Abstract
How the geochemical characteristics of organic matter shale for the carbonate platform facies remain uncertain, which restricts an integrated reconstruction of the model of organic matter accumulation and preservation. Here, we present new results from element geochemical fingerprinting and integrated analyses of paleoclimate, paleoproductivity, paleoredox environment, and terrigenous input of the targeted Middle Devonian Tanshanping and Dacaozi Formations in the Ninglang-Yanyuan Basin, Southwest China. It is worth noting that although the carbonate platform connects with the open sea partially, the redox environment will not be completely controlled by relative sea level variations. Specially, the paleoclimate, paleoproductivity, and paleoredox conditions are the main controlling factors of the accumulation and preservation of organic matter. In view of the paleoclimate indexes, we suggest that both a relatively warm-humid climate characterized by intensified chemical weathering conditions and a higher terrigenous input are identified as two major drivers forcing the reductive environment in the sedimentary waterbody. Finally, a comprehensive model is established for providing new insights into the mechanism of organic matter accumulation and preservation for the carbonate platform facies. The paleoredox environment, paleoproductivity, paleoclimate, and terrigenous input are believed to have exerted a very considerable force on reconstructing the model of organic matter accumulation and preservation for the carbonate platform facies. Specially, the coupling interactions between the paleoproductivity and redox condition are thus also stressed. We found that the preservation condition is much more important than the paleoproductivity, resulting in the degree of organic matter enrichment. Even if the paleoproductivity of a sedimentary waterbody of a depositional period of the Dacaozi Formation was higher, the TOC concentrations were relatively low due to the poor preservation condition by fall of the sea level and increase of the terrigenous input. In another aspect, the better preservation condition of the Tanshanping Formation makes the TOC concentrations higher in the case of lower paleoproductivity in the sedimentary waterbody.
Highlights
The mud shale of the Tanshanping Formation in the Ninglang-Yanyuan area is mainly composed of clay minerals and carbonate minerals, and the mud shale of the Dacaozi Formation is mainly composed of clay minerals and siliceous minerals
From the above discussion of the values (CIA, ∑rare earth element (REE)) from the Dacaozi Formation to the Tanshanping Formation, we have found that the climate is relatively warm-humid during the deposition of the Dacaozi Formation, the weak preservation condition leads to destroy the accumulation of organic matter, which shows the lower TOC concentrations in the end
The sedimentary position of the study area is different from the previous researches, which belongs to the carbonate platform facies (Figure 7)
Summary
The mud shale of the Tanshanping Formation in the Ninglang-Yanyuan area is mainly composed of clay minerals and carbonate minerals, and the mud shale of the Dacaozi Formation is mainly composed of clay minerals and siliceous minerals. Previous studies have proved that the Tanshanping Formation and the Dacaozi Formation are the shaleconcentrated sections in the Ninglang-Yanyuan Basin, and they have been important favorable exploration layers for shale gas, which is characterized by large thickness, high maturity, and high organic carbon content. As the development of the shale exploration in the Tanshanping Formation and the Dacaozi Formation, combined with the deployment of two-dimensional seismic data, we chose a structure favorable site (Lizihe anticline) to deploy a geological survey well called Well Yunningdi-1. The total organic carbon content in the Tanshanping Formation was between 0.09% and 1.83% (with an average of 1.14%), and the organic carbon content in the Dacaozi Formation was between 0.32% and 3.48% (with an average of 1.05%)
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