Majiang paleo-oil reservoir is one of the largest marine paleo-oil reservoirs in South China.There is debate about its forming time.This paper investigates the hydrocarbon accumulation period by analyzing the geochemical characteristics and genesis of reservoir-bitumen.The study shows that the distribution of n-alkanes of saturated hydrocarbon chromatogram from the reservoir-bitumen is complete

but the base line is up convex

and 25-norhopane exists.It indicates that crude oil charging occurred at least in two stages in this paleo-oil reservoir.The reservoir-bitumen in Ordovician and Devonian originated from the same source rocks

indicating that the massive crude oil charging occurred after deposition of the Devonian.Reservoir-fluid inclusion thermometry and K-Ar dating of autogenous illite

in combination with thermal history of source rocks

were used to investigate the hydrocarbon accumulation phases.The results shows that Majiang paleo-oil reservoir experienced small-scale hydrocarbon accumulation and damage during the Late Caledonian time

large-scale hydrocarbon accumulation during the Hercynian time

oil-gas conversion in the IndoChinese time

and large scale damage during the Yanshan time

and the Hercynian time is suggested as the main accumulation stage of Majiang paleo-oil reservoir.