Influence of ionic composition on minerals and source rocks: An investigation between carbonate-type and sulfate-type lacustrine sediments based on hydrochemical classification.

The geochemical characteristics and distribution of saline lacustrine source rocks are shaped up by the environment and conditions under which they develop. Such environments, in turn, are often heavily influenced by various salt minerals and clay minerals. Therefore, the current study aims to investigate the potential impact of common salt minerals and clays on the formation and chemical changes of source rocks in salt lakes. Instead of relying on conventional classification methods that emphasize salinity, this study divides common petroleum-bearing saline lacustrine sedimentary basins into two types, the carbonate-type and the sulfate-type, based on the hydrochemistry of salt lakes. Experimental comparison of inorganic geochemistry between samples from modern and ancient saline lacustrine sedimentary systems revealed key differences in ion composition, and resultantly in the sedimentary sequences of salt minerals, the transformation of clay minerals, and the evolution of source rocks. It is found that CaCO 3 -bearing minerals are the first to precipitate in both types of salt lakes. However, secondary mineral deposition in carbonate-type saline lacustrine systems features the precipitation of Na 2 CO 3 -bearing alkaline minerals due to the depletion of Ca2+. Meanwhile, the clay in carbonate-type salt lakes contains a relatively high level of smectite and mixed-layer illite/smectite, as well as low content of illite, implying an impediment in smectite illitization. In contrast, sulfate-type salt lakes are enriched in Ca2+ but depleted in CO 3 2−, leading to the abundant development of Ca-bearing sulfate mineral deposits such as gypsum. These geochemical differences could be responsible for the propensity of carbonate-type salt lakes to harbor well-developed oil shale, and for the tendency of source rocks deposited in sulfate-type lacustrine systems to produce oils at the immature stage. • Hydrochemical classification is proposed to study saline lacustrine source rocks. • CO 3 2− delays carbonate-type lacustrine sedimentary system's smectite illitization. • CO 3 2− and Ca-bearing sulfate minerals may play key role in hydrocarbon generation. [ABSTRACT FROM AUTHOR]