engleski

Results of geostatistical porosity mapping in Western Drava Depression fields (Molve, Kalinovac, Stari Gradac)

Porosity values of the gas and gas-condensate reservoirs of the western Drava depression (Molve, Kalinovac, Stari Gradac) are geostatistically analysed, in four hydrodynamically connected reservoir lithofacies. Lithofacies are (from the oldest to youngest): lithofacies IV of prePz-D age, lithofacies III of P+T1 age, lithofacies II of T2, 3+J age and lithofacies I of M1, 2 age. Laterally, the lithofacies are very similar, with minor differences depending on the observed field. The deepest and the oldest lithofacies IV consist of different metamorphic and magmatic rock, mostly diaftorite and volcanites. The next lithofacies III (Clastites), of Permotriassic age, includes metasandstone and quartzite, and less portion of dolomite, dolomitic breccia, slate, and at the northeast of Stari Gradac also volcanite. Lithofacies II (Dolomites) mostly consists of different dolomite and dolomitic breccia, shale, and sporadic intrusions of pyroclastite and diabase. Lithofacies I is named as: Limestone in the Molve field, Breccia in Kalinovac, and Clastite in Stari Gradac. The age is Lower and Middle Miocene, i.e. Badenian. These sediments are represented in Molve and Kalinovac by calcareous clastite, calcitic matrix, and some conglomerates and sandstone. The youngest are biocalcarenite and biocalcrudite. In Stari Gradac, such sediments are represented by different breccia, conglomerate and coarse-grained sandstone. Geostatistical mapping included variogram analyses, estimation and simulations. Four "average" variogram ranges are determined, one by each lithofacies: 5807x4921 ft (IV), 9285x6600 ft (III), 7244x3035 ft (II) and 7520x3773 ft (I). All are approximated with the spherical theoretical model. The most appropriate estimation technique was Ordinary Kriging. The most appropriate interpolations are achived in lithofacies IV and I in the Molve field, and in lithofacies III and II in Kalinovac field. Results of stochastical sequential simulations are expressed as interval of minimum, mean, and maximum values for each lithofacies. New mean porosities by lithofacies and fields are calculated based on 100 stochastic realizations. P1 and P99 realizations and interpolated maps are the base for outlining promising areas in different lithofacies regarding higher porosity. Lithofacies III is the most promising regarding higher porosity values mapped by presented methods, and compared with solutions from previous studies. Such promising areas were outlined in Stari Gradac on the northwestern (Stg-1z, 2 wells), central (around Stg-1, 4, 5 and Bny-5 wells) and southeastern (Bny-1, 3, 7 wells) parts of the field. The area with higher porosity, in Kalinovac field, is outlined along the central fault zone, among Kal-2, 6 and 12 wells. The central block in Molve field, bordered by margin faults is described as the most promising area.

porosity; kriging; stochastic simulations; Drava depression

The paper is written both in English and Croatian.