tricyclic terpane maturity parameters response to heating by an igneous intrusion the effects of thermal maturation upon the abundance and composition of the tricyclic terpanes have been investigated within a single sedimentary horizon within the dun caan shale member isle of skye scotland intruded by a 0 9 m thick tertiary dolerite dyke heating by the igneous body caused the concentrations of all 13 h 14 h and 13 h 14 h tricyclic terpanes to increase towards or within the effective oil window recording generation from abundant non hydrocarbon precursors such as kerogen and asphaltene bound components progressive changes in tricyclic terpane composition accompany these concentration changes an increase in the c23 maturity parameter towards the dyke resulted from the relatively greater rate of generation of the isomer combined with the earlier decline in concentration by degradation or isomerisation of the component the tricyclics tricyclics hopanes parameter shows a dramatic increase within the latter part of the oil window due to the greater relative thermal stability of the tricyclic terpanes the quantitative data demonstrate the importance of the processes of generation from non hydrocarbon fractions of the sedimentary organic matter and thermal degradation in the operation of tricyclic terpane maturity parameters

biomarker maturity parameters for high maturities calibration of the working range up to the oil condensate threshold abstract in a study of oils and source rocks from the kimmeridge clay united kingdom and the hanifa qatar formations several biomarker maturity parameters with working ranges up to vr e vitrinite reflectance equivalent 1 0 oil condensate threshold have been identified and calibrated this represents a significant improvement compared to commonly used biomarker maturity tools sterane and hopane isomerisation with their maturity maximum of approximately vr e 0 7 early part of oil window these new parameters are 1 the absolute concentrations of steranes and hopanes decreasing gradually with increasing maturity 2 the ratio of tricyclic terpanes tricyclic terpanes hopanes increasing with maturity 3 the ratio of diasteranes 14 h 17 h steranes depending both on maturity and on source rock lithology and 4 the ratio of t s t m 22 29 30 trisnorneohopane 22 29 30 trisnorhopane increasing rapidly at very high maturities during artificial maturation experiments the biomarker maturities described in the natural samples were also known to be time temperature dependent up to rather severe laboratory heating conditions this corresponds with the working range up to high maturity levels as observed in the natural samples

australasian asphaltite strandings their origin reviewed in light of the effects of weathering and biodegradation on their biomarker and isotopic profiles abstract asphaltites long known to strand along the coastline of southern australia and as distantly as new zealand and macquarie island are widely regarded as artefacts of submarine oil seepage their remarkably uniform composition suggests a common source marine shale containing sulphur rich type ii kerogen probably deposited during an early cretaceous oceanic anoxic event oae suitable hydrocarbon kitchens may exist in the offshore bight and otway basins the physical character of the asphaltites including laminations and flow structures and their degree of alteration which is not the result of biodegradation or extensive water washing suggest an origin from subsurface tar mats subsequently exposed by the incision of submarine canyons with the possible formation of asphaltic volcanoes api gravities of 4 18 impart quasi neutral buoyancy implying many asphaltites were submerged drifters prior to stranding their degree of weathering reflecting at least in part the residence time in the marine environment for any individual asphaltite specimen this will depend on the proximity of the seafloor seep to the stranding site an important consideration when attempting to locate their point of origin this study investigates the hydrocarbon biomarker signatures and n alkane 13 c profiles of asphaltite specimens from stranding sites on the eyre peninsula n 2 kangaroo island n 4 and the limestone coast n 3 south australia and the south island of new zealand n 2 sub samples of the interior and weathered surface of each specimen were analysed no distinction could be made between strandings based on their source dependent molecular and isotopic signatures confirming their common origin comparison of the interior and exterior sub samples revealed subtle although consistent differences given their degree of degradation and isotopic variance these australasian asphaltites seem to be products of low intensity seeps in the ceduna sub basin of the bight basin and or morum sub basin of the otway basin

origin distribution and alteration of asphalts at chapopote knoll southern gulf of mexico abstract following the discovery of asphalt volcanism in the campeche knolls a research cruise was carried out in 2006 to unravel the nature of the asphalt deposits at chapopote the novel results support the concept that the asphalt deposits at the seafloor in 3000 m of water depth originate from the seepage of heavy petroleum with a density slightly greater than water the released petroleum forms characteristic flow structures at the seafloor with surfaces that are ropy or rough similar to magmatic lava flows the surface structures indicate that the viscosity of the heavy petroleum rapidly increases after extrusion due to loss of volatiles consequently the heavy petroleum forms the observed asphalt deposit and solidifies detailed survey with a remotely operated vehicle revealed that the asphalts are subject to sequential alterations e g volume reduction leading to the formation of visible cracks in the asphalt surface followed by fragmentation of the entire deposit while relatively fresh asphalt samples were gooey and sticky older fragmented pieces were found to be brittle without residual stickiness furthermore there is evidence for petroleum seepage from below the asphalt deposits leading to local up doming and sometimes to whip shaped extrusions extensive mapping by tv guided tools of chapopote asphalt volcano indicates that the main asphalt deposits occur at the south western rim that borders a central crater like depression the most recent asphalt deposit at chapopote is the main asphalt field covering an area of 2000 m2 asphalt volcanism is distinct from oil and gas seepage previously described in the gulf of mexico and elsewhere because it is characterized by episodic intrusions of semi solid hydrocarbons that spread laterally over a substantial area and produce structures with significant vertical relief as chapopote occurs at the crest of a salt structure it is inferred that asphalt volcanism is a secondary result of salt tectonism

continental transform tectonics great slave lake shear zone ca 1 9 ga northwest canada great slave lake shear zone gslsz is a northeast trending dextral mylonite zone across which the coeval north trending thelon and taltson magmatic zones on the west margin of the churchill province are offset 300 700 km it is proposed that the gslsz is a continental transform structure related to oblique collisional indentation of the active margin of the churchill province by the microcontinental slave province this accounts for geologic and geophysical differences between the thelon and taltson zones and explains the localization of a tibetan type queen maud uplift north of the gslsz and the abrupt loss of slip on the gslsz east of the uplift it also provides a new model for the athapuscow aulacogen analogous to the modern bay of bengal indo burman ranges sagaing transform andaman sea situation

origin of tar mats in petroleum reservoirs part ii formation mechanisms for tar mats abstract tar mats and oil legs from several marine source rock derived oil fields including the ula and oseberg fields in the north sea and an unidentified north american example have been analysed geochemically in all cases the tar mats were derived from the same petroleum system as the oil leg oils and were found in the most porous and permeable portions of the reservoir comparing case history data with published data on the subsurface solubility characteristics of asphaltenes in petroleums it is possible to eliminate some potential tar mat formation mechanisms and suggest the most likely causative mechanisms thus the adsorption of asphaltenes on reservoir clays and light to moderate levels of biodegradation are considered unlikely to contribute to tar mat formation in the ula field thermal degradation of the oil in the carrier and reservoir causing asphaltene precipitation may be a major contributing factor to the tar mats there in the oseberg field increased gas solution in the oil leg may have contributed to deasphaltation tentative mass balance calculations the extreme pressure dependence of asphaltene solubility in subsurface crude oils the occurrence of tar mats in zones of elevated permeability plus a simple transport model suggest that during secondary migration an asphaltene rich petroleum phase may be produced in dipping carrier beds and may be carried into the reservoir contributing in some instances to tar mat formation

marine 187os 188os isotope stratigraphy reveals the interaction of volcanism and ocean circulation during oceanic anoxic event 2 high resolution osmium os isotope stratigraphy across the cenomanian turonian boundary interval from 6 sections for four transcontinental settings has produced a record of seawater chemistry that demonstrates regional variability as a function of terrestrial and hydrothermal inputs revealing the impact of palaeoenvironmental processes in every section the 187os 188os profiles show a comparable trend radiogenic values in the lead up to oceanic anoxic event 2 oae 2 an abrupt unradiogenic trend at the onset of oae 2 an unradiogenic interval during the first part of oae 2 and a return to radiogenic values towards the end of the event above the cenomanian turonian boundary the unradiogenic trend in 187os 188os is synchronous in all sections previous work suggests that activity of the caribbean lip large igneous province was the source of unradiogenic os across the oae 2 and possibly an instigator of anoxia in the oceans here we assess this hypothesis and consider the influence of activity from other lips such as the high arctic lip a brief shift to high radiogenic 187os 188os values occurred in the western interior seaway before the onset of oae 2 we evaluate this trend and suggest that a combination of factors collectively played critical roles in the initiation of oae 2 differential input of nutrients from continental and volcanogenic sources coupled with efficient palaeocirculation of the global ocean and epeiric seas enhanced productivity due to higher nutrient availability which permitted penecontemporaneous transport of continental and lip derived nutrients to trans equatorial basins

carbon isotopic composition of hydrocarbons in ocean transported bitumens from the coastline of australia abstract bitumens stranding along the coastlines of the northern territory western australia south australia victoria and tasmania often have biomarker signatures which closely match those of oils of s e asian origin a suite of these bitumens was studied to determine the isotopic signatures of their alkanes and to compare them to similar oils from the central sumatra basin saturated hydrocarbons were separated into a fraction containing n alkanes and isoalkanes and one containing the multibranched cyclic components using adduction into silicalite gas chromatography isotope ratio mass spectrometry gc irms of the n alkanes revealed 13 c signatures covering a wide range of values from 25 5 to 29 pdb but generally falling between those of the botryococcane containing minas and duri oils from the central sumatra basin n alkanes of the minas oil are 3 4 lighter than their duri counterparts the isotopic compositions of pristane and phytane cover a similar range 13 c 24 to 27 botryococcane is consistently heavy with 13 c values in the range 11 to 14 one bitumen sample with no botryococcane but abundant bicadinanes showed n alkane 13 c values in the range 28 to 29 at the light extreme for waxy bitumens in our sample suite sample to sample differences in carbon isotopic signatures of n alkanes are attributed mainly to variations in primary source with weathering and biodegradation as minor causes instances of low precision for replicate analyses 2 are caused by the relative abundance of co eluting isoalkanes overall the isotopic patterns of waxy bitumens from the australian coastline provide independent confirmation of their similarity to central sumatran lacustrine oils on the other hand asphaltic bitumens from the southern australian coastline are isotopically light with n alkane 13 c values in the range 31 5 to 33 this distinctive feature may assist identification of their source

origin of tar mats in petroleum reservoirs part i introduction and case studies abstract three case studies of petroleum reservoirs containing tar mats oseberg field and ula field both from the north sea and a north american field have been studied using a variety of organic geochemical techniques with special emphasis on the geochemistry of the asphaltenes the results were evaluated in combination with the available geological data and with numerical simulations to evaluate possible mechanisms of tar mat formation part ii wilhelms and larter 1994 in part i the petroleum geochemistry of three sets of tar mats is described the tar mat extracts were characterized by a sharp increase in the asphaltene content 20 60 wt compared with the oil leg extracts 1 5 wt the extract yields kg eom t rock in the tar mats ca 4 46 for the oseberg field 8 18 for the ula field and 8 43 for the north american case study are significantly higher than in the corresponding oil leg extracts which have extract yields of ca 1 14 kg t rock the tar mat extracts are geochemically related to the pooled oil as revealed by geochemical fingerprinting including biomarker analysis the tar mats are situated in high porosity and high horizontal permeability sandstone layers above permeability barriers several zones of low extract yield and immature petroleum geochemical characteristics were observed within the oil legs and between tar mats which might represent minor amounts of petroleum generated locally from indigenous organic matter

sequence stratigraphy depositional history and middle to late quaternary sea levels of the ventura shelf california abstract high resolution shallow seismic profiles from the ventura shelf reveal two contrasting structurally controlled styles of deposition on the northern shelf folded plio pleistocene strata comprising the south limb of the ventura avenue anticline are unconformably overlain by undeformed late quaternary strata in contrast a thicker section of undeformed prograding deltaic sequences preserved on the subsiding southern shelf contains a detailed record of at least five middle and late quaternary glacioeustatic sea level oscillations a transgressive surface at 140 m in upper pleistocene saugus deposits is overlain by transgressive and highstand systems tracts a shelf margin systems tract subsequently developed as the sea level lowered marking the end of saugus deposition thereafter over 50 m of post saugus upper pleistocene sediment accumulated during a period of smaller scale sea level oscillations followed by the development of a pair of marine terraces presently at 50 and 33 m and a large fluvial channel now buried at 113 m correlations of saugus and terrace marker horizons with the middle to late quaternary paleosea level curve suggest development of the saugus marker horizons during oxygen isotopic stages 7 5e and formation of the terraces during wisconsinan high sea levels maximum rates of vertical separation on the oak ridge and pitas point faults are estimated to be 0 13 and 0 30 m 1000 yr respectively

convergence rates across a displacement transfer zone in the western transverse ranges ventura basin california three cross sections are balanced and retrodeformed to the top of the pleistocene saugus formation and to a horizon close to the base of the jaramillo subchron to yield crustal shortening and shortening rates for the western transverse ranges of california the cross sections compare the shortening that occurs along a transfer zone in which displacement is transferred eastward from a surface reverse fault red mountain fault to a blind thrust and from that to a combination of both a surface reverse fault san cayetano fault and the blind thrust using an age of 250 50 ka for the top of the saugus formation based on amino acid racemization of fossil mollusks crustal shortening rates appear to have abruptly accelerated through time from 3 4 3 to 5 4 mm yr between 250 and 975 ka to 20 6 to 28 1 mm yr since 250 50 ka using an age of 500 ka for the top of the saugus formation based on paleomagnetic stratigraphy there is little increase in deformation rates through time ranging from 5 5 to 8 6 mm yr between 975 500 ka and from 10 3 to 14 1 mm yr since 500 ka more crustal convergence rates determined by global positioning system surveys taken over 4 6 years indicate a shortening rate of 7 10 mm yr across the basin this is consistent with the slower rates of deformation calculated using an age of 500 ka for the top of the saugus formation less

scales of stress heterogeneity near active faults in the santa barbara channel southern california the santa barbara channel represents the offshore portion of the ventura basin in southern california ongoing transpression related to a regional left step in the san andreas fault has led to the formation of e w trending en echelon fault systems that accommodate localized shortening across the basin recent studies have suggested that faults within the channel could be capable of a multisegment rupture and producing a m w 7 7 8 1 tsunamigenic earthquake however dynamic rupture models producing these results do not account for stress heterogeneity with only sparse information available on the stress field in this region further borehole derived stress constraints are essential for obtaining a more comprehensive understanding of the hazards related to the complex fault systems we used caliper logs from 19 wells obtained from industry to identify stress induced borehole breakouts beneath the holly and gail oil platforms in the channel our newly developed forward modeling technique provides constraints on the orientations and relative magnitudes of the three principal stresses at gail we determine a reverse faulting stress regime s hmax 1 7 s hmin 1 6 sv 1 0 and an s hmax azimuth of n45 e our results are consistent with local structures which reflect deeper regional scale trends and with similar studies onshore nearby at holly an s hmax rotation from n36 w to n57 e occurs across 100 m depth in a single well and differs from nearby results indicating that short length scale 10 km laterally and 1 km in depth stress heterogeneity is associated with complex changes in fault geometry

biomarker composition of some asphaltic coastal bitumens from tasmania australia abstract ten bitumen samples collected between 1880 and 1915 from the west and southwest coasts of tasmania three from flinders and king islands in bass strait and one each from preolenna in northwestern tasmania cape jaffa in south australia and a beach in the south of western australia were analysed as part of a study to re evaluate the petroleum prospectivity of tasmania and adjacent areas in southeastern australia the bitumens dissolved completely in chloroform and contained aliphatic hydrocarbons 12 3 17 3 aromatic hydrocarbons 4 4 10 6 and uncharacterized asphaltenes 75 6 82 9 the distributions of aliphatic and aromatic biomarkers were remarkably similar in all of the bitumens indicating that they were derived from a common or very similar source biomarker data confirmed that the bitumens are not related to land plant derived oils presently recovered from the nearby bass otway and gippsland basins of southern australia the high proportions of c 27 steranes and presence of c 30 steranes including dinosterane suggested that the bitumens were derived from a marine source rock containing mainly marine organic matter biomarker maturity parameters were also very similar in all bitumens and consistent with those found in crude oils generated at an equivalent vitrinite reflectance of about 0 75 hopanes showed a high proportion of c 29 and c 30 17 h 21 h hopanes with very minor amounts of the corresponding moretanes the unusual absence of 2 methylhopanes and low abundance of tricyclic alkanes ruled out carbonates or the tasmanite oil shales respectively as sources if the source of the bitumens can be related to local geological features these new findings could be significant for future oil exploration in tasmania and adjacent offshore areas

viscoelastic cycle model of interseismic deformation in the northwestern united states summary we apply a viscoelastic cycle model to a compilation of gps velocity fields in order to address the kinematics of deformation in the northwestern united states a viscoelastic cycle model accounts for time dependent deformation following large crustal earthquakes and is an alternative to block models for explaining the interseismic crustal velocity field building on the approach taken in pollitz et al we construct a deformation model for the entire western united states based on combined fault slip and distributed deformation and focus on the implications for the mendocino triple junction mtj cascadia megathrust and western washington we find significant partitioning between strike slip and dip slip motion near the mtj as the tectonic environment shifts from northwest directed shear along the san andreas fault system to east west convergence along the juan de fuca plate by better accounting for the budget of aseismic and seismic slip along the cascadia subduction interface in conjunction with an assumed rheology we revise a previous model of slip for the m 9 1700 cascadia earthquake in western washington we infer slip rates on a number of strike slip and dip slip faults that accommodate northward convergence of the oregon coast block and northwestward convergence of the juan de fuca plate lateral variations in first order mechanical properties e g mantle viscosity vertically averaged rigidity explain to a large extent crustal strain that cannot be rationalized with cyclic deformation on a laterally homogeneous viscoelastic structure our analysis also shows that present crustal deformation measurements particularly with the addition of the plate boundary observatory can constrain such lateral variations

australian offshore natural hydrocarbon seepage studies a review and re evaluation abstract surprisingly few natural hydrocarbon seeps have been identified in australia s offshore basins despite studies spanning thirty years early studies of natural hydrocarbon seepage around the australian margin were generally based on the geochemical analysis of stranded bitumens water column geochemical sniffer sampling synthetic aperture radar or airborne laser fluorosensor later studies involved the integration of these remote sensing and geochemical techniques with multi channel and shallow seismic a review of these earlier studies indicates that many seepage interpretations need to be re evaluated and that previous data sets when placed in a global context often represent normal background hydrocarbon levels low recent burial and subsidence rates are not favourable for high rates of seepage there are also difficulties in proving seepage on high energy shallow carbonate shelves where seabed features may be rapidly re worked and modern marine signatures are overprinted on authigenic seep carbonates thus the relatively few sites of proven natural hydrocarbon seepage in australia s offshore sedimentary basins can be reconciled relative to their geological occurrences and the dominantly passive margin setting active thermogenic methane seepage on the yampi shelf the only proven documented occurrence in australia is driven by deposition of a thick late tertiary carbonate succession and late miocene tectonic reactivation therefore to increase the success of detecting and correctly interpreting natural hydrocarbon seepage data need to be analysed and integrated within the context of the local geological setting and with an understanding of what is observed globally

map restoration of folded and faulted late cenozoic strata across the oak ridge fault onshore and offshore ventura basin california the ventura basin lies within the east west trending active fold and thrust belt of the western transverse ranges california this basin has been the site of significant earthquakes on structures within it and bordering it the purpose of our study is to identify the main structures in the basin and its borders and to quantify their rate of deformation our study includes the onshore and offshore ventura basin the arcuate basin bounding oak ridge reverse fault and the oxnard shelf to the south shortening fault slip and crustal block motions were studied using a three dimensional map restoration technique structure contour maps on the 6 ma surface and other horizons were digitized and restored to the initial horizontal state by unfolding them using the computer program unfold and then fitting the unfolded surfaces across faults comparing the restored and present configuration allows us to estimate total net finite displacements relative to a fixed horizontal reference line average post 5 ma shortening rates estimated from our restoration are slower than both post 1 ma rates and present rates determined by global positioning systems most shortening due to folding in the onshore basin is post 1 ma although slip on the oak ridge fault has occurred both before and after 1 ma displacement due to faulting and folding includes left lateral strike slip motion on the northeast southwest coastal segment of the oak ridge fault and associated clockwise rotation of the adjacent ventura basin the oxnard shelf is bordered to the south by mountains and islands that have been previously interpreted as folds above thrust fault ramps this onshore offshore block moves as one continuous thrust sheet similarly beyond the well studied onshore fault a kinematically continuous offshore oak ridge mid channel left oblique fault system is interpreted to continue at least an additional 100 km westward beneath the santa barbara channel

relationship between rank and composition of aromatic hydrocarbons for coals of different origins distributions of di and tricyclic aromatics were determined for upper carboniferous to oligocene coals of various origins in addition to upper carboniferous ruhr coals wealden lower creataceous coals from north west germany lower cretaceous coals from western canada and tertiary coals from japan and colombia were studied the coals ranged from sub bituminous to anthracite in rank and showed vitrinite reflectance from 0 4 to 4 9 rm the previously established relationship between the rank of coal in terms of vitrinite reflectance and the distribution patterns of individual aromatic hydrocarbon isomers has been verified and refined it is demonstrated how the above relationships can be used to calculate vitrinite reflectance from the methylphenanthrene index mpi 1 the chemical rank levels of coals can now be defined in terms of the calculated vitrinite reflectance rc

late quaternary history of the ventura mainland shelf california abstract the ventura mainland shelf off california provides an interesting example of sedimentary response to tectonic activity and eustatic sea level fluctuations during the late quaternary a network of 370 km of high resolution seismic reflection profiles provides the basis for the study the high local rates of uplift as well as local basin subsidence have resulted in two distinct styles of sedimentation a late pleistocene angular unconformity separates folded plio pleistocene strata from undeformed upper pleistocene and holocene strata on the northern shelf in contrast the southern shelf is characterized by a thick section of stacked progradational deltaic sequences containing a well preserved record of late quaternary sedimentation sediment overlying the unconformity north of the pitas point fault ranges from 10 to 40 m in thickness whereas the equivalent sequence in the adjacent southern shelf is 80 85 m thick the distribution of upper pleistocene and holocene deposits is controlled by structural features including the pitas point and oak ridge faults and the ventura avenue anticline as well as proximity to fluvial input from the santa clara and ventura rivers within upper pleistocene deposits a pair of buried marine terraces occur at depths of 33 and 50 m below present sea level b p s l terrace deposits consist of a basal coarse grained transgressive deposit which is transitional into a finer grained marine facies these deposits are associated with a period of aggradation during minor late wisconsinan sea level cycles deltaic deposits beneath the southern shelf 4 km north of hueneme submarine canyon suggest that the santa clara river debouched into the sea at this location during the late pleistocene the santa clara river shifted southward during the late wisconsinan and cut a large channel now buried near hueneme canyon this channel indicates that relative sea level was at least 113 m b p s l during the late wisconsinan a smaller channel indicates a fall in sea level to 46 m b p s l during the holocene the oak ridge fault displays a general increase in offset seaward across the shelf with a maximum vertical displacement of 17 m on the saugus upper pleistocene contact displacements on the pitas point fault increase shoreward attaining as much as 40 m of vertical displacement of the late pleistocene unconformity nearshore the pitas point fault vertically displaces the seafloor by 4 m

geochemistry and origin of sour gas accumulations in the northeastern sichuan basin sw china significant natural gas reserves have recently been discovered in the lower triassic oolitic reservoirs from northeastern sichuan basin sw china in the wake of the december 2003 sour gas well blow out this study presents an overview on the petroleum geology and geochemistry of the sour gas accumulations in the study area two types of natural gas accumulations were identified in the lower triassic oolitic reservoirs both containing highly mature thermogenic gases with their hydrocarbon source rocks in upper permian strata natural gases from the area south of the ancient kaijiang liangping seaway are generally sweet gases formed as the result of thermal maturation whereas those discovered from north of the seaway are products of both thermal maturation and thermochemical sulfate reduction of early accumulated oils in the feixianguan formation reservoirs the proposed origins of the gases are supported by their chemical and stable carbon isotope compositions as well as the presence or absence of pyrobitumens in the reservoir the distribution of gas accumulations is controlled predominantly by the combination of lithologic and structural factors the regional variation in the concentrations of h2s in the gases appears related to the presence and thickness of anhydrite bearing evaporitic rocks interbedded or intercalated with the oolitic reservoirs

methylhopane biomarker hydrocarbons in hamersley province sediments provide evidence for neoarchean aerobiosis abstract bacteriohopanepolyols with a ring methyl substituents are bacterial lipid biomarkers that are presently known to originate from just a few specific taxa when preserved in ancient rocks the fossilized hydrocarbon skeletons of these molecules have the potential to provide biogeochemical records of those taxa and their respective physiologies considering both their occurrences in modern organisms and their proterozoic and phanerozoic sedimentary distributions hopanes carrying 2 methyl or 3 methyl substituents are proposed to be derived from cyanobacteria and oxygen respiring methanotrophs respectively here we report the distribution of methylhopanes in 2 72 2 56 billion year old neoarchean rocks from the hamersley province on the pilbara craton the relative abundance of c 31 3 methylhopane but not that of c 31 2 methylhopane shows a strong correlation to the carbon isotopic composition of co occurring kerogen insoluble particulate organic matter the unanticipated nature of this relationship provides evidence for a shallow water locus of carbon cycling through aerobic oxidation of ch 4 and coincidentally a means to demonstrate biomarker syngenicity 2 methylhopanes are most abundant in both shale and carbonate from shallow water sediments and a positive correlation to carbonate abundance contrasts to variable but generally lower 2 methylhopane abundances of deeper water facies these observations are consistent with their origin from cyanobacteria which were likely the cornerstone of microbial communities in shallow water ecosystems providing molecular oxygen fixed carbon and possibly fixed nitrogen

botryococcane in a new class of australian non marine crude oils there are few documented examples of source specific hydrocarbons in non marine petroleum and most of these are related to land plant precursors1 3 one such biological marker of algal origin is botryococcane c34h70 a geologically rare acyclic alkane derived from the freshwater green alga botryococcus braunii4 5 first discovered in several sumatran crude oils6 7 and here identified in a suite of coastal bitumens from 10 different stranding sites between kingston south australia and portland victoria table 1 figs 1 2 biological marker and carbon isotope analyses of these bitumens provide evidence for a new class of australian non marine crude oils the bitumens are inspissated waxy residues of paraffinic and paraffinic naphthenic crude oils that emanate from submarine seeps in the western otway basin8 12 and are characterized by high concentrations of botryococcane unlike other high wax australian oils13 14 the coastal bitumens contain up to 2 6 sulphur and seem to be primarily of algal origin their geochemistry can be used to predict the occurrence of extensive organic rich source beds that were deposited in deep meromictic lakes along australia s southern continental margin

neotectonics of the offshore oak ridge fault near ventura southern california the oak ridge fault is a large offset south dipping reverse fault that forms the south boundary of the ventura basin in southern california previous research indicates that the oak ridge fault south of the town of ventura has been inactive since 200 400 ka ago and that the fault tip is buried by 1 km of quaternary sediment however very high resolution and medium resolution seismic reflection data presented here show a south dipping fault on strike with the oak ridge fault that is truncated at 80 m depth by an unconformity that is probably at the base of late pleistocene and holocene sediment furthermore if vertically aligned features in seismic reflection data are eroded remnants of fault scarps then a subsidiary fault within the oak ridge system deforms the shallowest imaged sediment layers we propose that this subsidiary fault has mainly left slip offset these observations of holocene slip on the oak ridge fault system suggest that revision of the earthquake hazard for the densely populated santa clara river valley and the oxnard coastal plain may be needed

the influence of thermochemical sulphate reduction on hydrocarbon composition in nisku reservoirs brazeau river area alberta canada abstract the upper devonian nisku formation reservoirs of the brazeau river area of west central alberta canada produce oil and sweet and sour gas condensate generally oil pools are located updip in the study area and sour 6 31 h2s gas condensate downdip h2s in the study area is formed by thermochemical sulphate reduction tsr all liquid hydrocarbons probably have one source with the duvernay formation being the most likely candidate assessing the relative maturity of the oils and condensates is difficult because of the wide variation in thermal maturity and the effects of tsr but the ratio of pristane n heptadecane does appear to decrease with increasing maturity for this sample set with increasing tsr the following changes were noted decrease in the saturate aromatic hydrocarbon ratio increase in the relative abundance of organo sulphur compounds e g benzothiophenes 34s values of liquid hydrocarbon samples approached the values for anhydrite of the nisku formation in the study area and an increase in 13c of the saturate fraction h2s concentrations in hydrodynamically isolated pools provide a good estimate of the extent of tsr in these reservoirs however other pools have anomalously high h2s concentrations for their depth suggesting that the h2s was generated at greater depths and migrated updip into these pools

biological marker characteristics of oils and asphalts from carbonate source rocks in a rapidly subsiding graben dead sea israel abstract a detailed gc ms study of biological marker compounds in the saturated and aromatic hydrocarbon fractions of oils and asphalts from the dead sea area israel provided decisive information to the solution of a long lasting controversy by showing that the asphalts are products of early generation in an immature stage from the same type of carbonate source rock which generated more mature oils the asphalts are not biodegraded residues of the oils oils from six different wells and asphalts from wells outcrops and a floating block from the dead sea all have very similar sterane and triterpane patterns they all lack rearranged steranes diasteranes indicating a carbonate source matrix and compare reasonably well with a sample of upper cretaceous bituminous chalk from nebi musa the main difference between the oils and the asphalts is a significantly higher triaromatic to mono plus triaromatic steroid hydrocarbon ratio in the former this is explained as a result of rapid subsidence and heating of their source rock close to the deep parts of the dead sea graben the oils thus were generated in the more deeply buried source rock blocks under the graben fill whereas the asphalts either originate from an immature source rock section closer to the graben rims or represent an earlier phase of generation and expulsion this study also provides general information on the evolution of biological markers in carbonate source rocks low activation energy processes like isomerisation of steranes appear to occur much faster at low temperatures than in shales the high sulfur content and less cross linking of the biogenic organic matter into a complex kerogen structure are suggested to be responsible for this care should be taken when using only sterane isomerisation to assess the maturity of hydrocarbons from carbonate rocks and of carbonate derived oils

the oak ridge fault system and the 1994 northridge earthquake the 17 january 1994 northridge earthquake moment magnitude m w 6 7 in the san fernando valley in southern california illuminated a hitherto unrecognized blind reverse fault with a moderate southward dip 1 this fault lies beneath the active north dipping santa susana fault system and uplifted both the footwall and the hanging wall of the santa susana fault during the earthquake1 here we argue that footwall uplift on the santa susana fault before the earthquake could have been used to identify the northridge blind thrust as an active fault moreover we propose that the northridge earthquake occurred on a continuation of the oak ridge fault system which reaches the surface in the ventura basin to the west slip rates on the western part of this fault system 2 3 are nearly three times larger than on the northridge blind thrust 4 increasing the probability of a n or t bridge sized earthquake in the heavily populated ventura basin

biomarker and n alkane isotope profiles for tertiary oils relationship to source rock depositional setting abstract biomarker and n alkane isotope profiles have been measured for 29 late cretaceous tertiary oils from se asia china papua new guinea new zealand and the u s a the results are interpreted with respect to six kinds of source rock depositional setting fluvio deltaic fd freshwater transitional tr lacustrine lac saline lacustrine sal marine deltaic md and marine carbonate mc a comparison of biomarker and n alkane isotope results suggests that parameters such as the oleanane hopane ratio may overestimate the higher plant contribution to marine oils the abundance of bicadinanes is much more variable than that of oleanane probably because of a specific association with the highly resinous dipterocarp hardwoods of se asia however traces do occur in an oil from outside the paleogeographic range of the dipterocarp family confirming that there is also a more general angiosperm source the bicadinane isomeric distribution is shown to depend on maturity leading to new maturity parameters which are resistant to even very heavy biodegradation retention indices based on the regular hopane series are provided for the main bicadinane and methyl bicadinane isomers and for a group of oleanane related triterpanes of unknown structure the presence of the latter compounds may indicate locally deposited as opposed to transported higher plant matter in the source source rock depositional setting is the primary control on the shape of the n alkane isotope profile with negatively sloping curves being characteristic of fd and tr oils and flat or positively sloping curves typical of marine oils the difference is probably related to the bacterial reworking of higher plant matter in the fd environment a slight isotopic anomaly at n c 17 correlates with the abundance of algal derived steranes and may indicate a minor marine contribution to the source of an fd oil

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

late quaternary activity and seismic potential of the santa monica fault system los angeles california the santa monica fault is a 40 km long oblique left lateral reverse fault that extends through the densely urbanized northwestern los angeles california area and offshore parallel to the malibu coast the fault exhibits near surface strain partitioning in paleoseismologic trenches and has undergone at least six surface ruptures during the past 50 k y although events may be missing from the older part of the record and the ages of older events are based on poorly constrained soil age estimates at least two and probably three events occurred after the well dated burial of a prominent paleosol ca 16 17 ka the resulting 7 8 k y latest pleistocene holocene average recurrence interval for events large enough to cause surface rupture is much longer than the 1 9 3 3 k y maximum interval we calculate for hypothetical m w 6 9 7 0 earthquakes generated by rupture of the entire santa monica fault the pronounced disparity between the measured and calculated average recurrence intervals suggests that the santa monica fault undergoes infrequent and therefore probably very large earthquakes m w 7 0 if such large earthquakes have occurred we speculate that they may have been generated by simultaneous rupture of the santa monica fault with other faults in the transverse ranges southern boundary fault system such as the hollywood or anacapa dume faults the most recent definite santa monica fault surface rupture occurred between ca 10 and 17 ka although a more recent event probably occurred between ca 1 and 3 ka

geodetic measurement of deformation in the ventura basin region southern california we have measured the deformation in the ventura basin region southern california with global positioning system gps measurements carried out over 4 6 years between 1987 and 1992 the deformation within our network is spatically variable on scales of tens of kilometers with strain rates reaching 0 6 1 micro rad yr in the east central basin blocklike rotations are observed south and northwest of the basin where the maximum shear strain rates are an order of magnitude lower 0 06 1 micro rad yr to the south we also observed clockwise rotations of 1 deg 7 deg m y shear strain rates determined by comparing angle changes from historical triangulation spanning several decades and gps measurements give consistent though less precise results the geodetic rates of shortening across the basin and western transverse ranges are lower than those estimated from geological observations but the patterns of deformation from the two methods agree qualitatively

evidence for multiple stages of oil cracking and thermochemical sulfate reduction in the puguang gas field sichuan basin china the puguang gas field is the largest gas field found in marine carbonates in china marine carbonate reservoirs in this field were buried to a depth of about 7000 m 22 966 ft and experienced maximum temperature up to 220c before uplift to the present day depth of 5000 5500 m 16 404 18 045 ft with present day thermal maturity between 2 0 and 3 0 equivalent vitrinite reflectance ro sulfur rich pyrobitumens with reflectance up to 3 5 are widespread in the reservoirs and resulted from thermal cracking of crude oils most likely generated from upper permian source rocks and thermochemical sulfate reduction tsr natural gases in the puguang gas field have wide variations in nonhydrocarbon gas contents with h2s contents between 5 1 and 58 3 and co2 contents between 7 9 and 18 0 the hydrocarbon gases originated mainly from thermal cracking of accumulated oil but were altered by tsr thermochemical sulfate reduction in the study area exerted different effects on the isotope compositions of different hydrocarbon gas components at different tsr stages the differential increase of 13c values for different gas components reflects transformation from a heavy hydrocarbon gas dominated tsr stage to a methane dominated tsr stage this caused a decrease of 13cmethane13cethane values and a corresponding conversion from reversed to normal isotope distributions thermochemical sulfate reduction in the study area appears to have been limited by sulfate concentrations in the reservoirs a successive three stage tsr series namely liquid hydrocarbon involved tsr heavy hydrocarbon gas dominated tsr and methane dominated tsr occurred in reservoirs with sufficient sulfate concentration methane can be the dominant organic reactant for tsr but only at elevated temperature and after most c2 hydrocarbons are exhausted

evaluation of the rhenium osmium geochronometer in the phosphoria petroleum system bighorn basin of wyoming and montana usa rhenium osmium re os geochronometry is applied to crude oils derived from the permian phosphoria formation of the bighorn basin in wyoming and montana to determine whether the radiogenic age reflects the timing of petroleum generation timing of migration age of the source rock or the timing of thermochemical sulfate reduction tsr the oils selected for this study are interpreted to be derived from the meade peak phosphatic shale and retort phosphatic shale members of the phosphoria formation based on oil oil and oil source rock correlations utilizing bulk properties elemental composition 13c and 34s values and biomarker distributions the 34s values of the oils range from 6 2 to 5 7 with oils heavier than 2 interpreted to be indicative of tsr the re and os isotope data of the phosphoria oils plot in two general trends 1 the main trend n 15 oils yielding a triassic age 239 43 ma with an initial 187os 188os value of 0 85 0 42 and a mean square weighted deviation mswd of 1596 and 2 the torchlight trend n 4 oils yielding a miocene age 9 24 0 39 ma with an initial 187os 188os value of 1 88 0 01 and a mswd of 0 05 the scatter high mswd in the main trend regression is due in part to tsr in reservoirs along the eastern margin of the basin excluding oils that have experienced tsr the regression is significantly improved yielding an age of 211 21 ma with a mswd of 148 this revised age is consistent with some studies that have proposed late triassic as the beginning of phosphoria oil generation and migration and does not seem to reflect the source rock age permian or the timing of re migration late cretaceous to eocene associated with the laramide orogeny the low precision of the revised regression 21 ma is not unexpected for this oil family given the long duration of generation from a large geographic area of mature phosphoria source rock and the possible range in the initial 187os 188os values of the meade peak and retort source units effects of re migration may have contributed to the scatter but thermal cracking and biodegradation likely have had minimal or no effect on the main trend regression the four phosphoria sourced oils from torchlight and lamb fields yield a precise miocene age re os isochron that may reflect the end of tsr in the reservoir due to cooling below a threshold temperature in the last 10 m y from uplift and erosion of overlying rocks the mechanism for the formation of a re os isotopic relationship in a family of crude oils may involve multiple steps in the petroleum generation process bitumen generation from the source rock kerogen may provide a reset of the isotopic chronometer and incremental expulsion of oil over the duration of the oil window may provide some of the variation seen in 187re 188os values from an oil family

kerogen asphaltene sulfur relationships in sulfur rich monterey oils abstract monterey oils in the santa maria basin onshore and offshore are generally low maturity oils because of low thermal exposure relatively shallow burial for short times they are generated as heavy high sulfur oils because of the composition of the unusual type ii source kerogens which are exceptionally rich in organic sulfur 8 14 these high sulfur kerogens which we designate type ii s appear to generate oil at significantly lower thermal exposures than ordinary type ii kerogens with less than 6 s in contrast to common type ii kerogens which generate oil largely by cleavage at oxygen bridge groups and carbon carbon bonds eliminating co2 h2o and hydrocarbon fragments type ii s kerogens have an additional complement of weak bonds associated with bound sulfur the preferential cleavage at weak sulfur linkages tends to produce larger fragments leading to high initial amounts of asphaltenes resins and sulfur rich aromatics together with smaller amounts of saturated hydrocarbons elemental analyses of monterey kerogens and asphaltenes clearly support the concept that the asphaltenes are essentially fragments of kerogen cleaved sufficiently to be soluble or peptized in this heavy oil system solubilization of asphaltenes is attributed to their interactions with the abundant resins and aromatics this sysstem is sufficiently stable under source and reservoir conditions to allow expulsion and migration for reasonable distance without significant fractionation or changes in gross properties initial asphaltenes have essentially the same atomic h c ratios as the source kerogen but have significantlyy lower o c ratios and slightly lower s c ratios the n c ratios are nearly the same for kerogens and their derived asphaltenes with increasing maturity the h c ratios in asphaltenes decrease much like maturation changes in kerogen the s c ratio also decreases significantly however the o c ratios initially low in asphaltenes and n c ratios increase slightly due to material balance caused by other losses these element composition and atomic ratio relationships allow some important inferences concerning source kerogen composition from analysis of asphaltenes alone this can be useful when potential source rock samples are not available for analysis and or when a choice between alternate source rock possibilities must be made

geomorphic indicators of active fold growth south mountain oak ridge anticline ventura basin southern california south mountain oak ridge near ventura california is an asymmetric anticlinal uplift forming at the present time above the active buried oak ridge reverse fault shortening along the oak ridge fault accumulated largely in quaternary time and is responsible for the growth and present topography of the westernmost 15 km of the ridge during the past 0 5 m y tectonic geomorphic analysis using several indices of active tectonics provides information concerning fold growth stream gradient indices are relatively high in the northern eroded fold scarp of the ridge a pattern consistent with the existence of active rapid slip on the oak ridge fault mountain front sinuosity along the northern slope of the anticlinal ridge roughly decreases from 2 to 1 toward the westernmost 10 km of observed surface folding valley floor width to valley height ratios along the northern flank of the ridge generally decrease westward from 1 5 to 0 5 values of the hypsometric integral along the northern flank increase significantly from 0 35 to 0 4 maximum 0 55 from east to west drainage density varies from 4 to 6 km km2 along both flanks of the south mountain oak ridge anticline entrenchment of streams into the southern backlimb of the fold along the westernmost 9 km of the structure decreases from 20 m to 1 m from east to west apparent backlimb rotation as measured by dip of strata along the westernmost 7 km of the fold decreases from east to west from 35 to 20 fold growth of the south mountain oak ridge anticline occurred during the past 0 5 ma following deposition of the saugus formation lateral and vertical fold growth were likely produced by westward decrease in fault slip along the buried oak ridge fault

late quaternary slip rate on the oak ridge fault transverse ranges california implications for seismic risk abstract the slip rate on the oak ridge fault at south mountain in the densely populated lower santa clara valley california is estimated as 6 15 to 12 3 mm yr since the end of saugus deposition at 0 4 to 0 2 ma slightly lower slip rate estimates result from a different projection of the top of the saugus in the hanging wall block of the fault an assumed displacement of 3 m per event gives an average recurrence interval of 250 to 500 yr with the major uncertainty being the age of the top of the saugus displacement of 2460 m of the top of the saugus at south mountain includes piercing point displacement and distributed displacement from drag folds near the fault both are the near surface expression of faulting at potential mainshock depths beneath well control slip rates for the nearby san cayetano and red mountain faults are not well constrained because post miocene strata are absent in their hanging wall blocks but available evidence suggests that their slip rates are about the same as that for the oak ridge fault earthquake return times measured in hundreds of years have been shown by others for the 1971 san fernando fault and a small fault near the red mountain fault and comparable return times are suggested for a normal fault in the hanging wall block of the oak ridge fault a trench on the harmon alluvial fan near the ventura county government center revealed evidence of cracks filled with sediments from below suggesting liquefaction during an earthquake the lower santa clara valley has not had a large damaging earthquake in 200 years of record keeping but one is expected there in the near future the december 21 1812 earthquake may have occurred on the offshore continuation of the oak ridge or red mountain fault

tetracyclic polyprenoids indicators of freshwater lacustrine algal input because of the great variety of lacustrine source depositional characteristics and biological consortia identification of a universally applicable indicator of freshwater or brackish water organic input for crude oils and rock extracts has proven elusive tetracyclic polyprenoid compounds tpp are present in relatively high concentrations in oils and associated source rocks deposited under freshwater or brackish water conditions but typically are present in relatively low concentrations in samples from marine or coal forming depositional environments a tpp ratio constructed from the gas chromatography tandem mass spectrometry peak areas of a c30 tetracyclic polyprenoid divided by the sum of the peak areas of the c26 27 norcholestanes revealed great specificity in identifying freshwater or brackish water algal input particularly in crude oils

molecular fossils in coastal bitumens from southern australia signatures of precursor biota and source rock environments abstract weathered waxy crude oil and black asphaltite tar are regularly washed ashore along the coastlines of south australia and western victoria these bitumens generally strand on medium to high energy sandy beaches in west or southwest facing bays some very weathered bitumens have been discovered in holocene dune deposits up to 200 m from the present day coastline differences in the sulphur content and carbon hydrogen and sulphur isotopic compositions of the coastal bitumens suggest that they belong to at least four oil families one of which may be emanating from submarine seeps along australia s highly faulted southern continental margin the remainder appear to be flotsam carried south from s e asia possibly sumatra the waxy bitumens families 1 3 range from paraffinic to aromatic intermediate in bulk composition and contain up to 2 6 s their density 13 38 api dictates that they are near surface drifters they have low pristane phytane ratios pr ph 2 abundant 4 methyl steranes c 30 with 24 ethyl substitution and in most cases high concentrations of botryococcane this biomarker assemblage indicates accumulation of freshwater algal source material including the remains of botryococcus sp and dinoflagellates under anoxic to suboxic conditions the additional presence of 28 30 bisnorhopane dinosterane and 24 n propylcholestane among the molecular fossils in family 3 bitumens suggests derivation from a lacustrine organic facies subject to marine incursions or alternatively mixing in the reservoir of two end member lacustrine and marine oil types trace amounts of oleanane and isomeric bicadinanes in the waxy bitumens constrain the age of their source rocks late cretaceous or younger and also preclude their local derivation an indonesian origin is considered likely implying long distance transport by surface ocean currents the family 4 bitumens 5 api are bottom drifters these sulphur rich 3 6 s aromatic asphaltic crudes have molecular signatures that include pr ph 1 c 27 c 29 c 28 c 30 desmethyl steranes dinosterane dominant over 4 methyl 24 ethylcholestane an enhanced concentration of 28 30 bisnorhopane relative to hopane and the absence of 2 or 3 methylhopanes a distal anoxic marine facies of the late cretaceous belfast mudstone in the eastern otway basin or its lithostratigraphic equivalent in the duntroon basin are possible local sources of the asphaltic bitumens

coastal bitumens from western australia long distance transport by ocean currents abstract samples of weathered bitumens were collected from granite rocks and quartz sand beaches near the high water mark along the west and south coasts of western australia samples were subjected to analysis using gc and gc ms techniques the proportions of n alkanes and branched and cyclic compounds in the samples were variable and the variation was attributed to differences in sample weathering more detailed analysis of the bitumens for biomarkers revealed the presence of botryococcane 4 methylsteranes bicadinanes and oleanane the occurrence of these compounds has also been observed in crude oils from the s e asian region the presence of bicadinanes has been attributed to a source contribution from an angiosperm currently confined to tropical regions and indicates that the source rocks from which the bitumens were derived were deposited after late cretaceous times the absence of late cretaceous and tertiary sediments deposited in lower latitudes in the northwest and western offshore basins of australia suggests a non local source of the bitumens a mechanism involving the leeuwin current has been proposed to account for transport of these bitumens which are presumably sourced from locations in s e asia

oak ridge fault ventura fold belt and the sisar decollement ventura basin california abstract the rootless ventura avenue san miguelito and rincon anticlines ventura fold belt in pliocene pleistocene turbidites are related to south dipping reverse faults rising from a decollement in underlying miocene shale to the east sulphur mountain anticlinorium comprises the upper plate of a south dipping set of thrusts sisar big canyon lion that merge downward into a decollement in mudstone of the rincon formation named here the sisar decollement shortening of the miocene and younger sequence is 3 km greater than that of the underlying competent vaqueros sespe and marine eocene sequence in the ventura fold belt and 7 km greater farther east at sulphur mountain cross section balancing requires that this difference be taken up by the paleogene sequence at the oak ridge fault to the south convergence is ne to nne based on earthquake focal mechanisms and piercing point offset of the south mountain seaknoll by the oak ridge fault a ne trending line connecting the west end of oak ridge and the east end of the sisar fault separates an eastern domain where late quaternary displacement is taken up entirely on the oak ridge fault and a western domain where displacement is transferred to the sisar decollement and its overlying rootless folds this implies that 1 the oak ridge fault near the coast presents as much seismic risk as it does farther east despite negligible shallow post 0 2 ma movement 2 ground rupture hazard is high for the sisar fault set in the upper ojai valley and 3 the decollement could fail on a low angle thrust at depth and produce an earthquake analogous to the 1987 whittier narrows event in los angeles

a second type of highly asphaltic crude oil seepage stranded on the south australian coastline abstract strandings of semi solid to solid asphaltic bitumen along the coastline of south australia have been reported as far back as the late 1800s hitherto only a single variety now referred to as asphaltite has been attributed to seepage from the nearby bight basin the geochemistry of the asphaltites suggest they were derived from a marine source rock deposited under anoxic or euxinic conditions most likely a cretaceous ocean anoxic event and were generated within the early main oil window here we identify a new type of semi solid asphaltic bitumen collected following a severe storm event in 2016 termed asphaltic tars these viscous oils bear a strong geochemical resemblance to the asphaltites both oil types have high asphaltene contents identical n alkane carbon isotopic profiles and near identical source specific sterane distributions however several notable geochemical variations can distinguish these new strandings from the asphaltites the most notable of these differences include heavier bulk sulphur isotopic composition extremely high abundances of re and os with distinct 187re 188os and 187os 188os values and thermal maturity parameters consistent with generation in the late oil window the differences in sulphur isotopic composition and re os systematics could be considered evidence that despite their other source specific similarities the asphaltic tars originated from a different source rock however alteration of these two parameters can occur due to thermochemical sulphate reduction conclusive identification of this alteration process typically relies on further diagnostic parameters which are unfortunately not available in the case of coastal oil strandings this introduces uncertainty to the correlation of these two types of asphaltic oil in either scenario the similarities between these two types of oil suggest their source rock s contained highly comparable organic matter inputs we therefore attribute the origin of these new asphaltic tar strandings to natural seepage from the offshore bight basin

dinosterane and other steroidal hydrocarbons of dinoflagellate origin in sediments and petroleum abstract the steroidal alkanes of a selection of sediments and oils have been examined by gc ms with multiple metastable reaction monitoring specific 4 methyl sterane isomers have been identified by comparison with isomers synthesized from sterols isolated from dinoflagellates an immature marine oil shale and two mature marine oils of triassic to early cretaceous age contained high concentrations of c30 steranes comprising desmethyl 24 ethyl 4 methylcholestane and 4 23 24 trimethylcholestane dinosterane isomers an immature non marine oil shale and two non marine oils of cretaceous to eocene age contained stereoisomers of 24 ethyl 4 methylcholestane as the dominant c30 steranes reaction monitoring analyses in gc ms are particularly suited to unravelling complex distributions of homologous and stereoisomeric steroids encountered in oils and their source rocks

large scale quaternary detachments in ventura basin southern california the ventura basin is an east north east trending trough in the california transverse ranges which records major quaternary detachment faulting at three levels the earliest thrusting occurred along weak siltstone interbeds in a sequence dominated by competent basin plain turbidite sandstone because sedimentation continued during thrusting the age rate and direction of thrusting can be worked out faulting began 1 3 m y ago and ceased 0 65 m y ago with a maximum slip rate of 2 8 mm yr to the southeast the fault set moved up a 45 ramp and ended as a blind thrust the ramp had topographic expression on the seafloor diverting turbidites around the ramp and preserving ash beds along with other hemipelagic sediments on its crest following the end of deposition 0 2 m y ago the competent basin plain turbidites underwent flexural slip folding over an incompetent miocene sequence dominated by shale underlying competent paleogene strata were not folded the south flank of the ventura avenue anticline tilted at 3 4 rad yr the anticlinal crest rose at a rate of 15 16 mm yr decelerating to 4 3 5 2 mm yr and the anticline and an adjacent syncline shortened at a rate of 20 mm yr the high rate of folding in the ventura avenue oil field resulted in overpressured sandstone reservoirs and oil water interfaces which have not had time to reach gravity equilibrium the red mountain san cayetano and santa susana faults mark the surface expression of a seismically active midcrustal detachment which produced convergence across the ventura basin at rates as high as 23 mm yr total convergence across the eastern san cayetano fault near fillmore is 11 600 2000 m in the last million years

recent deformation along the offshore malibu coast dume and related faults west of point dume southern california offshore faults west of point dume southern california are part of an important regional fault system that extends for about 200 km from near the city of los angeles westward along the south flank of the santa monica mountains and through the northern channel islands this boundary fault system separates the western transverse ranges on the north from the california continental borderland on the south previous research showed that the fault system includes many active fault strands consequently the entire system is considered a serious potential earthquake hazard to nearby los angeles we present an integrated analysis of multichannel seismic and high resolution seismic reflection data and multibeam bathymetric information to focus on the central part of the fault system that lies west of point dume we show that some of the main offshore faults have cumulative displacements of 3 5 km and many faults are currently active because they deform the seafloor or very shallow sediment layers the main offshore fault is the dume fault a large north dipping reverse fault in the eastern part of the study area this fault offsets the seafloor showing holocene displacement onshore the malibu coast fault dips steeply north is active and shows left oblique slip the probable offshore extension of this fault is a large fault that dips steeply in its upper part but flattens at depth high resolution seismic data show that this fault deforms shallow sediment making up the hueneme fan complex indicating holocene activity a structure near sycamore knoll strikes transversely to the main faults and could be important to the analysis of the regional earthquake hazard because the structure might form a boundary between earthquake rupture segments

active faulting and growth folding in the eastern santa barbara channel california we develop new methods to identify blind thrust fault systems determine fault slip rates and estimate potential earthquake magnitudes and recurrence intervals in active fold and thrust belts these methods are applied to compressive folds along the offshore oak ridge and blue bottle trends which overlie active blind thrust faults in the eastern santa barbara channel these folds and their causative faults are interpreted using fault bend fold theory and are represented in balanced models and cross sections that integrate surface and subsurface data the structures are mapped using a new technique of axial surface mapping in seismic reflection grids which defines three dimensional structural geometries and shows changes in slip and subsurface fault geometry along strike analysis of syntectonic growth sediments yields pliocene and quaternary fault slip rates of 1 3 mm yr on a deep thrust 16 km and 1 3 mm yr on shallower faults 2 5 km the combined 2 6 mm yr slip rate represents only part of the 6 mm yr of shortening measured by geodesy across the channel and estimated from relative pacific north american plate motions across the transverse ranges additional shortening is probably accommodated on other active thrusts in the western transverse ranges and in the northern channel along the santa barbara coast deformed seafloor sediments and a swarm of axial surface seismicity along the fold trends indicate that the underlying thrusts are active and may pose significant earthquake hazards to coastal southern california unsegmented fault surfaces are used through empirical relationships between fault surface area and rupture magnitude to estimate the sizes of potential earthquakes this analysis suggests that a ramp in the channel islands fault beneath the offshore oak ridge trend is capable of rupturing in a m s 7 2 earthquake earthquakes of this magnitude may release 2 m of slip which when combined with the estimated slip rate 1 3 mm yr yields an earthquake recurrence interval of 1500 yr for this channel islands fault ramp