GEOCHEMISTRY OF THE PERMIAN ORGANIC-RICH MUDSTONES, CENTRAL QIANGTANG BASIN, NORTHERN TIBET, CHINA: IMPLICATION FOR PROVENANCE, PALEOCLIMATE AND TECTONIC SETTING

Qiangtang Basin is located in the Tibetan plateau and covers the eastern section of the Tethys structural domain, which is known to be rich in oil and gas reserves. This large, onshore, petroleum-bearing basin has been the subject of low degree of research and been considered as the most potential area for exploration and development in Tibet.

On the other hand, reconstruction and research of this region could have role in restoring the East Tethys tectonic geological evolution. A＞500km-long high pressure metamorphic belt occurs in the middle of the Basin (the central uplift belt) and has been attributed to the subduction of the Paleo-Tethys Ocean in this area. Two contrasting hypotheses of its formation are lasting for 20 years.

Qiangzi 5 well is the first well that revealed the Late Paleozoic sediments in Qiangtang Basin, and found the Permian hydrocarbon source rocks for the first time in the central uplift belt of Qiangtang Basin, which has important value for both structural geology and petroleum geology research in this area.

Geochemical analysis and organic geochemical data of mudstones from cores of Qiangzi 5 well were used for identification of Permian geochemical strata characteristics, provenance, paleoclimate and tectonic setting of Qiangtang Basin.

Bulk- rock geochemistry analysis indicates that they are mainly arkose and subarkose, derived from dominantly andesitic rocks, probably a mixing of felsic and mafic provenance. The CIA, PIA and ICV values of these samples suggest that the mudstones are compositionally immature and related to moderate weathering. The Sr/Ba, Ceanom and δEu are used to reveal the redox of paleo-ocean, their ratios show that the sedimentary environment during Middle Permian was mainly oxidizing marine environment. The tectonic discrimination diagrams, as well as critical trace and REE characteristic parameters imply an oceanic arc or continental arc setting for the depositional basin of mudstones, probably back-arc basin. This study supports an occurrence of archipelagic oceanic basins in the Jiaomuri area in Middle Permian, which provides evidence for the research of the Longmu Co- Shuanghu-Lancangjiang suture.