Reservoir sedimentary layers

Oil reservoirs are layers of porous sandstone or carbonate rock, usually sedimentary. Impermeable rock layers, usually shales, and faults trap the oil in the reservoir. The oil exists in microscopic pores in rock. Various gases and water also occupy rock pores and are often in contact with the oil. These pores are intercoimected with a compHcated network of microscopic flow channels. The weight of ovedaying rock layers places these duids under pressure. When a well penetrates the rock formation, this pressure drives the duids into the wellbore. The dow channel size, wettabiUty of dow channel rock surfaces, oil viscosity, and other properties of the cmde oil determine the rate of this primary oil production. 

The sedimentary layer of the Earth s crust is the main carbon reservoir. The Cc and Co concentrations in the sedimentary layer are by an order of magnitude higher than in granite and basalt layers of lithosphere. The volume of sedimentary shell is about... 

Ronov (1976) estimated the average CaO content in sedimentary layer of 15.91 %, and in granite layer, of 2.71 %. Accordingly, the calcium reservoir in sedimentary shell is 272.8 X 10 - tons, and in the granite pool is 222.8 x lO tons. The weathering and metamorphosis of deep-layer silicates is accompanied by the formation of clay minerals with release of calcium available for plant and microbial uptake. 

The theoretical subsidence in various areas over time can be reconstituted with reference to the Hercynian unconformity taken for a zero level by adding successive layers of Mesozoic sedimentary layers. Since the Silurian source rocks (and, occasionally, the Devonian) closely underlie the Hercynian unconformity in the northern Sahara, the subsidence depth relative to the Hercynian surface (the bottom of a Triassic reservoir) is in fact the depth of source rock subsidence for a given area. 

In these examples as well as for most aquatic sediments, the principal diagenetic reactions that occur in these sediments are aerobic respiration and the reduction of Mn and Fe oxides. Under the slower sedimentation conditions in natural lakes and estuaries, there is sufficient time (years) for particulate organic matter to decompose and create a diagenetic environment where metal oxides may not be stable. When faster sedimentation prevails, such as in reservoirs, there is less time (months) for bacteria to perform their metabohc functions due to the fact that the organisms do not occupy a sediment layer for any length of time before a new sediment is added (Callender, 2000). Also, sedimentary organic matter in reservoir sediments is considerably more recalcitrant than that in natural lacustrine and estuarine sediments as reservoirs receive more terrestrial organic matter (Callender, 2000). 

These reasons are connected with S biogeochemical fluxes and pools in biosphere, atmosphere and hydrosphere. However, the main reservoirs are related to lithosphere. According to Ronov (1976) and Dobrovolsky (1994), the average concentration of sulfide sulfur in sedimentary shell is 0.183% and the total amount of sulfur is 9.3 x 10 tons. In addition, the granite crustal layer contains 8.6 x 10 - tons of S. Totally in the Earth s crust there is around 94% of the global S mass (Table 24). 

Due to their low density, freshly formed oil and natural gas deposits migrate upwards through porous strata. Eventually, a large proportion of the hydrocarbons that were formed in the geological past reaches the surface, is lost to the atmosphere and degraded in an aerobic environment. Only those fractions that were trapped beneath an impervious layer of sedimentary rock remain to form oil and gas reserves, captured within the microscopic pore spaces of so-called reservoir rocks. 

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