Sedimentary characteristics of the Lower Cambrian Niutitang shale in the southeast margin of Sichuan Basin, China

Abstract

The Lower Cambrian Niutitang formation shale in the Sichuan Basin is not only a source rock for the Sinian conventional gas reservoir but also a possible reservoir for shale gas. However, very limited work have been focused on the detailed sedimentary environment related to shale gas exploration. In this study, detailed field geological investigation, continuous sampling and analyses by SEM, EDS XRD, XRF and ICP-OES, were used to probe provenance and sedimentary environment of the Niutitang formation shale in the southeast margin of Sichuan Basin. Both field outcrop observation and laboratory experimental results indicate: 1) the presence of framboidal pyrite; 2) δU ≥ 1; and 3) a negative δCe index anomaly. The detrital mineral properties and major element analyses indicate that the geotectonic background of the sediment sources for the Niutitang formation shale can be linked to the neritic zone and slope zone. The horizontal bedding indicates that suspended sediments were involved in the sedimentation. The Ba minerals, REE characteristics and trace element index (U/Th) demonstrate that most of the study area was formed in a normal sedimentary environment, but several isolated zones were affected by submarine hydrothermal sedimentation. The sedimentation of the Niutitang shale most likely occurred in a weak hydrodynamic, deep water, oxygen-deficient reducing environment. The major sediment sources were continental margins, such as the neritic and continental slope zones.