Polish Baltic Basin Hydrocarbon Potential.
Baltic Shale oil/gas exploration and production concessions are located in the central part of the on-shore Polish Baltic Basin. Total petroleum system of the lower Paleozoic Polish Baltic Basin is composed of unconventional and conventional oil and gas plays. It comprises a few individual source rocks, reservoir rocks and seal formations. Its specific feature is a wealth of organic-rich shale formations, being the same time unconventional shale oil/gas reservoirs. These are the Piaśnica Formation (upper Cambrian - equivalent of the Alum Shale), the Sasino Formation (upper Ordovician – stratigraphic similarity to the Utica Formation in the Appallachian basin), and the Jantar Formation (lower Silurian).
The shale oil and gas prospects were explored in the Polish Baltic Basin during 2010-2015 but were subsequently terminated, mainly due to global oil price decline. In the Baltic Basin alone 38 new shale oil/gas exploration wells were drilled, and considerable amount of 2D and 3D seismic data was acquired. The location of sweet spots within the basin has been defined as dictated by exceptionally high net pay thickness, high average TOC content, and favourable mineralogical composition and geomechanical properties, as well as favourable porosity and permeability. With this sweet spot definition the Żarnowiec and Wejherowo concession blocks appear to represent the best acreage within the Basin.
The Piasnica Formation, the Sasino Formation, and the Jantar Formation reach maximum organic-rich section thickness of 22 m, 36 m, and 24 m respectively. At most of the Żarnowiec and Wejherowo acreage, at least two of the above formations might be fractured at one time, therefore cumulative net organic-rich sediment thickness might be as high as 50 m at maximum. The three above mentioned formations are characterized by maximum average TOC content equal to 5.0-9.5 wt. %.
Within the sweet spots the shale formations achieved thermal maturity level equivalent of volatile oil window in the East, through condensate and wet gas window, to dry gas window in the West. Oil Saturation Index for the shale located in the oil and condensate window is high, predominantly in a range of 100-220. Burial depth of the formations in that zone is in a range of 2600-3700 m.
The lower Paleozoic complex of the Polish Baltic Basin is characterized by low degree of tectonic deformation, favourable for unconventional wells drilling and completion. Hydrocarbons were generated during few phases, mainly during the late Devonian to early Carboniferous, as well as late Mesozoic. Oil and gas are of very good quality, devoid of any detrimental components, and its composition in a given location depends on thermal maturity. At most of the area a normal reservoir pressure, or slight overpressure is observed. An exception is the dry gas window, where overpressure of up to 0.6 psi/ft were reported.
(after Poprawa, 2020
Conventional petroleum plays of the Baltic Basin
Active petroleum system of the lower Paleozoic Baltic Basin is confirmed by presence of numerous small to mid-size oil and condensate fields, as well as widespread oil and gas shows. The key conventional reservoir rock is the middle Cambrian sandstone. Its reservoir properties solely depend on degree of quartz cementation, due to which in the central and south-western part of the basin it is developed as a tight reservoir. The subordinate conventional reservoir rock is the Upper Ordovician to Silurian carbonate build-ups, and to lesser degree the Lower to Middle Ordovician carbonates. Locally, the Rotliegend sandstone and Zechstein Main Dolomite might have good reservoir properties, although no hydrocarbon accumulations were identified in these formations.
Hydrocarbon kitchen is composed of three main organic-rich shale formations. Historically, the Piaśnica Shale (upper Cambrian to lower Tremadocian) was considered as the main source rock for hydrocarbons, however, recently the Sasino Shale (Caradoc) and the Jantar Shale (lower Llandovery) are also regarded as the source rocks. Hydrocarbon kitchen is of high quality, which is expressed by thick cumulative thickness of source rock intervals, their high to moderate average TOC content, and thermal maturity being equivalent of oil, wet gas or dry gas window.
The regional scale seal for the Lower Paleozoic hydrocarbon accumulations is the Silurian claystone and mudstone, characterized by very high thickness, and the Zechstein evaporates. The Middle Cambrian reservoir is sealed locally by the same age mudstone and/or by the upper Cambrian claystone. The Ordovician and Silurian carbonate build-ups are sealed by the same age shale.
There are a few individual conventional hydrocarbon plays developed in the lower Paleozoic Baltic Basin. Majority of the oil and condensate fields in the Baltic Province are developed in structural traps, being anticlines related to reversed or transpressional faults of limited offset (faulted anticline play), involving the middle Cambrian reservoir. The other are the drape/onlap play with depositional pinch-out of successively younger sediments against topographic/bathymetric high, as well as the stratigraphic pinch-out play, where sediments pinch-out along margin of actively subsiding basin. The next one is a weathered basement play, in which deeply weathered and naturally fractured crystalline basement became a conventional reservoir. Moreover, in the eastern part of the basin, the several oil fields related to reef play and inter-reef/back-reef play developed.
