Depositional System And Lithologic Traps In The Third Member Of The Shahejie Formation, South Of Dongpu Sag

Located in the southern end of Bohai bay basin, Dongpu sag has been experienced oil-gas exploration for more than 30 years, mainly concentrated in structural traps. So far, its proven oil reserves amounted to 5.4×108t, and proven reserves of natural gas is 615×108m3. However, 96% oil and 93.5% gas are dictrubuted in the northern part of Dongpu sag, which is a 2000km2 halite developed zone. The southern part of Dongpu sag, whose area ia about 3000km2, is larger than the north. As a matter of fact, its proved reserves of oil and gas is less than 10% of total reserves, mainy located in structural traps. Yet, after five years’s exproration practice, the lithologic traps is well developed in the south of Dongpu sag, especially in deep area.In this paper, guided by sequence stratigraphy and reservoir sedimentary theory, based on the core, well logging, seismic and analysis laboratory data, combined with regional geological background and structural development, the tactics of point (individual well)â†'line (well correlation cross section)â†'plane (map)â†'domain (evolution) was sdopt to analyze the structural system, explain the seismic stratigraphic framework, compile the depositional system maps, showing the structural controls of depositional system, the spread of depositional system and evolution, in order to guide the exploration for lithologic trap. Five main conclutions are obtained as follow:1. On the basis of former structural analysis, according to the accommodation zone theory and fault combination relation, types of accommodation zone developed in south of Dongpu sag are clearly pointed out, then their controls on depositional system and sand body are carefully discussed for the aim to direct the accomplish of depositional facies maps.2. By means of paleo geomorphology in research area, in the Shahejie formation depositional period, the sourthern part of Lanliao fault became active strongly and formed a sedimentary center in the southeastern part; Subsequently, faults in the western slope began forming and devepoled a double break structure cross section, at the same time central uplift appeared; at the end of Shahejie formation depositional period, there were a south weak, north strong structural configuration, maximun bed thickness formed in Qianliyuan and Huzhuangji-Haitongji area. In summary, sedimentary center moved from south to north.3. Types of depositional system are determined. There are 6 types of depositional system developed in south of Dongpu sag, including shore-shallow lacustrine, deep lake, fan delta, meandering river delta, turbidite and beach-bar. These depositional systems reflect the terrain gradient, the provenance system, structural system and their controls.4. Distribution features of depositional system in south of Dongpu sag are summerized. The topography is steep in east and mild in wast, which makes fan delta developed in east while meandering river delta in west. Maogang and Baimiao fan delta formed in steep slope zone controled by Lanliao fault; broad spreaded and fine granularity meandering river delta, such as Fangliji and Haitongji delta, developed in the western slope zone.5. Type of lithologic trap and their favorable form zone are summerized. Fine types of of lithologic trap are distinguished in south of Dongpu sag, including sandbody pointed out, sandstone lenticular-body, small conglomerate fan, isolated channel sand body and fault-lithologic complex. Sanchunji, Dongming-Xichengji, Menggangji deep subsag is favorable zone for lithologic trap development.