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 reservoirs are identified: the Piaśnica, Sasino and Jantar formations, as well as the Mingajny shale. These units were diachronously deposited during the starved stages of Caledonian foredeep basin development, in the course of rising or high eustatic sea level. Across most of the basin, the shale formations analysed 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 Biłgoraj-Narol Zone. The shale analysed might be generally classified as a moderate to low quality, and locally high quality, unconventional reservoir. Within the shale net pay zones, the average TOC content is 2-5 wt.% TOC. The exceptions are the Piaśnica 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 favourable locations, mainly on the Łeba 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 commonly exceeds 6 %, reaching up to 10% at maximum, which might be considered as moderate to good. The clay minerals content is moderate to high (30-50%), and geomechanical characteristics of the shale formations are intermediate between brittle and ductile. No overpressure occurs in the basin, except for a dry gas zone in the SW Baltic Basin. In the Biłgoraj-Narol Zone, and to a lesser degree also in the Lublin region, pronounced tectonic deformation significantly limits shale gas/oil potential. Among 66 exploration boreholes drilled in the basin so far, only 5 were lateral boreholes with representative 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 (~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 progress in the exploitation of shale basins during the last 5 years. Commercially viable production might be achieved for a single borehole with estimated ultimate recovery exceeding 30-50 thousand tons of oil, or 60-90 million m3 of gas.