Depositional Environment Variation and Organic Matter Accumulation Mechanism of Marine–Continental Transitional Shale in the Upper Permian Longtan Formation, Sichuan Basin, SW China

Abstract

Marine−continental transitional shale gas exploration is one of the hottest topics in the academic and industrial community. Frequent sedimentary environment changes lead to substantial heterogeneity and complex organic matter enrichment mechanisms, and it is of great practical significance to clarify the paleoenvironment of marine−continental transitional shale and reveal the enrichment mechanism of organic matter for subsequent shale gas exploration. However, strong heterogeneity creates divergence for our understanding of organic matter enrichment mechanisms and resource potential. This paper selected the marine−continental transitional shale of the Permian Longtan Formation in the southeastern Sichuan Basin as an example. Through TOC analysis, XRD measurements, and major and trace element analyses, we identify the depositional cycle and lithofacies, characterize the paleosedimentary environments of different lithofacies, establish depositional models, and clarify the organic matter enrichment mechanism. Four lithofacies can be identified in the Longtan Formation in the study area: organic−lean carbonate shale (LC), organic−lean mixed shale (LM), organic−lean argillaceous shale (LCM), and organic−rich argillaceous shale (RCM). The terrestrial detrital influx of the LCM and RCM shales is obviously strong, while the RCM shale was deposited in a relatively dysoxic and humid environment. For the Longtan shale, the terrestrial detrital influx and redox environment determine the enrichment of organic matter. In the lagoon sediment system, due to the high terrestrial detrital influx, the argillaceous shale in the transitional shale gas system should be given more attention in future exploration. For different kinds of shale gas systems (marine, marine− continental transitional, and lacustrine), the effect of the terrestrial detrital influx is the main difference. In marine−continental and lacustrine shale gas systems, shales with high TOC values can still exist in the context of oxic sedimentary environments. The research results of this paper can provide a theoretical basis for future marine−continental transitional shale gas sweet spot prediction and exploration from geochemical perspectives.

Favorable Exploration Lithofacies and Their Formation Mechanisms in Lacustrine Shales Deposited under Different Salinity Conditions: Insights into Organic Matter Accumulation and Pore Systems

“…Shale is defined as fine-grained sedimentary rocks with a grain size of <0.0039 mm. , Currently, there is no standardized classification for shale lithofacies. Various classification methods for fine-grained sediments consider factors such as the mineral composition, , OM content, − and sedimentary texture. , Lithofacies reflects the depositional environment and characteristics of rocks . Herein, lithofacies are identified using the mineral composition and TOC content.…”

“…Various classification methods for fine-grained sediments consider factors such as the mineral composition, 19,20 OM content, 20−22 and sedimentary texture. 20,23 Lithofacies reflects the depositional environment and characteristics of rocks. 24 Herein, lithofacies are identified using the mineral composition and TOC content.…”

Origins of Siliceous Minerals and Their Influences on Organic Matter Enrichment and Reservoir Physical Properties of Deep Marine Shale in the Sichuan Basin, South China

“…Terrigenous detrital input affects organic matter accumulation generally through (1) controlling the deposition rate and organic degradation efficiency and (2) changing the state of the organic matter in the marine environment. Terrigenous debris input can not only provide nutrients for enhancing primary productivity but also dilute the relative proportion of organic matter in sediments, , and the more rapid increase of detrital input does not promote organic matter accumulation and may even play a negative role . On the other hand, the ballast effect from the combination of organic matter and terrigenous clastic particles promotes the accelerated precipitation of organic matter .…”

“…Terrigenous debris input can not only provide nutrients for enhancing primary productivity but also dilute the relative proportion of organic matter in sediments, 64,65 and the more rapid increase of detrital input does not promote organic matter accumulation and may even play a negative role. 66 On the other hand, the ballast effect from the combination of organic matter and terrigenous clastic particles promotes the accelerated precipitation of organic matter. 67 In this process, the interlayer domains within clay mineral (e.g., smectite) structures can provide an important adsorption site for organic compounds, which is beneficial for reducing the loss of organic matter during precipitation.…”

Organic Geochemical Characteristics and Organic Matter Enrichment of the Upper Permian Longtan Formation Black Shale in Southern Anhui Province, South China

“…Over the past few decades, extensive research works have been conducted on the geochemical characteristics and organic matter enrichment in marine sediments. − It is widely accepted that special geological conditions are required for organic matter accumulation in marine sediments, and no single process or model can explain organic matter accumulation in all sedimentary environments due to the complex geological history. Moreover, environmental fluctuations are more common in transitional settings than in marine systems, likely due to the relatively small sizes of water reservoirs. ,, The Longtan Formation transitional shales, which were deposited in delta and tidal flat-lagoon environments, exhibit strong vertical and horizontal heterogeneities in geochemical characteristics, ,,, making them an invaluable opportunity to study organic geochemical characteristics and explore organic matter enrichment in such environments. , Specifically, the biomarker distribution can provide molecular-level information on organic matter sources, thermal maturity, and organic matter enrichment. − …”