directions for continental south america and the inferred major patterns of the regional intraplate stress field are presented stress orientations are based primarily on earthquake focal mechanisms and quaternary fault slip inversion published in the literature four new focal mechanisms based on short period p wave modeling and selected centroid moment tensor solutions published by u s geological survey consistent with p wave first motions at south american and other world wide standard seismograph network stations are also included in the data base the observed patterns of intraplate stresses may be useful in constraining numerical models of the plate driving forces in the south american plate in the andean plateau altitudes greater than 3000 m n s extensional stresses predominate e w compressional stresses are observed in the sub andean and platform regions extending up to about 1000 km east of the andes the maximum horizontal stress shmax is uniformly oriented in the e w direction throughout western south america averages of the shmax direction at grid points spaced 2 5 o were taken as representing the regional field the e w direction of this regional field is not affected by the change of strike of the andean chain nor by the contact of a flat subducted slab underneath the regional shmax direction is oriented 15 o more clockwise than the direction of the nazca plate convergence the difference between the regional shmax orientations and the absolute plate motion may be only about 6 o this may indicate that contact forces with the nazca plate may not be the only major contributor to the intraplate stresses in western south america the eastern limit of this andean stress province seems to coincide with aseismic regions in the upper amazon basin and in the paraml basin in the central amazonian region seismicity and stress data suggest a seismic province with n s compressional stresses the origin of these stresses is not yet clearly understood but could possibly be related to lower crustal loading along the middle amazon basin in northeastern brazil seismicity is characterized by upper crustal strike slip earthquakes bordering the potiguar marginal basin a model is proposed for this region in which the stress field is the result of a superposition of regional e w compressional stresses and local extensional stresses oriented perpendicular to the continental margin possibly related to density contrasts and sediment loading in the continental shelf

influence of borehole eccentred tools on wireline and logging while drilling sonic logging measurements we describe a numerical study to quantify the influence of tool eccentricity on wireline wl and logging while drilling lwd sonic logging measurements simulations are performed with a height polynomial adaptive hp fourier finite element method that delivers highly accurate solutions of linear visco elasto acoustic problems in the frequency domain the analysis focuses on wl instruments equipped with monopole or dipole sources and lwd instruments with monopole excitation analysis of the main propagation modes obtained from frequency dispersion curves indicates that the additional high order modes arising as a result of borehole eccentricity interfere with the main modes i e stoneley pseudo rayleigh and flexural this often modifies decreases the estimation of shear and compressional formation velocities which should be corrected increased to account for borehole eccentricity effects undesired interferences between different modes can occur at different frequencies depending upon the properties of the formation and fluid annulus size which may difficult the estimation of the formation velocities

an efficient method to compute the dynamic response of a fluid filled crack summary the vibration of a fluid filled crack is considered to be one of the most plausible source mechanisms for the long period events and volcanic tremors occurring around volcanoes as a tool for the quantitative interpretation of source process of such volcanic seismic signals we propose a method to numerically simulate the dynamic response of a fluid filled crack in this method we formulate the motions of the fluid inside and the elastic solid outside of the crack using boundary integrals in the frequency domain and solve the dynamic interactions between the fluid and the elastic solid using the point collocation method the present method is more efficient compared with the time domain finite difference method which has been used in simulations of a fluid filled crack and enables us to study the dynamics of a fluid filled crack over a wide range of physical parameters the method also allows us direct calculation of the attenuation quality factor of the crack resonance which is an indispensable parameter for estimating the properties of the fluid inside the crack the method is also designed to be flexible to many applications which may be encountered in volcano seismology and thus extensions of the method to more complicated problems are promising

multiple pulses of neogene compressive deformation in the ayacucho intermontane basin andes of central peru strata of the neogene ayacucho intermontane basin central peru can be subdivided into three tectonostratigraphic units the larampuquio volcanics consisting largely of intermediate lavas and coarse conglomerates of similar lithology the unconformably overlying huanta formation consisting of a lower part formed predominantly of lacustrine and volcaniclastic sedimentary rocks and an upper part composed largely of intermediate lavas tuffs and conglomerates and the unconformably overlying ayacucho formation consisting of complexly interfingering volcaniclastic sedimentary rocks silicic tuffs and lavas of intermediate to silicic composition radiometric ages of 17 3 0 2 m y and 18 3 0 6 m y have been obtained on tuffs from the upper and lower parts of the larampuquio volcanics determinations on 2 rocks from the lower member of the huanta formation indicate an age of 11 4 0 5 m y and a rock from the upper member is dated at 9 3 0 3 m y b p units of the ayacucho formation yield ages showing that it was deposited between 7 7 and 6 m y ago the intervening molinoyocc and puchcas volcanics were erupted between 9 9 and 7 4 m y ago these units may postdate the huanta formation or may be associated in part with the huanta and in part with the ayacucho formation coarse conglomerates of the larampuquio volcanics represent debris eroded from highlands uplifted during the first and most intense pulse of the late cenozoic quechuan phase of andean compressive tectonism that began in the early miocene folding of the larampuquio volcanics may have taken place during this or a subsequent pulse of compression a marked unconformity between the huanta and ayacucho formations demonstrates a period of deformation between 8 5 and 9 5 m y ago compression also was active during at least part of the deposition of the ayacucho formation as shown by inter fingering wedges of coarse clastic debris along the northeastern margin of the basin and by angular unconformities within the formation folded rocks of the ayacucho formation and its equivalents to the west are overlain by undeformed volcanic units dated at 3 8 0 4 m y b p and 5 5 m y b p bracketing a well defined pulse of deformation slightly younger than 6 m y these subsequent episodes of compressive deformation represent recurrent pulses of the quechuan phase folding related to these late quechuan pulses is restricted in the andes to northwest trending belts corresponding to reactivated fault zones in the pre neogene substratum a system of north trending dikes in the huachocolpa district 70 km west of ayacucho suggests relaxation of compression in the pliocene and compression of quaternary age has been recognized in the huancayo area 170 km northwest of ayacucho taken together with the first pulse of quechuan deformation in the early and middle miocene these data indicate at least four pulses of crustal compression during the late cenozoic separated by periods during which compressional stresses were at least partly relaxed and at times possibly replaced by extension

organic geochemistry of mid continent middle and late ordovician oils ordovician oils in mohawkian and cincinnatian reservoirs of the united states mid continent retain the biochemical imprint of middle and upper ordovician oceanic life before the evolution of land plants and most vertebrates thus these oils have some geochemical features that distinguish them from younger oils these features include 1 a predominance of n c15 n c17 and n c19 alkanes in the saturated hydrocarbon fraction 2 relatively low amounts of longer chain n alkanes 3 low amounts of chlorophyll derived isoprenoids such as pristane and phytane and 4 abundant c29 sterane relative to c27 with rearranged forms diasteranes predominant over normal steranes ordovician oils also generally contain little sulfur and have a somewhat variable light stable carbon isotopic composition with dgr13csat and dgr13caro values of 28 to 31 pmil pdb but these features are typical of many marine oils the unusual chemistry of these ordovician oils supports the interpretation of reed illich and horsfield 1986 that prokaryotic organisms provided the organic matter for most ordovician oils although their claim for gloeocapsamorpha a problematic unicellular prokaryote possibly a blue green alga or an unusually large bacterium cannot be proven from oil chemistry alone knowing that indigenous mid continent ordovician oils were derived from prokaryotic organisms may aid in future exploration for these reservoirs

flash pyrolysis of kerogens from algal rich oil shales from the eocene huadian formation ne china abstract the hydrocarbon composition of the kerogen fractions of two samples hd 20 and hd 21 from oil shale layer 4 in the eocene huadian formation ne china were investigated by analytical flash pyrolysis 650 c 10 s followed by gas chromatography mass spectrometry py gc ms organic petrography showed that the two kerogens were almost entirely derived from algal remains but contained very different algal maceral compositions with 40 of the macerals in hd 20 being of macroalgal origin that were not present in hd 21 py gc ms yielded high concentrations of n alkanes from both kerogens but with different molecular weight profiles due to the different algal contributors to the two kerogen samples the hydrocarbon pyrolysates generated at 650 c from hd 21 in which the green microalga botryococcus braunii was identified showed a higher proportion of longer chain alkanes and alkenes presumably from cracking of the botryococcus algaenan we also identified a c40 monoaromatic lycopane derivative which was absent in the hd 20 the high hydrocarbon potential of both kerogens can be attributed to common microalgal sources whereas the macroalgae which is abundant in hd 20 makes only a minor contribution to the hydrocarbon products

global carbon isotopic events associated with mass extinction and glaciation in the late ordovician mass extinctions and glacioeustatic sea level changes in the lower part of the hirnantian final stage of the ashgill are accompanied by shifts in marine stable isotope compositions previously published stable isotope changes have been used to identify the onset and demise of the gondwana glaciation and to suggest relationships between biotic changes and carbon cycling within the oceans however the existing isotopic data set had limitations because it was derived from ordovician low latitude settings and from carbonates or organic carbon in separate areas we report new data from ordovician high latitude carbonates and demonstrate parallel shifts in organic and carbonate 13c from baltica brachiopod shells from a high palaeolatitude periglacial setting in argentina have elevated 13c values similar to those described previously from low latitude sites the new data demonstrate that the positive hirnantian 13c excursion previously only recognised from low palaeolatitude areas was widespread and probably global in extent the poor preservation state of the brachiopods unfortunately prevented the determination of a reliable oxygen isotopic value from the same material preliminary carbon isotopic data from thermally immature organic matter from estonia provide the first indication of a synchronous shift in organic and inorganic 13c in sediments from the same basin this work provides new data of critical importance for constraining models of end ordovician palaeoceanography and climate change