Expulsinator assessment of oil/ gas generation and expulsion characteristics of different source rocks

Abstract Investigation of expulsion and primary migration usually suffers from inadequate methodologies failing to simulate natural conditions prevailing during oil and gas formation. The Expulsinator device provides a unique methodology to study these processes under near-natural conditions of pressure as well as near-natural constellation of source rock and reservoir. Eight source rocks, differing in lithology and kerogen type ((type-I, type-II, type-IIS and type-III) were treated with the Expulsinator to simulate oil and gas generation and expulsion with increasing subsidence. A standardised pressure and temperature program covering the main oil window was used, simulating pressure regimes, corresponding to 2000 m, 2500 m and 3000 m of depth at pyrolysis temperatures of 300 °C, 330 °C and 360 °C, respectively. The semi-open setup of the Expulsinator enabled the sampling of the products generated during the experiment, providing unique generation/expulsion profiles for each source rock. Expulsinator treatment by continuous withdrawal prevents the formation of condensation products and solid pyrobitumen, which are common artefacts of closed-mode pyrolysis and misleading in maturity assessment. In particular, the Rock-Eval Tmax-value is affected by pyrobitumen formation. The oil and gas generation/expulsion profiles show large differences between the source rocks, controlled by the organic matter composition, as well as the lithological characteristics. Earliest onset of hydrocarbon generation and expulsion was observed for source rocks containing kerogen type-III, followed by sulfur-rich type-IIS and type-II. Lacustrine type-I source rocks showed the latest onset of hydrocarbon generation. Lithology was an important factor for the observed generation/ expulsion behaviour under the experimental pressure and temperature conditions. Competent lithologies with a high degree of cementation and/or low permeability showed limited expulsion efficiency; in contrast, expulsion was enhanced by pressure-induced “squeezing” effects in ductile lithologies. A highly permeable but competent sample reached only moderate expulsion efficiency due to low “squeezing” response upon burial pressure.