Fluid Flow in Sedimentary Basins: Flow Along Fractures

Tom Pedersen Institute for Energy Technology, P.O. Box 40, N-2007 Kjeller, Norway


Knut Bjørlykke Dept. of Geology, University of Oslo. P.O. Box 1047 Blindern, N-0316 Oslo, Norway

Harald Johansen Institute for Energy Technology, P.O. Box 40, N-2007 Kjeller, Norway

Magnus Wangen Institute for Energy Technology, P.O. Box 40, N-2007 Kjeller, Norway

Quantification of fluid flow and its effects in sedimentary basins is important in the realms of diagenesis, hydrocarbon migration and formation of ore minerals in compaction driven systems. Fractures represent when they are open high-permeability conduits for fluid flow. Sandstones and mudstones which are not carbonate cemented may remain ductile to 24 km depth. In such rocks, flow is mostly through the inter-granular network and fractures will tend to close due to horizontal stress. Carbonate cemented rocks and limestones may become brittle at very shallow depth and most of the flow will occur along fractures. Brittle properties due to quartz cementation usually requires 3-4 km of overburden at normal geothermal gradients. In over-pressured basins, fluid flow and oil migration may to a large extent be controlled by hydro-fracturing. Due to the very low compressibility of water, the fluid volumes resulting from the decompression of pore water itself is small. However, when over-pressure bleeds off, sediment compaction will occur, increasing the total volume with a factor approximately equal to the ratio of sediment to water compressibilities. Compaction of large volumes of sediments is a slow process, and sudden episodes of hot compaction driven flow is unlikely. Besides at depth where the temperature is 150-200°C (4-6 km with normal geothermal gradients), the porosity is low, and the volume of hot water is relatively small. Sedimentary ores are therefore not likely to form from compaction driven flow, but by hydrothermal water during uplifts and fracturing. Near the surface, rapid cooling will occur due to boiling or mixing with cold water and cause concentrated precipitation of ores. Where an over-pressured sedimentary bed is intersected
by an opening fracture, the initial fluid velocity may be
very high. However. the velocity decreases rapidly with a characteristic time proportional to the square of the typical horizontal distance between the fractures. Focusing of flow in aquifers into fractures may produce high velocities as well as high total fluid fluxes. Upwards flowing fluids will lose heat to the wall rocks and cool. We estimate that in most cases, the fluid temperature in a fracture will follow rather closely the geothermal gradient in the basin; this is certainly the case for compaction driven flow in fractures for the vast majority of cases. One-dimensional models may strongly overestimate the thermal anomalies caused by fluid flow through fractures. Cooling of upwards moving fluid can cause precipitation of silica due to its prograde solubility. Our results indicate that silica precipitation is an inefficient mechanism in terms of fracture cementation in sedimentary basins. Due to the low solubility of silica, large volumes of fluid must be transported through the fracture for significant precipitation to occur. However, because of the slow kinetic rates of silica precipitation at temperatures characteristic for sedimentary basins, as well as the small specific surface of mm-cm wide fractures, very little precipitation will take place under such circumstances. This phenomenon may explain the relative rare occurrence of quartz cement in fractures formed at low temperatures. Carbonate is the dominant fracture cement in the upper few kilometres of sedimentary basins. Kinetic inhibition of silica precipitation is much reduced under metamorphic conditions, making quartz cemented fractures common under such conditions.