求翻译：(1) The oil originates in a source bed, and a marine shale, once a black mud rich in organic compounds, is thought to be a common source rock. (2) The oil then migrates to a permeable reservoir rock, and to do this it may travel for long distances both vertically and horizontally. Oil cannot move through the tiny openings of the shale source beds rapidly enough to be extracted profitably. (3) A nonpermeable layer must occur above a reservoir bed. Since oil is lighter than water, it tends to move upward through openings and cracks until it encounters impervious beds that it cannot penetrate. The oil may then accumulate beneath the impervious layers. Some gas occurs in solution within the oil, and if enough is present it separates out to occupy the uppermost region of such a trap. (4) A favorable structure must exist to concentrate the oil and anticlines, salt plugs, and faults are common examples.是什么意思？

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