A method for estimation of fluid saturation Sfl in a subsurface reservoir using at least three well-logging probes lowered into at least one well nearest to the subsurface reservoir (111), the method comprising:
- a) obtaining pairs of sonic velocities and bulk density values from well log data (114), wherein the sonic velocities are obtained from sonic interval transit times (115), wherein the pairs of sonic velocities and bulk density values are obtained for various depths of the at least one well, wherein the sonic velocities are obtained using a first well-logging probe of the at least three well-logging probes, and the bulk density values are obtained using a second well-logging probe of the at least three well-logging probes,
- b) obtaining a series of acoustic impedances (116) for the at least one well by calculating a product for each respective pair of sonic velocity and bulk density value, thereby obtaining a series of sonic velocity-bulk density value products with each product corresponding to an acoustic impedance, wherein the series of acoustic impedances develop in a same direction in response to a volumetric change of water or target fluid in sedimentary rocks,
- c) obtaining a resistivity ratio function (117) for the at least one well by measuring a series of resistivity values from a third well-logging probe of the at least three well-logging probes, the resistivity ratio function being defined as square roots of ratios between a first value for resistivity of water and each measured resistivity value of the series of resistivity values, wherein the first value for resistivity of water is an arbitrarily chosen initial value, and wherein the resistivity ratio function develops in different directions in response to the volumetric change of water versus target fluid,
- d) obtaining resulting pairs, wherein the resulting pairs define an acoustic impedance-resistivity ratio plane (118) such that the resulting pairs within the acoustic impedance-resistivity ratio plane correspond to an equal fluid saturation, the equal fluid saturation associated respectively with the sedimentary rocks, wherein the sedimentary rocks comprising a given percentage of rock matrix or water are equally represented by one pair of values of parameters of 100% fluid saturation, creating a system of sets of pairs of values of the parameters, to obtain a continuous representation of the fluid saturation of a formation of interest penetrated by the at least one well (119),
- e) estimating resistivity background within the formation of interest (120), and simultaneously obtaining a second value for resistivity of water (121),
- f) obtaining inverted controlled-source electromagnetic (CSEM) survey data (112) from the formation of interest,
- g) bringing the inverted CSEM survey data and acoustic impedances inverted from seismic data (113) into a same domain (122), the same domain being depth or time,
- h) estimating the fluid saturation Sfl (124) using an equation (123) by inputting the acoustic impedances inverted from the seismic data (113), the inverted CSEM survey data (112), and the second resistivity of water (121) whereby Sfl=1−Sw, where, Sw is water saturation.