In a D/E (deltaic coals as source rock), or a system with multiple source rocks, the most useful concept comes from Sales 1997 paper, that fluid phase often is controlled by trap closure and seal strength, as shown in this figure:
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| In a dual HC phase system (reservoir pressure < Psat), if a trap's closure is less than the maximum column of gas the seal can hold, it will spill the oil phase and contain only gas (class 1). If the seal strength is less than a full oil column, it will retain the oil column, and leak off the gas (class 3). If closure is greater than the maximum gas column but less than the maximum oil column, it will end up with both phases (class 2). After J, Sales, 1997. |
Structure closure is often known at time of prospecting. Seal strength is not, nor is the fluid entering the trap, so the method we use is a Monte Carlo model that describes the unknow parameters, most importantly seal strength, GOR of incoming fluids (which depends on source rock type, maturation and migration process), and the saturation pressure of the fluids, with a distribution. The outcome of the modeling (Trinity software) is a probability of fluid type for each prospect/trap.
The classic examples are found in deltaic basins in South East Asia. The discovery of Kikeh field rekindled the talk on this concept. Not too far, in the Mahakam delta, Kutei basin, Indonesia, there are several large/giant fields that are perfectly explained by this model. The structure closures range from 10s of meters to greater than 500 meters. The source rock is deltaic and fields are a mix of oil, gas and oil/gas. The figure below shows the model prediction using the same assumptions on 4 different fields.
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| Trinity 3D phase risking results of four fields along the cross section (blue line) with same input parameters on charge (1000-10000 scf/bbl) and seal (25 to 120 psi seal Pc). Only variable is closure height among the traps. Map courtesy of Ramdhan and Goulty , 2018 |
It is important to note that the results are very good and not so dependent on the incoming fluid type. With the typical seal strength, the traps can be charged with 1500 scf/bbl black oil, or a 15,000 scf/bbl gas condensate, the end results is nearly the same. The high relief structures consistently yield oil phase or oil phase with a gas cap, and vise versa, that low relief structures are most often end up with a gas phase.
I make this post because It seems to me that the last 25 years since the paper was published, there have not been enough application of this simple and very useful concept. Now we have a map based risking tool for easily making such predictions. Hope this will get more application of this unbelievably useful concepts.
Discussions:
To be more generic, we can describe Sales classes mathematically between capillary entry pressure, fluid density and trap closure. For any given structure closure that is in the two-phase region (reservoir pressure below bubble or due point) of a petroleum system, the seal capacity dictates which phase ultimately remain in the trap, assuming charge volume is sufficient.
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Where, Pc is the capillary seal capacity of the shale, H is the closure (crest to spill point) of the trap, ⍴w, ⍴o, and ⍴g are the in-situ densities of the water, oil and gas columns, respectively. |
Class 3 traps may often include a small gas column due to differences in interfacial tension between oil-water and gas water. Theoretically this may be 15 to 20% of the column see earlier post here. Some class 3 traps offshore Sabah (Kikeh and nearby fields) have variable small gas caps in stacked reservoirs.
In the real world, many factors can affect the contacts and phase proportions, faults are prevalent in deltaic systems and can complicate leak/spill/closure relationships greatly, especially 3 way traps. Column may be dynamic from both rate of charge/leakage and changing composition of charge where gas is often not equilibrated with the oil column below. There is rarely enough data to validate, let alone predict such details before drilling. However, for exploration purposes, Sales' concept, especially when combined with our probabilistic approach hold very well against observations. It is possible to include considerations of non-capillary seal, faults, and other factors in the input distributions.
References:
Sales, J.K., 1997, Seal strength vs. trap closure ----, fundamental control on the distribution of oil and gas, in R.C. Surdam, ed., Seals, traps, and the petroleum system: AAPG Memoir 67, p. 57-83
Ramdhan1, A. & N. R. Goulty, 2018, Two-step wireline log analysis of overpressure in the Bekapai Field, Lower Kutai Basin, Indonesia. Petroleum Geoscience online article.
