An Identified Method for Lacustrine Shale Gas Reservoir LithofaciesUsing Logs: A Case Study for No. 7 Section in Yanchang Formation inOrdos Basin

摘要

Unconventional reservoirs are keys to oil and gas exploration and development, especially shale gas reservoirs.Discriminated shale gas reservoir lithofacies are, in particular, a primary problem in shale gas reservoir engineering. Themineral composition will affect both absorbed and free gas contents, therefore their identification is important. The mineralcomposition is one part of lithofacies. The shale content has always been used in previous lithological identifications:this method is effective in sand reservoirs; however, it is not suitable for use in shale gas reservoirs. This paper takes No.7section in Yanchang formation in Ordos basin as an example. Through a lithological analysis, it was concluded that overlapmethod and cross-plot method are not also inappropriate for shale gas reservoirs. The Ordos basin shale gas reservoiris divided into seven lithofacies. We form a mathematical method and apply it to shale gas reservoirs using the shale volumeand ΔlgR which are available from conventional well logging and reflect organic matter in the processed dataset. Decisiontree is used here. However, there were too many parameters to discriminate all lithofacies precisely. Principal componentanalysis (PCA) is a technique used to reduce multidimensional data sets to lower dimensions for analysis. Thistechnique can be useful in petro-physics and geology as a preliminary method of combining multiple logs into a single entityor two logs without losing information. Combining PCA and a decision tree algorithm, the lithofacies of a shale gasreservoir were accurately discriminated.