Methods of tracing fracking liquid in oil or gas bearing formations using plural unique oligonucleotide markers. Method includes pumping the plural volumes of fracking liquid, each marked with a unique oligonucleotide, into the formation, thereby defining plural fracture zones in the formation, and, pumping fluids out of the formation while taking plural fluid samples. Then, analyzing the concentration of the unique oligonucleotides in each of the plural fluid samples, and, calculating the ratio of each of the plural volumes of fracking liquid recovered for each of the plural fluid samples according to the concentration of the unique oligonucleotides present in each of the plural samples. And, then, establishing the quantity of the plural volumes of fracking liquids removed from the plural fracture zones.