Molecular and bulk isotopic analyses of organic matter in marls of the Mulhouse Basin (Tertiary, Alsace, France)

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.     

Organic matter of the Mulhouse Basin, France: a synthesis

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.     

Sedimentation, distribution and diagenesis of organic matter in a recent carbonate environment, Abu Dhabi, U.A.E.

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.     

Experiments and geochemical modelling of CO2 sequestration by olivine: Potential,quantification

Aqueous solutions equilibrated with supercritical CO₂ (150 °C and total pressure of 150 bar) were investigated in order to characterize their respective conditions of carbonation. Dissolution of olivine and subsequent precipitation of magnesite with a net consumption of CO₂ were expected. A quantified pure mineral phase (powders with different olivine grain diameter [20–80 μm], [80–125 μm], [125–200 μm] and [>200 μm]), and CO₂ (as dried ice) were placed in closed-batch reactors (soft Au tubes) in the presence of solutions. Different salinities (from 0 to 3400 mM) and different ratios of solution/solid (mineral phase) (from 0.1 to 10) were investigated. Experiments were performed over periods from 2 to 8 weeks. Final solid products were quantified by the Rock-Eval 6 technique, and identified using X-ray diffraction, Raman spectroscopy, electron microprobe and scanning electron microscopy. Gaseous compounds were quantified by a vacuum line equipped with a Toepler pump and identified and measured by gas chromatography (GC). Carbon mass balances were calculated.     

Molecular and isotopic fractionation of light hydrocarbons between oil and gas phases

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 C₁₄-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 ¹³C 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 ¹³C 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.     

The organic matter of a gulf coast well studied by a thermal analysis-gas chromatography technique

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 C₇–C₁₄ 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.     

Generation and migration of hydrocarbons in offshore South Texas Gulf Coast sediments

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.     

Microanalysis of primary fluid inclusions in halite: constraints for an evaporitic sedimentation modeling. Application to the Mulhouse Basin (France)

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.