Abstract
The Longtan Formation of the Upper Permian in the Sichuan Basin has become a significant target for shale gas exploration in recent years. Multiple methods, including outcrop observations, thin sections, total organic matter content, X-ray diffraction and scanning electron microscopy were used to investigate the mineralogy, shale lithofacies assemblages and their relationships with the deposition environment. The mineral composition of the Longtan Formation has strong mineral heterogeneity. The TOC values of the Longtan Formation have a wide distribution range from 0.07% to 74.67% with an average value of 5.73%. Four types of shale lithofacies assemblages of the Longtan Formation could be distinguished, as clayey mudstone (CLS), carbonaceous shale (CAS), siliceous shale (SS) and mixed shale (MS) on the basis of mineral compositions. The TOC values of various types of shale lithofacies assemblages in the Longtan Formation varied widely. The shore swamp of the Longtan Formation is most influenced by the terrestrial input and mainly develops CLS and MS. The tidal flat is influenced by the terrestrial input and can also deposit carbonate minerals, developing CLS, CAS and MS. The shallow water melanged accumulation shelf develops CAS and MS, dominated by clay and carbonate minerals. The deep water miscible shelf develops CLS and SS, whose mineral composition is similar to that of the shore swamp, but the quartz minerals are mainly formed by chemical and biological reactions, which are related to the Permian global chert event. The depositional environment of the Longtan Formation controls the shale mineral assemblage of the Longtan Formation and also influences the TOC content.
Highlights
The XRD results of the Longtan Formation show that the content of quartz ranges from 7.8% to 39% with an average value of 20.3%, clay minerals content quartz ranges from 7.8% to 39% with an average value of 20.3%, clay minerals content ranges from 1.4% to 20.1% with an average value of 10.3%, potash feldspar content ranges ranges from 1.4% to 20.1% with an average value of 10.3%, potash feldspar content ranges from 0 to 0.9% with an average value of 0.2%, plagioclase content ranges from 0 to 11.3%
With an average value of 2.5%, calcite content ranges from 3.8% to 59.7% with an average with an average value of 2.5%, calcite content ranges from 3.8% to 59.7% with an average value of 32.2%, dolomite content ranges from 3% to 15% with an average value of 8.4%, value of 32.2%, dolomite content ranges from 3% to 15% with an average value of 8.4%, pyrite content ranges from 0 to 7.1% with an average value of 2.9%, siderite content ranges pyrite content ranges from 0 to 7.1% with an average value of 2.9%, siderite content ranges from 0 to 30.4% with an average value of 5.5%
The total organic carbon (TOC) value of clayey mudstone (CLS) has a large distribution range (Figure 7), from less than 0.5% to higher than 20%, which was related to the widest distribution area
Summary
In the past few years, huge breakthroughs have been undertaken in shale gas exploration in Sichuan Basin [1,2,3,4,5,6]. Following the Niutitang Formation and Wufeng- Longmaxi. Formation, another set of Paleozoic mudstone in the Sichuan Basin, the Longtan Formation, has attracted more and more attention [4,7,8,9]. Previous scholars have studied the geochemical characteristics and evaluated the potential of shale gas of the Longtan shale, and the results show enormous potential and exploration prospects [2,4,10,11,12,13,14].
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