Abstract: Clastic rock in Paleozoic is a new field for the petroleum exploration in the Rub al Khali Basin，Qatar. Based on
the regional geology study，the source-reservoir-cap assemblage in Paleozoic，especially the development characteristic of cap
rocks，is studied using petroleum geology and reservoir geology theory. The results show that the Palaeozoic group in the Rub
al Khali Basin is well-developed source-reservoir-cap assemblage. The Qusaiba hot shales in Silurian are the major source
rocks. The littoral sandstones of Jauf in Devonian and fluvial-deltaic sandstones of Unayzah in Permian are major reservoirs.
The Gypsum rocks and basal transgressive mudstones of Khuff are the regional cap rocks，with several sets of local cap rocks.
The seal abilities of cap rocks is one of the main risks for the Paleozoic petroleum exploration in this region. The distribution
of the regional cap rocks are stable on plane，which contains gypsum rocks. The seal ability is good as a whole，but local cap
rocks with minor dolomites and a few of siltstone are thin and impurity，which have an adverse effect on sealing abilities. The
cap risks should be taken in consideration in petroleum exploration.

Key words: source-reservoir-cap assemblages, clastic rocks, Paleozoic, the Rub al Khali Basin, Qatar