Abstract: A new natural gas exploration breakthrough has been gained in the fine-grained tight dolostone reservoirs of the Upper
Assemblage of the Ma-5 Member in the Yan’an area of the Ordos Basin. This paper presents a detailed research on the characteristics
of low-porosity and low-permeability dolostone reservoirs and the genesis of porosities. Through core observation, thin section analysis
and cathodoluminescence testing, combined with rock physical property analysis, the porosity-permeability characteristics of the
fine-grained dolostone reservoirs and the favorable reservoirs′ evolution process and controlling factors are clarified. Results show that
the lithology of fine-grained dolostone reservoirs is mainly composed of gypsum pseudocrystal dolomite, fine-grained dolomite and karst
breccia dolomite. The porosity types contain gypsum pseudocrystal dissolved pores, intercrystalline dissolved pores and non-selective
breccia dissolved pores, with local fractures developing. Thus, it is a pore type reservoir with low porosity and permeability. The
mould-pore-contained gypsodolomite flat, which is located at the high position and transformed by multi-stage karsts
(penecontemporaneous, epigenetic and burial), is the main distribution area of favorable reservoirs of the fine-grained dolomite in the
study area.

Key words: fine-grained dolostone reservoir, controls on reservoir quality, evolution mode of reservoir, Ordos Basin, fifth member of
the Majiagou Formation (Ma-5 Member)