The Halyard gas discovery: a case study

Abstract

The Halyard—1 exploration well discovered gas in a new and separate reservoir unit at the top of the Barrow Group along the outer margin of the Barrow Delta. The well is located in WA—13—L, approximately 15 km northwest of the East Spar Field and 4 km east-northeast of the Spar—1 well. It was drilled in February 2008 and intersected 27.8 m of net gas pay in a Valanginian age Barrow Group reservoir interpreted to be a wave-dominated shoreface sandstone. The nearby Spar—1 well was drilled in 1976 and intersected 18.4 m of net gas pay in an older Barrow Group Sandstone within a distinctly separate, delta-front turbidite setting. The Halyard and Spar fields are located within the overall confines of a structural closure at the Top Barrow Formation level. Earlier reservoir models dating back to the 1970s assumed connectivity between all permeable sandstones within the Top Barrow closure. Critical analysis and integration of all subsurface and engineering data sets inclusive of modular formation dynamics tester (MDT) pressures, condensate gas-ratios, production test data, post-drill amplitude variation with offset (AVO) modelling and seismic facies mapping, however, indicate these reservoirs are distinct and separate stratigraphic traps formed by successive, sequentially sealed depositional units within a larger delta slope edge sequence. Collection of this data has yielded considerable insight into what historically have been viewed as simple traps. In our view and experience, the Barrow Group geology is much more complex with many traps typified by critical stratigraphic components in their trapping mechanism. The Halyard discovery is an example of the type of trap we expect to host much of the remaining reserve potential in the proximal Barrow Delta.