Sonic slownesses are sensitive to the type of formation pore fluid. The sensitivity is large in the case of unconsolidated sands and when gas is present in the formation. The influence of fluid type is reduced in more compact formations. Up to now the use of sonic slownesses for fluid type identification was recommended only in sands with a porosity of at least 20% and to differentiate between liquid and gas. The present study reports how sonic slownesses were successfully used to identify hydrocarbons and to distinguish between oil- and gas-bearing intervals in well-compacted sands in the Neuquen basin in Argentina. The reservoir environment is fluvio-deltaic with the fluid distribution controlled not only by the structure, but also by stratigraphy, and it is common to find anomalous fluid distributions, with oil zones overlaying gas zones. The shallow, low porosity sands produce under various degrees of depletion. In these conditions the traditional neutron-density technique often has difficulties in distinguishing between oil and gas, probably because of intense flushing of the hydrocarbons in the invaded zone. A crossplot technique of Vp/Vs versus compressional ?t is successfully used to identify the hydrocarbon type. This is done at the wellsite and is available during logging to decide on further testing and completion programs. A more detailed evaluation is made on a workstation system at the office. The technique has been used with good success both in open and cased hole conditions. The predictions have been confirmed by production tests and by correlation on more than 20 wells, Examples of evaluations in different wells are presented and compared with other openhole logs. These results show that the use of sonic slownesses for hydrocarbon identification can be extended in some instances to the case of compact, depleted sandstones.

This content is only available via PDF.
You can access this article if you purchase or spend a download.