Log-Evaluation Of Low-Resistivity Pay Sands In The Gulf Coast

Gulf Coast pay sands are not always apparent on resistivity logs. High porosities, low formation water resistivities, and shaliness are common. The fine grain and silty sands are characterized by high irreductible water saturations. The resistivities of clean water sands range from approximately 0.2 to 1.0 ohm meter; shaliness increases this RO value. Silty pay sands may exhibit resistivities within this same range. Thus, recognizing pay zones with only a resistivity log is often difficult - if not impossible. The problem can be solved by a judicious combination of resistivity and porosity logs. Three types of porosity logs are now available: The BHC (sonic), FDC (density) and SNP (Neutron). In addition, SP and Gamma Ray curves, and sidewall samples, are beneficial in the study of shaly sands. When all three porosity logs are available, the effective porosity, shale content, and the distinction between gas and oil are possible. Resistivity values can then provide a reasonable estimation of water saturation. When no gas is present, two porosity logs, i. e. density with either sonic or neutron, permit an estimation of effective porosity and shaliness. To a great extend, the neutron and sonic give nearly the same values in the absence of gas, and any one of the two can be combined with the density. The neutron has the advantage over the sonic at shallow depths, but unaccounted gas will make the neutron pessimistic. When only one porosity log is available, a minimum of information will be obtained. An Rwa log incorporating sonic and resistivity data has proved to be very effective for finding pay zones, whether gas or oil bearing, but good values of porosity cannot be obtained in shaly sands. The neutron is an effective substitute for the sonic except in gas sands. The use of density permits good porosity values, but the combination resistivity - density is pessimistic in shaly sands and may miss some shaly producing intervals. Because the equations used in quantitative evaluation of shaly sands are complex, the computations are best handled by computer.