Sedimentary facies and their evolution in the lower submember of the 2nd member of the Shahejie Formation in the Wennan Oil Field

The sedimentary characteristics are described,on the basis of cores and well logs,with the emphasis on the division of the sedimentary facies in the lower submember of the 2nd member of the Shahejie Formation in the Wennan Oil Field.The sediments in this submember are assigned to the ephemeral lacustrine-delta deposits laid down under the dry climatic conditions.During the early stage of the deposition of the lower submember of the 2nd member of the Shahejie Formation in the Wennan Oil Field,the shallow lake was accentuated,followed by the littoral-shallow lake till the deposition of the 6th sand sets.The spreading of the lake basin initiated during the deposition of the 5th sand sets,and became the maximum area during the deposition of the 4th sand sets.Finally,the lake basin was subjected to the collapse till the end of the deposition of the 1st sand sets.