Page 196 - Geology and Geochemistry of Oil and Gas

PRIMARY ACCUMULATION AND FREE PHASE MIGRATION 165
At the first stage of formation of accumulations, the traps at great depth, in the
way of migrating hydrocarbons, are filled with gas and oil. The traps higher up the
slope are filled with water. If the regional structure changes later, the uplifted traps
may contain large hydrocarbon reserves. Peripheral accumulations may be totally
destroyed in the case of very significant uplifts. In the case of favorable scenario,
however, large oil fields or bitumen deposits could form (e.g. Devonian reservoirs in
the Romashkino Field in Russia and the Athabasca bitumens in Canada).
The density differentiation occurs easier in the moving fluids. Individual oil or gas
droplets float to the water surface, coalesce and, if the conditions are favorable, form
a large mass of hydrocarbons. The water is capable of displacing this mass by the
pressure of the hydraulic head.
It is important to elaborate here on the tilted water–oil contacts. Capillary
pressure curves, along with relative permeability curves, can be employed to explain
tilted water table conditions in the absence of hydrodynamic causes. In the
hypothetical example given by Arps (1964) in Fig. 9.4, the porosity and the
permeability of the formation decrease from left to right with resulting increase in
the transition zone thickness, as shown by the capillary pressure curves.
Some generalization of spatial pattern of changes in the properties of fluids (first
of all, density) was proposed by Buryakovsky (1973, 1974). The space pattern of
changes may be subdivided into the changes of the first and second order.

Fig. 9.4. Tilted oil–water contacts with lateral variation in rock characteristics (after Arps, 1964, Fig. 3;
courtesy of AAPG).