Basin Framework Reconstruction Of The Kong-2 Member Of Konidian Formation In Kongnan Area, Huanghua Depression

Kongnan area, controlled by the western Cangdong fault and the eastern Xuxi fault, lies in the southeastern part of the Huanghua Depression of the Bohai Bay basin, and is more than 4700 km2 in area.The Paleogene Kongdian Formation contains the main source rocks and reservoirs, as seen in Zaoyuan, Wangguantun, Shenusi, Wumaying, Xiaoji oilfields. Thickness of Kong-2 Member of Kongdian Formation consists of more than 400 meters of dark mudstone and oil shale, and oil shale occupies half of the total thickness of this member. Kong-2 is the main source rock of the area.As a result of the structural activity along Cangdong and Xuxi faults, the southern part of the Huanghua depression was converted to uplifts and sags bounded one side by faults, and the depocenter was in the Cangdong sag. As the Kong-2 Member reflects reformation by fault movement especially along the Cangdong and Xuxi faults, the original basin framework is hardly recognized. Based on isochronic stratigraphy correlation, and a comprehensive study of the thickness, lithofacies, sediment facies, structure and sediment system, the framework of the basin during the deposition of Kong-2 Member has been restored. During sedimentation of the Kongdian Formation, this area was a fault depression in early times and became a subsiding depression in later times.The Kong-2 Member is characterized by the following sedimentary and structural features:1. The Kong-2 is thicker in the central basin, that is, the Wangguantun-Shenjiapu- Fenghuadian area; whereas on the two sides of the above belt, the thickness of this member becomes thinner significantly.2. In the central part of Kongnan area, the sediments are rich in mud; whereas along the two marginal faults, the sediments are mainly sandstone. For example, Wang22-1, Kong19, Zao64, show more than 30% sand in the sediments. In Shenusi area near Cangdong fault, the sediments are also rich in sandstone. In the central uplift area, such as Jia6, Guan101, Guan181 and Guan185 wells, thick layers of dark mudstone and oil shale were penetrated.3. The lacustrine facies is well developed in Kong-2 member. The deltaic front and prodelta subfacies can be seen along the marginal faults, whereas the deltaic plain subfacies cannot be found in the studied area. This subfacies should be distributed far beyond the region controlled by the Cangdong and Xuxi faults.4. The Kongdian uplift, located in the central part of the basin, is a structure formed later than the sedimentation of Kong-1 and Kong-2. There is no change of the thicknesses of these members on both sides of the Kongdong and Kongxi faults. The conclusion of this study shows that: 1. The Cangdong and Xuxi faults did not control the basin development during the sedimentation of Kong-2 member; 2. The depocenter was in the central part of the basin where the Kongdian uplift is located, and this structure is a later structure; and 3. The sedimentary area of Kong-2 period was larger than that the current basin, far beyond the region bounded by Cangdong and Xuxi faults.