Abstract
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. The mineral composition will affect both absorbed and free gas contents, therefore their identification is important. The mineral composition 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.7 section in Yanchang formation in Ordos basin as an example. Through a lithological analysis, it was concluded that overlap method and cross-plot method are not also inappropriate for shale gas reservoirs. The Ordos basin shale gas reservoir is divided into seven lithofacies. We form a mathematical method and apply it to shale gas reservoirs using the shale volume and ΔlgR which are available from conventional well logging and reflect organic matter in the processed dataset. Decision tree is used here. However, there were too many parameters to discriminate all lithofacies precisely. Principal component analysis (PCA) is a technique used to reduce multidimensional data sets to lower dimensions for analysis. This technique can be useful in petro-physics and geology as a preliminary method of combining multiple logs into a single entity or two logs without losing information. Combining PCA and a decision tree algorithm, the lithofacies of a shale gas reservoir were accurately discriminated.
Highlights
In the mid-late period of oil and gas field development, unconventional reservoirs are important for increasing hydrocarbon reserves and production
This research selected different baselines according to the different wells and used (2) to calculate lgR which showed a linear correlation with total organic carbon (TOC) content
Principal component analysis (PCA) and Decision tree (DT) were applied to data from the Ordos Basin lacustrine deposit shale gas well and the results were displayed in Fig. (10)
Summary
In the mid-late period of oil and gas field development, unconventional reservoirs are important for increasing hydrocarbon reserves and production. Shale gas reservoirs are self-generating and self-accumulating Their stratification comprises fine mixtures composed of organic matter, hydrocarbons, and rock debris, any physical analysis and testing thereof is complicated. The research takes the Yanchang formation of triassic period in the south-eastern Ordos Basin as an example (Fig. 1): previous research and field tests show that this area has lacustrine shale gas accumulation conditions. The south-eastern region of the Yanchang formation in the Ordos Basin is a shale gas range with potential profitability ( data suggest it represents an improvement over some marine shale deposits). A decision tree algorithm was used to discriminate between the lithofacies using PCA parameters
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