Increased heavy gas reading Investigation

Bacem Ben Abdallah (Advanced Services Manager at Geolog International B.V.)

We encountered a very strange gas reading starting form the top of shallow Formation to TD. We record a higher heavy gas percentage, light gas ( C1, C2) become less than heavy which give the below figure :
C3>=iC4>=iC5>=nC4>=nC5> C2>C1 ?

We started Investigation From Gas Chromatograph Calibration has been check in, it was confirm to the standard gas calibration and to the original calibration and setting.
• We inject pure Methane in Gas line we got a good reading.
• We decide then with the client after many meeting to Change the gas Chromatograph for the next well
• To send Vaccutainer gas sample next well to take gas sample for further analysis. Give same result as chromatograph field reading.

With the calibration bottle C1 is around 10% and iC5 is 0.7% (ratio is 1 for 13), you Inject mixture gas in gas line you obtain at your chromatograph end a C1 of 1.4% and a iC5 of 0.3% (ratio is 1 for 2.5) how we explain this by gas equipments mal function or the presence of overpressure source rock transition zone?