The automatic decoding method for analysis of the territories occupied by natural and anthropogenictransformed ecosystems in the northern taiga of Western Siberia is offered. The fifty oil and gas fields which are in the different stages of development and located on the territory of Purovsk district of Yamalo-Nenets Autonomous Okrug have been analyzed. Based on the developed algorithm by the «Decision Tree» method in the software package ITTVIS ENVI 4.8 the normalized vegetation index and Tasseled Cap transformation results have been analyzed using summer cloudless satellite images obtained from Landsat 5 TM and Landsat 7 ETM +. Also, the accuracy evaluation of the algorithm has been made by comparing the results of the automatic decoding method with the results of the visual pattern recognition of the same satellite image. High efficiency of the offered method is shown at recognition of the water and forest territories, especially the most valuable needle-leaved forests. Also the possibility of its application at the space monitoring of the other territories: deciduous forest, wetlands, industries, degraded lands is discussed.