ĐỊA CHẤT DẦU KHÍ ( PETROLEUM GEOLOGY ) - CHƯƠNG 4 potx

Seal is impermeable rock that forms barrier on top of the reservoir rock of an oil and/or gas reservoir... 4.1 TYPES: the seal is commonly:° Best SEAL: Formed by ductile sedimentary roc

CHAPTER 04

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 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 tention between the fluids, and the wettability

° Seal need to be evaluated at two different (micro and macro) scales.

4.3 MICRO PROPERTIES OF SEAL

Pc= 2γcosθ/ R

γ: Hydrocarbon –water interfacial tension;

θ: Wettability;

R: Radius largest pore throats

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 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 CHARACTERISTIC OF SEAL

and organic-rich rocks.

 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 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 evaporite rock group make good ductile seal under overburden of several thousand feet, but can quite brittle at shallow depths.

SEAL LITHOLOGIES ARRANGED BY

DUCTILITY

(most ductile lithologies at top of column)

° 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.

STABILITY

QUESTION TO DISCUSS

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