Sedimentary Facies of the Longmaxi Formation Shale Gas Reservoir in the Weiyuan Area Based on Elemental Characteristics

Abstract

The Lower Silurian Longmaxi Formation in the southern Sichuan Basin is composed of a series of dark carbonaceous shales deposited in a hydrostatic shelf reduction environment. In this study, the ratio of uranium to thorium (U/Th), the total organic carbon (TOC), and the biological silicon content (SiBio) were selected as the characteristic parameters to precisely analyze the sedimentary environment and its impact on reservoir quality. The results show that the Weiyuan area in the Early Silurian Longmaxi period experienced two transgression-regression cycles, forming two third-class sequences, SSQ1 and SSQ2, which can be divided into six sedimentary microfacies: organic-rich siliceous argillaceous shelf, organic-rich silicon-containing argillaceous shelf, organic-rich silty argillaceous shelf, deep-water silty argillaceous shelf, shallow-water silty argillaceous shelf, and shallow-water argillaceous silty shelf microfacies. The organic-rich siliceous argillaceous shelf and organic-rich silicon-containing argillaceous shelf microfacies developed in the deepest transgressive system tract (TST1), with high U/Th, high TOC, and high SiBio, which are identified as the main control facies for reservoir development. These two microfacies are located in the middle of the study area, while a transition occurs in the east affected by the Neijiang Uplift. According to the classification criteria proposed in this article, the favourable shale gas reservoirs in Weiyuan area are characterized with high U/Th (>1.25), high TOC (>3%), and high SiBio (>15%). This paper proposed an evaluation method for shale sedimentary facies based on elemental and electrical logging characteristics, avoiding the limitations of core samples, which makes the quantitative division of shale sediments and the efficient recognition of high-quality reservoirs available. It is of great significance for delineating the potential production areas in the study area and beneficial for the scaled development of shale gas reservoirs.

Geological Characteristics and Controlling Factors of Enrichment of Deep Shale Gas in the East Weiyuan–North Rongchang Area, Sichuan Basin, China

“…The Zhongaoxi buried structure is in the middle, the Luoguanshan structure is in the south, and the Xishan structure is connected in the east, showing the structural characteristics of a wide syncline and a narrow anticline. Faults are developed and mostly distributed in the NE-SW direction (Figure 1) (Wu et al, 2021;Zhao et al, 2021). The Late Ordovician-Early Silurian is the turning period of sedimentarytectonic evolution of the Yangtze Block.…”

“…The shale of the Wufeng Formation-Longmaxi Formation in the East Weiyuan-North Rongchang block was deposited in an organic-rich shelf environment (Wu et al, 2021;Zhao et al, 2021). In this study, conventional logging and imaging logging data of eight wells, including R202, R233H, W231, W232, and W206, were collected to analyze the characteristics of the reservoir lithology, reservoir space, brittle minerals, and gasbearing.…”

An Analysis of Silurian Paleo–Tethys Hydrocarbon Source Rock Characteristics in North Africa, the Middle East, and South China

“…The black shale of the shelf facies, which is rich in fossils, such as graptolites, and formed in a deep-water shelf-anoxic environment [18], is widely distributed in the Paleozoic "Pan-craton basin" in North Africa and the Middle East. In North Africa, apart from the coastal-delta facies that developed in the peripheral areas north of the Eglab-Hoggar Shield, the shallow marine shelf facies are widespread across vast regions of the Tindouf, Timimoun, Triassic/Ghadames, Murzuq, and Sirt Basins located in the central and western parts of the Sahara Platform [19,20].…”