Rock migration

Gas from a source rock migrates vertically upwards into a trap which is closed by a seal. 

Keywords plate tectonics, sedimentary basins, source rocks, maturation, migration, reservoir rocks, traps, seismic, gravity survey, magnetic survey, geochemistry, mudlogs, field studies. 

Migration describes the process which has transported the generated hydrocarbons into a porous type of sediment, the reservoir rock. Only if the reservoir is deformed in a favourable shape or if it is laterally grading into an impermeable formation does a trap for the migrating hydrocarbons exist. 

The maturation of source rocks is followed by the migration of the produced hydrocarbons from the deeper, hotter parts of the basin into suitable structures. Hydrocarbons are lighter than water and will therefore move upwards through permeable strata. 

The pores between the rock components, e.g. the sand grains in a sandstone reservoir, will initially be filled with the pore water. The migrating hydrocarbons will displace the water and thus gradually fill the reservoir. For a reservoir to be effective, the pores need to be in communication to allow migration, and also need to allow flow towards the borehole once a well is drilled into the structure. The pore space is referred to as porosity in oil field terms. Permeability measures the ability of a rock to allow fluid flow through its pore system. A reservoir rock which has some porosity but too low a permeability to allow fluid flow is termed tight . 

Objecfive Method Basin Source Rock Maturation Migration Rese rvoir Rock Trap Fluids oil, gas, water... 

The magnesium ion is made available by migrating pore waters. If the process is continuous on a geologic time scale more and more Mg + is introduced to the system and the porosity reduces again. The rock has been over-dolomitised. 

Nearly all reservoirs are water bearing prior to hydrocarbon charge. As hydrocarbons migrate into a trap they displace the water from the reservoir, but not completely. Water remains trapped in small pore throats and pore spaces. In 1942 Arch/ e developed an equation describing the relationship between the electrical conductivity of reservoir rock and the properties of its pore system and pore fluids. 

Migration 244. Accumulation 245. Structural Geology 246. Traps 251. Basic Engineering Rock Properties 254, Basic Etigineering Soil Properties 266. 

The hydrocarbons in some altered form migrate from the source beds through other more porous and permeable beds to eventually accumulate in a rock called the reservoir rock. The initially altered (i.e., within the source beds) organic material may continue to alter as the material migrates. The hydrocarbon movement is probably the result of hydrodynamic pressure and gravity forces. As the source beds are compacted by increased burial pressures, the water and altered organic material are expelled. Water movement carries the hydrocarbons from the source beds into the reservoir, where the hydrocarbon establishes a position of equilibrium for the hydrodynamic and structural conditions [26-29]. 

Fault traps—involve the movement of the reservoir rock formation to a position where the formation across the fault plane provides a seal preventing further migration of hydrocarbons (see Figure 2-48). 

Solution porosity refers to voids formed by the solution of the more soluble portions of the rock in the presence of subsurface migrating (or surface percolating) waters containing carbonic and other organic acids. Solution porosity is also called vugularporosity where individual holes are called vugs. 

This equation has been used for estimating migration velocities of radionuclides (e.g. 66). Here Pr is the density of the rock (kg/m3), p the density of water, e the fissure porosity, af the specific surface of fissures in the bedrock (m2/m3) and ap the specific surface of particles used in the Kd determinations (m2/m3). The distribution coefficient Kd represents ar. equilibrium value for the particular rock under the pertinent conditions. 

If, hypothetically, the whole rock was taker to be porous, a /a would be 1 (volume sorption) this is typical for the migration in a chromatographic column. In reality, only part of the rock matrix could be available for sorption, typically giving aj/ap = 10-3 (surface sorption) for the conditions used in our studies (66). Other representative values are p =2500 kg/m3, p =1000 kg/n and e 0.001. r W... 

The assumptions behind this calculated example are a considerable oversimplification of the real migration behavior. Effects such as diffusion into the rock matrix, dilution with inflowing water, dispersion of the migration front, channeling of the host... 

Shikazono et al. (2002) considered the depositional mechanism of quartz and cristobalite and the change in silica concentration of fluid migrating through the altered rocks in the Hishikari mine district based on kinetics-fluid flow mixing model. Their discussion is summarized below. 

Hydrothermal solution containing appreciable amounts of dissolved silica migrates through andesitic volcanic rocks, accompanying Si02 precipitation. 

J. and Tanimura, S. (1973) Kind of volcanic rocks related to formation of the black ore deposits, and migration of some elements in the volcanic rocks during the mineralization. Mining Geology, 23, 337-346 (in Japanese). 

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