Abstract
Thermal maturation in the shale oil/gas system is inherently complex due to the competitive interplays between hydrocarbon generation and retention processes. To study hydrocarbon generation characteristics from shales within different stages of thermal maturation under the influence of retained oil, we performed Micro-Scale Sealed Vessels (MSSV) pyrolysis on a set of artificially matured lacustrine shale samples from the Shahejie Formation in the Dongpu Depression in Bohai Bay Basin, China. Experimental results show that hydrocarbon yields of shale samples with or without retained oil at various thermal maturities follow different evolution paths. Heavy components (C15+) in samples crack at high temperatures and generally follow a sequence, where they first transform into C6–14 then to C2–5 and C1. Methane accounts for most of the gaseous products at high temperatures in all samples, with different origins. The cracking of C2–5 is the main methane-generating process in samples with retained oil, whereas the source of methane in samples without retained oil is kerogen. In the studied shales, retained oils at early-mature stage retard the transformation of liquid to gaseous hydrocarbon and prompt the cracking of C2–5 to C1 to some extent. TSR reaction related to gypsum in the studied samples is the primary reason that can explain the loss of hydrocarbon yields, especially at high temperatures. In addition, transformation of volatile hydrocarbons to gas and coke also accounts for the loss of generated hydrocarbon, as a secondary factor.
Highlights
Hydrocarbon generation has been a topic of long-standing interest of petroleum geologists, and researchers have earned a lot of achievements in the last few decades (Tissot and Edited by Jie HaoState Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, ChinaCollege of Geosciences, China University of Petroleum, Beijing 102249, ChinaState Key Laboratory of Shale Oil and Shale Gas Resources and Effective Development, SINOPEC Petroleum Exploration and Production Research Institute, Beijing 100083, ChinaSINOPEC Key Laboratory of Petroleum Accumulation Mechanisms, Wuxi Institute of Petroleum Geology, Wuxi 214126, Jiangsu, ChinaWelte 1978; Lewan and Roy 2011; Sondergeld et al 2013)
Keywords Micro-Scale Sealed Vessels (MSSV) pyrolysis · Hydrocarbon generation · Lacustrine shales · Thermal maturation · Retained oil Hydrocarbon generation has been a topic of long-standing interest of petroleum geologists, and researchers have earned a lot of achievements in the last few decades
By conducting MSSV pyrolysis experiments on shale samples with or without retained oil, the purpose of this research is attained from three aspects: (1) investigating hydrocarbon generation characteristics of a lacustrine shale throughout the thermal maturation process; (2) finding out the compositional differences between hydrocarbons generated from extracted and unextracted samples; and (3) studying how hydrocarbon yields from samples within different thermal maturity evolve when heating to different temperatures
Summary
Hydrocarbon generation has been a topic of long-standing interest of petroleum geologists, and researchers have earned a lot of achievements in the last few decades State Key Laboratory of Shale Oil and Shale Gas Resources and Effective Development, SINOPEC Petroleum Exploration and Production Research Institute, Beijing 100083, China. The general scheme proposed by Tissot and Welte (1978) describing hydrocarbon formation during burial of source rocks has been widely accepted, and numerous studies were carried out on this basis. Techniques have been improved greatly these years to analyze hydrocarbon generation from source rocks. Thanks to the efforts that researchers have taken, we have a generally clear understanding on the properties of kerogens and hydrocarbon products at all stages of thermal maturation.
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