Mechanisms and evolution of overpressure for Miocene Huangliu Formation in the Ledong Slope Belt,Yinggehai Basin

Overpressure is a common feature in the Huangliu Formation of the Ledong Slope Zone, Yinggehai Basin, with pressure coefficients reaching up to 2.31. In this study, overpressure mechanisms in mudstone intervals of the Huangliu Formation (Upper Miocene) are investigated and their importance reconstructed in time. Acoustic and resistivity logs reveal characteristic responses to overpressure in mudstones, leading to its accurate prediction. As a result, this study shows that hydrocarbon generation is responsible for the recorded overpressures as proven by several lines of evidence. First, overpressured mudstones in the Huangliu Formation have anomalously high acoustic and low resistivity values. Low density values are not recorded suggesting that disequilibrium compaction is not responsible for the observed overpressure. A positive correlation is also lacking amongst vertical effective stress, acoustic velocity and density values - overpressured mudstones recording low effective stress and high density – a character indicating that overpressure is mainly caused by fluid expansion. Overpressured mudstones deviate from the normally loading curve and fall on the unloading curve. Secondly, overpressured mudstones are buried at depths above 3300-4300 m, and subjected to formation temperatures of 135-200℃. Corresponding vitrinite reflectance equivalents are 0.7%-1.3%, supporting that mudstones are in the oil generation window and generate large quantities of hydrocarbons. Overpressured reservoirs are also charged by gas/water mixtures and gas; overpressure among these reservoirs is attributed to pressure transfer during gas charge. Thirdly, the depth for transformation of clay types do not correlate with overpressure generation in the mudstones, suggesting that clay transformation is not the main mechanism promoting local overpressures. Models of maturity and hydrocarbon generation history for the study area show that the source rocks within the Huangliu Formation started to become overpressured at 3.5 Ma and that pore pressure is still increasing. Overpressure increased rapidly between 2.5 Ma and 1.0 Ma, and modelling results are consistent with the present-day values recorded on acoustic logs. For different types of inclusions, we established different paleo-pressure restoration model to quantitatively recover the trapping pressures of reservoir fluid inclusions. The reconstructed paleo-pressures for the four stages of natural gas charging are 29.5 – 41.5 MPa, 45.03 – 47.94 MPa, 35.0 – 100.7 MPa, and 97.22 – 104.31 MPa, with the paleo-pressure coefficients of 0.84 – 1.19, 1.31 – 1.39, 1.41 – 2.32, and 2.38 – 2.56, respectively. Quantitative models further indicate that the mudstones of interest can generate large quantities of hydrocarbons at present to maintain the recorded pore overpressures.