Log evaluation of deep Ellenburger gas zones is complicated by the low porosities encountered, and by formation matrix and fluid effects on log responses. In these very low porosities even small inaccuracies in porosity determination produce proportionally large errors in reservoir evaluation.
Each of the logging devices commonly used for porosity measurement—sonic, density, and neutron—is affected by variations in formation lithology and fluid content. While the deep Ellenburger is primarily dolomite, variations in lithology produce inaccuracies in the porosities determined from any one log. Furthermore, gas saturation in the formation close to the borehole affects the neutron and density responses; the neutron gives values of porosity that are too low; the density, too high. While no gas effects have been observed on the sonic response this tool characteristically is pessimistic in zones of secondary porosity. Therefore, no single device reliably affords accurate porosity determination.
The most reliable values of porosity are obtained through a complex interpretation procedure using data from all three logs—sonic, density, and neutron. Computer processing makes this procedure feasible. This computer processing gives not only an improved value of porosity, but also the major lithologic components of the formations, and the gas and liquid saturations of the zones close to the borehole.
Resistivity logs are used in conjunction with the previously mentioned porosity logs. Resistivity values are expressed in terms of bulk volume of water, as in the MOP technique, for comparison with the porosity data.
The analog machine printout of the computed data simplifies the use of logs for formation evaluation and selection of intervals of completion.
The deep Ellenburger gas play is characterized by extremes. The wells are extremely deep—and are correspondingly expensive to drill and complete.
