The South Belridge Opal-A diatomite reservoir is under waterflood and steamflood development. Laboratory measured compositional and viscosity data, API gravity distribution from commingled produced oil, as well as field production performance all suggest the existence of more than one oil type: light oil (29°API) and intermediate gravity oil (22°API), regionally separated by faulting. Accurate PVT and viscosity models are essential for use in thermal compositional reservoir simulation to quantify two major steam flood mechanisms: oil distillation and viscosity reduction. It is a challenge to develop PVT models with limited laboratory measurements of the initial, limited-interval, oil samples. Validation of legacy compositional measurements against mol-fraction distribution vs. SCN (Single Carbon Number) is critical for obtaining a meaningful PVT property model. A new approach of developing viscosity temperature relationship for each pseudo component based on PVT property modeling, and the use of the ASTM viscosity model with limited high temperature viscosity measurements is proposed. Integrated investigation of faulting from static geological modeling and API oil gravity measurements from commingled produced oil and sidewall core oil samples were used to define the boundary of two PVT regions. Waterflood and steam flood project responses were analyzed to assess the impact of fluid types (light or intermediate oil) on waterflood and steam flood performance. Field-scale reservoir simulation of both waterflood and steam flood processes demonstrate the success of applying the new approach for field-scale multiple PVT model zone studies.