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.