The basal Silurian hot shale (BSHS) is believed to be the main source rock for unconventional gas discovered in Paleozoic reservoirs. A challenge is to determine the maturity of these Paleozoic gases, which can help identify sweet spots. Geochemical and isotope analyses and modeling have been utilized to help characterize these unconventional gases. The BSHS-sourced gases are characterized by a wide range of wetness and varying carbon isotopes from a normal trend to partial and complete reversal. The isotope rollover and reversal most likely derived from mixing gases generated from kerogen, oil and wet gas cracking. Contribution from inorganic processes, particularly for C2+, may have also enhanced the isotope reversal in the highly mature gas, as suggested by high CO2 content. Based on the variation and patterns of δ13C1, δ13C2, and δ13C3 from the BSHS-sourced gas and the well-studied unconventional gases from Canada, China, and USA, it is believed that C-isotopes of unconventional gas depend more on maturity, less on source rocks as maturity increases. Four levels of maturation are identified from these gas fields as Ro 0.5-1.5% (I), 1.5-2.0% (II), 2.0-3.5% (III), and 3.5-5.0% (IV). The BSHS-sourced gas falls mainly in levels I, II and III. High gas production potential may be located in a zone between level II and level III. The implication is that maturity of mature gas can be well established and utilized to help plan exploration and identify high production zones.
AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018