Revisiting Rhenium-Osmium Isotopic Investigations of Petroleum Systems: From Geochemical Behaviours to Geological Interpretations

Recent decades have witnessed an increasing number of studies investigating petroleum systems with the application of rhenium-osmium(Re-Os) isotopic geochemistry. Here, we review the use of the 187 Re-187 Os geochronometer with respect to the geochemical behaviour of rhenium and osmium in hydrocarbon-related geological processes. The Re-Os budget in hydrocarbon source rock predominantly originates from natural water columns during its deposition. Open seawater tends to have a homogeneous Os isotopic composition because its residence time in seawater is longer than the time taken for ocean mixing. On the contrary, restricted water bodies(e.g., lakes) may have heterogeneous Os isotopic compositions due to the greater amount of terrigenous input. Hydrogenous Re and Os atoms are sequestered from the water body into sedimentary organic matter and transferred into crude oil through thermal maturation of organic matter. Thermal maturation likely does not significantly alter the Re-Os isotopic systematics of the source rock as a Re-Os isochron age of 442±21 Ma(2σ) is yielded in this study for over matured source rocks within the Silurian Longmaxi Formation from the Sichuan Basin. Re-Os atoms are mainly hosted by the highly polar/aggregating/aromatic asphaltenes in hydrocarbons, possibly chelating with organic complexes or occurring as metalloporphyrins. Resin and aromatic hydrocarbons also contribute to the Re-Os budget, but are 2 to 3 orders of magnitude lower than that of asphaltenes, whereas saturates do not contain appreciable Re-Os contents. The distribution of Re-Os atoms in hydrocarbons is heterogeneous because the duplicate analysis of pure single bitumen samples yields similar ^(187)Os/^(188)Os ratios whereas variable ^(187)Re/^(188)Os ratios. The Re-Os system in crude oils can be reset during transport away from the source rocks, with Os-rich organic fractions more readily expelled than Re-rich fractions. Contact with metal-rich fluids(e.g., hydrothermal fluid) or compositional changes related to asphaltene contents(e.g., deasphalting, biodegradation, thermal cracking and thermochemical sulphate reduction) are also likely to alter the Re-Os systematics in hydrocarbons. These geochemical features enable the ^(187)Re-^(187)Os isotopic system to have robust applicability for petroleum system investigations, which may use the Re-Os radiometric tool for:(1) stratigraphic correlation of source rocks,(2) dating geological events altering the asphaltene content in hydrocarbon such as hydrocarbon generation, thermochemical sulphate reduction, etc., and,(3) fingerprinting hydrocarbons. Regardless of the robustness of the ^(187)Re-^(187)Os geochronometer for petroleum system investigations, there are several pending questions such as partitioning between solid organic species or between organic matter and sulphide, chelating sites in hydrocarbons and Os isotopic equilibration between hydrocarbon subfractions. To improve the understanding of the Re-Os behaviour in petroleum systems, we underscore multi-proxies-based geochemistry(e.g., inorganic-organic geochemistry) and experimental studies(e.g., hydrous pyrolysis).

Accumulation conditions and key technologies for exploration and development of Qinshui coalbed methane field

The Qinshui Basin has been explored for more than 60 years through two stages of oil and gas reconnaissance survey and exploration&development of coalbed methane(CBM),it has become the largest CBM industrialization base in China and also is a model which successfully realize commercialization of CBM of high rank coal-bearing basin in the world.Although the high-rank coal field is characterized by low pressure,low permeability,low saturation and strong heterogeneity,the exploration practice and research show that the accumulation conditions of CBM reservoir in the Qinshui Basin are superior.As main productive intervals,No.15 coal seam of Taiyuan Formation and No.3 coal seam of Shanxi Formation respectively belong to the epicontinental-sea carbonate platform sedimentary system and the epicontinental-sea shallow-water delta sedimentary system.The coal seam has large thickness,and is mostly composed of humic coal and mainly contains vitrinite.Affected by tectonic thermal events in the Yanshanian period,the coal rank is high,the adsorption capacity is strong,and the gas content is large.Formation of the CBM reservoir goes through three stages including two stages of hydrocarbon generation,gas phase transformation and sealing of hydrodynamics and roof and floor.In view of the characteristics of the Qinshui Basin topography and the high rank coal,a series of key technologies for exploration and development are developed,including mountainous region seismic acquisition,processing and interpretation technology,drilling and completion technology of multiple wells,drilling and completion technology of multiple horizontal wells dominated by compound V type,deplugging secondary fracturing stimulation technology,control technology of high rank CBM drainage,and CBM gathering and transportation technology,which effectively supports the scale and industrialization development of high rank CBM in the Qinshui Basin.