Kinetic Simulation Of Thermogenic Natural Gas Generation And Its Geological Applications

It is presently a hot spot to quantificationally study the formation, migration and accumulation of natural gas. The kinetic simulation of natural gas generation, based on pyrolysis experiments and by means of hydrocarbons generation kinetics and carbon isotope kinetics, can quantificationally predict the compositions of gas components and stable carbon isotope and reveal the history of gas accumulation according to the evolution history of a basin. Recently it is widely applied in the geochemical study of natural gas. In this paper kinetic pyrolyses on four types of organic matter (n-octadecane, coal, mudstone, carbonate rock)were carried out in gold tube confining-system, and the behaviour of natural gas generation are discussed combined with the geological condition.The kinetic study on the pyrolysis of n-octadecane shows that secondary cracking of pyrolysates from n-octadecane largely contributes to the amount of methane generation, much more than primary cracking of n-octadecane. The large shift in the carbon isotope values of the residual alkanes from the initial n-octodecane precursor indicates that the δ¹³C values of over mature solid bitumen thermally generated in paleo-oil reservoir should not be directly used for oil and source rock correlation. Cracking and polymerization in the relatively low temperatures and disproportionation reactions leading to light hydrocarbons and polyaromatic hydrocarbons at high temperatures are probably causes for the carbon isotope reversal of gaseous hydrocarbons which is commonly observed in pyrolysis experiments. The kinetic study of methane generation implicates that methane from n-alkane hydrocarbons cracking begins to generate at 170 ℃ and the main peak of the generation is at 200 ℃ in sedimentary basins.Pyrolysis experiments on two type of Paleozoic source rock in Ordos Basin show that both the Upper Paleozoic coal-measure source rock and Lower Paleozoic marine carbonate source rock have larger gas producing capacity. Pyrolysis gases of the source rocks are dominated by dry gases (methane accounts for over 90% of the total gases). Among them the Permian coal have the biggest gas producing capacity with the highest yield of 270 mL/g TOC. Pyrolysis yields were used to model methane generation with a series of parallel, first-order reactions with activations energies between 40 and 63 kcal/mol and a single frequency factor of 1.01 X 1011 s"1 for Permian coal, activations energies between 40 and 64 kcal/mol and a single frequency factor of 1.51 X 10n s"1 for Permian mudstone and activations energies between 39 and 63 kcal/mol and a single frequency factor of 6.51 X 1010 s"1 for Ordovician limestone. Based on these kinetic parameters, the carbon isotope kinetic parameters were calculated for methane.Extrapolating these parameters to the geological condition in Ordos Basin, hydrocarbon generation history of different source rocks were simulated through hydrocarbon-generating kinetic calculation. Results show that the Permian coal-measure source rock in Ordos Basin entered the "main gas window" between the Late Jurassic and the Early Cretaceous, while Ordovician limestone in the center of Ordos Basin entered the "main gas window" between the Middle Jurassic and the Early Cretaceous. The Permian coal-measure source rock can still generate a significant amount of gas in the process of tectonic uplifting. Natural gases in C-P reservoir are coal-formed gas derived from the C-P coal-measure source rock. The gases in the Ordovician weathering crust are mixed gases which were mainly derived from Upper Paleozoic coal-measure source rock. The quantificational calculation result implicates that over 70% of gas in the weathering crust in the Central gas field is from Carboniferous-Permian beds.