Upper Devonian–Carboniferous Unconventional Gas Plays in Poland
Since 2006 the Lower Paleozoic Baltic-Podlasie-Lublin Basin (BPLB) become a subject for intensive exploration for shale gas/oil. With some 40 legacy key wells with abundant core samples and some 70 new exploration wells (including several lateral wells with multi-fracturing test) the shale gas/oil potential remains reasonably well documented. The basin is a flexural foredeep of Caledonian collision zone. Its characteristic feature is an exceptionally high volumetric contribution of mudstone and claystone (partly marly), being up to 1000-2000 m thick. Within this section there are a few individual shale formations identified, which are characterized by elevated TOC contents (mostly II type of kerogen) and shale gas/oil potential. These are present in a lower part of the section within interval of no more than 200 m.
There is a general diachronism of organic rich deposition from NW towards East and SE observed in the basin. The deepest in a stratigraphic section is the Alum shale of the (Middle-?) Upper Cambrian to Lower Tremadocian age. This shale is organic rich (5-10 % TOC av.), however is present only in the NW part of the BPLB. In the Danish or Scanian (SW Sweden) sector of the Baltic Basin (BB – northern segment of the BPLB) it is thicker than 100 m, while in the Polish offshore it is reduced to some 30 m. Onshore in Poland it is represented only in the SW part of the BB where it has thickness mostly of a few meters, though alone it is hardly a target for commercial production. In the central part of the onshore BB and in the western part of the Podlasie Depression (PD – middle sector of the BPLB) the main exploration target is the Upper Ordovician (Caradoc) shale. The net pay thickness of this formation is often several meters, with maximum 22-24 m in the northern onshore BB. Its average TOC contents is in a range of 2-4 %. Up section yet another exploration target is the Llandovery shale, properly developed in the central part of the onshore BB, west of Mazury uplift (Plock Trough), the central part of the PD, and the NW part of the Lublin region (LR – southern segment of the BPLB). This formation is very inhomogeneous. Its lower-most few meters is a hot shale very rich in TOC (up 20 %), developed due to eustatic sea level rise. Up section within Llandovery TOC decreases. The net pay thickness is often in a range of several meters, except of the Bilgoraj-Narol zone (western part of the LR), where the Llandovery net pay is exceptionally thick (>20-30 m). Since the Caradoc and the Llandovery net pays are divided only by a few to several meters thick Ashgill marl or limestone acting as a frack barrier, common current attempt is to cover with individual fracturing both the formations at once. In a such case the cumulative net pay thickness usually is several to more than 30 m. The last up-section stratigraphic interval being an exploration target is the (upper-most Llandovery-?) lower Wenlock. This formation is usually 40-70 m thick, though is lean in TOC (1,5 % av.). It is in its the best development in the central and eastern BB, the central PD and in the LR.
New thermal maturity measurements indicate that the shale reservoirs are more wet than initially anticipated based on legacy data. At a limited shale reservoir burial depth levels (<4000-4500 m) majority of the basin is in the oil window (eastern part) or liquids window. This is confirmed by both archival record of oil and gas shows and HC composition in the basin, as well as by composition of HC produced during tests from new exploration wells. Dry gas zone is of limited acreage, and is characterized by high burial depth (4000-4500 m), resulting with high drilling costs. The advantage of the deep dry gas zone is however presence of overpressure.
Brittleness of the Caradoc shale is relatively high (0.6-0.7) and is based mostly on high Silica contents. The Llandovery shale is less brittle (0.5-0.65) and is characterized by higher contents of Carbonates, which is even higher in the Welock shale. Brittleness factor for the later one is highly variable (0.4-0.75). Porosity of the Lower Paleozoic shale in the BPLB in general is moderate (5-10 %), while permeability is usually relatively low (120-250 nD).
The currently identified major exploration risk factors, presumably responsible for low gas/oil flows during the tests (up to 400-500 scf/d and 100-150 b/d, respectively), are: the relatively low net pay thickness, the lean TOC, the low permeability, and in the Lublin region also the tectonic deformation.
AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015