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Mandarin East, Norwegian Southern North Sea: Is Source Effectiveness the Reason for Failure of 1/3-12 S?


Post well analyses proposed that the most plausible reason for failure of the 1/3-12 S Mandarin East well was a lack of charge despite the Jurassic/Triassic reservoir being directly overlain by the Upper Jurassic Volgian/Kimmeridgian and Oxfordian source intervals. In order to test this hypothesis, 2D Petroleum Systems Modeling (PSM) was performed on an N to S line and a W to E line across the Mandarin structure. A point to note is that prior to drilling, PSM had identified three kitchens lying to the N, S and W of the Mandarin structure and charge was not considered the critical risk. During drilling, high gas readings were encountered in the Volgian/Kimmeridgian and Oxfordian source intervals but not in the Jurassic/Triassic reservoir section. Post well Fluid Inclusion Stratigraphy (FIS) was somewhat inconclusive and probably effected by drilling; however the results suggested that no indigenous hydrocarbons had migrated into the reservoir. For post well PSM, maturity and temperature calibration of the models was achieved by using %Ro and BHT from the offset wells 30/8-1 and 30/8-3. Unfortunately the %Ro data from the 1/3-12 S well was unreliable with wide ranging values for a given depth and could not be utilized. Source rock parameters for the Upper Jurassic source interval were taken directly from those measured in 1/3-12 S and were corrected to their likely immature values. Essentially the Volgian/Kimmeridgian source interval was modelled as having oil and gas prone potential whilst the Oxfordian was modelled as having only gas prone potential. As no direct pressure measurements were taken in the reservoir, pressure calibration was achieved by extrapolating the pressure measured from a gas event close to the top of the Kimmeridgian interval down to the reservoir horizons. On simulation, both models showed that there was a minor gas pool on-structure up-dip of the well. Source tracking in the models indicated that the gas prone Oxfordian is the effective source for Mandarin East, rather than the Volgian and that the reservoir is charged by downward migration. The charge volume on Mandarin East is limited by sealing faults, with this structure being unable to access further charge from the source kitchens to the N and S. This is also likely to be the case for the Mandarin West Horst structure. In conclusion, post well PSM has confirmed that the failure of Mandarin East is most likely due to an effective charge mechanism.