Chapter 4

CHAPTER 04 SEAL (ROOF OR CAP ROCK) HCMUT UA 2011 CHAPTER 04 SEAL (ROOF OR CAP ROCK) HCMUT UA 2011 CONTENT 4 1 TYPES 4 2 GENERAL PROPERTIES 4 3 MICRO PROPERTIES OF SEAL 4 4 MACRO CHARACTERISTICS OF SEA[.]

CHAPTER 04

SEAL (ROOF OR CAP ROCK)

HCMUT-UA-2011

4.1 TYPES

4.2 GENERAL PROPERTIES

4.3 MICRO PROPERTIES OF SEAL

4.4 MACRO CHARACTERISTICS OF SEAL

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; ρhc: 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:

pore, network of the contacting seal

4.4 MACRO CHARACTERISTICS OF

SEAL

 LITHOLOGY.

 DUCTILITY.

 THICKNESS.

 STABILITY.

 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

without visible fracturing that varies with pressure and temperature (burial depth) as well as with lithology.

under deformation, whereas brittle lithologies develop fractures.

seal under overburden of several thousand feet, but can quite brittle at shallow depths.

theoretically adequate to trap very large column

only a few inches thick could be continuous, unbroken, unbreached, and maintain stable lithoic character over a sizable accumulation.

 STABILITY IN LITHOLOGY

 STABILITY IN THICKNESS

Exercise