MASS-TRANSFER AND FLUID CONNECTIONS IN THE NORTH SEA HYDROCARBON BASIN

When sandstones are buried in hydrocarbon basins, they become cemented, and economic porosity decreases. How important is porewater motion to supply ions during oilfield cementation? Quartz is a common cement below 2.5km and can originate from diverse sources. We show in the Leman field (4km max, now inverted) that cement does not originate from stylolites, that mass-balance does not occur on a 50mm scale, but cement mass-balance does occur on a >10m scale by grain-to-grain dissolution and redistribution from fine to coarse sand. In the 4km Miller oilfield, quartz cementation rate has been reduced from the time when oil filled pores progressively throughout the trap. Quartz cementation occurred by diffusion, without a requirement for porewater motion. Evidence from oxygen and silicon isotopes, together with fluid inclusions, shows that in both cases, complete changes of porewater have occurred, but silica was local.

During deep burial, from 4km to over 6km, excess geopressures 10-30MPa occur. Geopressure provides a drive for fluid movement, and present day pressures indicate fluid is connected across the 50km rift basin. During deep burial, feldspar dissolves-- progressively changing the bulk rock from arkose to quartzite. K and Al are exported from sandstones, presumably into enclosing mudrocks. Feldspar dissolution seems to be enhanced at structural highs, and in the upper part of sandstones. Present day 10-25C temperature anomalies show that porefluid is migrating vertically up through the basin at these points, and also communicates buoyant oil 2-4km vertically. Adjacent 6km deep Erskine and 3km deep Machar fields enable calculation of aqueous fluid flux, by deep-derived Sr replacing anhydrite to form celestite at shallower depth. However only minor transport of Al can be demonstrated over that distance.

Thus significant movements of water and hydrocarbon occur vertically from 6km towards the surface. However advective mass-transfer of minerals or ions is minor; diffusion seems to be more important.