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PSOverpressured Gas Systems Previous HitModelingNext Hit in the Neuquen Basin Center*

By

F. Rodriguez1, Graciela Olea1, Daniel Delpino1, Roger Baudino2, Mirta Suarez1

 

Search and Discovery Article #10149 (2008)

Posted November 13, 2008

 

*Adapted from poster presentation at AAPG Annual Convention, San Antonio, Texas, April 20-23, 2008

 

1Gerencia de Exploración Argentina Onshore, Repsol-YPF, Talero 360, Q8300IEH Neuquén, Argentina ([email protected])

2Gerencia de Exploración ABB, Repsol-YPF Exploración, Esmeralda 255, C1035ABE Buenos Aires, Argentina

 

Abstract 

Complex overpressured systems of the Neuquen Basin center are discussed in this study with a basin-Previous HitmodelingNext Hit approach. Enormous amounts of hydrocarbons have been generated by the Jurassic Los Molles Formation in this area. This type III marine source rock reaches thicknesses of more than 1000 m 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 found in the overlying Jurassic sediments. Lajas Formation tight sands are the main reservoirs. Some recent oil and gas discoveries have also been made in fractured carbonates and Tertiary sills. The Cretaceous Auquilco Formation 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. Tertiary sills intruded in the Los Molles-Lajas formations contact were charged by retarded hydrocarbon expulsion related to source rock undercompaction. 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 allows for overpressure dissipation. Fluid inclusions and geochemical oils and gas analyses show several migration pulses of a wide maturity range of products. Diamondoids and GC analyses done in oils from shallow reservoirs are proof of mixing processes with high maturity hydrocarbon. These as evidence suggest the probable occurrence of deep accumulations in undrilled old structures located in the migration pathway. Basin Previous HitmodelingNext Hit and geochemical analyses were helpful in calculating generated volumes, understanding charge of young traps, and predicting long distance accumulations that opened new exploration opportunities in this basin.

 

 

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Selected Figures

Regional setting.

Basin center stratigraphic column.

W-E schematic cross-section. The Jurassic Cuyo and Lotena sedimentary sequences form an overpressured basin-centered gas system. It is semi-confined by regional evaporites underneath. The main source rock is the 1000 m thick type III Los Molles Formation. The main reservoirs are tight deltaic sandstones, fractured carbonates, and Tertiary sills.

Los Molles source rock on interpreted SW-NE Previous HitseismicTop line.

Pressure (Mpa) vs. time in basin-center well.

Geochemical evidence (geochemistry of gases).

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Conclusions 

Enormous amounts of hydrocarbons have been generated by the Jurassic 1000-m-thick Los Molles Formation in the Neuquen Basin. The development of a semi-confined petroleum system in the basin-centered subsalt sediments results in 15 MPa overpressure. Basin models show that these anomalous pressures are being dissipated by the migration of fluids (water and HC) towards the basin edge until present time, and it will continue in the future. Although most of the hydrocarbons are probably accumulated in the basin-centered tight sandstones, new sedimentary models suggest that good quality subsalt fluvial sandstones and carbonates should be present in old structures 3000-4000 m deep. These new migration and geological models supported by recent geochemical analyses suggest a promising exploration upside in the main structures of the basin that remain undrilled for these deep objectives.

 

References 

Dahl, J.E., J.M. Moldowan, K.E. Peters, G.E. Claypool, M.A. Rooney, G.E. Michael, M.R. Mello, and M.E.L. Kohnen, 1999. Diamondoid hydrocarbons as indicators of natural oil cracking: Nature v. 399, p. 54-57.

Uliana, M.A., and L. Legarreta, 1993. Hydrocarbons habitat in a Triassic to Cretaceous sub-Andean setting: Neuquén Basin, Argentina: Journal of Petroleum Geology, v. 16, p. 397-420.

Urien, C.M., and J.J. Zambrano, 1994. Petroleum systems in the Neuquen Basin, Argentina, in L.B. Magoon and W.G. Dow, eds., The Petroleum System - from Source to Trap: AAPG Memoir 60, p. 513-534.

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