Abstract
Lithofacies are the fundamental geological units for shale oil and gas exploration and development, and soluble organic matter (SOM) is most similar to crude oil in composition. Both aspects attract our attention in the interpretation of SOM in different lithofacies, which can provide direct evidence to predict shale oil “sweet spots”. Here, twenty-five shale samples were collected from the Eocene Shahejie Formation in the Dongying Sag and were subjected to X-ray diffraction, Rock-Eval pyrolysis, and SOM characterization. Comparison of the SOM contents in shales with different lithofacies revealed remarkable differences: 1) The contents of SOM, saturates and total hydrocarbons (THC) showed the order of detrital massive mudstone < homogenous massive mudstone < wide laminated shale < discontinuous laminated shale < fine laminated shale < gypsum-bearing mudstone, and the SOM content was controlled by lithofacies through differences in both OM and minerals. 2) The SOM in detrital and homogenous massive mudstones was mainly composed of saturates and resins. Saturates were the main component in wide and fine laminated shales. The SOM in discontinuous laminated shale was mainly composed of saturates and aromatics. The SOM in gypsum-bearing mudstone was mainly composed of saturates, and the percentage of asphaltenes was quite high. Based on the evaluation parameters of high-quality lithofacies in terms of abundance (i.e., SOM, THC or saturate contents) and quality (i.e., quality index and asphaltene percentage), the fine, wide and discontinuous laminated shales were regarded as relatively favorable lithofacies. Based on the lithofacies combination, the fine, wide and discontinuous laminated shales in Es3x and Es4ss (upper section of Es4s) in the Dongying Sag were interpreted as “sweet spots” for shale oil exploration and development. Thus, it is of great significance to study the characteristics of SOM in shale with different lithofacies for shale oil exploration and development.
Highlights
Shale oil and gas has become a hot topic in the exploration and development of unconventional oil and gas resources around the world and has attracted wide attention from petroleum geologists (Jarvie, 2012; Wang et al, 2015; Misch et al, 2016; Kumar et al, 2017; Chen et al, 2018; Wang et al, 2020; Zou et al, 2020; Hou et al, 2021a)
Among the different shale lithofacies, the mineral compositions or constituents varied greatly (Figure 3B): 1) The detrital massive mudstone was characterized by high contents of clay minerals and detrital minerals and low contents of carbonate minerals
1) In terms of the contents of soluble organic matter (SOM), saturates and THC, the lithofacies showed the order of detrital massive mudstone < homogenous massive mudstone < wide laminated shale < discontinuous laminated shale < fine laminated shale < gypsum-bearing mudstone
Summary
Shale oil and gas has become a hot topic in the exploration and development of unconventional oil and gas resources around the world and has attracted wide attention from petroleum geologists (Jarvie, 2012; Wang et al, 2015; Misch et al, 2016; Kumar et al, 2017; Chen et al, 2018; Wang et al, 2020; Zou et al, 2020; Hou et al, 2021a). As fundamental units representing the sedimentary environment, lithofacies are mainly identified by rock color, composition, structure and other characteristics (Xie, 2009; Zhang J et al, 2016). Many researchers have classified shale lithofacies under different rules and discussed their origin, sedimentary environment, hydrocarbon generation potential, reservoir space and diagenetic evolution (Hickey and Henk, 2007; Loucks and Ruppel, 2007; Xie, 2009; Abouelresh and Slatt, 2012; Fu et al, 2013; Wang et al, 2014; Wang et al, 2016; Zhang J et al, 2016; Zeng et al, 2017; Liang et al, 2018). The lithofacies that can generate high-quality shale oil is termed high-quality lithofacies
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