Evaluation of thermal maturity of source rock based on geochemical method, a case study of Late Oligocene source rock in Hoa Tra field, Cuu Long basin, Vietnam

Abstract

The  Hoa Tra field is located in the southeast of the Cuu Long basin which has great potential for oil and gas. The source rocks in this area are mainly fine-grained sediments from the late Oligocene and early Miocene, which are located in a continuous deposition zone and under anaerobic conditions. This paper focuses on studying the maturity level of late Oligocene source rock through geochemical analysis methods. Specifically, the paper uses indicators such as T­, R_o, MPI-1, T_s/(T_s+T_m) and C₂₉ steran 20S/(20S+20R) from Rock-Eval Pyrolysis, Vitrinite reflectance and Gas chromatography-Mass spectrometry methods, respectively, to evaluate the level of thermal maturity of the source rock. The source rocks in the Hoa Tra field mainly contain types I and II, which are poor in vitrinite particles, leading to low R_o measurement results. However, when R_o is calculated from T_max and (MPI-1), it indicates that the thermal maturity of source rock from the upper part of the late Oligocene is in the early mature stage (R_{o avg}= 0.56%), while those from the lower part of late Oligocene are mature and in the phase of oil generation (R_{o avg}= 0.65%). These results are similar to those obtained from assessing thermal maturity using T_s/(T_s+T_m and C₂₉ 20S/(20S+20R) indicator (the average values for the respective parameters are 0.32 and 0.31 for the upper part, while those are 0.39 and 0.42 for the lower part of late Oligocene). The article also shows that the contribution to the  Hoa Tra field is not only from in-situ generated oil but also from migratory oil coming from nearby troughs