Standard interpretation

Standard interpretation is the process of determining volumes of hydrocarbons in place from wireline logs, or log interpretation. This process requires four basic steps:

Determine the volume of shale. Shale affects the response of the various logging devices. To interpret the response for porosity or saturation, the volume of shale must be determined.

Determine the porosity. Porosity is the fraction of the total rock available for the storage of fluids.

Determine the formation water resistivity (Rw). The resistivity of the water (without hydrocarbons) is used to interpret the formation resistivity for saturation.

Determine the water saturation (Sw). A resistivity model is interpreted for saturation. This model relates water saturation, porosity, water resistivity, and volume shale.

Estimation of shale volume

The volume of shale (Vsh) is best estimated by logging measurements that respond primarily to shale, in particular, gamma ray and spontaneous potential (SP). The most common methods for estimating shale volumes from gamma ray and SP logs are outlined here. Other measurements can be used under special conditions to estimate shale volumes, such as the resistivity in very high resistivity formations, the compensated neutron in very low porosity formations, or density versus neutron crossplots in known lithologies.

Shale volume from gamma ray

Figure 1 Nonlinear shale volume chart. Copyright: Western Atlas International, 1985; courtesy of Atlas Wireline Services Division of Western Atlas International, Inc.

Application

The gamma ray can be an excellent estimator of shale volume in areas with little uranium and where radioactive salts are associated primarily with clay minerals.

Method

The two most frequently used methods of estimating shale volume from the gamma ray log are the linear and the nonlinear estimators. Both methods require determining the gamma ray response (GR) at the depth of interest and determining the response associated with a clean reservoir having no shale (GRcl) and a zone of 100% shale (GRsh).

The linear method is the simplest, but it tends to overestimate shale in the intermediate ranges of shale volumes. The linear response equation is

The nonlinear method begins by estimating the shale volume from the linear equation and then correcting that estimation using a chart as shown in Figure 1. First, one marks the horizontal axis (labeled “radioactivity index”) with the linear estimation of shale volume. One then proceeds vertically to the appropriate rocks and then horizontally to read the corrected volume of shale.

Advantages

The gamma ray method is very simple, fast, and generally the most reliable. It can be used with the potassium or thorium curves and with the uranium corrected total gamma ray curve from the spectral gamma ray.

Limitations

Gamma ray readings must be corrected for hole size first. This method does not work well in areas where radioactivity is not primarily associated with the clays, such as in feldspathic sands.

Shale volume from spontaneous potential (SP)

Application

The SP can be an fair estimator of shale volume in areas where mud filtrate and formation water resistivities contrast.

Method

The estimation of volume shale from the SP requires determining the SP response at the depth of interest and determining the response associated with a clean reservoir with no shale (SPcl) and a zone of 100% shale (SPsh). The response equation is

Advantages

The SP method is very simple and fast.

Limitations

This method does not have good vertical resolution. It overestimates shale volume in hydrocarbon-bearing zones, and it is ensitive to the selection of clean reservoir and shale points. It will not work in zones where Rw ≈ Rmf or in oil-based muds.