Chemostratigraphic Analysis of Wufeng and Longmaxi Formation in Changning, Sichuan, China: Achieved by Principal Component and Constrained Clustering Analysis

Zhifeng Zhang,Wei Liu,Chengshan Wang,Xiang Li,Yongjian Huang,Bo Ran

doi:10.3390/en14217048

Abstract

The increasing proportion of unconventional worldwide energy demands have consistently promoted the necessity for exploring a precise, high-resolution, objective, and quantitative stratigraphic division method for macroscopically homogeneous mudstone successions. The chemostratigraphy can resolve this problem well, although it has been applied successfully in North America, but not systematically studied in China for shale gas exploration and development. This work has conducted a chemostratigraphic analysis of Wufeng and Longmaxi Formation on the Changning section of Sichuan Province, southwestern China, to testify its applicability for shale gas exploration in China. Principal component analysis (PCA) was first employed to reduce the dimensionality of datasets. Three chemofacies, including detrital (K, Ti, Fe, Al, Na, Mg, Cr, Zr, Rb), authigenic (Ca, Sr, Mn, Si, S, Ba), and redox-organic (P, V, Ni, Zn, Cu, TOC), were found. Subsequently, constrained clustering analysis was utilized for the zonation of each chemofacies into chemozones. Consequently, the whole Changning section was divided into twelve chemozones (CZ I–CZ Ⅻ). The geochemical interpretation for these chemozones can be resolved from the regional changes in paleogeography and paleoceanography during the Late Ordovician to Early Silurian period. Thus, a three-stage geochemical evolution along the Changning section can be classified: (1) the siliceous and anoxic deposits of Wufeng Formation (CZ I–CZ III) with high TOC contents; (2) the siliceous and anoxic sedimentary rocks of bottom Longmaxi Formation with still higher TOC (CZ Ⅳ); (3) the calcarous-detrital and oxic sediments for the rest of Longmaxi Formation (CZ Ⅴ–CZ Ⅻ). In considering their high content of TOC and abundant brittle siliceous minerals, the CZ (I–Ⅳ, 0 m–33.6 m) are thought to be the most preferable sweet spot for shale gas exploration.

Highlights

The production of unconventional energy, especially shale gas, has increased dramatically in recent years to satisfy the growing demand for energy worldwide [1]
The macroscopically homogeneous attributes and limited fossil constraints of shales hinder the availability of biostratigraphy and lithostratigraphy [3]
The chemostratigraphic chemostratigraphic technique technique presented presented in this paper paper has has provided provided aa convenient convenient and methodfor fordivision divisionofoffine-grained fine-grained successions

Summary

Introduction

The production of unconventional energy, especially shale gas, has increased dramatically in recent years to satisfy the growing demand for energy worldwide [1]. Chemostratigraphy, which divides and correlates the succession based on the subtle variations in the elemental composition of the sediments, can deal with this matter quite well and has been applied widely and successfully for developing unconventional oil and gas in North America [9–16]. The rationale behind this fact is that the heterogeneity of macroscopically homogeneous mudstones can be best resolved by the chemical composition, which may reflect the climate variation in the source region, productivity in the ocean surface, and redox change at the bottom water, or even the hydrothermal input into the sediments [10,11,17–20]. The multivariate-statistically based chemostratigraphic technique is a new trend that can provide more objective and quantitative results [4,10,20,21]

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Discussion

Conclusion