1962 Paper D

INTERPRETATION OF THE DAKOTA SANDSTONE OF THE SAN JUAN BASIN

R. P. Alger

Schlumberger Well Surveying Corporation Houston, Texas

ABSTRACT

Problems of log interpretation for the Dakota sandstone in the San Juan Basin arise from the character of the formation. Grain-size variations affect the permeability, and thus the water saturation and resistivity measured. Varying amounts of shale affect the determination of porosity from the Sonic Log. Often, the SP is severely dampened due to shaliness, presence of gas, and limited invasion. Rt is easy to measure with the Induction Log, but its value is also affected by the presence of shale. Thus, proper evaluation of both the porosity and water saturation depends upon the use of suitable techniques to eliminate the effects of shaliness.

Studies have shown that the Gamma Ray can be used to correct Sonic-derived porosities. Proportional Gamma Ray deflections are assumed to indicate shale percentages; with these data and knowledge of Dt for pure shales, the effective matrix velocity can be deduced in order to obtain effective porosity from the Sonic travel time.

Two techniques are developed for obtaining water saturation: One uses the effective porosity along with an Rt corrected for the amount of shale present; the other is simpler and uses uncorrected values of Sonic data and Rt. The results are essentially comparable for both methods; so the simpler technique is recommended.

Finally, having both porosity and water saturation, permeability is estimated. A capacity index for the entire Dakota formation can be derived and used as a means of predicting the productivity of the well.