Three gas fields sourced from Pliocene-Quaternary Qigequan Formation have been found in Sanhu depression in southeastern Qaidam basin.It is a misunderstanding to take the base of Qigequan Formation in Sanhu depression as the boundary of Quaternary and Neogene.Based on logging

seismic stratigraphic and magnetobiostratigraphic studies

the age of the base of Qigequan Formation in Sanhu depression is believed to be 2.80 Ma

it is at a depth of 1 747 m in Xinshen 1 well drilled in Sebei area.There are three different opinions on the Quaternary-Neogene boundary in Qaidam basin in literatures.①The end of Olduvai

1.67 Ma

(1.80 Ma); it is at a depth of 710 m in Sezhong 6 well

i.e.19 m above the log-marker K

3

.②The boundary of Matsuyama and Gauss

2.48 Ma(2.60 Ma);it is at a depth of 1460 m deep in Seshen 1 well

i.e.37 m below the log-marker K

10

.③The end of Mammoth

3.05 Ma;it is 2 779 m deep in Dacan 1 well.None of the above three boundaries correlates with the base of Qigequan Formation.The definition of Quaternary-Neogene boundary

with an age of 1.80 Ma

has been adopted by the International Commission on Stratigraphy(ICS) through voting in 1998

and accepted by the International Union of Geological Sciences(IUGS) in 1999.According to this definition

most of the gas source rocks in Sanhu depression were deposited during Late Neogene to Early Quaternary

especially in Pliocene.It is suggested in this paper that the stratigraphic unit name of Qigequan Formation rather than the symbol "Q

1+2

" should be used in petroleum exploration and development research in Qaidam basin.Correlation of the seismic reflector T

0

(log-marker K

13

) and the base of Qigequan Formation with local unconformities at the edge of the basin is very useful for study of tectonic movement

gas generation and reservoiring. As the base of Qigequan Formation

the correlation of seismic reflector T

0

is characterized by practicability

isochroneity and operability in central Sanhu depression in Qaidam basin.