Characteristics and origin of the Fengcheng Formation water in Mahu depression and its significance to hydrocarbon migration and accumulation

Formation water is the power and carrier of oil and gas generation, migration and accumulation, which has important significance for oil and gas migration, accumulation and preservation. The groundwater characteristics and genetic mechanism of the Permian Fengcheng Formation in Mahu sag are mainly studied through the chemical composition, isotope test, inclusion temperature measurement and salinity analysis. The study shows that the formation water of Fengcheng Formation in Mahu sag has special hydrochemical characteristics of high salinity, high HCO3− concentration, low Ca2+, Mg2+ concentration and high pH value NaHCO3 water type. The characteristics of inclusion temperature measurement, carbon and oxygen isotope, strontium isotope and boron isotope show that the Fengcheng Formation fluid has deep hydrothermal source, and volcanism contributes significantly to its CO2 source. The source of deep hydrothermal solution, volcanism and a large amount of hydrocarbon generation and expulsion from source rocks may be the main reasons for high HCO3− content in formation water. The Fengcheng Formation in Mahu sag is an alkalized lake deposit. The highest alkalization degree of water in the study area may be distributed in the southwest slope of Mahu lake. From the evolution stage of the alkali lake, the first member of the wind is the initial stage of the formation of the alkali lake, corresponding to the late stage of the high water level of the lake, and the water salinity is low; the second member of the wind is the strong alkali forming stage, corresponding to the Lake recession period, the water surface drops, the water body is strongly concentrated, and the salinity is high; the third member of the wind is the fading period of the alkali lake, corresponding to the early stage of the lake advance, the water salinity gradually decreases. On the whole, the groundwater mineralization degree of Fengcheng Formation in Mahu depression is high, and the formation sealing property is relatively good. The side near the edge of the basin is affected by the infiltration of atmospheric water, which results in the decrease of mineralization degree and the appearance of Na2SO4 type water. The water head decreases gradually from Mahu sag to northwest margin, and the gradient of water head near Wuerhe in the north section is relatively strong, which is not conducive to hydrocarbon accumulation and preservation. Baikouquan and Karamay areas are low potential areas, and the wate