Characterization of Hith-Gotnia Formation Through Advanced Gas Analysis and Interpretation Using a Single Mass Spectrometer

The Hith and Gotnia Formations comprising of alternating halite and anhydrite plus limestone intervals traditionally considered as regional seals for lower Jurassic hydrocarbon bearing reservoirs. Sharp contrast in lithology of halite-anhydrite and high pressure in interbedded limestone layers make this formation extremely challenging to drill and evaluate. Casing is set immediately after drilling, limiting open hole data acquisition. Due to complex lithology and low porosity, a comprehensive acquisition and interpretation methodology was required to further characterize and assess the potential of this formation. A single mass spectrometer combined with a dynamic oil base mud discriminator was used during drilling. The OBM isolates and quantifies formation fluids from drilling fluid and aids in better characterization of fluid in reservoir matrix.

Comparison of oil signatures in the three zones reveals a decrease in fluid density from top to bottom. The difference in fluid density for zones and sealing efficiency of the interbedded halite suggests that there is no direct fluid communication between the individual halite beds. The last anhydrite has signatures similar to the lower reservoirs.

Calibrating these findings with other wells and finally the entire region, will give a better understanding on the overpressure within the carbonates and its distribution.