Oil Cracking of Deep Petroleum in Minfeng Sag in North Dongying Depression,Bohai Bay Basin,China:Evidence from Natural Fluid Inclusions

A fluid inclusion fluorescence and microthermometric study was performed on sandstones from the deep Es4 reservoir rocks of the Minfeng(民丰) sag,north of Dongying(东营) depression.Two types of oil inclusions(yellow and blue white fluorescence),one type of gas inclusions(blue white fluorescence),and bitumen inclusions(no fluorescence) were detected within quartz and feldspar minerals.The evolution of hydrocarbon fluid inclusions in the lower Es4 sequence indicates that present oil accumulation was predominantly thermal stress controlled.Homogenization temperatures of aqueous fluid inclusions coexisting with gas-bearing and bitumen-bearing fluid inclusions indicate that oil cracking occurred at temperatures up to 160 ℃,primary condensate or wet gas generation occurred during 170-195 ℃.Oil has cracked into condensate or wet gas in the depth of 4 300-4 410 m and dry gas and abundant pyrobitumen in the depth of more than 4 410 m in the geological history based on the fluid inclusion extrapolation.Secondary oil cracking is undergoing in present day when the depth of reservoir is more than 4 150 m whose temperature is the threshold temperature of oil cracking(160 ℃).However,because of the consumption of oil in the first oil cracking process,it may have few chances to find liquid petroleum,and only natural gas can be found when the depth of reservoir is more than 4 410 m,where oil cracks into condensate gas or wet gas according to present-day formation temperature.This study is preliminary but foreshadows a new insight into oil cracking using natural fluid inclusions to trace hydrocarbon evolution in sedimentary basins.