Experimental Study on Geochemical Characteristics of Paleogene Source Rocks

The evaluation of geochemical properties of source rocks can provide an important basis for the accurate evaluation of hydrocarbon resources. In this paper, taking the Tanhai area of the Damintun Sag of Liaohe Oilfield as an example, the chemical properties of source rocks of the Dongying and Shahejie Formations of Paleogene are compared using a large number of geochemical and hydrocarbon generation thermal simulation results. The results show that the organic carbon content of the Dongying and Shahejie Formations in the study area ranges from 0.4% to 3.2%. It is considered that the source rock of the She 3 Member in Paleogene is good source rock. The Dong 3 and Sha 1 Members are the relatively good source rocks. While the Dong 2 Member is a relatively poor source rock, and the Dong 1 Member is the non-source rock. The vitrinite reflectance of Paleogene mudstone in the study area ranges from 0.33 to 0.93%, with an average value of 0.57%, which belongs to the immature to mature stage. Considering that the organic matter types in this area are mainly type IIB–III, the oil generation threshold is set at 2850 m. It is particularly noteworthy that thick coal measure strata are developed in the lower part of the Dong 3 Member and the upper part of the Sha 3 Member, although they are of poor type as oil source rocks, however, they are of great potential value as gas source rocks. The Shahejie source rock has the closest affinity with crude oil. Natural gas in the study area is mainly wet gas and a small amount is dry gas. The ratio of isobutane to n-butane (iC₄ /nC₄ ) in natural gas increased with decreasing depth, showing a certain migration differentiation effect. It is speculated that the Shahejie Formation source rock is the main source of natural gas in the Taiyangdao – Kuihuadao Structure.