Integration of resistivity, neutron, and density logs for detecting, identifying, and calculating of groundwater over-pressuring phenomenon at Siwa Oasis Depression, Egypt

Abstract

Along with Siwa Oasis depression, there are several ponds, lakes, and Sabkhas features resulted from the current flows through the springs due to the over-pressure of deep sandstone aquifer. The Nubia Sandstone aquifer (NSA) and Carbonate aquifer are the main two water-bearing units. They are separated by a huge thickness of shale and controlled by complicated structural faults. There are ten wells including gamma-ray, short and long resistivity, caliper, neutron, and density logs. These logs were used in identifying the lithology, calculating the porosities, water saturation percentages, and effective permeability. The long resistivity, neutron, and density logs were used to detect, identify, and calculate this over-pressure and its zones. The shale resistivity, porosity, and density of the Nubia Sandstone formation were used in determining the expected normal compaction trend. The top of over-pressuring, over-pressuring forewarning, over-pressuring transition, and over-pressuring zones were detected. The formation pressure and its gradient were calculated at the first depth in the over-pressuring zone. The piezometric heads and pressures were calculated and mapped. The depths to the top of over-pressures, formation pressures, and their gradient maps were constructed. The statistical analyses between the two pressures were carried out. Accordingly, the thickness and type of shale, the effective porosity, the compaction of the deep aquifer, the huge thickness of the overlaid formation and its porosities, and the complicated structural conditions are considered the main factors in generating the over-pressuring phenomenon at Siwa Oasis. Therefore, this study considers that the used logs are compatible powerful tools in studying this phenomenon.