Experimental Simulation of Hydrocarbon Expulsion

Investigating generation, expulsion and primary migration usually suffers from inadequate methodologies, failing in the provision of natural conditions, prevailing during the genesis of oil and gas. Destructive sample preparation, inappropriate pressure regimes and pyrolysis in closed mode and/or in the absence of water caused results not representative for natural processes. To overcome these limitations, a newly designed apparatus was developed and built, capable to perform pyrolysis experiments with intact rock samples under pressure regimes, prevailing during catagenesis. In detail, lithostatic (or overburden-) pressures and hydrostatic (or pore-) pressures, corresponding to 3000 m depth and beyond, can be simulated by this apparatus, the “Expulsinator” device. Additionally, the experiments are conducted as hydrous, semi-open pyrolysis, allowing the time-based sampling of the expelled products. Thus, generation/expulsion profiles are generated for each investigated source-rock. Comparison of generation and expulsion efficiency of the Expulsinator with established pyrolysis methodologies (MSSV, HyPy, Rock Eval and closed small vessel pyrolysis (CSVP)) reveals striking differences: Expulsinator experiments yielded more bitumen and released lower gas amounts than classic pyrolysis. This is caused by secondary alteration of products in case of classic pyrolysis, e.g. oil to gas cracking at higher temperatures and polymerization to pyrobitumen. The open setup of the Expulsinator experiments prevents successfully secondary alteration, increasing the liquid product yields and lowering the gas formation. This is mirrored in TOC conversion rates as well: Expulsinator conversion exceeds 81 %, whereas those of hydrous CSVP remains at 65 %. Further, interactions between kerogen, bitumen, pyrobitumen and pyrite are reduced in case of Expulsinator experiments. In contrast, CSVP experiments residues show enrichment of nitrogen and oxygen and depletion of sulphur, indicating intense interaction between the mentioned components. Investigating the impact of catagenesis onto the yields and composition of expelled products was carried out by a stepwise experimental setup, simulating burial depths of ~2000 m, ~2500 m and 3000 m, implementing overburden pressures from 600 bar to 900 bar and hydrostatic pressures from 200 bar to 300 bar. Each experiment step shows a distinct expulsion maximum of liquid hydrocarbons, reaching the total maximum at 3000 m. Confrontation of Expulsinator results with comparative CSVP and MSSV experiments reveal expulsion and primary migration effects onto the n-alkane distribution in dependence of maturation and subsidence. An n-alkane increase towards larger molecular size with ongoing expulsion is associated with molecular size controlled retention effects. The expulsion progress is mirrored in trends of isoprenoid-ratios (pristane vs. phytane) and isoprenoid – n-alkane-ratios (pristane vs. n-C17), caused mainly by generation controlled effects. However, both ratios qualify as expulsion indicator, taking into account generation and expulsion are concurrent processes. Generation/expulsion classification is done with eight different source-rocks (two type-I, type-II (-I/II), type-IIS and type-III kerogens, each) with different lithologies, using the catagenesis experiment setup described above. Again, formation of pyrobitumen shows to affect the maturity assessment of the source-rock, increasing the measured Tmax in case of kerogen type-I samples, but decreasing the measured value for type-II and –IIS kerogens. Type-III kerogen shows earliest onset of generation, followed by type-IIS and –II kerogen and type-I kerogen with the latest generation onset. The early generation and the concomitant high expulsion rate, observed for type-III kerogen, is contrasted by results of natural systems where this kerogen type usually shows very low expulsion rates. The Expulsinator setup provides excellent expulsion conditions, leading to early release of generated hydrocarbons. In contrast, accumulation within the kerogen pore space and subsequent condensation to solid bitumen leads to low generation and expulsion in natural systems. Differences between the generation/expulsion profiles of the different source-rocks are not only caused by the nature of the organic matter, but the lithology of the rock sample as well. Sturdy lithologies with very high pressure resistance and low permeability lithologies show lowered expulsion efficiency. An increase of the CO2-yields with increasing temperatures and pressures was observed for lithologies with high inorganic carbon content.