predicting gas oil and water intervals in niger delta reservoirs using gas chromatography formation evaluation experts usually have little difficulty in interpreting wireline logs to assess the type of reservoir fluid oil gas water in sand shale sequences this assessment is usually accomplished by a combination neutron density tool that detects low hydrogen and low electron densities typical of gas zones and the repeat formation tester rft which uses both the pressure gradient and sample acquisition techniques to evaluate reservoir fluid in the niger delta however many of the sands exhibit a poor neutron density response to gas and rft testing has been largely eliminated because poor hole conditions commonly result in stuck tools oil fingerprinting of residual hydrocarbons from sidewall core extracts can provide an independent means of identifying eservoir fluid type in conjunction with the nigerian national petroleum company we completed numerous chromatographic analyses on solvent extracts of sidewall core samples from various wells in the mefa field offshore west niger delta oil fingerprints of sidewall core extracts clearly reflect the type of hydrocarbons in the reservoir fingerprints of oil saturated sands are typical of topped c15 whole oils whereas fingerprints of hydrocarbons extracted from the gas dominated sands show an abbreviated hydrocarbon distribution more typical of gas condensates fingerprints of extracts from water productive intervals show a further abbreviated signature suggesting that higher molecular weight hydrocarbons did not migrate into these intervals these signatures are distinct and intact despit poor sample preservation due to prolonged storage of up to 3 yr additional applications of this technique include the reevaluation of older wells that lack modern wireline measurements but have sidewall or conventional core material available and in thin or low permeability sands that may not yield definitive rft or wireline log responses application potential for this technology is worldwide and includes the gulf coast of the united states canada indonesia and papua new guinea

the residence time of the freshwater component in the arctic ocean the time function of bomb tritium concentrations in river runoff to the arctic ocean has been reconstructed from published data on tritium in precipitation 1959 1975 tritium measurements on oceanic samples through the halocline exhibit strong linear relationships between tritium concentrations tu values and salinity these waters thus look like binary mixtures of atlantic source water and freshwater runoff combining these data the vintage of the freshwater component in the arctic basin has been determined assuming no other major tritium source the relation indicates the average age of the freshwater component to be 11 1 years in the nansen basin and the outflow and somewhat higher in the canada basin according to tritium salinity data a surface layer of 10 60 m is affected by sea ice melting and freezing in the nansen basin and the thickness of this layer increases to 150 170 m toward the canada basin there is tritium also in the deeper waters the unmixed atlantic water which points at residence times for that water not to exceed 17 years the appendix is available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j81 012 1 00 payment must accompany order

formation and evolution of silurian paleo oil pools in the tarim basin nw china the formation and evolution of silurian sandstone reservoirs which are characterized by the coexistence of reservoir bitumen with different types of commercial oil pools in a very extensive area around the slope and uplift areas of the manjiaer depression in the tarim basin nw china are still in question in this paper we report our investigations on the geological and geochemical characteristics of this reservoir the results show there are four types of oil shows in the reservoir severely biodegraded solid bitumen biodegraded heavy oil normal oils and light oils with slight biodegradation three suites of petroleum inclusions were identified in the reservoir which were formed during the early devonian triassic to cretaceous and late tertiary respectively petroleum in the silurian sandstone reservoirs mainly originated from the mid lower cambrian source rock and mid upper ordovician source rock the thermal maturity evolution of the source rocks in slope areas has shown that there were three main stages of oil generation that controlled the formation of the above three stages of petroleum inclusions this evidence indicates that the silurian reservoir has experienced three petroleum charge events during its geological history the first charge event occurred during the early devonian and it resulted in giant paleopools which gradually evolved into the presently widespread bitumen due to the combination of biodegradation and thermal alteration the second charge covered a long period from triassic to cretaceous and probably was related to both the mid lower cambrian source rock and the mid upper ordovician source rock the petroleum generated during this stage mixed and interacted with the low matured residual bitumen and eventually formed the present soft bitumen and some residual heavy oils in the reservoirs the third charge event mainly associated with the mid upper ordovician source rocks occurred mainly during late tertiary and accounted for most of the present commercial petroleum pools mainly normal and light oils

petroleum source in the tazhong uplift tarim basin new insights from geochemical and fluid inclusion data abstract a total of 108 drill stem test dst crude oil samples and 10 reservoir fluid inclusion samples were investigated geochemically to identify the characteristics and origin of the crude oil in the tazhong uplift tarim basin nw china results show that the majority of oils share typical biomarker characteristics from the middle upper ordovician o2 3 source rock and related crude oil features these features include a distinct v shape in the relative abundance of c27 c28 and c29 regular steranes and low abundances of dinosterane 24 norcholestanes triaromatic dinosteroids and gammacerane only a few oils display typical biomarker features indicating a cambrian lower ordovician o1 genetic affinity such as linear or anti l shape c27 c28 and c29 regular sterane distributions and relatively high concentrations of dinosterane 24 norcholestanes triaromatic dinosteroids and gammacerane it appears that most of the oils studied were derived from the o2 3 intervals as suggested by previous studies however the 13c values of individual n alkanes indicate that most of the crude oils in the tazhong uplift represent a mixture of two end member oils an o1 derived oil such as from well td2 or tz62 s and an o2 3 derived oil such as from well ym2 the data suggest that most of the oils in the uplift have a mixed origin and do not originated from the middle upper ordovician strata alone this conclusion is supported by data on the molecular composition of petroleum inclusions this new oil mixing model is critical for reconstructing the hydrocarbon formation and accumulation history for the region and may have important implications for further petroleum exploration in the tarim basin

36cl as a tracer in geothermal systems example from valles caldera new mexico the chlorine 36 content of three thermal waters from valles caldera new mexico has been analyzed to investigate the use of chlorine 36 as a geothermal tracer

patterns of incidence of oil reserves in niger delta basin data on oil reserves in the niger delta basin show a bimodal lognormal distribution with the break in slope reflecting variation in exploration maturity quality of prospect and distribution of giant oil fields several areas of anomalous reserves are defined by the selected threshold oil reserves density of 200 000 bbl km2 five major prolific centers with reserves contrast 3 are aligned in an arcuate belt prolific belt which is an image of the present coastline the prolific belt marks the transition zone between oceanic and continental crusts a zone of weakness characterized by increased tectonic activity during passage through the belt the niger delta system was predominantly constructive and consisted of at least five delta complexes which now define the prolific centers beyond the prolific belt predominance of destructive processes resulted in merging of the delta complexes in these areas the distribution of zones of major concentration of reserves developed in response to alternate channel switching and extension superimposed on growth fault activity the prolific belt and offshore zone coincide with a hot geothermal belt that frames a geothermal minimum at the delta center the coincidence together with the progressively younger sediments from north to south suggests that the maturity per unit depth is less in the offshore zone and central delta nose than in the prolific belt implied are deeper levels of hydrocarbon occurrence as well as a lesser amount of generated hydrocarbons offshore and on the central delta nose than in the prolific belt these factors and the relatively shallow depth of investigation probably account for the present absence of prolific centers south of the prolific belt

analysis of secondary migration of hydrocarbons in the ordovician carbonate reservoirs in the tazhong uplift tarim basin china the process and mechanisms of secondary hydrocarbon migration in the tazhong uplift tarim basin were investigated based on the analysis of the regional structure and by integrating geologic hydrodynamic and geochemical parameters parameters successfully analyzed included the fluid potential fluid properties production outputs and diamantane index the results indicated that hydrocarbons migrated into the tazhong uplift from the northern part of the manjiaer depression through a series of injection points ips during four orogenies that is the early caledonian 510 ma the late caledonian 439 ma the late hercynian indosinian 290 ma and the yanshanian himalayan 208 ma a total of six ips were identified at the intersections of the northeast trending faults and the northwest trending flower strike faults the hydrocarbons migrated from the ips into traps along regional trends from northwest to southeast and from northeast to southwest the hydrocarbon migration process and patterns determined the distribution of hydrocarbon properties and production rates in the tazhong uplift with increasing distance from the ips daily hydrocarbon production decreases and the hydrocarbons become progressively heavier and display lower gas oil ratios

genetic type and maturity of lower paleozoic marine hydrocarbon gases in the eastern tarim basin abstract hydrocarbon compositions and their carbon isotopic data for 94 natural gas samples were combined with the results of thermal simulation experiments to complete a comprehensive study of maturity genetic types and sources of lower paleozoic marine hydrocarbon gases from the eastern tarim basin all of the lower paleozoic gases are relatively dry dryness coefficient 0 9 and belong to the marine sapropelic type of cracked gases these gases are characterized by low 13 c 1 and low 13 c 2 13 c 1 values due to a lack of terrestrial source input thermal simulation experiments on upper proterozoic to lower paleozoic source rocks resulted in a 13 c 1 r o regression equation that was used to discriminate the maturities and sources of the natural gases these natural gases are divided into four genetic types 1 humic coal derived pyrolytic gas 2 lacustrine sapropelic oil cracking gas 3 the marine sapropelic highly mature oil cracking gas and 4 overmature oil cracking gas types 1 and 2 were mainly generated from the upper triassic middle and lower jurassic types 3 and 4 are mainly generated from the lower paleozoic the data can be used to describe multiple sources and multiple generating stages of natural gas

application of diamondoids to distinguish source rock facies abstract diamondoid distributions in solvent extracts from marine siliciclastic shales marine carbonates and coals of a wide variety of maturities were determined by gc msd for this sample set published diamondoid maturity parameters did not change systematically throughout the oil window it was found that several novel diamondoid ratios were indicative of source it was possible to distinguish between terrigenous marine carbonate and marine siliciclastic facies the proposed dimethyl diamantane facies ratios are virtually unaffected by maturity while the ethyl adamantane ratio appears to be affected by maturity in the late gas window range

prediction of oil gravity prior to drill stem testing in monterey formation reservoirs offshore california this paper discusses empirical geochemical correlations used to predict api oil gravities prior to drill stem testing in monterey formation reservoirs offshore southern california the primary objective was to eliminate expensive well testing by identifying intervals that contain low gravity nonproducible oil usually 14 api however the correlations proved very successful in accurately predicting within 4 api oil gravities that range from 5 to 35 api throughout the offshore santa barbara and santa maria areas the primary data are weight percent sulfur and rock eval pyrolysis of bitumen chemically extracted from reservoir rock samples in general reservoirs that contain higher gravity producible oil have bitumen organic sulfur contents of less than 5 wt rock eval bitumen s1 values greater than 450 mg hc g bitumen and rock eval bitumen s1 s2 ratios greater than 1 0 these data are usually supplemented with rock eval pyrolysis of the reservoir rock where whole rock s1 s2 ratios greater than 0 30 usually indicate associated oil gravities greater than 14 api this analytical mix gives a multiple approach for estimating reservoir oil gravities within proposed drill stem test dst intervals using this approach oil gravities of more than 50 dsts have been accurately predicted in the offshore southern california area the technique is also useful for reevaluating api gravities in older wells where monterey reservoirs were not the primary target moreover the technique should have application elsewhere provided the range of oil gravities are not the result of obvious biodegradation and sufficient rock and oil samples are available to establish pertinent correlations

organic geochemical identification of reservoir oil gas water layers in the junggar basin nw china abstract the identification of reservoir oil gas water layers is a fundamental task in petroleum exploration and exploitation but is difficult especially in cases of complex hydrocarbon migration and accumulation in such cases hydrocarbon remigration and dysmigration take place very commonly leading to the presence of residual or paleo oil accumulations and layers which cannot be easily identified or misinterpreted as oil layers by conventional logging and geophysical data in this paper based on a case study in the luxi area of the central junggar basin nw china we seek to characterize such layers in terms of organic geochemistry we suggest specific indicator parameters of organic geochemistry such as the chloroform bitumen content of reservoir extracts which is usually 1 0 in oil layers we explore the application of grains containing oil inclusions goi the ratio of mineral grains containing oil inclusions to the total number of mineral grains for the identification of oil gas water layers in the junggar basin for the first time this method has been used elsewhere the maximum goi values for the oil layers oil water layers water layers and dry layers are 11 7 11 6 7 and c21 c23 in contrast the typical characteristic of the heavy oil layers is the presence of 25 norhopanes in reservoir bitumen extracts these specific indicators can be applied in the junggar basin and in similar settings elsewhere

geology water geochemistry and geothermal potential of the jemez springs area canon de san diego new mexico abstract studies of the geology geochemistry of thermal waters and of one exploratory geothermal well show that two related hot spring systems discharge in canon de san diego at soda dam 48 c and jemez springs 72 c the hot springs discharge from separate strands of the jemez fault zone which trends northeastward towards the center of valles caldera exploration drilling to precambrian basement beneath jemez springs encountered a hot aquifer 68 c at the top of paleozoic limestone of appropriate temperature and composition to be the local source of the fluids in the surface hot springs at jemez springs comparisons of the soluble elements na li cl and b arguments based on isotopic evidence and chemical geothermometry indicate that the hot spring fluids are derivatives of the deep geothermal fluid within valles caldera no hot aquifer was discovered in or on top of precambrian basement it appears that low to moderate temperature geothermal reservoirs

the abundance and distribution of diamondoids in biodegraded oils from the san joaquin valley implications for biodegradation of diamondoids in petroleum reservoirs abstract the biodegradability of diamondoids was investigated using a collection of crude oil samples from the san joaquin valley california that had been biodegraded to varying extent in the reservoir our results show that diamondoids are subjected to biodegradation which is selective as well as stepwise adamantanes are generally more susceptible to biodegradation than other diamondoids such as diamantanes and triamantanes we report a possible pathway for the microbial degradation of adamantane this cage hydrocarbon possibly breaks down to a metabolic intermediate through the action of microbes at higher levels of biodegradation in petroleum reservoirs microbial alteration has only a minor effect on diamondoid abundance in oil at low levels of biodegradation our results suggest that most diamondoids with the exception of adamantane are resistant to biodegradation like the polycyclic terpanes e g c19 c24 tricyclic terpanes hopanes gammacerane oleananes ts tm c29 ts steranes and diasteranes microbial alteration of diamondoids has a negligible impact on the quantification of oil cracking achieved using the diamondoid biomarker method

new insights into the formation mechanism of high hydrogen sulfide bearing gas condensates case study of lower ordovician dolomite reservoirs in the tazhong uplift tarim basin huge high gas oil ratio hydrogen sulfide h2s bearing gas condensate accumulations were recently discovered in the ordovician carbonate reservoirs of the tazhong uplift in the tarim basin northwest china distinct differences exist between the eastern and western condensates in terms of chemical and isotopic compositions condensates from the western part of the uplift were characterized by high dibenzothiophenes generally 500 g g a high h2s concentration 7 vol vol and relatively depleted 13c methane 13c1 55 5 to 36 the h2s concentration in the tazhong gas condensates shows a positive correlation to mg2 concentration in the formation water formation water in lower ordovician cambrian strata in the tazhong uplift is rich in mg2 which facilitates the thermochemical sulfate reduction tsr of sulfate contact ion pairs cips to produce h2s and dibenzothiophenes a detailed comparison of the chemical compositions of the formation waters in different strata indicates that a high h2s concentration in the tazhong gas condensates originates from the tsr of sulfate cips in the lower ordovician cambrian strata where a primary oil accumulation may have existed the concentrations of 3 and 4 methyldiamantanes in the western condensates 80 to 150 g g are relatively lower than those from the eastern part of the uplift also the 13c1 in the western h2s bearing gas condensates was more negative and the 13c2 13c1 value was larger than that from typical tsr altered gases these features indicate that the western tazhong samples had just entered the initial stage of tsr according to the pressure volume temperature pvt phase diagram the lower paleozoic section was quickly buried after the tortonian high h2s hydrocarbon inclusions formed during the last 10 m y when paleotemperatures reached 140 c 284 f because the reaction rate of the sulfate cips oxidation was relatively slower than that of h2s autocatalysis during the entire tsr process advanced tsr has not been accomplished yet it is also inferred that the tortonian was the key period for accumulation of secondary h2s bearing gas condensates resulting from abundant gas washing along deep fractures and charging in the early reservoirs an increased aromaticity parameter toluene n heptane and an increased fractionation index from east to west indicate an intensified degree of gas washing different gas washing intensities in the eastern and western gas condensates led to diverse pvt states as well deep strata in the tazhong uplift were characterized by multiple charges and mixing coupled with periodic tsr leading to the occurrence of variable h2s bearing gas condensates

implications of halide leaching on 36cl studies at yucca mountain nevada 1 chlorine 36 was generated from nuclear tests in the 1950s and 1960s and has been used to identify fast flow paths at yucca mountain the proposed repository for high level nuclear waste fabryka martin et al 1997 1998 bomb pulse 36cl brought into the subsurface by infiltrating rainwater presumably resides along fracture surfaces because of the extremely low rock matrix permeability however leaching a rock sample to extract this salt inevitably extracts pore water chloride cl and rock matrix chloride thereby making it difficult to obtain reproducible measurements or detect the specific bomb pulse signatures complexities introduced by these sources of older chloride include dilution of bomb pulse 36cl cl ratios for samples from strata with a high cl concentration variations in measured ratios as a function of leaching time rock chip size and the differing effects of active leaching from those of passive leaching this work provides both a conceptual model and a mathematical solution for the leaching processes and examines the role of sample leaching in the 36cl studies of yucca mountain rocks an analytical solution is derived for the diffusion of cl and 36cl in composite media rock matrix and water to accommodate variable diffusivity this solution is subsequently used to develop a leaching model that takes into account bomb pulse signal matrix pore water and relatively hard to leach components isolated fluid inclusion and mineral boundary salts the model is then applied to samples from stratigraphic units at yucca mountain to obtain leachate concentrations from different setup methods protocols including duration chip size and gravitational settling of the water rock mixture the model results show that the probability of detecting a 36cl cl bomb pulse signal is severely diminished under longer leaching times and smaller rock fragment sizes and that leaching times of 1 to 10 hours are most likely to be successful in detecting a bomb pulse signal bomb pulse 36cl cl ratios are more likely to be observed if pore water cl concentrations were initially low prior to the introduction of bomb pulse carrying water

predicting oil quality from sidewall cores using pfid tec and nir analytical techniques in sandstone reservoirs rio del rey basin cameroon abstract the expense and uncertainty of obtaining fluid samples from mdt tools provide a strong incentive for an alternative method for assessing oil character sidewall cores which are inexpensive and easy to obtain contain sufficient oil for microanalyses that can provide indications of oil quality degree of degradation api gravity and numerous additional characteristics including sulfur content acid number and viscosity the present study is concerned with the application of the micro techniques pfid pyrolysis flame ionization detection tec thermal extraction chromatography and nir near infra red spectroscopy for the characterization of oil from sidewall cores taken from two exploration wells in the rio del rey basin of cameroon in 1993

diamondoid hydrocarbon ratios novel maturity indices for highly mature crude oils abstract diamondoid hydrocarbons have been identified in oil and source rock samples from the tarim yinggehai qiongdongnan and other chinese basins two diamondoid hydrocarbon ratios have been used as novel high maturity indices to evaluate the maturation and evolution of crude oils and condensates in these chinese basins the two ratios 1 ma 1 ma 2 ma mai methyl adamantane index and 4 md 1 md 3 md 4 md mdi methyl diamantane index were found to increase with increasing vitrinite reflectance the initial values for the ratios are ca 50 and 30 respectively diamondoid hydrocarbons are products of rearrangement of polycyclic hydrocarbons and their ratios are found to be affected by organic input to a minor extent based on this study crude oils from the tarim basin are generally believed to be very mature ro 1 1 the cambrian ordovician source beds are the major source rocks of crude oils and natural gases in this basin condensates from the tarim yinggehai and qiongdongnan basins have higher maturities equivalent to ro values of 1 6 2 0 gases and condensates from the yinggehai and qiongdongnan basins are generated mainly from tertiary source rocks below 4000 5000 m

improved identification of pay zones through integration of geochemical and log data a case study from upper assam basin india the upper paleocene lower eocene clastic reservoirs constitute one of the most important hydrocarbon producing horizons in the upper assam basin of northeast india these reservoirs are characterized by the presence of 1 normal gravity oil 23 31 api 2 low to intermediate gravity oil 13 23 api and 3 gas and light oil condensate 31 api the sandstone reservoirs show complex wire line log signatures which are commonly misleading the main impediments to understanding the nature of reservoir fluid include 1 distinguishing low gravity intermediate gravity and normal gravity oil bearing zones and 2 resolving the problem of density rhob neutron porosity nphi crossover for gas bearing as well as oil bearing reservoirs geochemical analyses of sidewall core extracts by thin layer chromatography with flame ionization detection tlc fid and gas chromatography gc provide valuable cost effective input for reservoir fluid characterization the important parameters needed to identify the nature and composition of the reservoir fluid include 1 the bulk composition of the sidewall core extract i e saturated hydrocarbons aromatic hydrocarbons and resins asphaltenes 2 amount of extract in mg g of rock 3 gc fingerprint and 4 the ratio of pristane to n c17 these parameters integrated with geological and geophysical wire line logs evidence yield more accurate and reliable formation evaluation criteria the technique is simple and inexpensive and may find application as an additional formation evaluation tool in any geological setting

molecular and isotopic compositions of bitumens in silurian tar sands from the tarim basin nw china characterizing biodegradation and hydrocarbon charging in an old composite basin abstract biodegradation and oil mixing in silurian sandstone reservoirs of the tarim basin one of the largest composite basins in china were investigated by analyzing the molecular characteristics and stable carbon isotopic signatures of low molecular weight lmw saturated hydrocarbons and high molecular weight hmw asphaltenes detection of 25 norhopanes and 17 nortricyclic terpanes in most silurian tar sands from the tabei uplift in the tarim basin suggests a much greater degree of biodegradation here than in the tazhong uplift this explains the relatively more abundant tricyclic terpanes gammacerane pregnane and diasteranes in tar sands from the tabei uplift than in those from the tazhong uplift hence care must be taken when assigning oil source correlations using biomarkers in tar sands because of the biodegradation and mixing of oils derived from multiple sources in such an old composite basin asphaltenes in the tar sands seem to be part of the oil charge before biodegradation depending on the relative anti biodegradation characteristics of asphaltenes the similarity in carbon isotopic signatures for asphaltenes and their pyrolysates and the consistent product distribution for flash pyrolysis and for regular steranes in asphaltene pyrolysates regardless of whether the tar sands were charged with fresh oil according to the relative distributions of regular steranes and the relatively abundant 1 2 3 4 tetramethylbenzene significantly enriched in 13 c the oil sources for asphaltenes in the tar sands might be related to lower paleozoic marine source rocks formed in euxinic conditions nevertheless the relatively low abundance of gammacerane and c 28 regular steranes observed in asphaltene pyrolysates and residual hydrocarbons within limited samples investigated in this work made a direct correlation of oils originally charged into silurian tar sands with those cambrian source rocks reported so far seem not to be possible comparison of carbon isotopic signatures of n alkanes in asphaltene pyrolysates with those of lmw saturated hydrocarbons is helpful in determining if the abundant n alkanes in tar sands are derived from fresh oil charges after biodegradation the limited carbon isotopic data for n alkanes in lmw saturated hydrocarbons from the tar sands can be used to classify oils charged after biodegradation in the composite basin into four distinct groups

petroleum source bed evaluation of tertiary niger delta reply a total of 63 sidewall and drill cutting shale samples obtained from a depth slice of 800 to 3650 m in two onshore and two offshore wells located respectively in the eastern and western niger delta basin nigeria were analyzed for organic matter type concentration and thermal maturity dominant sedimentary kerogens were the humic and mixed types organic carbon contents of the sediments ranged from 0 4 to 4 4 and the degree of thermal evolution as evidenced by vitrinite reflectance r sub 0 odd even ratios oer 2c sub 29 c sub 28 c sub 30 and variations of the ratios of the concentration of soluble organic matter to total organic carbon content som toc were within the ranges 0 2 to 0 7 4 0 to 1 0 and 10 to 240 respectively on the basis of the variation of the maturation parameters the onset of mature source beds in the onshore and offshore delta were delimited at approximately 3375 and 2900 m respectively the inferred threshold temperature was about 95 sup 0 c the results of the present study support the hypothesis that the main source beds of the niger delta basin are the deeply buried paralic or paralic marine facies

paleozoic oil source rock correlations in the tarim basin nw china we studied a suite of 40 oils and extracts of purported source rocks from the tarim basin in nw china the main group of oils comes from tazhong and tabei wells which sample the largest known petroleum accumulations in the basin these oils can be statistically correlated with extracts of ordovician rocks based upon high relative concentrations of 24 isopropylcholestanes and low relative concentrations of dinosteranes triaromatic dinosteroids and 24 norcholestanes in contrast extracts from cambrian rocks have low relative concentrations of 24 isopropylcholestanes with high relative concentrations of dinosteranes triaromatic dinosteroids and 24 norcholestanes although some tarim basin cambrian rocks yield high total organic carbon contents we see little evidence in the analyzed oil samples to suggest that they came from cambrian source rocks

correlation of crude oils and oil components from reservoirs and source rocks using carbon isotopic compositions of individual n alkanes in the tazhong and tabei uplift of the tarim basin china abstract carbon isotopic compositions were determined by gc irms for individual n alkanes in crude oils and the free adsorbed and inclusion oils recovered by sequential extraction from reservoir rocks in the tazhong uplift and tahe oilfield in the tabei uplift of tarim basin as well as extracts of the cambrian ordovician source rocks in the basin the variations of the 13 c values of individual n alkanes among the 15 oils from the tazhong uplift and among the 15 oils from the triassic and carboniferous sandstone reservoirs and the 21 oils from the ordovician carbonate reservoirs in the tahe oilfield demonstrate that these marine oils are derived from two end member source rocks the major proportion of these marine oils is derived from the type a source rocks with low 13 c values while a minor proportion is derived from the type b source rocks with high 13 c values type a source rocks are within either the cambrian lower ordovician or the middle upper ordovician strata not drilled so far while type b source rocks are within the cambrian lower ordovician strata as found in boreholes td2 and fang 1 in addition the three oils from the cretaceous sandstone reservoirs in the tahe oilfield with exceptionally high pr ph ratio and 13 c values of individual n alkanes are derived or mainly derived from the triassic jurassic terrigenous source rocks located in quka depression the difference of the 13 c values of individual n alkanes among the free adsorbed and inclusion oils in the reservoir rocks and corresponding crude oils reflects source variation during the reservoir filling process in general the initial oil charge is derived from the type b source rocks with high 13 c values while the later oil charge is derived from the type a source rocks with low 13 c values the 13 c values of individual n alkanes do not simply correlate with the biomarker parameters for the marine oils in the tazhong uplift and tahe oilfield suggesting that molecular parameters alone are not adequate for reliable oil source correlation for high maturity oils with complex mixing

applications of petroleum geochemistry to exploration and reservoir management abstract petroleum geochemistry improves exploration efficiency by accounting for many of the variables that control the volumes of crude oil and natural gas available for entrapment including source rock distribution richness and quality thermal maturity and the timing of generation migration accumulation relative to trap formation it is most powerful when used with other disciplines such as seismic sequence stratigraphy and reservoir characterization four key technology milestones form the basis for most modern applications of geochemistry to exploration these are the concepts and applications of 1 petroleum systems and exploration risk 2 biomarkers stable isotopes and multivariate statistics for genetic oil oil and oil source rock correlation 3 calibrated three dimensional thermal and fluid flow modeling and 4 controls on petroleum composition by secondary processes petroleum geochemistry offers rapid low cost evaluation tools to aid in understanding development and production problems some technology milestones in reservoir geochemistry include 1 assessment of vertical and lateral fluid continuity 2 determination of proportions of commingled production from multiple zones and leaky casing 3 prediction of oil quality in reservoir zones and 4 prediction of gas oil and oil water contact locations as described in the conclusions future research will continue a trend toward predictive geochemistry examples of predictive tools that draw major research support include piston core surveys to assess deepwater petroleum systems prior to drilling and three dimensional basin modeling to predict the regional timing of generation migration and accumulation of petroleum among other research objectives models are needed to better predict the distribution and quality of petroleum in reservoirs

sedimentology and characteristics of dispersed organic matter in tertiary niger delta origin of source rocks in a deltaic environment organic matter in tertiary strata of the niger delta is mainly a mixture of types ii and iii kerogen as defined by pyrolysis has a high pristane phytane ratio 1 and is composed of the macerals vitrinite and minor liptinite the main palynofacies are structured woody material opaque organic matter cuticles pollen and spores and minor amorphous organic matter the geochemistry petrology palynofacies and distribution data indicate a predominantly terrestrial and detrital source of the organic matter irrespective of facies formation or age the distribution and abundance of the organic matter are related to both the age of the strata and depositional environment the most significant variation is the decline in mean total organic carbon toc content from late eocene 2 2 toc to pliocene strata 90 toc and an associated general decline in pyrolysis defined hydrogen index hi and pristane phytane ratio the decrease in toc and hi in younger strata reflects increased dilution of a nearly constant supply of terrestrial organic matter with the generally higher sedimentation rates of younger strata a simple linear dilution model accounts for about 50 of the variance in measured toc the low pristane phytane ratio of younger strata may reflect less oxidizing depositional conditions the variation in organic matter with depositional environments and environmental depth zones defined by foraminiferal assemblages is significant although less important than variation due to age the highest toc and hi values occur in nonmarine swamp marsh and floodplain deposits reflecting proximity to the organic source beach barrier and offshore bar facies have lower toc and hi values due to selective winnowing of organic matter and greater oxidation from continental to transition marine depth zones a marked decline in toc and hi occurs as a result of dilution selective transport and oxidation of the organic matter from transitional to bathyal depth zones little variation is found in toc but a general increase is found in hi which reflects a relative increase in more hydrogen rich autochthonous organic matter in all facies autochthonous organic matter comprises less than 20 of the toc no interpretable variation appears in maceral or palynofacies abundance with sediment type which probably reflects the homogeneity of the organic matter paradoxically no rich source rocks occur in the tertiary succession in the niger delta and as conventionally measured the strata have little or no oil generating potential the poor quality of the source rocks has been more than compensated for by their great volume by the excellent migration routes provided by interbedded permeable sands and possibly by the relatively rapid rate of maturation niger delta type of source rock although an end member in terms of general source rock composition appears to be relatively typical of tertiary deltas

groundwater dilution and residence times and constraints on chloride source in the mokai geothermal system new zealand from chemical stable isotope tritium and 36cl data the mokai geothermal system new zealand is located within a volcanic collapse structure comprised of flat lying silicic rocks the deep chloride fluid rises in the south inside the caldera margin near the surface the fluid flows laterally more than 12 km to the north because of the hydraulic gradient caused by 300 m of relief the maximum measured temperature of the production wells is near 323 c at 2500 m depth whereas temperatures in two wells on the northern margin of the upflow are 200 c with thermal inversions at 350 and 1000 m indicating lateral flow these two marginal wells have about 50 of the chloride of the deep fluid indicating dilution by cold ground water chloride bearing springs discharge 6 km north of the zone of upflow and are even further diluted by cold groundwater than the wells on the margin of the system the tritium data in this hydrologically rather simple system indicate that there is a mean residence time of greater than 100 years for the groundwater diluent also older water is mixed with 25 of young thermonuclear post 1955 water before the mixture dilutes outflowing chloride fluid from the deep system the deep chloride fluid has a 36cl cl ratio of 2 6 10 15 and a 36cl concentration of 106 atoms per 10 6 kg this fluid has a lower ratio by a factor of 90 and a higher absolute concentration by a factor of 7 than present day stream water however the marginal wells have been diluted by groundwater with at least 500 36cl atoms per 10 6 kg 30 times richer in 36cl than present day stream waters this spike is most likely due to circulation of precipitation formed during thermonuclear testing to depths of 1000 m though the component with the 36cl spike only makes a moderate 25 contribution to the groundwater diluent most of the diluent is much older pre 1955 the amount of 36cl in the deep fluid at mokai is 5 10 times less than that predicted from a calculation of the neutron fluxes in the silicic reservoir and underlying greywacke basement rocks this indicates that the chloride was not derived from host rock leaching in the upper 3 km rather the fluids have interacted with more primitive rocks below the drilled depths of the taupo volcanic zone with the chloride possibly derived from a magmatic brine

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

geochemistry of palaeozoic marine petroleum from the tarim basin nw china part 2 maturity assessment abstract the thermal maturity of forty three crude oil samples collected from the cratonic portion of the tarim basin nw china has been assessed using a variety of parameters while compound ratios of diamondoids alkylated naphthalenes methyl phenanthrenes and methyl dibenzothiophenes showed consistent trends with increasing maturity a large discrepancy was observed for the maturity levels of the oils determined from these parameters equivalent vitrinite reflectance values calculated from methylphenanthrene index 1 mpi1 methyldibenzothiophene ratio mdr and methyldiamontane index mdi were in the range of 0 66 0 97 1 11 1 64 and 0 89 1 74 ro respectively while we caution that some of the discrepancy may be related to calibration problems the difference observed from different compound classes could be real reflecting the multiple charge to petroleum accumulations as the diamondoid hydrocarbons were possibly generated at later maturation stages than most aromatic hydrocarbons as diamondoids with methyl substitution on the bridgehead positions are more stable than those with substitution on other positions several compound ratios based on ethyl admantanes dimethyl admantanes trimethyl admantanes and dimethyl diamantanes also correlated with the established maturity indicators such as mdr and mdi while condensates with high maturity levels represent the late charge hydrocarbons some of them with relatively low maturity levels may be affected by evaporative fractionation processes the normal gravity oils cover a wide maturity range consistent with multiple charges and in reservoir mixing while the waxy oils with high apparent maturity levels may be the products of wax precipitation due to a later gas injection into an oil column heavy oils with low apparent maturity levels could be the residue of oil that has lost its light end

geochemistry of palaeozoic marine petroleum from the tarim basin nw china part 3 thermal cracking of liquid hydrocarbons and gas washing as the major mechanisms for deep gas condensate accumulations abstract the lunnan lower uplift in the tarim basin nw china contains a composite petroleum system with mainly biodegraded heavy oil in the west normal oil in the center and gas condensate in the east twenty three gas samples and 37 oil samples taken from three major hydrocarbon bearing intervals in the lunnan lower bulge were analyzed for their stable carbon isotopes and molecular biomarkers on the basis of their isotopic compositions biomarkers especially diamondoids and integrating the physical properties of the hydrocarbons including densities gor and pvt relationships it has been concluded that the recently discovered deep 6500 m eastern lungu giant ordovician gas condensate pool with an estimated reserve of 723 million bbl oil equivalent is a secondary hydrocarbon accumulation derived from the mixing of an early formed oil and a late formed gas the extremely dry gases with a gas dryness of 0 98 were derived from thermal cracking of crude oils and charged to an existing oil reservoir causing extensive gas washing and secondary alteration compared with most of the hydrocarbons in the lunnan area the gas from the eastern lungu gas condensate pool has a heavier carbon isotopic composition higher dryness and a higher maturity level the unique physical chemical and isotopic compositions of the gas condensate are believed to be a direct result of gas flushing of an early formed eastern lungu oil pool by a late formed dry gas from oil cracking the eastern lungu gas condensate pool is presently characterized by containing dry gas heavy oil abundant aromatics and high wax

the fate of diamondoids in coals and sedimentary rocks diamondoids were detected in the extracts of a series of coals and rocks varying in maturity lithology source input and depositional environment at the same maturity level diamondoids are generally about a magnitude more abundant in source rocks than in coals the concentrations of diamondoids are maturity dependent however while diamondoids become more abundant with the increasing thermal maturity a diminution in diamondoid concentrations is observed at the maturity value of about r sub o 4 0 in both coals and rocks the occurrence of diamantane destruction at 550 sup o c during pyrolysis suggests that diamondoids may be eventually destroyed at high temperatures in the earth here we propose three main phases of diamondoid life in nature diamondoid generation phase i r sub o 4 0

reservoir alteration of crude oils in the junggar basin northwest china insights from diamondoid indices abstract reservoir alteration of crude oils is ubiquitous in petroleum reservoirs alteration can change the composition and physicochemical properties of crude oils and complicate oil source correlations and origin studies of crude oils thus the identification and assessment of reservoir alteration could provide a better understanding of the formation and evolution of crude oils in this study the effects of reservoir alteration e g biodegradation gas washing water washing and mixing on oils in the junggar basin northwest china were identified and assessed by combining diamondoid indices with other commonly studied crude oil properties the results indicate that 1 diamondoid indices can be used to evaluate the intensity of reservoir alteration on oils e g slight biodegradation and water washing has no effect on diamondoids moderate biodegradation and water washing can result in a marked increase in diamondoid concentrations severe biodegradation and water washing significantly changes diamondoid concentration and isomerization ratios 2 diamondoid indices can assist in determining the type of reservoir alteration of oils e g condensate formed by gas washing has a lower diamondoid concentration than that generated by thermal cracking differing from biodegradation and water washing gas washing can affect diamondoid ratios mixing can be identified by a combination of diamondoid concentrations and other parameters such as api gravity and biomarker indices in oils therefore diamondoids provide specific information as to the reservoir alteration of oils which is useful in further understanding of the origin and evolution of crude oils in the junggar basin

organic matter of agbada formation niger delta nigeria the status of the agbada formation as a major source rock for oil and gas in the niger delta is debatable an organic matter survey of the formation in three exploratory wells makaraba 26 abiteye 13 and okan 25 shows that on the average amorphous organic matter constitutes 63 of the kerogens whereas terrestrial organic matter woody remains spores and pollen and recycled coaly organic matter make up 30 and 7 respectively pyrolysis data indicate essentially type iii kerogen for the agbada shales thermal alteration index tai and pyrolysis data show that the kerogens are mature in the three wells we estimate that about 2 000 4 000 ft 600 1 200 m of mature agbada shales are present in the western part of the niger delta results of organic carbon determinations show that the agbada shales contain moderate to high amounts of organic matter which could have sourced some of the oils found in the makaraba abiteye and okan fields although this view is valid for the western part of the niger delta the situation in the other parts of the delta might be different

source and thermal maturity of crude oils in the junggar basin in northwest china determined from the concentration and distribution of diamondoids abstract here we discuss three types of diamondoid parameters for oils from the junggar basin these are absolute concentrations concentration ratios and isomerization ratios according to the absolute diamondoid concentrations the oils collected from different areas of the basin were broadly divided into three maturity stages 1 low mature 1000 ppm adamantanes 50 ppm diamantanes the oils in the northwestern region of the junggar basin are in the low mature to mature stages based on a combination of diamondoid concentration ratios and biomarker indices these oils can be divided into three groups i e group i oils in the wuxia zone derived from the lower permian fengcheng formation p1f group ii oils in the kebai zone sourced from middle permian lower wuerhe formation p2w and group iii oils in the mahu depression generated from more mature source rocks of jiamuhe formation p1j or the p1f diamondoid concentrations and isomerization ratios were used to precisely evaluate the thermal maturity of mature oils and highly mature condensates respectively our results indicate that the oils in the central part of the basin have decreasing thermal maturity from south to north whereas the oils in the kelameili area display increasing thermal maturity from east to west in this study we found that different diamondoid indices are useful only in certain thermal maturity ranges therefore at least for the junggar basin it is crucial to know which thermal region one is in before using diamondoid ratios for maturity assessment

isotopic compositions and biomarkers in crude oils from the tarim basin oil maturity and oil mixing abstract hydrogen and carbon isotopic compositions of n alkanes as well as biomarkers in various types of oil from the tarim basin were analyzed data were used to discuss the occurrence and formation of marine oils from this old composite basin of which we have limited understanding first hydrogen and carbon compositions of n alkanes clearly classify the collected oils into two distinct groups supporting the reported oil classifications based on biomarkers group i the majority of oil samples is characterized by relative enrichment in d and n alkane d values vary from 110 to 75 group ii includes two heavy oils and d values of n alkanes range from 145 to 142 second variations in the 13c value of n alkanes 2 3 show good correlations with the maturity indices indicating that kinetic isotopic fractionation is one control on the 13c value of n alkanes in the maturation process d values of n alkanes in most oils are also evidently influenced by the oil maturity finally by combining the isotopic compositions with biomarkers of oils two distinct oil mixing processes were recognized in the oils from the tabei and tazhong uplifts in this old composite basin the former process is the mixing of oils from the same source at different maturity stages while the latter involves the mixing of oils from distinct sources at different maturity stages biodegraded oils may contribute to the total oils but their proportions are minor

molecular organic geochemistry of the tarim basin northwest china we conducted organic geochemical analyses on the largest suite of oils and source rock extracts from the tarim basin northwest china currently available statistical cluster analysis of the entire suite of tarim oils distinguishes at least seven genetic groups of oils the largest group of oils was collected from the tazhong and tabei uplifts and originated from marine middle upper ordovician anoxic marls that mark slope facies at the margins of structural uplifts two other genetic groups most likely originated from marine middle upper ordovician source rocks but of distinct facies with one an oxic shale rich source west of the bachu uplift and the other an anoxic shale source at tazhong

methyl diamantane index mdi as a maturity parameter for lower palaeozoic carbonate rocks at high maturity and overmaturity diamantanes were identified in extracts from the lower ordovician majiagou formations o1m5 of the central gas field shanganning basin china at 3100 3800 m corresponding to ro 1 9 3 9 the mdi methyl diamantane index 4 md 1 md 3 md 4 md ranges from 40 to 65 for the source rock extracts changes in the mdi index in the very mature sections ro 2 0 are relatively small and no linear correlation between mdi and ro o r mdi and depth is noted as previously reported suggesting the possibility that the mdi may have limited applicability in maturity determinations 2000 elsevier science ltd all rights reserved

the application of diamondoid indices in the tarim oils this study presents new data for the identification of the source and assessment of the thermal maturity of oils based on the diamondoid indices of oils from the tazhong and luntai uplifts in the tarim basin in northwest china the oil samples were divided into three groups according to biomarker characteristics and the abundance and distribution of diamondoids group i oils are located along the tazhong no 1 fault zone which contained abundant diamondoids and are of high thermal maturity suggesting they are late charged hydrocarbons that were derived from a middle upper ordovician source group ii oils are mainly located in blocks close to the tazhong no 1 fault zone and are dominated by early formed hydrocarbons from cambrian lower ordovician source rocks group iii lacustrine oils comprise relatively low concentrations of diamondoids compared with group i and group ii oils group iii oils were sourced from jurassic or possibly triassic units and are hosted by the low relief yingmaili section of the luntai uplift the thermal maturity of the oils in each group was evaluated using diamondoid parameters a few group i oils exhibit intense thermal cracking to estimate the extent of oil cracking using the concentrations of diamondoids in oils this study proposes a practical approach that facilitates the determination of baseline 4 and 3 methyldiamantane concentrations application of this method indicates that the tarim basin marine oils contain a baseline concentration of approximately 69 ppm

molecular correlation of free oil adsorbed oil and inclusion oil of reservoir rocks in the tazhong uplift of the tarim basin china the free adsorbed and inclusion oils were recovered by sequential extraction from eleven oil and tar containing reservoir rocks in the tazhong uplift of tarim basin the results of gas chromatography gc and gc mass spectrometry analyses of these oil components and seven crude oils collected from this region reveal multiple oil charges derived from different source rocks for these oil reservoirs the initially charged oils show strong predominance of even over odd n alkanes in the range n c12 to n c20 and have ordinary maturities while the later charged oils do not exhibit any predominance of n alkanes and have high maturities the adsorbed and inclusion oils of the reservoir rocks generally have high relative concentrations of gammacerane and c28 steranes similar to the cambrian lower ordovician source rocks in contrast the free oils of these reservoir rocks generally have low relative concentrations of gammacerane and c28 steranes similar to the middle upper ordovician source rocks there are two interpretations of this result 1 the initially charged oils are derived from the cambrian lower ordovician source rocks while the later charged oils are derived from the middle upper ordovician source rocks and 2 both the initially and later charged oils are mainly derived from the cambrian lower ordovician source rocks but the later charged oils are contaminated by the oil components from the silurian tar sandstones and the middle upper ordovician source rocks

source facies of the paleozoic petroleum systems in the tabei uplift tarim basin nw china implications from aryl isoprenoids in crude oils abstract aryl isoprenoids have been detected for the first time in crude oils from paleozoic petroleum systems in the tabei uplift tarim basin northwestern china the principal compounds possess the 1 alkyl 2 3 6 trimethyl substitution pattern characteristic of diaromatic carotenoids found in the chlorobiaceae family of photosynthetic sulfur bacteria with a predominance of c13 c23 homologues in these samples flash pyrolysates of the asphaltene fractions isolated from crude oils show an unusual abundance of 1 2 3 4 tetramethylbenzene an indicator for a significant contribution of diaromatic carotenoids to the source kerogen of these oils the wide distribution of these compounds in crude oils reveals that penetration of the photic zone by sulfidic waters during deposition enabled prolific growth of photosynthetic green sulphur bacteria chlorobiaceae this suggests that the source rocks for the paleozoic petroleum systems in the tabei uplift were likely deposited under euxinic conditions with sulfate and sulfide rich water bodies which doesn t support previously published conclusions of a middle upper ordovician source that marks the slope facies at the margins of structural uplifts

the tazhong hybrid petroleum system tarim basin china abstract the tazhong petroleum system is geographically located in the central tarim basin including the tazhong uplift the north slope and the southern area of the manjiaer depression the total thickness of early palaeozoic to cenozoic sediments reaches 6500 9500 m the manjiaer depression and the north slope are the two major petroleum source areas with three sets of source rocks cambrian black shales and ordovician dark carbonates of the manjiaer depression and ordovician argillaceous limestones of the north slope there are three sets of reservoir cap rock combinations in this petroleum system the main periods for oil generation from these source rocks occurred during the middle silurian to early devonian late carboniferous to early triassic and tertiary there were three corresponding phases of petroleum migration well developed thrust faults and unconformity surfaces in this area provided the main pathways for petroleum migration as some of the faults cut through all the lower palaeozoic strata the oil in any given reservoir rock may be actually a mixture of oils from different sources complicating the origin and properties of the hydrocarbons in the reservoir and forming a hybrid petroleum system the formation of a typical oil and gas pool in this petroleum system may be attributed to one or all of three petroleum events which were closely associated with the regional tectonics the first event occurred in the early palaeozoic tectonic cycle and the generated oil accumulated in lower silurian sandstone the second event occurred in the late palaeozoic tectonic cycle and formed light oil pools mainly in lower silurian and ordovician reservoirs the third event took place during the cenozoic tectonic cycle and formed normal crude oils the oil and gas generated from different sources in different phases may co exist with one another in a reservoir rock however the commercial oil and gas pools are related to the cenozoic and late palaeozoic events

129i and 36cl concentrations in waters of the eastern clear lake area california residence times and source ages of hydrothermal fluids abstract the clear lake area of northern california is the location of hot spring activity some of which is associated with the formation of au and hg deposits we measured 129 i i and 36 cl cl ratios in nine warm springs formation springs six cold springs with elevated levels of cl and i mineral springs and five springs of recent meteoric origin dating of the i in the formation waters indicated that sources of i in these waters are formations with minimum ages between 60 and 80 ma this age range is in good agreement with the ages of the franciscan complex and the great valley sequence the dominant formations in this area because the mineral waters are essentially formation waters diluted with meteoric water i in these waters is of the same origin residence times of the waters were calculated based on the build up of129i and 36cl as a consequence of the presence of u and th in the crust the residence time of the formation waters in the great valley sequence the location for most of these springs was probably not longer than 84 000 yr the concentrations found for 36cl and 129i in the mineral waters indicate that these waters have residence times of similar magnitude in formations such as the franciscan complex or the clear lake volcanics which have slightly higher levels of u and th than the great valley sequence