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Hydrogen sulphide is likely to become more common in produced hydrocarbon fluids, as the exploitation of deep reservoirs increases and unconventional resources get recovered significantly, such as heavy oils or bitumen. Hydrogen sulphide presence in produced oil and gas results in operational, environmental and treatment problems. Therefore, understanding the origin and the amount of hydrogen sulphide in petroleum reservoirs has great importance for petroleum engineers. Three natural processes are set forth to explain the generation of H 2 S in reservoirs: bacterial sulphate reduction, thermal cracking and thermochemical sulphate reduction (TSR). It is the TSR that leads to the largest amount of H 2 S. This phenomenon involves hydrocarbon oxidation and sulphate reduction and produces as by-products, hydrogen sulphide, carbon dioxide, carbonate minerals and heavy organo-sulphur compounds. The reaction mechanisms of TSR, as well as its kinetics, are not yet fully understood. In this paper, we checked the thermodynamic feasibility of TSR, at temperatures prevailing in the reservoirs where TSR is encountered. Firstly, we calculated the Gibbs energy of the reactions proposed by Worden and Smalley (Worden R.H. and Smalley P.C., 1996, H 2 S producing reactions in deep carbonate gas reservoirs: Khuff Formation, Abu Dhabi, Chem. Geol., 133, p. 157–171). We concluded that they are thermodynamically possible from 25 °C, confirming thermodynamic data published by Anisimov (Anisimov L., 1978, Conditions of abiogenic reduction of sulfates in oil and gas bearing basins, Geochem. Int., 15, p. 63) and Yue and co-workers (Yue C., Li S., Ding K., Zhong N., 2003, Study of thermodynamics and kinetics of CH4–CaSO 4 and H 2 S–Fe 2 O 3 systems, Chinese J. chem. Eng., 11, (6), p.696–700., Yue C., Li S., Ding K., Zhong N., 2006, Thermodynamics and kinetics of reaction between C1–C3 hydrocarbons and calcium sulfate in deep carbonate reservoirs, Geochem. J., 40, 87–94). Secondly, we used a non-stoichiometric approach without any pre-requisite chemical scheme this time. We calculated the Gibbs Energy of chemical systems composed by hydrocarbons, sulphur, anhydrite and water. The minimization of the Gibbs Energy lead to find the most probable chemical systems at steady state. Our theoretical results are consistent with the chemical schemes set forth for TSR by Orr (Orr W., 1977, Changes in Sulfur Content and Isotopic Ratios of Sulfur during Petroleum Maturation — study of Big Horn Basin Paleozoic Oils, in R. Campo and J. Goni Eds, Advances in onorganic geochemistry, Madrid Spain, Enadimsa, p. 571–595), by Worden and Smalley (Worden R.H. and Smalley P.C., 1996, H 2 S producing reactions in deep carbonate gas reservoirs: Khuff Formation, Abu Dhabi, Chem. Geol., 133, p. 157–171) and by Machel (Machel H.G., 2001, Bacterial and thermochemical sulfate reduction in diagenetic settings — old and new insights, Sedimentary Geology, 140, p. 143–175). Moreover, they are in concordance with some in-situ observations: anhydrite and hydrocarbon consumption with simultaneous formation of calcite, hydrogen sulphide and water. Our results showed as well that the larger the number of the carbon atoms in the reactant hydrocarbons, the more irreversible the reaction is.

n-Alkanes, highly branched isoprenoids, monomethylalkanes (MMAs), dimethylalkanes (DMAs), and trimethylalkanes (TMAs) are the most abundant components in the hydrocarbon fractions of extracts of four modern and two Holocene cyanobacterial mats (t1500 and 5110 ± 170 y bp) collected in Abu Dhabi (United Arab Emirates). The homologous families of MMAs, DMAs, and TMAs were identified by comparison of mass spectral and relative retention time data with published data. DMAs were also identified by synthesis of authentic standards, 3,9-dimethyltricosane, 5,9-dimethyltricosane, and 11,15-dimethylheptacosane. MMAs, DMAs, and TMAs of the cyanobacterial mats can be separated into two groups on the basis of their distribution patterns and structures. MMAs and DMAs in the C16-C22 range are characterized by methyl substituents mainly located at C-6 (or ω6) and C-7 (or ω7) and are derived from cyanobacteria. They are relatively abundant components in the modern cyanobacterial mats, but with increasing age of the mats they become much less abundant. On the contrary MMAs, DMAs, and TMAs in the C24-C45 range are exclusively found in the Holocene cyanobacterial mats. Their longest chains mainly contain an odd number of carbon atoms and they always carry the methyl substituents at odd numbered carbon atoms. The similarity in composition of this very specific group of branched alkanes with that encountered in insect epicuticular waxes suggests that these sedimentary hydrocarbons originate from insects, which probably grazed on the cyanobacterial mats.

The occurrence of tar mats, their lateral extent and volume in place is important for reservoir assessment since it may provide information to improve reservoir management, help to assess the producible reserves and my help to increase the oil recovery. We discuss here currently available tools to locate tar mats in petroleum reservoirs. Dominantly core based detection methods are discussed and several log based methods will be briefly described. A newly developed core based method and a new log method are introduced: The low cost core density approach is based on routinely determined core petrophysical data (porosity, grain density). Porosity and rock density are usually measured on plugs which have been rinsed using organic solvents. In tar mat intervals the petroleum present in the core plug is frequently incompletely extracted and the petrophysical data result in artificially lower porosities and grain densities. The core density approach uses the difference between the real grain density and the calculated grain density to detect the presence of heavy oil or tar mat. The dipmeter approach makes use of the high vertical resolution resistivity measurements to detect thin tar mats which are characterized by a very high resistivity. Both new tar mat detection methods, i.e., the core based core density approach and the log based dipmeter approach allow tar mats to be readily identified and may serve as exploratory screening tools for more detailed organic geochemical analysis.

Contents of 13C in kerogens and carbonates in 21 samples from a core of the MAX borehole, Mulhouse Evaporite Basin, range from -27.3 to -23.5 and -3.7 to -1.8% vs PDB, respectively. Organic nitrogen in the same samples is enriched in 15N relative to atmospheric N2 by 12.2-15.7%. Hydrogen indices and delta values for kerogens vary systematically with facies, averaging 493 mg HC/g Corg and -25.7% in the most saline facies (dominated by inputs from aquatic sources) and 267 mg HC/g Corg and -23.7% in the least saline facies (50/50 aquatic/terrigenous). Values of delta were measured for individual aliphatic hydrocarbons from three samples representing three different organic facies. For all samples, terrigenous inputs were unusually rich in 13C, the estimated delta value for bulk terrigenous debris, apparently derived partly from CAM plants, being -22.5%. In the most saline facies, isotopic evidence indicates the mixing of 13C-depleted products of photosynthetic bacteria with 13C-enriched products of halotolerant eukaryotic algae. At lower salinities, a change in the producer community is marked by a decrease in the 13C content of algal lipids. The content of 13C in algal lipids increases in the least saline facies, due either to succession of different organisms or to decreased concentrations of dissolved CO2.

The chemistry of the waters from which the evaporitic minerals crystallized in the lower part of the Salt IV unit of the Mulhouse Basin (France) has been investigated. The lithological and fluid inclusion data have been compared to the output data of the mass transfer program NEWEQUI which simulates the change in brine composition during its evaporation advancement. The results show that the evolution of the composition of the brine is compatible with the calculated sequence of evaporation of a parent water corresponding to a seawater modified by CaCl2-rich hydrothermal water. In the studied sequence fluid inclusion data provide useful constraints for reconstructing the paleoenvironment controlling the evaporitic sedimentation. They also provide data on the water stratification in the basin and on its connectivity with the open sea, and important parameters controlling the productivity (possible sources for the nutrients) and the preservation of organic matter in the sediment.

In the Upper Eocene to Lower Oligocene Mulhouse Basin (France, Upper Rhine graben) an organic-rich evaporitic series has been deposited. First and second order cyclicities can be defined at the scale of the study: a short period cyclicity corresponding to an alternation of marl and evaporitic layers; a long period cyclicity corresponding to an evolution of the evaporitic layers from an anhydritic to halitic (or sylvitic) composition. The characteristics and abundance of organic matter in the marl also show cyclic variations in amount and origin. A geological scenario, based on numerical modeling, served for testing relationships between the environmental evolution (including state of stratification of the basin, sea level change and water balance variations) and the organic sedimentation. Phytoplankton productivity as well as terrestrial organic matter input have been simulated and compared to the organic data. The model results account for the distribution of organic matter in the basin and are in agreement with the geological and geochemical data. They serve to place the paleo-environment elaborated by Hofmann et al. (1993) within a more dynamic sedimentological framework. This work demonstrates the benefit of numerical modeling in tentatively identifying the respective influence of eustasy and climate on the organic sedimentation in an evaporitic basin.

The lower Oligocene evaporite sequence of the Mulhouse Basin (France) contains organic matter-rich marl deposits. These marls display an overall cyclic variation of sedimentation rate, organic carbon content, hydrogen index and selected molecular parameters over a 30 m thick stratigraphic interval. The integration of all sedimentological and geochemical parameters has allowed the reconstruction and characterization of the paleoenvironment of deposition. The marls were deposited in a perennial lake that was at times connected to the sea. Two organic facies end members could be assigned to a lake stage with a marine connection and a lake stage that received dominantly continental water input. The overall stratigraphic variation in the organic matter content is interpreted to reflect the adaptation of the Oligocene flora to the changing paleoclimate and environmental conditions.

A novel hexane-soluble nonpolar macromolecular fraction (NPMF) has been found to occur in substantial amounts (up to 32%) in sulfur-rich crude oils and a rock extract. It is highly aliphatic and has a molecular weight culminating at several thousand mass units, as proven by spectroscopic and molecular weight studies. C-S bond hydrogenolysis of NPMF with Raney nickel as a catalyst yields high proportions of aliphatic hydrocarbons in which long linear, acyclic polyisoprenoid and carotenoid chains usually predominate (except in one case) over polycyclic structures, such as steroids and hopanoids. Hence, NPMF consists mainly of macromolecules composed of low molecular weight hydrocarbon subunits cross-linked with sulfide bridges. Use of deuterated Raney nickel indicated in one case (Rozel Point oil) that the long chains and some hopanoids are multiattached to the macromolecular network, whereas other structural subunits, such as steroids or gammacerane, are essentially monoattached. Detailed structural determinations of the hydrocarbon “building blocks” of NPMF give information on their origin and the mode of formation of these macromolecules in the subsurface. Indeed, most of the building blocks can be related to algal (e.g., long linear chains, steroids, β-carotene, and related carotenoids) or bacterial (e.g., acyclic and monocyclic carotenoids, long-chain acyclic isoprenoids) precursors which essentially exist in living organisms as monounsaturated or polyunsaturated species or are easily transformed into such species by diagenetic processes (e.g., steroids). It appears that these alkenes or polyenes become selectively trapped into a macromolecular network by reaction with inorganic sulfur species produced by bacteria in a kind of natural, low-temperature, vulcanization process. This process could start at early diagenesis already in the water column or eventually continue in the bottom sediment. Although its exact nature is yet unknown, it seems likely that the cross-linking reaction can be initiated by the cleavage of sulfur species in a radical type mechanism. The alkanes formed upon desulfurization of NPMF usually represent much higher amounts than the free alkanes of the samples and show a dramatically different composition. They may deliver very useful, complementary information in studies related to source and palaeoenvironment.

In the modern hypersaline carbonate lagoon and sabkha sedimentary environments of Abu Dhabi (United Arab Emirates), three types of organic matter originate respectively from microbial mat, Avicennia mangrove, and Halodule lagoonal seagrass. The study of recent sedimentary processes and cross sections through the sabkha sediments lead to the definition of organo-sedimentary facies based on geochemical and sedimentological criteria. This permits the construction of an organo-sedimentary sequence which expresses the Holocene sedimentary record involving a transgressive and a regressive sequence. The various organic facies occur in both sequences. Heterogeneity within the individual organic facies reflects several factors, including sedimentation dynamics, mineral matrix, oxidation and reduction, and selective organic and mineral diagenesis. These parameters are discussed in terms of depositional environment and location within the organo-sedimentary sequence. Changes in distribution, quantity, and preservation potential of the buried organic matter are discussed in terms of sea level changes and sedimentary accretion rates.

The Jurassic Dorset Formation consists of argillaceous to calcareous rocks that outcrop on the South Coast of Great Britain. This formation exhibits large variations in T.O.C. content (from 2 to about 50%). The organic matter is essentially algal, withh a small contribution of detrital organic debris (mostly ligneous). All the samples are thermally immature. The present study aims firstly to visualize the distribution of organic matter in the rock on a submicroscopic scale, and secondly to characterize its spatial relationships with the mineral matrix. Three samples with increasing T.O.C. contents, from about 9 to 48%, were studied using a combination of several microscopic technics: transmitted, reflected and fluorescence light observations on thin slides and polished sections; optical observations on isolated organic matter (palynologic residue) on strewn slides; scanning electron microscopy (SEM) observations (secondary electrons) on polished sections; pin-point analysis and multi-elemental mapping using EDS and image analysis coupled with SEM (secondary electrons) on polished sections; and Carbon mapping using EDS and image analysis coupled with SEM (back-scattered electrons) on polished sections. The interpretations are based on the comparison and the integration of the different photonic and electron microscopy images. The results are given in the form of composite maps describing the mutual spatial distribution of both the organic and mineral elements in the rocks. The organic matter is found distributed as either well individualized particles, such as ligneous continental fragments and planktonic algal bodies, or as diffuse unfigured organic matter. At the investigated scale, the diffuse organic matter constitutes a continous network through the organo-mineral groundmass. A part of it originates probably from the destruction of algal bodies through bacterial activity and compaction while another part should be attributed to the coccolith sedimentation. The sediment microstructure whatever the organic carbon content might be, exhibits small inorganic zones parallel to the stratification. These are interpreted as microburrows from which the organic matter has been removed. The abundance and distribution of sulphur (pyrite and organic sulphur) suggests that the rate of suphate-reducing activity is not simply related to the supply of metabolizable organic matter.

Reservoir geochemistry has proved to be a valuable approach to constrain reservoir model and predict, or evaluate, potential production problems at an early stage in field development. The now classical reservoir geochemistry methodology is based on the interpretation of compositional variations detected in oil or in soluble bitumen from rock reservoir including gross composition and molecular distribution1,2. These variations are usually related to long term geological processes (biodegradation, water-washing, gas solubilization, thermal sulfate reduction, and thermodynamic disequilibrium)3 which alter the original oil composition inherited from the source-rock. In the presence of efficient permeability barriers which prevent homogenization of the fluids by diffusion processes, the compositional variations can be eventually preserved over geological time. Additionally, by the possibility of combining molecular information on test fluid samples and reservoir rock organic content, reservoir geochemistry can lead to a better evaluation of the reservoir fluid stability and detection of internal reservoir heterogeneity such as bitumen rich levels or pyrobitumen occurrence4–5. Because such molecular information is available from the first obtained test oil or core samples, the occurrence of difference in fluid compositions across a field can be detected in advance of any production. Such information can be incorporated into the appraisal of the reservoir communicability and taken into account in the very first development scheme. Additionally, molecular fingerprints from production fluids can be used to calculate the relative contribution of individual reservoir in case of multi-reservoir single completion1 or oil mud contamination. In parallel to this pragmatic methodology, a new reservoir organic geochemistry approach is emerging. The objective is to integrate the reservoir fluid in the geological dimension of the petroleum system. This is done through the understanding, the formalization and the modeling of the different geological processes which alter the original composition of the fluid. The aim is to calculate the effect, at the molecular level, of the mechanisms potentially responsible for the modifification of the fluid composition (thermodynamic equilibrium, thermal cracking, water-washing and biodegradation), in the context of the geological environment. Using these simulations, it becomes possible to identify the process(es) which is (are) responsible for the recognized fluid composition and eventual heterogeneity. In this respect, reservoir geochemistry becomes predictive in term of fluid distribution at the reservoir and regional scale.

As complexity of exploration problems increases, petroleum geologists tend to consider more carefully oil formation and migration in terms of quantity, quality, and timing, in order to assess more accurately the history of hydrocarbons in a given basin. The present knowledge concerning the behavior of source rocks (quantity of hydrocarbons generated, composition of hydrocarbons, kinetics, expulsion efficiency) is that differences can be attributed to a large extent to organic properties of sediments, such as organic-matter content, organic-matter quality, and the pattern of the microdistribution of organic matter in the mineral fabric (layered, lenslike, intergranular). Moreover, field observations show that organic facies change at various scales in space and time, in re ponse to variation in depositional environments and sedimentary processes. The effects of these factors on the source rocks emphasize, beyond the efforts to achieve comprehensive descriptions of source beds (Rock-Eval pyrolysis, microscopy, typing of organic matter, measurement of kinetic parameters of carefully selected kerogens, use of wireline logs, seismic information), the needs to increase our conceptual understanding of deposition of organic facies in order to improve our ability to predict the petroleum potential of sedimentary basins.

Three series of sediments containing land plant-derived organic matter were studied. They include wells from Mahakam delta, Indonesia (Miocene), Douala basin, Cameroon (Upper Cretaceous) and eastern Paris basin, France (Paleozoic). All of the kerogens and coals isolated from these series can be related to type III in a Van Krevelen diagram, however they obviously differ in terms of chemical structure as reflected by their pyrolysis-GC patterns, and by their bitumen composition. These observations indicate the variations in the nature of land plant-derived sedimentary organic matter and thus differences in quantity and composition of the hydrocarbons which are are released upon maturation. Based on hydrocarbons potential of the insoluble organic matter and on pyrolysis-GC, there is no reason to disregard coals and to favor shales as potential source rocks in these two deltaic sequences. Moreover we assume that the bitumen depletion in coal samples is a reflection of loss by migration which is favored in coals when compared to shales. The GC patterns of the residual saturates, compared to the pyrolysis-GC patterns of the related insoluble coals suggest that migration of hydrocarbons proceeds in these coals without any significant alteration of their distribution, at least in the C15+ range.

Under mild conditions humic acids persist in ancient sediments (up to 70% of the total organic matter). In most samples studied, there is a relation between humic acids and associated kerogens, and this is a possible way to obtain more information on the structure of kerogens. The occurrence of humic compounds characterizes the immature stage; they rapidly disappear with natural evolution, hence the humic content would be an interesting marker of evolution during the immature stage. Moreover these compounds may be compared with humic acids occurring in recent deposits; thus they are a valuable source of information about the original organic matter and deposit media.

The hydrocarbon content of two thick Tertiary sequences from the offshore Gulf Coast (South Padre Island and Mustang Island) was studied using a headspace technique, thermal distillation, pyrolysis and solvent extraction. The threshold of oil generation was determined to occur in the range of 3050 m (10,000 ft; 120°C) in Miocene sediments. In the South Padre Island well, the distribution of the different classes of hydrocarbons along the sedimentary column suggests some updip migration processes are occurring.

In the Ragusa Basin (Southeastern Sicily), the Noto (Late Triassic) and Streppenosa (Early Liassic) formations, considered as the main source-rock of oil, are composed of millimetric to metric recurrences of clays, marls and carbonates. Together with a sedimentological and mineralogical description, a characterization of the organic matter is performed, at the same scale of investigation. Organic matter is analyzed in immature samples, using Rock-Eval technique and isolated kerogens. Generally, in terms of Total Organic Carbon, the richest sediments (TOC > 3%) are the most clayey. But very often, in all the encountered lithotypes, the Hydrogen Index reaches very high values (HI up to 900), and the H/C atomic ratio of kerogens ranges between 1.2 and 1.6. On the other hand, the kerogens present a rather large distribution of O/C atomic ratios, and it is possible to define two poles of composition. The kinetic parameters of cracking of representative organic samples of each pole are calculated by the OPTIM method. The variations of organic matter quantity and quality are discussed with respect to a typification of the sediments, especially the laminites, based upon observations performed with optical microscope and SEM. A tentative paleoenvironmental approach is proposed, and the most favourable conditions for organic enrichment are outlined. The general context is a local collapse of the Triassic carbonate platform. The highest petroleum potentials are neither associated to the algal-sedimentary laminites of the Noto formation nor to the basinal silty facies of the Streppenosa formation, but to the black-shales intercalated within these facies and to the laminites rich in clay.

Analytical techniques such as elemental analysis, infrared spectroscopy, ESR, dark-field electron microscopy and reflectance analysis applied to kerogens enables the geochemist to describe how they evolve at depth. Simulation of this evolution is obtained by the temperature-programmed heating of immature samples in a stream of inert gas. Comparison of naturally occurring samples at increasing stages of maturation, and artificially matured samples demonstrates that this simulation generally reproduces the overall characteristics of evolution at depth. However, agreement is better for hydrogen-rich than for oxygen-rich kerogens. In fact, oxygen is eliminated in nature, and in the laboratory, by different mechanisms. In nature, elimination takes place mainly at the beginning of burial. Temperatures used in the laboratory are necessarily much higher and appear to cause secondary reactions which favour water formation. Elimination of hydrogen occurs, during natural maturation, at greater depths and temperatures--i.e. in the oil and gas formation zone. Therefore, laboratory simulation is better for kerogens which have already reached the oil formation stage. Hydrocarbons released by natural and laboratory-simulated maturation are compared. Although quantities and distributions are different, similarities exist which can be used, to a first approximation, to predict the distribution of hydrocarbons obtained from naturally-evolving kerogens.

Hue, A.Y., Hunt, J.M. and Whelan, J.K., 1981. The organic matter of a Gulf Coast well studied by a thermal analysis–gas chromatography technique. In: A.W. Rose and H. Gundlach (Editors), Geochemical Exploration 1980. J. Geochem. Explor., 15: 671–681. A thermal analysis–gas chromatography technique, previously described by Whelan et al. has been used to analyze cuttings from a Continental Offshore Stratigraphic Test (COST No. 1) drilled in the Gulf Coast of the U.S.A. The data allowed an evaluation of the degree of maturation of the organic matter and provided an accurate determination of the depth of the threshold of intense oil generation at 3048 m (10,000 ft.). Qualitative changes of hydrocarbons in the C 7 ,–C 14 range were determined by gas chromatography and confirmed by gas chromatography–mass spectroscopy. These data are discussed in terms of generation and migration processes. The catagenetic evolution results in a strong tendency for a proportional increase in n-alkanes. Mass transfer phenomena may be responsible for updip movement of the lighter hydrocarbons.

Light hydrocarbon (C 1 -C 8 ) profiles are compared for three wells of varying maturities: two immature DSDP wells (Site 397 near the Canary Islands and Site 530A near the Walvis Ridge in the south-east Atlantic) and a mature well, the East Cameron well in the Texas Gulf Coast. Primary migration of C 1 and C 2 appears to be occurring in all of the sedimentary rocks examined. Primary migration of C 3+ components becomes important only as fine-grained sedimentary rocks enter the catagenetic hydrocarbon generation zone or over short distances in more permeable sections. Lateral migration along bedding planes was more important than vertical migration in sedimentary rocks of all maturities. The lightest (methane, ethane and propane gases) hydrocarbon show greater fractionation than do the C 4 -C 8 alkanes which generally show minimal fractionation during the migrational process. Subsurface diffusion coefficients for these p.p.b. quantities of C 2 -C 5 alkanes from immature sediments from DSDP Site 530 are estimated to be several orders of magnitude less than values reported in the literature for diffusion of much larger amounts of these compounds from mature water wet sediments into air or sandstones. Since our calculations suggest light hydrocarbons are present in amounts less than their reported solubilities in pure water at 25°C, we postulate that the sediment organic matter has a substantial effect on retarding the movement of these light hydrocarbons.

Quantitative evaluation of source rocks is a prerequisite for assessing realistic petroleum potential in target basins. Such an evaluation includes information about the actual occurrence of organic-rich beds, their stratigraphic placement, specific locations in sedimentary sequences, lateral extension, geometry at the scale of the basin and their organic facies variations (richness and quality). To achieve this evaluation, current methods rely on analytical data alone. For several years an attempt has been made to elaborate a predictive approach based on the integration of the various factors controlling the accumulation and distribution of organic matter in sedimentary basins in order to try to model the process. The main effort has been devoted to the control of the production of biomass according to paleogeography, and paleoclimatology, and to the control of the degradation of organic matter according to the development of oxygen-starved water bodies. Depositional processes which are certainly a key factor to understand the internal variations and the distribution of organic bearing sediments have been largely overlooked: Examples of Modern environments (e.g. Black Sea, Caspian Sea, Lake Bogoria) show that the distribution of the organic matter in anoxic basins is generally organized in a concentric pattern, with a centripetal increase of organic content towards the depocenters where the sediment thickness is the greatest. Such a feature, which can be related to the hydrodynamic properties of the organic particles, is of paramount importance as far as the distribution of petroleum potential is concerned. This “centripetal concept,” with its implications for source rocks evaluation, is well exemplified in the Lower Jurassic of the North Sea and in the Lias of the Paris basin. However, this pattern can be strongly affected in anoxic basins when extensive gravity transport mechanisms cause redeposition of sediments. In such a situation sediments, including their specific organic contents, are redistributed along transit pathways. In Lake Tanganyika, these modes of transport drive a significant amount of altered organic matter towards the deepest parts of the basin resulting in a marked lateral heterogeneity of the organic facies in the bottom sediments. Similar processes might contribute to the complex pattern of the distribution of organic facies seen in the Upper Jurassic of the North Sea. On the other hand, gravity flows can introduce organic-rich sediments into basins where environmental conditions would, otherwise have been unfavorable for organic accumulation. A dramatic example of such a situation is provided by the several hundred metres thick sequence of organic-rich turbidites and debris flows deposited since the Late Miocene at the base of the continental slope off South-West Africa.

Well cuttings from two offshore Gulf Coast stratigraphic test wells (South Padre and Mustang Island) have been analyzed for C/sub 7/-C/sub 15/ hydrocarbon distributions by thermal distillation-pyrolysis GC, for C/sub 1/-C/sub 7/ hydrocarbons by headspace analysis and for C/sub 15//sup +/ hydrocarbons by extraction. The results from the South Padre well show that the threshold of intensive hydrocarbon generation starts around 10,000 feet and peaks around 14,000 feet. The distribution of individual homologs within each hydrocarbon group indicates that the more volatile members are migrating upward from the generation zone. Thus, n-pentane (C/sub 5/) peaks at 10,000 feet, and the higher homologs, C/sub 5/, C/sub 6/, C/sub 7/, C/sub 8/ and C/sub 10/ peak at successively greater depths to 15,000 feet. Benzene also peaks before toluene which peaks before the xylenes. This phenomena was not observed in the Mustang Island well where organic carbon contents and hydrocarbon yields were lower, apparently too low to induce migration. C/sub 1/-C/sub 7/ hydrocarbon distributions also were studied by the headspace technique in offshore samples from the East Coast, Gulf of California and Gulf of Maine. Hydrocarbon yields from wells east of Cape Hatteras and on the Blake-Bahama Plateau were much lower than inmore » most other areas analyzed to date. Some Gulf of California samples had high gas yields due to hydrothermal intrusions and igneous dikes raising sediment temperatures. Gulf of Maine samples were unique in having no aromatic hydrocarbons.« less

Oil-gas phase equilibria is likely to represent one of the main causes of observed compositional heterogeneities in fields. Our research has been focused mainly on the light C 14 -hydrocarbons fraction and can be divided into two parts. The first part was performed with a fluid mixture of a Middle East crude oil and a synthetic gas, and includes PVT experiments, high resolution-gas chromatography (HR-GC) and 13 C gas chromatography-combustion-isotopic ratio mass spectrometry (GC-C-IR-MS) analyses. Our observations show that under a given set of PT conditions there is a preferential separation of aromatics into the oil phase and isoalkanes into the gas phase. Moreover, the data from 13 C GC-C-IRMS suggest a small isotopic fractionation between the two phases, with the gas being relatively isotopically lighter. The second part is theoretical, and uses Equation of State (EOS) calculations which successfully simulate the previous experiment.