Reservoir Characterization & Geological Modeling
RESERVOIR CHARACTERIZATION & GEOLOGICAL MODELING (QCB4043)
Department of Petroleum Geoscience, Universiti Teknologi PETRONAS
DR. A K M EAHSANUL HAQUE, Senior Lecturer
E-mail: eahsanul.Haque@utp.edu.my
SEAL QUALITY & CAPACITY (Lect-5/6-2)
Page 2: Reservoir Geology Structure
Juxtaposition Seal:
A lateral seal formed when sand is placed against shale.
Fault-Rock Seal:
Predominantly controlled by the properties of the fault zone.
Page 3: Outline of Lecture
Seal Definition and Illustration
Sealing Mechanisms and Quality
Evaluation of Sealing Capacity
Stacking Reservoirs and Trap Models
Sealing Functions in Trap Classification
Page 4: Reservoir Compartmentalization
Reservoir Compartmentalization:
Defined as the segregation of a petroleum accumulation into multiple distinct fluid or pressure compartments.
Occurs when flow is inhibited across sealed boundaries within the reservoir, which can be caused by various geological and fluid dynamic factors.
Types of Seals:
Static Seals:
Completely sealed and capable of trapping petroleum columns over geological time.
Dynamic Seals:
Low to very low permeability flow barriers that significantly reduce petroleum crossflow to near-zero rates.
Page 5: Seals and Reservoir Presence
Presence of Seal and Reservoir Rocks:
Found across structural, stratigraphic, and hydrodynamic domains.
Types of Reservoir Closures:
Vertical and lateral closures categorized into:
4-way closure: Most efficient trapping/sealing mechanism.
3-way and 2-way closures respectively with decreasing sealing effectiveness.
Hydrodynamic Reservoir:
Results from internal pressure variations often correlated with lithology variations, affecting reservoir geometry related to depositional facies, particularly evident in deltaic and fluvial environments.
Page 6: Seal Definition & Meaning
Seal/Cap Rock:
A fine-grained rock formation that acts to prevent oil from migrating to the surface.
Phenomenon:
Describes the trapping of hydrocarbons during secondary migration processes.
Page 7: Seals Overview
Characteristics of Seals:
Typically thick, laterally continuous ductile rocks that allow fluid entry but prevent easy passage.
Pore throats are too small for hydrocarbons to pass through unless overridden by buoyancy pressure.
Role in Hydrocarbon Retention:
Major control mechanism over the retention of hydrocarbons.
Leakage Dynamics:
Many seals leak intermittently.
Various mechanisms spell leakage, reinforcing their dynamic nature.
Page 8: Traps and Sealing Mechanisms
Anticline Structure:
Strong sealing characteristics around structures with defined spill points present for hydrocarbon leakage if relief is minimal.
Stratigraphic Trap:
Good lateral seals due to porosity changes or unconformities, ensuring effective trapping.
Hydrodynamic Trap:
Hydrocarbon migration counter to descending water flows in aquifers, often influenced by structural and stratigraphic traps.
Active hydrodynamic flow can yield tilted oil-water contacts, impacting hydrocarbon accumulation dynamics according to vectors and equipotential lines.
Page 9: Capillary Seals
Membrane (Capillary) Seals:
Defined by capillary entry pressure, influenced by pore throat diameter, and pressure dynamics between hydrocarbon and water phases.
depend on pore throat size contrasts.
Reservoir Rock: Larger pores, lower capillary entry pressure → hydrocarbons can occupy them.
Seal Rock (e.g., shale, tight carbonate): Very small pores, high capillary entry pressure → hydrocarbons cannot displace water in these pores.
Leakage Conditions:
A capillary seal leaks when the vertically directed buoyancy force exceeds the capillary entry pressure, determined by the largest interconnected pore throat system.
Capillary Entry Pressure Equation:
P_1 = rac{2Y ext{ cos } heta}{R}
where:$P_1$ = capillary entry pressure
$Y$ = hydrocarbon-water interfacial tension
$ heta$ = wettability inverse measure
$R$ = radius of the largest pore throats.
Page 11: Salient Points about Seals and Cap Rocks
Seal Quality/Efficiency based on Lithology:
Excellent: Salt, ductile clay
Good: Anhydrite, ductile clay, shale
Fair: Marl, shale, siltstone
Poor: Argillaceous limestone, siltstone, argillaceous sandstone
Continuity Effect:
Regional seals create continuous covers over widespread reservoirs, mitigating migration loss.
Intraformational seals composed of site-specific seal facies, generally linked to stacked columns.
Influence of Faulting and Fracturing:
Faults may create pathways for hydrocarbon leakage.
If fault throw exceeds seal thickness, seals can be compromised.
Ductility Dependence:
Varies with lithology, temperature, and pressure, ranking from most ductile (Salt) to least (Chert).
High ductility (e.g., salt, shale):
Rock can deform plastically under stress.
Maintains continuity even during tectonic movement.
Less prone to brittle fracturing → better long-term seal quality.
Low ductility (e.g., chert, brittle carbonates):
Rock fractures easily under stress.
Fractures create leakage pathways.
Seal quality is compromised.
Thickness Factoring:
Thicker seals are less likely to be broken, though thickness alone is not the sole determinant of seal integrity.
Page 12: Traps and Fault Sealing Mechanisms
Fluid Flow Dynamics around Faults:
Faults can operate either as barriers or conduits for fluid flow.
Fault Trap Types:
Shale Juxtaposition Trap:
Shale Smear Trap:
Hydrodynamic Fracturing Trap:
Page 13: Fault and Seal Interaction
Illustrated relationships between sands and shales in terms of juxtaposition, fault-rock seals, and fault reactivation effects.
Page 14: Differential Leakage of Oil and Gas
Gas Above Oil:
Indicates conditions where oil and gas charge is abundant.
Oil Above Gas:
Shows similar conditions with potential for leakage influenced by different pressures at the top of gas and oil columns.
Leakage Dynamics:
Gas typically operates under higher pressure, leading to preferential leakage over oil.
Page 15: Evaluating Seal Capacity
Factors Influencing Seal Capacity:
Structural geometry
Fault geometry and relief
Stratigraphic considerations
Page 17: Hydrocarbon Accumulation Control
Factors Influencing Hydrocarbon Accumulation:
Height of traps and seals impact the accumulation of hydrocarbons. - Strategies and classifications of traps include;
Capillary Limited Trap:
Height > Maximum Hydrocarbon Height leading to buoyancy exceeds capillary entry pressure, hydrocarbons leak through the seal.
Spill-point Limited Trap:
Height < Maximum Hydrocarbon Height.
Referenced from Sawamura and Nakayama (2005).
Page 18: Trap Classification
Classification by Trap Types and Hydrocarbon Types:
Pure Capillary Limited Trap (Oil Prone)
Capillary/Spill-point Mixed Trap (Oil & Gas)
Pure Spill-point Limited Trap (Gas Prone)
Page 21: Classification of Traps by Sealing Functions
Capillary Limited Traps
Anticline
Stratigraphic (Various configurations)
Fault (Different block arrangements)
Page 22: Further Trap Classification
Spilling point limited traps
Anticlines, Faults, and additional mechanisms affected by hydrodynamic fracturing effects.
Page 23: Summary on Sealing Capacity and Quality
Key Insights:
Seals function to create hydrocarbon fluid compartments controlled by buoyancy and capillary pressure interactions.
Quality of sealing relies heavily on overburden magnitude, fault seals, and geometric configurations.
The discussion of reservoir stacking and hydrocarbon accumulation depends on the height of traps interacting with sealing capacities.
Traps are fundamentally classified by their capillary, spill-point, and hydrodynamic characteristics.