Reservoir Partitioning

COUPLES, GARY D., University of Glasgow, Glasgow, Scotland, HELEN LEWIS, Resource Consulting Group, Helensburgh, Scotland, and COLIN P. NORTH, University of Aberdeen, Aberdeen, Scotland

The main causes of reservoir partitioning are primary sedimentation, diagenesis, hydrocarbon separation, production activities, and structural effects. Our primary thesis is that reservoir behavior depends on the interaction of these factors, and that single-topic explanations are generally faulty. Our secondary thesis is that structural influences are universal, even if not dominant.

Most significant reservoirs are in layered rocks, sedimentation yielding the original stratigraphic succession as well as lateral and vertical inhomogeneities within layers. All subsequent processes are consequent to this original distribution of material elements.

Diagenesis operates on the exact rock mass and pore networks; these might be the result of original sedimentation, or of previous diagenesis, fluid separations, and structure. Typically, diagenesis alters the distribution of porosity and permeability both within layers, and at their interfaces. Reservoir fluids also can affect the distribution of permeabilities. For certain conditions and hydrocarbon compositions, separation, or degradation of viscous phases can create barriers. Production operations also can lead to separation of fluids, as when a two-phase system changes to three phases; relative permeability alterations are the result.

Only in the rarest circumstance could a reservoir develop in a totally undeformed rock mass; even where deformation is slight, essentially all reservoirs are deformed. Because the rocks are layered, deformation style is, for nearly all reservoirs, partially controlled by this mechanical layering. Deformation patterns and active mechanisms (enhancing or degrading porosity and permiability), depend on both loading conditions and material properties. Thus, the distribution of structural effects is fundamentally linked to the original lithesomes as well as all subsequent alterations to them. Since most reservoirs are filled after structural development, partitions caused by fluid separations might well be themselves associated with deformed zones, as might additional phases of diagenesis. /P>

The collage of these various effects constitute the permeability architecture of a reservoir. Understanding the effects on flow of the barriers and conduits is attained using a simulator, although present products are generally too simplified to account for the empirical knowledge of complex reservoir partitions.

AAPG Search and Discovery Article #91007© 1991 AAPG International Conference, London, England, September 29-October 2, 1991 (2009)