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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.

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.

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.

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.

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.