Extended abstract

At the beginning of the year 2016, teams of INA's experts started the re-evaluation of Sava, Drava and Mura Depressions. The purpose of these re-evaluations was regional studies, using Play Based Exploration methodology, with the main goal of defining new exploration plays, prospects and seismic acquisition polygons in zones of interests.

The Play Based Exploration is a method (Figure 1) used for building an understanding of the basins and petroleum systems in which we work, and the geological plays they contain. The benefits of this method is in providing better early focus to range an exploration activities and most valuable plays. The whole approach provides technical precision and more confident decisions, even with the partial or incomplete data. The power of Play Based Exploration methodology is in possibility of ranking of explored area, in extraction of basin and play parts for further evaluation, by elimination of time and budget lost.

Figure 1: General schematic illustration of Play Based Exploration methodology

The best illustration of Play Based Exploration (PBE) methodology is the exploration pyramid (Figure 1), which represents the workflow of regional exploration defining, basin determination and consequently better analysis of the working petroleum systems.

Petroleum system understanding forms the basis for the subsequent play focus - quantifying the different aspects of the system within each play and using tools such as common risk segment mapping (CRS maps) to highlight sweet spots within each play (Figure 2).

When the plays have been mapped and where it is possible, quantified, then the focus shifts again to use more detailed geological and geophysical analysis. This is essential to define prospects within each play, and build the portfolio, including estimation of volumetric, risk and uncertainty.

PBE, requires upfront investment of time if the regional play framework has not already been defined. However, that investment will be repaid by swifter and simpler assessment of individual prospects, and in the quality of subsequent decisions.

Figure 2: Schematic workflow

Last year, the expert teams in INA have successfully added significant benefit, using for the first time a play-based approach, which has been used in oil industry in the world for more than 20 years. Play Based Exploration is recognised as a crucial method in the fast and effective evaluation of new ventures. It have also a vital role in evaluating new plays in our existing areas. The most important effort of PBA is in rebuilding of a play-based approach to exploration, better understanding of petroleum systems and better risking.

Workflow (Figure 2)

Steps 1-2 

Regional tectono-stratigraphic segment, structural analysis and seismic interpretation

The tectono-stratigraphic segment of a basin is a primary control on the structural architecture, stratigraphic fill, source rock distribution, thermal history and understanding of petroleum systems, plays and prospectivity. All next steps in play-based exploration are built upon this step. Early identification of Mega-sequences is an essential part of the basin analysis (Figure 3). Mega-sequences are the groups of related depositional sequences, which are bounded by regional unconformities (e.g. Pre-rift, Syn-rift.) and which have the priority in play identification. All further identified units within basin are smaller than Mega-sequences. Also, the palinspastic reconstruction, as a part of structural analysis, can provide the information on the timing of structural development.

Figure 3: Mega-sequences in tectono-stratigraphic evaluation

Steps 3-4

Seismic characterisation and stratigraphic modelling

Seismic characterisation and stratigraphic modelling in PBE methodology commonly share a sequence stratigraphic controlled horizon. Plays are defined stratigraphic by a significant change in the play elements-major bounding seal and the gross depositional environment in the section (Figure 4 and Figure 5). Within previous identified Mega-sequences, the stratigraphic definition smaller sequences (plays or group of plays) is much easier and faster, where we save the time and consequently, the budget. Depositional model (Figure 6) is one of the main inputs for further steps in PBE method, petroleum system modelling. It is the result of all seismic and stratigraphic data, where the seals and reservoirs can be defined as potential traps for HC migration.

Figure 4: Spectral decomposition

Figure5: GDE map

Figure 6: Depositional model-schematic

Step 5

Petroleum system modelling

Initial steps involve integrating all the hydrocarbon occurrence data such as source rock observations from well penetrations and outcrops, oil and gas in wells, piston core extracts, seeps, slicks, hydrates and direct hydrocarbon indicators from seismic data. Present day maturity can tell us the integrated history of a source interval. To better understand the petroleum system, the hydrocarbon flux through time have been looked. 

Figure 7: Petroleum system chart (example)

Figure 8: Maturity map (example)

Petroleum systems analysis in our studies involves more than traditional charge modelling. A complete understanding of charge modelling covers aspects of reservoir conditions through time. This is taken into account by using programs such as B-mod and Per-media.  The one of the results of modelling (Figure 7) by B-mod is the maturity map (Figure 8) The petroleum system can be characterised simply by an events chart.  The events chart helps identify the critical moment when all elements of a viable petroleum system are first in place.

Step 6 

Play identification, CRS maps

CRS mapping and risking uses all available geological and exploration data to create a view of the play-scale risks.  Play chance maps are provided upon reservoir and seals GDE maps, SR distribution and maturity, migration fairways, GDE maps, etc. Maps should highlight high-grade areas and further exploration focus.

In the last step of CRS Risking we multiply the maps together to create Composite Common Risk Segment (CCRS) maps (Figure 9). A reality check against known data is then preformed. By multiplying the play and lead POs (Probability of Success) CCRS maps, we can approximate total geological POS.

Figure 9: Chance maps (CRS) overlying - result in CCRS map

Step 7

Yet-to-Find (YTF), Creaming Curves, Probability of Success (POS), Prospect ranking

The last step of the workflow in PBE methodology is the prospect portfolio. When evaluating prospect portfolio, we place the prospects in a play context. This gives us the strategic information which is needed for play de-risking and prospect’s ability to build up the portfolio (Scope for Recovery volumes). A prospect portfolio should correspond to Composite Common Risk Segment (CCRS) maps. CCRS maps help identify drill candidate prospects in segments with acceptable chances of success. Prospect delineation on basis of seismic interpretation, attribute analysis, petrophysics, field, proven and speculative play analogy will be provided

Well log and petrophysical parameters for volumetric calculation have to be analysed. Stochastic volumetric assessment have to be done through Rose & Associates MMRA^® software (Figure 10). Ranking according to OHIP and POS is expected.

Ultimately, PBE is the method for better understanding the additional potential (Yet-to find, YTF) within the same play or the play segment. It can shorten prospect evaluation by giving the focus to exploration.

Also, some statistical methods such as Field Size Distribution (FSD) in Creaming Curves can provide the information about the play. Creaming curves represent the history of discovery volumes, where we can also predict the further discoveries within one play or play segment (Figure 11).

Figure 10: Geological resources vs chance of exceeding chart (MMRA) EXAMPLE

Figure 11: Creaming curve (example)

References:

Ahmad, Nadeem and Khan, Moin R.(2010): Play Fairway Analysis and Crs Mapping within a Sequence Stratigraphic Framework: ScreeningTools for Geological Risk constrained Exploration

Allen, P.A. & Allen, J.R. (2005): Basin Analysis: Principles and Applications, Blackwell Science, Malden, MA, USA

Castagna, J., S. Sun & R. Siegfried (2003): Instantaneous spectral analysis: Detection of low-frequency shadows associated with hydrocarbons. The Leading Edge, Feb 2003

Royal Dutch Shell: A guide for AAPG’S imperial barrel award participants, Play based exploration guide