The disclosure features methods of analyzing a fluid extracted from a reservoir, the methods including introducing a first composition featuring a first complexing agent into a reservoir at a first location, extracting a fluid from the reservoir at a second location different from the first location, combining the fluid with a second composition featuring a concentration of a lanthanide ion to form a third composition featuring a concentration of a complex formed by the first complexing agent and the lanthanide ion, exposing a quantity of the complex to electromagnetic radiation for a first time period ending at a time t.sub.0, detecting fluorescence emission from the quantity of the complex for a second time period starting at a time t.sub.1>t.sub.0, where t.sub.1t.sub.0 is greater than 2 microseconds, and determining information about a fluid flow path between the first location and the second location.

The disclosure features methods of analyzing a fluid extracted from a reservoir, the methods including introducing a first composition featuring a first complexing agent into a reservoir at a first location, extracting a fluid from the reservoir at a second location different from the first location, combining the fluid with a second composition featuring a concentration of a lanthanide ion to form a third composition featuring a concentration of a complex formed by the first complexing agent and the lanthanide ion, exposing a quantity of the complex to electromagnetic radiation for a first time period ending at a time t.sub.0, detecting fluorescence emission from the quantity of the complex for a second time period starting at a time t.sub.1>t.sub.0, where t.sub.0t.sub.0 is greater than 2 microseconds, and determining information about a fluid flow path between the first location and the second location.

Disclosed is an apparatus and method for fluorescence excitation and detection. The apparatus comprises one or more light sources for providing excitation light for fluorescence excitation at an observation spot along an optical axis for excitation, an optical collection element for collecting fluorescence light generated by the excitation light at two or more different observation spots into two or more different measurement channels with an optical axis for collecting non-parallel to the optical axis for excitation of each of the one or more light sources, and, for each of the two or more measurement channels and thereby for each of the two or more observation spots, a dedicated optical detector for detecting fluorescence from the fluorescence light collected by the optical collection element.

The disclosure features methods of analyzing a fluid extracted from a reservoir, the methods including introducing a first composition featuring a first complexing agent into a reservoir at a first location, extracting a fluid from the reservoir at a second location different from the first location, combining the fluid with a second composition featuring a concentration of a lanthanide ion to form a third composition featuring a concentration of a complex formed by the first complexing agent and the lanthanide ion, exposing a quantity of the complex to electromagnetic radiation for a first time period ending at a time t.sub.0, detecting fluorescence emission from the quantity of the complex for a second time period starting at a time t.sub.1>t.sub.0, where t.sub.1t.sub.0 is greater than 2 microseconds, and determining information about a fluid flow path between the first location and the second location.