fluid inclusions and hydrothermal alteration on the dixie valley fault nevada footwall rocks of the 1954 rupture segment of the dixie valley fault show extensive hydrothermal alteration related to fluids that were present on the fault during tectonic events hydrothermal alteration of granitic host rocks consists of temporally and spatially overlapping mineral assemblages an early biotite feldspar assemblage is followed by later fe chlorite and epidote both chlorite and epidote are replaced by hydrothermal sericite and cross cut by calcite hematite and quartz calcite veins biotite is partially replaced by prehnite the latest hydrothermal minerals are stilbite laumontite kaolinite alunite smectite illite and pervasive replacement of rock units with fine grained quartz chalcedony and opal secondary fluid inclusions trapped in healed microfractures in igneous quartz include type i inclusions that contain a moderate salinity aqueous liquid and vapor type ii inclusions that contain a moderate salinity aqueous liquid and co2 type iii inclusions that show eutectic melting temperatures below the nacl h2o eutectic and contain substantial cacl2 and type iv inclusions containing halite and other daughter minerals microthermometric measurements on these inclusions yield variable compositions and homogenization temperatures salinities of type i inclusions vary from 0 1 to 12 9 wt nacl with the mode in the interval 0 to 1 salinities of type ii co2 bearing inclusions range from 0 62 to 6 81 wt nacl relative to h2o and salinities of type iii inclusions with low eutectic melting temperatures are 12 9 to 25 3 nacl equivalent wt salinities of halite bearing inclusions are 30 1 to 39 2 wt nacl homogenization temperatures span the range 120 to 400 c the processes of isochemical cooling with upward displacement of the footwall mixing of cool low salinity water with hotter components and mixing of cool evaporite brine with hotter components could be responsible for variable fluid inclusion compositions homogenization temperatures and densities the p t path of the fault fluids is established by mineral equilibria and fluid inclusion characteristics the path includes a lithostatic fluid pressure at 305 c and 1570 bars along with cooling and escape of co2 from fluids the fluid p t path probably approaches hydrostatic pressure conditions at lower temperatures hydrothermal alteration product minerals fluid temperatures pressures and compositions in the footwall of the dixie valley fault constrain minimum fault age to 20 to 25 ma displacement to 6 km with about 3 km of pre 10 to 13 ma and 3 km of post 10 ma uplift fluid compositions and p t data suggest the following mechanism for rupture initiation and arrest ruptures may be initiated as a result of high fluid pressures then opening of dilatant fractures causes drastic decrease in fluid pressure separation of steam and co2 the drastic reduction in fluid bulk modulus that accompanies volatile phase separation permits propagation of the ruptures even though fluid pressure is reduced in areas where fluid pressure reduction is not accompanied by phase separation fractures are arrested by dilatant hardening

seismic and aseismic slip indicators in serpentinite gouge structures observed within experimentally deformed serpentinite gouges provide information that may be used to identify the seismic behavior of natural fault zones in laboratory friction experiments serpentinite exhibits two modes of behavior one can only result in stable fault creep and the other may result in stable slip but has the potential for earthquakes the microstructures that form during these experiments reflect the deformation style of the serpentinite distributed deformation results from aseismic fault creep and localized deformation results from conditions favorable for seismic slip distributed deformation produces a crystallographic preferred orientation of the serpentine grains s foliation localized deformation forms riedel shears similar structures occur within a natural serpentinite shear zone from monterey county california and suggest a history of stable fault creep with intermittent seismic events

ultracataclasite structure and friction processes of the punchbowl fault san andreas system california abstract the punchbowl fault is an exhumed 40 km displacement fault of the san andreas system in the devil s punchbowl the fault contains a continuous ultracataclasite layer along which the punchbowl formation sandstone and an igneous and metamorphic basement complex are juxtaposed the fabric of the ultracataclasite layer and surrounding rock indicate that nearly all of the fault displacement occurred in the layer by analogy with nearby active faults we assume that the punchbowl fault was seismogenic and that the ultracataclasite structure records the passage of numerous earthquake ruptures we have mapped the ultracataclasite layer at 1 1 and 1 10 to determine the mode of failure and to constrain the processes of seismic slip on the basis of color cohesion fracture and vein fabric and porphyroclast lithology two main types of ultracataclasite are distinguished in the layer an olive black ultracataclasite in contact with the basement and a dark yellowish brown ultracataclasite in contact with the sandstone the two are juxtaposed along a continuous contact that is often coincident with a single continuous nearly planar prominent fracture surface pfs that extends the length of the ultracataclasite layer in all exposures no significant mixing of the brown and black ultracataclasites occurred by offset on anastomosing shear surfaces that cut the contact or by mobilization and injection of one ultracataclasite into the other the ultracataclasites are cohesive throughout except for thin accumulations of less cohesive reworked ultracataclasite along the pfs structural relations suggest that 1 the black and brown ultracataclasite were derived from the basement and sandstone respectively 2 the black and brown ultracataclasites were juxtaposed along the pfs 3 the subsequent final several kilometers of slip on the punchbowl fault occurred along the pfs and 4 earthquake ruptures followed the pfs without significant branching or jumping to other locations in the ultracataclasite by comparison with rock friction experiments the slip localization along the pfs in the ultracataclasite implies rate weakening behavior with a critical slip distance similar to laboratory values and thus relatively small nucleation and breakdown dimensions for earthquake ruptures of the various mechanisms proposed to explain the low strength of the san andreas and to produce dynamic weakening of faults those that require or assume extreme localization of slip are most compatible with our observations

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

origin and significance of redistributional secondary porosity abstract redistributional secondary rds porosity is created by the dissolution of framework grains and cements and the simultaneous precipitation of reaction products in the nearby pore space aluminosilicate minerals and in particular feldspars are the usual precursors of rds porosity in sandstones common product minerals are authigenic quartz and clay minerals usually kaolinite this type of secondary porosity is almost ubiquitous in sandstones rds porosity has little effect on the total porosity of reservoir rocks and contrary to popular opinion cannot give rise to higher than expected porosities at depth petrographic criteria alone are an insufficient measure that the rock has undergone leaching resulting in porosity enhancement to demonstrate porosity enhancement due to leaching it is necessary to show that the porosity of the rock is statistically higher than would otherwise be the case for a rock of similar characteristics that has not been leached the dissolution precipitation reactions that give rise to rds arise from the inherent thermodynamic instability of minerals such as the feldspars and other aluminosilicates which formed under magmatic or metamorphic conditions when these minerals dissolve they add aluminium and silica to the pore fluid however the concentration of aluminium and silica in the pore fluid is buffered by clay minerals and quartz respectively during burial diagenesis thus any increase in the concentration of these components leads to the precipitation of an authigenic clay mineral and authigenic quartz carboxylic acids are claimed to have the capacity to complex aluminium and so increase the solubility of aluminium and eliminate the precipitation of a clay phase however they are unlikely to have any major effect on aluminium solubility in natural i e complex pore fluids in fact dissolution will occur in acid neutral or alkaline conditions at a rate that is kinetically controlled reaction rates speed up exponentially with increasing temperature and therefore depth resulting in the destruction of more and more unstable components this increase in the rate of destruction of unstable components at depth can give rise to what appears to be a late leaching event what is seen in fact is the natural culmination of the destruction of thermodynamically unstable phases at an ever increasing rate during burial thus for a rock of a given composition the fraction of the total porosity that is secondary will increase with burial depth until all labile components are consumed the behaviour of mineral mixtures in the presence of a moving pore fluid is an additional contributing factor to the ubiquity of rds porosity where two or more minerals add the same ion to the pore fluid then the steady state concentration of that ion which is reached at some distance down the flow path theoretically an infinite distance is a weighted average of the equilibrium solubilities of that ion with respect to the minerals that contribute the ion thus as the pore fluid approaches a steady state concentration of the ion it will be undersaturated with respect to the least stable minerals and oversaturated with respect to the most stable mineral hence rds porosity must form as a result of the dissolution of the least stable mineral and the precipitation of the most stable mineral although kaolinite is the most common dissolution product of feldspars with increasing temperature its stability field shrinks with respect to other clay minerals such as illite thus it is possible for feldspars and other aluminosilicates to decompose at depth to give illite and authigenic quartz the growth of fibrous illite together with the change from a dominantly primary to a dominantly secondary pore structure can give rise to a rapid loss of permeability with depth where changes in the clay mineral type do not occur the impact of rds porosity on the porosity permeability behaviour of sandstones is minimal up to secondary porosities of about 10 beyond this figure there is at present too little data to make a clear statement

shear fracture pattern and microstructural evolution in transpressional fault zones from field and laboratory studies abstract zones of transpressional shear deformation accommodate strike slip and oblique slip displacements field work in a transpressive shear zone and transpressional analogue clay box modelling show that a p oriented foliation and associated p shears are preferentially developed over the more common r 1 riedel shears the carboneras fault system cfs in se spain is a left lateral transpressional shear zone with an internal geometry characterized by first order y oriented faults and widespread p oriented second order faults the mesoscopic to microscopic gouge fabric reflects the regional architecture of the shear zone being dominated by a pervasive poriented foliation and discrete y and p shears friction experiments carried out to investigate the textural evolution of gouge fabrics showed four textural stages of fabric development from foliation formation to extreme shear localization resulting in cross gouge failure transpression clay box models favoured the formation of secondary p oriented shear fractures and p oriented shear lenses further deformation caused differential shear lens rotation and shear lens orientations closer to the mean displacement direction our field studies and laboratory analogue experiments indicate that shear zones dominated by p shears are diagnostic of a transpressional deformation regime

channelized fluid flow and mass transfer along a rift related detachment fault eastern alps southeast switzerland remnants of a rift related low angle detachment system are spectacularly exposed in the continental err nappe and occur discontinuously in the underlying oceanic platta nappe in southeast switzerland rocks along the detachment fault exhibit systematic changes in mineralogy fabrics and structures that are related to deformation and hydration of granite to form cataclasite and subsequent gouge structural mineralogical and geochemical data indicate a continuous evolution of fluid flow and mass transfer along the detachment fault mass balance calculations together with stable isotope data suggest 1 an altered seawater source and local interaction with mantle rocks 2 limited fluid rock interaction and mass transfer during initial cataclasis at deeper crustal levels and temperatures of 150 to 200 c 3 increased fluid flux and mass transfer at slightly lower temperatures 100 150 c during gouge formation at shallower crustal levels and 4 strongly localized fluid rock interaction at low temperatures 65 80 c near the sea floor at a final stage of detachment faulting we propose a model in which seawater dominated fluids penetrated and locally interacted with mantle rocks before they were caught and channelized along large scale detachment faults flow direction is upward along the detachment from higher to lower temperatures suggesting that the driving force of fluid flow was thermal fluid convection associated with mantle exhumation our study suggests that this large scale detachment fault is a good analog for extensional structures along present day nonvolcanic rifted margins such structures along ocean continent transitions are important in controlling fluid pathways and mass transfer between mantle and continental crust

quantitative analyses of porosity evolution in tight grainstones a case study of the triassic feixianguan formation in the jiannan gas field sichuan basin china abstract tight grainstones although widespread throughout the lower triassic feixianguan formation in the sichuan basin have received little attention in part due to their lower porosity and greater heterogeneity relative to their dolostone counterparts based on data from cores and thin sections as well as petrophysical properties the feixianguan grainstones representing a major gas reservoir in the jiannan gas field were systemically analysed to better understand porosity evolution in tight carbonates that have experienced original oil accumulation and subsequent thermal cracking during progressive burial the grainstones were divided into two types according to whether pyrobitumen was present and their porosity evolutions were quantitatively reconstructed taking 40 as the original porosity the grainstones without pyrobitumen which were ineffective palaeo oil reservoirs lost 21 94 and 3 13 of their porosities through marine and burial calcite cementation respectively and 13 34 by compaction and have a current porosity of 1 59 thus allowing them to serve as major present day gas reservoirs comparatively pyrobitumen bearing grainstones which were once palaeo oil reservoirs lost 23 96 and 2 36 of their porosities through marine and burial calcite cementation respectively 11 4 by compaction and 1 44 by pyrobitumen and have a current porosity of 0 84 thus making them ineffective gas reservoirs this study provides a quantitative understanding of the close association between porosity evolution and reservoir effectiveness for the palaeo oil charge and present day gas accumulation with respect to diagenetic history which is useful for the future exploration in tight gas limestone reservoirs

the death valley turtlebacks reinterpreted as miocene pliocene folds of a major detachment surface determining the origin of extension parallel folds in metamorphic core complexes is fundamental to understanding the development of detachment faults an excellent example of such a feature occurs in the death valley region of california where a major undulatory detachment fault is exposed along the well known turtleback antiformal surfaces of the black mountains in the hanging wall of this detachment fault are deformed strata of the copper canyon formation new age constraints indicate that the copper canyon formation was deposited from 6 to 3 ma the formation was folded during deposition into a se plunging syncline with an axial surface coplanar with that of a synform in the underlying detachment this relation suggests the turtlebacks are a folded detachment surface formed during large scale extension in an overall constrictional strain field the present more planar black mountains frontal fault system may be the result of out stepping of a normal fault system away from an older detachment fa

earthquake faulting as a structural process abstract structural geology is concerned with the history of movement in the earth s crust and the processes by which displacements occur in the upper one third to one half of deforming continental crust displacement is accommodated largely by seismic slip increments on existing faults it follows that earthquakes and related processes are an integral part of structural geology traditionally structural geologists have been preoccupied with the complexity of the finite deformation within fault zones and with the stress states prevailing at the initiation of faults in intact crust future structural work should be directed more towards understanding the dynamic character of fault reactivation during incremental slip and related effects questions of interest include rheological and geometrical controls on the initiation perturbation and termination of ruptures directivity effects associated with rupture propagation the recognition of structures resulting from repeated stress cycling within seismogenic crust and identification of structural features diagnostic of shear stress levels during faulting structures arising from the inter relationships between slip episodes and induced fluid flow are of special importance because these dynamic fault processes appear influential in the development of much fault hosted mineralization mesothermal gold quartz lodes hosted in high angle reverse shear zones of mixed brittle ductile character form illustrative examples of structures that arguably can only be interpreted by seismo structural analysis embodying the concepts listed above

evidence for paleoseismic slip on a continental low angle normal fault tectonic pseudotachylyte from the west salton detachment fault ca usa abstract the potential of continental low angle normal faults lanf to nucleate large 6 0 m w earthquakes at low angles remains unclear despite much focused research we document evidence for ancient seismicity along a continental lanf detachment fault that formed and slipped at low angles and produced tectonic pseudotachylyte these thick and laterally persistent pseudotachylyte accumulations along the west salton detachment fault wsdf salton trough usa preserve convincing evidence for a frictional melt origin including spherulitic microlites ductile flow structures preservation of high temperature phases as clasts and injection veins cumulative thickness of pseudotachylyte along the fault ranges from 0 1 to 1 5 m and pseudotachylyte cataclasite in the fault core and damage zone are exposed along 2 6 km length of the fault reworked fragments of pseudotachylyte in cataclasites and multiple generations of cataclasites provide evidence for the preservation of multiple earthquake cycles the limited exposure

frictional properties of natural fault gouge from a low angle normal fault panamint valley california 1 we investigate the relationship between frictional strength and clay mineralogy of natural fault gouge from a low angle normal fault in panamint valley california gouge samples were collected from the fault zone at five locations along a north south transect of the range bounding fault system spanning a variety of bedrock lithologies samples were powdered and sheared in the double direct shear configuration at room temperature and humidity the coefficient of friction was measured at a range of normal stresses from 5 to 150 mpa for all samples our results reinforce the intuitive understanding that natural fault gouge zones are inherently heterogeneous samples from a single location exhibit dramatic differences in behavior yet all three were collected within a meter of the fault core for most of the samples friction varies from 0 6 to 0 7 consistent with byerlee s law however samples with greater than 50 wt total clay content were much weaker 0 2 0 4 expandable clay content of the samples ranged from 10 to 40 wt frictional weakness did not correlate with expandable clays our results indicate that friction decreases with increasing total clay content rather than with the abundance of expandable clays the combination of field relations analytical results and friction measurements indicates a positive correlation between clay content fabric intensity and localization of strain in the fault core a mechanism based upon foliations enveloping angular elements to reduce friction is suggested for weakening of fault gouge composed of mixed clay and granular material we provide broad constraints of 1 5 km on the depth of gouge generation and the depth at which fault weakness initiates we show that slip on the panamint valley fault and similar low angle normal faults is mechanically feasible in the mid upper crust if the strength of the fault is limited by weak clay rich fault gouge

a kinematic model for deformation within brittle shear zones textural analysis of fault gouge indicates that displacement of particles within brittle shear zones occurs by three diaerent modes synthetic riedel shears antithetic riedel shears and shearing particulate flow a kinematic model is developed which incorporates these three slip modes and constrains the combined deformation to be simple shear parallel to the shear zone boundaries penetrative fabric p foliation and shape preferred orientation of survivor grains commonly observed in foliated fault gouge is assumed to be a result of the shearing particulate flow only the model which relates penetrative fabric orientation to the relative shear rates on riedel shears and shearing particulate flow reconciles inclined p foliation and high g 10 shear strain 1999 elsevier science ltd all rights reserved

high angle origin of the currently low angle badwater turtleback fault death valley california the late cenozoic badwater turtleback fault separates an upper plate of volcanic and sedimentary rocks from a lower plate of predominantly mylonitic plutonic and metamorphic rocks the turtleback fault however is not a single continuous surface but consists of at least three generations of faults these faults occur as discrete crosscutting segments that progressively decrease in age and increase in dip to the west therefore they probably began at moderate to steep angles but rotated to lower angles with extensional strain if so lower plate mylonitic rocks also restore to steeper dips and suggest that transport of the upper plate occurred on moderate to steeply dipping surfaces in the middle and upper crust the crosscutting nature of the fault segments and their initial moderate to steep dips plus a possible offset marker on one of the segments are most consistent with moderate amounts of extension in the death valley region

continuous hydrocarbon accumulation over a large area as a distinguishing characteristic of unconventional petroleum the ordos basin north central china abstract global petroleum exploration is currently undergoing a strategic shift from conventional to unconventional hydrocarbon resources unconventional hydrocarbons in tight reservoirs show characteristics distinct from those of conventional hydrocarbon sources hosted in structural and stratigraphic traps the characteristic features include the following a hydrocarbon source and reservoir coexist porosity and permeability are ultra low nano pore throats are widely distributed hydrocarbon bearing reservoir bodies are continuously distributed there is no obvious trap boundary buoyancy and hydrodynamics have only a minor effect and darcy s law does not apply phase separation is poor there is no uniform oil gas water interface or pressure system and oil or gas saturation varies examples of unconventional hydrocarbon accumulations are the mesozoic tight sandstone oil province and the upper paleozoic tight sandstone gas province in the ordos basin north central china generally continuous hydrocarbon accumulation over a large area is a distinguishing characteristic of unconventional hydrocarbon sources because of the great potential of unconventional petroleum resources it is believed that research on such resources will be at the forefront of the future development of petroleum geology

patterns of mineral transformations in clay gouge with examples from low angle normal fault rocks in the western usa abstract neoformed minerals in shallow fault rocks are increasingly recognized as key to the behavior of faults in the elasto frictional regime but neither the conditions nor the processes which wall rock is transformed into clay minerals are well understood yet understanding of these mineral transformations is required to predict the mechanical and seismogenic behavior of faults we therefore present a systematic study of clay gouge mineralogy from 30 outcrops of 17 low angle normal faults lanf s in the american cordillera to demonstrate the range and type of clay transformations in natural fault gouges the sampled faults juxtapose a wide and representative range of wall rock types including sedimentary metamorphic and igneous rocks under shallow crustal conditions clay mineral transformations were observed in all but one of 28 faults one fault contains only mechanically derived clay rich gouge which formed entirely by cataclasis clay mineral transformations observed in gouges show four general patterns 1 growth of authigenic 1m d illite either by transformation of fragmental 2m 1 illite or muscovite or growth after the dissolution of k feldspar illitization of fragmental illite smectite is observed in lanf gouges but is less common than reported from faults with sedimentary wall rocks 2 retrograde diagenesis of an early mechanically derived chlorite rich gouge to authigenic chlorite smectite and saponite mg rich tri octahedral smectite 3 reaction of mechanically derived chlorite rich gouges with mg rich fluids at low temperatures 50 150 c to produce localized lenses of one of two assemblages sepiolite saponite talc lizardite or palygorskite chlorite quartz and 4 growth of authigenic di octahedral smectite from alteration of acidic volcanic wall rocks these transformation groups are consistent with patterns observed in fault rocks elsewhere the main controls for the type of neoformed clay in gouge appear to be wall rock chemistry and fluid chemistry and temperatures in the range of 60 180 c

fault zone architecture and deformation processes within evaporitic rocks in the upper crust 1 recently in the northern apennines geophysical data have identified the triassic evaporites te anhydrites and dolomites as the source region of the major extensional earthquakes of the area m 6 in order to characterize fault zone architecture and deformation processes within the te we have studied exhumed evaporite bearing normal faults within the upper crust the structure of large displacement 100 m normal faults is given by 1 a zoned fault core with a wider portion of fault parallel foliated ca sulphates ductile deformation overprinted by an inner fault core ifc of localized brittle deformation and 2 wide dolostones to absent ca sulphates damage zones of fault fracture patterns fault rock assemblage within the ifc is characterized by fault breccia gouge and cataclasites of different grain size most of the deformation within the ifc is localized along thin and fault parallel principal slip surfaces pss made of dolomite rich fine grained cataclasite sem analyses show an evolution from ca to st to gypsum rich mineralization due to episodic fluid flow events channeled along the fault zones during different stages of fault exhumation the development of the observed fault geometry can be explained by a mechanical fault evolution model where initial faulting occurs along broad and ductile shear zones within the anhydrites and causes fracturing within the dolostones progressive deformation within the fault core leads to the development of fault parallel dolomite rich cataclastic layers their reactivation coupled with transient fluid overpressures can produce embrittlement and localization of brittle deformation within the ifc

anisotropy pore fluid pressure and low angle normal faults low angle normal faults lanfs are commonly associated with high angle normal faults high pore fluid pressures and foliated fault cores explanations for lanf slip based on high pore fluid pressure critically depend on stress rotations in the fault core zone which have been based on an assumption of isotropy anisotropy fundamentally changes the coupling between pore fluid pressure and effective stress the response of saturated fault core rocks in lanfs can be explored by combining the effective medium theory with anisotropic poroelasticity the nucleation and evolution of lanfs are investigated in a framework of local changes in rock properties and the interaction of a pressurised pore fluid with intrinsic lithological and extrinsic damage anisotropies this quantitative approach enables comparison with detailed field and laboratory measurements of fault rock properties

quaternary low angle slip on detachment faults in death valley california detachment faults on the west flank of the black mountains nevada and california dip 29 36 and cut subhorizontal layers of the 0 77 ma bishop ash steeply dipping normal faults confined to the hanging walls of the detachments offset layers of the 0 64 ma lava creek b tephra and the base of 0 12 0 18 ma lake manly gravel these faults sole into and do not cut the low angle detachments therefore the detachments accrued any measurable slip across the kinematically linked hanging wall faults an analysis of the orientations of hundreds of the hanging wall faults shows that extension occurred at modest slip rates 1 mm yr under a steep to vertically oriented maximum principal stress the black mountain detachments are appropriately described as the basal detachments of near critical coulomb wedges we infer that the formation of late pleistocene and holocene range front fault scarps accompanied seismogenic slip on the detachments

the effects of diagenesis on the reservoir characters in sandstones of the late cretaceous pab formation kirthar fold belt southern pakistan abstract the maastrichtian pab formation in the southern part of pakistan is composed of fine to very coarse grained texturally mature quartz arenite and subordinate sublitharenite varieties the sandstones have undergone intense and complex diagenetic episodes due to burial and uplift diagenetic modifications were dependent mainly on the clastic composition of sandstone burial depth and thrust tectonics diagenetic events identified include compaction precipitation of calcite quartz clay minerals and iron oxide hydroxide dissolution and alteration of unstable clastic grains as feldspar and volcaniclithic fragments as well as tectonically induced grain fracturing the unstable clastic grains like feldspar and lithic volcanic fragments suffered considerable alteration to kaolinite and chlorite dissolution and alteration of feldspar and volcanic lithic fragments and pressure solution were the main sources of quartz cements mechanical compaction and authigenic cements like calcite quartz and iron oxide hydroxide reduced the primary porosity whereas dissolution of clastic grains and cements has produced secondary porosity chlorite coatings on clastic grains have prevented quartz cementation coarse grained thick bedded packages of fluviodeltaic shelf delta lobe and submarine channels facies have higher average porosity than fine grained thin bedded and bioturbated sandstone of deeper shelf and abyssal plain environments and these facies are concluded to be possible future hydrocarbon prospects

weakness and mechanical anisotropy of phyllosilicate rich cataclasites developed after mylonites of a low angle normal fault simplon line western alps abstract the simplon fault zone is a late collisional low angle normal fault lanf of the western alps the hanging wall shows evidence of brittle deformation only while the footwall is characterized by a c 1 km thick shear zone the simplon fault zone which continuously evolved during exhumation and cooling from amphibolite facies conditions to brittle cataclastic deformations due to progressive localization of the active section of the shear zone the thermal rheological evolution of the footwall resulted in a layered structure with higher temperature mylonites preserved at the periphery of the shear zone and cataclasites occurring at the core indicated as the simplon line in order to investigate the weakness of the simplon line we studied the evolution of brittle cataclastic fault rocks from nucleation to the most mature ones cataclasites are superposed on greenschist facies mylonites and their nucleation can be studied at the periphery of the brittle fault zone this is characterized by fractures micro faults and foliated ultracataclasite seams that develop along the mylonitic scc fabric exploiting the weak phases mainly represented by muscovite and chlorite approaching the fault core both the thickness and frequency of cataclasite horizons increase and as their thickness increases they become less and less foliated the fault core itself is represented by a thicker non foliated cataclasite horizon no andersonian faults or fractures can be found in the footwall damage zone and core zone whilst they are present in the hanging wall and in the footwall further from the fault applying a stress model based on slip tendency we have been able to calculate that the friction coefficient of the simplon line cataclasites was

quantitative identification of diagenetic facies and controls on reservoir quality for tight sandstones a case study of the triassic chang 9 oil layer zhenjing area ordos basin abstract the identification of diagenetic facies is essential for prospecting hydrocarbon reserves and for hydrocarbon exploration in tight oil reservoirs a comprehensive method for determining quantitative index of reservoir diagenetic facies rdi using core analysis techniques hierarchical cluster analyses hca and grey correlation analyses gca was proposed the petrological characteristics physical properties and pore structures of the chang 9 tight oil sandstones are analysed based on thin section analysis x ray diffraction xrd scanning electron microscopy sem cathode luminescence cl and mercury injection capillary pressure micp tests the properties of the chang 9 sandstones were controlled by different forms of diagenesis compaction and cementation of calcite have reduced or occluded the pore volume in the sandstones but not all cementation is deleterious to reservoir quality chlorite cementation helped preserve relict primary porosity and the direct dissolution of feldspars and unstable lithic fragments has resulted in enhanced pore connectivity following petrographic observations and quantitative analysis the sandstones were divided into four diagenetic facies calcite cementation facies intense compaction facies porosity preserved by grain coating chlorites facies and dissolution facies the intense compaction facies and calcite cementation facies are responsible for the evolution of low quality reservoirs while the reservoirs with porosity preserved by grain coating chlorites facies and dissolution facies are of better porosity and permeability the distribution of fractures improves reservoir quality by enhancing the dissolution of unstable grains and providing convenient channels and storage spaces for oil and gas migration

the origin and charging directions of neogene biodegraded oils a geochemical study of large oil fields in the middle of the shijiutuo uplift bohai sea china abstract two large oil fields qhd32 6 and qhd33 1 located in the middle part of the shijiutuo uplift have generally suffered mild biodegradation based on multivariate statistical analysis of the biomarker parameters this study discussed the origin and charging directions for these two oil fields in contrast to ed 3 derived oil all available oil samples from these two large oil fields displayed low c 19 c 23 c 24 c 26 and high g h and 4 msi which are attributed to the mixtures of oils derived from the shahejie es 1 and es 3 source rocks oils in qhd32 6 which contain relatively more es 3 derived oil are called group i oils and most oils in qhd33 1 which share relatively more es 1 derived oil are called group ii oils our mixed oil experiments reveal the predominant es 3 and es 1 derived oil contribution for group i and group ii oil groups respectively however the selection of end member oils warrants further research based on comparisons of biomarker parameters the qhd32 6 oil field was mainly charged in the north by oil generated from shahejie formation source rocks in the bozhong depression however oils from the north of qhd32 6 field display a remarkable difference to the oils in the south of this field which may indicate that a charging pathway exists from the qhd33 1 field considering the variations in biomarker compositions in the west to east and northwest to southeast sections across the qhd33 1 and qhd32 6 oil fields it can be deduced that es 3 sourced oil migrated westward to the qhd32 6 traps and then charging by es 1 oil from the bozhong sag resulted in the qhd33 1 oil field being characterized by the mixture of es 3 and es 1 sourced oil moreover migration of es 1 derived oil from the qinnan sag was not identified implying that the qhd33 1 oil field is mainly charged from the northeast of the bozhong sag

clast fabric development in a shearing granular material implications for subglacial till and fault gouge elongate clasts in subglacial till and in fault gouge align during shearing but the relation between clast fabric strength and cumulative shear strain for such materials is effectively unknown this relation was explored in experiments with a large ring shear device in which a till and a viscous putty that contained isolated clasts were sheared to high strains as expected rotation of clasts in the putty is closely approximated by the theory of g b jeffery who derived the orbits of rigid ellipsoids in a slowly shearing fluid clast rotation in the till however is strikingly different rather than orbiting through the shear plane as predicted by jeffery most clasts rotate into the shear plane and remain there resulting in strong fabrics regardless of the aspect ratios and initial orientations of clasts this divergent behavior is likely due to slip of the till matrix along the surfaces of clasts which is a natural expectation in a granular material but violates the no slip condition of jeffery9s model these results do not support the widespread belief that subglacial till deformation results in weak clast fabrics thus many tills with weak fabrics thought to have been sheared subglacially to high strains like many basal tills of the laurentide ice sheet may have been sheared only slightly with little effect on either ice sheet dynamics or sediment transport in addition these results indicate that in simple shear the rotation of clasts in till and in fault gouge is best analyzed with the model of a march who treated inclusions as passive markers

controls on reservoir heterogeneity of tight sand oil reservoirs in upper triassic yanchang formation in longdong area southwest ordos basin china implications for reservoir quality prediction and oil accumulation abstract compared to conventional reservoirs pore structure and diagenetic alterations of unconventional tight sand oil reservoirs are highly heterogeneous the upper triassic yanchang formation is a major tight oil bearing formation in the ordos basin providing an opportunity to study the factors that control reservoir heterogeneity and the heterogeneity of oil accumulation in tight oil sandstones the chang 8 tight oil sandstone in the study area is comprised of fine to medium grained moderately to well sorted lithic arkose and feldspathic litharenite the reservoir quality is extremely heterogeneous due to large heterogeneities in the depositional facies pore structures and diagenetic alterations small throat size is believed to be responsible for the ultra low permeability in tight oil reservoirs most reservoirs with good reservoir quality larger pore throat size lower pore throat radius ratio and well pore connectivity were deposited in high energy environments such as distributary channels and mouth bars for a given depositional facies reservoir quality varies with the bedding structures massive or parallel bedded sandstones are more favorable for the development of porosity and permeability sweet zones for oil charging and accumulation than cross bedded sandstones authigenic chlorite rim cementation and dissolution of unstable detrital grains are two major diagenetic processes that preserve porosity and permeability sweet zones in oil bearing intervals nevertheless chlorite rims cannot effectively preserve porosity permeability when the chlorite content is greater than a threshold value of 7 and compaction played a minor role in porosity destruction in the situation intensive cementation of pore lining chlorites significantly reduces reservoir permeability by obstructing the pore throats and reducing their connectivity stratigraphically sandstones within 1 m from adjacent sandstone mudstone contacts are usually tightly cemented carbonate cement 10 with low porosity and permeability lower than 10 and 0 1 md respectively the carbonate cement most likely originates from external sources probably derived from the surrounding mudstone most late carbonate cements filled the previously dissolved intra feldspar pores and the residual intergranular pores and finally formed the tight reservoirs the petrophysical properties significantly control the fluid flow capability and the oil charging accumulation capability of the chang 8 tight sandstones oil layers usually have oil saturation greater than 40 a pore throat radius of less than 0 4 m is not effective for producible oil to flow and the cut off of porosity and permeability for the net pay are 7 and 0 1 md respectively

mesoscopic structure of the punchbowl fault southern california and the geologic and geophysical structure of active strike slip faults we examine the distribution density and orientation of outcrop scale structures related to the punchbowl fault an exhumed ancient trace of the san andreas fault southern california in order to determine the structure of the fault zone the punchbowl fault has 44 km of right lateral slip and cuts the cretaceous pelona schist in the study area the mesoscopic structures examined include fractures small faults and veins they were inventoried using scan lines at closely spaced stations along three strike perpendicular traverses 200 250 m long across the fault the fault zone thickness is a function of the type of structure measured slip along narrow 2 m wide ultracataclasite cores of the faults results in foliation reorientation over a distance of 50 m from the cores fracture and fault densities appear to increase 50 80 m from the fault cores and vein densities are highly variable across the fault zone fractures and faults in the damaged zone have a variety of orientations but most are at high angles to the main fault zone when coupled with previous geochemical and microstructural data these data show that large displacement faults of the san andreas system are up to 200 250 m thick and enclose zones of mineralogic and geochemical alteration that are 20 30 m thick extreme slip localization occurs over zones 1 5 m thick when reconciled with geophysical imaging our data suggest that trapped headwaves travel in the damaged zone and that some aftershock events produce slip on faults and fractures which often have orientations very diaerent from the principal slip surfaces 7 2000 elsevier science ltd all rights reserved

pore throat sizes in sandstones tight sandstones and shales pore throat sizes in siliciclastic rocks form a continuum from the submillimeter to the nanometer scale that continuum is documented in this article using previously published data on the pore and pore throat sizes of conventional reservoir rocks tight gas sandstones and shales for measures of central tendency mean mode median pore throatsizes diameters are generally greater than2 mm in conventionalreservoir rocks range from about 2 to 0 03 mm in tight gas sandstones and rangefrom0 1to0 005 mminshales hydrocarbonmolecules asphaltenes ring structures paraffins and methane form another continuum ranging from 100 a 0 01 mm for asphaltenes to 3 8 a 0 00038 mm for methane the pore throat size continuum provides a useful perspective for considering 1 the emplacement of petroleum in consolidated siliciclastics and 2 fluid flow through fine grained source rocks now being exploited as reservoirs

field tests of rolling hinges existence mechanical types and implications for extensional tectonics in rolling hinge models a locus of isostatic footwall uplift migrates together with the withdrawing hanging wall of a large displacement tens of kilometers normal fault a fault bend fold that migrates through the footwall results causing progressive deactivation of the updip part of the fault as it is tilted to a lower dip rolling hinge migration should leave a discernible geologic record but compelling tests are difficult to design because predictions of rolling hinge models are nonunique and alternative models commonly can explain individual data sets field tests based on thermochronology paleomagnetism and structural analysis are typically applied to footwall rocks whereas sedimentology geochronology of volcanic rocks and structural analysis are typically applied to upper plate rocks we review existing theoretical models and case studies in 14 detachment terranes some negative results are reasonably compelling but positive tests as yet tend to permit rather than demand a rolling hinge locally both positive and negative conclusions have been reached about the passage of a hinge within the same terrane such differences in interpretation may depend more on the scale of observation or on the specific model considered than on the geological development of the area subvertical simple shear and flexural failure mechanisms with or without significant induced flow of lower crustal material are thought to dominate hinge related footwall deformation in individual sites although rolling hinge models offer an alternative to low initial fault dips hinge related deformation does not demand a steep initial fault dip few areas in which rolling hinge tectonics are permissible had initially steep faults 35 rather where the evidence most strongly favors a rolling hinge detachment faults had initial dips 35 in the future conclusive results are most likely to be obtained if diverse tests are applied to both the upper and lower plates of detachment terranes where other geological sources of ambiguity are minimal at best the rolling hinge mechanism does not universally explain the observation that earthquakes are rare on low angle normal faults nor solve the mechanical problems of frictional slip on such faults

structural geology and kinematic history of rocks formed along low angle normal faults death valley california several late cenozoic low angle normal or detachment faults on the western flank of the black mountains are each characterized by the following in descending structural order 1 a hanging wall of upper tertiary to quaternary sedimentary and volcanic strata that displays little evidence for fault related damage other than widely spaced planar or listric normal faults 2 a sharp planar and locally striated principal slip plane forming the lower boundary of the hanging wall 3 an upper zone zone i of very fine grained fault generated rocks composed dominantly of gouge 4 a lower zone zone ii of coarser grained fault rocks consisting chiefly of foliated breccia and 5 a variably damaged footwall consisting of partly mylonitic precambrian units or tertiary plutonic rocks that has been exhumed from depths of 10 12 km since late miocene time the fault rocks which were mostly derived from the footwalls preserve evidence for cataclastic and particulate flow fault rocks contain authigenic minerals but lack the cyclically deformed mineral filled syntectonic veins that are abundant in some other late cenozoic high angle and strike slip faults meso and microscale fabrics in zones i and ii indicate that finite shear strains increase progressively upward toward the principal slip plane in a conceptual kinematic model of a shear box displacements of the hanging wall produced a shearing flow in the fault rocks below some of the slip on the principal slip plane was also partitioned into localized slip on discrete sliding surfaces in zones i and ii since 770 ka during the latest stages of incremental deformation in the brittle shear zones distributed flow in the fault rocks alternated with slip that was chiefly localized on the principal slip planes in this respect the detachment faults differ from inactive segments of the san andreas system along which displacements were progressively and irreversibly localized onto a single principal slip surface the strain gradients and corresponding changes in grain size in the shear zones resemble those in mylonitic shear zones except in their symmetry strain gradients in the black mountains are notably asymmetric they are present only below the principal slip planes not above

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

kinematics of conjugate shear zones displacement partitioning and fragmentation of the upper rigid crust during denudation of high p rocks pelagonian and sub pelagonian zones greece abstract in the sub pelagonian and pelagonian zones kinematic analyses on faults and cataclastic shear zones document an overall displacement vector towards nnw and wsw ssw for attica and western macedonia respectively this tectonic episode is interpreted as being triggered by the kinematically oligocene miocene detachment of the sub pelagonian and pelagonian zones from the high p rocks during this episode the sub pelagonian and pelagonian zones were decoupled along detachment zones fragmentised by brittle mechanisms and horizontally stretched in km scale the strain record of the cataclastically deformed lithologies of the upper plates reveals a complex kinematic pattern with displacement partitioning in an overall non coaxial regime these structural processes are considered to be kinematically linked to the exhumation of the high p rocks deforming the sub pelagonian and pelagonian zones

how clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones observations and experiments observations and hydrothermal experiments were used to derive new information about how clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones samples of deeply buried porous sandstones with different types of clay coats were split in two coats removed from one of each pair of splits and grain surfaces inspected with scanning electron microscopy quartz grains in a fluvial deltaic sandstone buried to 115c had no visible authigenic quartz on grain surfaces cleaned of diagenetic chlorite coats though well developed overgrowths occurred on nearby naturally uncoated grains however in similar sandstones buried to 164c quartz grain surfaces exposed by chlorite coat removal were covered with small 5 m mainly anhedral syntaxial quartz overgrowths similar overgrowths were observed under various detrital and diagenetic clay coats in porous eolian sandstones buried to temperatures up to 215c we conclude that clay coats may retard quartz nucleation at moderate temperatures but at high temperatures many coats permit quartz nucleation and preserve porosity by limiting cement growth to investigate cement growth limitation mechanisms samples with coats removed were subjected to quartz cementing conditions in a hydrothermal reactor during experiments the naturally occurring small overgrowths on clay cleaned grains coalesced and grew suggesting that clay particles in coats inhibit cement growth by forming barriers to early overgrowth coalescence although the fraction of grain surface coverage is the primary control on cement inhibition by coats cement growth interference textures vary with coat type providing a mechanism by which coat composition may be a secondary control on inhibitory effectiveness in deeply buried sandstones quartz cement can fill significant microporosity within diagenetic chlorite coats potentially affecting mechanical and petrophysical rock properties

tectonic evolution of a low angle extensional fault system from restored cross sections in the northern apennines italy 1 broad geological and geophysical documentation is available on regional extensional systems driven by low angle normal faults however little information exists about the three dimensional geometry and the offset distribution of such extensional structures we present a new set of balanced geological sections across the extensional fault system driven by the altotiberina low angle normal fault in the northern apennines of italy we document this extensional system throughout a large set of surface field surveys and geological maps and subsurface data seismic reflection profiles and boreholes the subsurface data allowed us to define the fault deep geometry and to obtain its structural contours the fault geometry is characterized by both along dip and along strike irregularities in cross section the fault displays a staircase trajectory with the shallowest part being dome shaped and flattened to horizontal this bending could be due to the footwall uplift triggered by a footwall uploading greater than about 115 mpa the sequential restoration of five geological cross sections yields a maximum extension of about 10 km accumulated over approximately 3 ma the resulting long term slip rate is about 3 mm yr which is of the same order as the present day extensional rate measured by gps 2 5 3 0 mm yr suggesting an almost steady state extension over the last 3 ma the distribution of the extension values along the fault strike is bell shaped as expected for a continuous surface

magnetic and clast fabrics as measurements of grain scale processes within the death valley shallow crustal detachment faults 1 the rock product of shallow crustal faulting includes fine grained breccia and clay rich gouge many gouges and breccias have a fabric produced by distributed deformation the orientation of fabric elements provides constraints on the kinematics of fault slip and is the structural record of intrafault strain not accommodated by planar and penetrative surfaces however it can be difficult to quantify the deformational fabric of fault rocks especially the preferred orientations of fine grained minerals or to uniquely determine the relationship between fabric geometry and finite strain here we present the results of a fabric study of gouge and breccia sampled from low angle normal detachment faults in the black mountains death valley ca we measured a preferred orientation of the long axes of the clasts inherited from the crystalline footwall of the fault and compared the shape preferred orientation to the anisotropy of magnetic susceptibility of the fault rocks the two measurements of fabric exhibit systematic similarities and differences in orientation and anisotropy that are compatible with the large scale kinematics of fault slip the dominant carriers of the magnetic susceptibility are micron and sub micron scale iron oxides and clay minerals therefore even the finest grains in the fault rock were sensitive to the distributed deformation and the micro mechanics of particle interaction must have departed from those assumed by the passive marker kinematic model that best explains the fabric

fault structure frictional properties and mixed mode fault slip behavior abstract recent high resolution gps and seismological data reveal that tectonic faults exhibit complex multi mode slip behavior including earthquakes creep events slow and silent earthquakes low frequency events and earthquake afterslip the physical processes responsible for this range of behavior and the mechanisms that dictate fault slip rate or rupture propagation velocity are poorly understood one avenue for improving knowledge of these mechanisms involves coupling direct observations of ancient faults exhumed at the earth s surface with laboratory experiments on the frictional properties of the fault rocks here we show that fault zone structure has an important influence on mixed mode fault slip behavior our field studies depict a complex fault zone structure where foliated horizons surround meter to decameter sized lenses of competent material the foliated rocks are composed of weak mineral phases possess low frictional strength and exhibit inherently stable velocity strengthening frictional behavior in contrast the competent lenses are made of strong minerals possess high frictional strength and exhibit potentially unstable velocity weakening frictional behavior tectonic loading of this heterogeneous fault zone may initially result in fault creep along the weak and frictionally stable foliated horizons with continued deformation fault creep will concentrate stress within and around the strong and potentially unstable competent lenses which may lead to earthquake nucleation our studies provide field and mechanical constraints for complex mixed mode fault slip behavior ranging from repeating earthquakes to transient slip episodic slow slip and creep events

microstructures of cataclasites in a limestone on shale thrust fault implications for low temperature recrystallization of calcite abstract the hunter valley thrust hvt a low temperature foreland thrust fault in the valley and ridge province southern appalachians produced two distinct cataclasites a limestone cataclasite derived predominantly from the hanging wall limestone and a shale cataclasite derived predominantly from the footwall shale the limestone cataclasite consists of fragments of calcite limestone pre existing limestone cataclasite quartz and quartz aggregates in a fine grained

ductility in fault gouge from a normal fault system death valley california a mechanism for fault zone strengthening and relevance to paleoseismicity fault gouge along the badwater turtleback fault system displays abundant evidence for mesoscopic ductility as foliation isolated and or rotated inclusions and isoclinal folds fault gouge thicknesses also vary dramatically at the outcrop scale gouge thicknesses can range from 0 to 1 m and at the 100 m scale they can range from 0 to 5 m because these variations are closely associated with other ductility related features they probably also formed because of ductile flow ductile flow appears to locally strengthen the fault zone by severe attenuation of the gouge and by transport of inclusions where gouge pinches out to zero thickness hanging wall rock rests directly on footwall rock a condition that produces an overall stronger fault zone as well as stick slip behavior in laboratory tests transport of inclusions within the gouge zone may create local inclusion jams which may also have a strengthening effect the presence of these stronger portions of the fault zone may give indirect evidence for ancient seismicity on the fault

spatial variability in microscopic deformation and composition of the punchbowl fault southern california implications for mechanisms fluid rock interaction and fault morphology abstract we examine the distribution and nature of microstructures geochemistry and mineralogy along two traverses across the punchbowl fault southern california to determine the morphology and deformation mechanisms of the fault zone in schistose rocks the punchbowl fault is an exhumed fault that has two main strands of slip localization and has a total of 44 km of right slip protolith in the study area consists of the pelona schist which is primarily a quartz albite muscovite actinolite schist with thin to medium banding and rare metabasalts the traverses are 1 3 km apart and were conducted at a site with a single fault strand and a site where both principal strands of the fault are exposed the fault zone thickness is a function of the type of measurement that is used to define it for the single strand site analysis of the distribution of microfractures shows that the fault zone consists of a roughly 40 m thick damaged zone adjacent to the fault core the damaged zone is marked by an increase in veins thin cataclasite bands inter and intragranular fracturing and alteration relative to the country rock brittle grain size reduction occurs in a zone 10 m thick as measured from the fault core which consists of a continuous 10 cm thick very fine grained cataclasite that experienced repeated alteration vein injection and grain size reduction whole rock geochemical analyses of the fault related rocks suggest that the geochemically defined fault zone is less than 10 m thick volume loss at the site with one fault strand appears to have been small the dominant alteration reactions associated with the fault core are the hydration of hornblende and actinolite accompanied by the alteration of muscovite to produce a quartz chlorite albite epidote ultracataclasite the composition of the fault core is variable and locally influenced by one of the adjacent protoliths the examination of the two fault strand sites shows that two damaged zone fault core structures are present the region between the two strands experienced a greater degree of deformation than the protolith but the total deformation is much less than immediately adjacent to the fault cores the total thickness of the damaged zone around the two strands is less than 200 m the fault core enveloped by a damaged zone morphology as well as the textures of the fault core rocks are similar to rocks associated with the north branch san gabriel fault which formed in crystalline rocks with a total displacement of 22 km fault thickness is less in the pelona schist than in the crystalline rocks perhaps owing to more efficient strain localization in the schists thus the faults of the san andreas system may be thinner in regions where schists or franciscan rocks are the protolith but the main fault core may be a constant feature of the fault zone transformation induced weakening is less important in this part of the punchbowl fault since the protolith has a large amount of mica the structure of the fault zone with two principal slip surfaces is marked by both chemical changes and microstructures and indicates that some parts of the san andreas fault system may consist of multiple slip surfaces each with a damaged zone that together may create a fault zone 100 m thick and in which slip is localized to zones meters to decimeters thick

continental rifting and metamorphic core complex formation ahead of the woodlark spreading ridge d entrecasteaux islands papua new guinea 1 we evaluate the role of a metamorphic core complex mcc on normanby island in the woodlark rift located 1 km thickness of blueschist derived mylonites formed in a midcrustal shear zone during the pliocene at 400 500 c this top to the north zone appears to have reactivated the gently dipping base of the papuan ophiolite papuan ultramafic body pub and its continued activity appears to control the north dipping asymmetry of active half grabens to the north of the mcc and rapid subsidence of the woodlark rise mylonites in the mcc s lower plate have been exhumed along a detachment as a result of 50 km of slip at rates of 12 mm yr the inactive back tilted detachment preserves fault surface megamullions and mylonitic lineations parallel to the plio pleistocene plate motion a second se vergent detachment has been established on the opposite flank of this rolling hinge style mcc probably since 0 8 at depth and provide a sufficient mechanism for activating low angle normal faults in the rift mcc inception was not localized to the tip of the woodlark mor instead extreme crustal thinning near the mcc preconditioned later continental breakup the lower crust appears to be weak thickening beneath unloaded footwalls to uplift mccs above sea level and flowing laterally to even out regional crustal thickness contrasts on a 1 6 m y timescale deep seated transforms separate rheologically distinct domains in which extension has been localized along the weak pub to cause mcc formation vs those in which slip is distributed across an imbricate zone of more uniform strength normal faults the trobriand fault connects in the eastern woodlark rift to the owen stanley fault in the papuan ranges which is probably moving at nearly the full plate velocity

stress state and fluid pressure level along the whipple detachment fault california brittle structures in the upper 200 m of the footwall of the whipple detachment fault are used to characterize the state of stress under which the detachment slipped they are younger than or coeval with chloride brecciation and those that record shear typically are either moderately northeast dipping normal structures or steep northwest striking southwest down structures that we interpret as conjugate shears from this we infer that the maximum principal stress was oriented 55 80 from the detachment while they were active the angle between the conjugate faults averages about 30 indicating failure in the transtensional regime during conjugate faulting the effective least principal stress was nearly equal to the tensile strength the whipple fault probably had a normal coefficient of static friction and it slipped under hydrostatic to only moderately elevated fluid pressure rapid mineralization in the surroundings of major fault zones allows transtensional failure there and permits differential stress to be higher than if cohesionless friction limits strength this causes resolved shear stress sufficient for slip on faults severely misoriented relative to the maximum principal stress

shear zones and rock history abstract shear zones that have widened with time can display margin to interior microstructural profiles unlike the real microstructural history of the interior rocks shear zones that have not widened with time can provide more reliable views of microstructural history and therefore of deformation processes even the latter zones can mislead however where there has been an annealing overprint during or following the shearing interval these sources of uncertainty need to be acknowledged more regularly and if possible eliminated by workers interpreting deformation processes from shear zone strain gradients

the oleaginous botryococcus from the triassic yanchang formation in ordos basin northwestern china morphology and its paleoenvironmental significance high abundance but rather low diversity algal fossils were found in the hydrocarbon source rocks of the ch 7 2 ch 7 3 section triassic yanchang formation in the xifeng area of southwest ordos basin which are mainly composed of prolific leiosphaeridia and botryococcus botryococcus colonies are of various forms the majority is nubbly with some of cluster and cotton shape the nubbly colonies appear globular cordiform ternate petal obtuse triangle chrysanthemum shape and so on most botryococcus are saffron or brown and are frequently covered with clay under transmission microscope and shows strong yellow and light brown under fluorescence microscope botryococcus could live in freshwater and brackish water the botryococcus colonies that lived in fresh water are small with small single cells arranged radially with undulant or indented edges the botryococcus colonies that lived in brackish water are bigger with larger single cells arranged irregularly with slippery contours the most of botryococcus are discovered from the organic rich argillaceous sediment with abundant pyrites in the semi and deep lake facies and shows they were preserved in low energy reducing environments taphonomic characteristics of various microfossils and the present of pediastrum in the phytoplankton flora indicate that they are in situ or near burial the lake area of the ordos basin was gradually expanding and reaching its most extensive flood surface in the ch 7 of yanchang formation interval during the middle and late triassic with warm climate plentiful rainfall and luxuriant vegetation as determined by the environmental analysis with botryococcus in xifeng area the presence of two ecological types of botryococcus indicates that the salinity of lake water was fluctuating in the ch 7 interval the occurrence of symbiotic acritarchs and geochemical salinity indices show that the ordos lake was a typical fresh water lake which was gradually desalted and its salinity fluctuation was narrow during the mid later triassic the ecological type of the palynological flora discovered from the ch 7 to ch 8 in xifeng area is similar to that from the fuxian lake with abundant botryococcus in the yungui plateau of china these findings imply that the ordos basin was in a lower latitude area of temperate to subtropical climate during the middle and late triassic

investigation of fluid inclusion and oil geochemistry to delineate the charging history of upper triassic chang 6 chang 8 and chang 9 tight oil reservoirs southeastern ordos basin china abstract the chang 6 chang 8 and chang 9 reservoirs of the upper triassic yanchang formation in the zhenjing area of the ordos basin are characterized by low porosity low permeability high oil saturation and large oil reserves which indicate huge tight oil potential to comprehensively investigate the hydrocarbon contributors and charging history fluid inclusions were systemically studied coupled with basin modeling and oil geochemical analysis according to the petrographic characteristics two episodes of fluid inclusions occurred in each of the three reservoirs microthermometry analysis suggests that the hydrocarbon inclusions of the first episode in each reservoir are distributed mainly along overgrowth quartz or secondary microcracks and exhibit light yellow or light brown fluorescence the coeval aqueous inclusions display low homogenization temperature th peaks with 80 100 c the hydrocarbon inclusions of the second episode in each reservoir are mainly distributed in microcracks that cut across or within quartz grains and exhibit light green or bluish green fluorescence the coeval aqueous inclusions are characterized by high th peaks with 110 120 c the different characteristics of the two episodes of the fluid inclusions imply the existence of two episodes of hydrocarbon charging events the burial and geothermal histories of the various reservoirs were reconstructed using one dimensional basin modeling and pressure volume temperature composition pvtx simulation coupled with the authigenic illite k ar dating the reconstruction implied the time of the two hydrocarbon charging episodes to be early cretaceous and late cretaceous oil source correlation studies illustrate that the chang 6 chang 8 and chang 9 crude oils all originate from the chang 7 source rock the migration characteristics and directions were comprehensively analyzed by using maturity parameters this explanation of the charging history and oil migration pathway of the upper triassic yanchang formation in the zhenjing area provides a theoretical basis for further exploration

rapid uplift and crustal growth in extensional environments an isotopic study from the death valley region california the willow spring diorite in the black mountains of the central death valley extended terrain yields a u pb zircon age of 11 6 plus minus 0 2 ma sup 40 ar sup 39 ar analyses of hornblende and u pb analyses of sphene from this sample give ages of about 10 ma indicating that the batholith remained above about 500 c for about 1 5 m y after crystallization geologic relations indicate that the diorite was exposed to erosion by about 5 ma bracketing the evolution of the diorite within the time between onset of extension and uplift of the black mountain crustal block initial sup 87 sr sup 86 sr ratios range from 0 7060 mafic diorite to 0 7083 felsic diorite in samples collected from an area 200 x 100 m these data combined with structural and petrologic evidence suggest that the batholith represents a rare view of a mid crustal zone of mixing between mantle derived magma and crustal material often suggested to exist on the basis of observations of intermediate volcanic rocks the black mountains may therefore expose a cross section through a continental rift magmatic system from partially contaminated mafic to intermediate intrusive rocks in the deep crust up to their volcanic equivalents the relatively low initial sup 87 sr sup 86 srmore and high epsilon sub nd minus 1 4 of the diorite which is within proterozoic basement with epsilon sub nd approximately minus 18 is consistent with significant amounts of mantle input in continental rifts inferred from geophysical data and measurement of he isotopic ratios such additions to the crust in continental rights may represent a significant process of crustal growth furthermore the emplacement of igneous bodies with a large mantle component may help reconcile the large crustal pull apart in the basin and range in excess of 140 km with the fact that the crust still has as normal thickness of about 30 35 km less

fracture detection in oil based drilling mud using a combination of borehole image and sonic logs abstract the deeply buried 5 5 7 1 km lower cretaceous bashijiqike sandstones are important fractured reservoirs in the kuqa depression in these high temperature high pressure drilling environments oil based muds are used to limit drilling risks and improve efficiency making facture detection using resistivity based image logs difficult the maximum horizontal stress sh max trends close to north south 0 10 based on orientations of the drilling induced fractures and borehole breakouts core descriptions thin sections and image logs used to describe the fractures in the bashijiqike sandstone reservoirs show that opening mode natural fractures are common and the fractures range from fully open to sealed by calcite and clay minerals the open fractures are dominantly steeply dipping or vertical and primarily strike north south locally horizontal fractures or fractures with low angle dip are detected we compare fracture visibility on image logs acquired in the same ks 2 2 12 key well in oil based mud and water based mud fmi hd logs successfully detect large 0 1 mm aperture vertical fractures in oil based mud better than obmi image logs and the two imaging tools have a similar ability in detecting the high dip angle fractures since the ubi tool is not affected by oil based muds most of the fractures can be picked out on ubi images minor negative deviations of bulk density could be observed in fractured zones and the presence of fractures obviously causes an increase in sonic transit time full waveforms from dsi tools combined with electrical image logs detect fractured zones both the velocity and amplitude of the sonic waves are sensitive to fractures and the fractured zones could be identified according to the attenuation of stoneley waves the combination of cores image logs and full wave sonic logs offers comprehensive detection of fractures in oil based mud environments

shear zones in clay rich fault gouge a laboratory study of fabric development and evolution abstract clay rich fault rocks have long been recognized to host distinctive fabric elements and fault rock fabric is increasingly thought to play a fundamental role in fault mechanical behaviour in the brittle regime although the geometries of fabric elements in fault gouges have been described for almost a century the genesis and evolution of these elements during shear and their links to bulk mechanical properties remain poorly understood we characterize the development and evolution of fabric elements with increasing shear in a variety of clay rich experimental gouges over shear strains of 20 and at normal stresses of 2 150 mpa in the double direct shear configuration in addition to sem observations of experiment products at a variety of shear strains we quantified clay fabric intensity and the degree of grain size reduction using x ray texture goniometry xtg and particle size distribution psd measurements we also measured p and s wave velocities during shear to further probe the evolution of shear fabric and gouge properties we find that clay fabric elements develop in a systematic manner regardless of the gouge material riedel shears in the r1 orientation and boundary parallel shears are the dominant fabric elements riedel shears nucleate at layer margins and propagate into the layer shortly after reaching yield stress clay particles rotate into the p orientation shortly after riedels propagate through the layer the riedel shears are through going but are 10 thinner than similar zones observed in coarser granular materials our results suggest that the weakness of clay rich fault gouge may be less a function of anisotropic crystal structure as has been suggested previously and more a consequence of very thin shear surfaces permitting deformation in clay rich materials with minimal dilation or cataclasis the very thin shear surfaces are a function of the fine grain size of the materials and possibly polymodal psd s

faulting at mormon point death valley california a low angle normal fault cut by high angle faults new geophysical and fault kinematic studies indicate that late cenozoic basin development in the mormon point area of death valley california was accommodated by fault rotations three of six fault segments recognized at mormon point are now inactive and have been rotated to low dips during extension the remaining three segments are now active and moderately to steeply dipping from the geophysical data one active segment appears to offset the low angle faults in the subsurface of death valley

interactions between low angle normal faults and plutonism in the upper crust insights from the island of elba italy the late miocene to early pliocene zuccale low angle normal fault on the island of elba italy was one of the principal structures active during extensional collapse of the apennine fold and thrust belt we investigate the relationships among the zuccale fault subsidiary footwall fault networks and igneous bodies that were intruded into the immediate footwall of the zuccale fault both brittle and ductile kinematic indicators found in association with fault zones and igneous bodies yield a consistent wnw ese extension direction suggesting that faulting and intrusion overlapped in time structure contour analysis indicates that the zuccale fault has a regional domal morphology the dimensions and spatial location of the dome correlate with the likely subsurface position of the porto azzurro pluton originally intruded at 6 km depth we propose that doming of the zuccale fault may have been related in part to emplacement of the porto azzurro pluton as a tabular intrusion involving some component of vertical inflation and roof uplift the immediate footwall of the zuccale fault is everywhere crosscut by a complex linked network of high and low angle extensional faults with observed displacements of

charging of carboniferous volcanic reservoirs in the eastern chepaizi uplift junggar basin northwestern china constrained by oil geochemistry and fluid inclusion substantial amounts of petroleum were recently discovered in the carboniferous andesite tuff breccia and basalt reservoirs of the chepaizi uplift in the western junggar basin however the charging history of the carboniferous petroleum reservoir is poorly understood oil oil correlation studies indicate that all of the oils were mainly derived from the middle permian wuerhe formation source rocks possibly mixed with a small contribution from carboniferous baogutu formation source rocks in the neighboring changji sag based on the petrographic and microthermometry of fluid inclusions two hydrocarbon charging episodes are defined these episodes were characterized by a low peak range homogenization temperature th distribution 80 c 90 c and high salinity 13 22 13 42 wt nacl and a high peak range th distribution 120 c 130 c and low salinity 4 89 11 72 wt nacl respectively through one dimensional basin modeling and pressure volume temperature composition simulation the burial thermal histories for wells p61 p66 p668 and p663 were reconstructed and their trapping temperatures of the hydrocarbon inclusions were calculated to be higher than their corresponding highest paleotemperature i e 56 8 c 53 7 c 60 9 c and 58 1 c respectively implying fast hydrocarbon charging processes promoted by deep hydrothermal fluids associated with the hydrocarbon generation history sealing process of the hongche fault and regional tectonic evolution these two hydrocarbon charging events were deduced as the adjustments of oils previously accumulated along the hongche fault zone because of the tectonic extension in the paleogene and regional tilting in the neogene respectively the general direction of oil charging was traced from south to north and from east to west as indicated by the molecular parameters of nitrogen bearing compounds and c20 c21 triaromatic steroids c20 c21 c26 c28 triaromatic steroids ta i ta i ii which roughly coincided with the active fracturing

early tertiary magmatism and probable mesozoic fabrics in the black mountains death valley california we report two early tertiary u pb zircon ages for pegmatite from the black mountains of death valley california these ages 54 7 0 6 ma and 56 3 ma are unique for much of southeastern california the samples belong to a pegmatite suite that occupies part of the footwall of the badwater turtleback a late tertiary extensional feature similar but undated pegmatite intrudes the footwalls of the copper canyon and mormon point turtlebacks farther south the pegmatite suite demonstrates that fabric development on the turtlebacks was at least a two stage process fabrics cut by these pegmatites likely formed during the mesozoic whereas those that involve them formed during late tertiary extension

open or closed geochemical systems during diagenesis in sedimentary basins constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs descriptions of mineralogy and textural relationships in sandstones and limestones have been used to establish a sequence of diagenetic events epigenesis involving mineral dissolution and precipitation which have been interpreted to have occurred during the burial history published epigenetic sequences commonly imply a geochemically open system with very significant changes in the bulk chemical composition of the sediments during burial near surface diagenetic reactions may be open involving significant changes in the sediment composition and formation of secondary porosity caused by high pore water flow rates of meteoric water or reactions with sea water near the sea floor calculations show that the bulk chemical composition of the sediments below the reach of high pore water flow rates of meteoric water or hydrothermal convection should remain nearly constant during progressive burial because of limited pore water flow mass transport between shales and sandstones is also limited because the pore water is in most cases buffered by the same minerals so that the concentration gradients are low recent studies show that silica released from clay mineral reactions in mudstones has been precipitated locally as small quartz crystals and not exported to adjacent sandstones if the geochemical constraints for mass transfer during burial diagenetic reactions are accepted the chemical reactions involved in diagenesis can be written as balanced equations this offers the possibility to make predictions about reservoir quality based on assumptions about primary sediment composition related to facies and provenance large scale changes in the bulk composition of sandstones and mudstones during burial diagenesis have been suggested but because such changes cannot be explained chemically and physically no predictions can be made burial diagenetic processes are in most cases not episodic but occur as slow adjustments to increased stress and temperature driving the sediments toward increased mechanical and thermodynamic stability as a result the porosity of a single lithology must decrease during progressive burial but each lithology has a different porosity curve this article discusses quantitative calculations and estimates that show clearly that burial diagenesis must represent geochemically nearly closed systems where mineral dissolution and precipitation must be balanced this provides a theoretical basis for the modeling and prediction of reservoir quality

control of reservoir properties of tensleep sandstone by depositional and diagenetic facies lost soldier field wyoming the pennsylvanian tensleep sandstone in the lost soldier field of sweetwater county wyoming is composed of texturally homogeneous fine grained sandstones and interbedded dolomite beds geologic setting and sedimentary features indicate that the tensleep was deposited in supratidal eolian environments detailed petrographic cathodoluminescence and sem analyses of core samples from a well in the lost soldier field show authigenic features which resulted from nine diagenetic events in the tensleep 1 pervasive anhydrite cementation 2 precipitation of feldspar overgrowths 3 poikilotopic nodular calcite cementation 4 nodular anhydrite cementation 5 fabric selective anhydrite cementation 6 precipitation of silica overgrowths and silicification of carbonates 7 alteration of feldspar 8 dolomite cementation and 9 hydrocarbon accumulation four diagenetic facies with their own specific patterns of alterations were identified 1 early carbonate precipitation facies 2 early anhydrite cementation facies 3 primary carbonate grain facies and 4 nodular cementation facies the carbonate rich diagenetic facies are closely associated with the supratidal sebkha depositional environment the anhydrite rich facies is related to the interdune sebkha depositional environment and the nodular cementation facies is found within rocks deposited in the eolian dune environment reservoir qualities i e porosity and permeability of the tensleep sandstone in the lost soldier field vary considerably both vertically and laterally porosity and permeability are primarily controlled by compaction and cementation good correlation exists between reservoir quality and volume of cements for samples from nodular cementation and primary carbonate grain diagenetic facies samples from other diagenetic facies can also be grouped into distinct areas on a cross plot of porosity versus volume of cements therefore the cross plot can be used as a tool to differentiate different diagenetic facies at least for the evaporitic eolian rock system

the coriolis effect on mass wasting during the rheasilvia impact on asteroid vesta we investigate the influence of the coriolis force on mass motion related to the rheasilvia impact on asteroid vesta observations by the nasa dawn mission revealed a pattern of curved radial ridges which are related to coriolis deflected mass wasting during the initial modification stage of the crater utilizing the projected curvature of the mass wasting trajectories we developed a method that enabled investigation of the initial mass wasting of the rheasilvia impact by observational means we demonstrate that the coriolis force can strongly affect the crater formation processes on rapidly rotating objects and we derive the material s velocities 28 9 22 5 m s viscosities 1 5 9 0 106 pa s and coefficients of friction 0 02 0 81 during the impact modification stage the duration of the impact modification stage could be estimated to 1 1 0 5 h by analyzing the velocity distribution with respect to the topography we deduce that the rheasilvia impactor hit a heterogeneous target and that the initial crater walls were significantly steeper during the modification stage

impact of sedimentology diagenesis and solid bitumen on the development of a tight gas grainstone reservoir in the feixianguan formation jiannan area china implications for gas exploration in tight carbonate reservoirs abstract tight gas grainstone reservoirs in the third member of the feixianguan formation jiannan area evolved from a paleo oil accumulation as evidenced from abundant solid reservoir bitumen porosity evolution of the grainstones was studied by evaluating relative influences of sedimentology diagenesis and solid bitumen formed during cracking of accumulated oils grainstones exhibited regional distinct effectiveness for paleo oil and present gas accumulations during oil window and subsequent gas window diagenesis in the southern zone where grainstones were not subjected to subaerial exposure and meteoric diagenesis in the early diagenetic stage paleoporosity at the time of oil charge was mainly controlled by sedimentologic factors e g grain size sorting and grain type and paleo oil reservoirs only occurred in the ooid dominated grainstones with good sorting and coarse grain size in contrast in the northern zone meteoric diagenesis was responsible for paleoporosity preservation due to the early mineral stabilization of grains and meteoric calcite cementation which caused grainstones greater resistance to compaction hence most of the grainstones in the northern zone regardless of textural variables formed effective reservoirs for paleo oil accumulation as the oil cracked to gas with increasing depth and temperature during the late oil window and initial gas window solid bitumen occluded reservoir pores to varying degrees and caused paleo oil reservoirs to be significantly heterogeneous or completely ineffective for gas accumulation in contrast most grainstones that were once ineffective oil reservoirs transformed into effective gas reservoirs due to no or minor influence of solid bitumen precipitation the model of reservoir transformation development of tight grainstones provides a plausible explanation for key observations concerning the diagenetic and distribution differences between paleo oil and present gas reservoirs it is useful in predicting the distribution of potential reservoirs in carbonate strata in future exploration

depositional facies diagenesis and their impact on the reservoir quality of silurian sandstones from tazhong area in central tarim basin western china abstract the silurian sandstones of the tazhong area are an important oil exploration target in the tarim basin western china this study deals with the lower silurian of the kalpingtag formation which was deposited in a braided delta system four major depositional microfacies are identified consisting of distributary channel interdistributary bay front bar and sand sheet in spite of their large lateral extent exploration of these sandstones is complicated by variation of sediment facies and intense diagenesis which strongly affected reservoir quality depositional environment exerted an essential control on the reservoir quality higher energy well sorted and relatively coarser grained sandstones have better quality than low energy fine grained clay rich sandstones diagenesis was mainly composed of mechanical and chemical compaction and cementation by quartz overgrowths carbonates and various clay minerals mechanical compaction was a significant porosity reducing agent in the silurian sandstones the presence of carbonate cement exerts a dominant influence on porosity and permeability in tazhong area other cements are less important but quartz cement is significant the third significant element is clay minerals the porosity reduction by compaction and cementation and the relative timing of them indicates that compaction was much more important than cementation in reducing porosity

quantitative impact of diagenesis on reservoir quality of the triassic chang 6 tight oil sandstones zhenjing area ordos basin china abstract the upper triassic chang 6 sandstone an important exploration target in the ordos basin is a typical tight oil reservoir reservoir quality is a critical factor for tight oil exploration based on thin sections scanning electron microscopy sem x ray diffraction xrd stable isotopes and fluid inclusions the diagenetic processes and their impact on the reservoir quality of the chang 6 sandstones in the zhenjing area were quantitatively analysed the initial porosity of the chang 6 sandstones is 39 2 as calculated from point counting and grain size analysis mechanical and chemical compaction are the dominant processes for the destruction of pore spaces leading to a porosity reduction of 14 2 20 2 during progressive burial the porosity continually decreased from 4 3 to 12 4 due to carbonate cementation quartz overgrowth and clay mineral precipitation diagenetic processes were influenced by grain size sorting and mineral compositions evaluation of petrographic observations indicates that different extents of compaction and calcite cementation are responsible for the formation of high porosity and low porosity reservoirs secondary porosity formed due to the burial dissolution of feldspar rock fragments and laumontite in the chang 6 sandstones however in a relatively closed geochemical system products of dissolution cannot be transported away over a long distance as a result they precipitated in nearby pores and pore throats in addition quantitative calculations showed that the dissolution and associated precipitation of products of dissolution were nearly balanced consequently the total porosity of the chang 6 sandstones increased slightly due to burial dissolution but the permeability decreased significantly because of the occlusion of pore throats by the dissolution associated precipitation of authigenic minerals therefore the limited increase in net porosity from dissolution combined with intense compaction and cementation account for the low permeability and strong heterogeneity in the chang 6 sandstones in the zhenjing area

thermal and barometric constraints on the intrusive and unroofing history of the black mountains implications for timing initial dip and kinematics of detachment faulting in the death valley region california unroofing of the black mountains death valley california has resulted in the exposure of 1 7 ga crystalline basement late precambrian amphibolite facies metasedimentary rocks and a tertiary magmatic complex the 40ar 39ar cooling ages obtained from samples collected across the entire length of the range 55 km combined with geobarometric results from synextensional intrusions provide time depth constraints on the miocene intrusive history and extensional unroofing of the black mountains data from the southeastern black mountains and adjacent greenwater range suggest unroofing from shallow depths between 9 and 10 ma to the northwest in the crystalline core of the range biotite plateau ages from 13 to 6 8 ma from rocks making up the death valley turtlebacks indicate a midcrustal residence with temperatures 300 c prior to extensional unroofing biotite 40ar 39ar ages from both precambrian basement and tertiary plutons reveal a diachronous cooling pattern of decreasing ages toward the northwest subparallel to the regional extension direction diachronous cooling was accompanied by dike intrusion which also decreases in age toward the northwest the cooling age pattern and geobarometric constraints in crystalline rocks of the black mountains suggest denudation of 10 15 km along a northwest directed detachment system consistent with regional reconstructions of tertiary extension and with unroofing of a northwest deepening crustal section mica cooling ages that deviate from the northwest younging trend are consistent with northwestward transport of rocks initially at shallower crustal levels onto deeper levels along splays of the detachment the well known amargosa chaos and perhaps the badwater turtleback are examples of this splaying process considering the current distance of the structurally deepest samples away from moderately to steeply east tilted tertiary strata in the southeastern black mountains these data indicate an average initial dip of the detachment system of the order of 20 similar to that determined for detachment faults in west central arizona and southeastern california beginning with an initially listric geometry a pattern of footwall unroofing accompanied by dike intrusion progresses northwestward this pattern may be explained by a model where migration of footwall flexures occur below a scoop shaped hanging wall block one consequence of this model is that gently dipping ductile fabrics developed in the middle crust steepen in the upper crust during unloading this process resolves the low initial dips obtained here with mapping which suggests transport of the upper plate on moderately to steeply dipping surfaces in the middle and upper crust

fluid pressure transients on seismogenic normal faults abstract fluid inclusions in hydrothermally altered footwall rocks of the dixie valley fault nevada and the wasatch fault utah indicate that pore fluid pressure fluctuated minimum entrapment pressures for fluid inclusions consisting of h2o co2 nacl ranged from 295 mpa to 60 mpa in the temperature range 350 to 170 c on the wasatch fault and from 158 to 35 mpa in the temperature range 350 to 200 c on the dixie valley fault scatter in the pressure estimates at constant temperature is interpreted as paleo fluid pressure transients at depths of up to 11 km on the wasatch fault and 3 to 5 km on the dixie valley fault observed pressure transients range from 5 mpa within the limits of error in pressure determination to 120 mpa on the wasatch fault and 7 to 126 mpa on the dixie valley fault the pressure transients are greatest on both faults in the temperature range 270 to 310 c the fluids represented by fluid inclusions play a key role in nucleation and propagation of earthquake ruptures high fluid pressures may initiate rupture then dilatancy pore pressure reduction and dilatant hardening may arrest the rupture however decompression of the fluids and phase separation produces a decrease in fluid bulk modulus of 41 to 90 which reduces the dilatant hardening effect and may permit ruptures to propagate

fault rock injections record paleo earthquakes fault rocks such as pseudotachylyte melt or granular gouge are sometimes injected into cracks off a fault surface the width to height ratio of the injection veins is a direct measure of the shear strain in the wall rock required to accommodate the injection so the aspect ratios preserve a record of the overpressure opening the cracks and injecting melt or fluidized granular material from the fault zone we measured the aspect ratios for 201 pseudotachylyte injections and 29 granular injections they have aspect ratios of 0 2 0 177 0 025 and 0 227 0 077 respectively within 99 confidence these are significantly proportionally wider than the aspect ratios of ordinary dikes from 10 4 to 10 3 the injection aspect ratios exceed the usual limit of elastic strain in the wallrock 1 so we infer that the injection veins were accommodated by permanent deformation by microcracking and slip on foliation surfaces in the host rock which is difficult to detect observationally the fully elastic model therefore offers an upper bound on the stress required to open these injection veins 10 8 210 10 pa with higher pressures in stiffer host rock lithologies these overpressures cannot be achieved by expansion of the fault rock fluids during melting or gouge fluidization mechanisms as the pressure along the fault is limited to 10 7

depositional process and astronomical forcing model of lacustrine fine grained sedimentary rocks a case study of the early paleogene in the dongying sag bohai bay basin abstract the paleogene strata of the bohai bay basin include multiple sets of lacustrine fine grained sediments which provide a detailed record of the climate changes that occurred in the lake basin early paleogene fine grained sedimentary rocks are the main target layers of continental shale oil exploration in eastern china therefore studies of the geological responses to astronomical factors have important theoretical and practical significance for understanding the depositional process of lacustrine fine grained sedimentary rocks in this paper we focus on the lower es3 es3l upper es4 es4u units in four shale oil wells in the dongying sag of the bohai bay basin based on the previous establishment of a high resolution astronomical time scale in this area we have further studied the petrological characteristics and lithofacies of the fine grained sedimentary rocks then the geological responses to astronomical cycles on different scales are investigated and the models for the depositional process of fine grained sedimentary rocks under the control of astronomical factors are revealed the results indicate that the cyclic changes in mineral content and lithofacies in fine grained sedimentary rocks in the dongying sag are controlled by short eccentricity and precession cycles periods near the maximum short eccentricity range had large precession amplitudes humid climates and the development of organic rich laminated calcareous mudstone lithofacies in contrast periods near the minimum short eccentricity range had small precession amplitudes a dry climates and the development of organic containing layered muddy limestone lithofacies this study explores the depositional process of fine grained sedimentary rocks from the perspective of astronomical cycles which is conducive to the comprehensive prediction of favorable shale oil layers

brittle faulting induced by ductile deformation of a rheologically stratified rock sequence badwater turtleback death valley california field relations in the footwall of the badwater turtleback fault provide evidence that macroscopically ductile deformation was directly responsible for early stages of brittle faulting during extensional denudation footwall rocks which are typically mylonitie consist of dominantly pegmatite quartzofeldspathic gneiss and dolomite and calcite marble because of the ductile deformation these rocks are widely variable in thickness bodies of rock that exceed 10 m in thickness may pinch out to zero thickness in distances of less than 100 m the mylonitic rocks also display crosscutting relations that indicate that onset of bulk brittle behavior occurred at different stages of uplift for different rock types calcite marble was the last to deform by predominantly crystal plastic mechanisms and therefore accommodated most of the shear strain during later stages of lower plate ductile deformation where the calcite marble pinched out into adjacent stronger rocks the high strains caused brittle faulting in the adjacent rocks the process described here of ductile deformation leading to extreme thickness variations and consequent brittle faulting directly links mid crustal ductile deformation with upper crustal brittle deformation it further demonstrates that brittle faulting can initiate at depths where ductile deformation accommodates the bulk strain

prediction of diagenetic facies using well logs a case study from the upper triassic yanchang formation ordos basin china abstract understanding diagenetic heterogeneity in tight sandstone reservoirs is vital for hydrocarbon exploration as a typical tight sandstone reservoir the seventh unit of the upper triassic yanchang formation in the ordos basin chang 7 unit central china is an important oil producing interval results of helium porosity and permeability and petrographic assessment from thin sections x ray diffraction scanning electron microscopy and cathodoluminescence analysis demonstrate that the sandstones have encountered various diagenetic processes encompassing mechanical and chemical compaction cementation by carbonate quartz clay minerals and dissolution of feldspar and lithic fragments the sandstones comprise silt to medium grained lithic arkoses to feldspathic litharenites and litharenites which have low porosity 0 5 13 6 with an average of 6 8 and low permeability 0 009 10 3 m 2 to 1 818 10 3 m 2 with an average of 0 106 10 3 m 2 this study suggests that diagenetic facies identified from petrographic observations can be up scaled by correlation with wire line log responses which can facilitate prediction of reservoir quality at a field scale four diagenetic facies are determined based on petrographic features including intensity of compaction cement types and amounts and degree of dissolution unstable and labile components of sandstones can be identified by low bulk density and low gamma ray log values and those sandstones show the highest reservoir quality tightly compacted sandstones siltstones which tend to have high gamma ray readings and relatively high bulk density values show the poorest reservoir quality a model based on principal component analysis pca is built and show better prediction of diagenetic facies than biplots of well logs the model is validated by blind testing log predicted diagenetic facies against petrographic features from core samples of the upper triassic yanchang formation in the ordos basin which indicates it is a helpful predictive model

outcrop architecture of a fluvio lacustrine succession upper triassic yanchang formation ordos basin china abstract although several studies have considered the sedimentary facies of the upper triassic yanchang formation in terms of their hydrocarbon potential in the petroliferous ordos basin such studies have mostly interpreted subsurface data and few have systematically examined the detailed outcrop based anatomy of this succession this study characterizes a series of well exposed outcrop sections along an 80 km long e w oriented series of road cuttings in the southeast ordos basin to reconstruct the sedimentary architecture of a fluvio lacustrine succession and determine the principal controls that governed the style of accumulation and preservation the yanchang formation is divided into 10 depositional units chang 10 base to chang 1 top that record the sedimentary evolution of a series of large scale fluvial systems that constructed delta front and pro delta bodies as they entered a large interior draining lake detailed analysis of the internal architectures of the delta front bodies records the mechanism of growth of a large scale shallow water delta succession lacustrine subaqueous distributary channels are composed of 3 15 m thick sandstone fills such channelized bodies are highly elongate and many can be traced several kilometres such bodies can be shown to be commonly associated with thin bedded mouth bar and subaqueous interdistributary bay deposition the progradational architecture of the delta front elements is difficult to recognize on outcrop scale deposits of the chang 10 to 1 units record 4 major lacustrine transgressive regressive cycles during the late triassic transgressive events are signified by the widespread occurrence of 10 20 m thick oil shale prone intervals regressive events are signified by the progradational stacking towards the basin centre of sandstone prone facies associations mainly accumulated in fluvial channel delta plain and upper delta front environments the uppermost chang 1 interval records late stage filling of the lacustrine basin the transgressive regressive cycles equate to sequences that can be correlated across the region development of which was controlled by a combination of changes in the rate of tectonic subsidence the rate of sediment delivery from the basin margin hinterland and climate factors that were themselves influenced by intracontinental growth of the neighbouring qinling orogen

impact of diagenesis on reservoir quality and heterogeneity of the upper triassic chang 8 tight oil sandstones in the zhenjing area ordos basin china abstract reservoir quality and heterogeneity are critical risk factors in tight oil exploration the integrated analysis of the petrographic characteristics and the types and distribution of diagenetic alterations in the chang 8 sandstones from the zhenjing area using core log thin section sem petrophysical and stable isotopic data provides insight into the factors responsible for variations in porosity and permeability in tight sandstones the results indicate that the chang 8 sandstones mainly from subaqueous distributary channel facies are mostly moderately well to well sorted fine grained feldspathic litharenites and lithic arkose the sandstones have ultra low permeabilities that are commonly less than 1 md a wide range of porosities from 0 3 to 18 1 and two distinct porosity permeability trends with a boundary of approximately 10 porosity these petrophysical features are closely related to the types and distribution of the diagenetic alterations compaction is a regional porosity reducing process that was responsible for a loss of more than half of the original porosity in nearly all of the samples the wide range of porosity is attributed to variations in calcite cementation and chlorite coatings the relatively high porosity reservoirs formed due to preservation of the primary intergranular pores by chlorite coatings rather than burial dissolution however the chlorites also obstruct pore throats which lead to the development of reservoirs with high porosity but low permeability in contrast calcite cementation is the dominant factor in the formation of low porosity ultra low permeability reservoirs by filling both the primary pores and the pore throats in the sandstones the eogenetic calcites are commonly concentrated in tightly cemented concretions or layers adjacent to sandstone mudstone contacts while the mesogenetic calcites were deposited in all of the intervals and led to further heterogeneity this study can be used as an analogue to understand the variations in the pathways of diagenetic evolution and their impacts on the reservoir quality and heterogeneity of sandstones and is useful for predicting the distribution of potential high quality reservoirs in similar geological settings

relation of deformation and multiple intrusion in the death valley extended region california with implications for magma entrapment mechanism the crystalline core of the black mountains crustal section death valley california exposes a tremendous volume of miocene plutonic rock intruded at a depth of 10 13 km few plutons were intruded above 10 km prior to unroofing by tectonic denudation in the eastern part of the core a brittle detachment fault separates predominantly miocene hanging wall strata from the midcrustal miocene 11 6 and 8 7 ma plutonic complex and early proterozoic basement in the west mylonitic lineations and foliations locally well developed within the 11 6 ma intermediate mafic willow spring pluton are cut by dikes which emanate from an 8 7 ma silicic plutonic complex the younger silicic complex throughout the crystalline core exhibits few ductile deformational structures published thermal and barometric studies from both plutonic bodies indicate similar midcrustal 10 13 km emplacement depths at ambient temperatures just above 300 c the significant difference in densities of these magmas argues against a density control for magma entrapment also the country rock above and below the plutonic complex contains no apparent differences from field observations that would suggest a change in density the depth of entrapment corresponds well with the expected depth for the crustal strength maximum determined from laboratory experiments the similar emplacement depths but contrasting styles of deformation of the two plutonic bodies further suggests that entrapment may have been controlled by a high strength barrier represented by the brittle ductile transition late hypabyssal intrusions and associated volcanism are linked to diachronous rapid unroofing of the range block all show a northwest progression paralleling the regional extension direction thus when migration of magma through the high strength barrier did occur it was apparently related to increased strain rates which allowed magma ascent by fracture exploitation rheological stratification of the crust may have played an important if not major role in trapping magmas in the middle crust in this area

strain and kinematic analysis in general shear zones we present a unifying theory for the development and distribution of strain markers and kinematic indicators in zones of general shear and thus provide a framework in which data previously considered contradictory may be understood rigid and deformable porphyroclast systems including and complex grains are potential indicators of both strain and flow the shapes and distribution of such porphyroclast systems may be used to distinguish among different tectonic regimes general shear is divided into two fields sub simple shear in which the rotational component of the strain is less than that for simple shear and super simple shear in which the rotational component is greater than for simple shear sub simple shear may involve narrowing or broadening of shear zones super simple shear regimes are possible in local regions such as the vicinities of deformable porphyroclasts but must be enclosed by regimes of sub simple shear the polar mohr constructions for finite deformation and flow are useful to analyze general shear in theory the hyperbolic net is employed for practical plotting of real data and derivation of the kinematic vorticity number wn this number represents the relative contributions of pure and simple shear in steady flow in nature deformation is thought to build up and decay by processes that may invalidate the assumption of constant flow regime we therefore introduce the concept of accelerating deformation and analyze the implications of non steady flow for the shearing histories of deformed objects

fault related rocks suggestions for terminology many traditional terms for fault related rocks have undergone recent dynamic metamorphism under high pressure discussions by various groups of specialists a generally acceptable simplified framework encompassing these and associated structural terms is now needed for many geologic engineering and legal purposes such a framework is proposed here focusing on a rate of strain versus rate of recovery diagram and relating this framework to the products of brittle and ductile deformation along faults

geologic evidence for multiple slip weakening mechanisms during seismic slip in crystalline rock we examine exhumed seismogenic faults to investigate the mechanisms that may have achieved dynamic fault weakening during ancient ruptures field and microscope observations imply more than one weakening mechanism must have been active during slip events on the faults pseudotachylytes that are continuous over the scale of field exposures are indicative of melt lubrication a fault breccia crosscutting earlier formed cataclasites was mobilized during faulting and possibly represents a pressurized fault rock that resulted from thermal pressurization or elastohydrodynamic lubrication in some faults pseudotachylytes are developed in patches several meters long cataclasites in which there is no evidence for melting are present immediately along strike from the pseudotachylyte patches by considering the energy required for melting we show that the pseudotachylytes must have formed during ruptures larger than the patches implying that the cataclasites also accommodated seismic slip the distribution of fault rock types shows that the frictional response to slip during a single event was spatially variable reworked pseudotachylytes also indicate that coseismic processes change over time at a point on a fault these observations emphasize that macroscopic dynamic fault weakening is a function of multiple coeval processes at microscales to mesoscales detailed observations of the discontinuous pseudotachylytes show that slip zone thickness is the critical parameter that controls active coseismic processes the frictional response of a fault to slip is therefore dependent on the internal structure of faults given the along strike heterogeneity of most mapped fault zones the coexistence of multiple slip weakening mechanisms in a single earthquake will be common

role of seismogenic processes in fault rock development an example from death valley california fault rocks developed along the mormon point turtleback of southern death valley suggest that a jog in the oblique slip death valley fault zone served as an ancient seismic barrier where dominantly strike slip ruptures were terminated at a dilatant jog dramatic spatial variations in fault rock thickness and type within the bend are interpreted as the products of 1 fault overshoot in which planar ruptures bypass the intersection of the two faults composing the bend and slice into the underlying footwall and 2 implosion brecciation in which coseismic ruptures arrested at a releasing bend in the fault lead to catastrophic collapse brecciation fluid influx and mineralization

faults smooth gradually as a function of slip abstract geometry and roughness of fault surfaces plays a central role in the dynamics and kinematics of faulting faults smooth with increasing slip but the degree of the smoothing has not previously been well constrained for natural faults we measure the roughness as a function of displacement for a suite of 16 faults with cumulative offsets ranging from 0 1 m to approximately 500 m we find that slip parallel roughness evolves gradually with slip for instance for segments of length 0 5 m h 2 10 3 d 0 1 where h is the rms roughness and d is the displacement on the fault strand with both quantities measured in meters the gradual nature of the smoothing is robust to varying lithology and erosion the weak function implies a decrease in the rate of gouge formation for a model with increasing slip for a model in which gouge is generated by abrading an asperity tip the relatively gradual evolution of roughness could be explained by lubrication by the accumulated gouge that mitigates the abrasional smoothing that occurs during slip and or re roughening processes

identification of sedimentary diagenetic facies and reservoir porosity and permeability prediction an example from the eocene beach bar sandstone in the dongying depression china abstract accurate prediction of reservoir porosity and permeability is essential for prospecting hydrocarbon reserves and petroleum production capacity we propose an innovative reservoir porosity and permeability prediction method through identifying sedimentary diagenetic facies determining the porosity permeability trends using core measurement data extrapolating the spatial distribution of the sedimentary diagenetic facies using log data through the bayes discriminant analysis and predicting the reservoir porosity and permeability the essence of the method was illustrated and its effectiveness was demonstrated using the eocene beach bar sandstones in the dongying depression bohai bay basin eastern china the eocene beach bar sandstones are classified into fine sandstone siltstone and argillaceous siltstone based on grain sizes sorting and matrix contents the major diagenetic processes that influence the porosity and permeability of the beach bar sandstones are compaction carbonate cementation and feldspar dissolution seven sedimentary diagenetic facies were identified in the beach bar sandstones based on the lithological types and their corresponding diagenesis and influence extent on reservoir properties the variation ranges of porosity and permeability log k of these sedimentary diagenetic facies are typically less than 6 and 1 2 respectively both the porosity and permeability have well defined functional relationships with depth the sedimentary diagenetic facies can be identified effectively from logging data through the use of the bayes discriminant analysis and corresponding cross plots the porosity and permeability log k of the well evaluated in the study were predicted with errors in the range of 3 and 0 6 respectively based on the distribution and the fitting equation of the trend lines for the seven sedimentary diagenetic facies the predicted porosities and permeability of sedimentary diagenetic facies match the measured porosities and permeability well

exhumation patterns along shallow low angle normal faults an example from the altotiberina active fault system northern apennines italy a multi method approach palaeothermal and thermochronological analyses thermal modelling is applied to reconstruct the exhumation history of the altotiberina fault atf a representative example of crustal scale active low angle normal faulting in the northern apennines italy thermal maturity and thermochronological data yield similar burial histories but different exhumation patterns for the sedimentary successions in the hangingwall and the footwall of the atf since 3 8 ma the atf footwall has exhumed at rates of 0 90 mm a 1 exhumation led to bending and deactivation of the atf uppermost portion as a result of tectonic unloading and isostatic adjustment followed by migration of extension and the development of a set of domino like east dipping normal faults rooting on the buried portion of the atf atf activity and isostatic rebound exhumed triassic rock units from depths of about 4 km we suggest that isostatic instability is accommodated at shallow crustal levels in a similar way to what is observed on larger structures at mid low crustal levels

relations between progressive deformation and fluid rock interaction during shear zone growth in a basement cored thrust sheet sevier orogenic belt utah variations in microtextures strain whole rock chemistry mineralogy fluid inclusion characteristics and fracture network properties record complex interactions between deformation and fluid rock processes during progressive growth of a shear zone within crystalline basement rocks this shear zone formed at a depth of about 15 kilometers t 350 c and at elevated fluid pressures the shear zone sz has a 10 meter thick core of highly deformed phyllonite and is bounded by a 3 to 8 meter thick transition zone tz of variably fractured chloritic gneiss which grades outward into relatively undeformed granitic gneiss granitic gneiss consists mostly of coarser grained feldspar and quartz chloritic gneiss consists of varying amounts of quartz feldspar and fine grained micaceous matrix produced by a mixture of cataclastic plastic and alteration processes and phyllonite consists mostly of very fine grained quartz mica rich matrix produced by pervasive plastic deformation and alteration processes estimated strain ratios are 10 1 within the sz whole rock chemical compositions record significant depletion of ca and na and enrichment of mg and h2o during progressive alteration of granitic gneiss into phyllonite similar average contents of al ti and fe as well as si indicate that alteration was about isovolumetric at outcrop scale although variations between individual samples record up to 20 percent local volume change alteration produced significant changes in mineral abundances and compositions with conversion of feldspar to muscovite mafic minerals to mg rich chlorite and dissolution precipitation of quartz fluid inclusion characteristics and mineral compositions indicate that sz fluids were moderately saline and became depleted in mg during alteration changes in mineralogy and fluid composition record large influxes of reactive fluids with geochemical fluid rock ratios on the order of 102 to 103 variably deformed cross cutting veins fractures and microcracks record repeated episodes of cataclasis fluid influx and sealing along complex networks fluid flow appears to have been concentrated within the sz along grain scale and vein networks with a component of outward flow into the tz along fracture and microcrack networks estimated average fluid fluxes are on the order of 10 7 to 10 10 m s in the sz and 10 9 to 10 12 m s in the tz consistent with average permeabilities of 10 14 to 10 17 m2 in the sz and 10 16 to 10 19 m2 in the tz for moderate fluid pressure gradients these average permeabilities are consistent with observed grain scale and fracture network properties for a range of sealing and fluid pressure histories permeabilities may have been transiently greater during episodes of very high fluid pressure but decreased as fluid pressure gradients equilibrated and fractures sealed strain softening in the sz was likely produced by reaction softening grain size reduction and hydrolytic weakening of quartz a model of shear zone growth involves 1 initial fracturing of relatively strong coarser grained quartz feldspar rich rock 2 episodic influx of reactive fluids during periods of fracturing and high fluid pressure with intervening periods of sealing and reduced fluid pressure 3 progressive alteration of feldspar and mafic minerals to micas and recrystallization of quartz to form relatively weak fine grained matrix and 4 concentrated deformation and focused fluid flow in a growing sz core