Abstract

The Lishui-Jiaojiang Depression, a northeast-southwest trending depression within the Taibei Trough, is currently the focus of exploration activity following CNOOC's Fourth Round of Offshore Bidding. Previous to this recent activity, only four wells had been drilled in the depression. These wells were drilled on the Yan Arch along the eastern margin of the depression. As such, there are no penetrations of the earliest fill sequences of the depression.

Hydrocarbon charge is the principle exploration risk in this region. Therefore it is important to understand the early-fill sequences, which are potentially source-prone. The focus of this study was to determine the likely depositional environment of the early-fill sequences, and to delineate the distribution of likely source-prone intervals within the sequence.

The study is based on sequence stratigraphic interpretation of 1984 and 1986 vintage seismic lines in the northern portion of the Lishui-Jiaojiang Depression in the vicinity of Shell's Block 26/19. Paleogeographic and depositional models are constructed from the interpretation.

The lowermost sequences thicken into the basin. They are generally characterized by low-amplitude discontinuous seismic events, although occasional continuous high-amplitude events are observed near the basin axis. The early-fill sequence is interpreted to be a lacustrine sequence and the high-amplitude continuous events observed near the basin axis to be lacustrine shales. By mapping the distribution of the latter seismic facies. hydrocarbon charge risk can be assessed.