Polymerisation Theory - Formation of Hydrocarbons in Sedimentary Strata (Hydrates, Clays, Sandstones, Carbonates, Evaporites, Volcanoclastics) from CH4 and C02: Part I: Polymerisation Concepts, Kinetics, Sources of Hydrogen, and Redox Environment

摘要

ymerisation processes, reaction routes, chain growth by polymerisation, polyaddition/condensation and cleavage concepts are summarised within the context of a 5 Stage (Stages 1 to 5) scheme for the formation of ature oil. The study focuses on the formation of n-alkanes in Stage 1 oils and defines polymerisation ratios (PR) for oils formed in constant and varying chemical environments. The relationship between H2 gas content and chain propagation is defined using examples from the Cascadia Gas Hydrate Field (eastern Pacific Ocean, continental slope, offshore USA/Canada) and the East Mariana Basin (abyssal plain, Central Pacific Ocean).Analysis of C2 - C8 from Recent to Cretaceous sediments in the Pacific and Atlantic Oceans demonstrated that PR is independent of depth, temperature and pressure over the temperature range 0 - 80° C; pressure range 10 to 70 MPa and depth range 0 -1500 mbsb. There is a close relationship between the equilibrium constant ratio, KIKC, with PR. This ratio is independent of temperature and pressure.The rate constants for polymerisation, the relationship between hydrate formation, gas hydrate stabilisation zone (GHSZ), and polymerisation, PR and CO2:CH4 ratio, H2:CO ratio, H2 concentration and chain propagation, and the variation in PR with carbon number were defined for the Cascadia Hydrate field.These observations were confirmed by analysis of Stage 1 oils from DSDP Site585 (East Mariana Basin (Ita Maitai Guyot, Magellan Seamount Chain, Western Pacific Ocean)), and DSDP Site 586 (Tertiary sediments from the Ontong Java Plateau (Western Pacific Ocean).The calculated rate constants are csistant with Fischer Tropsch (FT) rate constants for Fe catalyst. The distribution of n-alkanes in the Stage 1 oil is similar to the distributions produced experimentally at low temperatures using a Fe (FT) catalyst.The relationship between Eh and polymerisation ratio has been defined for the Cascadia Hydrate Field and the East Mariana Basin. The polymerisation ratio increases as the product of (Eh-Eh1) increases. Eh1 = the water stability fence. The relationship between pH, Eh and PR is defined.The potential sources of hydrogen (for the polymerisation of CO2) are considered. These include water, biogenic gas and thermogenic gases. Experimental analyses from low temperature (50 - 80° C) substoichiometric oxidation of organic matter are used to identify the composition of the liquids, oils and gases, which may be produced by thermogenic processes.