Regional overpressure genesis of carbonate formations in the northeastern Amu Darya Basin
Carbonate rock sedimentary formations are different from clastic rock formations,possessing highly heterogeneous overpressure characteristics.Regional overpressure phenomena are evident in the carbonate formation in the northeastern part of the Amu Darya Basin,and in this study,the measured formation pressure and the formation pressure calculated from drilling fluid density are used to determine the spatial distributions of overpressure anomalies in different stratigraphic layers of carbonate rocks,as well as their vertical superposition relationships with the formation pressure of overlying gypsum-salt rocks.By using methods such as the physical properties of carbonate rock,mercury analysis,hydrocarbon source rock analysis,hydrocarbon generation simulation,and tectonic extrusion pressure simulation,the origin of overpressure in the carbonate rocks is investigated by comparing the differential distributions of both longitudinal and transverse overpressure and normal pressure.The results show that the sub-salt carbonate rocks developed with both normal pressure and strong overpressure.There is a strong,continuous overpressure zone that spans a large area and is unrelated to local structures and fractures.The sub-salt dense carbonate rocks form a closed system.Although tectonic extrusion has resulted in the development of faults and fractures,the plasticity of the salt rocks has prevented the destruction of the sub-salt carbonate rock closed system,facilitating the continuous development of the overpressure area.Strong tectonic extrusion has also made a significant contribution to the development of overpressure.Due to the good closure of the gypsum-salt rocks,high-quality hydrocarbon source rocks(with TOC up to 6％)between the gypsum-salt rock layer and the carbonate rock layer generate liquid hydrocarbons,which are expulsed into the sub-salt carbonate rock,further promoting the development of overpressure.Dehydration of the gypsum minerals between the gypsum-salt rocks and the carbonate rocks is not the cause of carbonate overpressure.In fact,the effect of gypsum-forming dehydration on the open system of the sub-salt carbonates is weak.These findings are valuable for predicting anomalous overpressure in sub-salt formations.
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