Source and accumulation of oil and gas in Kashi Sag, Tarim Basin, NW China

The most important source rocks in Kashi Sag, Tarim Basin, are Lower Carboniferous type Ⅱ marine sediment, Lower-middle Jurassic type Ⅱ-Ⅲ lacustrine Yangye Formation and the Lower Jurassic coal-bearing strata. The oil-gas-source correlation suggests that the oil seepages found in Kelatuo, Yangye, etc., was derived from the Middle Jurassic Yangye Formation, and the gas in Well Ake 1 was derived from the Lower Carboniferous. The pool filling process expressed that the pool accumulated the gas from Carboniferous source rock at a stage with high R_o value of 1.5%-1.8%. This is the major reason for the heavier gas isotopic composition. The oil derived from Jurassic source accumulated in Early Pliocene and was destroyed to form oil seepages and oil sand on surface.