Assessment criteria,main types,basic features and resource prospects of the tight oil in China

The tight oil refers to an oil that accumulates in source rocks in a free or adsorbed state or in tight sandstones and carbonates interbedded with or adjacent to source rocks.Generally,this oil accumulation has not yet experienced a large-scale,long-distance migration.Based on the clarity of the tight oil concept and connotation,we proposed ten key indices to evaluate the tight oil.Tight oil reservoirs can be generally divided into 3 different types based on porosity and permeability,while the tight oil itself can be also classified into 3 types depending on the genetic relationship of a close contact between tight reservoirs and source rocks,i.e.①tight oil in lacustrine carbonate rocks;②tight oil in deep-lake gravity flow sandstones and ③tight oil in deep-lake delta sandstones.The tight oil is widely distributed in China,currently,a number of important exploration discoveries of the tight oil have been achieved in the Triassic Chang-6 and Chang-7 sections of Yanchang Formation in the Ordos Basin,the Permian Lucaogou Formation in the Junggar Basin,the Middle-Lower Jurassic of the Sichuan Basin and the Cretaceous Qingshankou-Quantou Formation of the Songliao Basin.To expect the tight oil prospect in China,we preliminary forecast that the tight oil geological resources in China are about(106.7~111.5) ×108t.Combined with the analysis of future prospects of petroleum development,we can come to a conclusion that the tight oil in China should be a realistic replacement resource of the conventional oil.