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Abstract The compositional and bulk isotopic signatures of hydrocarbon compounds are routinely characterized to constrain the origin of oils and natural gases in sedimentary basins. Recent developments in clumped isotope geochemistry have given rise to a suite of measurements that provide new information on the thermal history of low molecular weight hydrocarbon gases. However, to date no study has linked these compositional, bulk and clumped isotope constraints to any mechanistic understanding of how these signatures develop and evolve. In this study, we developed a kinetic Monte Carlo method to predict consistent and simultaneous molecular distributions, bulk isotopic content, and detailed (multiply substituted and site-specific) isotopic structures of hydrocarbons from a cracking model. The detailed isotopic structure of the source (initially modeled as long alkanes), the intermediates, and the product molecules is followed as a function of the level of conversion due to cracking reactions. The bulk 13C content of gaseous alkane products generated via the model is shown to follow the linear natural gas plot at low conversion, but to deviate at high conversions. As examples of the information generated, the populations of center vs. terminal 13C-substituted propane are reported as a function of the starting alkane chain length and conversion and the population of doubly-13C-substituted ethane is described as a function of conversion and for different 13C substitution patterns in the source. The results are compared to experiment where possible and highlight the possibility of constraining hydrocarbon source isotopic structure and the nature of the generation processes leading to the formation of natural hydrocarbon deposits. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hydrous pyrolysis experiments were conducted under diagenetic to catagenetic conditions. • Half-lives of organic acids were between 8.88 × 1010 and 2.81 × 10-3 years (80–200 °C, 20 MPa). • Pyrolytic organic acid generation might result in millimolar in situ concentrations in pore water. • Reaction rates allow the prediction of expected concentrations of LMWOA in pore water at elevated temperatures by geochemical modelling. Increasing temperature in claystone formations accelerates geochemical reactions between organic matter, water and mineral phases, promoting the generation and decomposition of organic compounds such as low-molecular-weight organic acids (LMWOA). LMWOA presence in claystone formations can enhance mineral dissolution, stimulate gas generation through thermal decomposition reactions, affect metal-ion complexation, alter sorption processes, and provide feedstock for microbial life. Hydrous pyrolysis experiments (80–200 °C and 20 MPa) using Dickson-type flexible gold-titanium reaction cells were conducted to investigate the thermal generation and decomposition of LMWOA in organic matter poor (0.64 wt%) Opalinus Clay. Maximum concentrations of dominant species were 44 µmol/kg clay formate (120 °C), 310 µmol/kg clay acetate (200 °C), and 74 µmol/kg clay oxalate (120 °C). Calculated half-live ranges of the dominant compounds were between 8.88 × 1010 and 2.81 × 10−3 years for the experimental conditions. Comparison of observed and calculated reaction rates suggests that the thermal generation and decomposition of LMWOA proceed simultaneously. Therefore, the observed generation rates in these and other previously published experiments are minimum estimates that need to be corrected for parallel decomposition reactions. The new quantitative reaction rate data presented here facilitate modelling of production/decomposition and local pore water concentrations of LMWOA by the extrapolation of kinetic data to temperatures encountered, for example, in natural heating during subsidence or by thermal intrusions. [ABSTRACT FROM AUTHOR]

• During soaking pyrolysis C 2 to C 5 gases decreased with increase in pyrolysis temperature. • For hydrous pyrolysis C 4 and C 5 gases increased with increase in pyrolysis temperature. • MW of compounds in oil from soaking and hydrous pyrolysis ranged from 86 to 376 g/mol. • The yield of the organic acids appeared to be higher in the presence of DI water. In the present study, soaking and hydrous pyrolysis experiments were performed on Indian oil shale in the presence of de-ionized (DI) water and brine solution under isothermal conditions of 300, 350, and 400 °C, and residence time of 72 h to simulate the effect of underground aquifers on the expulsion and composition of hydrocarbons from source rocks. During soaking pyrolysis, the oil yield increased minutely with the increase in soaking time from 1 to 4 weeks but was not influenced by the presence of brine solution (8.82 wt/wt%) or DI water (8.46 wt/wt%). A similar trend in the generation of oil yield was observed during hydrous pyrolysis in both DI water and brine solution. GC analysis of the gas samples showed methane (CH 4) as the prominent gas produced during soaking and hydrous pyrolysis. During soaking pyrolysis, the concentration of C 2 to C 5 gases decreased with increased pyrolysis temperature. However, during hydrous pyrolysis, the concentration of C 4 and C 5 gases increased with an increase in pyrolysis temperature. GC-SimDist analysis of the oil samples obtained from soaking pyrolysis at 400 °C showed the compounds ranged from C 8 to C 38 in brine solution and from C 6 to C 34 in DI water. Nevertheless, during hydrous pyrolysis in both DI water and brine solution, the carbon number distribution of the oil samples was observed to be in the range of C 8 to C 38. GC–MS analysis of the oil samples obtained from the soaking and hydrous pyrolysis experiments showed that the molecular weight of the identified compounds ranged from 86 to 380 g/mol. GC–MS, ATR, and 1H NMR analyses showed that the oil samples obtained from soaking and hydrous pyrolysis experiments were rich in aliphatic compounds. The organic acids content appeared to be higher in the presence of DI water as compared to the brine solution. [ABSTRACT FROM AUTHOR]

Abstract: The early Miocene Pedregoso Formation is one of the numerous formations rich in organic matter within the stratigraphic record of the Urumaco Trough, in the central area of the Falcón Basin. Due to its lithological characteristics and stratigraphic position, this formation is of great interest regarding the basin''s petroliferous systems. The evaluation of various inorganic and organic geochemical parameters indicates that the organic matter is primarily of marine origin, deposited in a marine carbonate environment typical of reefal systems, under oxic-to-dysoxic conditions. The low variability in the TOC concentrations and in the distributions of the biomarkers extracted from the samples suggests that the paleoenvironmental conditions and the organic-matter supply remained approximately constant throughout the sedimentation of this unit. The Pedregoso type-II organic matter (marine origin) and initial organic richness value (∼1.8%) suggest that this unit has probably generated hydrocarbons within the Urumaco Trough. However, present-day thermal maturity parameters reveal that the Pedregoso organic matter is overmature (dry gas window), indicating that this unit is only capable to generate gas. In addition, the geothermal gradient, maturity parameters, and the maximum paleotemperature estimated in this study suggest that the Pedregoso Formation reached a maximum burial depth the ∼6.5 km, consistent with the value obtained from data of stratigraphic thickness in the Urumaco Trough. This implies that the thermal anomaly that affected the basin during the Late Eocene–Early Miocene did not reach the central part of the basin, and therefore, the organic matter maturation in this unit is due to the sedimentary burial. [Copyright &y& Elsevier]

Coal-bearing deposits of the Tunas Formation suggest the existence of potential gas-prone source rocks in the Claromecó Basin, south of Buenos Aires province, Argentina. Two wells, PANG 0001 and PANG 0003, containing Permian rocks assigned to the Tunas Formation were studied through core observation, X-Ray diffraction, petrographic description, geochemical analysis, and organic petrography to investigate the hydrocarbon potential of the Claromecó Basin and define horizons with source rock potential. The analyzed sedimentary successions are composed of medium- to fine-grained sandstones interbedded with tuffs, mudrocks, carbonaceous mudrocks, and coal. Two main horizons with potential source rocks were determined: an upper horizon, at the top of the succession (300–400 m) and a lower one, at the base (740–850 m). These layers are mainly composed of mudrocks, carbonaceous mudrocks, and coal beds up to 4.50 m in cumulative thickness. Total organic carbon (TOC%) content is high, with most samples ranging from 0.5 to 2% in organic-rich mudrocks, and from 26 to 53.9% in carbonaceous mudrocks and coals, characterizing the coal-bearing layers as good-quality source rocks. The organic matter is of terrigenous origin as indicated by the abundance of woody plant debris. Predominant organic matter constituents belong to vitrinite group (type III kerogen), with a low abundance of the inertinite group macerals. Vitrinite reflectance (Ro) values in carbonaceous mudrocks and coals ranged from 1.3 to 1.6% for the upper horizon to 1.9–2.4% at the lower one, reflecting a catagenesis to late catagenesis stage within the wet to dry gas window. The organic matter quantity, type, and maturity in Tunas Formation coal-bearing deposits suggest that this unit has a good potential as gas-prone source rocks. • The hydrocarbon source rock potential of Tunas Formation coal-bearing deposits were evaluated. • Total organic carbon (TOC%) content is high, with an average of 2.69%. • Predominant organic matter is terrestrial (type III kerogen). • Vitrinite reflectance values range from 1.3 to 2.4%, indicating a mature to over mature stage within the wet to dry gas window. • The Tunas Formation coal-bearing deposits have a good potential as gas source rocks. [ABSTRACT FROM AUTHOR]

One of the biggest challenges in evaluating the transport and storage properties of shales is understanding how the pore structure and network evolve in the same stratigraphic shale during catagenesis. Using a combination of CO 2 adsorption, mercury intrusion porosimetry, as well as scanning electron microscopy, we have described the evolution and the abrupt changes (jump) of pore characteristics in lacustrine shale at oil and gas maturation. With an increase maturity, our sample set exhibited abrupt changes in porosity-related characteristics. Porosity and surface areas presented a decline and then an increase, and a jump occurred due to hydrocarbon generation, with the minimum value in late maturity (i.e. vitrinite reflectance of ~1.3%). These changes were accompanied by corresponding changes in pore-size distributions, surface area percentage, and multi-scale pores volumes. The trends in porosity variation were related to different organic matter and mineral contents in our samples, to some extent, with the degree of maturation being the dominant influence. We found that inorganic pores were the main contributor to the porosity of less mature shales, while porosity in gas-mature shales was dominated by organic matter pores. In this work, we showed that organic matter transformation was a centric component of pore evolution. Our findings may fill significant gaps in understanding and predicting pore development, with respect to the maturity of lacustrine shales. • Pore volume and surface area in lacustrine shales show jumps with maturity. • The porosity of oil- and gas- mature shales is governed by mineral interparticle pores and OM-hosted pores, respectively. • From oil-window to gas-window, porosity-related characteristics present a decline and then an abrupt increase. [ABSTRACT FROM AUTHOR]