Petrophysical Characteristics of the Paleocene Zelten Formation in the Gialo Oil Field, Sirte Basin, Libya

Abstract

This work evaluated the hydrocarbon potentiality and petrophysical properties of the Paleocene Zelten Formation in the Libyan Sirte Basin. It aimed to delineate the influence of the microfacies composition of the studied sequence on the reservoir characteristics. The study was based on petrographical and petrophysical data derived from six wells. The petrophysical data included well-logging data (gamma-ray, caliper, PEF, sonic, neutron porosity, density, spectral gamma-ray, and deep and shallow resistivity) and conventional core data (density, porosity, permeability, and fluid saturations). Lithologically, the carbonate Zelten reservoir sequence is composed of four non-clastic lithofacies: (1) argillaceous limestone; (2) calcareous shale; (3) fossiliferous limestone, sometimes slightly dolomitic; and (4) dolomite lithofacies. Petrographically, four microfacies were defined: (1) oolitic grainstone; (2) dolomitic bioclastic packstone; (3) dolomudstone; and (4) ferruginated bioclastic wackestone microfacies. The petrophysical characteristics of the studied sequence were deduced by analyzing well-logging data sets to evaluate the effective and total porosities, shale volume, fluids saturations, and thickness of the net pay. Moreover, detailed processing of the core dataset was applied to estimate the average reservoir pore radius (R₃₅) and the reservoir quality parameters. Petrophysically, the Zelten reservoir was sliced into four reservoir rock types (RRTs), with the RRT1 group having much better reservoir quality than the other RRTs; it forms the topmost parts of the Zelten Formation, averaging 78 ft thick and primarily composed of oolitic grainstone microfacies. It has fair to very good permeability (2.3–479.0 mD), poor to excellent porosity (8.1–41.8%), good to tight reservoir quality parameters, and micro- to meso-pore sizes (0.97–8.08 µm). Besides, the oil saturation was in the range of 0.70–44.6%. In contrast, the RRT4 is a compact reservoir rock type; it primarily consists of ferruginated bioclastic wackestone microfacies and is characterized by excellent porosity (10.5–34.8%), fair to tight permeability (0.013–1.4 mD), tight reservoir quality index (RQI) and flow zone indicator (FZI) values (0.011 and 0.153 µm, respectively), micropore sizes (0.05–0.34 µm), and 0.9–31.5% oil saturation. The petrophysical characters of the RRT2-3 samples have transitional reservoir quality (average porosity = 22.7 and 24.8 %, average permeability = 12.34 and 2.789 mD, RQI_av = 0.198 and 0.091 μm, FZI_av = 0.588 and 0.291 μm, and R₃₅ = 1.29 and 0.53 μm for RRT2 and RRT3, respectively) between the tight RRT4 and the best RRT1 reservoir samples. Also, the Zelten reservoir was sliced vertically into six zones, with the best reservoir quality assigned for zone 5 (net-pay thickness = 348.3 ft, average porosity = 18.7%, average water saturation = 48.3%, and shale volume = 27.9%). The proposed integrated petrophysical and petrographical workflow is applicable to other analogues in Sirte Basin and other basins in Libya and North Africa.