An analysis of sequence filling types and process of Shihezi Formation in northeast Ordos Basin

Abstract：Based on data from field outcrops，well logs and drilling cores, this paper studied the sequence filling types and evolution of Shihezi Formation in northeast Ordos Basin. In the early development stage of the continental depression basin, the braided river delta and the meandering river delta were successively filled, and in the mature stage of the continental depression basin, the anastomosed river delta was filled. These deltas, characterized by “big plain, small front”, offered space for river evolution and sand enrichment. Distributary channels（A1 type），submerged distributary channels（A2 type） microfacies of the braided river delta are considered to be the most favorable facies belts for the formation of reservoirs. Distributary channel（A2 type） and submerged distributary channel（A2 type） microfacies of the meandering river delta, and nodes of distributary channels（C1 type）of the anastomosed river delta can also become favorable reservoirs. In conclusion, the palaeophysiography and sequence filling controlled by base-level cycles must have had played an important role in forming lithologic reservoirs in Ordos Basin.