Mud logging involves rig-site monitoring , recording and assessment of various type of data related to borehole which comes to surface by drilling process.Mud logging is also called as surface logging.Borehole data is recorded by examining cutting data brought to surface and doing gas analysis.Mud log provides first glimpse of lithology and fluid content of borehole while drilling.Normally mud log data is provided by a mud logging contractor ,they set up a laboratory near drilling rig to do the above mentioned analysis. Main deliverable from mud log laboratory are depth wise hydrocarbon zones ,identify markers, Gas entering mud stream , lithology plotted together on log and finally provide information to driller to ensure safe and economical operations.In order to provide these deliverable mud loggers must do following things:-
Use of mud log for petrophysics interpretation focuses on hydrocarbon indicators from cutting samples,Gas detected from mud log are classified as Liberated gas,recycled gas,produced gas,contamination gas and trip gas.Total gas from mudlog is separated into C1,C2,C3,C4,C5 and C6 gas components using gas chromatograph.Ratio of these gas components are used to determine gas and oil quantity using pixler plot as given in below diagram:
Figure Pixler plot
In the absence of pixler plot ,basic rule of thumb is C1 should be very high in gas sands hence C1/C2, C1/C3,C1/C4 and C1/C5 will be having value higher than 50,Oil zones will be having ratios in the range of 2 to 20 and wetter gases could have ratios in 20-50.These values are just estimates and may differ in different reservoirs.
Usually as a quick look to identify hydrocarbon bearing zones ,petrophysicist plot Total gas readings and try to compare total gas reading in different zones with background total gas reading.
One major issue to consider in Mud logging is depth and lag time.Mud log measurements are done on surface but data is from subsurface depth which introduces some uncertainties in mud log depths.Depth information is provided by driller’s log,which is basically depth versus time of day.Lag time is time taken by mud sample to move from bit to surface,lag time depends upon velocity of mud in annulus between drill pipe and formation.If lag time gets much shorter than expected,it could be due to leaking drill pipe.It is recommended to use tracing method to establish lag time.