The decomposition of microscopic animals at the bottom of the sea results in an accumulation of .... in porous rocks.
the oil / oil
Which one is correct and why? I know when we are talking about a general abstract, we shouldn't use the before a non countable noun
As gotube points out, it is impossible to say which is correct without seeing the complete context, which you mentioned does exist.
For example, the passage could read:
There is sometimes oil present on the sea floor. The decomposition of microscopic animals at the bottom of the sea results in an accumulation of .... in porous rocks.
Here "the oil" would be correct, as it refers back to the specific oil that is the subject of the passage as a whole. This is an awkward way of writing it; given that the oil itself is the subject, I would expect it to come at the beginning of the sentence:
The oil accumulates in porous rocks due to the decomposition of microscopic animals at the bottom of the sea.
But if instead the full context was:
Many factors affect the makeup of materials on the sea floor. The decomposition of microscopic animals at the bottom of the sea results in an accumulation of .... in porous rocks. Ocean currents move scouring sand across the sea bed. From time to time, a dead whale will drop to the bottom and provide a both food and habitat for scavengers.
...then the oil mentioned is just one of several things, and is not the subject of the passage as a whole. In this case "oil" without the definite article would be correct.
The decomposition of microscopic animals at the bottom of the sea results in "the" accumulation of oil in porous rocks.
Shouldn't it be "the" since this accumulation is assumed to be a result of the decomposition?
