Selection of Normal Compaction Trend Line for Abnormal High Pressure Reservoirs in Fergana Basin

摘要

Fergana basin formed thick deposits in Cenozoic.Because the deposition rate is fast,the drainage of the formation is not smooth and the under-compaction leads to the abnormal high pressure in the reservoir,which brings great risks to the operation in the well.Due to the difference in formation pressure caused by the dehydration of the mineral like gypsum and the differential migration and distribution of the formation fluid in the later stage,it is very difficult to predict accurately the reservoir pressure.As a result,a few precious work has been done on it.In the work for pressure prediction,the key point is the determination of the normal compaction section of the formation,which is the selection of the normal compaction trend line on the acoustic lnDT-Depth diagram.In this study,the normal compaction trend line of the Upper Tertiary sand-shale formation was determined by multi-well comparison method.Three schemes were discussed for the determination of the normal compaction trend line of the Lower Tertiary carbonate formation.Scheme 1:select the overlying mudstone section to make the normal compaction trend line.Scheme 2:sort out the microcrystalline-micrite limestone in the carbonate stratum as the approximate mudstone to make compaction trend line.Scheme 3:considering the possibility of abnormal high pressure in the whole Lower Tertiary,the normal compaction trend line of the Upper Tertiary is uniformly used for the Lower Tertiary.The equivalent depth method is used in the formation pressure prediction,and the results are comparatively analyzed combined with the measured formation pressure data.Finally,the causes of abnormal high pressure in Fergana basin are discussed.