Whale oil is at the—well—heart of In the Heart of the Sea. At times, it represents straight-up cash money. At others, it represents the pitfalls of greed. Funny how that works out, huh?
To put it simply: whale oil = $$$. For the sailors who work on boats like the Essex, even the gruesome extraction process itself "seems to them a glorious representative of prospective dollars" (4.12). These dudes spend as long as two to three years away from their families, so each barrel of oil they fill brings them one step closer to home.
This symbolism shifts after the Essex disaster. Now the oil that the crew fought so hard to collect has been returned to whence it came: the ocean. Even worse, the barrels have burst open, leaving the "yellowish slime" to slop into their whaleboats—"the fluid that only a few days before had been their fortune, their obsession, was now their torment" (6.12). As we learn throughout In the Heart of the Sea, greed has a tendency to bite you in the backside when you least expect it.