Trap and Seal Analysis

Jan de Jager 
5 x 3 hours

Business context

Two recent comprehensive look-back studies concluded that absence of Trap and/or Seal are the main causes for exploration wells to be dry. Clearly, Trap and Seal analysis is an essential part of prospect evaluation. Furthermore, in the large majority of cases the Trap Volume uncertainty is the main factor controlling the uncertainty range of oil or gas volumes that may be discovered in a successful exploration well. 

Doing a good job in identifying valid potential traps requires creativity and resourcefulness, plus an excellent understanding of what may constitute a trap and where potential spill- and leak-points may be. Otherwise it is not possible to make meaningful predictions of the most likely column length and its uncertainty, and therefor also not of the HC volumes that may be present in an undrilled prospect. In mature basins and in basins with sub-optimal seismic data quality, where the more obvious traps have already been drilled, the search may be for alternative traps such as stratigraphic, deeper and/or basement traps.

Trap and seal analyses are discussed in this course from a pragmatic explorers point of view, based on the extensive experience of the instructor and on published case histories. Top seal and fault seal will both be addressed. The theoretical aspects of sealing that have been developed over the years will be explained, and traps and seals will be illustrated with a large variety of trap types from different basin settings.

Advice and practical guidelines will be given to help explorers in their day-to-day work.

Who should attend

This course is designed for exploration geoscientists working in prospect maturation roles.

Course content

In this 5-day VILT course the main aspects that need to be considered for trap and seal analysis will be discussed, including:

-    Understanding and identifying spill- and potential leak-points of traps

-    Theoretical aspects of seals and sealing capacity

-    The effectiveness of Top and Fault seals

-    Methods to assess the sealing potential of seals

-    The calculation of Shale Gouge Ratio and Clay Smear, as well as the construction of Allen (juxtaposition) plots will be explained and illustrated with several exercises

-    Trap identification and analysis in a variety of extensional and in compressional settings

-    Analysis and risking of stratigraphic traps

-    Unusual traps, such as basement traps

-    Sealing mechanisms and reasons of seal failure: permeability seals and hydraulic seals

-    Dealing with traps with multiple reservoir-seal pairs (in different basin settings)

-    Differential leakage of gas versus oil and its role on gas-oil mix in traps

-    How to treat trap and seal risks and uncertainty in a technically sound (and pragmatic) way in probabilistic risk and volume analysis tools; also for multiple reservoir-seal pair prospects

Examples of traps and sealing efficiency from around the world will be given throughout the various modules.

Learning, methods and tools

The course will be delivered online, in5 sessions of 3 hour each, with 2 breaks of 10 minutes per day. It is the intention to have at least 2 smaller exercises per day, plus a larger exercise as 'home assignment'; to be worked on after the online sessions and before the start of the next session. Time will be reserved for recapitulation, questions and discussions. 

The platform for the online course is either Zoom of Microsoft Teams: presenting material can easily be done on this platform; when participants want to ask a question they can raise their hand or interrupt the course instructor by microphone; the presenter can switch to a screen where he/she can see all participants (also when each is sitting in another location, such as at home); participants can ask questions by writing notes, but they can also be given the microphone; there is a whiteboard functionality that can be used as one would use a flip chart. Exercises will be done on the platform Mural, which gives each participants its private work area that can be accessed by the course instructor to discuss the exercise, as would also be done in a classroom course. 

Exercisesare focussed on understanding how traps and seals should assessed in a practical manner as a sound basis for realistic risk and volume assessments for prospects. 

At the end of the course the participants will understand the critical elements of hydrocarbon traps and how they affect the trap risk and volume of trapped hydrocarbons.  This should result enable participants to focus on realistic traps and in technically sound and realistic pre-drill estimates of prospect volumes. It should also result in a more focussed search for alternative trap types (deeper, stratigraphic and unusual) in mature basins.

Each topic is introduced by a lecture, and learning is re-enforced by practical exercises and discussions. Hand-out material in paper and/or electronic format will be provided.

Day by day programme

DAY ONE                     

Introduction and the basics

  • A brief introduction of what will be discussed in this course; followed by a summary of the main elements of the prospect maturation workflow, the dependency of trap types on the basin type and deformation history, the importance of recognising spill- and leak-points of traps
    • Exercise: determining spill- and leak-points from a prospect depth map

Trap styles and distributions

  • Typical basin development and deformation histories will be discussed for different basin types as a basis for understanding the trap types that may occur
    • Find the prospect(s) - based on a real depth map from a mature basin

Control on depth of water contacts

  • Illustration of how HC-water contacts are normally controlled by spill- and/or leak-points.
    • Exercise: Is this a realistic faultplane?

  

DAY TWO                    

Recap of Day 1

Sealing lithologies

  • Different sealing lithologies and their effectiveness will be discussed, from perfect seals like salt to rather poor silty or marly seals, with examples from different basins. Also unusual seals (e.g. basalt) will be presented. Emphasising the role of sealing on trapped hydrocarbons (oil vs gas, column length.

Seal theory

  • The theory of capillary entry pressure; Rate of leakage from theoretical considerations; Tools and methods to determine seal capacity; hydraulic seal failure; column length versus sealing capacity
    • Exercise: Is this a 'protected trap' (in pressure cell)

Top seals

  • The main geological controls on the effectiveness of top seals will be presented, and how this affects the HC column lengths that can be retained by top seals.
    • Exercise: Make a depth map and identify the prospect(s)

 

DAY THREE             

Recap of day 2

Faults and fault mapping

  • Typical characteristics of faults are presented, which should help their interpretation, especially in areas with limited or poor quality seismic data. The construction of Allen juxtaposition plots will be explained with several exercises
    • Exercise: construct a fault juxtaposition (Allen) plot

Fault seals

  • Different fault seal mechanisms will be discussed: Juxtaposition, Shale Gouge, Clay smear, Cataclasis and Diagenesis. Industry practices and formulas will be presented, and their application and validity will be discussed.
    • Exercise: Calculate Clay Smear Potential
    • Examples of top and fault seals in different basins, trap types and sealing lithologies: rift basins, deltas, deep water settings, carbonate provinces; Discussion on effects of seal thickness and the role of faults and fractures

 

DAY FOUR                  

Recap of day 3

Differential retention of gas and oil

  • The role of seals play on the gas-oil mix in traps will be discussed

Stratigraphic traps

  • Trapping geometries and aspects of sealing for stratigraphic traps; where can stratigraphic traps reasonably be expected to occur, and where are they highly unlikely. Examples will be given from different basin settings and depositional environments
    • Exercise: Is this a stratigraphic trap?
    • Exercise: when did these structures form?

Unusual traps

  • Basement traps, Volcanics, hydrodynamic traps

  

DAY FIVE                  

Recap of day 4

Traps and Seals in different basin types

  • Examples of conventional traps in rift basins, deltas, deep-water settings, foldbelts/foredeeps and salt basins
    • Where is the trap, and where would you drill it?

Risk & Volume analysis of traps and seals.

  • Guidelines and examples of prospects with different trap and seal risk profiles; Rules of thumb for trap and seal risking - related to trap type, seal lithology, depth of burial, pressure regime, degree of faulting / fracturing and data availability
  • Simple and pragmatic guidelines for deciding pre-drill uncertainty ranges of column-length (and area) and most likely column length.
    • Exercise: sketch column length uncertainty distribution for 2 prospects
    • For several basin types pragmatic rules of thumb will be given for how to assess the likelihood of the presence of effective seals and their roles on the distribution of oil and gas

Course summary