Ok so 15 bytes per second,
And the requirement to store for 20 years, Does this imply there are times where it is not logging?
Just to give clarification, 15 bytes per second, or 0.5KB/30 seconds, is 10 megabit every day, In other words your 256Kb memory would last 25 days, or roughly 1 month,
If you went with the maximum normal non nand flash size of 2Gb then it would last about 210 days, or just shy of a 2/3rd of a year.
I chose NOR flash because of the size you will likely require, most data logger projects end up being limited only be memory technologies, EEPROM and even SRAM the other 2 fairly low power options generally cap out at 64Mb, while NAND's low reliability doesn't meet your specs,
Can you store the data in a more efficient form? or sample less often, 20 years is difficult for your current requirements. which is closer to 10GB, now while an SD card may be able to manage it, I would expect you to struggle to make it both low power, and have a warrented reliability for that long.