Abstract
Distinguishing the differences of pore characters between different mixed lithofacies shales is helpful for improving shale gas development efficiencies. In this study, the targeted Longmaxi shale ( L1) of the Southern Sichuan Basin was selected as the research object. Two kinds of mixed lithofacies shale were identified by analyzing total organic carbon and X-ray diffraction results. The forming depositional environment of mixed lithofacies shale was researched by elements analyses. Pores in different mixed lithofacies shale were observed using field emission scanning electronic microscope. Low-field nuclear magnetic resonance and low-temperature nitrogen adsorption were conducted to analyze pore characters of different mixed lithofacies shale. The results showed that L1 were mainly composed of organic rich clay–siliceous mixed shales (OR-M-1) and organic extreme rich calcareous–siliceous mixed shales (OER-M-3). OR-M-1 and OER-M-3 were formed in high paleo-producing dysoxic–oxic seawater and anoxic waterbody, respectively. Micro-pore and meso-pore volumes of OER-M-3 were greater than those of OR-M-1 while macro-pore volume of OER-M-3 was lower than that of OR-M-1. Meso-pore surficial and structural complexities of OER-M-3 were greater than those of OR-M-1. OER-M-3 were greater in oil-wetting micro-pore structural complexities while lower in water-wetting micro-pore structural complexities, compared with OR-M-1. The inherent relationships between lithofacies type and sedimentary environment, total organic carbon, as well as pore characters, respectively, were quite close.
Highlights
Shale gas, with recoverable resources in China estimated to be 25.1 Â 1012 m3 (Hu et al, 2017), is regarded as a key replenishment for natural gas supply
Those of OER-M-3, primarily locating in L12, were ranged in 2.47 À 8.34, 5.49 À 16.30, and 0.68 À 4.04, respectively. These two phenomena indicated that OR-M-1 were formed in dysoxic–oxic seawater, while OER-M-3 were primarily formed in an anoxic waterbody (Figure 5)
The results showed that porosity proportions of OER-M-3 organic matter (OM) pores, ranging between 17.43% and 67.36%, were greater than those of OR-M-1 OM pores, which were in the range from 3.23% to 28.87%
Summary
With recoverable resources in China estimated to be 25.1 Â 1012 m3 (Hu et al, 2017), is regarded as a key replenishment for natural gas supply. Identification of shale lithofacies is important in predicting total organic carbon (TOC) content, designing completion strategies, and determining optimal horizontal well trajectories (Wang and Carr, 2012; Wang et al, 2016). Shale is a heterogeneous porous medium containing nanometer scaled pores (Yang et al, 2019), which provide spaces for gas adsorption and seepage. Considering that different lithofacies have differences in depositional environments and pore structure complexities (Li et al, 2019), joint research on shale lithofacies, depositional environment, and shale pores is significant for a reasonable determination of targeted shale reservoirs. Shale lithofacies classifications considering mineral composition and organic matter (OM) richness are proved to be significant and meaningful for identifying the reservoir with high content of gas (Tang et al, 2016) and brittle minerals (Wang et al, 2017). Shale lithofacies may vary successively in a vertical profile, and a comprehensive understanding on the variation can provide instructions on gas producing ability of different intervals
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