What makes an observable geological object a relevant reservoir analogue?

Reservoir modellers are often reminded to calibrate the static model with an analogue. In general, analogues are used to get a mental (3D) picture of the reservoir and help developing an understanding of the reservoir concepts. I have deliberately put 3D between brackets because it is physically not possible to observe an outcrop in 3D (either map view or cross section). The combination of several outcrops makes a pseudo 3D or what I like to label a “2.5D”. Willy-nilly it is still extremely valuable to develop a mental picture of the reservoir before attempting to model it. Even though the perfect analogue does not exist, it gives a pointer toward the key reservoir features not to be missed in your model.

However, every coin has two sides. It is important to keep in mind that analogues can be challenging and sometimes misleading.

Through the following three examples, I will elaborate on the advantages / inconvenient of using outcrop analogues for reservoir modelling.

Picture A is a quarry in Germany serving as analogue for Carboniferous fluviatile reservoirs in The Netherlands / Germany. The key learnings are the continuity of the coal layers (black) and the high degree of amalgamation of the channels. You can confidently assume that the channels expression is simplified to a layer cake type of reservoir body at the observed scale. The principal drawback is the lateral extent of the outcrop (circa 300 meters, the truck acts as scale), barely representing few grid cells. You can argue that the learning can only be applied to a tiny portion of the reservoir. It is plausible that the layer cake concept does not hold water if a larger outcrop area would have been exposed. The “2.5D” nature of this outcrop poses also a challenge. It is not possible to see how the channels evolve in the perpendicular direction assuming that the direction of the paleo flow perpendicular to the outcrop.

Picture B is an outcrop picture in Ras Al Khaimah (UAE) serving as analogue for the Khuff equivalent reservoirs in the Middle East (the tree at the bottom left corner is 2 meters high). The key learning is the density / spacing of the fractures and the ability to characterize them (open / closed / filled). The draw back is that fracture characteristics are often dependent on the observed location in the deformed object (intrados, extrados, fore limb). On a bigger scale, fractures represent the snapshot of a given stress history at a given location. In a thrust and fold belts setting, the stress distribution variability is highly dependent on the location. Although this picture gives important information, it is most likely not representative of the reservoir in the subsurface.

Picture C is a Google Earth picture of Pulau Mantabuan (offshore Sabah, Malaysia), a modern-day reef build-up serving as analogue for the South East Asia tertiary carbonates build-ups found mostly in Indonesia, Malaysia, the Philippines and Papua New Guinea. The key learning is the succession of facies belts with a clear reef rim, detrital back reef area and inner lagoon. The reef rim is more developed in the windward side of the reef, where the currents are stronger as opposed to a thinner expression in the leeward side.  The drawbacks are the scale of this observation. The size of this reef (2km X 2km) is a mere fraction of the Tertiary reef build-ups observed in South East Asia. Is it robust to extrapolate the same observation to a factor 10? On top of that, this photography is just a snapshot of the reef at a given time. Its analogy can only be applied to a limited time interval during a Tertiary build up evolution. As we know, reef build ups are dynamic. The facies belts are strongly dependent on relative sea level variations. If there is a possibility to slice through the reef, the facies belt arrangement may be more complex than it looks.

Through these 3 examples, the recurring challenge when using observable analogues to calibrate reservoir models is the scale of observation. In my humble opinion, the advantages outweigh the inconvenient. Nothing beat being able to see and “touch” the reservoir. Geology, in a subsurface reservoir modelling context, is largely conceptual. Whatever helps to better define and frame the reservoir concepts is always welcome. When I build or audit peer reservoir models, producing a mental picture of the reservoir (note: it does not have to be a piece of art, a schematic on a piece of paper is good enough!) is the first thing I always seek. Nothing is set in stone (note: except the reservoir), concepts can and will evolve when more data are available throughout the field life.

I am interested to read your experience on using observable analogues in a reservoir modelling context.

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