Model of Low Maturity Generation of Hydrocarbons Applied to the Carupano Basin, Offshore Venezuela
F. J. S. Schneider1, J. A. Noya2, and C. Magnier3
1Beicip-Franlab / PDVSA EFAI, Caracas, Venezuela
2PDVSA Exploracion, Caracas, Venezuela
3Institut Français du Pétrole, Rueil Malmaison, France
The Carupano basin is located in the northeastern offshore of Venezuela. This area is characterized by the interaction between the Caribbean and the South American Plate. It is composed by two structural highs, the Los Testigos High located in the northern limit of the basin and the Patao High, which is between the Caracolito sub-basin and the Paria sub-basin. The main gas fields are located in the latter area, but there are some gas/condensate shows in other wells.
The proven gas is characterized by a low maturity and it has been associated to biogenic processes because of its carbon isotopic signature. Nevertheless, the composition of the gases shows that there is an increase of the gas maturity which is correlated with the westward increase of the thermal gradient. In the western part of the basin where the thermal gradient is the highest, the gas is associated with condensate.
In order to understand the origin of the gas, the Carbolog (Carpentier el al., 1981) methodology has been used in order to define continuous TOC profiles from sonic and resistivity wire logs. As a result, we have shown that the whole column from the Eocene to the Pliocene could be considered as a source rock with average TOC content in the range 0.5 to 1.2 %. The geochemical study showed that the organic matter is mostly type III except in the Middle Miocene where part (~20%) of the organic matter is marine type II.
The thermal calibration and the basin modeling study shows that the Paria sub basin has reached the oil window while the Caracolito sub basin has reached the gas window. Nevertheless, the drainage areas of the fields were not able to catch what has been produced by these kitchens. As a direct consequence, the known gas fields cannot be filled if one uses classical kinetic parameters optimized from lab data (see results Figure 1).
One had to conclude that the classical kinetics parameters used for basin modeling are not able to reproduce the nature. Indeed, in our study, the main part of the drainage area is located in a low maturity domain where the vitrinite reflectance (Ro) is less than 0.6 %, but the kinetic parameters used were calibrated with kerogen samples for which Ro ~ 0.6 %.
Using a different approach and considering a classical type III as a starting point and using observed natural data such as extracts compositions and gas compositions, a new set of kinetic parameters were derived to account for low maturity. This new type III differs from the previous one by a 10 % increase of the HI and in turns produces about half the proportion of observed methane.
The simulations carried out with this new type III allowed to reproduce quite well the filling of the fields as well as the general state of the hydrocarbons (see results Figure 2).
One of the main conclusions of this study is the fact that more efforts should be done in the determination of the kinetic parameters of the source rocks. Obviously modeling petroleum systems with kinetics parameters determined on samples at the end of the diagenetic zone leads to an underestimation of the amount of hydrocarbons actually produced. It is then of great importance to account for phenomena which occur in the diagenetic zone (Ro < 0.6 %) and that may change in a significant way the mass balance.
Carpentier B., Huc A., Bessereau G. (1991). Wireline Logging and Source Rocks. Estimation of Organic Carbon by the CARBOLOG Method. The Log Analist, May-June, 1991, pp 279-297.
AAPG Search and Discovery Article #90091©2009 AAPG Hedberg Research Conference, May 3-7, 2009 - Napa, California, U.S.A.