Stratigraphic and Regional Distribution of TOC in the Vaca Muerta Formation

Abstract

Combining total organic carbon (TOC) values with ammonite zones and a sequence stratigraphic framework produces a detailed and well constrained map of TOC distribution across the Vaca Muerta Formation in the Neuquén Basin. The dataset consists of ~3000 samples for which TOC, carbonate content, δ¹³C_org, and g- log was measured. The samples are from four distinct areas within the Neuquén Basin. The locatins represent the southern shelf margin, the transition from shallow shore-face to shelf margin to basin, and two basin areas (outcrop and subsurface). Results presented here illustrate the heterogeneity of organic-rich strata in outcrops and subsurface. High TOC is prominent on the basal component of time-transgressive prograding clinoforms from the proximal shelf to the central basin.

Within the Neuquén Basin, two different laterally extensive intervals of high TOC exist. The older of these two intervals occurs in the Tithonian in the V. mendozanus, P. zitteli and A. proximus zones. It can be observed in all outcrops from the proximal to distal positions. In the prograding clinoforms the TOC-rich interval thins in the toe-of-slope and distal basin deposits. They are thickest, and have the highest TOC (up to 15%), in the more proximal setting (foresets and bottomsets). The younger organic rich interval is stratigraphically located within the S. damesi zone of the Lower Berriasian, and has TOC measurements of 5-10% in the more proximal subsurface, and 3-8% in the distal outcrops. Between these two high TOC intervals there are thick intervals (250-300 m) with an average TOC values of 3% exist from the base of the P. zitteli zone to the middle of the S. damesi ammonite zones, albeit limited in lateral consistency.

The prediction of high TOC intervals depends on basin location. In the south, only the older sediments contain high TOC intervals. In the younger southern strata, the absence of high TOC is a result of the shallowing of the depositional environment. This implies that the high TOC intervals are restricted to thebottomsets and lower foresets of the clinoforms. Stacking of these toe- and bottomset in the basin center results in a thick interval of organic rich strata in the basin center.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018