Elemental Capture Spectroscopy and Electrical Image Logs in Shale Gas Reservoir Characterization, Longmaxi Formation of Lower Silurian, China

The elemental capture spectroscopy and electrical image logs from four wells in the Fuling shale gas field in southern China indicate that the shale gas source rock contains 25-32% silicon, 1.6-6% calcium, 0.2-2% sulfur, and 3.8-5% iron and other elements. Additionally, the source rock is associated with a sedimentary structure of thinly laminated beds. Based on the elemental capture spectroscopy log processing, the main mineral compositions of the source rock are identified, including clay, carbonate, pyrite, and siliceous content that had previously been described as detrital silt. The minerals and thinly laminated beds suggest that the source rock was formed in an anoxic submarine sedimentary environment where organic-rich siliceous shale was deposited, which is interpreted as a deepwater shelf depositional environment. The shale gas reservoir in this case study is from the Longmaxi formation of Lower Silurian deepwater shelf sediments. The source rock contains 2.3-4% total organic content (TOC). In the past years, however, few mineral component analyses of the source rock were performed from core data, and most sedimentary analysis depended primarily on outcrop observation and core data. The detailed mineralogy from elemental capture spectroscopy logs helped to recognize the significant elemental concentration of silicon, calcium, sulfur and iron of source rock, which are closely related to the source rock's TOC and reservoir rock properties. The detailed sedimentary structure analysis on electrical images identifies the syndepositional structures, slump structures and horizontal laminated beds, which correspond to slope and deepwater anoxic euxinic basin. By the correlation of the four wells, both the horizontal thinly laminated beds and the elemental compositions indicate a shale gas reservoir. The presented case study demonstrates the integrated processing and interpretation of the elemental capture spectroscopy and electrical image logs. It provides a novel approach to combine both logs to perform shale gas reservoir characterization. The case study also shows that the mineralogy and sedimentology of the source rock from the continuous records of elemental component and geological sedimentary information are indispensable to unconventional shale gas reservoir characterization, particularly in new exploration wells.