Abstract
The overpressure formation in the Pannonian basin, Hungary, was investigated but has not been properly understood for the last 40 years because at least two different explanations were delineated. The first explanation considers the hydrocarbon generation as the main overpressure generation mechanism with some undercompaction contribution. On the contrary, another explanation assumes tectonic stress as the main trigger of abnormal pressure. The following research delivers a suitable workflow to understand which generation mechanisms were active in the study area and estimate the quantitative contribution of the mechanisms. The developed workflow relies on the basin modeling principles that were designed to simulate subsurface processes on a geological timeframe. Moreover, the uncertainty of input parameters was considered, and the joint application of a heuristic Monte Carlo simulation scheme and improved basin modeling resulted in stochastic pore pressure models. The most frequent value (MFV) method was applied on the simulated values to test a robust statistical method in pore pressure prediction. The study has identified not only the four main overpressure generation mechanisms, but it could calculate the individual contribution to the subsurface pressure. Finally, two independent and stochastic pore pressure prediction methods have been developed that could be used in the pre-drill well planning phase and the real-time prediction during drilling.
Highlights
Three main pressure types have been applied to describe subsurface pressure conditions: the hydrostatic, the pore, and the lithostatic pressure
The normal shale compaction trend has been identified based on the Vp -depth plots in case of the Late Miocene and Pliocene series that allow identifying the top of the overpressured zone [55]
The well log signature of Vp -depth plots suggests that the disequilibrium compaction is the main overpressure generation mechanism (Figure 10), because the measured compressional wave velocity values do not decrease with increasing depth
Summary
Three main pressure types have been applied to describe subsurface pressure conditions: the hydrostatic, the pore, and the lithostatic pressure. The hydrostatic pressure regime is present when the pore pressure is equal with the hydrostatic pressure. An abnormal pressure regime occurs when the pore pressure is smaller or higher than the hydrostatic pressure. Overpressure is present when the pore pressure exceeds the hydrostatic conditions; it is known from several young sedimentary basins [1,2,3,4,5,6]. The overpressure generation mechanisms have been divided into three groups in case of sedimentary basins. The first group is triggered by the increase of compressive stress, which inherits the disequilibrium compaction and the tectonic stress generated overpressure.
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