Deposition models for the widespread Eocene bedded halite in China and their implications for hydrocarbon potential of salt-associated mudstones

Abstract

Abstract A group of Eocene saline lakes were developed in the middle latitudinal arid zone across East Asia. In these lakes, bedded halite and laminated mudstones were alternately deposited in lake centers, leading to a long-standing controversy about their depositional water conditions. Here we selected three representative Eocene saline lakes, spanning east to west over China, and made comparisons of their minerology and minor element abundances. Minor elements of solid halite can reflect the relative abundances of impurities: quartz (Zr, Hf), K-silicates (Rb, Th, Cs, Pb, Zn), clays (Co, Cr, V, Ni, Li), carbonates (Sc, Cu, Sr), sulphates (U, Ba), and organic matter (Mo, Cu, V, Ni), and thus can indicate depositional environments. Halite in the east Dongpu Depression is mainly composed of chevron crystals and comparatively enriched in most elements. Halite in the central Qianjiang Depression mainly consists of tightly-cemented coarsely-crystalline mosaics and is depleted in most elements, except for elements Cu, V, and Ni. Whereas halite in the west Mangya Depression underwent syndepositional and burial recrystallization, and contains moderate abundances of most minor elements, with significant enrichments in elements Sc, Cu, and V. Saline lakes in the three depressions represent three typical continental settings for bedded halite deposition: a stratified shallow-water lake in the Dongpu Depression, a stratified deep-water lake in the Qianjiang Depression, and a mixed shallow-water lake in the Mangya Depression. These depositional models provide implications about the hydrocarbon exploration potential of salt-associated mudstones: highest in the stratified deep-water lakes, medium in the stratified shallow-water lakes and lowest in the mixed shallow-water lakes.