Characteristics And Evaluation Of The Organic-rich Shale Reservoirs Of Yanchang Formation, Southeast In Ordos Basin

Shale gas is a kind of arisen unconventional gas resources. The reservoir is different from conventional one in several ways, such as mineral composition, characteristics of organic matter, and pore structure, etc. Shale gas mainly adsorbed to the minute pores in nanometer order in shale, which makes the study of pore structure a significant role. Organic-rich shale in Yanchang Formation, southeast of Ordos Basin, is formed in deep lake environment. Based on the observation of outcrops and cores, and the analysis of the well-log data of more than 150 wells, we concluded that the target area mainly developed deltaic front and lacustrine facies. The lake basin located in the south side of the area, and the north side is controlled by the underwater distributary deposits. Moreover, to better understand the reservoir characteristics, pore structure, adsorptive capacity, and the controls of these features of the mentioned shale, cores of Yanchang Formation in southeast Ordos Basin are sampled, and a series of measurements including abundances of detrital mineral grains and clay minerals, mineral surface area, pore diameters and isothermal adsorption are carried. The results clearly show that the pore diameters of the target shale mainly vary between 2-5nm, and the mesopores volume accounts for 70% of the total volume. Quarts and clay minerals are the most important minerals of the target shale. Smectite mixed layer, with a 47.6% content, takes a significant position in clay minerals, and illite the next. The Illite-smectite ratio has a value of 18.89%, indicating that the target shale has reached the late diagenesis A2 subage period. The TOC varies from 0.16% to 6.6%, and the Ro values are from 0.56% to 1.4%, which indicate that the organic matters reach maturation stage. The correlations between pore volume and mineral content are random processes that in certain state spaces, macropore volume and quartz content are directly related, and so is micropore volume with clay content. In condition of low maturity (Ro<1.0%), pore volume is uncorrelated with TOC and Ro. Futhermore, saturated extent of adsorption is negatively related with micropore volume and quartz content, while positively correlated with clay content, TOC and Ro. In the end, to combine the research results to the application, we suggest that the shale gas reservoir should conclude not only the organic-rich shale, but also the sandy interlayer, which may have great contribution to the reserve of shale gas. Overall consideration of the reservoir thickness, interlayer ration, TOC, Ro, brittle mineral content and the total porosity, we consider the west side and the middle part of the Chang7 section, and the south area of the Chang9 section are the Favorable area of shale gas reservoir in this area.