Sedimentary Characteristics and Their Controlling Factors of Lower Cretaceous Fan Deltas in Saidong Sub-Sag of Saihantala Sag, Erlian Basin, Northeastern China

Fan deltas of the Lower Cretaceous area in Saihantala sag, Erlian Basin have been identified as major petroleum exploration opportunities. The sedimentary evolution is, however, still debatable, which hinders insights into its controlling factors. This research employed new core observations, thin section observations, and grain size analyses of 28 wells in the Saidong sub-sag, together with numerous borehole and seismic data points, to explore lithofacies types, subfacies, and microfacies characteristics, thus leading to a further investigation of the sedimentary facies evolution of the sag and its controlling factors. The findings showed there are 3 categories, 12 sub-categories, and 20 fine lithofacies types in the Saidong sub-sag. Additionally, various sand-conglomerate lithofacies were characterized by lower composition and texture maturity. With dentate-shaped, box-bell-shaped, and other morphological well-logging responses, fan deltas were mostly developed in the A'ershan Formation and the Tengge'er Formation, which could be subdivided into three subfacies and eight microfacies. Given the sedimentary features and lithofacies characteristics of each microfacies, it can be determined that three main stages occurred in formations from the A'ershan to the Tengge'er: the water transgression, the water oscillation, and the water regression. Moreover, fan delta deposits were regulated primarily by semi-arid hygrothermal and semi-arid paleoclimate and paleotectonic factors.