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Abstract

This 1D/2D petroleum system modelling study intended to reconstruct the temperature, maturation, migration and accumulation history within the „Tight Gas Area“ on the southern edge of the Pompeckj Block in northern Germany. To better understand hydrocarbon migration and to generate a more reliable predictive model of reservoir characteristics, we integrated detailed information from (i) sedimentological analysis of core and log data, and (ii) a structural reconstruction of the Rotliegend reservoir horizons. One cross section and 12 wells were examined using the 1D/2D simulation software PetroMod® 7.1 of IES GmbH, Jülich; subsequently the results of the 1D modelling were incorporated in the 2D model. The W-E section connects well-known gas fields of the Nordhannover concession and runs perpendicular to the prevalent N-S striking horst-and-graben structures within the Paleozoic rocks. The depth to the major source rocks within the area, the coal-bearing Westphalian strata, varies between 5800 m and around 4700 m in some areas (e.g. Top Westphalian B). Along the section, a pre-Permian erosion of between 400 to 1200 m of Upper Carboniferous sediments was assumed, the actual amount depending on the position relative to horst and grabens. During the Upper Carboniferous and the Rotliegend the temperatures within Westphalian B and Lower Westphalian C sediments remained below 100°C and 75°C, respectively. The temperatures within these sediments increased by about 50°C, before they decreased again to 125°C and 100°C, respectively, due to the Late Jurassic inversion. During on-going burial in the Upper Cretaceous, temperatures rose until they reached their present-day values of about 175°C in Lower Westfalian C, 210°C in Westfalian B, and more than 250°C in Namurian C sediments. According to the temperature history, the maturation of organic matter within these rocks occurred in two important stages. During the Triassic, vitrinite reflectance reached values of 1.5% and 1.0% in Westphalian B and Lower Westphalian C sediments, respectively. The second period of intense maturation was coeval with enhanced subsidence during the Upper Cretaceous, when vitrinite reflectance within Westfalian B and C sediments increased from to 3.0% and about 2.2%, respectively. Maturity within the horst block sediments remained lower by about 1.0%. Within Westphalian B sediments gas generation started during the Middle Triassic and lasted until the Late Tertiary. These rocks became overmature whilst Lower Westfalian C rocks generated gas until the present time and transformed 60-90% of their convertible kerogen into hydrocarbons, the actual amount depending on the structural position. During the Jurassic and the Triassic, when the regional stress field allowed N-S striking faults to be dilatant and to act as migration pathways, high gas columns along the fault zones led to a breakthrough and loss of gas, where thinned Zechstein salt exists.