Abstract
The effect of organic matter on hydrocarbon potential, storage space, and gas content of shale is well-known. Additionally, present-day content of sedimentary organic matter in shale is controlled by depositional and preservation processes. Therefore, a study of the enrichment mechanisms of sedimentary organic matter provides a scientific basis for the determination of favorable areas of shale gas. In this study the Upper Ordovician Xinkailing Fm. and the first member of the Lower Silurian Lishuwo Fm. were examined. Stratigraphic sequences were identified through conventional logs and elemental capture spectrum data. Oxygen isotope analysis was applied to recover paleotemperature of seawater in the study area. The excess silicon content was calculated and the origin of the silica was determined by the Fe-Al-Mn ternary plot. The enrichment mechanism of organic matter was analyzed by two aspects: redox conditions and paleoproductivity. As a result, the stratigraphic interval was divided into two 3rd-order sequences. Through oxygen isotope, the paleotemperature of seawater was 62.7–79.2 °C, providing evidence of the development of hydrothermal activity. Analysis of excess siliceous minerals identified two siliceous mineral origins: terrigenous and hydrothermal. It also revealed an upwards decreasing tendency in hydrothermal activity intensity. Strong hydrothermal activity during the Late Ordovician, recognized as TST1, formed a weak-oxidizing to poor-oxygen environment with high paleoproductivity, which promoted organic matter enrichment. During the Late Ordovician to the Early Silurian, identified as RST1, TST2, and RST2, weakening hydrothermal activity caused the decline of paleoproductivity and increased oxidation of bottom waters, leading to a relative decrease of organic matter content in the shale. Therefore, favorable areas of shale gas accumulation in the Upper Ordovician and Lower Silurian are determined stratigraphically as the TST1, with a high total organic carbonate content. Geographically, the hydrothermally-active area near the plate connection of the Yangtze and the Cathaysian is most favorable.
Highlights
Since 2000, the shale gas industry has achieved great success in the United States, thanks to new ideas and advanced technology [1,2,3]
Due to the weak response of the deep-water environment to changes of sea level, it is difficult to identify the boundary between the low-stand and high-stand system tracts
Since the absence of minerals resulting from diagenetic precipitation in the thin section observation and the low content of Fe and S shown in the ECS data, this temperature was more likely to represent a depositional signal rather than a diagenetic process, which basically verity the existence of hydrothermal activity during the period
Summary
Since 2000, the shale gas industry has achieved great success in the United States, thanks to new ideas and advanced technology [1,2,3]. Interconnection of the pores provides permeability [14,15,16,17,18] As such organic matter abundance is one of the key indicators of shale gas evaluation. Large-scale exploration and development of shale gas is enhanced by new technology, providing new methods and data for the research on sequence stratigraphy of shale gas and mechanisms of organic matter deposition. Stratigraphic sequences are divided mainly by well logs and ECS data to form a enrichment is further analyzed and a model is proposed to explain the organic matter enrichment in sequence stratigraphic framework for the study area. The excess silicon content is applied to study the geological factors controlling the original abundance of sedimentary organic matter. The southern part of the Lower Yangtze generally transformed from
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