Trap

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.3.1.Definitions and Concepts •A trap is subsurface configuration of reservoir rock and cap rock or seal that has potential to concentrate petroleum in the pores of a reservoir rock •A trap is a geological feature of a reservoir rock that restricts the flow of fluids •A trap can content one or more reservoirs ..• The highest point of the trap is the crest or culmination. • The lowest point is the spill point. A trap may or may not be full to the spill point.

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Nội dung Text: Trap

Chapter 3: TRAP
3.1 Definitions and Concepts 3.2 Classification: four major types: Structural, Stratigraphic, Stratigraphic, Hydrodynamic and Combination
3.1.Definitions and Concepts •A trap is subsurface configuration of reservoir rock and cap rock or seal that has potential to concentrate petroleum in the pores of a reservoir rock •A trap is a geological feature of a reservoir rock that restricts the flow of fluids •A trap can content one or more reservoirs
• The highest point of the trap is the crest or culmination. • The lowest point is the spill point. A trap may or may not be full to the spill point. • The horizontal plane through the spill point is called the spill plane. • The vertical distance from the high point at the crest to the low point at the spill point is the closure.
• The productive reservoir is the pay. • Its gross vertical interval is known as the gross pay. This can vary from only one or two meters in Texas to several hundred in the North Sea and Middle East. • Not all of the gross pay of a reservoir may be productive. For example, shale stringers within a reservoir unit contribute to gross pay but not to net pay • Net pay refers only to the possibly productive reservoir. (Figure 2, Facies change in an anticlinal trap, illustrating the difference between net pay and gross pay).
Figure 1: Nomenclature of a trap using a simple anticline as an example
Figure 2
• A trap may contain oil, gas or a combination of the two. The oil-water contact, OWC, is the deepest level of producible oil within an individual reservoir • ( Figure 3a , Fluid contacts within a reservoir in an oil-water system). • It marks the interface between predominately oil-saturated rocks and water-saturated rocks. Similarly, either the gas- water contact, GWC ( Figure 3b , Fluid contacts within a reservoir in a gas-water system), • or the gas-oil contact, GOC ( Figure 3c , Fluid contacts within a reservoir in a gas-oil-water system) is the lower level of the producible gas. The GWC or GOC marks the interface between predominately gas-saturated rocks and either water-saturated rocks, or oil-saturated rocks, as the case may be.
Figure 3
• Source rock chemistry and level of maturation, as well as the pressure and temperature of the reservoir itself, are important in determining whether a trap contains oil, gas or both. • In some oil fields (e.g. Sarir field in Libya), a mat of heavy tar is present at the oil-water contact. Degradation of the oil by bottom waters moving beneath the oil-water contact may cause this tar to form. Tar mats cause considerable production problems because they prevent water from moving upwards and from displacing the produced oil.
• Boundaries between oil, gas and water may be sharp ( Figure 4a , Transitional nature of fluid contacts within a reservoir- sharp contact • Gradational ( Figure 4b , Transitional nature of fluid contacts within a reservoir- gradational contact). An abrupt fluid contact usually indicates a permeable reservoir. Gradational contacts usually indicate low permeability reservoirs with high capillary pressure.
Figure 4
• Directly beneath the hydrocarbons is the zone of bottom water ( Figure 5 , Nomenclature of underlying reservoir waters). • The zone of edge water is adjacent to the reservoir.
Figure 5
• Fluid contacts in a trap are almost always planar but are by no means always horizontal. • Should a tilted fluid contact be present, its early recognition is essential for correct evaluation of reserves, and for the establishment of efficient production procedures. • One of the most common ways in which a tilted fluid contact may occur is through hydrodynamic flow of bottom waters ( Figure 6 , Tilted fluid contact caused by hydrodynamic flow)
Figure 6
• There may be one or more separate hydrocarbon pools, each with its own fluid contact, within the geographic limits of an oil or gas field ( Figure 7 , Multiple pools within an oil and gas field). Each individual pool may contain one or more pay zones.
Figure 7
3.2.Classification Basically, traps can be classified into four major types: Structural, Stratigraphic, Stratigraphic, Hydrodynamic and Combination