Lower Paleozoic oil and gas shale in the Baltic-Podlasie-Lublin Basin (central and eastern Europe) - a review

In the Baltic-Podlasie-Lublin Basin, four potential lower Paleozoic shale res ervoirs are identified: the Piasnica, Sasino and Jantar formations, as well as the Mingajny shale. These units were diachronously deposited dur ing the starved stages of Caledo nian foredeep basin development, in the course of rising or high eustatic sea level. Across most of the basin, the shale formations ana lysed are saturated with light oil and condensate, and they are buried to depths of 2300-3500 m. The shale reservoirs reach the wet gas window at burial depths of 2800-4000 m, while dry gas accumulations occur at depths exceeding 3500-5000 m, except in the Bilgoraj-Narol Zone. The shale analysed might be gener ally classified as a mod erate to low quality, and lo cally high quality, unconventional reservoir. Within the shale net pay zones, the average TOC content is 2-5 wt.% TOC. The exceptions are the Piasnica Formation, for which this is 5-12 wt.%, and the Mingajny shale, which is TOC-lean (1.4-1.7 wt.%). The thickness of the shale net pay intervals in the most favour able loca tions, mainly on the Leba Elevation, generally reaches 20 m, and locally exceeds 35 m. The shale reservoirs are saturated with hydrocarbons of good quality. Their permeability is low to moderate, often in the range of 150-200 mD, while total porosity average per borehole is com monly exceeds 6 %, reaching up to 10% at maximum, which might be con sidered as moderate to good. The clay minerals content is moderate to high (30-50%), and geomechanical characteristics of the shale forma tions are interme diate be tween brittle and ductile. No overpressure occurs in the basin, except for a dry gas zone in the SW Bal tic Basin. In the Bilgoraj-Narol Zone, and to a lesser degree also in the Lublin region, pronounced tectonic deformation significantly limits shale gas/oil potential. Among 66 explo ration bore holes drilled in the basin so far, only 5 were lateral boreholes with repre sentative production test results. Hydrocarbon flow from the best boreholes was low to moderate, equal to 11.2 to 15.6 thousand m3/day for gas, and 157 bbl/day (similar to 21.4 ton/day) for oil. There is, however, high potential to improve production flow rates, connected with the fracturing of two net pay intervals at one time, as well as with significant technological prog ress in the ex ploitation of shale basins dur ing the last 5 years. Commercially via ble pro duction might be achieved for a single borehole with es timated ultimate recovery exceeding 30-50 thousand tons of oil, or 60-90 million m(3) of gas.