Abstract

Unconventional reservoirs are often argillaceous shale formations that can be hundreds of meters thick exhibiting remarkable vertical variability of hydrocarbon fluid compositions. Some compartments of these low permeability shale layers can be hydraulically disconnected before being fracked and therefore it is important to allocate vertical differences in fluid compositions and qualities ahead of horizontal well placement. To better assess the producible fluid composition and quality from vertically distinct shale intervals we have developed a simple approach that is based on a systematic comparison of molecular compositions between rock extracts and produced hydrocarbon fluids. The rock and dead oil samples analyzed for this study are derived from different wells targeting the Upper Jurassic / Lower Cretaceous Vaca Muerta Formation in the Neuquén Basin, onshore Argentina. Rock and fluid samples analyzed for this study represent not only a regional maturity sequence ranging from the peak oil to gas / condensate window, but are also derived from vertically distinct intervals at well scale. Our results show that the extractable and producible fluid composition varies significantly in vertically distinct intervals of the Vaca Muerta Formation. We demonstrate that after a regional calibration a simple geochemical parameter can be used to assess the producible fluid quality coming from a specific vertical shale compartment. Our new geochemical parameter can be obtained from analyses of cuttings or side wall core (SWC) samples from development wells to estimate the composition and quality of the liquid hydrocarbon phase that would be produced prior to hydraulic fracturing.

Introduction

Shale formations can be characterized by significant vertical variability in mineralogy, petrophysical and geomechanical properties (Passey et al., 2010). In addition it was shown that kerogen and fluid quantities and qualities can also vertically vary in the Vaca Muerta unconventional play (Elias & Gelin, 2015). Such vertical fluid variability implies that some of the low permeability shale layers must be hydraulically disconnected before being fracked. As a consequence unconventional petroleum exploration and development nowadays aims to identify fluid composition and quality not only on a lateral but also on a vertical scale.

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