Abstract: Based on the principles, advantages and disadvantages as well as applicable conditions of oil and gas resources assessment method, and in accordance with the geological characteristics and exploration history of Tarim Basin, the adaptability of several conventional statistical methods is brought under discussion with the help of some examples. It is assumed that the discovery sequence method and statistical trend prediction method are not applicable in Tarim Basin while the saturation exploration method is not applicable to the composite hydrocarbon accumulation play or area mainly with structural traps. The size sequence method needs to be used flexibly in the concrete cases. The combination of resource calculating methods applicable to different geological types of calibrated units is established to lay a solid foundation for the fourth round of resources assessment in Tarim Basin. The paper points out that one of the most important principles that selection of calibration areas should follow is to further divide the composite hydrocarbon accumulation play into two or more calibration sub-areas, making the geological characteristics of the sub-areas more simplified and typical. The combination of resources calculating methods is selected according to the geological characteristics of the calibration sub-areas, thus leading to more accurate and reasonable calculation results. Take Tazhong north slope calibration area for example. It is divided into two calibration sub-areas—carbonate rock sub-area and clastic rock sub-area. The combination of calculation methods is used to recalculate the resources. The results are substantially improved as compared to the third round of resources assessment. The natural gas resources are raised several times and more compatible to the current exploration conditions.