LITHOFACIES CHARACTERISTICS OF GRAVITY FLOW DEPOSITS AND THEIR IMPACTS ON RESERVOIR QUALITY IN THE YANCHANG FORMATION, ORDOS BASIN

During the periods of Chang 7 and Chang 6 Members in Late Triassic, gravity flows developed widely in the deep water of Longdong area of the Ordos Basin. Based on the detailed study of 852.7m cores in addition to loggings and testing data, this paper classified the types of lithofacies, then summarized the sedimentary characteristics, forming process, distribution scale and reservoir quality of each lithofacies. Eight types of lithofacies are recognized, i.e., sliding sandstone (Sl), slumping sandstone (Ss), slumping mudstone (Sm), thin- to medium-bedded massive conglomeratic sandstone (Sc), thin- to medium-bedded massive pure sandstone (Sp), thick-bedded massive conglomeratic sandstone (St), massive conglomeratic mudstone (Mc), and thin- to medium-bedded sandstone with beddings (Tb), belonging to slides-dominated lithofacies (Sl); slumps-dominated lithofacies (Ss, Sm); sandy debris flow-dominated lithofacies (Sc, Sp, St); muddy debris flow-dominated lithofacies (Mc); and turbidity current-dominated lithofacies(Tb) respectively according to Shanmugam's classification. The distribution scale of different lithofacies varies greatly, St and Sp are the largest in terms of accumulated thickness and depositional frequency. All the sandstone lithofacies can be good reservoir, and lithofacies St is the best, followed by Sp and Sc. However, the distribution scale of Sp and Sc is much larger than St. It is generally believed that Sp and Sc are the most favorable targets in exploration of gravity flow reservoirs.