We applied a new processing algorithm to extract intrinsic attenuation (1/Q) from the head P-wave of a full waveform sonic log. The sonic log was acquired in an oil reservoir in northeast Texas (USA). The reservoir is a sand-shale sequence characterized at the pore, core, and borehole scales. We found that the attenuation correlates with the lithology, including a sandstone zone partially saturated with oil and water. A comparison of 1/Q with the microseismogram and the petrophysics of other well logs provides a way of evaluating the consistency of Q at each borehole depth location. An analysis of the results explains the different attenuation anomalies found in the Q log.