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CHAPTER 04 SEAL (ROOF OR CAP ROCK) CONTENT: 4.1 TYPES 4.2 GENERAL PROPERTIES 4.3 MICRO PROPERTIES OF SEAL 4.4 MACRO CHARACTERISTICS OF SEAL Definition: Seal is impermeable rock that forms barrier on top of the reservoir rock of an oil and/or gas reservoir In the case of anticlines ( Figure 1, A), only a vertical seal, or caprock, is required; but faults (Figure 1, B) and stratigraphic traps ( Figure 1, C,D) must be sealed both vertically and laterally Figure 1 4.1 TYPES: the seal is commonly: Best SEAL: Formed by ductile sedimentary rock: clay or shale (for most sandstone reservoir, >60% of known giant oilfields have shale seal) Shale is the dominant caprock of worldwide reserves (Figure 3) and is overwhelmingly the seal in basins rich in terrigenous sediments, where sandstones are the dominant reservoir rock ° ° Idea cap rock: evaporates (especially favorable where the reservoir rock are carbonates, its density being almost 3.0) Evaporites, however, are the most efficient caprock They are particularly common in carbonate-rich basins, and they often form seals for carbonate reservoirs Furthermore, evaporites commonly develop in restricted basin settings, where accumulations of organic-rich source rocks are also favored (Figure 2) •Third common type: Dense carbonates are the third most abundant caprock lithology and seal about 2% of the world's reserves, cemented rocks, argillaceous rocks, chalk… (Figure 3) The Seal • Commonly shales, evaporites, and dense carbonate • Relatively impermeable Figure 2 Figure 3 4.2 GENERAL PROPERTIES Permeability in seal are mostly < 10-4 darcies Seal are important and commonly overlooked component in the evaluation of a potential hydrocarbon accumulation Effective seals for hydrocarbon accumulation are typically thickness, laterally continuous, ductile rocks with high capillarity entry pressure To calculate the seal capacity, the geologist needs also to know the pore size and parameters permitting the fluids to pass through pores of that sizes, the fluid densities, the interfacial tension between the fluids, and the wettability Seal need to be evaluated at two different (micro and macro) scales 4.3 MICRO PROPERTIES OF SEAL Capillary pressure, Pc Pc= 2γcosθ/ R γ: Hydrocarbon –water interfacial tension; θ: Wettability; R: Radius largest pore throats Hydrocarbon pressure, P P = (ρw-ρhc) *g*h ρw: density of the water; ρ hc: density of the HC.; g: the acceleration of gravity; h: the height of HC column A seal is broken when P > Pc DIFFUSION LOSSES THROUGH SEALS Diffusion of Hydrocarbon through seals is dependent mainly on: Hydrocarbon type The characteristics of the water filled pore, network of the contacting seal Time available for diffusion 4.4 MACRO CHARACTERISTICS OF SEAL LITHOLOGY DUCTILITY THICKNESS STABILITY LITHOLOGY Almost effective seals are evaporate, fine grained classtics, and organic-rich rocks These lithologies are seen as seals because: Have high entry pressure Are laterally continuous Maintain stability of lithology over large areas Are relative ductile Are a significant portion of the fill of sedimentary basins DUCTILITY Ductility is a rock property to deform and flow without visible fracturing that varies with pressure and temperature (burial depth) as well as with lithology Ductile lithologies tend to flow plastically under deformation, whereas brittle lithologies develop fractures The evaporate rock group make good ductile seal under overburden of several thousand feet, but can quite brittle at shallow depths THICKNESS A few inches of ordinary clay shale are theoretically adequate to trap very large column heights of hydrocarbons (particle size of 4-10mm ⇒ have 600 psi ≅ 915m of hydrocarbon column) Unfortunately, there is a low probability that a zone only a few inches thick could be continuous, unbroken, unbreached, and maintain stable lithoic character over a sizable accumulation STABILITY STABILITY IN LITHOLOGY STABILITY IN THICKNESS Exercise ... almost 3.0) Evaporites, however, are the most efficient caprock They are particularly common in carbonate-rich basins, and they often form seals for carbonate reservoirs Furthermore, evaporites commonly... commonly: Best SEAL: Formed by ductile sedimentary rock: clay or shale (for most sandstone reservoir, >60% of known giant oilfields have shale seal) Shale is the dominant caprock of worldwide reserves... vertical seal, or caprock, is required; but faults (Figure 1, B) and stratigraphic traps ( Figure 1, C,D) must be sealed both vertically and laterally Figure 1 4.1 TYPES: the seal is commonly: Best SEAL:
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