Presently the carbonate rock subtle reservoir in the Tahe oilfield is the largest marine unchanged oilfield with large-scale dissolution fissure-cave subtle reservoir as the main pool type. Horizontally, the oil distribution is characterized by large superposed continuous area in which oil is distributed asymmetrically. Vertically, the oil is concentrated within a range of 400 m under the uniformity at the top of the Ordovician without universal oil-water contact, and controlled mainly by the development of strongly heterogeneous secondary porosity-cave-fissure reservoirs. The main factors controlling the formation of oil pools are as follows: (1) good collocation of reservoir rocks, whose reserving capacity are composed of porosities, caves and fissures formed by multistage multicyclic rock dissolution (paleo-surface water and deep water) and fracturing, and overlying effective sealed rocks, especially the Bachu formation mudstone and salt lime mudstone, are key to the formation of pools; (2) the long-term continuously developed Akekule uplift and multiple fracture systems control not only the scope of dissolution reservoir distribution, but also oil migration, accumulation and formation; (3) long-term hydrocarbon supply by multiple source rocks, reconstruction of early-stage pools and late-stage refilling and adjusting are the most important pool-formation mechanism in the Tahe oilfield.