High-quality marine shale reservoir prediction in the lower Silurian Longmaxi Formation, Sichuan Basin, China

Abstract

For the lower Silurian Longmaxi marine shale reservoir in the Jiaoshiba area, Sichuan Basin, China, the traditional P-impedance ([Formula: see text]) is insensitive for predicting good reservoirs. As an integral indicator of borehole radioactivity that is substantially corresponding to the organic richness, the uranium curve is qualified to identify the abundance of organic matter. Therefore, to efficiently comprehend the shale distribution in the Jiaoshiba area, we have adopted a so-called pseudo-P-impedance ([Formula: see text]), which is constructed from the P-impedance and the uranium (U) log and spatially computed from impedance inversion of the zero-offset seismic. The [Formula: see text] log shows an obvious correlation with the organic richness of the shale. The thickness of the high-quality shale derived from the inverted [Formula: see text] cube shows a higher correlation with the log data compared to the traditional P-impedance cube. The distribution of predicted high-quality shale is strongly influenced by the fault systems: (1) Although the fault development zone might lead to a distorted image of the seismic and weak shale evaluation, the shale gas cannot be efficiently preserved in these places, and (2) high-quality shale reservoirs are mainly distributed in the not-faulted area and show a general northeast–southwest trend. Generally, the introduced [Formula: see text] inversion is suitable for the seismic frequency range and could be applied to other similar exploration areas.