Depends what you mean by "refilling"

All oilfields have oil left in the ground when economic production ceases. - Not just bits you missed, but what we call "irreducible oil saturation", because rock grains are generally oil-wetting - the oil sticks to them in preference to water sticking to them (recall the horrible aftermath of oil washing up on the beach after Exxon Valdez, and adhering to the boulders). A thin film of oil remains, no matter how hard you suck, and there are always the pockets of oil that were too far away to reach from your wellbores.

This "bypassed" oil is sometimes closed off in fault blocks, but more often just too far away for the suck to reach.

However, if you leave such an oilfield shut-in for many years, the oil in it will gradually redistribute itself. A small fraction of the "irreducible" oil will in fact become mobile as grain surface chemistry and charge changes. Tiny pores that had trapped oil will allow it to slowly ooze out, and the bypassed oil will escape from its pockets and spread out sideways and upwards.

So if you come back after 10, or 20, or 50 years, you find "new" oil sitting at the top of the reservoir, or adjacent to jour existing wells, waiting to be produced. - But it isn't new oil at all. It's chemically and isotopically identical to the original oil, and beleive me, there are thousands of complex modified biological molecules that act as fingerprints for oils. Every single oil field is different in detail, but the oils group into families and petroleum geochemists can trace all this, accurately.

After a year or threee, production falls off to near-zero and you have to shut-in again for anothe 20 years.

Next time, you get a bit less "new" oil, and so on.

Exponentially decaying functions are great. There's always more oil to be had, but you have to wait longer and longer for less and less, and you still don't extract more than the original oil-in-place when the field was discovered.

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There are a couple of bizarre exceptions to this.

In Japan, there is a small gas field that has been going for over 30 years, and has produced its original volume 10 times over. This is now well understood. The field is a shallow gas pocket, collecting a seep escaping from a deeper gas pool.

Once this was discovered, the deeper pool was drilled and tapped. pressure fell off and the shallow pool stopped refilling.

In Formby, UK, there was a shallow oil field trapped in gravels, under surface peat. Again, this kept refilling because it was on the escape route from deeper pools. Again, the escape route was traced out and major new fields were found.

The surface seep is still active here, because the journey time from the big fields is ~thousands of years, but the volume and rate are tiny.

With these two well-understood exceptions, oil doesn't accumulate at a significant rate in existing oilfields on a human timescale.

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There is another process that could make people think that oil fields were "growing". As a combination of geological uncertainty and human behaviour, the size of an oilfield is reported differently through its life.

I'm an explorer for oil, and we tend to be wildly optimistic and report the maximum possible volume that could fit in the trap we map. We predict "possible" oil.

When this is drilled, we are like as not disappointed because conservative evaluation of what has actually been found - "proven", is much less than we claim. At the same time, it is likely that the pool is larger than that one well can see, so we describe "probable" reserves, but still in a very cautious way (you don't want to blow a multi-million or even billion dollar investment on something that mnight *not* be there, after all). And there is still a "possible" upside.

As appraisal drilling continues, each well proves reservoir continuity, producible oil, and delimits the exact shape of the "box" containing the oil. "proven" reserves increase, "probable" reserves either go down, as they get turned into "proven" or go up as more "possible" gets better defined. "possible" almost always goes down.

When the field is declared commercial, this is typically the moment in its life when "proven"+"probable" are at their all-time low!

As production continues, the predicted recovery of oil is confirmed. New wells are drilled that further reduce uncertainty. Production geologists are really proud of finding "new" or bypassed oil that was in a "hidden" pocket. In fact, this is oil that the explorer originally reckoned was inside his overall envelope, but was discounted during development. Additional reserves are booked, and the total of "produced"+"proven"+"Probable" increases.

Finally, the oil is exhausted. We shut in the field and dismantle the machinery for scrap or re-use elsewhere. We finally know how much oil came out of the field. In an ironic coincidence, this is often pretty close to the original prediction by the explorer.

Hence, the apparent "reserves growth" of established oilfields is NOT due to finding "new" oil that is "refilling" the field, but due to conservative bookkeeping by the engineers and accountants.

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So, in summary, yes I have heard of oilfields "refilling", but you need to check your source to see what exactly it refers to, and whether it is one of the very common scenarios above, rather than the unusual one of actual refill on a human timescale. The latter *does* happen, but very slowly - it may take a million years to fill an oilfield.