--> Abstract: Temperature, Maturity and Timing of Hydrocarbon Generation in the Central Graben and Terchelling Basin, Dutch Part of North Sea Basin, by M. S. C. Echternach, H. Verweij, A. Fattah, and P. David; #90091 (2009)

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Temperature, Maturity and Timing of Hydrocarbon Generation in the Central Graben and Terchelling Basin, Dutch Part of North Sea Basin

Monica Souto Carneiro Echternach, Hanneke Verweij, Abdul Fattah, and Petra David
TNO Geological survey of The Netherlands, PO Box 80015, 3508 TA Utrecht, The Netherlands

Several factors may affect temperature, maturity and, cosequently, the timing of hydrocarbons generation in a basin. The present work shows the impact of source depth location as well as salt structures on the time evolution of temperature, source maturation and hydrocarbon generation in a salt-dominenated basin in the Netherlands.

Introduction
TNO uses basin modelling approaches to integrate the wealth of data and information gathered and mapped in its detailed mapping programme of subareas of offshore Netherlands. The main objective is to evaluate the interdependencies of the different processes that affect rocks and fluids during its geological history. Special attention is paid to processes and conditions affecting hydrocarbon potential. Here we present selected results of the 3D reconstruction of the history of temperature, source rock maturity and timing of hydrocarbon generation in the Terschelling Basin and the southern part of the Dutch Central Graben from 320 Ma to present-day (Figure 1).

Basin modelling: Input and boundary conditions
The basin modelling programme used at this work was Petromod (version 10) of IES. Basic data requirements for the modelling include the present-day geometry, lithological properties, time-sequence of principal events during geological history, boundary conditions and calibration data.

The TNO mapping project provided the basic 3D stratigraphic model (depth and thicknesses) of 11 main stratigraphic groups from the Upper North Sea Group to Upper Rotliegend Group. This model was extended to greater depth with the Step Graben & Hospital Ground, Maurits, Ruurlo and Baarlo formations of the Limburg Group (Figure 1). The model was refined with additional Jurassic and Triassic reservoir units and Triassic seals. It includes 29 layers plus basement. The model takes 2 source rock intervals into account: the gas-prone coals of the Baarlo, Ruurlo and Maurits formations (source rocks of kerogen type III) and the oil-prone Posidonia Shale Formation (source rocks of kerogen type II), focus of the present work. The geological history incorporates the 3 main phases of erosion identified in the area: the Saalian, Mid-Kimmerian and Sub-Hercynian phases, and additional periods of non-deposition. The 3D thicknesses of eroded sediments were estimated for each period using the stratigraphic information from new well log interpretations and initial 1D basin modelling at well locations. This resulted in 11 erosion maps (corresponding to the 11 stratigraphic layers affected by erosion).

Paleo water depths and sediment water interface temperatures are representative for the whole area. For the heat flow we used a variable basal heat flow boundary condition estimated in-house Abdul Fattah et al with the 1D tectonic heat flow predictor Petroprob.

Results

Thermal history
For steady state Petromod simulations, the regional variations in temperature in a stratigraphic unit at a certain time in history result mainly from regional variation in: depth of burial, basal heat flow and bulk thermal conductivity sedimentary sequence. The simulations used here also incorporate effects of paleo boundary conditions and effects of rapid sedimentation or uplift on the temperature distribution at a certain time. The lateral variations in thermal and maturity history of a source rock in this salt dominated area also depend on its position relative to salt structures of high thermal conductivity.

Thermal and maturity history Posidonia Shale Formations
The burial history of the Posidonia Shale Formation in the Central Graben shows large differences resulting in very different temperature and associated maturity histories depending on its structural position. In order to compare those differences we did three 1D extractions at different Posidonia depths in the 3D model (figure 2). During the Latest Paleogene and subsequent Neogene times the temperatures in the source rock decrease also during times of increasing burial (Figures 2a and 2b) probably due to the sharply decreasing sediment water interface temperatures in combination with the relatively low basal heat flow. Only very recently, the temperatures in the source rock start to increase again. Present-day temperatures in the Posidonia Shale Formation are lower than previous values. The transformation ratio is the ratio of generated petroleum to potential petroleum in a source rock (figure 2d). The ratio indicates that in the southwestern part of block F17 the Posidonia Shale Formation already starts generating hydrocarbons in Cretaceous times with generation rates reaching their maximum values just before Late Cretaceous uplift (Figures 2a and 2d). The generation is resumed in Paleogene times and practically stops at the end of the Paleogene. At location 2 (block L02) the maturity of the Posidonia increases gradually and the source rock starts generating hydrocarbons not until the Paleogene and continues into the Neogene. In the inverted centre of the Dutch Central Graben (location 1) the Posidonia did not reach a mature state for hydrocarbon generation (Vr< 0.55; Figure 2c).

Conclusion and outlook
The data and information recently gathered and mapped in TNO’s mapping programme in combination with the newly reconstructed tectonic heat flow boundary conditions improved the 3D characterization and understanding of the thermal and maturity history in the study area and the timing of the main periods of hydrocarbon generation of Posidonia source rock. The 3D modelling revealed large lateral variations in maturity and hydrocarbon generation history related to: 1- structural positions of the source rocks and 2 - their position relative to large salt structures. The next phase of the basin modelling programme will focus on the masses of hydrocarbons generated and expelled from the source rocks and their subsequent migration and accumulation.

Figure 1. Location and present-day 3D geological section of the Dutch Central Graben and Terschelling Basin

Figure 2. 1D different Depth extractions of 3D mode and its temperature, maturity and Tranformation Rate time evolution.

 

 

AAPG Search and Discovery Article #90091©2009 AAPG Hedberg Research Conference, May 3-7, 2009 - Napa, California, U.S.A.