An Introduction to Play Fairway Evaluation (PFE)

Play fairway evaluation (PFE) or play fairway analysis (PFA) also known as play based exploration (PBE) is a way of systematically mapping the elements of a petroleum play spatially in order to focus hydrocarbon exploration efforts.

Let us first define what a petroleum play is.

The broadest petroleum bearing unit is the sedimentary basin. These are regions of the earth where long term subsidence, which has a variety of geological causes has created accommodation space which has been infilled by sedimentary rocks. The basins have a long geological life and can be infilled by sediments up to 20 km in thickness.

A petroleum system is the next level in the hierarchy. This is a combination of a source rock, reservoir rocks and seals. The focus of the petroleum system is the source rock which in the USGS world petroleum assessment is used to define and name each petroleum system. The source rock, which is a shale mudstone or organic rich carbonate, would generate hydrocarbons from the organic matter within it due to the heat and pressure it experiences from deep burial over time. These hydrocarbons then migrate away from the source kitchen due to buoyancy. Petroleum systems modelling is used to predict if a source rock has generated hydrocarbons, how much has been generated, what type of hydrocarbons have been generated and at what time. More complex 2D or 3D petroleum systems models predict the migration of hydrocarbons from the source kitchen towards any potential reservoirs.

A petroleum play is defined by a reservoir unit, the rock which stores the hydrocarbons, and its associated cap or seal rock which prevents hydrocarbons from escaping.  This reservoir unit will have a defined geological age and also a defined depositional environment. The reservoir should also have reasonably consistent rock properties such as porosity and permeability within a defined range. The USGS world petroleum assessment refers to this as an assessment unit.

A play would normally be connected to a petroleum system (source rock) but in some cases may be fed by several petroleum source systems. A single petroleum system can also feed several plays. 

At the most detailed level are the discoveries (fields), prospects (features defined by geologists but not drilled yet) and leads (figments of a geologists fevered imagination). To be grouped in a play the prospects must share the same reservoir, seal and source. Some geologists also group trap types with in a play as they may share common risks.

Figure 1. Diagram showing the basin - system - play - feature (discovery & prospect) hierarchy

Figure 2. Diagram showing a petroleum system.

Play Fairway Mapping

Play fairway evaluation (PFE) is a way of mapping the elements of a petroleum play spatially and using this information to qualify and quantify risks associated with a petroleum play.

Three main elements within a petroleum play are mapped in a holistic way

·        Reservoir

·        Top Seal (and side/base seal in stratigraphic trapped plays)

·        Source Rock (hydrocarbon charge)

Structural elements mapping is also used to constrain some of these features and define the locations of different structural features which may form traps

The play fairway is where reservoir, seal and hydrocarbon charge coincide and potential traps located in this area are more likely to contain hydrocarbons. Areas where any of the reservoir, seal or charge elements are likely to be absent are highly unlikely to contain hydrocarbons. This technique can therefore be used to focus exploration efforts on areas that are believed to be more prospective.

Play fairway analysis will also help with consistently risking prospects and leads and managing exploration portfolios.

Play Fairway Mapping Workflows

Mapping play fairways consists of several stages as shown in Figure 3 below.

The key tool for PFE is a geographic information system such as ESRI ArcGIS. This tool enables data to be displayed, combined overlain within a map. There are several commercial add on tools to arc GIS focussed on play fairway mapping which enable detailed analysis of the outputs. Examples that I am familiar with are Exprodat Exploration Analyst and GIS-Pax Player.

The first stage is gathering and synthesising available data. This would include a geographic information systems (GIS) databases of current licences and permits, bathymetry, topography and hydrography, locations of existing wells and fields, surface outcrop geology and any areas with limitations on activity. Geological data includes well data such as logs and reports, seismic data and third party reports whether done by government bodies such as geological surveys or outside consultants. Many consultants have GIS compatible versions of their reports, for example Getech, Corelab and CGG Robertson, which can be integrated with proprietary interpretations . There are also comprehensive global GIS compatible geological interpretation models such as Neftex (owned by Halliburton) and CGG Robertson’s Tellus which are commercially available.

The resultant interpretation products for PFE include; structure maps produced with in a unified interpretation framework, Depositional environment and facies maps for potential reservoir, seal and source units, a petroleum systems model to analyse potential charge from the source to reservoir and well results analysis to show why wells discovered hydrocarbons or were dry.

The final products are common risk segment maps (CRS) which define regions of high, mid and low potential for reservoir, seal and charge. There are also individual prospect analyses and a holistic overview of the prospects and plays in a portfolio to see what would be the impact of drilling a particular prospect.

Figure 3. Diagram showing some of the workflows involved in building a play fairway evaluation

Well Results Analysis

Analysing why wells failed and why they worked is a vital process which is challenging to do. This work gives a vital insight into what are the key risks in the play and how are they spatially distributed.

We would need to gather important data for the well such as well logs, reports and petrophysical analyses. We would need to have an interpretation of the seismic data over the drilled feature which is integrated into the overall basin interpretation. A key document would be the original well proposal document which might not be available for older wells. This gives an insight into the opinions of the geologists at the time the well was drilled. Some of this data may be difficult to obtain as it may be proprietary to other operators and still held as confidential. In some countries such as UK and Norway well data is released by the government several years after drilling and can be obtained by anyone. In other countries data is never released but may be purchased from the government regulator or specialist data brokers.

We would need to look at the key petroleum elements within each well and classify them as; good (present), bad (clearly absent) and intermediate or unknown. This scheme is shown in table 1 below. 

Table 1. An example of a well results analysis table

Figure 4 shows a well analysis for a fictional basin including a pie chart for main causes of well failure and a bar chart showing the rate at which key elements are present or absent in the wells.

Figure 4. Well results analysis from a fictional basin.

Introducing the fictional basin

Below is a map showing well locations for a fictional play in a fictional basin. Wells are classified as gas wells (it is a gas prone basin), dry wells aimed at the play and bypass wells which have gone through the unit on their way to deeper targets. The information from these wells is of course valuable

Figure 5 Basemap showing well locations from a fictional basin

Gross Depositional Environment (GDE) Maps

This is a map showing the environments and facies of the potential reservoir, source and seal rocks. They would be created by a geologist using analysis of well logs, well core (rock sample), palaeontology and seismic data. All of this interpretation needs to be synthesised into a map. Some examples of simplified GDE maps from a fictional basin are shown below.

Figure 6. Reservoir unit facies map from the fictional basin

Figure 7. Source rock map for the fictional basin

Figure 8. Top seal map for the fictional basin

Common Risk Segment (CRS) Maps

A common risk segment maps turns the GDE map into a “traffic light” map showing where the geologist believes that the petroleum element’s properties can be good, moderate or bad. Some geologists assign risks to these properties which will be discussed below.

Figure 9. Common risk segment (CRS) map for reservoir 

Figure 10. Common risk segment (CRS) map for source and charge

Figure 11. Common risk segment (CRS) map for top seal

Combining the CRS maps to build a combined common risk segment map

The Reservoir, source and seal maps are overlain to create a combined map, we can now see the sweet spot where the elements are all likely to be present.

Figure 12. Combined common risk segment map (CCRS) – the green blob is where all the elements line up and we are more likely to find hydrocarbons

Quantitative Play Fairway Evaluation

Traditionally PFE mapping was only used qualitatively in risking. Individual prospects within the play would be risked using the formula

Chance of Success (COS) = Probability of trap (Pt) x probability of top seal (Pts) x Probability of reservoir (Pr) x Probability of source (Ps) x Probability of Migration charge (Pc)

Or a variation of it as different companies use more elements or fewer. The play maps would influence the values but not directly inform them. The advantages of this system is that it is simpler and there is less risk of double risking

Some companies however choose to risk the play (i.e. apply probabilities to the blobs) and then modify the play risk by the prospect risk. This has some advantages: it can clarify thinking about the impact of drilling the prospects on the other features nearby by separating play risks (which have impact) from prospect risks which are locally specific. In this case the formula is

Reservoir (Play) x Charge (play) x Seal (play) x Trap (prospect) x Reservoir (prospect) x Charge (prospect) x Seal (prospect)

There are however drawbacks as this is more complex and there is a question of assigning risks, as to whether it is a general play risk or a local specific prospect risk. The danger of double dipping can be avoided by reasoned review.

A matrix for play risking is shown in table 2. The sector HHH represents the “green blob” where all elements are seen as likely to work (80 % chance each), LLL represents the area where all of the elements are seen as being unlikely to work (20 % chance each) 

Table 2. Probability matrix, numbers represent the chance of all three elements working.

Quantifying uncertainty in the play maps

Some areas are more certain than others. We can be reasonably sure in an area covered by good resolution 3D seismic with well control. We would be far less sure in an area with no wells and limited seismic control where the geological model is based on views which have little constraint. To show this we can add a data layer to the map with areas of high medium and low confidence. The resultant map can however be complex and resemble a Roman mosaic. A matrix for this is shown in table 3

Table 3 Quantifying both chance of presence and uncertainty for a single element

Summary

Play fairway mapping enables geologist to better understand the target play, and place detailed prospect level interpretations into context. It enables geological work to be structured with a set of consistent output products which try to address the key risks.

The maps are good visual tools to enable geologists to demonstrate and share concepts. The data is displayed spatially and can be shared between different software tools.

PFE analysis enables limited resources, people, data, budget and time to be focussed where they are more likely to yield positive results. This analysis is particularly useful in giving a head start during time limited evaluations such as licensing rounds or M&A activity.

But – it is still a geologist’s interpretation, with all of the limitations uncertainties and caveats which are inherent in any earth model – models should change when new data or new ideas appear, we must not be fixed on the green blobs.