Petroleum geologic characteristics and profitable exploration zone selection of Changtu Sag

The studies of petroleum geologic characteristics, structural unit division and profitable exploration zone selection of the Changtu Sag showed that Cretaceous was well developed in the study area, and the Lower Cretaceous was a main exploration target. The dark mudstones of the Jiufotang Formation, with high organic carbon abundance, type Ⅱ₂ kerogen and in mature stage, served as the main source rocks and mainly developed in the north of the sag. There were 3 sets of reservoirs including the Jiufotang, Shahai and Fuxin Formations. The Jiu-fotang reservoirs developed in the north and the south, and belonged to type Ⅱ. 2 kinds of petroleum systems were concluded. As to the 1st kind, hydrocarbon generated and self-stored in the Jiufotang and Shahai Formations. While for the 2nd kind, hydrocarbon generated from the mudstones in the Jiufotang Formation, stored in the sandstones in the Shahai Formation, and capped by the mudstones in the lower part of the Fuxin Formation. The Central Structural Zone, which was adjacent to hydrocarbon generation sag and had many traps, was the target for hydrocarbon migration for a long time, hence was a favorable petroleum exploration zone.