Wstępne wyniki modelowań przestrzennych (3D) parametrów petrofizycznych skał podczas poszukiwań stref występowania gazu zamkniętego w polskim basenie czerwonego spągowca

The Rotliegend Basin is filled with terrigenic complex over 1200 m in thickness. The complex comprises sediments of eolian, fluvial and playa depositional systems (Fig. 1). Reservoir volume of the pore space accessible for the reservoir media is ca. 830-990 km3. Gas exploration in this stratigraphic unit is now focused at depths of around 3000-3800 m b.s.l., in the near-top zone. Poor reservoir properties, especially permeabilities, are here the limiting factor for conventional exploration. A change in the prospecting strategy to comprise tight gas targets moves research into the deeper zone covering the whole profile of the Rotliegend. The paper presents preliminary results of 3D modeling of lithofacies and related petrophysical parameters variability. The static model was created with the use of Petrel 2009.2. Structural framework was built using regional structural, isopach and facies maps. It was relatively detailed, comprising 9 576 000 cells organized in 3 zones and 60 layers. To estimate facies model, the authors used results of integrated environmental analysis of core data and logs from 117 wells (Fig. 2 ). Models of clay content (Vsh) and porosity (PHI) were based on logs from 75 wells. The obtained results show that the northern margin of the Eastern Erg is characterized by presence of numerous eolian strata with porosity ranging from 5 to 15%. Their quality, quantity and thickness decrease toward the north, along with increase in depth. Modeling results indicate that the dominating porous layers of eolian sandstones and fluvial inserts are often intercalated with "non-reservoir" layers revealing porosity below 5%. In this zone, a deeper part of the Rotliegend section should be investigated more thoroughly (Fig. 5, 6, 9). The Pomeranian sector of the Central Basin is dominated by playa and fluvial sediments (Fig. 7, 8, 10). Slightly clayey eolian strata make a few, laterally discontinuous intercalations. Gas accumulations could be expected within local, laterally confined interlayers of eolian and fluvial sandstones with porosity of around 5-12%. Due to the location in the near-base part of the Rotliegend section, close to Carboniferous source rocks, they may be filled with gas, forming so-called sweet spots. Probability of gas occurrence in Pomerania is high as indicated by the Międzyzdroje gas field or small accumulation found in Piaski-PIG2 well. The presented preliminary study allowed to test usability of 3D modeling in tight gas prospecting. Fully reliable results will be obtained after increasing precision of the models comprising detailed seismic interpretation, the use of seismic attributes, and inclusion of quantitative data in diagenetic processes and sedimentology of layers in the modeling process.