Abstract:

Up to now, there are 7 wells with the Lower Paleozoic carbonate strata in eastern Tarim Basin （Tadong）. The trap drilling prospect is optimistic in these strata, especially in the Cambrian-Lower Ordovician. After analyzing comprehensively the data from maturity of organic matter in these 7 wells, the results show that the maturity of organic matter is normal thermal evolution and the sections look like a single beeline in 2 wells, and there are very distinct inflection points in the other 5 sections of maturity.The sections of maturity in 7 wells are all discontinuous between Paleozoic and Mesozoic-Cenozoic, that is to say,there is a fault between them. This denotes that the increase of maturity of Paleozoic organic matter doesn't owe to sedimentation and burial in the uplift and slope region in Tadong, but to the abnormally thermal events in the deep. According to the variety of maturity, the abnormally thermal events mainly distribute in the Lower Paleozoic in the vertical, and near the deep grand strike-slip fault zones in the lateral,and the sections of maturity are normal in those wells apart from the faults. All these show that the abnormally thermal events have a close relationship with the deep grand faults, so the author calls it tectono abnormal thermal events. It is the main thermal propulsion for cracking of Paleozoic crude oil in the Cenozoic in the uplift and slope region in Tadong. Multistage bitumen in these 7 wells proves clearly that there is a late cracking of crude oil in Tadong. The bitumen reflectivity has an obvious difference even in one sample,and this phenomenon shows that the bitumen is formed in different stages. Furthermore, there is no abnormally thermal event in the areas where only have one stage bitumen, and the bitumen is formed by the early sedimentation. There is one late abnormally thermal event in the areas where have two stages bitumen,and the time of thermal event is consistent with the movements （the terminal Late Jurassic）of the faults in this area.There are two late abnormally thermal events in the area where have three stages bitumen,and the time of thermal events is consistent with the movements （S₃—D₂, the terminal Late Jurassic）of Churchen Fault System.

Key words: tectono thermal event, cracking of crude oil, maturity of organic matter, bitumen reflectivity, Churchen Fault System, eastern Tarim Basin