Active source rocks of Chang 7 member and hydrocarbon generation and expulsion characteristics in Longdong area, Ordos Basin

The distribution, hydrocarbon generation and expulsion history and intensity of active source rocks in the Chang 7 member of the Yanchang Formation in the Longdong area of the Ordos Basin were studied based on geoche-mical data, TOC calculation by well logging, hydrocarbon generation potential and expulsion history simulation. Dark mudstone (including oil shale) thickness with an organic carbon content (w(TOC)) more than 1.0% in the Chang 7 member mainly ranges from 20-90 m, and the average w(TOC) is mainly between 2%-9%. Active source rocks of the Chang 7 member have a w(TOC) value over 1.0% and R_o value over 0.7%, and are widely distributed in the Longdong area. They became mature and generated hydrocarbon at the end of Jurassic, expelled hydrocarbon during the early stage of the Early Cretaceous (140-130 Ma,R_o≈0.7%), and reached peak at the end of the Early Cretaceous (100 Ma, R_o≈1.0%). At this time, the oil generation ratio reached 400 mg/g TOC, and the oil expulsion ratio could be 308 mg/g TOC. But oil generation was forced to stop as a result of regional tectonic uplift since the Late Cretaceous. The oil expulsion intensity is mainly (10-250)×10⁴ t/km², showing the characteristics of strong oil expulsion. Oil reservoirs of the Yanchang Formation dominated with tight oil of the Chang 6-Chang 8 members mainly developed in the area with an oil expulsion intensity over 20×10⁴ t/km², which indicated that oil accumulation in tight sandstones has characteristics of being near the oil source with a short migration distance.