Numerische Simulation der Kohlenwasserstoffgenese an der Subduktionszone vor Costa Rica
In this study sediment samples drilled during ODP Leg 170 at Sites 1039 and 1040 were analyzed with organic geochemical and organic petrological methods. These methods included determination of organic carbon, sulfur and iron contents, Rock-Eval pyrolysis on sediments and kerogen concentrates as well as gas chromatographic studies of the soluble organic matter n-alkane and fatty acid distributions. With these studies information about type, origin and preservation of the organic matter was obtained. Some results were used as input parameters for the numerical modeling of temperature and subsidence history as well as hydrocarbon generation. The seismic profile SO81-200 was used together with organic geochemical data for the numerical modeling of hydrocarbon generation and migration of different models with different heat flow histories. In an area off Costa Rica which was studied by 3D-seismic investigations (3D box) bottom simulating reflectors (BSRs) were found. These BSRs are an indicator for gas hydrates but they show a small-scale patchy distribution in the 3D box. Pressure and temperature was calculated to test the assumption that variations in pressure/temperature ratios are the cause for the patchy BSR distribution. Studies on the organic matter revealed that most of the organic matter incorporated in the sediments overlying the Cocos plate is of marine origin and has a suitable potential for generating hydrocarbons. For the generation of methane and oil kinetics for Type II kerogen were used. The subducting sediments were treated as one sedimentary unit with a mean organic carbon content (TOC) of 0.5% and a hydrogen index (HI) value of 270 mg HC/g TOC. Except for the wells drilled during ODP Leg 170 there are no other wells available which could contribute suitable calibration parameters. Therefore several models with changing input parameters were calculated to determine the influence of single parameters on the result and to give a range for the amount of hydrocarbons generated and their migration velocity. In addition the possible influence of frictional heating along the decollement (boundary separating the upper and lower converging plates) was studied. The calculated temperature distribution for the present day exhibits a disturbance in temperature equilibrium for all models. The isotherms are heavily bended downwards and especially those below the decollement do not show a balancing of the thermal field until the coast of Costa Rica is reached. The additional heat due to frictional heating between the two plates has no significant influence on temperature distribution and hence on hydrocarbon generation. The amount of organic carbon subducted off Costa Rica which could generate hydrocarbons is huge in all models. The amount of methane which could be generated from the organic matter which is in the field of hydrocarbon generation today or has already passed through this field within the studied profile is in the range of 0.2-0.3*10E+12 m^3 per kilometer of trench length. Migration velocities of hydrocarbons (oil and methane) are in the models with very high (120 mm/a) and high (90 mm/a) subduction velocities lower than the respective subduction velocity. Therefore migrating hydrocarbons do not reach the sea floor. For the model with a low subduction velocity (30 mm/a) migration velocity of methane is higher than the subduction velocity in some parts of the profile. Despite this fact methane does not reach the frontal zone of the subduction complex because the very high migration velocities only occur in some parts of the sediments and velocities are not constantly higher than the subduction velocity. This means that the oil reaches the gas zone and is converted to methane. Therefore oil discoveries cannot be expected at convergent margins with high subduction velocities, at least not in the subducting plate. Gas discoveries are possible, especially if the hanging wall rocks are of higher porosity and permeability. The patchy distribution of BSRs within the 3D box cannot be explained with sudden changes in temperature/pressure ratios, at least not for the studied model. At three points of the 3D box pseudo wells (vertical one-dimensional cuts) were constructed and temperature/pressure ratios determined. The temperature/pressure ratios show that large parts within the sediments are suitable for gas hydrate formation. Some parts of the upper region (to the coast) and some parts of the lower region (to the MAT) of the 3D box exhibit temperature/pressure ratios unsuitable for gas hydrate formation. Faults which could serve as migration paths for warm fluids which can destroy gas hydrates were not visible in the first seismic interpretations.
info:eu-repo/classification/ddc/550, Geowissenschaften, Costa Rica, Subduktion, Sediment, Kohlenwasserstoffe, Entstehung
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