The Origin of Silica of Marine Shale in the Upper Ordovician Wulalike Formation, Northwestern Ordos Basin, North China

Abstract

The shale of the Wulalike Formation developed in the northwestern Ordos Basin is considered to be an effective marine hydrocarbon source rock. One of the key factors for successful shale gas exploration in the Wufeng–Longmaxi Formation in the Sichuan Basin is the high content of biogenic silica. However, few people have studied the siliceous origin of the Wulalike shale. In this study, we used petrographic observation and element geochemistry to analyze the origin of silica in the Wulalike shale. The results show that the siliceous minerals are not affected by hydrothermal silica and mainly consist of biogenic and detrital silica. A large number of siliceous organisms, such as sponge spicules, radiolarians, and algae, are found under the microscope. It has been demonstrated that total organic carbon has a positive correlation with biogenic silica and a negative correlation with detrital silica, and biogenic silica is one of the effective indicators of paleoproductivity. Therefore, the enrichment of organic matter may be related to paleoproductivity. Through the calculation of element logging data in well A, it is found that biogenic silica is mainly distributed in the bottom of the Wulalike Formation, and the content of biogenic silica decreases, while the content of detrital silica increases upward of the Wulalike Formation. Biogenic silica mainly exists in the form of microcrystalline quartz, which can form an interconnected rigid framework to improve the hardness and brittleness of shale. Meanwhile, biogenic microcrystalline quartz can protect organic pores from mechanical compaction. Therefore, it may be easier to fracture the shale gas at the bottom of the Wulalike Formation in well A.

Quartz Origins and Paleoenvironmental Controls on Organic Matter Accumulation of Marine Shale in the Ordovician Wulalike Formation, Northwestern Ordos Basin, China: Significance for Shale Gas Exploration

“…In the mid-late Ordovician period, influenced by the expansion of the Helan trough, the main part of the Ordos Basin began to uplift, while the western and southern margins subsided substantially, forming an L-shaped marginal sea and depositing a suite of deep water facies shale [33]. The study area is situated in the Zhuozishan district in the northwestern margin of the Ordos Basin, and the Ordovician is composed of the Sandaokan Formation, Zhuozishan Formation, Kelimoli Formation, Wulalike Formation, and Lashizhong Formation in ascending order [6] (Figure 2A). The Wulalike Formation is the target layer characterized by the grey-black bearing graptolite shale and a small amount of limestone interlayers [34], which is distributed in the western and southern margins of the basin, with the thickness ranging from 20 to 120 m and the burial depths varying from 1500 to 4700 m, as well as a low TOC contents [3][4][5][6].…”

“…The TOC is the most critical index to assess the quality of source rocks [34,36]. The TOC content of the Wulalike shale is generally low, varying from 0.14% to 0.90%, with an average of 0.51% (Table 1).…”

“…Gas-bearing layers in the Sichuan Basin are mainly distributed in the quartz-rich organic-rich shales in the lower part of the Longmaxi Formation [5]. In contrast, the Wulalike shale has a high content of quartz (more than 45%) and numerous siliceous organisms, but a low TOC concentration (averaging 0.49%) [3,6].…”

“…Quartz is one of the most prevalent minerals in shale (usually greater than 30%) [6,7], and it is not only positively connected with TOC content [8,9], but it is also a major determinant of shale reservoir quality [10,11]. It is derived primarily from four sources: terrigenous clastic quartz, Si-bearing hydrothermal fluids input, conversion of smectite to illite formed quartz (S-I quartz), and biogenic quartz.…”

“…Furthermore, it is worth noting that biogenic quartz protects reservoir pores from destruction, but S-I quartz does not [14,17]. Biogenic quartz is formed by dissolution and recrystallization of siliceous plankton, such as radiolarians, sponge spicules, and algae [6,14]. Siliceous plankton is not only a source of high-quality hydrocarbon parent rock, but it also has an interior cavity that can help preserve organic matter (OM) by providing an OM-hosted space, which is linked to OM enrichment [14].…”