
To explain the distribution of trapped oil and gas in a basin or play we normally think about variations in hydrocarbon charge.
- Source rock types: there are source rocks that are more oil-prone (algal Type I source rocks) and others are more gas-prone (coaly type III).
- Maturity: many source rocks generate initially mainly oil, and more gas at higher maturities
But what about the role of seals?
We tend to take seals for granted; a fine-grained lithology (often shales) is either considered to be sealing or not. For reservoirs we have no problems accepting that there are better ones and poorer ones. Would that be different for seals? Of course not.
Just like reservoirs also seals can be good, or not so good. There is a whole spectrum of lithologies from excellent reservoirs to excellent seals. In fact, most seals leak. Some leak very fast, and then the seal is not effective; others leak not so fast or maybe very slowly, even at geological timescales, and then the seal is effective.
In some areas we find traps entirely oil-filled right next to traps with only gas, and others with gas and oil; such as for example in the Chalk fields of the Central North Sea (figure 1). There are numerous other basins where also seemingly haphazard distributions of oil and gas occur.

In some basins we find traps entirely oil-filled right next to traps with only gas, and others with gas and oil, such as for example in the Chalk fields of the Central North Sea (fig. 1). There are numerous other basins where also seemingly haphazard distributions of oil and gas occur.
Could this be explained by differences in charge?

In traps with several reservoir-seal pairs we may find that some reservoirs contain only oil, some only gas, some gas and oil, and others may not contain any hydrocarbons at all (fig. 2). This is quite common in deltas. In many of these cases it is difficult to envisage that these variations in hydrocarbon content can be the result of differences in charge.
I would encourage explorers to also consider the role of sealing to understand the distribution of trapped oil and gas. Several observations can be made that suggest that sealing can indeed influence the relative amounts of trapped oil and gas (see also John Sales, 1997 & Sam Algar, 2012):
- In several settings traps with more strongly faulted seals tend to contain a greater percentage of oil than nearby traps with less faulted seals. It seems that the faulting has made the seal less effective for sealing gas.
- In slope and deeper marine settings, you may find oil-filled reservoirs at shallow depth, with deeper traps containing more gas. This is the case when the shallowly buried clays in the seal have not yet been much compacted and are not yet very good seals.
- In several marine settings you may find relatively more gas in reservoirs directly below the thickest and best seals, while deeper reservoirs with the same trap geometry but below thinner intra-formational seals contain more oil (with shorter columns).
- In fault-bounded traps in deltas where clay smear and juxtaposition are the main factors controlling sealing, there is often a rather erratic distribution of oil and gas and dry reservoirs. Apparently, sometimes the seal works excellently (and we end up with a gas-filled reservoir), sometimes the seal doesn’t work (no trapped hydrocarbons) and sometimes the seal is not perfect, allowing gas to leak out, and we end up with oil-filled reservoirs or mixed gas-oil columns.
Such observations have also been made in areas with abundant gas charge, where with perfect seals all oil should have been pushed out of the traps and displaced by oil. It seems an inescapable conclusion that gas may leak preferentially to oil, or much faster than oil, making room for gas.
That preferential leakage of gas does indeed occur seems to be ‘proven’ (if we can ever say that in petroleum geology) by the fact that there are traps with only oil – no gas cap – with impressive gas chimneys above them (e.g. Ekofisk in the central North (see figures 1 and 4). Clearly, gas is being charged into the trap, but the seal is not good enough to keep it there. It may well be that oil is also leaking out, but because of its much higher viscosity this occurs at a much lower rate. For oil-bearing traps in settings with abundant mixed oil and gas charge, the rate of gas leakage must be higher than the rate of gas charge, and the rate of oil leakage lower than the rate of oil charge. It is all a matter of rates; rate of charge versus rate of leakage – for gas and for oil.

A fault seal may be very good or (somewhat) leaky. In cases with both oil and gas charge different HC mixes can be trapped, depending on the rates of leakage of oil and gas relative to the rate of charge of oil and gas (fig. 3; After Urai et al, 2008)
A gas chimney above a dip-closed trap may suggest that the seal is leaky. However, many traps with gas chimneys do contain hydrocarbons. Different mixes of hydrocarbons can be found, depending on the rates of gas and oil charge and the rates of leakage for oil and gas (fig.4).

I don’t want to underplay the role of variations in charge, which do of course occur both spatially and over time, and which do of course cause differences in amounts of oil and gas found in traps. Nevertheless, also preferential leakage of gas needs to be taken into account in several cases. Whenever I talk about these concepts during courses or in presentations, people seem to agree. Unfortunately, I rarely see this concept being applied for predicting oil and gas mix in undrilled prospects (see also an interview with Dick Bishop in the AAPG Explorer of July 2012).
I would be interested to know from others how they look at this. Have you come across differences in amounts of trapped oil and gas that are difficult to explain by variations in charge? Have you seen any examples of preferential leakage of gas?
References:
Algar, S., 2012: Big oil from “gas-prone” source rocks and leaking traps: Northwest Borneo. Search and Discovery article #10465.
Durham, L.., 2012: Is the trap half-full or half-empty? Interview with Dick Bishop. In: AAPG Explorer of July, 2012.
Sales, J.K., 1997: Seal strength vs Trap Closure – A Fundamental Control on the Dstribution of Oil and Gas. In: Surdam, R.C. (ed), Seals, raps and the petroleum system: AAPG Memoir 67, p. 57-83.
Urai, J.L., Nover, G., Zwach, C., Ondrak, R., Schöner, R. and Krooss, B.M., 2008: Transport processes. In: Geankoplis, C.J. (ed): Transport Processes and Separation Process Principles, Chapter 6.2 pp367-388.
Read the next volume of the blog series:
- Reporting Risk & Volume Assessments: Key Insights and Best Practices
- The Bull’s Head Risk Matrix: A Data-Driven Approach to Risk Assessment
- Prospect Police versus the Socratic Method
- Data Acquisition for Exploration: Risk Reduction or POS Polarisation?
- Exploring Stratigraphic and Combination Traps: Definitions, Risks, and Applications
- Optimizing Risk & Volume Assessments in Mixed Hydrocarbon Fields
View upcoming related courses:
Trap & Fault-Seal Analysis, Modeling for Oil & Gas and CO2