Abstract

In the well log analysis of carbonate reservoirs it is critical to determine the pore type and the wetting phase (i.e. water or oil). Of course the best way to determine this critical information is by petrographic analysis (i.e. LOOK at the ROCKS). However, there are times when an examination of the rock data is not possible, and therefore the geologist/engineer must use log data to determine pore type and wetting phase.

The first step is to determine multiple values for porosity including: 1.) total porosity from the neutron-density log, 2.) matrix porosity from the sonic log, and 3.) connected porosity from the shallow resistivity log. The second step is to calculate values for Archie water saturation (Swa). But what values are you going to use for a, m and n? Therefore, we must assume values for a, m and n, and the values we select are a=1.0, m=2.0 and n=2.0. When we select these values we have just concluded that the reservoir has intergranular or intercrystalline porosity, and is water-wet. The next step is to find out if our assumptions about pore type and wetting phase are correct. In order to do this we also need to calculate water saturation by the Ratio Method: Swr = [(Rxo/Rt)/(Rmf/Rw)]^0.625. Note in the equation for ratio water saturation there is no value for porosity, therefore the need for values of (a) and (m) is eliminated. In addition to what we have already calculated we also need to determine if the hydrocarbons in the reservoir are moveable by calculating the moveable hydrocarbon index: Sw/Sxo = [(Rxo/Rt)/(Rmf/Rw)]^0.5.

For example: if neutron-density porosity is equal to sonic porosity and resistivity porosity, and Archie and Ratio water saturations are equal, the reservoir has intergranular or intercrystalline porosity and is water-wet. In addition, if moveable hydrocarbons are indicated (Sw/Sxo<0.6) the reservoir should produce hydrocarbons. However, if neutron-density porosity is greater than both sonic porosity and resistivity porosity, and Archie water saturation is much less than Ratio water saturation, the reservoir has vuggy porosity and is water-wet. In addition if moveable hydrocarbons are not indicated (Sw/Sxo>0.6) the reservoir should produce water.

Nine examples will be presented to illustrate the following types of reservoirs: 1.) intergranular, 2.) intercrystalline, 3.) vuggy poorly connected, 4.) vuggy well connected, 5.) bimodal pore system, 6.) oil-wet, and 7.) fractured. The examples include: 1.) Cretaceous Glen Rose Limestone, 2.) Ordovician Gunton Dolomite, 3.) Pennsylvanian Virgilian Limestone, 4.) Siluro-Devonian Fusselman Dolomite, 5.) Permian Bader Limestone, 6.) Mississippian (Chester) limestones, 7.) Pennsylvanian Atoka Limestone, and 8.) Permian Clear Fork Dolomite. Two of these examples will be from wells with older logging suites (circa: 1957 and 1962).