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The estimation of time-to-depth relationships can prove challenging in regions with rare acoustic logs. This study focuses on the eastern part of the Drava Basin in north Croatia, chosen as a mature hydrocarbon exploration area with abundant geophysical and well data. As only a small portion of wells have well log measurements or seismic profiling performed, a time-to-depth extrapolation is often performed, which potentially results in the erroneous placement of well log markers in the time domain and affects the interpretation of seismic sections or volumes. This study proposes a novel methodology for predicting two-way travel time values in wells without vertical seismic profiling or acoustic logging. This research evaluates the parameters for the characterization of the velocity distribution in the subsurface and the efficiency of artificial neural networks versus conventional methods for this task. The constructed artificial neural network model has a correlation coefficient above 0.99 for the training, testing, and validation datasets, with a mean absolute error of approximately 25 milliseconds for each network. Artificial neural networks proved to have a lesser error in predicting the two-way time and are not sensitive to outlier values. [ABSTRACT FROM AUTHOR]

The deep saline aquifer (DSA) Poljana in the Upper Pannonian Poljana Sandstones of Sava depression, the SW part of the Pannonian basin system, was identified as a potential CO2 storage object in previous works. Its boundaries have been redefined and its general model further developed, including the areal distribution of porosity based on analyses of 23 well logs. The sandstones were deposited in turbiditic and deltaic facies that caused considerable variations of porosity, which was further influenced by diagenetic processes. A comparison of altogether 355 pairs of porosity and permeability measurements on core plugs from 16 wells indicated 2 different sets of samples: impermeable samples with effective porosities reaching 18% and permeable samples which showed correlation between porosity and permeability. Accordingly, the permeability model was developed as semi-categorical with two categories: the first category comprising parts of DSA Poljana with porosity values exceeding 18%, where permeability was correlated with porosity, although with limited reliability, and the second category comprising model cells with porosity values below the threshold of 18%, where permeability should not be correlated with porosity due to the appearance of impermeable values. This approach enabled delineation of areas where permeability can be estimated with greater certainty, which is of utmost importance for the planning and development of CO2 storage projects and/or energy storage projects with respect to fluid injectivity. This approach can be used in areas with similar geological settings and limited datasets as an important step from regional capacity estimations towards the detailed, local-scale investigations. [ABSTRACT FROM AUTHOR]

Presented work focuses on the importance of unconformity that separates the Neogene infill from older Palaeozoic and Mesozoic rocks in the Croatian part of Pannonian Basin. Structure map of this horizon nearly represents the thickness map of the Neogene and Quaternary basin fill. Rock formations just below the unconformity are significantly weathered, which results in favourable petrophysical properties, making them interesting from the aspect of geoenergy potential. The pre-Neogene surface was constructed in 1:400,000 scale using publicly available subsurface maps of different scale and different level of detail. Harmonization and compilation of these maps enabled construction of a structured surface with near-vertical fault planes. Supplemental maps were constructed via basin modelling, showing the temperature distribution in the subsurface, potential source rock maturity near the mapped horizon, surface heat flow and geothermal gradient distribution. Constructed maps illustrate the importance of the mapped interval for regional planning of future geoenergy-related research.. [ABSTRACT FROM AUTHOR]

The regular working method for well log correlation in the Neogene-Quaternary infill of the Croatian Pannonian Basin is based on visual identification of specific motifs that can be observed regionally through the sub-basins or depressions. This method is proven successful through exploration for petroleum accumulations in the aforementioned area but its application is limited outside of the interval of the Upper Miocene sediments and has proven to be subjective. Standard deviation values of well log data from conventional well logs (spontaneous potential, shallow and deep resistivity) were calculated and observed over the entire Neogene-Quaternary well interval for defining new correlation markers which correspond to the old well log marker horizons. Traditionally determined marker horizons did not coincide with those defined by the standard deviation technique. However, new regional horizons were established in the youngest part of the infill, which made more detailed correlation now possible. [ABSTRACT FROM AUTHOR]

Nitrates present one of the most common groundwater contaminants in the world and one of the five major groups of contaminants in the study area. Gaussian simulation (GS) algorithm was used for determining the spatial distribution of average nitrate concentrations from 2010 to 2015 on 95 sampling points. Results indicate two main focus areas of nitrate contamination, located on the left and right bank of the Sava River. Those areas generally extend according to groundwater flow, while areas near Sava River have much smaller concentrations. GS showed that they can be useful for this kind of mapping because they favor abrupt changes in data values which are in this case a result of heterogeneous lithological composition of the aquifer. [ABSTRACT FROM PUBLISHER]

Data from selected Lower Pontian sandstone reservoir in the Kloštar Field, situated in the Sava Depression (Northern Croatia), were used for mapping with Sequential Indicator Simulations rather than using a classical approach. Such approaches offer better insight in distribution of geological variables or zonal uncertainties in the cases with larger datasets (15 points or more). Obtained maps of porosity and reservoir thickness are presented here along with probability maps of certain selected cut of values of petrophysical parameters. Maps showed distinct sedimentological features that can clearly be observed on the both sets of maps. [ABSTRACT FROM AUTHOR]

The Kloštar oil field is situated in the northern part of the Sava Depression within the Croatian part of the Pannonian Basin. The major petroleum reserves are confined to Miocene sandstones that comprise two production units: the Lower Pontian I sandstone series and the Upper Pannonian II sandstone series. We used well logs from two wells through these sandstones as input data in the neural network analysis, and used spontaneous potential and resistivity logs (R16 and R64) as the input in network training. The first analysis included prediction of lithology, which was defined as either sandstone or marl. These two rock types were assigned categorical values of 1 or 0 which were then used in numerical analysis. The neural network was also used to predict hydrocarbon saturation in selected wells. The input dataset was extended to depth and categorical lithology. The prediction results were excellent, because the training and prediction dataset showed little disagreement between the true and predicted values. At present, this study represents the best and most useful application of neural networks in the Croatian part of the Pannonian Basin. [ABSTRACT FROM AUTHOR]

As natural gas reserves are generally decreasing there is a need to successfully characterize potential research objects using geophysical data. Presented is a study of amplitude vs. offset, attribute and artificial neural network analysis on a research area of a small gas field with one well with commercial accumulations and two wells with only gas shows. The purpose of the research is to aid in future well planning and to distinguish the geophysical data in dry well areas with those from an economically viable well. The amplitude vs. offset analysis shows the lack of anomaly in the wells with only gas shows while the anomaly is present in the economically viable well. The artificial neural network analysis did not aid in the process of distinguishing the possible gas accumulation but it can point out the sedimentological and structural elements within the seismic volume. [ABSTRACT FROM AUTHOR]

Croatia has both significant CO2 emissions from the point sources and a history of oil and gas exploration, and this is why the CCS technology surfaced as a viable solution for curbing CO2 emissions on a national level. Since approximately half of emissions from the stationary industrial sources occur along the Adriatic coastline, the entire offshore area became an exploration target. Regional studies revealed the potential storage plays, one of which is in the aquifer of the Mesozoic carbonate complex with dual porosity extending all along the Croatian offshore area. Three structures were chosen in its central part–Klara, Kate and Perina. For the first two, the models were constructed based on the data from old exploration wells and a regional structural map, while for the Perina structure, a new seismic interpretation was added to better characterise its properties. It came out that the Kate structure appears to be the most prospective in general (45 Mt), with neighbouring Klara as the second (39 Mt), while the initially promising Perina (7 Mt) turned out to be of far lesser importance. The Perina structure case is an example that new seismic interpretation can reduce the capacity estimate if it reveals certain limiting factors, in this case, the limitation of structural closure. [ABSTRACT FROM AUTHOR]

Exploration of Pliocene, Pleistocene and Holocene deposits in of the Croatian Northern Adriatic and Southern Dalmatia (coastline) offshore is presented in this paper. It was a way to describe the existing lithostratigraphic units in Croatian Po River sediments and to determine such units in the depositional area of Neretva River. Both systems have been influenced by sea-level fluctuations during the Quaternary, resulting in significantly decreased sea depths and increased subaerial exposure during glacial periods, especially in the Northern Adriatic. In the Croatian part of Po Depression, two new formations were mentioned - Istria (Pliocene) and Ivana (Pleistocene, Holocene). Three new lithostratigraphic members were described within Ivana Formation, namely Anamarija, Katarina and Izabela. Further south, in the Southern Dalmatia shallow offshore, Pliocene, Pleistocene and Holocene deposits were named the new Neretva Channel Formation. This formation was further divided into Neretva Sands (sometimes equivalent for entire formation) and the (Quaternary) Mali Ston Bay Member. No typical Pliocene lithotype has yet been recognised. [ABSTRACT FROM AUTHOR]

Approximately 104x106m³ of oil (39 fields), 6.93x106m³ of condensate (11 fields), and 64.92x109 msup3; of gas (52 fields), were recovered in the Croatian part of the Pannonian Basin System during 64 years of exploitation (1941-2005). The production peak was attained between 1980-1989, when exploitation began in 12 new fields. Based on their cumulative production, the Croatian oil and gas fields can be divided into four groups, and the condensate fields into three groups. Such a division has been supported by analysis of recovery, number of reservoirs, porosity and permeability, age and lithology of reservoir rocks. The longest production period is assumed for the first group of fields; for oil it is approximately 55 years, for condensate 46 and gas 36 years. In the favourable first group the average number of reservoirs is 16 for oil and 11 for gas. Lithological composition is highly favourable, because reservoirs are represented mostly by sandstones of Pannonian and Pontian age with high porosities and permeabilities. A relatively homcgeneous sandstone lithology, including good regional seals like marls, enables an increase in recovery through the use of secondary and tertiary recovery methods. Also, water-flooding will remain the dominant secondary-recovery method for increased production in the future. [ABSTRACT FROM AUTHOR]

Every country with a history of petroleum exploration has acquired geological knowledge of its sedimentary basins and might therefore make use of a newly emerging resource—as there is the potential to decarbonise energy and industry sectors by geological storage of CO2. To reduce its greenhouse gas emissions and contribute to meeting the Paris agreement targets, Croatia should map this potential. The most prospective region is the SW corner of the Pannonian basin, but there are also offshore opportunities in the Northern and Central Adriatic. Three "geological storage plays" are suggested for detailed exploration in this province. Firstly, there are three small gas fields (Ida, Ika and Marica) with Pliocene and Pleistocene reservoirs suitable for storage and they can be considered as the first option, but only upon expected end of production. Secondly, there are Miocene sediments in the Dugi otok basin whose potential is assessed herein as a regional deep saline aquifer. The third option would be to direct future exploration to anticlines composed of carbonate rocks with primary and secondary porosity, covered with impermeable Miocene to Holocene clastic sediments. Five closed structures of this type were contoured with a large total potential, but data on their reservoir properties allow only theoretical storage capacity estimates at this stage. [ABSTRACT FROM AUTHOR]

The Drava Basin in the SW Pannonian Basin System (PBS) was initially formed by passive rifting with accompanying sedimentary infill. Although this has been the subject of much previous work, an account of tectonic control has been lacking. Based on cores, wire logging, and seismic data, the tectonostratigraphic interpretation, depositional systems, and control on depositional systems of the syn-rift infill of the eastern part of the Drava Basin were studied. The rifting phase is characterised by the formation of half-grabens, grabens, a sag, and supradetachment basin structures with structural ramp and structural highs. The syn-rift infill can be divided into second-order tectonostratigraphic sequences corresponding to the early and late rift stages. The second-order sequences are further subdivided into third-order tectonostratigraphic sequences formed in response to higher-order tectonic events associated with local rift migration. In contrast to the early-stage structures, the late-stage rift structures are primarily controlled by extensional detachments that represent parts of the Drava Rift Fault System (DRFS). The early syn-rift is characterised by continental deposition through alluvial fans, fan deltas, and lacustrine environments. The late syn-rift stage is characterised by marine deposition in shallow water, fan deltas, and submarine slope-aprons, with deep marine sedimentation and intense volcanic activity. The ramp, basin slopes, and fault scarp slopes represent the major sediment transport pathways involved in the formation of alluvial fans, fan deltas, or submarine slope-aprons. Basinal sedimentation and major depocenters are located within synforms formed by structural lows in the geometry of extensional detachments. This study gives an example of syn-rift tectonic control for the SW part of the PBS and the influence of detachment geometry on basin fill. We have presented an approach based on 3D seismostratigraphic interpretation of tectonostratigraphic sequences, and the correlation of seismic facies with depositional environments developed in a back-arc setting. • The syn-rift infill of the Drava Basin is characterised by seismic facies and facies associations analysis. • The early rift stage is characterised by continental deposition and the late rift stage is by marine deposition. • Second-order and third-order sequences depict rift infill architecture. • Tectonic forces were the main controlling factors during sedimentation. [ABSTRACT FROM AUTHOR]