Log Evaluation Of Shale Gas Reservoir In Chang 7 Formation, Xiasiwan Area, Ordos Basin

It is acknowledged that Yanchang Formation of Ordos Basin has abundant shale gas resources. The first shale gas well LP177 was fractured in Xiasiwan area which had strongly verified the shale gas resource potential in this area. However, the geological features, reservoir conditions and log response characteristics of this area are different from normal natural gas resource. The lack of core data, the complexity of lithology and mineral components as well as the poor physical property had led to a biased estimate of the shale gas reservoir. Therefore, the study of the shale gas log response characteristics and well log interpretation of Chang 7 Formation in Xiasiwan area is very necessary in order to established logging evaluation method.The geological characteristic of the reservoir had been detailed analyzed based on the core observation, core test data analysis and gas production test data. The results have shown that the main lithologies of Chang 7 Formation are shale, mudstone, fine sandstone, argillaceous siltstone and silty mudstone. Besides, the mineral types of shale reservoir are dominated by clay minerals (illite/smectite mixed-layer and ammonium illite) and detrital minerals (quartz and feldspar). Pore types are intergranular pore, intracrystalline pore, intragranular pore, dissolved pore, organic pore and microfracture. The pore structure is complex while the physical property of the reservoir is poor. The majority gas in the reservoir is adsorbed gas with a relatively small amount of free gas accumulation.The logging response characteristics of shale reservoirs had been analyzed based on the comprehensive research of the geological characteristics. The optimized sensitivity logging parameters were selected and used in multiple methods to establish the qualitative identification and the quantitative interpretation model of lithology, property and other key parameters of the shall gas reservoir, the methods include the overlapping figure method, the crossplot method, multiple linear regression method, the multimineral model method and the normalized density and neutron difference method. The volume calculation model of shall adsorbed gas and free gas has been provided according to the isothermal adsorption experiment, rock electricity experiment and the log analysis. The total shale gas volume thus can also be calculated using this model. At the last, the calculation method of the lost gas volume can be known by means of the desorption experiments. Accordingly, the total shale gas volume is obtained combined with desorbed gas and residual gas volume.