Print this pagePore Pressure Assessment and Its Role in the Petroleum System Characterization in the South-East Part of the Maracaibo Basin
The Zulia Oriental area is located in the Maracaibo basin (Western Venezuela, South America). It has some of the biggest petroleum-producing fields in the western country but at the same time the more complex due to the pressure regimes that need to be drilled to reach the objectives. Some of these fields are: Mene Grande, Barua, Motatán, Ceuta, Tomoporo, and La Ceiba.
Different pressure regimes were identified in this area of the Maracaibo basin and they have been observed on certain levels which have been interpreted as seals in all these petroleum-producing fields. The main causes of the abnormal pressures found are the erosion and the uplift which led to compaction disequilibrium mostly in the trasgressive shales. Rock with abnormal pressure generated by nonequilibrium should be undercompacted and therefore have higher porosity than rocks that have undergone normal compaction. (Ortoleva, Peter J, 1994).
The area has one of the thickest sedimentary sequences in the Maracaibo basin where clastics deposition was influenced by synsedimentary faults (Escalona and Mann, 2006a).
This sedimentary sequence is subdivided into two tectonic settings, the Passive Margin (Early Aptian- Maastrichtian) and the Active Margin (Paleocene- Pleistocene)
The main commercial reservoirs (Eocene sandstones) have been interpreted as an aggradational sucession of fluvial sandstones (toward the bottom of the section) and as a rapidly prograding delta systems toward the top. Transgressive marine shales overlay the Eocene reservoirs.
A pore-pressure data base was made by gathering wireline and lithology logs, RFTs, DSTs, mud weights, and drillings reports; all this information was used to estimate the Pore Pressure in 20 wells by using the Eaton sonic and resistivity equations with exponent of 3.0 (Eaton 1972). By integrating 3-D seismic interpretation (884 km2) with geological model, several traps mechanisms were identified revealing seal thickness variation as well as the estimated Pore Pressure.
We have estimated an overpressure on the sealing surfaces of 13 ppg on inner shelf depositional environment and up to 15 ppg on the bathyal environment that represent respectively the least and the more deformed reservoirs structures in the area.
This abnormal pressure have helped to keep hydrocarbons trapped during millions of years, even in cases where the seal surface has less than 80 feet thickness.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil