the distribution and origin of dimethyldibenzothiophenes in sediment extracts from the liaohe basin east china abstract the distributions of dimethyldibenzothiophenes dmdbts the relationship between dmdbts and dimethylbiphenyls dmbps and the applications of dmdbts as maturity indicators in source rocks have been investigated in a set of 21 lacustrine shales from the eocene shahejie formation well sg1 in the western depression liaohe basin china all source rock samples are characterized by total organic carbon contents of 1 37 3 27 and type ii with minor type iii kerogen they were deposited in suboxic and brackish lacustrine environments and have maturities ranging from immature to mid mature the 3 3 dmbp isomer can potentially react to yield 4 6 dmdbt 2 6 dmdbt and 2 8 dmdbt by incorporating sulfur into biphenyl which may be supported by a strong positive correlation between the absolute concentration changes of 3 3 dmbp and those of 4 6 dmdbt 2 6 dmdbt plus 2 8 dmdbt the relative abundance of dmdbt isomers may be explained by the sulfur radical mechanism and are also controlled by steric hindrance and thermodynamic stability the 4 6 1 4 1 6 dmdbt ratio shows no regular trend with increasing maturity at the low stage and it should be used with caution as a maturity indicator for immature sediments however within the oil generation window the relative concentration of 4 6 dmdbt progressively increases with increasing maturity which can be explained by its higher thermodynamic stability relative to the 1 4 dmdbt isomer the 4 6 1 4 1 6 dmdbt ratio exhibits a linear increase with increasing thermal maturity of the sediments thus this ratio can be applied as an effective maturity indicator for source rocks within oil generation window

biogenic silica fluxes and accumulation rates in the gulf of california the gulf of california though small in size plays an important role in the global silica cycle the seasonal pattern of biogenic silica flux in the gulf is closely related to that of phytoplankton biomass levels and is controlled by changes in weather and hydrographic conditions the highest opal fluxes approximately 0 35 g center dot m sup minus 2 center dot d sup minus 1 occur during winter and spring and they are comparable to those measured in some of the most productive ecosystems of the world approximately 15 25 of the biogenic silica produced in surface waters is preserved in gulf sediments a figure significantly higher than the average global ocean preservation rate however the flux of opal at 500 m water depth is less than 25 of that being produced at the surface suggesting that most of the recycling of biogenic silica in the gulf of california occurs in the upper water column 28 refs 3 figs

alkyldibenzofurans in terrestrial rocks influence of organic facies and maturation abstract dibenzofuran dbf dibenzothiophene dbt and phenanthrene phe have been determined along with their methyl derivatives mdbfs mdbts mps in coals carbonaceous shales and mudstones of lower cretaceous to upper carboniferous ages from the elmworth gas field canada and from the sakoa madagascar and ruhr coal germany basins the samples contained mainly terrestrial organic matter with thermal maturities in the 0 7 1 9 mean vitrinite reflectance rr range the pronounced predominance of mdbfs over mdbts in the sakoa samples is typical of plant debris deposited in lacustrine swamp environments onset of intense mdbf generation occurs at fairly low rank 0 77 rr as indicated by elevated concentrations of 50 120 g mdbfs g total organic carbon toc influences of lithology organic matter type and maturity on distributions of the target compounds were extracted from the molecular concentration data by multivariate statistical methods such as principal components analysis pca and fuzzy c means cluster analysis with nonlinear mapping nlm a specific source effect on 1 mdbt concentrations suggests that lichens contributed to the senescent coal forming forest in the sakoa coal basin alkyldibenzofurans hence are potential lichen biomarkers mdbfs can be used as maturity indicators only beyond 1 0 rr where ruhr coals displayed a distinct increase in the 1 4 mdbf ratio

joint two dimensional dc resistivity and seismic travel time inversion with cross gradients constraints it is now common practice to perform collocated dc resistivity and seismic refraction surveys that complement each other in the search for more accurate characterization of the subsurface although conventional separate dc resistivity and seismic models can be diagnostic we posit that better results can be derived from jointly estimated models we make the assumption that both methods must be sensing the same underlying geology and have developed an innovative resistivity velocity cross gradients relationship to evaluate the structural features common to both methods the cross gradients function is incorporated as a constraint in a nonlinear least squares problem formulation which is solved using the lagrange multiplier method the resultant iterative two dimensional 2 d joint inversion scheme is successfully applied to synthetic data serving as validation tests here and to field data from collocated dc resistivity and seismic refraction profiling experiments and also compared to conventional separate inversion results the joint inversion results are shown to be superior to those from separate 2 d inversions of the respective data sets since our algorithm leads to resistivity and velocity models with remarkable structural agreement

biomarker 17α h diahopane a geochemical tool to study the petroleum system of a tertiary lacustrine basin northern south china sea abstract relatively abundant c30 17 h diahopane has been detected in oils and rock extracts in the fushan depression a tertiary lacustrine basin of the northern south china sea c30 17 h diahopane c30 17 h diahopane c30 17 h hopane ratios di increase with increased burial depth of the source rocks di values of most oils from the upper unit e 2 l 3 1 of the third member of the eocene liushagang formation e2l3 in the huachang oil and gas field show a positive correlation with other molecular maturity indicators such as the dnr dimethylnaphthalene ratio mdr methyldibenzothiophene ratio 4 6 1 4 dmdbt 4 6 1 4 dimethyldibenzothiophene mpr methylphenanthrene ratios or the mpi 1 methylphenanthrene index the inflection of the di depth curve occurring at about 3200 m corresponds to 0 55 ro the oils of the huachang oil and gas field have higher di values than the extracts of in situ source rocks indicating the oils might be sourced from deeply buried source rocks two petroleum systems can be distinguished in the huachang field in terms of the relative concentration of diahopane and other maturity parameters the oils in the upper petroleum system consisting of the reservoirs in e 2 l 3 1 are mainly sourced from the e2l source rocks in the bailian sag and are characterized by relatively higher maturity higher oil and gas yields and lower contents of diahopane and oleananes the oils in the lower system consisting of middle e 2 l 3 2 and lower units e 2 l 3 3 of e2l3 have higher contents of diahopane higher contents of co2 gas and lower maturity and oil and gas yields the occurrence and geochemical characteristics of the petroleum system are mainly controlled by the double fault system in the huachang uplift the c30 17 h diahopane is an effective biomarker for oil source rock correlation oil oil correlation maturity assessment and petroleum system study in some terrestrial lacustrine basins

source oil charging history and filling pathways of the ordovician carbonate reservoir in the halahatang oilfield tarim basin nw china abstract the ordovician reservoir in the halahatang depression is one of the most commercially prolific oil reservoirs in the tabei uplift of the tarim basin nw china for this study a total of 94 oil samples were collected from 88 exploration production wells in this oilfield all of the oils possess similar organic molecular compositions and belong to the same oil population oil to source correlation results indicate that oil accumulations discovered in halahatang oilfield were derived from the upper ordovician source rocks the homogenization temperatures of fluid inclusion in the ordovician reservoir in the halahatang oilfield range from 90 c to 100 c and from 140 c to 160 c the bimodal distribution pattern shown in a histogram of the homogenization temperatures indicates that the ordovician oil reservoir in the halahatang oilfield has been charged twice coupling this information with the reconstructed burial and geothermal histories by 1 d basin modeling the homogenization temperatures can be related to the relative geological ages of 419 ma to 410 ma and 16 ma to 8 ma namely the middle to late silurian and the middle miocene respectively representing the two phases of oil charging and entrapment the oil migration orientation and charging pathways can be traced using molecular indicators such as ts ts tm and tmnr parameters the overall oil migration orientation in the oilfield is generally from south to north therefore the source kitchen for the halahatang oilfield is predicted to be to the south side of the depression the southern part of the halahatang depression along the oil charging pathways is identified as the most promising region for oil exploration

geochemical cycles in sediments deposited on the slopes of the guaymas and carmen basins of the gulf of california over the last 180 years sediments deposited on the slopes of the guaymas and carmen basins in the central gulf of california were recovered in two box cores q mode factor analyses identified detrital clastic carbonate and redox associations in the elemental composition of these sediments the detrital clastic fraction appears to contain two source components a more mafic component presumably derived from the sierra madre occidental along the west coast of mexico and a more felsic component most likely derived from sedimentary rocks mostly sandstones of the colorado plateau and delivered by the colorado river the sediments also contain significant siliceous biogenic components and minor calcareous biogenic components but those components were not quantified in this study redox associations were identified in both cores based on relatively high concentrations of molybdenum which is indicative of deposition under conditions of sulfate reduction decreases in concentrations of molybdenum in younger sediments suggest that the bottom waters of the gulf have became more oxygenated over the last 100 years many geochemical components in both box cores exhibit distinct cyclicity with periodicities of 10 20 years the most striking are 20 year cycles in the more mafic components e g titanium particularly in sediments deposited during the 19th century in that century the titanium cycles are in very good agreement with warm phases of the pacific decadal oscillation implying that at times of greater influx of titanium rich

csem regularized seismic velocity inversion a multiscale hierarchical workflow for subsalt imaging seismic imaging in salt geology is complicated by highly contrasted velocity fields and irregular salt geometries which cause complex seismic wavefield scattering while the imaging challenges can be addressed by advanced imaging algorithms a fundamental problem remains in the determination of robust velocity fields in conditions of high noise conventional migration velocity analysis is often ineffective and even the most advanced methods for depth domain velocity analysis such as full waveform inversion require starting from a good initial estimate of the velocity model to converge to a correct result non seismic methods such as electromagnetics can help guide the generation of robust velocity models to be used for further processing using the multi physics data acquired in the deep water section of the red sea we apply a csem resistivity regularized seismic velocity inversion for enhancing the velocity model in a complex area dominated by nappe style salt tectonics the integration is achieved

source rock palaeoenvironments and controls on the distribution of dibenzothiophenes in lacustrine crude oils bohai bay basin eastern china abstract the applicability of sulphur aromatic parameters based on ratios of dibenzothiophene dbt and its alkylated homologues as indicators of oil maturity has been investigated in 27 oils from the damintun and zhuanhua depressions bohai bay basin the oils were characterised by bulk composition and environmentally sensitive aliphatic biomarkers and were derived from a wide range of lacustrine source depositional environments ranging from highly reducing hypersaline sulphur rich oils to terrestrial input dominated freshwater very high wax oils with maturities ranging from immature to probable mid mature dibenzothiophene based maturity parameters are not well correlated with sterane isomerization ratios and immature oils in particular could not be discriminated by these aromatic maturity indicators in contrast dbt based parameters are strongly correlated with pristane to phytane and ts tm ratios high methyldibenzothiophene ratios mdr are associated with fresh water source deposition whilst low mdrs are found in oils of hypersaline source affinity unusually high relative concentrations of 2 3 mdbt were observed in some of the immature oils inferred to be sourced by lacustrine organic laminites the data provide further evidence that thermal stress is not the sole factor controlling the distribution patterns of dibenzothiophenes in oils and that source depositional environments can exert a dominant influence when their range of variation is large absence of dbt in some biodegraded oils is tentatively attributed to its aqueous solubility during associated water washing

rhenium and molybdenum enrichments in sediments as indicators of oxic suboxic and sulfidic conditions of deposition abstract the trace elements re and mo both behave conservatively in seawater yet are strongly enriched in reducing sediments their potential for authigenic enrichment above crustal concentrations is greater than for many other elements due to the high ratio of metal sw metal crust we present sedimentary re and mo data from box and multi cores spanning a range of redox conditions from well oxygenated sites to locations with substantial sulfide concentrations at the oxic sites re and mo as expected accumulate at concentrations close to their crustal abundances re shows substantial enrichment in suboxic absence of o 2 and h 2 s sediments of the sea of japan in sediments within the oxygen minimum of the pakistan margin as well as in sediments underlying the sulfide bearing waters of the black sea and saanich inlet re enrichment occurs in these cores just below the depths of fe and u reduction only in the sediments underlying the sulfidic waters of the black sea and saanich inlet is there substantial authigenic mo accumulation absence of re and mo enrichments in sediment trap samples from the sulfide bearing basins suggests that the addition of both metals occurs at or below the sediment water interface the re and mo concentration profiles are modeled to hindcast the removal depth of re and mo in the sediments which along with the re mo ratio determine whether oxic suboxic or sulfidic conditions were present in the water column or in the sediments in the past using this approach historical redox conditions can be inferred even in environments such as continental margins where a substantial lithogenic component can obscure authigenic enrichments of other metals

molecular correlation of crude oils and oil components from reservoir rocks in the tazhong and tabei uplifts of the tarim basin china molecular data from a large set of source rock crude oil and oil containing reservoir rock samples from the tarim basin demonstrate multiple sources for the marine oils in the studied areas of this basin based on gammacerane c31 hopane and c28 c27 c 28 c 29 sterane ratios three of the fifteen crude oils from the tazhong uplift correlate with cambrian lower ordovician source rocks while the other crude oils from the tazhong uplift and all 39 crude oils from the tahe oilfield in the tabei uplift correlate with middle upper ordovician source rocks these two ratios further demonstrate that most of the free oils and nearly all of the adsorbed and inclusion oils in oil containing reservoir rocks from the tazhong uplift correlate with cambrian lower ordovician source rocks while the free and inclusion oils in oil containing carbonates from the tahe oilfield correlate mainly with middle upper ordovician source rocks this result suggests that crude oils in the tazhong uplift are partly derived from the cambrian lower ordovician source rocks while those in the ordovician carbonate reservoirs of tahe oilfield are overwhelmingly derived from the middle upper ordovician source rocks the scatter of c23 tricyclic terpane c23 tricyclic terpane c30 17a 21b h hopane and c21 c21 rc29 sterane ratios for the free and inclusion oils from oil containing carbonates in the tahe oilfield possibly reflects the subtle organofacies variations in the source rocks implying that the ordovician reservoirs in this oilfield are near the major source kitchen in contrast the close and positive relationship between these two ratios for oil components in the oil containing reservoir rocks from the tazhong uplift implies that they are far from the major source kitchen

water column anomalies associated with hydrothermal activity in the guaymas basin gulf of california abstract sea floor hydrothermal activity in the guaymas basin gulf of california is quite different from that associated with ridge crest spreading centers injection of hydrothermal fluids occurs in the bottom of a semi enclosed basin and water column anomalies produced by this activity increase to much higher values than in the open ocean in the guaymas basin the hydrothermal venting generates large clouds of fine suspended particulate matter spm 100 300 m above active mounds and chimneys these hydrothermal clouds have potential temperature anomalies of about 0 010 0 020 c are enriched in dissolved silica particulate manganese and depleted in dissolved oxygen relative to areas away from the vents the particulate manganese values increase from about 3 nmol kg at 1000 m well above the enclosing topography of the subsill basin to 100 150 nmol kg in the clouds of spm and in the bottom nepheloid layer the particulate mn in the hydrothermal clouds appears to originate from both direct precipitation of dissolved mn2 injected by the vents and entrainment of mn rich spm in the rising hydrothermal plumes injection of silica rich vent fluids into the basin bottom waters produces a silica anomaly of 10 15 mol relative to the other deep basins of the gulf of california spillover of guaymas basin deep water produces a silica plume just above the basin sill depth which is detectable to the mouth of the gulf a simple two endmember mixing model indicates that the deep waters of the guaymas basin contain approximately 0 1 hydrothermal fluid oxygen anomalies associated with the hydrothermal clouds are on the order of 5 mol relative to regions away from active vents the basin as a whole shows a depletion in oxygen of about 13 mol relative to the other deep basins of the gulf the mixing model shows that this oxygen consumption can be explained by the oxidation of dissolved sulfide and methane injected by the hydrothermal vents box models of the deep basins of the southern gulf of california indicate that the guaymas basin has a significantly higher source term for dissolved silica and sink term for dissolved oxygen than the other basins the calculated flux of hydrothermal fluids into the guaymas basin is 10 12 m3 s

fractionation of dibenzofurans during subsurface petroleum migration based on molecular dynamics simulation and reservoir geochemistry abstract dibenzofurans oxygen containing heterocyclic aromatics are ubiquitous in crude oils their geochemical implications in petroleum geochemistry have been studied in numerous previous publications here we present the distribution of dibenzofuran and its methylated homologues and calculate the polarity and solubility of methyldibenzofuran isomers based on the connolly surfaces and molecular dynamics simulation the results show that the geo chromatographic fractionation effect is evident due to absorption onto the water silica matrix carrier bed during the oil migration process two potential molecular indicators relative to dibenzofurans i e the total content of dibenzofurans and the relative abundances of 1 methyldibenzofuran to 4 methyldibenzofuran are proposed for tracing the oil migration orientation and filling pathways these two indicators were applied successfully in paleozoic marine carbonate and tertiary lacustrine sandstone reservoirs the variations of source input depositional environment and maturity for oils from same source bed kitchen appear to have no significant effect on these two indicators it suggested that these two dibenzofuran indicators are potentially effective molecular tracers for orientation and filling pathways in oil migration

calcite dissolution driven by benthic mineralization in the deep sea in situ measurements of ca 2 ph pco 2 and o 2 abstract in situ measured microprofiles of ca 2 pco 2 ph and o 2 were performed to quantify the caco 3 dissolution and organic matter mineralization in marine sediments in the eastern south atlantic a numerical model simulating the organic matter decay with oxygen was used to estimate the calcite dissolution rate from the oxygen microprofiles measured at four stations along a 1300 m isobath of the eastern african margin and one in front of the river niger at a water depth of 2200 m the diffusive oxygen uptake dou and oxygen penetration depth opd was calculated dou rates were in the range of 0 3 to 3 mmol m 2 d 1 and showed a decrease with increasing water depth corresponding to an increase in opd the calculated amount of degradated organic matter is in the range of 1 to 8 5 gc m 2 a 1 the metabolic co 2 released from mineralization of the organic matter drives calcite dissolution in these sediments overlain by calcite supersaturated water fluxes across the sediment water interface calculated from the in situ ca 2 microprofiles were 0 6 mmol m 2 d 1 for two stations at a water depth of 1300 m the ratio of calcite dissolution flux and organic c degradation is 0 53 and 0 97 respectively the microprofiles indicate that co 2 produced within the upper oxic sediment layer dissolves up to 85 of the calcite rain to the seafloor modeling our o 2 ph and ca 2 profiles from one station predicted a calcite dissolution rate constant for this calcite poor site of 1000 mol kgw 1 a 1 mol per kg water and year which equals 95 d 1 this rate constant is at the upper end of reported in situ values

the occurrence and distribution of phenylnaphthalenes terphenyls and quaterphenyls in selected lacustrine shales and related oils in china abstract phenylnaphthalenes phns terphenyls trps quaterphenyls qtps and triphenylbenzenes were identified by the use of authentic standards in gc ms analysis phenyl derivatives of fused ring polycyclic aromatic hydrocarbons phpahs and polyphenyls pphs were observed in selected lacustrine shales and related oils from the tarim liaohe and bebuwan basins in china all isomers of phenylnaphthalene and terphenyl were observed in these oils the more stable 2 phn and m trp p trp isomers are predominant among phenylnaphthalenes and terphenyls respectively the effect of maturity on the distribution patterns of phenylnaphthalenes and terphenyls in a set of lacustrine shales from the liaohe basin east china was investigated the depth trends of three currently used maturity indicators were investigated analysis was based on the relative abundance of phenylnaphthalene and terphenyl isomers namely phnr 2 phn 1 phn trp1 p trp o trp and trp2 m trp p trp o trp in lacustrine shales from well s202 in the liaohe basin east china the indicators increase slightly with relatively great deviations within low to intermediate maturation stages and then increase abruptly in high maturity stages therefore they are not suitable maturity indicators for lacustrine source rocks in this study an alternative thermal maturity indicator defined as p trp m trp was proposed it exhibits a uniform increase with increasing maturity and is probably a more suitable indicator for use in maturity assessment in lacustrine source rocks and related oils this study broadens the current understanding of the occurrence and distribution of phpahs and pphs in sedimentary organic matter of various origins

identification and distribution of chrysene methylchrysenes and their isomers in crude oils and rock extracts abstract chrysene its methylated homologues and their structural isomers are important polyaromatic compounds in crudes oils coals and sediment extracts chrysene benzo a anthracene methylchrysenes and methylbenzo a anthracenes were unequivocally identified in mass chromatograms by comparison with authentic standards and by retention indices reported in the literature the response factors relating those of chrysene and methylchrysenes to internal standards perdeuterated chrysene chrysene d12 and pyrene pyrene d10 were obtained by gc ms analyses of mixture solutions with different concentration ratios the oil families of ordovician reservoirs in the cratonic region of the tarim basin nw china are distinguished by the distribution patterns of chrysene methylchrysenes and their isomers the majority of the oil accumulations and sediment extracts from the middle upper ordovician source rocks are characterized by low concentrations of benzo a anthracene and methylbenzo a anthracenes relative to chrysene and methylchrysenes respectively in contrast some oils with relatively high abundances of benzo a anthracene and methylbenzo a anthracenes were derived from cambrian and lower ordovician source rocks the effect of maturity on the distribution patterns of methylchrysenes in tertiary lacustrine shales from the liaohe basin east china was investigated the 2 1 methylchrysene ratio keeps a nearly constant value with increasing maturity at the low maturity stages and then it abruptly increases corresponding approximately to the onset of intense c 15 hydrocarbon generation this indicates that chrysene and its methylated homologues are useful molecular indicators in petroleum geochemistry

occurrence and origin of carbon dioxide in the fushan depression beibuwan basin south china sea abstract natural gases produced from the fushan depression beibuwan basin south china sea are generally rich in carbon dioxide the 13 c values of methane in these gases are within the range of 41 18 to 47 00 this together with the linearity of the 13 c values of methane to butane in the natural gas plots indicates that the hydrocarbon gases are of organic origin the 13 c co 2 values of these gases are in the range of 5 1 to 7 5 suggesting an inorganic origin a dominant mantle source for the co 2 in these gases was determined on the basis of a simple binary mixing model using the 13 c co 2 values and 3 he 4 he ratios of associated helium in these gases the primary geological controls on the occurrence of co 2 rich gas accumulations in the fushan depression are discussed following a comparison with other sedimentary basins in the region

manganese geochemistry in the guaymas basin gulf of california manganese geochemistry in the guaymas basin gulf of california is dominated by hydrothermal processes the concentration of particulate mn in the subsill basin is about four times higher than in the non hydrothermally active basins of the southern gulf this excess mn closely matches the predicted anomaly from a model of hydrothermal fluxes based on dissolved silica and sup 3 he the hot spring flux of mn is about 4 times greater than the benthic flux from the sediments on the slopes of the basin dissolved hydrothermal mn has a residence time of about one week in the water column the particulate mn phase consist of 2 5 mu m size particles that are distinct from all other suspended particulate matter these mn rich particles are responsible for the observed turbidity in the water column hydrothermal clouds above vents and chimneys in the guaymas basin and bear a striking resemblance to the mn oxidizing bacterium metallogenium the rapid conversion of mn to a particulate phase in these clouds is consistent with bacterial catalysis this contrasts with the particles at 21 sup 0 n east pacific rise where mn is more slowly co precipitated with or adsorbed onto an iron oxyhydroxide phase

fractionation of azaarenes during oil migration abstract a comparative study of alkylbenzoquinolines in crude oils and rock extracts bitumens from japan and sumatra shows that the ratios of nitrogen masked isomers nmis to nitrogen exposed isomers neis and the ratio of higher homologs to lower homologs is higher in crude oils than in corresponding bitumens also a regular increase in the ratios of nmis to neis with increasing migration distance is observed for a series of crude oils from the sarukawa oil field northeastern japan these results are attributed to the preferential migration of nmis caused by their weak adsorption on clay minerals and or their low solubility in interstitial water and the selective removal of lower honologs caused by their irreversible adsorption onto clays during oil migration which are regarded as geochromatographic phenomena it is suggested that those ratios can be used to estimate the degree of fractionation of oils during primary and secondary migration

cross property relations between electrical conductivity and the seismic velocity of rocks cross property relations are useful when some rock properties can be measured more easily than other properties relations between electrical conductivity and seismic velocity stiffness moduli and density can be obtained by expressing the porosity in terms of those properties there are many possible ways to combine the constitutive equations to obtain a relation each one representing a given type of rock the relations depend on the assumptions to obtain the constitutive equations in the electromagnetic case the equations involve archie s law and its modifications for a conducting frame the hashin shtrikman hs bounds and the self similar and complex refraction index method crim models in the elastic case the stress strain relations are mainly based on the time average equation the hs bounds and the gassmann equation also expressions for dry rocks and for anisotropic media using backus averaging are analyzed the relations are applied to a shale saturated with brineoverburden and to a sandstone saturated with oilreservoir tests with sections of a north sea well log show that the best fit is given by the relation between the gassmann velocity and the crim selfsimilar and archie models for the conductivity

joint inversion of seismic traveltime and frequency domain airborne electromagnetic data for hydrocarbon exploration geophysical joint inversion requires setting of few parameters for the optimum performance of the process however there are yet no known detailed procedures for selecting the various parameters for performing the joint inversion previous works on the joint inversion of em and seismic data have reported parameter applications for both data sets acquired from the same dimensional geometry either both in 2d or in 3d and few on variant geometry but none has discussed the parameter selections for the joint inversion of methods from variant geometry for example a 2d seismic travel and pseudo 2d frequency domain em data with the advantage of affordable computational cost and the sufficient approximation of 1d em model in a horizontally layered sedimentary environment we are able to set optimum joint inversion parameters to perform structurally constrained joint 2d seismic traveltime and pseudo 2d em data for hydrocarbon exploration from the synthetic experiments even in the presence of noise we are a

geophysical modeling via simultaneous joint inversion of seismic gravity and electromagnetic data application to prestack depth imaging geophysical modeling of multiple geophysical parameters can take place reliably through the simultaneous joint inversion of the different geophysical domains the desired objective is the derivation of a model expressing different geophysical properties of the earth but at the same time showing consistency among the parameters this concept generally referred to as data integration is widespread in geophysics but the way of achieving it is not obvious several authors have proposed different approaches where data interpretation and experience all contributed to the task the result is that no general workflows could be defined that left ample margins for uncertainties and personal judgment

palaeozoic oil source correlation in the tarim basin nw china a review abstract oil source correlation in the cratonic region of the tarim basin nw china has long been controversial current knowledge of the potential source rock distribution end member selection oil source correlation and impacts of secondary alteration processes have been reviewed two source rock systems from the cambrian lower ordovician o 1 and middle upper ordovician o 2 3 potentially contributed to oil accumulations in the cratonic region geochemical correlations suggest that oils are dominantly derived from the o 2 3 source rocks while geological evidence and sulfur isotopic compositions supports the o 1 source rocks as a main contributor such inconsistency is rooted in the selection of end members and their characterization changes in some biomarker parameters such as sterane distribution gammacerane index and ratio of tricyclic terpanes to pentacyclic terpanes are mainly controlled by thermal maturation and biodegradation rather than source input and depositional environment variation no clear boundary between the two source systems can be established using biomarkers except for a few diagnostic components some o 1 source signatures such as even odd predominance of n alkanes unusual tricyclic terpane distribution unusually enriched 13 c carbon isotopic compositions of oils and kerogens and the occurrence of combustion related polycyclic aromatic hydrocarbons largely result from an abnormal heating influence either from igneous intrusion or hydrothermal fluid activity local thermochemical sulfate reduction can also remove most source related signatures these so called markers in currently selected end members result from extensive secondary alteration processes and do not reflect source input differences while more research is needed for full reconstruction of oil source correlation in the cratonic region of the tarim basin the main source rocks most likely reside in the o 1 succession rather than o 2 3

high resolution velocity modeling by seismic airborne tem joint inversion a new perspective for near surface characterization we discuss the use of helicopter borne transient electromagnetic htem for a high resolution spatial and vertical characterization of the near surface in a structure controlled wadi in central saudi arabia in this area seismic data quality is poor and seismic imaging suffers from a combination of scattering effects due to swarms of faults reaching the surface and large velocity variations occurring along subvertical boundaries between the wadi sediment infill and the surrounding carbonate plateaus htem was selected from a suite of nonseismic methods for multiparameter velocity model building to enhance the velocity estimation for the wadi and surrounding areas htem data were modeled by performing spatially constrained 1d resistivity inversion to obtain a high resolution image of the near surface with sensitivity to a depth of 400 500 m from the surface sharp boundaries of the wadi and fine vertical layering obtained from the htem inversion provide detailed information about the parameter vari

variations of upwelling intensity recorded in varved sediment from the gulf of california during the past 3 000 years circulation in the pacific ocean is periodically affected by el nino events which are a direct response to intensity changes in trade winds1 2 the appearance of anomalously warm surface water in the eastern equatorial pacific ocean is one of the most spectacular manifestations of this event warm surface water anomalies are also observed in the northern hemisphere along the californian coast and in the gulf of california in the gulf strengthened trade winds induce upwelling during the winter whereas in the summer prevailing southerly winds provoke an input of warm water from the south3 we have sampled laminated sediments from the slopes of guaymas basin to reconstruct with high resolution variations of the hydrology of surface waters in the central part of the gulf of california sedimentation rate is of the order of 4 mm yr 1 at the top and 1 2 mm yr 1 down core by analysing the oxygen isotope ratio of biogenic silica of diatoms4 we have recorded the surface water temperature variations in the guaymas basin the comparison of these fluctuations and variations of silicoflagellate abundance has allowed us to reconstruct changes in upwelling intensity during the past three thousand years the present conditions of low upwelling activity are seen to be abnormal in comparison with those prevailing in the past

geochemistry of palaeozoic marine petroleum from the tarim basin nw china part 1 oil family classification abstract a series of palaeozoic marine oils ranging from condensates to very heavy oils from the cratonic region of the tarim basin nw china have been examined to delineate oil families crude oils from the mesozoic foreland kuqa depression were also analyzed for comparison the genetic family was assessed using bulk hydrocarbon isotopic composition alkylation pattern of polycyclic aromatic hydrocarbons pahs and biomarker distributions detailed oil oil correlation suggests two distinct families occur in the cratonic region despite the diversity of the reservoir rocks and some variability in oil physical properties the tabei and tazhong oils are classified in general into a single family with a possible indication for secondary subdivision these oils are characterized by low pristane phytane pr ph ratio high dibenzothiophene phenanthrene dbt p ratio light bulk isotopic composition high degree of pah alkylation high proportion of c23 and c24 tricyclic terpanes and low abundance of c24 tetracyclic terpane and triaromatic dinosteroids this family is inferred to have a middle upper ordovician source rock the tadong family distinctly different from the tabei and tazhong families is characterized by relatively high pr ph ratio low dbt p ratio slightly heavier bulk isotopic compositions and low degree of pah alkylation it also has a unique biomarker signature with a high proportion of triaromatic dinosteroids possibly indicating cambrian lower ordovician source rocks the oils from the kuqa depression are easily distinguished from the other two families due to their terrigenous origin the major characteristics of the kuqa oils are high pr ph ratio very low dbt p ratio heavy bulk isotopic composition low pah alkylation degree high proportion of c20 and c21 tricyclic terpanes and c24 tetracylic terpane and abundance of triaromatic dinosteranes and 4 methyl 24 ethyltriaromatic cholestanes these characteristics are consistent with terrestrial organic source rocks deposited under sub oxic conditions

changes in biomarker abundances and sulfur isotopes of pyrite across the permian triassic p tr schuchert dal section east greenland abstract in this study we report on biomarker abundances through parts of the permian triassic boundary ptb of schuchert dal east greenland that contains rich marine faunal records and excellent terrestrial palynological records biomarker abundances and sulfur isotopes are used to correlate the series of events including changes in element cycling and associated redox conditions of the ocean surrounding the collapse of the marine and terrestrial ecosystems through this record of a major crisis of life on earth during a mass extinction episode the upper schuchert dal formation contains a low diversity palynological assemblage ascribed to arborescent cordiaite conifer pteridosperm vegetation samples from this pre collapse interval are characterised by high abundances of dibenzofuran dbf dibenzothiophene dbt and biphenyl since these compounds have similar base structures and show comparable abundance curves it is plausible that they probably derive from a common source we propose that phenolic compounds of lignin of the woody plants present during this period could be the source for dbf dbt and biphenyl the redox conditions during this period of time also support the formation of dbf and dbt just above the extinction interval there is a dramatic decrease in the abundances of dbf and dbt which occurs at the same time as a sudden change in the stable sulfur isotopic composition 34s of pyrite indicating a change in redox conditions from oxic to anoxic euxinic conditions 34s values leading up to the extinction are highly depleted in the heavy sulfur isotope about 40 vs vcdt whilst shortly after the extinction interval much more positive isotope values are observed about 25 an inferred change in the biogeochemical sulfur cycle is supported by facies evidence from similar neighbouring sections it is suggested that two processes are operating closely here 1 changes in redox conditions and 2 extinction and or transgression accounting for the absence of woody material

geo chromatographic fractionation effect of methyldibenzofuran in dolomite reservoirs and its application in tracing oil filling pathways in the sichuan basin abstract dibenzofuran and its methyl substituted homologues are the important oxygen containing polycyclic aromatic compounds in crude oils the ratio of 1 4 methyldibenzofuran has been successfully used in sandstone reservoir tracing oil migration orientation here we focus on the geo chromatographic fractionation effect of methyldibenzofuran mdbf in dolomite reservoirs we calculated the charge density differences between 1 methyldibenzofuran 1 mdbf and 4 methyldibenzofuran 4 mdbf based on density functional theory the e i n between mdbf isomers and dolomite 104 surface was also calculated according to molecular dynamics simulation the result show that comparing with the 4 mdbf there is strong interaction between 1 mdbf and dolomite 104 surface leading the ratio of 1 mdbf and 4 mdbf increase with the migration distance there is no significant correlation between bitumen reflectance and the ratio of 1 mdbf and 4 mdbf in our samples in addition molecular simulation results show that the mdbf isomers are thermodynamically stable and dissociation of ph ch3 is difficult to occur under geological condition all these demonstrate that 1 mdbf and 4 mdbf can be survived after oil cracking to gas and pyrobitumen therefore in theory the ratio of 1 4 methyldibenzofuran is also regarded as an index for oil migration orientation in dolomite reservoir gas chromatography mass spectrometry results show that mdbfs occur at relatively higher abundance in the bitumens of the ediacaran dengying formation z2dn4 and cambrian longwangmiao formation 1l in the sichuan basin the ratio of 1 4 methyldibenzofuran has been successfully used to trace the crude oil filling pathway in the z2dn4 and 1l dolomite reservoir in the sichuan basin in this study two preferential filling orientations i e from west to east and from north to south for paleo oil have been identified in the contour maps of 1 4 methyldibenzofuran in the z2dn4 and 1l which is consistent with the geological background

significance of polycyclic aromatic hydrocarbons pahs in permian triassic boundary sections abstract in this study the abundances of several polycyclic aromatic hydrocarbons pahs table 1 i xxi have been measured throughout three permian triassic p tr sections from meishan south china kap stosch area east greenland and peace river basin western canada dibenzothiophene i and dibenzofuran ii were found to decrease in abundance just before or shortly after the p tr transition in all three sections while perylene iii was observed to increase in abundance at the onset of the main extinction horizon bed 25 in meishan perylene has been attributed to a wood degrading fungal source and therefore it seems possible these phenomena are related to the demise of land plants further distinct patterns of various combustion derived pahs occurring in each section imply that forest fire events occurred within the late permian and early triassic in the meishan section high amounts of combustion derived pahs pyrene iv fluoranthene v benzo a anthracene vi benzofluoranthenes sum i e vii benzo a pyrene viii benzo e pyrene ix and coronene x occur within bed 25 also containing ash attributed to the fallout from massive volcanic eruptions in siberia and or china

pleistocene to holocene contrasts in organic matter production and preservation on the california continental margin organic matter in sediments from cores collected from the upper continental slope 200 2700 m off california and southern oregon shows marked differences in concentration and marine character between the last glacial interval ca 24 10 ka and either holocene time or last interstadial oxygen isotope stage 3 ca 60 24 ka in general sediments deposited during holocene time and stage 3 contain higher amounts of marine organic matter than those deposited during the last glacial interval and this contrast is greatest in cores collected off southern california the most profound differences in stage 3 sediments are between predominantly bioturbated sediments and occasional interbeds of laminated sediments the sediments are from cores collected within the present oxygen minimum zone on the upper continental slope from as far north as the oregon california border to as far south as point conception these upper pleistocene laminated sediments contain more abundant hydrogen rich type ii marine algal organic matter than even surface sediments that have large amounts of nonrefractory organic matter the stable carbon isotopic composition of the organic matter does not change with time between bioturbated and laminated sediments indicating that the greater abundance of type ii organic matter in the laminated sediments is not due to a change in source but rather represents a greater degree of production and preservation of marine organic matter the presence of abundant well preserved organic matter supports the theory that the oxygen minimum zone in the northeastern pacific ocean was more intense and possibly anoxic during late pleistocene time as a result of increased coastal upwelling that enhanced algal productivity

suboxic trace metal geochemistry in the eastern tropical north pacific abstract we analyzed al ti fe mn cu ba cd u mo v and re in water column settling particulate and sediment 0 to 22 cm samples from the intense oxygen minimum zone omz of the eastern tropical north pacific near mazatlan mexico the goal was to determine how the geochemistry of these elements was influenced by suboxic water column conditions and whether the sediments have a unique suboxic geochemical signature the water column was characterized by a mn maximum reaching 8 nmol kg 1 at 400 m concentrations of cu ba cd mo re u and v were unaffected by the low o2 conditions and were comparable to those of the open ocean sinking particles were composed of lithogenic particles of detrital origin and nonlithogenic particles of biogenic origin al ti and fe were mostly at least 79 lithogenic about 75 of the mn was nonlithogenic significant amounts at least 58 of cu ba cd and mo were nonlithogenic sediment geochemistry varied across the continental shelf and slope cadmium u and re have prominent maxima centered at 310 m with 12 3 ppm 10 9 ppm and 68 3 ppb respectively at the core top high values of mo averaging 6 8 ppm and v averaging 90 ppm are seen in omz surface sediment additional down core enrichment occurs for all redox sensitive elements in the top 10 cm for u mo v and re surface sediments are a poor indicator of metal enrichment comparison of the nonlithogenic composition of sediments with sinking particles suggests that direct input of plankton material enriched in metals makes a significant contribution to the total composition especially for cd u and mo we evaluated re mo and cd u ratios as tracers for redox environments rhenium and mo concentrations and re mo ratios do not lead to consistent conclusions concurrent enrichments of re and mo are an indicator of an anoxic depositional environment in contrast high re mo ratios are an indicator of suboxic conditions cadmium is enriched in surface sediments while u has considerable down core enrichment the concentrations of cd and u and the cd u ratio do not follow patterns predicted from thermodynamics though the water column is suboxic these four redox sensitive elements indicate that the sediments are anoxic the implication for paleostudies is that a trace metal sediment signature that indicates anoxic conditions is not necessarily attributable to an anoxic water column

oil filling history of the ordovician oil reservoir in the major part of the tahe oilfield tarim basin nw china abstract the ordovician oil reservoir in the tahe oilfield northern tarim basin is so far the largest marine oil reservoir in china ordovician oil was sampled from 73 exploration production wells in the oilfield according to oil oil correlation all the ordovician oil samples belong to the same oil population of mature to high mature phase based on the coexisting intact n alkane series evident ucm and 25 norhopanes in the oil as well as the bimodal distribution pattern of homogenization temperatures t h ranging from 80 to 100 c and from 115 to 135 c respectively in fluid inclusions within the reservoir rocks it is concluded that the oil reservoir has been twice charged during its oil filling history by means of 1d basin modeling the measured t h values can be related to relevant geological ages of 429 415 ma and of 8 2 ma respectively i e the middle to late silurian and the miocene to pliocene representing two phases of oil filling and entrapment using independent molecular parameters e g the 4 1 mdbt ratio and total concentrations of pyrrolic n compounds as indicators to reconstruct the oil filling process in the reservoir oil filling pathways can be traced and mapped the general orientation of oil filling is from south to north in the oil reservoir therefore it can be predicted that its source kitchen should be on the south side of the oilfield most probably at the shuntuoguole uplift between the awati and manjiaer depressions

effect of water washing on crude oil compositions crude oils from venezuela oklahoma and new mexico were water washed in the laboratory at temperatures from room temperature to 80 c and with water salinities from 0 to 300 000 mg l in experiments lasting from 7 to 338 hours the effects of water washing were determined by gas chromatographic and gas chromatography mass spectrometric analysis of the residual oil these experiments show that water washing is particularly effective for the c15 fraction and hydrocarbons are removed in the sequence aromatics then n alkanes then naphthenes in the c15 fraction no loss of pristane phytane steranes or terpanes occurs but some aromatics and sulfur compounds especially dibenzothiophene are depleted although water washing reduces api gravity water washing is unlikely to produce tar layers the effects of water washing were simulated with a numerical model based on the equation of diffusion predicted oil water ratios were in good agreement with the experimental values for the light ends and the model suggests that water washing is very fast in the subsurface comparison of oils that appeared to have been water washed in nature with the equivalent unwashed oils showed the expected compositional trends water washing is probably the dominant process affecting crude oil composition in the subsurface when water flows past oils under conditions where bacterial degradation is precluded by temperature 80 c or by lack of dissolved oxygen and the temperatures are too low for thermal cracking

biomarker distributions in crude oils and source rocks from different sedimentary environments abstract the saturated and aromatic hydrocarbons from a significant number of oils and source rocks derived from terrestrial and marine organic matter have been analysed by gc and gc ms results show that certain classes of biomarkers including bicyclic sesquiterpanes tricyclic terpanes pregnanes methylsteranes rearranged steranes hopane sulfides benzohopanes long chain alkylbenzenes and pentacyclic triterpanes can be used to distinguish differences in source input and or depositional environments variations in the distributions of the naphthalenes phenanthrenes fluorenes dibenzofurans dibenzothiophenes naphthobenzothiophenes and triaromatic steroid hydrocarbons may also reflect disparities in source input or sedimentary environments

the significance of 24 norcholestanes triaromatic steroids and dinosteroids in oils and cambrian ordovician source rocks from the cratonic region of the tarim basin nw china abstract two oil families in ordovician reservoirs from the cratonic region of the tarim basin are distinguished by the distribution of regular steranes triaromatic steroids norcholestanes and dinosteroids oils with relatively lower contents of c 28 regular steranes c 26 20s c 26 20r c 27 20s and c 27 20r regular triaromatic steroids dinosteranes 24 norcholestanes and triaromatic dinosteroids originated from middle upper ordovician source rocks in contrast oils with abnormally high abundances of the above compounds are derived from cambrian and lower ordovician source rocks only a few oils have previously been reported to be of cambrian and lower ordovician origin especially in the east region of the tarim basin this study further reports the discovery of oil accumulations of cambrian and lower ordovician origin in the tabei and tazhong uplifts which indicates a potential for further discoveries involving cambrian and lower ordovician sourced oils in the tarim basin dinosteroids in petroleum and ancient sediments are generally thought to be biomarkers for dinoflagellates and 24 norcholestanes for dinoflagellates and diatoms therefore the abnormally high abundance of these compounds in extracts from the organic rich sediments in the cambrian and lower ordovician and related oils in the cratonic region of the tarim basin suggests that phytoplankton algae related to dinoflagellates have appeared and might have flourished in the tarim basin during the cambrian period steroids with less common structural configurations are underutilized and can expand understanding of the early development history of organisms as well as define petroleum systems

quasi 3d modeling of airborne tem data by spatially constrained inversion we present a new methodology spatially constrained inversion sci that produces quasi 3d conductivity modeling of electromagnetic em data using a 1d forward solution spatial constraints are set between the model parameters of nearest neighboring soundings data sets models and spatial constraints are inverted as one system the constraints are built using delaunay triangulation which ensures automatic adaptation to data density variations model parameter information migrates horizontally through spatial constraints increasing the resolution of layers that would be poorly resolved locally sci produces laterally smooth results with sharp layer boundaries that respect the 3d geological variations of sedimentary settings sci also suppresses the elongated artifacts commonly seen in interpretation results of profile oriented data sets in this study sci is applied to airborne time domain em data but it can also be implemented with other ground based or airborne data types

water mass formation in the gulf of california the gulf of california is a long narrow marginal sea lying between the baja california peninsula and the mainland of mexico air sea fluxes of heat and moisture in the gulf are enhanced because of geographical isolation from the pacific provided by the mountainous baja california peninsula in the northern gulf annual evaporation rates are about 1 m y 1 unlike most evaporative basins however the gulf gains heat from the atmosphere at an annual average rate of 20 to 80 w m 2 bray 1988 given the unusual air sea forcing of the gulf what form or forms should water mass formation take the annual moisture loss and heat gain require that high salinity surface water be transported downward to an intermediate depth and that cold fresh inflow be transported upward to an intermediate depth this is accomplished through several mechanisms 1 winter convection this occurs only in a limited geographical region the wagner basin of the far northern gulf except in el nino southern oscillation years when convection appears to be more widespread 2 dispersion of convected water in small eddylike features this occurs within a large scale southward transport possibly driven by the large scale density gradient associated with atmospheric fluxes 3 an anticyclonic circulation in the northern gulf this is found south of the convection region and transports high salinity water off the shallow shelves and to substantial depths where it may mix with water of central gulf origin 4 tidal mixing most of the energy available for mixing in the northern gulf derives from the tides in particular tidal mixing over the sill in the island region is responsible for the substantial reduction in salinity of northern gulf waters as they enter the central gulf

sediment mixing and accumulation rates in the sulu and south china seas implications for organic carbon preservation in deep sea environments abstract recent rates of carbon burial in two deep sea basins the sulu and south china seas are contrasted to determine factors influencing organic carbon preservation in deep sea sediments the sulu sea is an oxygen deficient warm water basin whereas the adjacent south china sea is more typical of open ocean conditions high resolution sea bed profiles of 14c reveal that accumulation rates for the sulu sea are 2 3 times higher than the south china sea at any given water depth e g 1 3 g cm 2 kyr 1 versus 4 3 g cm 1 kyr 1 respectively at 3000 m higher accumulation rates for the sulu sea are attributed to higher terrigenous input and better carbonate preservation in the warmer bottom waters benthic mixing rates determined using 210pb geochronology generally are lower in the sulu sea 0 004 0 254 cm2 yr 1 as compared with the south china sea 0 17 1 57 cm2 yr 1 especially at the 3000 m and 4000 m sites in the sulu sea where benthic mixing is negligible despite the higher bottom water oxygen levels in the south china sea organic carbon concentration profiles are strikingly similar for the two basins at any given water depth with similar concentration profiles higher accumulation rates in the sulu sea result in equivalently higher carbon preservation rates as primary production is thought to be similar for both basins if anything slightly higher for the south china sea the production rate can not explain the observed differences in carbon preservation in addition benthic mixing rates apparently do not play a major role in carbon preservation as extremely low rates at the deeper sulu sea would predict relatively low carbon preservation contrary to our observations we conclude that sediment accumulation rate is the dominant control on carbon preservation in this area of the western equatorial pacific

dibenzothiophenes and benzo b naphthothiophenes molecular markers for tracing oil filling pathways in the carbonate reservoir of the tarim basin nw china abstract dibenzothiophenes dbts and benzo b naphthothiophenes bnts are the main sulfur containing heterocyclic aromatic compounds in crude oils and sedimentary rock extracts because of the relatively higher electronegativity and an unshared pair of electrons in the outer shell orbital of the sulfur atom hydrogen bonds can be formed between the sulfur and hydrogen atoms of the surrounding medium in carrier beds therefore dbts and bnts are potential geochemical markers for tracing oil filling pathways a correlation of oils in the halahatang oilfield of the tabei uplift in the tarim basin nw china shows that the molecular marker compositions of all oils are quite similar implying that these oils have been sourced from the same source kitchen and belong to a same oil family using molecular indicators relative to dbts and bnts namely 4 1 mdbt 4 1 methyldibenzothiophene 4 6 1 4 1 6 dmdbt 4 6 1 4 1 6 dimethyldibenzothiophene 2 6 3 6 1 4 1 6 dmdbt 2 6 3 6 1 4 1 6 dimethyldibenzothiophene and 2 1 bnt 2 1 bnt 1 2 bnt benzo b naphtho 2 1 d thiophene benzo b naphtho 2 1 d thiophene benzo b naphtho 1 2 d thiophene this paper traces the oil migration orientation and filling pathways in this oilfield the isopleth maps of these molecular indicators unequivocally reveal an overall oil migration filling points and preferred filling pathways orientation from south to north based on this a potential source kitchen is predicted in the southern part of the halahatang depression most probably at the manxi shuntuoguole low uplift just between the awati and manjiaer depressions the southern part of the halahatang depression along the oil charging pathways is predicted to be the most promising region for oil exploration

numerical simulation of petroleum generation and migration in the qingshui sag western depression of the liaohe basin northeast china a two dimensional basin modeling study was employed to analyze the petroleum generation and migration history of the qingshui sag in the western depression of the liaohe basin northeast china the eocene oligocene es4 and es3 members of the shahejie formation es are the most important source rocks responsible for the major hydrocarbon accumulations two sandstone beds of fan delta origin the dujiatai djt and the xinglongtai xlt are the primary reservoir rocks in the study area the model indicates that two migration systems can be differentiated they are separated by the thick shale beds of the middle es3 and es1 which acted as important regional seals for the lower and the upper systems respectively the djt bed is the most important pathway for migrating hydrocarbons in the lower system whereas the xlt bed is a significant factor in the upper system calibration of the model with thermal maturity data indicates that the geological evolution is consistent with typical heat flow values for rift basins in the range between 85 and 48 mw m2 all source units reached maturity during the major burial phase which extended from the oligocene to the eocene simulation results further indicate that after a cooling period renewed heating occurred and additional hydrocarbons were generated during the last 5 m y particularly in the southern part of the basin sensitivity tests indicated that the faults particularly the two main faults served as important conduits for petroleum throughout most of the geological history the main phase of migration occurred between the eocene and oligocene and was responsible for major accumulations in the west slope the central anticline and the central rise additional migration between 5 ma and the present is significant in the southwestern area and may have brought about petroleum accumulation in the central depression and central rise

nitrogen isotopic variations in the gulf of california since the last deglaciation response to global climate change high resolution records of the nitrogen isotopic composition of organic matter 15norg opal content and opal accumulation rates from the central gulf of california reveal large and abrupt variations during deglaciation and gradual holocene changes coincident with climatic changes recorded in the north atlantic homogenous sediments with relatively low 15norg values and low opal content were deposited at the end of the last glacial period during the younger dryas event and during the middle to late holocene in contrast laminated sediments deposited in the two deglacial stages are characterized by very high 15norg values 14 and opal accumulation rates 29 41 mg cm 2 yr 1 abrupt shifts in 15norg were driven by widespread changes in the extent of suboxic subsurface waters supporting denitrification and were amplified in the central gulf record due to variations in upwelling vertical mixing and or the latitudinal position of the intertropical convergence zone