Downhole Real-Time Filtrate Contamination Monitoring

A method, comprising:

• estimating a stock tank oil (STO) basis density of a fluid obtained from a subterranean formation based on data associated with the obtained fluid, wherein the data includes gas-oil ratio (GOR) data;
• fitting the estimated STO-basis density of the obtained fluid as a function of the GOR data to determine an STO-basis density of oil-based mud (OBM) filtrate contamination in the obtained fluid and a parameter relating the STO-basis density of the obtained fluid, the STO-basis density of the OBM filtrate contamination, and the GOR data;
• determining the STO-basis density of the obtained fluid based on the determined STO-basis density of the OBM filtrate contamination, the parameter, and the GOR data, thus obtaining a log of the STO-basis density of the obtained fluid with respect to volume of the obtained fluid or time elapsed during obtaining the obtained fluid;
• determining an API gravity log based on the log of the STO-basis density of the obtained fluid;
• and
• estimating a volume fraction of OBM filtrate contamination within the obtained fluid based on the API gravity log.

A method, comprising:

• determining a linear relation between formation volume factor (FVF) data of a fluid obtained from a subterranean formation and gas-oil ratio (GOR) data of the fluid, wherein data associated with the obtained fluid includes the FVF data and GOR data;
• determining stock tank oil (STO) basis density of native formation fluid within the obtained fluid based on a slope of the linear relation, a GOR of the native formation fluid, and a density of the native formation fluid;
• determining a parameter relating STO-basis density of the native formation fluid, STO-basis density of oil-based mud (OBM) filtrate contamination within the obtained fluid, and GOR of the native formation fluid;
• and
• determining STO-basis density of the obtained fluid based on the STO-basis density of OBM filtrate contamination, the parameter, and the GOR data.