Evolution of Cambrian mound-beach gas reservoirs in Gucheng platform margin zone, Tarim Basin

Gas has been discovered in the Cambrian mound-beach in the Gucheng platform margin zone of the Tarim Basin for the first time, opening up a new field of oil and gas exploration. The hydrocarbon accumulation conditions of four stages of Cambrian mound-beach in the Gucheng platform margin zone were analyzed based on core observation, cast thin section examination, geochemical analysis as well as logging and seismic data of exploration well CT 1. The results showed that the gas in the Cambrian mound-beach is mainly secondary gas from paleo-oil reservoirs. Oil accumulated during the middle and late Caledonian stage, which was cracked to gas during the Himalayan period. Mound-beach reservoirs formed due to weathering, leaching or organic acid dissolution, and the reservoir pro-perties of the central part is better than that of the periphery. On the top of the first and second stages of the mound-beach, direct caprock developed and configured well with the reservoir, which is favorable for independent accumulations. In the third and fourth stages, the top of mound-beach was exposed and eroded, and the sealing ability of direct cap rock is poor. The Upper Cambrian regional cap rocks are required for a useful reservoir. Fractures in the Gucheng platform margin zone mainly formed in the Cambrian, middle Ordovician and late Ordovician periods. Later tectonic activities were weak, which is favorable for gas accumulation.