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Temperature effects on geopressure during deposition and erosion

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... On the effect of crustal vertical uplift, the load of the formation decreased for its overlaying rocks are eroded, resulting in its porosity and formation fluid being uninstalled and expanded. Pore expansion will definitely lead to the reduction of formation pressure, the pore fluid expansion also occurres at − 568 − the same time, the formation pressure will increase to some extent [4,6,8] , ultimately, the final change of formation pressure will depend on the interaction of the two. Compression coefficient of sandstone is 1×10 −9 Pa −1 , and compression coefficient of water is 3×10 −10 Pa −1 according to Russell [15] , so during the process of formation uplift, the volume of formation water swelling is less than 1/3 of the volume of pore increment resulting in the pores swelling. ...
... In the middle-late period of the early Cretaceous, the Mesozoic reaches the maximum depth, and the formation temperature reaches the highest. According to a large number of drilling data, the average buried depths of main exploration strata of Chang 4 +5 , Chang 6 Homogenization temperature of inclusions represents the formation temperature when inclusions are captured. The statistics of homogenization temperature of fluid inclusions of the Yanchang Formation is done (Fig. 3), and the two high value intervals (120-130 °C and 130-140 °C) of homogenization temperature data are averaged to approximately represent the average maximum temperature that the Yanchang experienced. ...
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Abnormally low pressure is characteristic of the Mesozoic reservoirs in the Ordos Basin. Based on the study of low pressure distribution, the formation mechanisms of abnormally low pressure were quantitatively analyzed from several aspects, such as tectonic uplift, stratum temperature decrease and dissolution-increased porosity. The Mesozoic reservoir pressure experienced two major evolutionary stages: the pressure rising to maximum pressure stage from the Late Triassic to the Early Cretaceous, the pressure decreasing to abnormally low pressure stage from the end of Early Cretaceous to now. Since the late Early Cretaceous, porosity rebound caused by tectonic uplift, stratum temperature decrease and dissolution caused pressure decrease of up to 20–25 MPa, the decrease ratio is about 54%-77%. The three factors are the main reason for the pressure decrease of the Mesozoic in the Ordos Basin, and among them stratum temperature decrease is the most important factor. Combined with other factors, they made the formation pressure reduced gradually and eventually formed the abnormally low pressure distribution pattern of 10–18 MPa.
... The pore fluid contracts more than the rock skeleton with the temperature decreasing, thus resulting in the reduction of the pore pressure [64]. The stratum uplifting and denudation can lead to unloading of overburden pressure, which will increase the pore space and reduce the pore pressure [65]. Therefore, the depressurization resulting from pore rebound was closely related to the erosion thickness of the stratum [62,63]. ...
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The vague understanding of the coupling relationship among natural gas charging, reservoir densification, and pressure evolution restricted the tight gas exploration in the Lower Shihezi Formation of the Hangjinqi area, north Ordos Basin. In this study, the quantitative porosity evolution model, the pressure evolution process, and the natural gas charging history of tight sandstone reservoirs were constructed by integrated investigation of the reservoir property, the thin section, SEM and cathode luminescence observations, the fluid inclusion analysis and the 1D basin modeling. The results show that the compaction and cementation reduced the primary porosity by 21.79% and 12.41%, respectively. The densification of the reservoir occurred at circa 230 Ma, which was before the natural gas charging time from 192 to 132 Ma. The paleo-overpressure within the tight reservoirs occurred since the Middle Jurassic with the pressure coefficients between 1.1 and 1.55. The continuous uplifting since the Late Cretaceous resulted in the under- and normal-pressure of the Lower Shihezi Formation with the pressure coefficients ranging from 0.67 to 1.05. The results indicate that the densification of the reservoirs was conducive to the formation of paleo-pressure produced by gas generating. The gas predominantly migrated vertically, driven by gas expansion force rather than buoyance and displaced the pore water in the reservoirs near source rocks.
... If the pore is filled with water, pressure drop is dramatic, and underpressure occurs. If the pore is filled with dry gas, a significant pressure drop (lower than the hydrostatic pressure at the new depth) would not be expected, thus the reservoir would form or maintain overpressure 20,21 . The scheme of uplift effect on fluid pressure is shown in Figure 1. ...
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Underpressured reservoirs are widespread in the Huatugou (花土沟) oilfield of the western uplift in Qaidam basin, western China. At depths between 462 and 1 248 m, the pressure of Neogene reservoirs in the Huatugou oilfield is only about 40% to 80% of hydrostatic pressure. Based on a study of the geological characteristics of these underpressured reservoirs, this work used fluid inclusion analysis and numerical simulation to investigate the mechanism creating these abnormal pressures and to evaluate the characteristics of the hydrocarbon distribution. The results show that the underpressured reservoirs are all well-sealed by undercompacted and thick mudrocks. The large-scale tectonic uplift in the late Himalayan plays an important role in the generation of underpressure in the Huatugou oilfield. At the beginning of this movement, the field was overpressured due to episodic petroleum accumulation. Later, structural uplift and erosion led to porous rebound and a temperature decrease, which produced the underpressure. Key Wordsunderpressure–formation mechanism–structural uplift–Huatugou oilfield–Qaidam basin
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