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Unconventional geological play concept of southwestern part of Neogene Pannonian basin (Croatia)

Sava and Drava Neogene basin sedimentary infill was submitted to fraccing campaign in 2015/2016 on 5 vertical wells. Results gave crucial input data for start of an integrated, multidisciplinary study of unconventional play potential in Croatia. Exploration approach to unconventional reservoirs with knowhow transfer was established with engagement of experienced consulting company. Research focused on regional present source rocks and over-pressured oil & gas accumulations in low permeability reservoirs. Complete in-deep analysis from database, basin scale, play and prospect based analysis to the final drilling location was performed in one-year joint project. Composite dataset analyzed archive studies, published articles, well data (mud logging, TOC (Rock-Eval), core data, well tests, vitrinite reflectance - Ro), wirelogs, basin modeling data and seismics (two 3D cubes, >400 2D seismic lines, full TWT interpretation coverage, full coverage of structural depth maps and fault polygons interpretation). Two major unconventional plays were identified, shale and tight oil/gas plays. Further analysis characterized 6 main regional plays: Panonian low permeability play, Panonian shale oil play, Badenian low permeability play/tight carbonates, Badenian shale play, Karpatian low permeability play and Lower Miocene shale play. Altogether 20 potential locations were analyzed on play level while only 5 top-most ranked were upgraded to prospect level – two shale and three tight plays. Ranking was performed by implementing following criteria: stratigraphy, target depth, depositional environment/sedimentology, area extent, thickness, TOC, trapping, HC shows, maturity and migration, porosity and data quality. Unconventional shale prospect analysis revealed that the gas/oil content per rock unit is very low, which predominantly relates to the low TOC content in the interlayered shale/marls. “Sweet spot” pilot area analysis reached best well recovery of 0.018 BCM which is significantly lower than required well economic cut-off. The studied shale plays are regarded as a poor unconventional play and at present day conditions the economic exploitation is unlikely. The most prolific prospects identified in the study areas are tight gas plays regarded as moderate prolific and moderate risk prospects. For these plays an appraisal concept was developed together with multistage stimulation/fraccing designs and development scenarios. Review of drilling and stimulation technologies for low permeability reservoirs and develop alternative cost schemes can make potential “game changers” out of tight gas plays in mature Pannonian basin petroleum system.

Unconventional play concept, Neogene, Sava and Drava basin

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