The discovery and significance of the large-scale asphalt veins in the Qiangtang Basin, northern Xizang

The asphalt appeared on the Earth's surface may provide an important information about oil and gas occurrence for the early petroleum exploration. Abundant oil shows were discovered in the Qiangtang Basin, northern Xizang, where the large-scale asphalt veins were recognized in 2002 in the Late Jurassic strata in the northern part of the basin. The asphalt veins occur mostly in the Upper Jurassic Suowa Formation that is underlain by the salt domes locally with oil shows within the Middle Jurassic Xiali Formation. The components of the asphalt group consist of saturated hydrocarbon (5.72%), aromatic hydrocarbon (18.46%), asphalt (52.37%) and nonhydrocarbon components (23.45%). The above-mentioned observations show that the asphalt veins have been subjected to migration and oxidation. The occurrence of the asphalt veins within composite anticlines facilitates the accumulation of oil and gas. The discovery is of significance, implying that the basin is expected to be highly prospective for the oil and gas deposits.