Types of DeepGWater Gravity Flow Deposits and Related Reservoirs: A Casefrom the Lashizhong Formation of Ordovician onthe Western Margin of Ordors Basin

The depositional types and reservoir properties of deep-water gravity flow deposits have been the focus of consideration of sedimentology for long. Based on the field study of outcrops and laboratory examination of thin-sections, these questions are studied in details in this paper for the Ordovician Lashizhong Formation on the western margin of the Ordos Basin. The Lashizhong Formation mainly consists of grayish-green sandstone and dark-gray mudstone, with a little of siltstone and conglomerate. Flute casts, cross bedding, graded bedding and convolute bedding are well developed. Based on the integrated study of lithologic characters, sedimentary structures and fossils, it is inferred that the Lashizhong Formation is deposited in an environment of deep-water, dominated by submarine fan with channel and lobe microfacies. The deep-water channel could be further divided into complex, vertically accretionary and migrational channel deposits. The complex channel is dominated by coarse sandstone with well developed secondary channels. The vertical accretionary channel is mainly filled by fine sandstone and siltstone as the result of vertical accretion. As to the migrational channel deposits, it is characterized by unidirectional migration and lateral accretionary bodies are well developed. From bottom to top, the sandstone decreases first and then increases, as the results of relative sea level fluctuation, and the submarine fans as a whole are in a sequence of retrogradation. The laboratory examination of porosity and permeability shows that the complex channel deposits have best reservoir properties, followed by lobe and migrational channel deposits.