Evidences of chief source rock in Tarim Basin

Comprehensive analysis on the sea level changes, sedimentary facies, seismic reflection characteristics, and relationship between hydrocarbon-generation and reservoir formation of the Paleozoic in Tarim Basin shows that the area from the upper of the Lower Ordovician to the Middle Ordovician developed chief marine source rock. The evidences include following aspects: ①The upper of the Lower Ordovician to the Middle Ordovician is the stratum with the deepest sedimentary water in the Sinian to Ordovician of Tarim Basin. The average content of sedimentary total organic carbon (TOC) is the highest. ②The seismic response from the upper of the Lower Ordovician to the lower of the Middle Ordovician in the northern depression of Tarim Basin is characterized by weak or blank reflection and similar to that of the Lower Cambrian mudstone source rock. It is speculated that the stratum is made of source rock. ③The analysis on sea level change and sedimentary facies indicates that the source rocks from the upper of the Lower Ordovician to the Middle Ordovician were mainly formed in starved basin to marginal basin with deoxidization environment. The source rocks may be distributed in all area of the northern depression of Tarim Basin. ④The analysis on relationship between hydrocarbon-generation and reservoir formation shows that the formations of primary oil reservoirs of chief marine in the central and northern uplift of Tarim Basin are in the stage from the Permian to the Cretaceous, which is coupling to the chief oil-generation stage of source rock from the upper of the Lower Ordovician to the Middle Ordovician, but not coupling to the oil-generation stage of source rock of the Lower Cambrian and the Late Ordovician. ⑤The distribution of oil-gas reservoirs discovered in the central and northern uplift of Tarim Basin indicates that oil and gas should be mainly migrated from the distributing region of basin facies' source rocks in the northern depression.