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A New Rock-Eval Methodology for Tight Oils

Abstract

The Rock-Eval is a versatile, high-tech instrument for geochemical assessment of fossil organic matter. It imposed itself as a standard technic which has evolved over the last decades in terms of hardware, software and interpretation capabilities. Primarily, in a conventional petroleum system perspective, it has been designed as a screening tool assessing basic organic attributes of source rocks, such as occurrence of organic matter, TOC, petroleum potential and thermal maturity (Espitalié and Bordenave 1993). Further improvements were developed in order to determine kinetics parameters (e.g. Ungerer 1986, 1987) and more recently carbonate content and type (Pillot et al 2013). Moreover, several of these attributes are instrumental inputs for any numerical basin modeling. Due to operational constraints, sampling is often poor and relies mainly on cuttings which are mixing lithologies and are often stored in non-adequate conditions, implying an uncertain assessment of free hydrocarbons content (S1). The Rock-Eval program developed was then not specifically directed to measure this parameter with accuracy. For instance, the initial heating temperature of the standard Rock-Eval program (set à 300°C) leads to a loss of the lighter end of free hydrocarbons. Consequently with this standard methodology the S1 parameter is generally not seriously considered. With the rise of shale plays interest, a specific focus is needed in order to quantify the occurring free hydrocarbons and assess their nature and properties. In this respect, a new Rock-Eval method is proposed to provide this kind of information. This method, presented using examples, implies a low initial thermovaporization temperature which allows to minimize the loss of the lightest hydrocarbons. Consequently, it provides a more realistic value for the free hydrocarbons occurring in shale play samples. This method is also designed to generate a double peak for the free hydrocarbons, with the aim to tentatively provide an insight in the quality of the hydrocarbon fluids. This double peak is artificially induced by introducing a selected temperature plateau during the step of hydrocarbons thermovaporization. Information on the nature of hydrocarbon fluids and operational proxies, i.e. API of the fluid in the rock, are derived from a processing of the relative importance of these two peaks. In addition, this new methodology intents to prevent the potential interference of heavy hydrocarbons with the peak resulting from the pyrolysis of the kerogen (S2). This interference is more likely to occur in oil-rich shale plays and are likely to bias parameters such as residual HI and Tmax.