Gas and water reservoir differentiation by time-frequency analysis: a case study in southwest China

Since the favorable conditions of gas migration and accumulation in the deep Xujiahe formation of west Sichuan depression, a couple of high-yielding gas wells have been developed successfully, prime examples include X851, X856, X2, X3 and X10, which have confirmed the great exploration potential within the area. However, the appearance of some water production wells, such as CX565 and X201, has led to uncertainty within the reservoir prediction. Seismic signal based Time-frequency analysis has been used to distinguish the gas reservoir from water-bearing reservoir in Xujiahe formation. When the seismic wave propagates in the gas and water reservoir, it is difficult to identify the dynamic differences, in terms of amplitude, frequency and energy in the time field. Given that time-frequency analysis allows transformation of the seismic data from the time field into the time-frequency field, it is employed to highlight the seismic responses of gas reservoirs. Usually, the characteristics of ‘low-frequency shadows’ and ‘high-frequency attenuation’ are very obvious in gas reservoirs, while there is no such indication in water reservoirs, which can help to distinguish the gas reservoir from water reservoir. In this paper, we present the theoretical approaches and the in-situ model analyzing results, as well as, a case study. The application of time-frequency analysis on the deep Xujiahe formation in the western Sichuan Depression, provides robust indications for reservoir prediction of gas and water differentiation and validates investigation.