apparatus and method for providing in - situ data of formation fluids at true reservoir conditions . the apparatus has nmr chamber of the flow - through type , so it is no longer necessary to divert a sample for the basic nmr analysis or transfer it uphole for laboratory analysis . the apparatus is preferably modular and is compatible with wireline tools configurable for a variety of sampling , testing and monitoring purposes . fluid properties such as viscosity can be derived from diffusivities and relaxation times , as measured by nmr . additional accuracy is provided in the downhole sampling process , and in the interpretation of nmr logs in real time at true reservoir conditions .