Thermal Maturity Distribution of Dispersed Organic Matter in Probable Silurian Gas Shale in the Baltic and Lublin Basins (Poland) by means of 3D Petroleum System Modeling
Silvia Carozzo¹, Sveva Corrado¹, Francesca Gaeta¹, and Domenico Grigo²
¹Dipartimento di Scienze Geologiche Università degli Studi Roma Tre, Roma, Italy
²ENI spa- Exploration & Production Division, Milan, Italy
In the last decade, unconventional resources have turn out to be a topic of great interest worldwide in the production of hydrocarbons. Building on the success in the U.S., exploration projects have been recently started in Eastern and Northern Europe, including Poland.
The aim of this contribution is to provide a reconstruction of the thermal evolution of the Polish Baltic and Lublin Basins: two NW-SE hundreds of kilometers long basins, whose subsidence, active since at least Lower Paleozoic times and prevailing on denudation events, was mainly driven by main orogenic cycles’ dynamics. Thermal maturity distribution was focused in the Baltic Basin and in the Lublin Basin on the Silurian (Wenlokian and Llandoverian) mainly clayey interval stratigraphic record to provide a reference framework for potential shale gas evaluation.
3D burial and thermal modeling was performed by means of Simba software integrating classical stratigraphical data with organic petrography (Ro%) and geochemistry (T.O.C. and Tmax) data derived from seven public wells in the Baltic Basin and nine public wells for the Lublin Basin, as well as present-day temperature and structural maps. Modeling allowed to perform a series of geological and paleo-thermal scenarios that helped in the identification of areas of limited extension within each basin that turn out to be particularly attractive for further gas exploration.
These models have allowed the identification of the major geological and thermal events that have shaped the history of both Baltic and Lublin Basins.
For the Baltic Basin two main subsidence stages have been defined: the first occurring in the Early Paleozoic is characterized by high sedimentation rates in the western portion of the basin (with more than 3,000 m thick sequences) which decrease moving toward the East at the time of the evolution of the foredeep of the Caledonian orogenic system. The second event recorded throughout the Mesozoic with much lower sedimentation rates and maximum thicknesses preserved in the central-southern portion of the basin. Between these two main depositional events, during Carboniferous times, the area underwent a regional uplift which led to erosion of the Devonian stratigraphic record.
During the first burial event, organic matter dispersed in lower Silurian sediments reached in most of the basin thermal maturity levels corresponding to the oil generation window. After Late Paleozoic uplift and cooling, during the Mesozoic, temperatures gradually increased culminating in an Upper Cretaceous thermal event (characterised by up to 2 HFU) that caused a further increase in thermal maturity of organic matter dispersed in Lower Silurian that reached the gas window and the overmature stage in the thickest portion of the basin.
The burial and thermal evolution of the Lublin Basin as well is driven by two main subsidence stages: the first occurring during most of the Paleozoic accompanied by variable, but generally high sedimentation rates. These rates are recorded by sedimentary sequences locally exceeding 6,000 m and decreasing from SW to NE. The second event recorded throughout most of the Mesozoic with much lower sedimentation rates and greater thicknesses preserved in the north-western portion of the basin. Between these two main depositional events, in Permian-Lower Triassic times the area underwent a generalized uplift and erosion. Also here a generalized Mesozoic thermal event cannot be excluded, but the lack of Mesozoic maturity data do not permit to validate the existence of it.
The comparison between thermal and burial trends in the Lublin Basin shows that the thermal evolution of organic matter of Upper Silurian shales got faster and reached gas window and/or overmature stage when burial rate was higher and a first thermal peak (not exceeding 2HFU) occurred in Upper Paleozoic times. The time-space maps of vitrinite reflectance for the Upper Silurian shales show that the transition from oil to gas window was located in basin depocentre just after the occurrence of Upper Paleozoic thermal peak. Later on, after the occurrence of an Upper Mesozoic thermal peak, this boundary migrated towards NE with a NW-SE trend.
In the end for both the Baltic and Lublin Basins other features related to the probable gas shale such as T.O.C., thickness and depth values have been mapped against thermal maturity distribution in order to further constrain location and extension of the most promising areas according to these preliminary models constrained by a limited dataset.
AAPG Search and Discovery Article #120098©2013 AAPG Hedberg Conference Petroleum Systems: Modeling the Past, Planning the Future, Nice, France, October 1-5, 2012