Shale oil potential and depositional environment of the Lower Toarcian Posidonia Shale in NW Europe
Year: 2016
Type: dissertation
Abstract: This doctoral thesis deals with a suite of drilling cores and outcrop samples of the Lower Toarcian Posidonia Shale (PS) collected from multiple locations covering a vast area including the Swabian Alb (SA) and Franconian Alb (FA) of Southwest-Germany, Wenzen well (WEN) of Hils syncline, northern Germany, Runswick Bay of UK, Loon op Zand well (LOZ-1) of the West Netherlands Basin (WNB) and the Esch-sur-Alzette, Luxembourg (LU). Based on a series of laboratory measurements, firstly, shale oil potential was assessed including thermal maturity. In order to quantify the petroleum generation potential of the rocks a large set of experimental analyses was conducted including elemental analysis, Rock-Eval pyrolysis, organic petrographic investigations and molecular hydrocarbon analyses. Secondly, three types of depositional environments of the Posidonia Shale from these six different sampling localities were deduced mainly based on organic geochemical characteristics.The Lower Toarcian Shale is more carbonate-rich (about 30 % on average) in SW-Germany, NW-Germany and Luxembourg, and is instead more silicate-rich in UK and NL with carbonate contents of about 15% on average. However, HI values are similar, approx. 500-700 mg HC/g TOC on average at all sampling localities, exhibiting typical type II kerogen with excellent hydrocarbon generation potential. Samples from outcrops of the Lower Toarcian Shale from Runswick Bay, UK, have reached the early stage of the oil window, with quite uniform Tmax values (425-438°C, avg. 433 °C) and a narrow range of PI (production index) values (0.10-0.19), substantiated by mature fluorescing characteristics of telalginite and lamalginite and vitrinite reflectance values of 0.6-0.7%. Wider ranges of Tmax values (421-443°C, avg. 427°C), vitrinite reflectance (0.4-0.8%) and production indices (0.06-0.29) suggest more variable thermal maturation ranging from immature to almost peak oil generation at the LOZ-1 well, NL, although only some intervals indicate advanced thermal maturity there. Non-indigenous light hydrocarbons occur at the LOZ-1 well, NL, evidenced by unexpectedly high production indices and abundant low molecular weight, short-chain n-alkanes extending from n-C10 to n-C16 together with a sharp decrease of n-alkanes concentration towards the heavier, long-chain components (>n-C20). In contrast, the Lower Toarcian marlstones from the Swabian and Franconian Alb of SW-Germany are clearly immature, similar to the samples from Luxembourg. In NW-Germany, the Posidonia Shale covers a wide spectrum of maturity ranging from immature to overmature.Organic geochemical characteristics were comprehensively analyzed and compared for these six different sampling localities on a series of sterane, hopane and aromatic biomarkers. Results indicate that highest thermal maturity occurs in the PS of Runswick Bay, UK, equivalent to ca. 0.7% VRr, whereas samples from other sampling localities did not exceed the early stage of oil generation or are immature with merely slight variations. Comparable, immature to marginally mature source rocks make it possible to compare their depositional environments based on biomarker characteristics. Accordingly, three different types of redox conditions are established on the basis of biomarker indicators for these six sampling localities during deposition of the PS in NW Europe. (1) Ferruginous conditions are discriminated by low dibenzothiophene/ phenanthrene (<0.2) and gammacerane index (<0.1) values in Runswick Bay, UK, and possibly also LOZ-1, NL, especially when accompanied by high sulfur content and abundant tiny pyrites (>95% of < 7 µm in diameter) which are postulated to be partly generated within the water column due to immediate reaction of Fe2+ and H2S derived from bacterial sulfate reduction. (2) In contrast to the iron-rich, low-sulfidic conditions, sulfidic (euxinic) bottom waters possibly with enhanced water salinity and stratified water column prevailed due to restricted iron supply in the Esch-sur-Alzette of LU. Similar but slightly less reducing and less sulfidic conditions occurred in the FA of SW-Germany as well as WEN of northern Germany which was also probably less saline. (3) For the PS of SA, a more oxygenated (dysoxic) water column associated with anoxic sediments (anoxic/dysoxic boundary close to the sediment/water interface) is indicated.
Source: RWTH Publications (RWTH Aachen)
Authors: Jinli Song, Ralf Littke, D. Leythaeuser
Language: English
Subfield: Geology
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2772288723