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

Abstract

The elemental capture spectroscopy and electrical image logs from four wells in a field in southern China indicate that the shale gas source rock contains 9–11% calcium, 3–6% iron, and 40–45% silicon and other elements. Additionally, the source rock associates with thin laminated beddings of sedimentary structure. Based on the elemental capture spectroscopy log processing, the main mineral components of the source rock are identified, including clay, quartz, carbonate, and pyrite. The minerals and thin laminated beddings suggest that the source rock was formed in an anoxic submarine sedimentary environment where thick accumulations of organic-rich silty shale and calcareous shale were deposited. The shale gas reservoir in this case study is from the Longmaxi formation of Lower Silurian neritic 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 mainly depended on outcrop observation and core data. The detailed mineralogy from elemental capture spectroscopy logs helped to recognize the significant elemental concentration of calcium 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 structure and horizontal laminated beddings, which correspond to deep-water slope and deep-water anoxia euxinic basin. By the correlation of the four wells, both of the horizontal thin laminated beddings of sedimentary structure and the element composites of calcium and iron indicate the shale gas reservoir distribution. 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 composites and geological sedimentary information are indispensable to unconventional shale gas reservoir characterization, particularly in new exploration wells.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015