Abstract:

Songpan-Aba area has experienced two major extensional-compressional tectonic stages during the regional tectonic evolution. The first stage began with the dismembering of late Proterozoic China ancient land. In Early Paleozoic, the Tianshan-Qilian-Qinling ocean was formed, which separated Yangtze landmass from North China landmass; and passive epicontinental basin was developed on the northern edge of Songpan-Aba area. The Early Paleozoic oceanic basins consumed along with the consumption of the proto-Tethys. Due to the Caledonian subduction, the Yangtze landmass collided with the North China landmass, which finally formed the Caledonian subducting orogenic belt, resulting in the inversion of the passive epicontinental basin. The second stage began in the Late Paleozoic, chasmic troughs and small oceanic crusts between them were developed on the western edge of Yangtze landmass along with the spreading of Paleo-Tethys, resulting in the formation of extensional and fractured continental margin and the development of turbidite sedimentary basin on the western and/or southern edge of Songpan-Aba area. In Late Triassic, the extensional and fractured continental margin was inverted during Indosinian movement along with the closure of the Paleo-Tethys, resulting in the formation of Indosinian folded orogen. Later, this area was uplifted together with Qinghai-Tibet plateau along with the subduction and consumption of Neo-Thetis and collision of Indian plate with Eurasia. Affected by the Longmenshan intracontinental subduction, an intracontinental folded orogen was developed in the eastern part, and finally formed the present tectonic framework. Therefore, the sedimentation types and tectonic evolution of the basins are characterized by their multicyclic features. As a whole, Songpan-Aba area is in the early stage of over-mature and begins to generate large amount of methane, thus it has gas potential. The available geochemical and non-seismic geophysical exploration results show that Norgai depression has the most favorable conditions for hydrocarbon preservation and is a favorable play in Songpan-Aba area.

Key words: Songpan-Aba area, basin evolution, hydrocarbon potential, proto-Tethys