Let’s suppose that you and your company have done a magnificent job, gotten a bit lucky, and discovered a new field in onshore or offshore Brazil. After five wells, it is likely that the discovery is commercial. You are very excited, and after massive celebrations featuring a few too many Caipirinhas, you are starting to think about where to put the next wells to optimally develop the field. You take all the engineering data, e.g., pressure vs. depth, and put them into a reservoir simulation model. The data from the five wells look pretty similar, so you model the reservoir as if it were one giant swimming pool with no barriers to fluid flow. You do this not only because it is what the data say, but also because a field with no compartments can be produced at the lowest cost with the highest recovery. Who wants to be the party pooper saying that the field might be divided into a lot of small compartments?
However, it might be good to know the truth, even if it hurts a little. Should the field be developed or sold to someone else? Highly compartmentalized fields can be very tricky and expensive to produce. So, to really understand your reservoir architecture, our advice is to think of wine and wine-tasting. Here is what we mean:
Let’s say we have a wine barrel and want to play a little game to challenge your taste buds. We are going to secretly divide the barrel into five compartments by inserting barriers to fluid flow, but we are not going to tell you how many compartments there are. Then, we are going to fill the compartments with wine in a manner similar to the filling an oil reservoir by its source rocks. Think you will be able to guess how many compartments there are in the barrel by doing a bit of wine-tasting?
Oil reservoirs are charged by one, or more commonly, multiple source rocks. For most of the Brazilian Margin basins, there are two main source rocks, the marine source and the underlying lacustrine sources. For our analogy, red wine will represent the marine-derived oils and white wine will represent the lacustrine-derived oils. Within the marine source, there are a variety of facies (e.g., shallow-water or deep-water), and each one generates distinctive oils (at least on a molecular level). Similarly, within the lacustrine source rocks of Brazil, there are also a wide variety of facies, including hypersaline, brackish-water and freshwater, etc., and each one also produces distinctive oils. So, for our source-rock facies analogy, we will use various types of red wine for the marine facies (e.g., Pinot Noirs, Cabernets) and several types of white wine for the lacustrine facies (e.g., Chardonnay, Pinot Grigio and Riesling).
As wine ages, the taste changes. This is due to molecular changes that occur with time and temperature. And time and temperature also affect the type of oil generated by a source rock. Just like wine, it’s due to molecular changes in the oil due to time and temperature. But in the case of oil, it’s mostly due to temperature because of the increasing source-rock burial.
Finally, we all know what happens if a wine barrel or bottle is not properly sealed, and it’s not a good thing. The wine goes bad. An unsealed wine is similar to a shallow oil reservoir in which bacteria can degrade and “spoil” the oil. Both wine “spoiling” and oil “spoiling” are due to chemical changes stemming from less-than-optimal storage.
For our game, we fill each of the five compartments in the barrel with different amounts of the various wines in a manner similar to how oils might fill the various compartments of an oil reservoir. We fill Compartments #1 and #2 exclusively with red wine (analogous to reservoir compartments filled only by oil from marine source rocks). To imitate charging from different source facies, we fill Compartment #1 with 60% Cabernet and 40% Pinot and Compartment #2 with 80% Pinot and 20% Cabernet. In Compartment #3, we put a mixture or red wine and white wine to imitate co-sourced oils having both a marine and lacustrine source, which is quite common in post-salt reservoirs. In Compartment #4, we put the same wine mixture that we put into Compartment #3, but we use a spoiled version to imitate a biodegraded oil. Finally, in Compartment #5, we put an oil similar to that in Compartment #3, but we use a version that has matured a little bit more (i.e., aged with time), imitating late charging of more mature oils.
Next, we fill eight glasses of wine from the five compartments and give them to you so you can tell me how many compartments are in the barrel and which wines are in those respective compartments. After smelling and tasting you correctly deduce that there are five compartments and that the three bad-tasting, smelly wines are in the same compartment and the two wines that taste exactly alike (the predominantly Cabernet from Compartment #1) are from the same compartment. The other three wines, each with unique smells and taste, are from their own separate compartments.
You win the challenge because the molecular makeup of the wine from each compartment is different, and you can smell and taste these differences. But you could have also won the challenge by chemical analyses to determine the relative ratios of flavor-imparting molecules in each of the wines using a gas chromatograph (GC). These molecular ratios would have been identical in the three wines from Compartment #4, identical for the two wines in Compartment #1, and unique for the wines from the other three compartments. Many wineries actually hire chemists who analyze the molecular makeups of wines using GC and GC-MS (gas chromatography-mass spectrometry) to determine which molecular ratios make the best wines. Interestingly, the GC analysis of a wine and that of an oil are quite similar. In fact, one of the “Fathers of Molecular Geochemistry”, Prof. Pierre Albrecht, was both an oil and wine chemist, employing many of the same analytical techniques, including GC and GC-MS. Not surprisingly he was also French.
So, how did you win the wine-tasting game? Basically, you reasoned that wines that were identical were in the same compartment and those that were different were in different compartments. And you can do exactly the same thing with oil reservoirs, although we would suggest you opt for chemical analyses (GC and GCMS) rather than taste testing! Oils in the same compartments have identical molecular makeups due to mixing over time, and oils from different compartments have different molecular makeups because there are barriers to mixing. As with many tenets related to petroleum exploration and production, there are exceptions, but not many.
So, next time you are thinking about reservoir architecture, think about wine and wine-tasting. By chemically analyzing your oils, you can be a lot more certain about continuity and where barriers to fluid flow might be located than you can by using engineering data alone. Your reservoir model and simulations will be drastically improved for the low cost of a few analyses.
As with most data and interpretation, it is imperative to get the best, otherwise you can be led to incorrect conclusions (i.e., no data is better than bad data). BPS and its partners have excellent instrumentation and undoubtedly the best geochemists in Brazil. So, next time you wonder about oil mixtures and reservoir architecture in your field, think about BPS and wine-tasting. Your models and simulations will be vastly improved.