Geochemical features of Eurasian evaporites in the context of the chemical evolution of seawater in Phanerozoic

Abstract

Studies of evaporites provide new data to characterize the seawater chemistry in the Early Paleozoic and Middle Mesozoic. In particular, we studied the fluid inclusions in halite from Ordovician (China) and Cretaceous (Laos) evaporites. The corresponding sections on the plot of Ca/SO4 oscillations curve in the Phanerozoic seawater are updated. The calcium content in seawater concentrated to halite precipitation stage was 45.6 mol %, 485 million years ago and 24.3 mol % 112.2–93.5 million years ago. By analyzing the previously published and new factual material, it is established that in Permian evaporites the sulfur isotopic composition is inversely correlated with the sulfate ion content in evaporite basin brines. Thus, the evolution of seawater chemistry in Permian is confirmed by the evolution of the isotopic composition of dissolved seawater sulfate. According to the generalization of 38 Phanerozoic marine evaporite formations, it was found that the peculiarities of the clay minerals associations correlate with the change of the seawater chemical type. Clay minerals associations precipitated from the SO4-rich seawater are characterized by a larger set of minerals, among which smectite and mixed-layered minerals often occur; Mg-rich clay minerals (corensite, paligorskite, sepiolite, talc) also occur. Instead, in the associations of evaporite clay minerals formed from the Ca-rich seawater are represented by the smaller amount of minerals, and Mg-rich minerals are extremely rare. The increased content of magnesium in seawater of SO4-rich type is the main factor in the formation of Mg-rich silicates in evaporites. The composition of clay minerals associations depends on the evaporate basin brine concentration; with its increase, unstable minerals are transformed, which theoretically leads to a decrease in the number of minerals in the associations. However, it was found that evaporite deposits of higher stages of brine concentration often still contain unstable clay minerals – products of incomplete transformation of a significant amount of pyroclastic material from coeval volcanic activity. The main factor determining the composition of clay minerals associations of Phanerozoic evaporites was the seawater (and basin brines) chemical type. Geochemical studies of scattered organic matter and fluid inclusions with hydrocarbon phase in evaporites of the Upper Pechora Basin (overlying oil and gas deposits) indicate the presence of allochthonous bitumoids and allow to use this method to predict oil and gas potential of other areas. Analysis of the results of oil and gas exploration in a number of areas of the Transcarpathian Trough indicates the presence of fluid-saturated reservoirs and the prospects for the discovery of new accumulations of hydrocarbons. Geochemical studies proved the effectiveness of gas-flow survey method for oil and gas exploration, assessing the prospects for fluid saturation of seismic structures.