New workflow for petrophysical evaluation of Visean unconventional organic rich reservoirs within Dnipro-Donets Depression

Increasing the resource base and extraction of hydrocarbons remains one of the main tasks of engineers and scientists, and this is especially relevant in the war conditions in Ukraine. Ultra-deep drilling (>6 km) and the extraction of hydrocarbons from low-permeability unconventional reservoirs are possible directions for the implementation of these tasks, especially taking into account the high depletion of active deposits (of about 75 %) and the good cover of the Dnipro-Donets Basin by exploratory drilling. The results of laboratory studies and the interpretation of well logging data of unconventional reservoirs of Visean sediments (V-23 and V-24-25) indicated that these intervals are promising. An important feature is that these layers are located within the northwestern and southeastern parts of the Dnipro-Donets Basin, with a total area of promising areas of about 8,000 km2. However, ultra-deep drilling and extraction of hydrocarbons from unconventional reservoirs requires modern technological approaches and solutions. In order to choose the optimal location (a «sweet spot»), to investigate the optimal program of drilling, geological study, and completion of the studied wells, it is necessary to construct the most accurate geological model of this reservoir. This paper constructed a petrophysical model of unconventional-type reservoirs with high total organic content (TOC) based on well logging data of old fund wells with a typical complex of well logging. The model was calibrated on the data of modern geophysical complexes (SGR, neutron-gamma spectroscopy, NMR, lithodensity, etc.) and special core studies (XRD, LECO, TOC, etc.). The equation for calculating the target formations’ total porosity, TOC, and mineralogical composition was derived using the method of multilinear regressions in the «Techlog» wellbore interpretation software complex. The results showed good correlations for unconventional reservoirs of Dnipro-Donets Basin North-Western Play (Glynsko-Solokhivskiy oil and gas region) and Dnipro-Donets Basin Southeastern Flank Play. The magnetic susceptibility of the target reservoirs was researched, and its correlation dependences with other physical parameters of the rocks were also investigated. In particular, a direct correlation of magnetic susceptibility and porosity was determined. This article is a continuation of a series of publications on the study of Visean unconventional reservoirs within the DDB.