Sand-gathering and reservoir-controlling mechanisms of Paleogene slope-break system in Qikou Sag, Huanghua Depression, Bohai Bay Basin

Based on tectonic background and taking into consideration of sedimentary element,the concept of slope-break system has been proposed.In the Qikou Sag,5 types of slope-break zone have been found in the study area,including tectonic (steep and gentle slopes),flexural,sedimentary,ancient landscape and erosive ones.The distribution of slope-break zone is influenced by ancient tectonic background,fault activity and sedimentary diagenesis.Similar to sedimentary system,various types of slope-break in the Qikou Sag match with each other both horizontally and vertically,accounting for over 70% of the sag area.Slope-break provides underwater channel for source supply,and also offers accommodation,resulting in sand assemblage in slope and depression.Sand distribution is controlled by the following mechanism:source providing sand,groove transporting sand,slope-break controlling sand and source type determining sand gathering.Thanks to wide-distributed reservoir,petroleum has been found all over the sag.The Qibei and Binhai slopes and the Banqiao subsag are the favorable exploration regions for Paleogene lithologic reservoir in the Qikou Sag.