Shale oil potential and thermal maturity of the Lower Toarcian Posidonia Shale in NW Europe

Abstract

A suite of drilling cores and outcrop samples of the Lower Toarcian Posidonia Shale (PS) were collected from multiple locations including the Swabian Alb and Franconian Alb of Southwest-Germany, Runswick Bay of UK and Loon op Zand well (LOZ-1) of the West Netherlands Basin. In order to assess the thermal maturity and quantify the petroleum generation potential of the rocks, elemental analysis, Rock-Eval pyrolysis, organic petrographic investigations and molecular hydrocarbon analyses were performed. Available literature data of the Posidonia Shale from Luxembourg and northwest Germany were used for comparison in the discussion.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. This coincides with the relative distances of the depositional settings to the clastic source areas on the continents during the early Jurassic. However, HI values are similar, approx. 500–700mgHC/g TOC on average at all sampling localities, exhibiting typical type II kerogen with excellent hydrocarbon generation potential. Differences in relative abundance of terrigenous organic matter input are deduced from molecular indicators, Rock-Eval data and organic petrography. The highest abundance of terrigenous organic matter is evident for Runswick Bay, UK.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. 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.Source potential index (SPI) indicates the highest value in the LOZ-1, NL followed by Wickensen, NW-Germany and Esch-sur-Alzette, LU, mainly due to differences in thickness of the studied sections. The lowest value was observed in SW-Germany. The Lower Toarcian (Jet Rock) in Runswick Bay, UK, is assumed to have a high shale oil potential because of its advanced thermal maturity, although its SPI is not high due to the limited thickness of the section studied. The PS in SW-Germany has low shale oil potential due to low thermal maturity and limited thickness.