Vertical Heterogeneity of the Shale Reservoir in the Lower Silurian Longmaxi Formation: Analogy between the Southeastern and Northeastern Sichuan Basin, SW China

Abstract

Lower Silurian Longmaxi formation (LSL) shale is widely and continuously distributed in the northeastern Sichuan Basin and, based on structural analogies with the gas producing LSL formation in the southeastern Sichuan Basin, has significant potential for shale gas exploration. However, limited research has been performed to evaluate the shale gas potential in this region. Samples from a recently completed exploratory well (Well-WQ2) in the northeastern Sichuan Basin indicate that the LSL shale has a vertical property sequence that closely resembles the vertical property sequences in wells in the gas-producing sections of the southeastern Sichuan Basin. The continuous sampling and analyses of Well-WQ2 have allowed a detailed investigation of the vertical variations in lithofacies, mineral characteristics, pore structures, and organic geochemical characteristics. The Longmaxi formation was divided into two third-order sequences (SQ1 and SQ2) based on systematic core observations and well logging analyses. Both SQ1 and SQ2 include a transgressive system tract (TST) and a high-stand system tract (HST). The lithofacies exhibit an upward decrease in the organic content. From SQ1 to SQ2, the quartz content, in situ graptolite content, total organic carbon (TOC) content, and brittleness index decrease, but the clay mineral content increases. The LSL shale sections from depths of 1204 to 1214 m and from 1271 to 1282 m possess well-developed fractures and high permeability. Additionally, the average porosity and permeability in SQ1 are higher than those in SQ2. In addition, the positive correlation between the TOC and quartz contents of the assayed samples suggests that much of the quartz is of biogenic origin. Changes in the sedimentary and diagenetic environments during deposition are two key factors that contribute to the observed vertical heterogeneity of the Longmaxi formation. In conclusion, the shale sections of the lower part of the SQ1, like their analogs in the southeastern Sichuan Basin, are the most favorable targets for shale gas production in the northeastern Sichuan Basin.

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