Qualitative–Quantitative Analyses of the Influence of Depth and Lithological Composition on Lower Pontian Sandstone Porosity in the Central Part of Bjelovar Sag (Croatia)
<div>Results of several tests (porosity–depth graphic relation, t-test, Ftest,</div><div>Pearson’s R), were used to analyse and interpret the regularity</div><div>in porosity values of Lower Pontian oil-bearing sandstones from the</div><div&...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
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Croatian Geological Survey
2005-06-01
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| Series: | Geologia Croatica |
| Subjects: | |
| Online Access: | http://www.geologia-croatica.hr/ojs/index.php/GC/article/view/199 |
| Summary: | <div>Results of several tests (porosity–depth graphic relation, t-test, Ftest,</div><div>Pearson’s R), were used to analyse and interpret the regularity</div><div>in porosity values of Lower Pontian oil-bearing sandstones from the</div><div>central part of Bjelovar sag. Data came from 7 cored intervals within</div><div>5 wells in the Pepelana member and from 6 cored intervals (in 4</div><div>wells) in the Poljana member. The expected porosity decrease with</div><div>increasing depth was checked against lithological variations.</div><div>Sandstones are mostly fine-grained lithoarenites. The detritus is</div><div>composed of quartz, various micas, carbonate fragments and feldspars.</div><div>Across the study area, the depth range of the sandstones varies</div><div>from 430 m (top of the Pepelana member near Šandrovac) to 2046 m</div><div>(base of the Poljana member near Velika Ciglena). Cores included in</div><div>the analyses vary between 2.75 to 15.5 m in length.</div><div>Mean porosity and relative depth data were collected for two</div><div>groups: Group 1 comprised wells Pav–1, Pav–2, Rov–1, Ša–5, Ša–</div><div>35, and Group 2 included well VC–1. These groups were subdivided</div><div>for analysis into two (litho)stratigraphic units (Pepelana and Poljana</div><div>ss.). Porosity variation within each group is explained with reference</div><div>to the silt or clay fraction. Differences between the porosities of the</div><div>two groups (~10% lower absolute porosity near Velika Ciglena) is the</div><div>result of compaction and other processes.</div><div>Interval of 400 m thickness in particular sandstone member is set</div><div>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.</div><div>The analysis was improved by statistical calculation of Pearson’s</div><div>R, t-test and F-test, which more precisely described the relationship</div><div>between porosity and depth. Using these statistical tests and regression</div><div>equitation, the depth difference is calculated as 621 m in the</div><div>Pepelana and 667 m in the Poljana sandstones, as the limits when the</div><div>influence of compaction in the porosity–depth relationship could be</div><div>noticed.</div><div>Compaction was observed, in the study area, when data from</div><div>Velika Ciglena are compared to data from the other wells.</div> |
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| ISSN: | 1330-030X 1333-4875 |