There are certainly oilfields under the sea floor off the coast of Sakhalin, that are being actively exploited. This map shows their locations: https://www.researchgate.net/figure/Map-of-Sakhalin-Island-showing-offshore-oil-and-gas-blocks-and-study-locations-Okha_fig1_248393621
But one thing that jumps out at me about that map is that it seems to also be showing some oilfields on the island itself.
If that is the case, would those not be easier to reach than the offshore ones, therefore exploited first? Yet I cannot find any mention of that in e.g. https://en.wikipedia.org/wiki/Sakhalin-I
Am I misreading the map, is there some other difficulty, or have onshore oil fields indeed been exploited?