Automated Well Log-Based Prediction of Formation Tops: Case Study of Norway Offshore Data

Abstract

Abstract A geological formation represents a combination of certain genetic types of rocks associated with the proximity of the sedimentation conditions occurring at certain stages of development of the main structural elements of the Earth's crust. In petroleum engineering, the knowledge on spatial distribution of formations is necessary for many applied tasks including construction of hydrocarbon field geological model, petrophysical modelling, basin and petroleum system modelling, calculation of hydrocarbon reserves, planning field development strategy, and many others [Ahmed, 2018; Garb, 1985]. There are different approaches in upstream industry (mainly in geophysics) to determine spatial distribution of formations and depth intervals of wells where the formations are altered. One of the main geophysical methods that enables reliable prediction of spatial distribution of the formation is seismic survey. The main benefit of the seismic survey is that it covers large areas and has solid geomechanical background to detect formation boundaries [Vail, 1987]. However, the ground seismic data has two advantages: (1) relatively low vertical resolution (~ ±20 m) and (2) requires well logging data to convert recorded signal (arrival time of sonic waves) to the depth values. During drilling of boreholes, the information on formation tops can be inferred from visual analysis of drill cuttings that are recovered from wells. Although that approach is cost-effective and has a good potential to be automated in real-time mode [Ismailova et al., 2022; Tolstaya et al., 2023], there are several disadvantages to be considered. The process of recovering drill cuttings from in situ conditions to the surface is complicated by many aspects, including well inclination, distribution of cuttings sizes, properties of drilling fluids, etc. [Zakerian et al., 2018]. Thus, this approach exhibits high uncertainties in absolute values of depth intervals. Moreover, in many cases the alteration of formations cannot be detected visually and requires laboratory support. To overcome the problems mentioned above, petroleum engineers conduct well logging. The main advantages of well logging are (1) high vertical resolution (~ ± 0.5 m) and (2) coverage of large depth intervals during investigations.