The idea and interest of studying the unconventional hydrocarbon reservoirs in the Panonian Basin System, abbr. PBS, (the Drava, Mura and Zala Depressions) are achieved by defining the joint research project carried out by the multidisciplinary team of MOL and INA petroleum companies. This analysis is performed in the Croatian part of the Panonian Basin System (CPBS). Eight areas with potential existence of unconventional reservoirs were examined with focus on Tight Gas Sands and Gas Shales.

The primary object in this project stage is the estimation of possible unconventional reserves of gas (or Original Gas in Place, abbr. OGIP). Reserves are defined by area and reservoir porosity, saturation and net pay. They are usually estimated from well logging data and core laboratory and hydrodynamic data. Some difficulties and inabilities of accurate, i.e. professionally acceptable reservoir evaluation, were noticed. The reason is inadequate or incomplete well logging suite and inadequate formation evaluation work flow. Therefore, evaluation concepts from unconventional reservoirs presented in North American petroleum provinces could not be directly applied in our case. It was inevitable to use other data source, especially the Mud Logging Data to quantify net pay and qualify saturation. The rate of penetration, abbr. ROP, gas indications while drilling, the presence of hydrocarbon in rock samples, fracture systems on cores, inflows, eruptions and mud losses as well as the interpretation of overpressure using D exponent, abbr. Dcs method, significantly facilitated the evaluation of necessary parameters.

It is crucial to improve economics of hydrocarbon production from any basin through operational efficiency, well productivity as well as new analytical models. Here presented evaluation method of potential hydrocarbon reserves is applicable in any similar case. It provides a highly acceptable professional credibility and can be very useful in situations with incomplete and inadequate Well Logging Suite facilitating identification and categorization of unconventional reservoirs.

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