•900 bar water pressure increased the viscosity of oil via combination reaction.•Oil and n-C16 cracking to gas is retarded at 900 bar compared to lower pressures.•Oil may be more thermally stable in deep petroleum basins than currently thought.This study reports a laboratory pyrolysis experimental study on oil and n-hexadecane to rationalise the thermal stability of oil in deep petroleum reservoirs. Using a 25 ml Hastelloy pressure vessel, a 35° API North Sea oil (Oseberg) and n-hexadecane (n-C16), were pyrolysed separately under non-hydrous (20 bar), low pressure hydrous (175 bar) and high liquid water pressure (500 and 900 bar) at 350 °C for 24 h. This study shows that the initial cracking of oil and n-hexadecane to hydrocarbon gases was retarded in the presence of water (175 bar hydrous conditions) compared to low pressures in the absence of water (non-hydrous conditions). At 900 bar water pressure, the retardation of oil and n-hexadecane cracking was more significant compared to 175 bar hydrous and 500 bar water pressure conditions. Combination reactions have been observed for the first time in pressurised water experiments during the initial stages of cracking, resulting in the increased abundance of heavier n-alkane hydrocarbons (> C20), the amount of unresolved complex material (UCM), as well as the asphaltene content of the oil. These reactions, favoured by increasing water pressure provide a new mechanism for rationalising the thermal stability of oils, and for producing heavy oils at temperatures above which biodegradation can occur. Indeed, we demonstrate that bitumen from the high pressure Gulf of Mexico basin has been formed from lighter oil components and it possesses similar characteristics to the laboratory oils generated.

•We report the first study using water pressure of 900 bar and temperature of 420 °C.•The gas yield from coal was retarded at 900 bar compared to lower pressures.•Oil yield and VR for coal were reduced at 500 and 900 bar compared to low pressures.•Oil cracking to gas was retarded at high pressures compared to low pressures.This study investigates the effect of water pressure on hydrocarbon generation and source rock maturation at high maturities for a perhydrous Tertiary Arctic coal, Svalbard. Using a 25 ml Hastalloy vessel, the coal was pyrolysed under low water pressure (230–300 bar) and high water pressure (500, 700 and 900 bar) conditions between 380 °C and 420 °C for 24 h. At 380 °C and 420 °C, gas yields were not affected by pressure up to 700 bar, but were reduced slightly at 900 bar. At 380 °C, the expelled oil yield was highest at 230 bar, but reduced significantly at 900 bar. At 420 °C cracking of expelled oil to gas was retarded at 700 and 900 bar. As well as direct cracking of the coal, the main source of gas generation at high pressure at both 380 °C and 420 °C is from bitumen trapped in the coal, indicating that this is a key mechanism in high pressure geological basins. Vitrinite reflectance (VR) was reduced by 0.16 %Ro at 380 °C and by 0.27 %Ro at 420 °C at 900 bar compared to the low pressure runs, indicating that source rock maturation will be more retarded at higher maturities in high pressure geological basins.

In order to ascertain whether kerogen and the heavy oil or bitumen generated during the initial stages of source rock maturation then go on to produce similar gas yields compared to the natural situation when they are present in source rocks, a study has been conducted on an immature type II Kimmeridge Clay Formation (KCF) source rock (Dorset, United Kingdom), with an initial vitrinite reflectance (VR) of 0.31% Ro and total organic carbon (TOC) content of 14.4%. Water (hydrous) pyrolysis experiments were conducted on the whole immature source rock, the isolated partially matured bitumen-extracted source rock and the isolated bitumen (bitumen generated and extracted from the initial source rock) and mixtures of the isolated partially matured source rock and bitumen using a 22 ml Hastalloy vessel at 320–420 °C for 7–48 h over a pressure range of 115–500 bar. Hydrocarbon gas yields and the increase in VR were greater for the whole rock experiments compared to the experiments on the isolated partially matured source rock and isolated bitumen (no mineral matter present), as well as the experiments on mixtures of the isolated partially matured isolated source rock and bitumen combined. Hydrocarbon gas yields and VR values were found to decrease at 500 bar compared to 180 bar at 350 °C where the effects of pressure retardation were found to be much more significant for the partially matured source rock maturation and bitumen in isolation than for the whole source rock. At 420 °C, gas generation was not affected by pressure, but VR decreased going from 310 bar to 450 bar. The results obtained demonstrate that the interplay of inherent mineral matter, reactant phase, source rock/kerogen, bitumen and pressure are key factors in determining the extent of hydrocarbon generation and source rock maturation in geological basins.Highlights► Gas yields and VR were higher for whole rock compared to isolated rock. ► VR were reduced at 450 and 500 bar compared to 180 and 310 bar at 350 and 420 °C. ► Gas yields were reduced by water pressure at 350 °C but not at 420 °C. ► Pressure and reactant phase were found to be important during maturation reactions.

This study investigates the effect of water pressure on hydrocarbon generation and maturation of coals. Using a 25 ml Hastalloy pressure vessel, two high-volatile coals (Longannet, UK 0.75% Ro, and perhydrous Svalbard (Spitsbergen), Norway 0.68% Ro) were pyrolysed under non-hydrous, hydrous at 175 bar pressure, and high water pressure hydrous (500 bar and 900 bar) conditions at 350 °C for 24 h. The bitumen yield obtained during pyrolysis, together with the Rock–Eval S2, hydrogen index (HI) and vitrinite reflectance (VR) results from the pyrolysed coal residues indicated that water under relatively low pressure (175 bar) hydrous conditions promoted hydrocarbon generation and coal maturation in relation to non-hydrous conditions, consistent with previous work. However, under high water pressure (500 and 900 bar) conditions, a combination of the hydrocarbon gas (C1–C4) and bitumen yields, Rock–Eval S2, HI, VR and solid state 13C NMR results demonstrated that the changes in reaction pathways occurring with increasing pressure resulted in both hydrocarbon generation and maturation being retarded. The observed effect of pressure implies that for Type III source rocks, hydrocarbon generation will be retarded in high pressure geological basins, with gas yields being proportionally reduced more than bitumen yields. Source rock maturation (or coalification) is also retarded, with the decreases in vitrinite reflectance and carbon aromaticity being relatively small but significant in terms of explaining retardation in geological basins.Highlights► Gas yields from two coals are significantly retarded by increasing water pressure. ► Bitumen yield and VR for coals were reduced at 500 and 900 bar compared to 175 bar. ► Residual Rock-Eval S2 and HI for coals decreased at 175 bar compared to 500 and 900 bar. ► If pressure was insignificant results should be similar at 175, 500 and 900 bar.