Abstract: Four petroleum realms of the world have been distinguished by H. D. Klemme and G. F. Ulmishek (1991): Tethyan (dominantly in the Middle East), 17% of the total area and 68% of the original petroleum reserves; Boreal, 28% of the total area and 23% of the original petroleum reserves, South Gondwana, 38% of the total area and 4% of the original petroleum reserves and Pacific, 17% of the total area and 5% of the original petroleum reserves.
Recent advances in plate tectonics provide insight into new areas of research within the Tethyan realm. The Tethyan Ocean is an intercontinental plate characteristic of the oceanic lithospheric crust, and may be composed, according to its development, at least of the Palaeo-Tethys (from the Devonian or Carboniferous to Permian) and Neo-Tethys (from the Triassic to Tertiary). There exists the Cimmerian-Cathaysian palaeocontinental group between Laurasia and Gondwana land. The Alpine-Himalayan orogenic belts are believed to be developed following the termination of the Neo-Tethyan Ocean. However, the temporal and spatial extent of the Palaeo-Tethys and Cimmerides-presented in the text has gone beyond that defined by Suess, i.e. an extension of the area into the mid-latitude regions in Asia and of the time dating back to the Palaeozoic. Furthermore, the Qinghai-Xizang Plateau, Yangtze, North China and Tarim landmasses are all assigned to the Tethyan realm. Although the individual parts within the giant Tethyan tectonic domain are evidently different, they may be compared each other. For example, the Neo-Tethys represented by Zagros may be compared with the Himalayan Neo-Tethys. The striking similarities are outlined as follows. (1) The Cambrian to Tertiary marine sediments 5000-12000 m thick are widely developed, typical of the passive continental margin sediments. The Jurassic to Paleogene marine sediments also occur on a wide range of scales in these areas; (2) The representative sediments recognized in these areas include-reefal limestones during the Devonian, Late Jurassic, Cretaceous and Tertiary; dolostones, anhydrite, rock salts and beach rocks during the Permian, Triassic, Jurassic, Cretaceous and Tertiary, and oil shales during the Late Jurassic, Early Cretaceous and Eocene of the Paleogene, and (3) The multiple hydrocarbon-bearing stratigraphic associations and source-reservoir-seal associations indicate a history of polycyclic evolution from the Early Palaeozoic to Cenozoic.
The Qinghai-Xizang Plateau is occupied by the tremendous thickness of the marine sediments especially the Triassic, Jurassic, Cretaceous and Paleogene strata. The major petroleum basins with an area of more than 10⁴ km² include the mediumand large-sized basins such as Qiangtang, Qamdo, Coqen, Biru, Xigaze, GambaTingri, Ngangla, Kumkol, Hoh Xil, Lanping-Simao, Xichang and Chuxiong Basins. These basins immediately overlain upon stable landmasses are characterized by thick sedimentary rock series (more than ten thousand metres) and effective petroleum source rocks with several source-reservoir-seal associations, and thus exhibit excellent geological conditions for oil and gas accumulation. The northern plateau has been uplifted by 4000 m since the late Pliocene, and the uplift is still going on within a relatively enclosed system. According to the first law of thermodynamics, it is im possible for the energy and matter to be created and eliminated in an enclosed system. In this respect, particular attention should be drawn to the Palaeo-Tethyan realm for the exploration plays of oil and gas throughout the Tethyan realm.