Application of GEM elemental logging in shale reservoir sedimentary environment identification

Abstract: Background Element capture logging can be used to determine the elemental contents of rocks in formations.  Purpose This study focused on the BY2 well, the inaugural parameter well for shale gas in the Cambrian Niutitang Formation of the Baojing Block, located in the Middle Yangtze region. The aim is to investigate the developmental and distributional characteristics of shale gas reservoirs in this formation.  Methods By interpreting and processing elemental capture logging data, precise elemental compositions of the shale reservoirs were determined. This analysis led to the creation of a comprehensive geochemical index profile for the Niutitang Formation. Additionally, elemental geochemical indicators were used to identify and reconstruct the paleosedimentary environments.  Results The analysis revealed that the predominant elements in the Niutitang Formation’s shale were Si, Al, and Fe, accompanied by lower amounts of K, Ca, Mg, and S. The shale featured relatively high concentrations of Si, Fe, and S, which contributed to its enhanced fracturing ability. The sedimentation process of this shale could be categorized as active continental margin sedimentation. The source material for the sedimentary rocks originated from the Kangdian ancient land, located in the northwest. The sedimentation was primarily normal but was influenced by the presence of hydrothermal fluids in the region’s active tectonic zone.  Conclusions The upper section of the Niutitang formation was subject to a dry climate during its depositional period, featuring gentle slope sedimentation at the periphery of a stagnant basin and a lack of oxygen. High water salinity, ample land supply, and low water body paleoproductivity characterized this region. Conversely, the lower section experienced a humid climate and served as a deep-water retention basin. Here, the land supply was limited, but water salinity and paleoproductivity were high, leading to the accumulation of organic matter. The aquatic setting was primarily anaerobic, conditions that were conducive to the preservation of organic matter, and provided an optimal sedimentary environment for the generation and concentration of shale gas.