Abstract
To better understand potential vertical variability of the original kerogen quality in an unconventional reservoir we have carried out organic geochemical analyses on hundreds of rock and fluid samples coming from the Upper Jurassic / Lower Cretaceous Vaca Muerta Formation (Neuquén Basin, Argentina). The studied samples are derived from six wells located in different parts of the Neuquén "embayment" where the Vaca Muerta Formation reached distinct maximum thermal maturities. This natural maturity sequence ranges from the peak oil to dry gas window (VReq ~ 0.7 % to 1.8 %). Applied geochemical analyses comprise high-frequency TOC measurements on a centimeter scale, conventional Rock-Eval analyses on a decimeter scale, and meter by meter molecular and isotopic mud gas analyses. In addition we have performed molecular and isotopic analyses on several hydrocarbon liquid extracts and produced oils. Our data clearly indicate highly heterogeneous vertical distributions of organic matter contents on centimeter scales reflecting the fine shale laminations in the Vaca Muerta Formation. Despite the regional control of thermal maturity on liquid and gaseous hydrocarbon compositions we also observe at well scale some remarkable vertical compositional kerogen and fluid variability. Molecular and isotopic analyses on liquid hydrocarbon extracts document vertical differences in the source organofacies. These vertical organofacies variances are responsible for the distinct compositions/properties of the produced fluids at well scale.