Shale Reservoir Characterization
The schematic is much more complicated for unconventional reservoirs:
In these types of reservoirs, we calculate the four different porosity components:
- Effective porosity
- Total organic carbon (TOC)
- Clay porosity
- Free shale porosity
A standard petrophysical interpretation is used to define basic components of the reservoir:
TOC is calculated from either the Passey or Schmoker techniques:
Passey, et al
In the Passey technique the responses of the resistivity and porosity logs are identified in organic-lean intervals:
These are then use to calculate a DlogR difference, which represents the TOC contribution. This approach was created from clastic sequences, and may not be appropriate in carbonates.
Schmoker
This model calculates a TOC from RhoB directly, and has three different correlations available:
- High Appalachian
- Low Appalachian
- Williston Basin Bakken
These models are more appropriate in carbonates.
Shales
The next step is to estimate clay porosity using the shale only density/neutron cross plot.
The free shale porosity can then be calculated as the remaining porosity after removing effective and clay porosities from the total porosity.
A check on the validity of the model is made by comparing free available porosity with effective porosity:
Free hydrocarbons are located in the available free porosity, which is defined as the effective porosity plus the free shale porosity. Adsorbed hydrobarbons are calculated using published epirical relations. Comparisons can then be made on the cumulative OIP of each:
Example
See Also