Abstract:

The beach-bar deposits developed in the 2^nd member of the Paleogene Shahejie Formation （Sha 2 Member， Es₂） in the northern Liaoxi Sag， have great exploration potential as proved by the testing results of exploratory wells. However， there is little insight obtained on the mechanism of beach-bar deposition， seriously restricting the exploration and evaluation of this kind of deposits. Therefore， it is necessary to analyze the controlling factors of beach-bar sand deposition from the perspective of source-to-sink system. The source-to-sink system of the Es² in the northern Liaoxi Sag and its control on beach-bar deposition are studied by integrating the data of core observation， logging， grain size and casting thin section， combined with 3D seismic and regional geological data. The results are shown as follows. First， during the deposition of the Es² in the study area， the Xingchenghe river system and the Yuehe river system in the Yanshan uplift transported sediments to the braided river deltas， which in turn served to supply sediments for the beach bar. Because the relatively small catchment area of the two river systems together with the shorter length of main river， the braided river deltas formed are relatively small in scale， and their capability to provide sediments for beach bars is thereby relatively weak. Second， during the deposition of Es²， a wide and gentle palaeo-uplift was developed in the northern Liaoxi Sag， providing favorable sedimentary palaeo-geomorphic conditions for beach bar deposition. Third， during the same period， the study area was characterized by shallow water body and weak sediment supply capacity， resulting in the transportation of sand from the braided river delta front towards the lacustrine basin with waves. Blocked by the higher landform of the palaeo-uplift zone， the sand bodies deposited in the palaeo-uplift zone and formed fine-grained sandy beach bars， dominated by fine-grained sandstone and siltstone. As the lake level gradually rises， the development of the beach bars gets weakened. Fourth， Yanshan uplift， Xingchenghe-Yuehe river systems， palaeo-uplift in the central Liaoxi Sag and lake level fluctuation jointly control the lithology and scale of beach-bar sands， and they are the key factors in making breakthrough in hydrocarbon exploration in the study area. The understanding obtained in the study is of referential significance to the prediction of beach-bar sandstone reservoirs in the Paleogene hydrocarbon exploration in the Bohai Sea.

Key words: provenance, parent rock, river system, beach-bar sandstone, source-to-sink system, Paleogene, Liaoxi Sag, Bohai Sea