Paleogeotemperature and maturity evolutionary history of the source rocks in the Ordos Basin

Tectonothermal evolution has an important control on maturity and hydrocarbon-generation of source rocks. On the basis of analysis of the present-day geothermal gradient, rock thermophysical parameters and heat flow, the thickness contour and vitrinite reflectance can be identified, and the distribution of thermal anomaly was discussed. The paleogeotemperature and paleogeothermal gradients are calculated after the recovery of the erosional thickness. With the theory of the recovering of the superposed and modified sedimentary basin, the evolutionary history since the end of Triassic, the end of Jurassic, the Late Early Cretaceous is presented for two sets of source rocks recovered in various perspectives of plans, cross-sections, important wells, and typical areas. The impact of tectonothermal evolution on the maturity and hydrocarbon-generation history of the source rocks is discussed. The paleogeothermal gradient (4.0 degrees C/100m on average) is higher than the present one (2.9 degrees C/100m on average). With the slightly continuous increase since the Late Paleozoic, the paleogeotemperature of the Ordos Basin reached its maximum in Late Early Cretaceous and decreased continuously from then on. The paleogeotemperature and maturity of the source rocks continuously increased before Late Early Cretaceous. The tectonothermal event during Late Jurassic to Early Cretaceous controlled the source rocks with the high temperature and maturity in the Wuqi-Qingyang-Fuxian. The temperature decreased, and the maturity hardly changed after the Late Cretaceous. The source rocks of the Shanxi Formation reached maturity, high maturity and over-maturity at the Late Triassic, the Late Jurassic, and the Early Cretaceous, respectively. The source rocks of the Yanchang Formation reached low maturity and maturity sequentially all in the Early Cretaceous.