SEQUENCE STRATIGRAPHY AND STRUCTURAL SETTING OF PALEOGENE RIFT BASINS, EAST CHINA: THE FORMATION OF LACUSTRINE STRATIGRAPHIC TRAPS

The Tertiary rifted basins of East China are all fault-sag structural basins. Their formation was initiated by the subduction of the west Pacific Plate during the early Paleocene. The Paleogene sedimentary fill was primarily lacustrine in origin. Three main styles of rifted basins are present; those with: (1) a steeply dipping boundary fault, (2) a listric boundary fault, and (3) complex rift faulting with later strike-slip movement superposed. Sequence stratigraphic concepts were used to analyze the lacustrine strata in eight rifted basins, resulting in at least ten petroleum discoveries in stratigraphic traps during last seven years. Lacustrine sediments were commonly derived from different sides of the basin at the same time. Consequently, during lower lake levels, sands were deposited in bathymetric lows. With rising lake levels, the reservoir sands became isolated from the lacustrine shorelines. Thus, most reservoirs are deep-lacustrine or slope-related sands that were isolated in the lowstand or highstand systems tracts. The locations of these sand bodies are controlled by different structural features and their occurrence within the sequence stratigraphic framework.. Structural elements include boundary faults of the basin, smaller syndepositional faults that form half-grabens within the basin, and abrupt changes in regional dip of the basin. Stratigraphic controls include localized pre-existing channelized surfaces, paleobathymetry (deepest parts of the lake), and onlap onto clinoforms. These eight basins illustrate many kinds of stratigraphic traps that can form in lacustrine settings.