Qualitative–Quantitative Analyses of the Influence of Depth and Lithological Composition on Lower Pontian Sandstone Porosity in the Central Part of Bjelovar Sag (Croatia)

Results of several tests (porosity–depth graphic relation, t-test, F-test, Pearson’s R), were used to analyse and interpret the regularity in porosity values of Lower Pontian oil-bearing sandstones from the central part of Bjelovar sag. Data came from 7 cored intervals within 5 wells in the Pepelana member and from 6 cored intervals (in 4 wells) in the Poljana member. The expected porosity decrease with increasing depth was checked against lithological variations. Sandstones are mostly fine-grained lithoarenites. The detritus is composed of quartz, various micas, carbonate fragments and feldspars. Across the study area, the depth range of the sandstones varies from 430 m (top of the Pepelana member near Sandrovac) to 2046 m (base of the Poljana member near Velika Ciglena). Cores included in the analyses vary between 2.75 to 15.5 m in length. Mean porosity and relative depth data were collected for two groups: Group 1 comprised wells Pav–1, Pav–2, Rov–1, Sa–5, Sa–35, and Group 2 included well VC–1. These groups were subdivided for analysis into two (litho)stratigraphic units (Pepelana and Poljana ss.). Porosity variation within each group is explained with reference to the silt or clay fraction. Differences between the porosities of the two groups (~10% lower absolute porosity near Velika Ciglena) is the result of compaction and other processes. Interval of 400 m thickness in particular sandstone member is set as minimum value for observing influence of compaction. Such statement is based on sandstone’s tops and bottoms comparison as well as graphical presentation of relation core porosity–relative depth interval. The analysis was improved by statistical calculation of Pearson’s R, t-test and F-test, which more precisely described the relationship between porosity and depth. Using these statistical tests and regression equitation, the depth difference is calculated as 621 m in the Pepelana and 667 m in the Poljana sandstones, as the limits when the influence of compaction in the porosity–depth relationship could be noticed. Compaction was observed, in the study area, when data from Velika Ciglena are compared to data from the other wells

Lithologic Composition and Stratigraphy of Quaternary Sediments in the Area of the “Jakusevec” Waste Depository (Zagreb, Northern Croatia)

In the area covered by the “Jakusevec” waste depository, to a depth of 101 m, six lithological units were determined based on fieldwork and laboratory geologic-geophysical investigations. It was discovered that the silty-clayey units (units 1, 3 and 5) are covered by sandy-gravely (units 2 and 4) and gravely ones (unit 6), respectively. These units constitute the sediments of the Middle and Upper Pleistocene and Holocene and are separated by erosional unconformities. The Pleistocene gravels are predominantly of quartz-quartzite composition, while the Holocene ones are composed of carbonate cobbles and pebbles. In contrast, the sands exhibit a fairly uniform mineral composition throughout the column. The Pleistocene silt and clay are mostly composed of muscovite-illite and quartz with lesser amounts of chlorite, kaolinite and smectite. There is a difference in composition of this fraction in unit 6, where the quartz, calcite and dolomite particles prevail and smectite and illite/smectite are absent. Unit 3 is characterised by the goethite content. The Pleistocene layers were formed in a lacustrine-marshy environment while the Holocene sediments are fluviatile. This sedimentary sequence is interrupted by occasional terrestrial phases, or drying-up periods, dependent on the palaeoclimate conditions, particularly the interchange of cold and dry glacials with the warmer and more humid interglacial stage

Neural networks in petroleum geology as interpretation tools

Abstract Three examples of the use of neural networks in analyses of geologic data from hydrocarbon reservoirs are presented. All networks are trained with data originating from clastic reservoirs of Neogene age located in the Croatian part of the Pannonian Basin. Training always included similar reservoir variables, i.e. electric logs (resistivity, spontaneous potential) and lithology determined from cores or logs and described as sandstone or marl, with categorical values in intervals. Selected variables also include hydrocarbon saturation, also represented by a categorical variable, average reservoir porosity calculated from interpreted well logs, and seismic attributes. In all three neural models some of the mentioned inputs were used for analyzing data collected from three different oil fields in the Croatian part of the Pannonian Basin. It is shown that selection of geologically and physically linked variables play a key role in the process of network training, validating and processing. The aim of this study was to establish relationships between log-derived data, core data, and seismic attributes. Three case studies are described in this paper to illustrate the use of neural network prediction of sandstone-marl facies (Case Study # 1, Okoli Field), prediction of carbonate breccia porosity (Case Study # 2, Beničanci Field), and prediction of lithology and saturation (Case Study # 3, Kloštar Field). The results of these studies indicate that this method is capable of providing better understanding of some clastic Neogene reservoirs in the Croatian part of the Pannonian Basin

Characterization of clastic sedimentary enviroments by clustering algorithm and several statistical approaches — case study, Sava Depression in Northern Croatia

Abstract This study demonstrates a method to identify and characterize some facies of turbiditic depositional environments. The study area is a hydrocarbon field in the Sava Depression (Northern Croatia). Its Upper Miocene reservoirs have been proved to represent a lacustrine turbidite system. In the workflow, first an unsupervised neural network was applied as clustering method for two sandstone reservoirs. The elements of the input vectors were the basic petrophysical parameters. In the second step autocorrelation surfaces were used to reveal the hidden anisotropy of the grid. This anisotropy is supposed to identify the main continuity directions in the geometrical analyses of sandstone bodies. Finally, in the description of clusters several parametric and nonparametric statistics were used to characterize the identified facies. Obtained results correspond to the previously published interpretation of those reservoir facies

Review of Miocene shallow marine and lacustrine depositional environments in Northern Croatia

The Neogene and Quaternary depositional and structural characteristics of the southwestern Croatian Pannonian Basin System (CPBS) are unique within the Pannonian Basin System. Due to its position at the basin margin, the area was covered by shallow and partially isolated parts of the Pannonian Sea in the Badenian (Middle Miocene). Later, in the Pannonian and Pontian ages, the area contained several brackish lakes, the largest being Sava Lake and Drava Lake. Tectonic events, sedimentation and depositional mechanisms occurring during the Neogene in the CPBS have revealed that those areas can be considered as former shallow seas or lakes dominated by clastic sedimentation. Marine coarse-grained clastic sedimentation took place during the Badenian, with local sources of material and numerous alluvial fans developed during the first transtensional phase. In the Pannonian and Pontian (Late Miocene), sediments were deposited by turbidity currents from a single, distal material source located in the Eastern Alps during the second transtensional tectonic phase

Local sediment sources and palaeoflow directions in Upper Miocene turbidites of the Pannonian Basin System (Croatian part), based on mapping of reservoir properties

The source of the clastic sediments in the Croatian part of the Pannonian Basin System (CPBS) during the Late Pannonian and Early Pontian was the Eastern Alps. Clastic sediments were redeposited several times before they reached the Sava Depression. The depositional environment and sediment transport mechanisms have been subject to detailed analysis described in many publications, and this study builds on previous research. We have carried out geostatistical mapping of selected Upper Pannonian and Lower Pontian reservoir variables of the Kloštar Field, located to the west of the Moslavačka gora Mt. (Croatia). This has shown that the Moslavačka gora Mt. was a secondary, local source of sediment, in contrast to the previous interpretation of a single, distant clastic source (Eastern Alps) for the CPBS during the Late Miocene. As the mineralogical composition of the Moslavačka gora Mt. and the Eastern Alps is very similar, the dominant direction of turbidity currents obtained by sequential indicator simulations are used to suggest that a modest amount of detritus was eroded from the Moslavačka gora Mt. and mixed with detritus sourced from the Eastern Alps