求翻译：A fault zone may itself be impervious, or faulting may have shifted an impervious bed so that it now blocks a reservoir bed. Stratigraphic traps tend to be more difficult to locate and may form where tilted reservoir beds are overlain unconformably by impervious layers or where the reservoir beds become thinner up-dip and wedge out within enclosing impervious beds. Thus oil that was once distributed in sparse amounts throughout a very large volume of rock may now be richly concentrated within the uppermost portions of favorable reservoir rocks.是什么意思？

待解决 悬赏分：1 - 离问题结束还有