Integrated Trap Analysis in High Pressure High Temperature Fields; Central North Sea

The Central North Sea (CNS) is a complex and mature hydrocarbon province with over 40 years of production. Significant new discoveries (e.g. Catcher and Culzean) continue to be made, and are appealing due to their close proximity to existing infrastructure. The CNS fields have had a complex structural history of burial, extension, inversion and halokinesis. Fields located in the large CNS graben have a wide range of trap styles and sediment juxtaposition. Consequently, a study was initiated to understand the controls on hydrocarbon column height at selected fields, identify reservoir juxtaposition, and define an ideal workflow.

Nine fields were investigated and the hydrocarbon accumulations were found to be controlled by: structural spill, top seal capacity, or low Shale Gauge Ratio (fault leakage). Field studies began by using depth converted seismic and well data to create integrated static models. Trap analysis and SGR calculations were performed to determine the most probable control on accumulation; assuming source and charge is abundant. Estimates of the fault seal capacity were made from Petrel SGR calculations, and compared to known global fault seal capacity in both hydropressured and geopressured reservoirs. All the traps studied with sand-on-sand fault contacts are sealing hydrocarbon column. Estimates of hydrocarbon column height tend to be reasonable for most of the fields studied, but an uncertainty analysis workflow should be followed to ensure a realistic range of possible outcomes for exploration settings.

In addition to a static model, a top seal analysis was completed for each field with available pressure data. This type of analysis gives a best estimate of the maximum reservoir pressure a field can attain before top seal failure. It was determined that the Marnock field, which appears under-filled with respect to dip and fault closures, is top seal limited. This method of analysis should be included in all future work in the CNS and requires a normalization of pressure data and respective depths, to depth below mud line.

Through this approach three fields were found to be filled to fault tips, two filled to dip spill and only one field top seal limited. The remaining accumulations are maintained by fault seal. This approach can be used globally and requires integrating all available data into a detailed static model. Additional uncertainty analysis improves the results and should always be completed where data is sparse.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California