Analysis of Brittleness in Mudstone Caprock Using Conventional Well Logs: A Case Study from Northern Margin of Qaidam Basin

Abstract

Brittleness index, generally estimated from array sonic logs or triaxial tests, is commonly used to quantitatively characterize the possible failure features in shale reservoirs. Because of the similarities in mineralogical composition between shale reservoirs and mudstone caprock, it is valuable to utilize brittleness index derived from conventional well logs for evaluating the brittleness of mudstone caprock. With the research object of Tertiary mudstone caprock in northern margin of Qaidam basin, core samples of mudstone were taken from several significant wells. To construct the ternary diagram of mineralogy of study area, the X-ray diffraction analysis was used for obtaining relative content of minerals (including quartz, carbonate and clay). Then the brittleness index was calculated from mineralogical composition. Since some kind of conventional well logging signatures are sensitive to variation in mineral constituent. Statistical regression analysis was carried out to probe into such a corresponding relationship between brittleness index and conventional well logs (such as GR, SP, Pe etc.). The relationship between the brittleness index and logs data, like GR, Pe, were established. The results indicate that the ratio of GR to Pe has the best corresponding effect to brittleness index. Therefore, the brittleness index could be calculated from conventional logs. The brittleness of different formations mainly range from 60%-70%, among which Shangganchaigou Formation (N1) and Lulehe Formation (E1+2) are the highest while Xiaganchaigou Formation (E32) is the lowest. The brittleness of different districts are various, the highest of which are in Lengdong and Lenghu No.5 structure belt, about 60%, while the brittleness of Lenghu No.4 structure belt is lowest, around 47%. Therefore, E32 brittleness is comparatively low so that natural fractures are not easy to form. Besides, the brittleness of Lenghu No.4 structure belt is lower, so it is more difficult to form natural fractures than the other part of research area. Thus, E32 caprock in Lenghu No.4 structure belt could be high-quality caprock. The analysis result is consistent with oil test results. To conclude, the brittleness index could be estimated using conventional well logs and could be used as a new parameter to evaluate sealing capacity of mudstone caprock.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017