Model of Low-Maturity Generation of Hydrocarbons Applied to the Carupano Basin, Offshore Venezuela
Frederic Jean Simon Schneider, Caroline Magnier, Jose A. Noya, 2012. "Model of Low-Maturity Generation of Hydrocarbons Applied to the Carupano Basin, Offshore Venezuela", Basin Modeling: New Horizons in Research and Applications, Kenneth E. Peters, David J. Curry, Marek Kacewicz
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The Carupano Basin is located in northeastern offshore Venezuela. This area is characterized by the interaction between the Caribbean and the South American Plate. It has two structural highs, the Los Testigos High located in the northern limit of the basin and the Patao High, which is between the Caracolito subbasin and the Paria subbasin. In the latter are located the main gas fields.
The generated gas is characterized by low maturity, and it has been attributed to biogenic processes because of its carbon isotopic signature. Nevertheless, gas compositions show that a thermogenic signature predominates with an increase of gas maturity from the east to the west, where condensates were found associated with gas.
To understand the origin of the gas, the total organic carbon (TOC)SR methodology was used to define continuous TOC profiles from sonic and resistivity wirelogs. As a result, we have shown that the whole column from the Eocene to the Pliocene consists of a poor source rock, except the middle Miocene that could be considered as a good source rock. The average TOC content of the middle Miocene can reach values around 2.5%. The kerogen is mostly type III continental–derived organic matter.
The thermal calibration and the basin modeling study shows that the bottom of the Paria subbasin has reached the oil window, whereas the bottom of the Caracolito sub-basin has reached the gas window. Nevertheless, simulations of fluid-flow migration conducted using default type III kinetic parameters were not able to fill any of the known fields.
We conclude that the default kinetic parameters used for basin modeling are not able to reproduce the nature of these fluids. Indeed, in our study, the main part of the fields drainage area is in a low-maturity domain where the vitrinite reflectance (R0)is less than 0.6%, but the kinetic parameters used were calibrated with kerogen samples for which Ro was taken to be approximately 0.6%.
Considering default type III kerogen as a starting point and using observed natural data such as gas compositions, a new set of kinetic parameters were derived to account for low gas maturity. This modified type III kerogen differs from the previous one by a 13% increase of the hydrogen index. The simulations conducted with this modified type III scheme allowed us to reproduce quite well the filling of the fields, as well as the composition of the hydrocarbons.
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Temperature-time–based first-order kinetic models are currently used to predict hydrocarbon generation and maturation in basin modeling. Physical chemical theory, however, indicates that water pressure should exert significant control on the extent of these hydrocarbon generation and maturation reactions. We previously heated type II Kimmeridge Clay source rock in the range of 310 to 350°C at a water pressure of 500 bar to show that pressure retarded hydrocarbon generation. This study extended a previous study on hydrocarbon generation from the Kimmeridge Clay that investigated the effects of temperature in the range of 350 to 420°C at water pressures as much as 500 bar and for periods of 6, 12, and 24 hr. Although hydrocarbon generation reactions at temperatures of 420°C are controlled mostly by the high temperature, pressure is found to have a significant effect on the phase and the amounts of hydrocarbons generated.
In addition to hydrocarbon yields, this study also includes the effect of temperature, time, and pressure on maturation. Water pressure of 390 bar or higher retards the vitrinite reflectance by an average of ca. 0.3% Ro compared with the values obtained under low pressure hydrous conditions across the temperature range investigated. Temperature, pressure, and time all control the vitrinite reflectance. Therefore, models to predict hydrocarbon generation and maturation in geological basins must include pressure in the kinetic models used to predict the extent of these reactions.