The high pressure - high temperature Culzean field, UK Central North Sea, contains lean gas condensate in the Triassic Joanne sandstones and the Middle Jurassic Pentland sandstones. A comprehensive gas analysis programme was installed as an integrated part of field development in order to monitor gas composition, distribution and origin in the reservoirs and overburden pre- production start-up. Isotube OUT and isotube IN gas samples were collected. The isotube IN data show that some gas is recycled, including alkenes representing contamination from the degradation of mud additives; but concentrations are minor, and do not seem to affect the isotope values derived from the C-2 and C-3 isotube OUT gases significantly. C-13-enriched methane derived from drill-bit metamorphism is recorded in the isotube IN gas, but likewise in low concentrations. Gas data were also acquired from a Continuous Isotope Logging Tool (CILT) which measures real-time gas concentrations and isotope values of C-1-C-3 each foot through the entire drilled section. The CILT thus provides a continuous trend of methane isotope values versus depth, and this trend is useful in identifying changes in gas composition. However, concerns related to CILT include: (i) C-1-C-3 stable carbon isotope detection limits for isotube OUT gas analyses are considerable lower than for CILT; due to the lower isotube gas concentrations required for measurement of C-3 isotopes, isotubes are able to map a shallower vertical thermogenic gas migration front in the overburden. (ii) Discrepancies between isotube OUT and CILT isotope values may be significant and cannot be assigned to analytical uncertainty; by contrast, test gas and isotube OUT isotope values are comparable. Hence, CILT isotope values from specific depths cannot stand in isolation but must be complemented by isotube OUT isotope measurements. Gas in the Pentland reservoir is the most coaly in composition due to self-sourcing from the Pentland coals. The coals are the primary source rock for the gas encountered in the entire reservoir interval at Culzean, but the underlying Joanne sandstones contain contributions from highly mature marine shales in the Kimmeridge Clay Formation and/or Heather Formation. The Lower Cretaceous seal on top of the Pentland reservoir is relatively tight but some minor migration/leakage of thermogenic gas into the overburden is recorded by the detection of C-3 isotopes. Thermogenic gas also occurs in high porosity intervals in the Upper Cretaceous succession but this gas is interpreted to have migrated laterally in porous carrier beds and did not enter these intervals at the well locations.