Lee Sanders & Calvin Kessler, Halliburton Logging Services
Recent advances in the design of sonic logging tools and in the development of computer software have provided methods for the detection of hydrocarbons in porous formations. The detection of hydrocarbons is based on changes in the acoustical proper-ties of the formation, such as compressive wave travel time, Tc, shear wave travel time, Ts, attenuation of the acoustic energy, and changes in the frequency of the acoustic signal. The compressive and shear wave travel time to the compressive wave travel time is termed the velocity ratio. Extensive prior research has demonstrated that each rock type has a fairly specific velocity ratio. However, when compressible fluids occupy a portion of the pore space in the rock, the compressive wave travel time increases, which results in a lower than expected velocity ratio for a given lithology. One of the limitations of using the velocity ratio as a method for detection of compressible hydrocarbons is that the lithology must be known. Interpretation methods have been developed which combine the velocity ratio and Photoelectric absorption index, Pe. This technique enables the velocity ratio to be used for the detection of hydrocarbons in formations where there may be changes in the lithology. The velocity ratio has been compared to a synthetic velocity ratio which is calculated from the volumetric fractions of the various lithologies present in the formation. This method has been used successfully in formations composed of a mixture of many lithologies. Several log examples will be presented that show these techniques have been successful for the detection of hydrocarbons.