9 The Marathon 1 Mesquite well was drilled in Hamilton County, 10 Texas, targeting the Barnett Shale with late oil window maturity. 11 Combining a large suite of petrologic and high-resolution organic 12 geochemical analyses on 120 core samples, we have been able to 13 document qualitatively and quantitatively the effects of petroleum 14 retention and expulsion within and from five intervals within the 15 Barnett Shale. Lithological heterogeneities control the composi-16 tion and amount of retained fluids; the sorption of oil by solid 17 organic matter is important in all intervals. Applying empirical 18 formulas, we have been able to demonstrate not only that retention 19 is primarily controlled by total organic carbon (TOC), but also 20 that the "live" or "labile" component, rather than "dead" or "inert" 21 carbon, constitutes the most active sorptive sites. Additional reten-22 tion in the micropores provided by biogenic microcrystalline 23 quartz (sponge spicules) accounts for the sweet spot defined by 24 an "oil crossover" in the 914-m (30-ft) thick second interval. The 25 fluorescing oil occurring in the axial chamber of the sponge spic-26 ules and that sorbed on organic particles are together enriched in 27 saturated hydrocarbons, whereas the dispersed oil from the adja-28 cent interval 3 is depleted in this compound class. Mass-balance 29 calculations reveal that short-distance migration of petroleum into 30 this "reservoir" interval (second) fractionates the generated oil 31 into a higher quality oil by preferential retention in the order polar 32 compounds > aromatic hydrocarbons > saturated hydrocarbons 33 within the underlying organic matter and clay-rich third interval This has to be remedied because even subtle changes in bulk fluid M51South Africa, China, Australia, and North M52Africa are key activities in service and M53research at the present time.We wish to thank D. M. Jarvie and R. J. Hill M56for their careful reviews and helpful M57comments. The editorial comments and M58suggestions by Terrilyn Olson are also M59gratefully acknowledged. Parts of this study M60were carried out during the course of the M61"GASH-Gas Shales in Europe" initiative. The M62authors wish to thank the sponsors of this M63project, especially Marathon Petroleum M64Corporation, for providing sample material. M65We are also grateful to Prof. (Bowker, 2007;Jarvie et al., 2007; Bunting and 139 Breyer, 2012) as well as in reservoir diagenesis 140 (Milliken et al., 2012 (Bowker, 2003). Rather than detrital quartz, micro-147 crystalline quartz is the major component of the sili-148 ceous Barnett facies (Loucks and Ruppel, 2007). 149 Milliken et al. (2007) interpreted the microcrystalline 150 quartz in Barnett Shale as being derived from mainly 151 agglutinated foraminifera, whereas others have 152 emphasized the importance of siliceous sponge spic-153 ules (Hickey and Henk, 2007; Slatt and O Brien, 154 2011; Abouelresh and Slatt, 2012). 155 In this paper, we (1) assess the amount and quality 156 of retained hydr...

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Predicting gas-to-oil ratio (GOR) and bulk petroleum properties is of paramount concern when developing production strategies for unconventional resource plays. Subtle changes in bulk fluid composition can result in large differences in phase envelope geometry and predicted fluid types, which can cause unfavorable pressure drawdown during production. Here we present new insights into the thermal evolution of petroleum compositions in conventional and unconventional reservoirs, focusing on polar compounds, i.e., nitrogen-, sulfur-, and oxygen-containing (NSO) compounds. These compounds feature functional groups that strongly influence the sorption, solubility, and partitioning behavior of petroleum constituents in unconventional shale systems. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) is a powerful tool to rapidly characterize NSO-compounds in complex mixtures and was used to compare the polar compounds in (1) extracts of six Posidonia Shale source rock samples with maturity levels between 0.48 and 1.45% vitrinite reflectance (Ro), (2) open-system pyrolysates of those six source rocks, and (3) four Posidonia Shale-sourced medium-gravity conventional crude oils. The aromaticity and degree of condensation were found to increase much more pronouncedly with increasing maturity for retained than for expelled oil NSO compounds. Pyrolysate NSO compounds have compositions intermediate between those of retained and expelled NSO compounds, pointing to preferential expulsion of smaller compounds in the crudes and enhanced cyclization and aromatization within retained fluids. Aromatization was shown to occur at the expense of aliphatic carbon. A genetic link between the fluids as well as the likely timing of petroleum expulsion is revealed by comparing the carbon number distributions of compounds from the N1 elemental class, that is, compounds that contain one nitrogen atom (e.g., carbazoles). The here documented chemical differences between the investigated fluid types are significant and need to be taken into account when formulating production strategies.

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