Trap and Seal Analysis

Trainer(s): Jan de Jager
Duration: 5 days

About this training course

Trap and Seal analysis is an essential element of the evaluation of oil and gas prospects. Several comprehensive look-back studies have concluded that absence of Trap and/or Seal are the main causes for exploration wells to be dry. Furthermore, in the large majority of prospect evaluations, the Trap Volume uncertainty is the main factor controlling the total uncertainty range of probabilistically calculated oil and gas volumes of exploration prospects. 

Doing a good job in identifying valid potential subsurface traps requires creativity and resourcefulness, plus an excellent understanding of what may constitute a trap, what may cause a trap to leak 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 in undrilled exploration prospects, and therefor also not of the HC volumes that may be present. 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 case histories from different basin settings will be presented.

For subsurface CO2 storage, trap and seal analysis is equally important as it is not only essential to know how much pore space is available, but also what the risks are for leakage of CO2 and for induced fault reactivation and earthquakes. The geomechanical aspects of containment of CO2 will be presented (online) by industry expert Peter van den Bogert.

Participants will be provided with the opportunity to use the Ariane software package (ÓArianeLogic) to investigate the effect of different seal qualities of top and fault seal on trapped column lengths and the oil and gas mix. The president of ArianeLogix (Martin Neumaier) will be present online to explain the software and provide assistance

The main objective of this course is to provide advice and practical guidelines on trap and seal analysis that will help explorers in their day-to-day work.


What this course will cover in 5 days
In this 5-day 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
  • Geomechanical aspects of CO2 containment
  • Trap identification and analysis in a variety of extensional and in compressional settings
  • Analysis and risking of stratigraphic traps
  • Unusual traps, such as basement and hydrodynamic 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
  • Quantitative analysis of trapped HC columns and gas-oil mix in based on seal qualities of top and fault seal using the Ariane software package (ÓArianeLogic)
  • 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.


Who should attend
This course is designed as an advanced module of the Prospect Maturation course by the same instructor, but can also be seen as a stand-alone course; the relevant messages of the Prospect Maturation course will be briefly summarised at the start of the course.  

The course is designed for explorers with at least several years of experience in exploration; both geologists and seismic interpreters, and their supervisors / team leaders.


Learning, methods and tools

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, including hands-on use of specialist software for seal analysis, and discussions. Hand-out material in paper and/or electronic format will be provided.



Day-to-day Programme


DAY ONE                     

  1. Introduction and the basics
    • Summary of the main relevant learnings from the Prospect Maturation course (dependency of trap types on the basin type and deformation history; importance of recognising the spill- and leak-points of traps) followed by a brief introduction of what will be discussed in this course
  2. Trap styles and distributions
    • Typical basin development and deformation histories will be discussed of different basin types as a basis for understanding what type of traps may occur, and the setting (location within the basin) where these may occur.
    • Illustrated with examples of trap densities and field sizes in different basin settings, and of variations of these parameters within basins
  3. Control on depth of water contacts in worldwide examples
  • Recapitulation of how HC-water contacts are normally controlled by spill- and/or leak-points, followed by 1 or 2 exercises where participants need to propose considered uncertainty distributions for HC column length.
  1. Prospect identification from seismic sections and maps (exercises)
  • Short presentation followed with exercises on conceptual cross-sections and ‘real’ maps and seismic lines on identification of potential prospective zones and leads. The data to be used for these exercises comes from different basin types, including deltas, rift basins, passive margins and deep water foldbelts.


DAY TWO                    

Recap of Day 1

  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.
  1. 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
  1. 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.

4.     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


DAY THREE             

Recap of day 2

  1. 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 a topic for discussion. 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
  1. Differential retention of gas and oil
  • The role of seals play on the gas-oil mix in traps will be discussed
  1. Geomechanical assessment of CCS containment
    • This module will be presented on-line by the industry expert on this topic: Peter van den Bogert. The main leakage mechanisms for CO2 storage and the Geomechanical metrics used to assess these risks. The Geomechanical differences between CO2 storage in virgin reservoirs and depleted HC fields. Explanation of the input data for Geomechanical assessments.


DAY FOUR                  

Recap of day 3

          1. 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
          1. Unusual traps
  • Basement traps, Volcanics, hydrodynamic traps
          1. Traps in Rift basins
  • Examples of conventional traps in the syn-rift and post-rift: simple horsts and tilted fault blocks as well as flower structures and anticlinal traps in inversion structures.
  • Examples of stratigraphic traps
  • Examples where deeper traps have been successfully explored for (deeper syn-rift and pre-rift.
  1. Modelling of top and lateral seals
    • The Ariane softwarepackage (ÓArianeLogic) will be used to investigate the effect of top seal and side seal strength on trapped HCs. Participants will be given the opportunity to use the software, changing the inputs for charge mix (gas and oil) and seal strengths, and see how this affects the gas-oil mix in traps. This will be followed by a group discussion on observations and learnings. The president of ArianeLogix (Martin Neumaier) will be present online to explain the software and provide assistance


DAY FIVE                  

Recap of day 4

  1. Traps and seals in different basin settings
  • A comprehensive overview of traps and seals in different basin settings will be available in the provided course material. Based on the interest of the participants a few of these settings will be discussed in detail. Several exercises can be chosen from to deepen the understanding of trap types and seal issues in these basins:
    • Delta settings: NW Bornea, Nigeria
    • Deep-water passive margins: Surinam-Guyana, Ghana, xNamibia
    • Deep-water foldbelts: NW Borneo, Nigeria
    • Orogenic foldbelts: Andes, Alps, Apennines
    • Salt basins: Gulf of Mexico, South Atlantic and Southern Permian Basin
    • Carbonate settings: Arabian platform and Central Luconia province

2.    Risk & Volume analysis of traps and seals

    1. Dependencies for prospects with more than 1 reservoir-seal pair; what are the consequences of a proven seal for a deeper reservoir on the chances for the shallower reservoirs.
    2. 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
    3. Simple and pragmatic guidelines for deciding pre-drill uncertainty ranges of column-length (and area) and most likely column length.
    4. For several basin types pragmatic rules of thumb will be presented for how to assess the likelihood of the presence of effective seals and their roles on the distribution of oil and gas
  1. Course summary and close-out