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Challenges in the research of the unconventional tight oil play in SW Pannonian basin (Croatia)

Abstract

Over the years INA researchers have reinterpreted almost every play in Croatia and finally came out with three promising areas for unconventional exploration.

One of them is Sava basin within the exploration oil filed Ivanic, where completely new borehole has been set, primary targeted to examine Middle Miocene deposits of marl (marlstone) intercalated with thin layers of several types of sandstone which by the reservoir characteristics belong to the unconventional reservoirs.

Unconventional resource invites unconventional methods. Unconventional reservoirs must be stimulated to enhance the hydrocarbon production in order to reach economically acceptable amounts. We have decided to engage hydraulic fracturing, totally aware of the fact that this means not only implementation of new technologies in well construction and operations, but responsible and safe attitude to the nature respectively.

INA's recent well Iva-2D is designed to fulfill all planed technical and environmental demands, together with numerous research tasks. Its main purpose is to verify seismic projection, correlate and calibrate various wireline logs and gain as much data as possible to help prepare successful hydraulic fracturing of tight-oil/source rock prospect. Besides many ordinary and extraordinary activities, three continuous oriented cores were drilled.

Total of 27.2 meters were recovered (interval: 2450.0 – 2477.2 m). Cores were approached multidisciplinary.

After simple technical activities (e.g. cleaning, marking, photographing…), petrophysical measurements (total and spectral gamma, porosity, permeability, bulk and grain density) were carried out. Complete examination under the ultraviolet light to detect the presence of hydrocarbons followed.

Geochemical investigations, predominantly in order to define organic facies, kerogen type and maturity as well as generative potential were performed. Cores and drill cuttings were analyzed for total organic carbon and all samples with TOC ≥ 0.3% were pyrolysed. The most interesting samples were submitted to extraction, column liquid chromatography, gas chromatography, microscopic examination, and analysis of sulphur and stable carbon isotopes in kerogen and bitumen. The acid solubility test was made on particular samples.

Detailed geological analyses were performed as well. Petrographical analyses of thin sections gathered information on mineralogy, diagenesis, lithofacies and even microporosity by the analyses of slides impregnated with blue dye epoxy resin. Biostratigtraphical (micropaleontological and palynological) analyses provided the information on age, fossil assemblage and palaeoenvironment. Together with sedimentological observations, the depositional environment has been established.

Certain core samples were analyzed by SEM to define mineral composition, clay minerals and pore space geometry. According to crystallographic forms of the minerals and identification of peaks in the energetic spectrum of x-rays (EDS) mineral identification has been done.

The same samples were submitted to the semi-quantitative XRD analyses. By the X-ray powder diffraction analyses mineral content and more importantly, clay mineral identification and quantification in core samples were defined.

Geomechanical ultrasonic velocity measurements were carried out on selected core plugs. Dynamic elastic constants, dynamic Poisson's ratio (νd), dynamic shear modulus (Gd) and Young's modulus (Ed), were calculated from the test results.

Cores were oriented and therefore recorded on DMT CoreScan II. Obtained data were analyzed in CoreLog Integra presenting the results of the structural element analyses.

All data were evaluated and then integrated in the complex final interpretation of Iva-2D unconventional Tight Oil Play.