Overpressured Gas Systems Modeling in the Neuquen Basin Center

Felipe Rodriguez Monreal¹, Graciela Olea¹, Daniel Delpino¹, and Roger Baudino^2
1Exploration Onshore Argentina, Repsol-YPF, Neuquén, Argentina
²Gerencia Exploración AB, Repsol-YPF, Buenos Aires, Argentina

Complex overpressured systems of the Neuquen Basin center are discussed in this study with a basin modeling approach. Enormous amount of hydrocarbons have been generated by the Jurassic Los Molles Fm. in this area. This type III marine source rock reaches thicknesses of more than 1000 m in this area with TOC values in the range of 1-3.5 %. Vitrinite reflectance values of 1-2.5 % show present time overmaturity of the source rock. Widespread overpressured gas accumulations have been founded in the overlying Jurassic sediments. Lajas Fm. tight sands are the main reservoirs. Some recent oil and gas discoveries have also been made in fractured carbonates and Tertiary sills. Cretaceous evaporites act as an effective regional seal for this petroleum system.

Multi-1D and 2D regional models were satisfactorily calibrated for temperature, maturity, pressure and known accumulations. Models showed that generation occurred between 150 and 50 Ma. Up to 500 billion barrels of oil equivalent were generated in this area. Deposition of a Cretaceous evaporitic regional seal and a later overburden developed a highly overpressured system. Despite the lack of effective migration pathways, long distance migration occurred and it is still active. Fluids migrate towards the basin edge where the pinch out of the evaporites allow for overpressure dissipatation. Fluid inclusions and petroleum geochemical analyses showed several migration pulses of a wide maturity range of products. Tertiary sills intruded in the Los Molles-Lajas Fms. contact were charge by retarded hydrocarbon expulsion related to source rock undercompaction. Basin modeling and geochemical analyses were helpful to calculate generated volumes, to understand young traps charge and to predict long distance accumulations that opened new exploration opportunities in this basin.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas