Abstract
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 Šandrovac) 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, Ša–5, Ša–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.
Highlights
Reservoir characterization mostly comprises detailed evaluation of petrophysical parameters, i.e. porosity, permeability and saturation
Compaction was assumed at significant lower depth differences (150 m at Okoli field) than it was supposed for equivalent lithostratigraphic members in the central part of Bjelovar sag
A detailed study of porosity and depth in the sandstone members of the Kloštar-Ivanić formation (Lower Pontian) was performed using the data obtained from core analysis
Summary
Reservoir characterization mostly comprises detailed evaluation of petrophysical parameters, i.e. porosity, permeability and saturation. Accuracy depends on the source, amount of data, and method applied for their comparison and evaluation in places where the parameters are not measured. Porosity is selected as the most important variable for reservoir characterization as it has a direct influence on permeability and saturation values. Input data were porosity values measured in vertical cores, performed in the INA-Naftaplin laboratory and noted in well files. The majority of the data in Bjelovar sag (SW margin of Pannonian basin), belongs to the Pepelana and Poljana sandstones These units were chosen for analysis because they occur in the central part of the sag
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
