A basement Carboniferous carbonate reservoir is discovered with production rate of 226 m3/d through acid-fracturing production test in Well Sai-51 in Saihan Tal Sag, Erlian Basin. It has been the highest production rate since large-scale petroleum exploration was initiated in Erlian basin in 1979 and it also becomes an important new field of oil and gas exploration as the first discovery in eastern China. Sai-51 reservoir is located at a multiple-fault zone on the gentle slope of the sag that is adjacent to the main oil-generating trough through which hydrocarbon is sufficiently supplied by two kinds of oil supplying modes including unconformity surface and faults. Favorable anticlinal traps and developed pores, fractures and caves are common in the Carboniferous carbonate reservoir that is capped by tight volcanic andesite rock, which is the key for rich hydrocarbon accumulation in the reservoir and high oil yield. The discovery of Sai-51 reservoir plays an important role for the areas with similar geologic settings.