Display Lithology Crossplots

Fundamental of any log interpretation of carbonate formations is the determination of lithology. Because carbonate reservoirs often contain more than one mineral, it is vital to correctly define the lithology for accurate porosity calculations. Several crossplots are available to aid in determining lithology. The crossplots include dual porosity plots (density-neutron, sonic-neutron, and density neutron) tri-porosity plots (M&N plots and MID) and the PE Plot using the lithodensity log.  For each crossplot lithology lines or mineralogy points are displayed and users can highlight points in specified depth ranges as desired to see those points fall in the crossplot. To help identify zones, the crossplots are accompanied with well logs showing Vcl and porosity.

From these crossplots users can quickly identify the lithology. Notes about the crossplots are available which provide insight into ways to verify lithology such as gas effects, or vuggy carbonates moving points in the lithology plots.

Specify Lithology and Compute Porosity

Analysts can use CBA's lithology crossplots as guidelines for selecting a formations lithology. The user has the option of specifying the lithology as a single matrix, dual matrix, or tri-matrix system with matrix components of sandstone, limestone, dolomite, or anhydrite.  For formations with shale, clay volume can be computed from a variety of models and an effective porosity computed corrected for shale.

Once lithology is defined, porosity can be computed from the porosity logs or resistivity logs. A table of results including volume of each matrix and porosity is displayed along with two computed logs. The first log shows the different volumes of lithology (red pluses for anhydrite, purple slanted bricks for dolomite, blue brickes for limestone, yellow dots for limestone and black dashes for shale). The second log shows porosity. This display provides a quick look at the matrix makeup of the formation.

Pore Type Crossplot

Although determining the correct lithology is important in the porosity equations, calculating reliable water saturations in carbonate reservoirs requires accurate values of the cementation exponent, m. If core data is available, a por-m relationship can be developed, then m can be computed using log porosities and the por-m relationship. Without core data, several models are available for calculating m from logging data.  Choosing which variable m model to use requires some knowledge of the pore type. Helping the analyst determine pore type from logging data is a significant feature of the CBA program.

In carbonate reservoirs, pore type is recognized as being  intercrystalline/intergranular, fracture, vuggy, or bimodel. In CBA, the dominant pore types can be distinguished for different zones by using a series of logging crossplots.  The five crossplots available for selecting pore type are the SwArchie vs. SwRatio, Porsonic vs. Portotal, Porres vs. Portotal, M-N, and Dew plot. Where logging data falls in a crossplot indicates the pore type. To assist the user, the different pore type regions are color coded on the crossplot and color coded op the well log shown next to the crossplot easily identifying which zones are vuggy, fractured or other pre type. Through these crossplots both the logging expert and occasional log analyst can easily determine pore type.