First things first: the oil pumps serve as the burial ground for Rusty's murdered body. Now there's all kinds of symbolic baggage there. But let's look beyond the obvious for a second, and take a look at two important descriptions that Marlowe gives us of the Sternwood oil pumps. The first description comes at the very beginning of the novel, as Marlowe is leaving the Sternwood mansion after meeting with the General. Standing outside on the front doorsteps of the mansion, Marlowe gazes past the fence surrounding the house and looks down at the oilfield:
[…] far off I could barely see some of the old wooden derricks of the oilfield from which the Sternwoods had made their money. Most of the field was public park now, cleaned up and donated to the city by General Sternwood. But a little of it was still producing in groups of wells pumping five or six barrels a day. The Sternwoods, having moved up the hill, could no longer smell the stale sump water or the oil, but they could still look out of their front windows and see what had made them rich. If they wanted to. I didn't suppose they would want to. (3.18)
So you've heard of the phrase "filthy rich," right? Well, this description gives it a whole new meaning. The oilfields are what made the Sternwoods richer than their wildest dreams, but we get the sense here that Marlowe thinks that no one can get that rich without getting their hands dirty. We know that Marlowe is pretty contemptuous of the wealthy upper class, and as the novel progresses, we learn that the Sternwood family is mixed up in some very corrupt stuff.
See, having money means having power in the world of The Big Sleep. And Marlowe recognizes that the Sternwood family thinks they can buy their way through life without having to look down at the dirty oilfield where they got that money from. Put simply, the Sternwoods tend to turn a blind eye to their own corruption, living in denial that their money is tainted.
The second description of the oilfield comes at the very end of the novel when Marlowe drives Carmen out to the oilfield to teach her how to use a gun. As they approach the foothill ranch, Marlowe sees the oil pumps:
Then the oil-stained, motionless walking beam of a squat wooden derrick stuck up over a branch. I could see the rusty old steel cable that connected this walking-beam with a half a dozen others. The beams didn't move, probably hadn't moved for a year. The wells were no longer pumping. There was a pile of rusted pipe, a loading platform that sagged at one end, half a dozen empty oil drums lying in a ragged pile. There was the stagnant, oil-scummed water of an old sump iridescent in the sunlight. (31.32)
In this description, there is a much clearer connection made between the oil sumps and the corruption of money. Notice the dirty imagery that Chandler uses: the "oil-stained" beam," the "rusty" steel cable and "rusted" pipe, and the "oil-scummed water" of an old sump. Everything here is falling apart and decaying. The beams don't function anymore and the wells are no longer pumping oil. This scene reeks of death and filth, which is of course appropriate since this is where Rusty is buried. Marlowe can sense the nastiness of the oilfield when he says, "the smell of that sump would poison a herd of goats" (31.35). For Marlowe, the oilfields are the ultimate representation of the idea that money corrupts.