Sedimentary characteristics and types of hydrocarbon source rocks in the tertiary semiarid to arid lake basins of northwest China
Abstract The climate in Tertiary times was semiarid to arid in northwest China. The basic characteristics of the Tertiary lakes in this area are as follows. The lakes were formed and evolved in a single cycle. From Oligocene to Miocene times, all the lake basins subsided permanently and formed stable depressions. The lakes expanded and contracted considerably, with the lake surface area changing every few ten to hundred thousand of years as a result of short-term cyclical variations in climate. The different rise and subsidence of lake basins and mountains around the lakes resulted in the migration of the depressions and depocenters with time. Consequently, the areas with the thickest hydrocarbon source rockes were also change. Two periods of evaporite formation of saline lake facies in the Tertiary strata took place due to the arid climate, producing saline lake sulphate facies. The salinity of the lakes and the content of lacustrine carbonate rocks are high. Carbonate sediments are controlled by both the salinity and the depth of the water body. The salinity increases from the margin to the center of the lakes, as do the water depth and the content of carbonate rocks. It is proposed that the lacustrine carbonate rocks could also be source rocks for oil and gas, and that there are two types of lacustrine carbonate hydrocarbon source rocks.