Petroleum Seal Rocks

Seal—Rock made of fine particles and having little pore space or connection between pores that prevents fluids from leaking out of a reservoir rock. Shale and salt provide some of the best seals for petroleum reservoirs. 

Drilling is successful when the well strikes petroleum deposits below their seal rock. Natural pressure ejects gas and oil through the pipe. A wellhead is placed on the top of the well, and natural gas is separated from crude oil in a separator unit. Both products are collected and transported by intermediary pipelines connecting the many wells of a single field. 

Figure I. Elements of a Petroleum System. All petroleum systems contain 1. at least one formation of organic-rich sediments that has been buried to a sufficient depth by overburden rock such that petroleum is generated and expelled, 2. Pathways (permeable strata and faults) that allow the petroleum to migrate, 3. Reservoir rocks with sufficient porosity and permeability to accumulate economically significant quantities of petroleum, and 4. Sealing rock (low permeability) and structures that retain migrated petroleum within the reservoir rock. The top and bottom of the oil window is approximated as a function of burial depth. In actual basins, these depths are not uniform and vary as a function of organic matter type, regional heat flow from basement, in thermal conductivity of the different lithologies, and burial history (e.g., deposition rates, uplift, erosion, and hiatus events).

The movement of petroleum from the place of its origin to the traps where accumulations are found is believed to have occurred in an upward direction. This movement took place as the result of the tendency for oil and gas to rise through the ancient seawater with which the pore spaces of the sedimentary foimations were filled when originally laid down. An underground porous formation or series of rocks which occur in some shape favorable to the trapping of oil and gas must also be covered or adjoined by a layer or rock that provides a coveting or seal for the trap. A seal of this type, frequently called a cap rock, stops further upward movement of petroleum through the pore spaces. 

Schlomer, S. and Krooss, B.M. (1997) Experimental characterisation of the hydrocarbon sealing efficiency of cap rocks. Marine and Petroleum Geology 14, 565-580... 

In order for a reservoir to contain petroleum, the reservoir must be shaped and sealed like a container. Good petroleum reservoirs are sealed by a less porous and permeable rock known as a seal or cap rock. The... 

Reservoir rocks and seals work together to form a trap for petroleum. Typical traps for petroleum include hills shaped like upside-down bowls below the surface of the earth, known as anticlines, or traps formed by faults. Abrupt changes in rock type can form good traps, such as sandstone deposits next to shale deposits, especially if a sand deposit is encased in a rock that is sufficiently rich in organic matter to act as a petroleum source and endowed with the properties of a good seal. 

An important aspect of the formation of petroleum accumulations is timing. The reservoir must have been deposited prior to petroleum migrating from the source rock to the reservoir rock. The seal and trap must have been developed prior to petroleum accumulating in the reservoir, or else the petroleum would have migrated farther. The source rock must have been exposed to the appropriate temperature and pressure conditions over long periods of time to change the organic matter to petroleum. The necessary coincidence of several conditions is difficult to achieve in nature. 

Watts, N.L., 1987. Theoretical aspects of cap-rock and fault seals for single- and two-phase hydrocarbon columns. Marine and Petroleum Geology, Vol. 4, November 1987, pp. 274-307 Weber, K.J., 1982. Influence of common sedimentary structures on fluid flow in reservoir models. Journal of Petroleum Technology, March 1982, pp. 665-672 Weber, K.J., 1987. Hydrocarbon distribution patterns in Nigerian growth fault structures controlled by structural style and stratigraphy. Journal of Petroleum Science and Engineering, 1, pp. 91-104... 

The rocks in which large volumes of petroleum are able to accumulate are termed reservoir rocks. They require suitable porosity (typically 10-25%) and permeability (typically 1-1000mD 1 mD or milliDarcy = c.10 9m2), with reasonably sized pores and an impermeable cap rock or seal to prevent escape of petroleum over geological time periods. They must also be in place before the onset of oil generation. Sandstones often provide suitable reservoir characteristics. More than 60% of all oil occurrences are in clastic rocks, while carbonate reservoirs account for c.30%. The smaller molecules present in gases can escape through narrower pores than oil components, and seals are often slightly leaky with respect to gas. 

Another possible explanation of the low GORs in the Field is the imperfect seal over the reservoir. Shows of petroleum resembling the Snorre population have been found several hundreds of metres over the reservoir interval (Caillet 1993 Leith et al. 1993 Leith Fallick 1995), even though the field probably has never reached fracture pressure. It is clear that the poor cap rock quality on the relatively shallow and underfilled Snorre structure makes this a perfect candidate for a Type 3 trap as explicitly classified by Sales (1997), i.e. the GOR of the Snorre petroleum charge is determined more by the cap rock properties than by SR quality and its kinetic properties. The oil inferred by Leith et al. (1993) may have leaked from the reservoir as the cap rock section has very low pore entry pressures and micro fracturing is also observed in the cap rock section. One can therefore speculate that the leakage may be a prerequisite to maintain black oil in the Snorre Field. 

Furthermore, the undersaturated nature of the petroleum is suggested not to be primarily controlled by the maturity of the source rock, but rather by the cap rock properties (Sales 1997) and the burial (pressure) history of the accumulation. The same mechanism is suggested to cause more shallow fields in the area to maintain a black oil preference as lower boiling compounds are rapidly leaking from the relatively poor seals. 

This pattern of repeated filling events from a progressively maturing source rock basin, structural trap tilting, and migration up the fault zone, coupled with phase fractionation and separation of petroleum into oil and gas phases in the reservoir intervals (Fig. 32), which were subsequently progressively sealed off due to increased quartz cementation in response to increased burial, may explain the complex patterns of variable GOR, API and carbon isotope values observed in Smorbukk (cf. Fig. 14). 

Historically, a BCGS has been defined in terms of the fundamental petroleum system elements and processes associated with development and formation of this resource. Similar to conventional hydrocarbon reservoirs, the BCGS process requirements generally include deposition of the reservoir rock, hydrocarbon generation, migration and entrapment by geological structural elements and/or seal capacity genesis. 

Over time, the primitive oceans dried up. The sand and mud that had accumulated on the ocean floors changed into rock. The natural gas and liquid petroleum that had formed on the ocean floor was trapped in the rock. It flowed through cracks in the rock until it reached porous rock that acted like a sponge and soaked up the petroleum and natural gas. These fossil fuels remain trapped in the porous rock by non-porous layers of rock that act like caps or seals on the porous rocks. 

The third essential requirement for a commercial accumulation of petroleum is a cap rock or seal. This is a sedimentary stratum that immediately overlies the reservoir and inhibits further upward movement. A cap rock need have only one property It must be impermeable. It can have porosity, and may indeed even contain petroleum, but it must not permit fluid to move through it. Theoretically atty impermeable rock may serve as a seal. In practice it is the shales and evaporites that provide most examples. Shales are probably the commonest, but evaporites are the more effective. We saw earlier how mud is compacted during burial into mudstone, or shale. These rocks are commonly porous, but because of the narrow diameter of the pore tluoats, they have negligible permeability. Thus shales generally make excellent seals to stop petroleum migration. When strata are folded or faulted, however, brittle shales may fracture. As described earlier, iiactures enhance permeability most dramatically. In such instances, petroleum may leak from an underlying reservoir and ultimately escape to the surface of the earth. 

Petroleum, which is generated over millions of years from organic matter that is buried deep in the earth, is usually trapped in underground structures formed by porous rocks. These rocks usually are sandstone or fissured limestone. The petroleum, which is produced in deep-lying source rocks (source reservoir), migrates to these structures and is trapped there, if these structures are sealed at the top. These structures then form what is called a conventional oil field. It is important to note that the source rock will thus be expected to contain many times larger amounts of oil than as one finds in conventional reservoirs. 

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