DISTRIBUTIONS OF THE MAIN MESOZOIC HYDROCARBON SOURCE ROCKS IN THE QIANGTANG BASIN OF THE QINGHAI-TIBET PLATEAU

Qiangtang Basin in Qinghai-Tibet plateau is a Mesozoic marine residual basin.Geologic survey and analysis of the hydrocarbon source rock samples show that there are four hyddrocarbon source rocks longitudinally developing in T₃x,J₂b,J₂x and J₃s formations in Qiangtang Basin.The main hydrocarbonsource rocks in the different depressions or sags are different.The hydrocarbon source rocks develop in J₃s,J₂b and J₂q-T₃x formations in the east of South Qiangtang Depression and in J₂x,J₂b and T₃x formations in the centre and west of South Qiangtang Depression.In North Qiangtang Depression,the hydrocarbon source rocks develop in T₃x and J₂b formations in the east and in J₂x,T₃x,J₂b and J₃s formations in the west.The thicknesses of the effective Jurassic muddy hydrocarbon source rock and carbonate hydrocarbon source rock are generally 100 to 500 meters and 200 to 600 meters,respectively.There are hydrocarbon-rich rocks with 200 to 400 meters thick devceloping in J₂x and J₃s formations.The thickness of the good-quality hydrocarbon source rock developing in T₃x formation is generally 500 to 700 meters.And in North Qiangtang Depression,100 to 300 meters thick of good-quality hydrocarbon sourck rock developing in T₃x formation.The maxima of residual TOC of the J₂x shale,J₃s marl and T₃x shale are 4.15%,1.26% and 1.5%,respectively.However,the values of residual TOC of the J₂x gray and gray-green mudstone as well as J₃s and J₂b limestone are mostly less than 0.5% and 0.2%, respectively.The value of residual TOC of the T₃x marine shale is generally in the range of 0.5% to 1%.