Stratigraphic Distribution of Source Rock Intervals in the Vaca Muerta Formation

Abstract

Detailed outcrop and geochemical analyses of organically rich source rocks of the Tithonian – Berriasian Vaca Muerta Formation in the Neuquén Basin show that organic rich zones occur multiple times within the basin, and are not a unique stratigraphic horizon. In the Neuquén Basin, sections in two outcrop areas were measured to observe variations in Total organic carbon (TOC). The basin margin is located in the Sierra de la Vaca Muerta (SdlVM) area, while the Puerta Curaco section east of Chos Malal provides information of the basin center. TOC content varies (<0.1-∼16%) throughout the entire basin with several distinctive peaks at varying stratigraphic intervals. The highest value (15.56%) is found in a marginal position, while in the basin several peaks in TOC occur at separate stratigraphic intervals reaching above 8% TOC by weight. At the base of the sections the δ13Corganic is heavier indicating a terrestrial source. In the basin center a shift to lighter values in younger sequences indicate a transition to more marine organic material. In both reference areas, the distribution of TOC is strongly influenced by stratigraphic positioning. In the SdlVM, the high TOC (2-∼16%) values are seen only in lowermost sequence. Subsequent sequences are all organically poor with less than 2% TOC. At Puerta Curaco, the lower most sequence in this location lacks a TOC maximum, however above this sequence a 240m zone of greater than 2% TOC exists. Within each sequence, local maxima coincide with maximum flooding intervals. Further subdivision can be observed in the basin center, where higher values in TOC occur just after each parasequence flooding surface and TOC minima can be located at parasequences boundaries. The outcrop data corroborate the subsurface findings that high TOC is not at a unique stratigraphic level within the basin but the positioning of organic rich intervals becomes stratigraphically higher and chronologically younger towards the basin center. This distribution of TOC parallels the general time-transgressive facies evolution in the basin due to the prograding pattern of the Vaca Muerta Quintuco in the Neuquén Basin. This pattern of TOC is also observed on a sequence and parasequence scale, which implies that TOC variations are related to the sequence stratigraphic framework on all levels.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016