# Water saturation distribution in a reservoir

Series Exploring for Oil and Gas Traps Treatise in Petroleum Geology Predicting the occurrence of oil and gas traps Predicting reservoir system quality and performance Dan J. Hartmann, Edward A. Beaumont Web page AAPG Store

The distribution of water saturation (Sw) values within a reservoir depends on the height above free water, hydrocarbon type, pore throat-size distribution, and pore geometry. Mapping Sw distribution in a reservoir helps us predict trap boundaries.

## BVW

Bulk volume water (BVW) equals porosity (Φ) × Sw. In zones with the same pore type and geometry, BVW is a function of the height above the free water level. Above the transition zone, BVW is fairly constant. Below the transition zone, BVW is variable.

A Buckles plot is a plot of Sw vs. porosity. Contours of equal BVW are drawn on the plot.

• Points plot on a hyperbolic BVW line where the formation is near immobile water if the points come from a reservoir with consistent pore type and pore geometry.
• Points scatter on a Buckles plot where the formation falls below the top of the transition zone.

Figure 1 shows how a Buckles plot relates to capillary pressure, fluid distribution, and fluid recovery in a reservoir.

## Limitations of BVW

BVW and Buckles plots can be confusing in interbedded lithologies or in areas where facies changes occur because of changing pore types.

## Sw–depth plots

These illustrate how Sw varies within a hydrocarbon-bearing zone. Variations reflect different pore types and/or height above free water. An Sw–depth plot can be used to delineate three things:

Individual plots can be prepared for wells along dip and strike and correlated to show Sw changes across a reservoir or field. Figure 2 is a hypothetical example of an Sw–depth plot with estimated Sw distribution curves for several flow units for a hydrocarbon-bearing zone in a well.

## Height–sw–pore type diagram

The empirical ternary diagram in Figure 3 is handy for estimating either height above free water, pore type (r35), or Sw for a flow unit when the other two variables are known. For example, if Sw for a flow unit is 20% and the pore type is macro with a port size of approximately 3μ, then the height above free water for the flow unit is approximately 100 ft30.48 m
1,200.001 in
. Assumptions for the diagram include 30°APIAmerican Petroleum Institute gravity oil, saline formation water, and a water-wet reservoir.