Chapter 8_Trapping Structure and Seal Systems PDF

MKPP 1213 APPLIED GEOSCIENCE & GEOPHYSICS LECTURER: DR. CHONG AIK SHYE CHAPTER 8 Trapping Mechanisms 3 The Petroleum System Traps • A trap • In order to prevent the hydrocarbons rising to the surface and escaping they must be caught in a confined space, termed a trap. i.e. the source, reservo...

MKPP 1213 APPLIED GEOSCIENCE & GEOPHYSICS LECTURER: DR. CHONG AIK SHYE CHAPTER 8 Trapping Mechanisms 3 The Petroleum System Traps • A trap • In order to prevent the hydrocarbons rising to the surface and escaping they must be caught in a confined space, termed a trap. i.e. the source, reservoir and seal must be arranged in such a way that the petroleum is trapped. Petroleum Traps Traps • For a trap to be efficient and commercially viable, a large variety of factors have to be considered. These include: • The presence of a positive porous permeable structure • The imperviousness of the seal • The absence of leaking faults • The migration of sufficient quantities of HC The Trap • Classified (broadly) into: – Structural Traps • Formed by tectonic deformation, diapirism, gravitational and compactional processes, e.g. folds and faults. – Stratigraphic Traps • Caused by depositional differences between adjacent rock types, e.g. pinchout and permeability traps – Hydrodynamic Traps • Caused by water flow A simplified map and cross-section of the Zagros orogenic belt (Iran) – Compressive tectonic Structural Traps • Structural traps are formed where the space for petroleum is limited by a structural feature  Anticlinal traps (or Convex Trap Reservoirs) are formed by folding in the rocks. – Porosity extends in all directions beyond the reservoir – Reservoir is surrounded by water (edge water) Structural Traps Convex Trap Reservoirs Anticlinal Trap Reservoirs Structural Traps Tilted fault-block traps are formed where the upward flow of the petroleum is prevented by impermeability along the fault plane and by an overlying cap or seal. • Reservoir is defined partly by edge water and partly by a fault boundary. Fault Traps Structural Traps Unconformity traps are generated where an erosional break in the stratigraphic succession is followed by impermeable strata. Unconformity Traps Structural Traps • Piercement Trap Reservoirs Formed by diapirs or volcanic necks Reservoir defined by edge water and a piercement contact. Examples of Structural Traps : Iran Stratigraphic Traps • Stratigraphic traps are traps created by the limits of the reservoir rock itself, without any structural control. Pinchout Trap Reservoirs Permeability Trap Reservoirs Pinchout Trap Reservoirs • Formed by lenticular structures (e.g. reefs) • Periphery defined by edge water and the pinchout of the reservoir bed • Here is an example of a reef trap. The diagram shows a vertical slice (cross-section) through the reservoir and overlying rocks. Pinchout Trap Reservoirs • If a porous reservoir rock is encased within an impermeable seal, such as shale or salt, then a trap may form at the updip pinchout of the reservoir (i.e., where the reservoir thickness decreases to zero). Permeability Trap Reservoirs • Form due to changes in reservoir power • Reservoir partly defined by edge water and partly by a permeability barrier Permeability Trap Reservoirs • Stratigraphic traps are also formed in clastic rocks: here, in a cross-section through a continental margin, two sandstone beds form traps within muddy coastal deposits. • River channels may form long, thin traps corresponding to the former position of the river or delta distributary. Beach sands may form sheet-like bodies along an ancient shoreline etc. Hydrodynamic and Combination Traps • Hydrodynamic traps occur where the downward movement of water prevents the upward movement of oil. • Such traps are rare. • Combination traps are formed by a combination of two or more of the previously defined genetic processes. Hydrodynamic Traps Combination Traps • This type of structural trap is very common in fold-and- thrust belts at the front of mountain ranges like the Rocky Mountains of Alberta, where older rocks are pushed sideways over younger rocks (e.g., the yellow unit is here pushed over the light-blue unit). • Oil is pooled in anticlinal folds. • The traps may also be partly faulted, as in the upper one shown here. Combination Traps Combination Traps Cross-section of the Ras Gharib field, West coast of Suez graben, Egypt. Oil is found in faults and unconformity traps of the Carboniferous Nubian sandstone. Petroleum Traps Next Class Chapter 9 Subsurface Geology 32 THANK YOU In the Name of God for Mankind www.utm.my