The Eogene volcanism were quite often in Sanshui Basin.Various lavas and volcanic clastics

totalling about 2

000m thick

were formed in more than one hundred times of volcanic eruption.They distribute mainly in the central part of the basin which measures 45 km long NS and 8-12 km wide EW

and formed in the process of deposition of the Buxin Formation(E

1-2

b)

the source rock

and also in the generation and migration process of oil and gas in it.As a result

they have exerted great influence both on the accumulation and preserva-tion of oil and gas.The high ancient geotherm derived from the volcanism had accelerated the evolution of source rocks

so the depth of oil generating threshold is 800-1

200mshallower than that in basins elsewhere in the eastern part of China.The abundance of organic matters

such as organic carbon

chloroform asphalt and hydrocarbon

decreased remarkably due to volcanism within this area

resulting in poor oil and gas showing.Rocks rich in organic matters are distributed in a horse-shoe belt surrounding the centre of volcanics

showing clear relationship to the distance away from the area of volcanic eruption centres or subvolcanic intrusions and also to the shape and volume of lavas.In fact

almost all oil and gas fields and wells with good showings are located around the area of volcanic activities.Volcanism occurred during the stage of oii and gas migra-tion

for example

formed in the deposition process of the Huayong Formation (E

2-3

b)

had brought serious damage to oil and gas.Diapiric and draping structures were formed due to volcanic activities in San-shui Basin

but they bear no significance for oil and gas accumulation

because of their delay in formation.The highly pure carbon dioxide(99.5%)pools discovered in the present area are closely related to volcanism

they distribute in or close to the terrain of volcanic rocks.