EFFECTS OF EFFLUENTS AND WATER-PRESSURE ON OIL GENERATION DURING CONFINED PYROLYSIS AND HIGH-PRESSURE HYDROUS PYROLYSIS

In order to test experimentally the effects of hydrostatic pressure on oil generation from source rocks, immature Woodford shale was pyrolyzed in confined (Au cells + external pressure = effluents pressure) and hydrous (water pressure) conditions (260-365 degrees C during 72 h) at pressures ranging between 220 and 1300 bars. Comparisons of the results obtained in both systems also allowed the effect of the presence of water to be investigated. Increasing the external hydrostatic pressure in confined pyrolysis retards the oil potential consumption in the kerogen. However, the effect of pressure on the yields of extractable organic matter is limited. In hydrous pyrolysis, increasing water pressure significantly lowers the total yields of bitumen + expelled fraction and the consumption of the oil potential in the extracted solid residue is correspondingly reduced. The quantitative composition of the pyrolyzates is significantly affected, although, the C15(+) n-alkane distributions do not show significant variations. The total yields of pyrolyzates are similar in confined pyrolysis and hydrous pyrolysis performed between 300 and 700 bars. However, during equivalent time-temperature-pressure experiments, the degradation rates of kerogen and asphaltenes is lower in hydrous pyrolysis than in confined pyrolysis. The very different effects of increasing effluent pressure (confined pyrolysis) and water pressure on oil generation indicate that the nature of the pressurizing medium is an important factor. Tentative explanations are given to elucidate these differences, as well as to estimate the:importance of the effects of pressure in the natural environment.