In: Physics
In principle a density log measures the electron density of any formation. It has a contact tool that emits gamma rays from a source. Emitted gamma rays collide with formation electrons and scatter. A detector, located a fixed distance from the tool source, counts the number of returning gamma rays. The number of returning gamma rays is an indicator of formation bulk density.
Formation bulk density is a function of matrix density, porosity, and fluids contained in the pore space. Formation bulk density measured by the log must be corrected for borehole irregularities. Calculate porosity from bulk density using this equation:
where:
The neutron log mainly measures hydrogen concentration in a formation. The logging device is a noncontact tool that emits neutrons from a source. Emitted neutrons collide with nuclei of the formation and lose some of their energy. Maximum energy loss occurs when emitted neutrons collide with hydrogen atoms because a neutron and a hydrogen atom have almost the same mass. Therefore, most neutron energy loss occurs in the part of the formation that has the highest hydrogen concentration.
The density-neutron crossplot helps determine lithology of oil- or water-filled formations that are pure lithologies like sandstone, limestone, or dolomite. The density–neutron crossplot analysis can be ambiguous when the formation is of mixed mineralogies, like a dolomite-cemented sandstone.