Fracture cementation in the North Sea

Fluid migration and distribution are important aspects of the evolution of sedimentary basins. An essential parameter affecting fluid flow is permeability. Even mm thick fractures have permeabilit ies many orders of magnitude above surrounding sediments, and thus have the potential of focusing fluid flow. It is a major task to establish the importance of these fractures as fluid conduits. One aspect is to determine the mechanisms of mass transport and precipitation of the fracture cements. Basically the question is whether the mineral cementing fluids are locally sourced from the wall rock by diffusion or local fluid advection, or have been externally imported and focused into the fractures, or both. Detailed investigation of vein mineral and wall rock may help in establishing the relative importance of the two major cementing processes. Here, we will focus on the use of petrographical observations. In addition, we use microscale stable isotope analysis of carbonate cemented fractures obtained with the laser ablation method (O and C) in order to establish the direction of material transfer. Fracture cements. Cemented fractures occur in various lithologies of Tertiary to Palaeozoic ages within the North Sea, including sandstones, mudrocks, chalk, limestones and volcanics. The fractures are commonly subvertical and 1-2 mm thick, rarely exceeding 1-2 cm, and they are in many cases reactivated. The fractures are cemented by a variety of minerals. However, the about 150 fracture occurrences recorded in our North Sea fracture database are dominantly cemented by carbonates. Other minerals include kaoline, baryte, pyrite and only occasional quartz. This observation contrasts with the common occurrence of quartz veins in metamorphic rocks. Bitumen is also sometimes seen as a fracture filling material. Cemented fractures may be monomineralic, or they are seen to be filled with more than one mineral type. Carbonate cement. Calcite, dolomite/ankerite and siderite are all observed. The grain size of the fracture carbonates is in most cases notably coarser than the grain size in wall rock cements.The wall rock cement is generally also carbonate, but chemically distinct from the vein mineral (e.g. siderite versus dolomite or calcite). Multiple generations of carbonate cementation in fractures occur as well, and also precipitation by carbonate preceding a quite different phase as kaoline is occasionally observed. Carbonate fractures in sandstones which are not cemented by carbonates are recorded in a few cases. Pyrite cement. Pyrite fracture cements are less common, but are observed both within sandstones and altered volcanics. Some fractures have walls lined by early pyrite, and central parts cemented by carbonate, thus documenting the presence of compositionally different fluids at various stages of fracture cementation. Kaoline cement. Fractures cemented by kaoline occur within sandstones and interlayered igneous rocks.The texture is fine grained and appear similar to pore-filling, blocky kaoline cement. Wall rock cements recorded are carbonates or clays. Baryte cement. Baryte cemented fractures are observed within all lithologies. The grains are commonly coarse compared to wall rock, which is cemented by either clays or carbonates. Both equant and elongated grains occur. Dendritic growth is recorded in one case. Quartz cement. Quartz cemented fractures are rarely observed and pure quartz veins are only found within altered igneous rocks. These are probably due to hydrothermal activities. Chert veins are seen within chalk, and carbonate fractures hosted in sandstones occasionally contain a few grains of coarse quartz, which evidence the presence of silicate fluids. Isotopes. The laser ablation method allows for very small-scale recording of variations in isotope composition (C, O) across a carbonate filled fracture. One example of such a carbon isotope profile is given in Fig. 1. The carbon isotopes demonstrate that the carbon source for fracture mineralisation was different from that of the local wall rock.If the fracture fillings has been formed by wall rock dissolution and redistribution, the 613C value of the precipitating mineral would be close to the average value of the