Seal and Reservoir in CO2 Long-Term Natural Analogues: North Sea Miller Oilfield

R. Stuart Haszeldine, Jiemin Lu, Mark Wilkinson, and Gordon Macleod
The University of Edinburgh, Edinburgh, United Kingdom

The UK North Sea contains a cluster of hydrocarbon fields with naturally elevated CO2 contents of 20mol% in the oils. These hydrocarbon fields are buried to about 4km, and include Miller and the Brae cluster. Miller is proposed by BP and Scottish ands Southern Energy to be the world's first site to take onshore CO2 for offshore Enhanced Oil Recovery and CO2 storage. This is the same geological province, which hosts Statoil's Sleipner West, and supplied CO2 to the offshore SACS demonstration. We are examining Miller using approaches established in studies of oilfield reservoir quality.

Wireline logs show that extensive concretionary carbonate cements exist. Some of these appear to form palaeo-horizontal contacts, and may indicate old CO2 charge deeper than the present oil. Isotopic and fluid-inclusion analyses are underway to test this possibility. The reservoir sandstone is unusually quartz-rich, but with a low abundance of diagenetic clay cement, it is equivocal if large–scale dissolution of depositional feldspar has occurred. Initial results from analysis of mudrock mineralogy suggest that systematic differences exist between mudrocks interbedded the oil zone, and those interbedded with the aquifer or forming the overlying seal. A young calcite cement exists in mudrock within 5 metres of the top-reservoir, isotopic analysis is underway to compare with CO2 signatures. This pattern of mineral cements is unlike that predicted by published geochemical modelling.