04 Chapter4 Seal

CHAPTER 04CHAPTER 04

SEALSEAL((ROOFROOF OR OR CAPCAP ROCK) ROCK)

DefinitionDefinition:: Seal is impermeable rock that Seal is impermeable rock that

forms barrier on top of the reservoir forms barrier on top of the reservoir rock of an oil and/or gas reservoir.rock of an oil and/or gas reservoir.

In the case of anticlines ( In the case of anticlines ( Figure 1Figure 1, (a)), only a vertical , (a)), only a vertical seal, or caprock, is required; but faults (seal, or caprock, is required; but faults (Figure 1Figure 1, (b)) , (b)) and stratigraphic traps ( and stratigraphic traps ( Figure 1Figure 1, (c,d)) must be , (c,d)) must be sealed both vertically and laterally.sealed both vertically and laterally.

Figure 1

4.1 TYPES: the seal is 4.1 TYPES: the seal is 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 (Figure 2) 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 2)

Figure 2

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 4.3 MICRO PROPERTIES OF SEALSEAL Capillary pressure, Pc

Pc= 2γcosθ/ R γ: Hydrocarbon –water interfacial tension;

θ: Wettability; R: Radius largest pore throats.

Hydrocarbon pressure, P P = (ρw-ρhc) ×gh 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 DIFFUSION LOSSES THROUGH SEALSSEALS

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 CHARACTERISTIC OF 4.4 MACRO CHARACTERISTIC OF SEALSEAL

LITHOLOGY. DUCTILITY. THICKNESS. STABILITY.

LITHOLOGYLITHOLOGY 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.

DUCTILITYDUCTILITY

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.

SEAL LITHOLOGIES ARRANGED BY SEAL LITHOLOGIES ARRANGED BY DUCTILITYDUCTILITY

° SALT

° ANHYDRITE

° •KEROGEN – RICH SHALES

° •CLAY SHALES

° •SILTY SHALES

° •CARBONATE MUDSTONES

° •CHERTS

(most ductile lithologies at top of column)

THICKNESSTHICKNESS

A few inches of ordinary clay shale are

theoretically adequate to trap very large column

heights of hydrocarbons (particle size of 10-4mm

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.

STABILITYSTABILITY

STABILITY IN LITHOLOGY

STABILITY IN THICKNESS

QUESTION TO DISCUSSQUESTION TO DISCUSS

Determine the type and characteristics (petrographic content, thickness, colour, main minerals, rock facies, original, tectonics) of below seal rock (Cuu Long Basin)