Abstract: The Ordovician Wulalike Formation in the northwestern margin of Ordos Basin is an important field for shale gas exploration in Northern China. However, the geological conditions and gas enrichment characteristics have not been fully recognized. By using basic data such as core, lab test, logging and seismic data, key wells including Li 105 and Zhongping 1 and regional geological data are analyzed to study the enrichment conditions and accumulation patterns of shale gas in Wulalike Formation. The study results show that the favorable source rock,reservoir and cap rock combination was controlled slope facies zones. The deep water slope facies siliceous shale, calcareous siliceous mixed shale and clayey siliceous mixed shale were developed in the lower section, forming the main source rock and reservoir. The shallow water slope facies clayey shale, calcareous mudstone or marl were deposited in the upper section, forming good cap rocks. The geochemical index indicates a medium-good source rock, with mainly self-generated oil cracking gas, and good gas source conditions. The reservoir is generally characterized by high brittleness, with an average brittleness of 51.6%. The reservoir space is dominated by matrix pore and bedding fracture system, with an average total porosity of 4.7%, and an average fracture porosity of up to 1.8% (accounting for more than 38%). The roof generally has good sealing capacity, with clayey shale cap rocks in the middle and south regions, and marl cap rock in the north region. Shale gas is dominated by free gas, accounting for more than 64%. The comprehensive analysis shows that shale gas enrichment in Wulalike Formation was controlled by the consistent distribution of highly brittle shale in the lower section, gas source conditions, tectonic background and fracture development degree, and the favorable cap rock in the upper section. The fractures were mainly developed from the middle stage of Early Cretaceous to the present, which was consistent with the peak of gas generation. There are two main modes of shale gas accumulation. A fractured type shale gas reservoir controlled by the structure was dominant in the south region, and mainly accumulated in fault nose and fault anticline sealed by counter-inclined faults. While a continuous mixed shale gas reservoir was developed in the north region and dominated by a large area of continuous accumulation. In summary, Wulalike Formation has good exploration prospects, and the south and north sections are favorable areas for shale gas exploration.

Key words: Ordos Basin, northwestern margin, Ordovician, Wulalike Formation, shale gas, geological condition, accumulation pattern