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Major hydrocarbon-producing trends of the Neuquen basin occur along its northeastern margin (Eastern Shelf) and along an east-west-trending structural high in the southern half of the basin (Neuquen Dorsal). Sediment thickness increases northward from the Neuquen Dorsal and westward from the Eastern Shelf into the Neuquen Embayment, a region that has been relatively unproductive for hydrocarbons.
Major source rocks are the Lower to Middle Jurassic Los Molles Formation and the Upper Jurassic to Lower Cretaceous Vaca Muerta Formation. Los Molles shales are immature to moderately mature on the Neuquen Dorsal: vitrinite reflectance (Ro) = 0.3-0.8%; thermal alteration index (TAI) = 1+ to 2; total organic carbon (TOC) = 2.0-5.0%. However, they are severely altered in the deepest part of the Neuquen Embayment (Ro = 2.5-3.0%; TAI = 3+ to 4; TOC = 1.1%). Organic matter is woody on the Neuquen Dorsal but is coaly in the Neuquen Embayment. Clay minerals are smectite and mixed layer illite-smectite on the Neuquen Dorsal whereas illite and chlorite are present in the Neuquen Embayment.
Similarly, Vaca Muerta shales are immature on the Neuquen Dorsal and along much of the Eastern Shelf (Ro = 0.3-0.5%; TAI = 1 to 2-), but are mature throughout the Neuquen Embayment (Ro = 0.7-1.3%; TAI = 2-3). The lower part of the unit is a bituminous black shale (TOC = 2.5-6.5%). The dominant visual kerogen type is amorphous (Al). Clay minerals change from smectite to mixed layer illite-smectite with decreasing expandability toward the deepest part of the Neuquen Embayment.
The lack of correlation between areas of source rock maturity and major hydrocarbon production suggests long-distance fluid transport out of deep portions of the basin.
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