Abstract
Subsurface pore pressure affects the direction of hydrocarbon migration, determines the distribution of the hydrocarbon reservoir, and provides scientific reference for drilling planning. Overpressures are widespread in the Paleogene Shahejie Formation in the Linnan Sag, which is closely related to the distribution of oil reservoir. However, the overpressure generation mechanisms are undefined, let alone the relationship between the evolution of paleo-overpressure and hydrocarbon migration in the Linnan Sag, which brings great challenges for the understanding of oil accumulation and future oil exploration. Basin modeling was carried out to solve the issue of quantitative evaluation of overpressure mechanisms and to restore the overpressure evolution of the Paleogene source rocks. The implications for the pore pressure prediction and oil migration in the Linnan Sag were further discussed. The modeling results show that the disequilibrium compaction of mudstones is a dominated overpressure mechanism of source rocks in the Linnan Sag, which accounts for approximately 90% of the measured overpressure in the region. The remainder part of overpressure was generated by hydrocarbon generation; however, the effects of hydrocarbon generation on overpressure evolution were limited in the intervals deeper than 4000 m. The significance of the overpressure mechanism is that the porosity-dependent method will give a satisfactory pressure prediction result in the current exploration depth range (3800–4300 m). The overpressure evolution of the source rock has undergone a cycle of “accumulation-dissipation-reaccumulation,” which corresponds to the age of 45.5–24.0 Ma (Es3-Ed period), 24.6–14.0 Ma (Ed period), and 14.0–0 Ma (Ng-Qp period). The oil potential of the Es3l shows good inheritance with the overpressure in the source rock, indicating overpressure increased the driving force for oil migration. The oil released from the source rock has a trend to migration from the center of the sag to the uplift belt, which is also indicated by the physical properties of crude oil. The knowledge of the generation and evolution of overpressure has great significance for further exploration in the Linnan Sag and other extensional basins.
Highlights
Abnormal pore pressure in the basins is closely related to the formation of hydrocarbon reservoirs, including generation, migration, accumulation, preservation of hydrocarbon, and even reservoir diagenesis (Hunt, 1990; Osborne and Swarbrick, 1997; Hao et al, 2007; Stricker et al, 2016; Feng et al, 2019; Li et al, 2021)
The overpressure points with depths greater than 4,000 m fall on the unloading curves, which manifested as density is maintained stable, whereas the sonic transit time increases sharply, indicating overpressures generated by both disequilibrium compaction and fluid expansion (Figure 11)
This study carried out basin modeling to quantify the overpressure mechanisms, restore the evolution of paleo-overpressure of the source rocks, and discuss the implications on pressure prediction and dynamic conditions of hydrocarbon accumulation
Summary
Abnormal pore pressure in the basins is closely related to the formation of hydrocarbon reservoirs, including generation, migration, accumulation, preservation of hydrocarbon, and even reservoir diagenesis (Hunt, 1990; Osborne and Swarbrick, 1997; Hao et al, 2007; Stricker et al, 2016; Feng et al, 2019; Li et al, 2021). The identification of generation mechanisms and construction of the evolution history of overpressure are key issues of hydrocarbon migration and accumulation dynamics (Borge, 2002; Luo et al, 2007; Chan et al, 2016; Radwan et al, 2019; Nifuku et al, 2020; Li et al, 2022). Researchers have accepted that disequilibrium compaction and fluid expansion (especially gas generation) are two most common reasons for large-scale overpressure in extensional basins (Luo and Vasseur, 1992; Osborne and Swarbrick, 1997; Ruth et al, 2004; Tingay et al, 2013). Basin modeling is an important tool for overpressure research; numerical models have been established to describe almost all overpressure mechanisms, making it possible to determine the overpressure mechanisms and their contributions at the same time (Wangen, 2001; Bolås et al, 2004; Madon, 2007; Nguyen et al, 2016; Qiu et al, 2020; Radwan, 2021)
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