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

• Main Authors: Tomislav Malvić, Josipa Velić, Zoran Peh
• Format: Article
• Language: English
• Published: Croatian Geological Survey 2005-06-01
• Series: Geologia Croatica
• Subjects: Croatia
• Online Access: http://www.geologia-croatica.hr/ojs/index.php/GC/article/view/199

<div>Results of several tests (porosity&ndash;depth graphic relation, t-test, Ftest,</div><div>Pearson&rsquo;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 &Scaron;androvac) 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&ndash;1, Pav&ndash;2, Rov&ndash;1, &Scaron;a&ndash;5, &Scaron;a&ndash;</div><div>35, and Group 2 included well VC&ndash;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&nbsp;is based on sandstone&rsquo;s tops and bottoms comparison as well as&nbsp;graphical presentation of relation core porosity&ndash;relative depth interval.</div><div>The analysis was improved by statistical calculation of Pearson&rsquo;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&ndash;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>