Organic geochemistry, hydrocarbon generation, and expulsion characteristics and their impact on tight oil: A case study of the third member of the Upper Palaeogene Shahejie Formation, Nanpu Sag, Bohai Bay Basin, NE China

Abstract

In recent years, exploration of tight oil in the Nanpu Sag has made significant progress in the third member of the Upper Palaeogene Shahejie Formation (Es3) in the Nanpu Sag, Bohai Bay Basin, Northeast China, showing excellent prospects. However, owing to insufficient understanding of the hydrocarbon generation and expulsion characteristics of Es3 source rocks and their matching relationship with the reservoir tightening time, the genetic type and resource potential of tight oil systems are not precise, which limits the further exploration of tight oil. In this study, the geological and geochemical characteristics of Es3 source rock are identified through a comprehensive geological and organic geochemistry analysis. In addition, hydrocarbon generation and expulsion patterns are established using a hydrocarbon generation potential model. On the basis of these results, combined with sandstone reservoir evolution histories, we have identified the genetic type and resource potential of the tight oil. The results show that the kerogen types of the Es3 source rocks of the Nanpu Sag, are widely distributed in an area of nearly 1,000 km2 and have a large thickness of more than 700 m, are mainly Types II and III with a high average total organic carbon (TOC) value of 1.63%. Moreover, the source rocks are primarily in the mature stage, with an average vitrinite reflectance (Ro) of 1.06%. The Es3 source rock reached the hydrocarbon expulsion threshold at Ro = 0.7%, and the hydrocarbon expulsion efficiency was approximately 70%. Two hydrocarbon generation and expulsion centres are identified, the Gaoshangpu and Laoyemiao structural belts, with maximum hydrocarbon generation and expulsion intensities of 100 × 106 t/km2 and 80 × 106 t/km2, respectively. In addition, two genetic types of tight oil are identified with accumulation before and after tightening, respectively. The resources of these types are 2.3 × 108 t and 20.4 × 108 t, respectively, with a total of 22.7 × 108 t, which indicates significant prospects for tight oil exploration.

Pore Structure Characteristics and Controlling Factors of a Tight Sandstone Reservoir in the Paleogene Shahejie Formation, Nanpu Sag, Bohai Bay Basin, China

“…During the oil gas filling period (approximately 25.0 Ma), higher formation temperatures caused by volcanic activity could inhibit the dissolution of carbonate cements, and the overall effect of dissolution on the Es 3 reservoir is not as strong as that of Es 1 . Continuous volcanic activity played an important role in these different phenomena.…”

“…Generally, we observed only one cement phase, and this conclusion is consistent with the previous understanding of the study area. 34,58 In the same period, the Es 1 reservoirs experienced late cementation, and some pores were blocked but the intensity was not high due to the lack of a material basis transformed by volcanic activity.…”