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Geochemical characteristics and Thermal Evolution of Paleogene Source Rocks in Lunpola Basin, Tibet Plateau

Abstract Lunpola Basin is the first exploration area with industrial oil flow in the hinterland of Tibetan Plateau and the target horizon is the Paleogene Niubao formation and Dingqinghu formation. Based on drilling data from multiple wells, the organic matter abundance, type and maturity of Paleogene source rocks in Lunpola Basin were measured and analyzed using organic geochemistry method. The 67 samples data from middle Niubao formation show that the average of TOC (wt.) is 3.50, S1+S2 is 30.55 mg/g; and the Tmax is 427 ∼ 442 °C, δ13CPDB is -25.6‰ ∼ -29.3‰ and Ro is 1.37%∼1.84%. The 53 samples data from upper Niubao formation show that the average of TOC (wt.) is 4.16%, S1+S2 is 30.16 mg/g. and the Tmax is 421 ∼ 440 °C, δ13CPDB is -26.9‰ ∼ -28.7‰ and Ro is 0.61%∼1.35%. The 70 samples data from lower Dingqinghu formation show that the average of TOC (wt.) is 4.16%, S1+S2 is 30.16 mg/g and Ro is 0.58%, and the Tmax is 421 ∼ 440 °C, δ13CPDB is -26.9‰ ∼ -28.7‰. These three sets of source rocks have high organic abundance and kerogen type of I- II1. Based on the previous studies of W-1 well in JiangjiaCo depression, XL-3 well in JiangriaCo depression and Z-1 well PaCo depression, a 2D model of north-south tectonic and thermal evolution in Lunpola basin is simulated by Schlumberger's PetroMod method. The results show that the XL-5 well of Niubao formation in Diezong area of Lunpola basin began to generate oil at 41.3Ma and reached the peak of hydrocarbon generation at 36.4Ma; and upper Niubao formation began to generate oil at 38.4Ma and hydrocarbon generation peaked at 31.9Ma. The lower Dingqinghu formation began to generate oil at 24.2Ma. At the end of upper Dingqinghu formation, the basin began the uplift and denudation stage at 23Ma, but the middle and upper Niubao formation and the Lower Dingqinghu formation are still active for hydrocarbon generation. The simulation also shows that the primary deposits in main source rocks are thin in the thrust uplift belt of southern basin and have not entered the stage of oil generation, and that the main oil source area is the Paleogene depression in central basin.