The Study On The Sealing Capacity Of Caprock In Northern Qaidam Basin

The Northern Qaidam Basin is one of the structure cells in the Qaidam Basin where located in Qingzang Plat., with an area of 3 X 104km2. This paper presents a systematic research on oil and gas sealing conditions of the Northern Qaidam Basin based on the seismic profiles, well logs, drilling, sample analyses, mechanism of sediment and the theory of oil and gas preservation.Based on the previous work, the main results are as follows:1. The sealing mechanism can be classified as capillary sealing and concentration sealing after systematical analyzing the sealing mechanism of the sealing formation. In fact, both the pressure sealing and the hydraulic sealing are belong to capillary sealing, they are special forms of capillary sealing;2. According to the sedimentary mechanism, the effects of sedimentary environments, structural movements and changes of the sea level on the development of the cap rock were analyzed, the distribution regularity was obtained;3. The high pressure and high temperature experimental tests have been done to measure the breakthrough pressure of the cap rock. According to the special and routine results, the sealing capacity of the mudstone at reservoir conditions and the affecting factors were analyzed. The evaluation criterion of the cap rock was established, combining the geology feature of the Northern Qaidam Basin;4. A method of synthesized evaluation on sealing capacity was proposed, the sealing capacity of cap rocks by use of simulation test technology was analyzed, and the cap rock was evaluated by means of the feature of the cap rock distribution;5. By means of Well Log Evaluation System, the cap rock distribution and combining reservoir sealing of single well were analyzed;6. The comprehension sealing capacity of each formation group has been studied by using the physical properties and the formation distribution. Study on the package relationship of source rocks, reservoir and cap rocks, the oil shows, the development of faults, and the development of traps, the prospecting area have been predicted in the Northern Qaidam Basin;7. The sealing capacity is better with two sets of area cap rock and several sets of regional cap rock. The most important regional cap rocks of the Northern Qaidam Basin are the mudstones of upper and lower Ganchaigou Formation. The mudstones of upper Youshashan Formation are the local cap rocks for the upper secondary reservoir. The argillaceous rocks of each formation are the direct or indirect cap rocks for the underlying reservoirs and have good sealing qualities;8. The most favorable regions for cap rocks are evaluated. The comprehensive result is that the Lenghu structural belt and the Nanbaxian-Mahai structural belt are the best ones of oil and gas accumulation conditions.