Karst carbonate reservoir division and contrast based on natural gamma ray spectrometry logging in Tahe Oilfield

Key words:  Tahe Oilfield  carbonate rock  natural gamma ray spectrometry logging  energy spectrum ratio and intersection technique  sedimentary facies identification  strata division

Abstract:

Carbonate reservoir in Tahe Oilfield is characterized by large burial depth, great thickness variations and strong anisotropism which result in the difficulty of makitig layer contrast. In order to recognize the distribution regularity of hydrocarbon， a logging-ratio characteristic parameter method based on natural gamma ray spectrometry lc^ data ratio computing and overlapping technology was presented to study the deposition characteristics of the carbonate reservoir. By using the method, the strata division mode of natural gamma ray spectrometry logging was established in Ordovician system of the region. The results show the relative content of U, which is derived from natural gamma ray spectrometry logging, is greater than Th-U ratio. So the region develops karst leached zone, and karst reservoirs should be found in the belt. The Qarbag formation of upper Ordovician has steady deposition and is r^ional criteria layer. The upside of Yijianfang formation develops fracture-pore reservoirs of reef-beech facies, and is characterized by continuous distribution. The downside develops tight schollen micrite limestone, and is separated by the Yingshan formation of lower Ordovician. By contrast of well tie, it is found that the belt on top surface of Ordovician(T7) has karst platform, and that of middle and lower Ordovician(T7) has changeful karst ground feature. These characteristics reflect that the carbonate reservoirs in the upside of Ordovician and the Yijianfang formation and Yingshan formation of middle Ordovician probably possess hydrocarbon available. It also indicates that Tahe Oilfield has a wide development space of hydrocarbon. The application to more than fifty wells proves that the method using more parameters to determine reservoir type is effective. And the method can be applied to reservoir evaluating of new-drilled well and further analyzing of old-drilled well in oil region.