Quantitative analysis of top-seal capacity: offshore Netherlands, southern North Sea

Sealing characteristics of the caprock to the Main Buntsandstein reservoir sands in the P blocks in the Dutch offshore are assessed based on a core from the P15 well. The core which represents the Main Buntsandstein, Selling, Rot, and Muschelkalk interval has been analyzed using standard geologic and petrophysical techniques including mercury-injection capillary-pressure tests. The caprock to the Main Buntsandstein reservoir sands consists of anhydritic and/or dolomitic sandstone to argillaceous siltstone, silty shale, and dolostone. Early emplacement of nodular anhydrite followed by cementation and replacement by anhydrite, dolomite, and siderite has resulted in tight, submicroporous pore geometries which act as good to excellent seals. Capillary entry pressures of the best caprock lithologies are such that gas columns of the order of 300 m could potentially be trapped. The actual column height in P15 is about 125 m as indicated by RFT data. The presence of significant gas accumulations in the Main Buntsandstein in the P12, P14, P15, P18 and adjoining Q8 and Q16 blocks suggests that the P15 top-seal quality may be representative for that area.