Source rocks expulsion from

INAN, S., Yalcin, M. N. Mann, U. 1998. Expulsion of oil from petroleum source rocks, inferences from pyrolysis of samples of unconventional grain size. In Horsfield, B., Radke, M., Schaefer, R. G. Wilkes, H. (eds) Advances in Organic Geochemistry 1997. Organic Geochemistry, 29,45-61. 

Rather than estimating reservoir trapping efficiency from seal capacity measurements, gas volumes expelled from the source rock (equation (13)) were compared with gas volumes computed from conventional reservoir engineering approach (equation (14)). These comparisons provide an indication of the range of possible trapping efficiencies in each prospect area. Additionally, the distribution of source rock expulsion efficiency was computed using a Monte Carlo simulation technique and equation... 

Dilppenbecker, S. J. Welte, D. H. Petroleum expulsion from source rocks insights from geology, geochemistry and computerized numerical modelling. Thirteenth World Petroleum Congress, 1992 165-177. 

After primary migration has taken place, a certain proportion of the generated hydrocarbons remains in the pore system of the source rock (Hunt, 1979). The oil fraction that remains in the source rock will be cracked to gas as the source rock is buried to greater depths and temperatures (Section 3.1.5). The effect of primary migration of hydrocarbons can be indicated by the expulsion efficiency. The petroleum expulsion efficiency is the ratio of the expelled petroleum and the sum of the generated and initial petroleum and can vary from zero (no expulsion) to 1.0 (complete expulsion) (Cooles et al., 1986). The expulsion efficiencies are not uniform in time and space (Leythaeuser et al. 1987b). They depend on the tsrpe of source rock, its richness and thermal maturity and the primary migration mechanism. 

The expulsion of gas is probably very efficient. Altebdumer (1982, in Cooles et al., 1986), for instance, showed that gas expulsion from the gas-prone Lias 5 shales, N.W. Germany, is very efficient with up to 95% of generated gas being expelled from the source rock. As outlined in Section 3.2.2.1, gas generation... 

The rate of hydrocarbon expulsion from mature rich oil-prone source rocks is about 8 x l(>-i to 8 x 10- m m- s-, according to a rough estimate made by England et al. (1987). England et al. s calculations are based on the subsurface conditions given in Table 3.4. 

Oil expulsion from good oil-prone source rocks is very efficient, whereas oil expulsion from leaner source rocks is relatively inefficient. Probably, most of the oil generated in leaner oil-prone source rocks will remain in the source rock and be cracked to gas at higher temperatures and expelled as gas condensate followed by dry gas. Gaseous solution cem be an effective migration mechanism for oil generated from mature type III kerogen. The expulsion of gas is very efficient. 

Secondary hydrocarbon migration is the movement of hydrocarbons after expulsion from a source rock through carrier and reservoir rocks or fault and fracture systems. 

These estimated specific discharges for the buoyancy-driven migration of oil through a carrier rock are about 5 orders of magnitude greater than the estimated expulsion rates for oil from source rocks as given in Chapter 3. 

In addition to the losses of hydrocarbons during secondary migration indicated by Equation 4.25, additional losses of hydrocarbons can be expected to occur by accumulation of small non-economic volumes of hydrocarbons in miniature traps present along the migration path, and by dissolution and diffusion of especially hydrocarbon gases after their expulsion from the source rock (Chapter 3.2 Slvdjk and Nederlof, 1984). 

Under hydrostatic conditions the basin-wide secondary hydrocarbon migration patterns and consequently also the final distribution of the oil and gas accumulations in a sedimentary basin are closely linked to the stable basin geometry present during hydrocarbon expulsion from the source rocks. 

The hydrocarbons expelled from the mature source rocks in separate phase, may initially occur in a very finely dispersed state. At depths corresponding to the peak phase of hydrocarbon expulsion in actively filling and subsiding basins, the hydrodynamic condition is characterized by the intermediate or the deep subsystem of burial-induced groundwater flow. Initially, the very finely dispersed hydrocarbons will move along with the burial-induced groundwater... 

Between 120 and 150 °C the oil expulsion is very eflScient (c. 60 - 90%) for good oil-prone source rocks with initial petroleum potentials greater than c. 0.01 kg/kg of rock (Mackenzie and Quigley, 1988). The oil expulsion from leaner oil... 

The precise mechanisms by which expulsion of petroleum occurs are not fully understood, although pressure and to some extent temperature are of importance (England et al. 1987). Different mechanisms may operate in different types of source rock (Stainforth Reinders 1990). One possibility is that hydrocarbons move through microfractures in the source rock under the influence of over-pressure (Tissot Welte 1984), and compaction plays a part (Braun Burnham 1992). An increase in volume during the liberation of hydrocarbon fluids from the solid kerogen matrix would contribute to over-pressure development, but evidence for it is equivocal (Osborne Swarbrick 1997). Microfrac-turing will reduce capillary pressure and so reheve over-pressure by allowing the escape of hydrocarbons. 

Generation and expulsion. In the western source kitchen, petroleum generation from Upper Jurassic source rocks commenced towards the end of the Cretaceous (56.5 Ma bp) and was almost complete by mid Miocene times (10.4 Ma bp) when transformation ratios of 0.9 were reached. In the easterly kitchen, generation from the Kimmeridge Clay Formation (KCF) source rock was only just starting at this time (TR<0.3). At 10.4 Ma bp, no generation has occurred from Upper Jurassic... 

---