Experimental Insights on Sources, Amounts, and Kinetics of Thermogenic Gas
Michael D. Lewan
U. S. Geological Survey, Box 25046, MS 977, Denver Federal Center, Denver, CO 80225
The familiar paradigm for thermogenic gas generation suggests that, with increasing thermal maturity source rocks first generate oil and then generate thermogenic gas. This gas may be supplemented with additional thermogenic gas generated by the cracking of oil in deeply buried reservoirs. Various nuances of this sequence for thermogenic-gas generation and the required thermal-stress levels at which they occur have been proposed. However, this paradigm is based on intuitive interpretations of limited subsurface data. As a result, a quantitative understanding of sources, amounts, and kinetics of thermogenic gas in the subsurface has not fully developed into scientific concepts that can be applied to the assessment and exploration of conventional- and unconventional-gas resources. These limited subsurface data in the vastness of sedimentary basins, and the mobility of gas and oil to migrate through out a basin, makes laboratory pyrolysis experiments critical to developing a scientific understanding of thermogenic-gas generation. It is important that laboratory pyrolysis experiments be scrutinized in terms of how well their conditions and products simulate the natural process of thermogenic-gas generation. When unanticipated pyrolysis results present themselves, one must ask whether the results are revealing a new concept not previously considered in interpreting the limited subsurface data or are they simply an artifact of experimental conditions employed in the laboratory. This quandary can be minimized by conducting laboratory pyrolysis experiments as close to natural conditions as possible in order to understand reactions and mechanisms responsible for natural thermogenic-gas generation. With water being ubiquitous in the subsurface, pyrolysis experiments conducted in the presence of water at the lowest possible temperatures have proven to best simulate natural petroleum generation. These hydrous pyrolysis experiments provide insights on the sources, amounts, and kinetics of thermogenic gas that facilitate the development of scientifically sound concepts.
The two major sources of thermogenic gas typically considered in pyrolysis experiments are maturing source rocks and cracking of reservoir oils. Hydrous pyrolysis experiments on oil-prone source rocks indicate gas and oil generation is synchronous, which results in low ultimate gas:oil ratios (GORs<1,500 scf/bbl). These results indicate that basins with higher GORs (>1,500 scf/bbl) are sourced from gas-prone source rocks and (or) from cracking of oil in deeply buried reservoirs. Hydrous pyrolysis experiments also provide insights on the amount of thermogenic gas generated from source rocks containing different types of kerogen (i.e., Type-I, -II, -IIS, and III). Counter to our intuitive concepts, pyrolysis experiments indicate oil-prone kerogen (Type-I, II, and IIS) generates significantly more thermogenic gas than gas-prone kerogen (Type-III) on an organic-carbon basis. Type-IIS kerogen generates almost 2.5 times more thermogenic gas than Type-III kerogen, and Type-I and -II kerogen generates 1.8 times more thermogenic gas than Type-III kerogen. These results indicate that our designation of oil-prone and gas-prone source rocks are in reference to the proportions of gas to oil generated and not necessarily the amount of gas generated. The amounts of these different kerogen types needed to deem a rock, as a gas-prone source remains an issue that is currently being investigated by pyrolysis experiments. Results from hydrous pyrolysis experiments also show that on an organic-carbon basis the amount of thermogenic gas generated from thermal cracking of reservoir oil may be 3 to 4 times greater than thermogenic gas generation from source rocks. The significance of this gas source in a basin depends on oil remaining in coherent traps that are deeply buried in high thermal-maturity regimes.
Kinetics determines the timing and thermal maturity level of thermogenic gas generation from different sources. A consensus on appropriate kinetic parameters for thermogenic gas generation has not emerged, and a better scientific understanding of precursors, reactions, and mechanisms of gas generation under geological conditions is needed. Figure 1 shows generation curves based on published hydrous- and hydrothermal-pyrolysis kinetic parameters for generation of oil, source-rock gas, and oil-cracking gas at the base of the Mowry Shale at the Eagle Nest location in the Greater Green River Basin. Based on the vitrinite reflectance attained at the base of the Mowry Shale, 95% of oil generation is complete by 1.1 %Ro, 90% of source-rock gas generation is complete by 1.5 %Ro, and 10% of gas generation from oil cracking is complete by 2.1 %Ro. These results indicate that there is a significant gap (1.5 and 2.1 %Ro) in gas generation between maturing source rocks and oil cracking that current paradigms for gas generation have not considered. This maturity gap has been shown to have important implications in some areas and attests to the need for more experimental kinetic studies on gas generation.
Figure 1.Generation curves for oil (type-II), source-rock gas (C1+C2), and oil-cracking gas at the base of the Mowry Shale at the Eagle Nest location in the Greater Green River Basin. Curves are based on hydrous and hydrothermal pyrolysis kinetics, EASY%Ro, a 26.8°C thermal gradient, and a 4°C surface temperature.