Distribution Of Natural Hydraulic Fractures (Beef Veins) Within the Vaca Muerta Formation, Neuquén Basin, Argentina

Abstract

Bedded-parallel natural hydraulic fractures are common in sedimentary basins worldwide, especially within organic-rich petroleum source rocks. The preservation of such fractures in the sedimentary record is due to the precipitation of fibrous calcite, forming calcite veins known as “beef”. Such veins result from natural hydraulic fracturing of organic-rich shales due to fluid overpressures interpreted to be caused by hydrocarbons generations and/or migrations. However, their distribution through the source rock sequence remains poorly studied, including in the Vaca Muerta Formation (VMF) where such veins are commonly observed on cores and outcrops.

To investigate the parameters controlling the distribution of beef veins in the VMF, we proceeded to high-resolution outcrop logging. We carried out mineralogical, total organic carbon (TOC), Rock-Eval and scanning microscope electron (SEM) analyses on shales and beef sampled during the field trip. Two deep-marine sedimentary facies are recognized: (i) a distal one marked by basinal facies with black mudstone displaying high TOC values and (ii) a more proximal one offshore) developed on top of prograding sequences with preserved turbidites and storm facies displaying lower TOC values. In the studied VMF outcrops, we observed that beef veins have a highest occurrence frequency in distal environments compared to the proximal ones. This differential repartition illustrates that the depositional facies and the TOC must influence the beef distribution. In parallel, two different types of beef veins are recognized: (i) continuous and (ii) discontinuous beef. We observed that the continuous beef is preferentially represented in distal facies whereas the discontinuous beef type predominates in proximal facies. Such observations suggest that processes of natural hydraulic fracturing are more efficient in organic-rich distal sedimentary facies.

In addition, the preferential location of beef at ash beds or calcareous concretions interfaces highlights the influence geomechanical interfaces in the process and localization of natural hydraulic fractures. Finally, we started to apply statistical models to quantify the link between several shale parameters (lithological, mineralogical and petrophysical) and the VMF beef distribution.

We intend to compare our results to our previous results obtain on cores to be able to identify the petrophysical parameters involved in the natural hydraulic fractures distribution in source rocks.

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