Changes

Particle size can be determined from gamma ray, “porosity”, and resistivity logs. Grain-supported rocks commonly have lower gamma ray activity then do mud-supported rocks. Other fine-grained rocks, such as shaley and organic-rich carbonates, commonly have the highest gamma ray activity. However, the level of gamma ray activity in some carbonates (dolostones in particular) is not related to particle size because of the presence of anomalous concentrations of uranium. Water saturation is a function of particle size and interparticle porosity, and crossplots of porosity, water saturation, and reservoir height can be used to determine particle size.

Particle size can be determined from gamma ray, “porosity”, and resistivity logs. Grain-supported rocks commonly have lower gamma ray activity then do mud-supported rocks. Other fine-grained rocks, such as shaley and organic-rich carbonates, commonly have the highest gamma ray activity. However, the level of gamma ray activity in some carbonates (dolostones in particular) is not related to particle size because of the presence of anomalous concentrations of uranium. Water saturation is a function of particle size and interparticle porosity, and crossplots of porosity, water saturation, and reservoir height can be used to determine particle size.

Interparticle porosity can be determined by subtracting separate vug porosity from total porosity. Total porosity can be calculated from porosity logs, while separate vug porosity can be estimated from crossplots of acoustic transit time versus crosspiot porosity. Touching vug pore systems can be identified using borehole televiewer and resistivity scanner logs. (For more details, see the chapter on “[[Borehole imaging devices]]” in Part 4.)

Interparticle porosity can be determined by subtracting separate vug porosity from total porosity. Total porosity can be calculated from porosity logs, while separate vug porosity can be estimated from crossplots of acoustic transit time versus crosspiot porosity. Touching vug pore systems can be identified using borehole televiewer and resistivity scanner logs. (For more details, see [[Borehole imaging devices]].)

Lithology can have a major effect on porosity logs. Gypsum is of particular concern in carbonate reservoirs because it contains large quantities of bound water. The bound water is seen by neutron logs as porosity, resulting in erroneously high porosity indications.

Lithology can have a major effect on porosity logs. Gypsum is of particular concern in carbonate reservoirs because it contains large quantities of bound water. The bound water is seen by neutron logs as porosity, resulting in erroneously high porosity indications.