Predicting oil production from chalk reservoirs relies on quantifying contributions from fracture and matrix porosity, which change during compaction. Cl isotopes in water characterise fluid transport processes. Chalk matrix microporosity and fractures will give different values. We will combine two novel methods, trace water extraction from dry oil and Cl isotopes to characterise porosity from produced fluid. High pressure lab. experiments carrying fracture/matrix porosity in cores will give characteristic brine geochemistry related to poroperm to calibrate field values. We will compare fluid derived porosity regime with values measures on field core. Porosity changes from geochemistry will be compared with compaction in the field.