Pyrolysis and TOC Identification of Tight Oil Sweet Spots

Abstract

URTeC 1578754 An interpretation of Bakken Formation's pyrolysis and Total Organic Carbon (TOC) data analyzed from 8 wells that are located in North Dakota within Williston Basin (North Dakota Geological Survey, 2011), has been done. The pyrolysis data comprises of S1, S2, S3 and Tmax. TOC is the organic carbon content of the rock. Wildcat Technologies' HAWK instrument analyzes for both pyrolysis and TOC parameters directly at the well site. Interpretation of the pyrolysis and TOC data was done in a lithofacies framework that demarcates the Upper Bakken Shale, the Middle Bakken Member and the Lower Bakken Shale. Also compiled were graphical plots of TOC, S1, S2, S3, Tmax, Hydrogen Index, Oxygen Index, Oil Saturation Index and Vitrinite Reflectance equivalent (Roe) derived from Tmax as per the equation given by Jarvie, et. al., 2007). A plot of Carbonate Carbon is also included. The pyrolysis and TOC data interpretation was contrasted with previously generated Bakken Formation Isopach (LeFever, 2009), API gravity and Hydrogen Index maps (Kuhn et. al., 2009). The graphical plots demarcation of formation tops was contrasted with that done using wire line logging tools (Hess, 2011). Interpretation of pyrolysis and TOC data using the compiled graphical plots is a suitable tool for geosteering while drilling for tight oil targets because it provides a means for predicting sweet spots and also delineating formation tops. Middle Bakken Member oil fairway running through sweet spots that are anticipated to have highest APIs has been delineated using pyrolysis and TOC data. These sweet spots exhibit Oil Saturation Index values exceeding 100 over a continuous multi-depth interval and not just single depth streaks and are indicative of productive oil intervals in the Middle Bakken Member. They tend to occur within the sandy lithofacies at locations whereby the Middle Bakken Member overlies Lower Bakken Shale of Roe 0.90 – 1.00 and Hydrogen Index of about 100. This pyrolysis and TOC data study is envisaged to facilitate geosteering that can be done relatively inexpensively and still yield similar results to those obtained from the more expensive wire line logging methods. Whereas wire line logging would require down time at the well site, using pyrolysis and TOC data for geosteering is a real-time application as drilling continues. This study also provides a new way of demarcating formation tops and provides a means for mapping sweet spots in tight oil exploration ventures