Many wells have encountered the Qrebake

Lianglitaga and Sangtamu formations in Upper Ordovician of northern Tarim Basin.A systematic analysis of source rock samples from the sequence indicates that it was characterized by low abundance of organic matters and mixed organic types.In southern Tahe area

the Upper Ordovician source rock is poorly developed with a vitrinite reflectance (R

o

) of about 1%

thus could not be the source of commercial oil and gas reservoirs.In contrast

in northern Tarim Basin

the Upper Ordovician carbo-nates are both source rocks and reservoirs

making conventional oil-source correlation impossible.The study adopted two oil-source correlation methods to compare the properties of direct extracts from source rocks and their pyrolysis yields with that of crude oil from the reservoirs.The result shows that there is no source-product relationship between the crude from Tahe oilfield and source rocks from the Upper Ordovician.The geochemical characteristics of direct extracts from the Upper Ordovician source rocks are something between that of crude oil and pyrolysis yield

while the geochemical characteristics of crude oil and the pyrolysis yield are quite different.The new method of oil-source correlation solved the problem of the conventional oil-source correlation method not applicable to the Upper Ordovician as it is both source rocks and reservoirs at the same time.