Entrapment of petroleum

Figure 4.7 Densities of subsurface oils and gases under equilibrium phase conditions, as a function of pressure and temperature (after England et al., 1987, The movement and entrapment of petroleum fluids in the subsurface . Journal of the Geological Society, London, Volume 144. Reprinted by permission of Blackwell Scientific Publications Ltd.).

Hubbert, M.K. 1953. Entrapment of petroleum under hydrodynamic conditions. Am. Assoc. Pet. Geol. Bull., 37 1954-2026. 

Hubbert, M. King, Entrapment of Petroleum under Hydrodynamic Con-... 

It is clear that the entrapment of petroleum in inclusions is likely to result in gross compositional fractionation, i.e. resulting in the entrapped petroleum becoming enriched in asphaltenes and resins, and such has been observed (Karlsen et al. 1993 Wilhelms et al. 1996). However, there is no reason to assume, nor observations to suggest, that the fractionation affects the typically employed molecular ratios between isomers of biomarkers and related compounds. 

Formation of clean fractures represents an extremely attractive site for entrapment of petroleum inclusions as the surfaces are clean, i.e. without impurities on the quartz surface, poisoning minerals or organic matter, and inclusion formation proceeds rapidly. 

The preceding account has described the fundamental facts and concepts concerning the generation, migration, and entrapment of petroleum in the earth s cmst. It does not tell you how to find and oil field In the old days, petroleum was found by wandering about with a naked light, a spirit of optimism, and a sense of adventure. 

Natural Gas Natural gas is a combustible gas that occurs in porous rock of the earth s crust and is found with or near accumulations of crude oil. It may occur alone in separate reservoirs, but more commonly it forms a gas cap entrapped between petroleum and an impervious, capping rock layer in a petroleum reservoir. Under high-pressure conditions, it is mixed with or dissolved in crude oil. Natural gas termed dry has less than 0.013 dmVm (0.1 gaLlOOO fF) of gasoline. Above this amount, it is termed wet. 

Figure 5.8 Spill differential entrapment of oil and gas according to Gussow (1954) (top) Leak differential entrapment of oil and gas (bottom) (after Schowalter, 1979. Reprinted by permission of the American Association of Petroleum Geologists).

Allan, U.S., 1989. Model for hydrocarbon migration and entrapment within faulted structures. The American Association of Petroleum Geologists Bulletin, Vol. 73, no. 7, pp. 803-811... 

Darcy, H., 1856. Les fontaines publiques de la ville de Dijon. Dalmont, Paris Davis, J.C., 1986. Statistics and data analysis in geology. John Wiley, New York, 646 pp. Davis, R.W., 1987. Analysis of hydrodynamic factors in petroleum migration and entrapment. The American Association of Petroleum Geologists Bulletin, Vol. 71, no. 6, pp. 643-649... 

The relatively smaller proportion of later formed inclusions may reflect the presence of fewer lattice defects in quartz at the higher temperatures when diagenesis has proceeded for substantial time. Thus, diagenesis has already covered up irregularities on the detrital grains with their mixed wettabilty characteristics where inclusions tend to form (Karlsen et al. 1993 Nedkvitne et al. 1993). The net result of this process would be the reduced possibihty of petroleum inclusion entrapment. 

We could not detect fluorescent inclusions in the other dry wells, 6406/8-1, 6506/11-3, nor in well 6507/5-1, aU of which contain quartz overgrowths, suggesting that fluorescent petroleum inclusions could have been entrapped had petroleum occurred in these structures. However, a lack of observable fluorescent inclusions alone cannot be taken as evidence for lack of petroleum in the investigated sandstones. The conclusions drawn are based upon comparison, in all wells, of inclusions in the same or related formations, at virtually the same depths. The samples selected for the analysis are uniform in grain size, degree of polygrains and overall sedimento-logical characteristics. 

These cases may serve to underUne the dynamic nature of petroleum entrapment. 

In the case of petroleum inclusions the amount of water is most likely so minute as to not be readily observed during microscopy and one may speculate as to why more three-phase inclusions (containing gas, petroleum and water) are not observed. It seems from microscope observations tempting to suggest that the inclusion mechanism is to some extent binary , i.e. water either with or without HCs in solution is entrapped as inclusions, or pure petroleum inclusions form. In the case of the petroleum inclusions is it likely that the bulk of the water which is not lining the inside of the inclusion is simply partly dissolved in the oil or exists as part of the gaseous phase in the gas bubble under ambient conditions. It would take much more water to make the phase a discrete microscopically observable one and such quantities seem empirically unlikely. 

In most of our reservoir studies on clastic reservoirs we come across cases with generally more petroleum inclusions having formed in the upper and most permeable and porous layers of the reservoirs, and this may reflect that oil has occurred for more time in the upper part of the reservoir, giving inclusions more time to form than deeper down. Still, it could also simply be that the higher pressures in the oil column in the top of the reservoir as compared with farther down forces the oil into more intimate contact with the cementing quartz grains and that this results in more entrapment of oil as inclusions higher up in the oil column. 

Gussov, W. C. 1954. Differential entrapment of oil and gas, a fundamental principle. American Association of Petroleum Geologists Bulletin, 38, 816-853. 

Some anti-foam materials are based on silicone resins, others are based on petroleum oils, and all of them are intended to depress the surface tension of the latex emulsion. The anti-foam agents delay the entrapment of air in mixes which is always a possibility vdien mixing liquids and solids, such as a mortar in a cement mixer. This results in a denser mortar which has better abrasion resistance, better resistance to water absorption, and higher compressive, tensile and bond strengths. 

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