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Sequential Indicator simulations of sandstone reservoir thickness, Sava Depression, Croatia (SW Part of Pannonian Basin)

Located 35 km east of the Croatian capitol Zagreb, in the regional geological unit of called the Sava depression, the Kloštar oil field covers an area of some 30 km2. Its hydrocarbon reservoirs were discovered in Palaeozoic igneous rocks and Miocene sediments. These reservoirs are subdivided into a number of operational units called ‘reservoir series’, among which the most important are fine- to medium-grained Early Pontian sandstones, called ‘1st sandstone series’ that contain most of the oil reserves. Thickness of this reservoir was analysed using geostatistical simulation method called Sequential Indicator Simulations (abbr. SIS). The input dataset consisting of 19 hard data (wells) with average reservoir thicknesses ranging from 1 to 25m was used for making histogram in Excel(TM) and six thickness classes were defined with width of 4m. However, histogram analysis on such a small data set is not reliable and transformation had to be done using programs Surfer 8.0(TM) (licensed) and Grid HistogramTM (freeware). Original dataset had been interpolated by Inverse Distance Weighting method, obtaining grid with 7900 values. From this new set calculated a new histogram was drawn, confirming previously defined six classes and constructing cumulative density function (CDF). Classes resulted in defining of six cut-offs as follows: 5, 9, 13, 17, 21 and 25 m. Omnidirectional indicator semivariograms were calculated for each cut-off and approximated (using Variowin 2.21TM) with standardized theoretical models with sill=1 and nugget=0.6. It means that 3rd order stationarity is assumed, accepting only the intrinsic hypothesis as the minimum for dataset (mean is independent and semivariograms exist). SIS realizations were based on Simple Kriging techniques. Final model contained 630010 cells in each of 100 realisations created for thickness. Results show that variations in thickness do not follow the field’s structure, i.e. cell values are approximately isotropically dispersed along entire study area. Thickness mostly varies between 10 and 25 meters. If connected with depositional setting, it indicates on approximately similar depositional condition in the largest part of the mapped reservoir, where larger thicknesses are connected along “stripes” oriented NW-SE or NNW-SSE.

Sequential Indicator Simulations; thickness; sandstone reservoirs; Sava Depression; Pannonian Basin

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