Abstract: The exploration practice in recent years shows that not only sedimentary facies exist in the oxidized environment but also in the semi-reduced and reduced environment in the early stage of the faulted basin. In order to find out the relatively complete sedimentary sequence and its petroleum geological significance in the early stage of the faulted basin, taking the lower Es₄ (the fourth member of Shahejie Formation) in Bonan sag as an example, comprehensive analysis has been carried out on drilling, logging and experimental data. The results show that in the early stage of the faulted basin, in addition to the alluvial fan and the braided river understood traditionally, river delta, fan delta and lake were deposited. Original sedimentary boundary restoration and sedimentary center analysis indicate that the primary direction of the alluvial fan and the braided river in the lower Es₄ is NNE; the braided river delta formed by the braided river into the lake is in the NW banded lake basin; and under the control of the "source-sink" system, a relatively complete sedimentary sequence of alluvial fan-braided river-braided river delta-lake could be developed in the early stage of the faulted basin. The re-understanding of the alluvial-lake sedimentary system not only makes the originally "barren" red layer have a certain potential for hydrocarbon generation, and the newly identified braided river delta may provide favorable reservoirs and traps, thus expanding the exploration space of the "red layer".

Key words: sedimentary sequence, early fault basin, lower Es₄, Bonan sag, petroleum geological significance