The dolomite reservoirs in middle reservoir-source rock combination of the Ordovician Majiagou Formation on the east side of paleo-uplift in Ordos Basin are obviously different from the weathering crust reservoirs at the top of the Ordovician in Jinbian gas field.A systematic study had been carried out on the geological conditions for hydrocarbon accumulation in these dolomite reservoirs in respect of source rocks

reservoirs

sedimentation based on results of reservoir lithology characterization and fluid inclusion testing as well as analysis of drilling data

geochemical and testing data.The results show that the middle reservoir-source rock combination of the Formation experienced strong hydrodynamics against a brief regression sedimentary settings.Beach facies were well developed and formed dolomite reservoirs.The dolomite features in highly automorphic coarse powder and fine-crystal texture

uniform grains

excellent connectivity and well-developed intercrystal pores.Affected by tectonic uplifting during the late Caledonian

the top Ordovician at eastern side of the paleo-uplift were denuded and came into contact with coal-measure source rocks

providing favorable source-reservoir combination.Controlled by lithofacies transformation at updip direction

the dolomites of beach facies in the middle reservoir-source rock combination of the Formation change gradually into micrite

forming discontinuous lithological traps.