Rock-Eval data

carbon and oxygen isotopes and molecular geochemical parameters of the source rocks in Es

4

(the 4

th

member of Paleogene Shahejie Fm)-the lower Es

3

(the 3

rd

member of Paleogene Shahejie Fm)in Langgu Sag were integrated to study the formation mechanism of lacustrine source rocks with low abundance under different tectonic and climatic conditions.Rock pyrolysis and kerogens' maceral composition analysis revealed that the Es

4

and the lower Es

3

are the similar in the major kerogen type of Ⅱ

but are quite different in organic matter abundance.Based on analyses of carbon and oxygen isotopes in the context of regional tectonic and palaeo-climate settings

we established depositional models for the source rocks in Es

4

-lower Es

3

.The source rock in the Es

4

was deposited in a lake basin at the initial stage of rifting

with low width/depth ratio

arid climate

and brackish water or saline water with stable salinity stratification.Its

13

C-depleted of the inorganic and organic matter and lower Pr/Ph might reflect relatively low primary productivity and better organic preservation.In contrast

the source rock in the lower Es

3

was deposited in a lake basin in the intense rifting stage

with a high width/depth ratio

warm and humid climate

fresh water

and frequent water-column overturning with unstable temperature stratification.Its

13

C enrichment of the inorganic and organic matter and C

27

/C

29

sterane ratios were interpreted to be caused by higher primary productivity and more terrigenous organic matter input.