pattern and geological meaning of oblique progradational filling within the late triassic lake in the ordos basin

1. Abstract With the technical progress of seismic exploration in the Loess Plateau, a number of large-scale oblique Progradational sequence reflections have been seen in the deep-water area of the lacustrine Triassic Yanchang Formation of the Ordos Basin in central to western China. These Progradational sequences come from different margins of the lake: four from the southwest, two from the northeast, and one from the southeast. Each lenticular Progradational sequence can be divided vertically into three parts including topset, foreset and bottomset, consisting of delta, deeper lake and gravity flow depositional complexes, respectively. A Progradational sequence usually penetrates a few lithostratigraphic Mbrs from C7 to C3 of the Yanchang Formation that are divided and correlated using well data.  Seismic sections show that well data-defined lithostratigraphic units such as the C6, C4+5 or C3 Mbrs are in fact simultaneous deposits contained in the same Progradational sequence or they correlate to different Progradational sequences over long distances. The clinoform reflections show an ancient lake bed form in the southwest steep slope (3-5 ° ) and the northeast gentle slope (2-2.5 ° ) of the Yanchang Formation. The slope gradient controls the distribution of tight sandstone reservoirs.  Due to the actions of gravity or storms, the sandy sediments were more easily transported to the lower part of the foot of the southwest steep slope, so the sandstones of this slope foot are richer than those on the foot of the northeast gentle slope of the lake. The oblique filling pattern of Progradational sequences might exert an important influence over the effectiveness of water driving during oil pool development. 2. Keywords: Potential Field; Sand Reservoir; Seismic Interpretation