engleski

Using Ultraviolet Light for Fast Sedimentological Analysis and Characterization of Reservoir Rocks: A Case Study of the Upper Miocene Sediments from the Sava Depression, Croatia

Ultraviolet (UV) light, a part of the electromagnetic radiation spectrum, can produce different physical and chemical reactions in various materials. This especially applies to minerals and organic compounds that exhibit fluorescence. UV lamps have been used in the exploration of hydrocarbons for many years. Examination of mud, drill cuttings and cores under UV light often indicates oil in small amounts which might not be detected by other means. This work demonstrates how UV radiation can also be used for sedimentological analysis, as well as for estimating reservoir quality. To demonstrate the aforementioned, UV light analysis was conducted on the Upper Miocene sediments from the Sava Depression in Croatia, SW part of the Pannonian Basin System. In order to confirm UV light defined features, additional detailed analyses have been conducted and the results have subsequently been compared. The following methods have been applied: UVA lamp Fluotest Forte (365 nm) ; description of sedimentary structures and textures ; thin section analyses with additional staining and impregnation (Olympus BX51) ; and SEM-EDS analysis (JEOL JSM-6510 LV equipped with Oxford INCA X-act). Due to weathering and chemical alteration or contamination, seemingly massive and homogenous rocks can have diverse textures and structures that can often be hard to notice under visible light, which is the case with the cored Upper Miocene reservoir sediments. Hydrocarbons in these rocks emit certain colors caused by fluorescence. Consequently, bedding, lamination, erosional or post-depositional structures, and bioturbation are easier to observe. UV light makes it simpler to gain insights into changes of sedimentary succession which help to precisely identify depositional environments. It can also help in measuring dip and strike directions of beds, current flow directions and directions of interior fractures. Seeing how certain minerals fluoresce in different colors as well, areas with different amounts of distinctive mineral components can accordingly be distinguished. Areas of varying rock porosity caused by diagenesis, with zones of reduced or enhanced mineral cementation between and within grains, accompanied by oil occupied pores, can be better detected under UV radiation. Furthermore, detrital fragments of organic matter and particular fossils will occasionally fluoresce, providing a better additional assumption of the depositional settings. Different lithological units, sedimentary characteristics, mineral composition and description of diagenetic processes of the analyzed cored sediments discerned by the fast UV light analysis match the detailed results of rock examinations, showing that the sediments were deposited in a system of lacustrine delta environment. The UV light can be of great help in sedimentological interpretation and accordingly in assessing the position of the most favorable reservoirs in a basin. Besides, this valuable method can be used for choosing representative samples for further detailed mineralogical, petrographic and geochemical analyses or petrophysical measurements. Data regarding mineralogy, lithology, porosity, and permeability of rocks, as well as type and amount of hydrocarbons, is directly connected to the extraction quantity of a reservoir. A fast estimate of characteristics of drilled rocks is crucial for procedures in exploration, development and, moreover, production of oil fields.

UV light, sedimentology, reservoir characterization, Sava depression

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