Abstract:

Lowstand systems tract (LST) is a concept used in sequence stratigraphy.It is referred to as sediments deposited above the type I sequence boundary,and below the first flooding surface.It is usually composed of basin-floor fan,slope fan and lowstand prograding wedge.When the low-water datum declines down below the slope-break,then the area above the slope-break will become exposed or denudation,and there will be unconformity and incised valley;while that below the slop-break will become a depositional area deposited with aktian deposit, such as basin-floor fan,slope fan,and lowstand wedges, All these will constitute a typical LST deposited in a basin with shelf slope-break margin or growth fault margin.Since there are various types of slope-breaks controlling the development of LST,there will also be different compositions of litholosomic bodies in the LST. On the basis of a vast amount of 3D seismic interpretation and analyses of well and log data, the LST developed in the Paleogene Shahejie Fm in Dongying sag has been studied and identified.It is suggested that the LST developed in the slop-break with steep slope faults in northern Dongying sag is characterized by alluvial fan,small lowstand delta,nearshore submerged fan,and the LST developed in the gentle slope belt in southern Dongying sag is mainly composed of lowstand delta and coastal shallow lacustrine sediments.Being situated in a specific position in the stratigraphic sequences and characterized by the development of its lithosomic body,the LST would be in an extremely favorable position for the LST is usually developed on the sequence interface or uncomformity interface.These interfaces are mostly good migration pathways, oil and gas can migrate along these interfaces,and would enter and accumulated in the sand bodies in the LST;secondly,the deposition of lowstand wedges,lowstand fans and basin-floor fans in the LST are mostly related to tectonic movement,therefore the dimension of single sand body would often be bigger than the other sand bodies within the sequence,and have relatively good poroperm characteristics,so they can be good reservoirs.In addition, the sand bodies in the LST are located on the transgressive and highstand system tracts (TST and HST) of the former sequence and under the TST and HST of its own sequence,i.e. they would be in interrelated between the mudstone deposited in the two sets of TST and HST,and thus form a good vertical sealing.All these would be favorable for the development of nonstructural traps in the LST.During this research, it is found that various types of nonstructural traps have been developed in the LSTs in Dongying sag,such as the developing stratigraphic traps in northern and southern slope-breaks,and the lithologic traps in the eastern depositional slope-break. Through detailed description and preliminary assessment of these traps, it is believed that the hydrocarbon prospects are optimistic, and it is very important to study the LST through research of slope-break and unconformity in Dongying sag.A large contribution would be made in the exploration of nonstructural traps in Dongying sag.

Key words: lowstand system tract (LST), deposit, Dongying sag, nonstructural trap