THE ORGANIC FACIES DISTRIBUTION AND HYDROCARBON-FORMING MODEL IN THE QIANGTANG BASIN, QINGHAI-TIBET PLATEAU

The main Mesozoic marine hydrocarbon source rocks developed in J_3x,J₂b,J_2x and J_3s formations in the Qiangtang Basin,Qinghai-Tibet Plateau.Macro-algae are the main hydrocarbon-forming organisms in these hydrocarbon source formations.The Ⅱ₁ and Ⅱ₂ organic facies developed in the J_3s and J₃b formations.The Ⅱ₁,Ⅱ₂ and lignosa organic facies developed in the J_2x formation.They are getting poorer and poorer from the center of the basin or depressions to the northeast of the basin.In the J_3x formation all types of above mentioned organic facies developed.The Ⅰ or Ⅱ₁ organic facies gradually transited into lignosa organic facies from the south and north of the basin or depressions towards the central uplifted area.The source rocks in the center of the Qiangtang Basin are in the middle to late stages of mature and the Jurassic Ⅱ₁ organic facies are the most favorable oil-generation facies.The source rocks in the center and west of the north Qiangtang Depression are in the stage of high-mature and the Jurassic Ⅱ₁ organic facies are the favorable condensate oil-or light oil-and gas-generation facies.The Ⅱ₁ organic facies in the J_3x formation in the Qiangtang Basin are over mature at present.They are the favorable gas-generation facies.