Abstract:

The lower to middle carbonate-evaporite assemblages of the Ordovician Majiagou Formation in the Ordos Basin are potential exploration targets but questions have been raised concerning whether there are source rocks developed， how well they are developed and if they match well with reservoirs. Detailed observation of cores and analyses of high-frequency sedimentary cycles from wells Shan 473 and Tao 112 encountering the Majiagou Formation show that： ① Carbonate-evaporite assemblages consist two kinds/eight types of sedimentary cycles （recording gradually dry-up climate） of limestone （without dolomitization）， dolostone， thin dolomite-evaporite interbeds， and evaporites， of which the dolostone and thin dolomite-evaporite interbeds are further recognized to contain 10 types/111 high-frequency sedimentary cycles characterized with frequent cycles and thin single layers. ② Based on analysis of these high-frequency sedimentary cycles， two types of source rocks are identified： the high-quality source rocks consisting of black argillaceous dolostone in millimeter-scale lamina with high TOC （>2.50 %） and the medium-quality source rocks consisting of grey-black dolostone with low TOC （1.21 %）. The high-quality source rocks are facies and horizon conditioned and mostly distributed in the interbedded thin dolomite-evaporite sections around the evaporitic lake and with gas resource amounting up to 850.494 BCM. ③ Two types of facies-controlled dolostone reservoirs are recognized： the dolo-grainstone reservoir on the paleo-uplift and its periphery， and the microbial dolostone reservoir around the evaporite lake， both are favorably located with source rocks. The above understandings may be used as guidance to the future exploration in the lower and middle assemblages of Majiagou Formation， Ordos Basin.

Key words: carbonate-evaporite assemblage, high-frequency sedimentary cycles, argillaceous dolostone source rock, microbial dolostone reservoir, Majiagou Formation, Ordovician, Ordos Basin