Late Jurassic Arwa Member in south-eastern Al-Jawf sub-basin, NW Sabatayn Basin of Yemen: Geochemistry and basin modeling reveal shale-gas potential

Abstract

The current study evaluated the source rock characteristics, thickness, and lithology of the Late Jurassic Arwa Member to provide information about both conventional and unconventional resource systems in the Al-Jawf sub-basin. Three exploratory wells in the south-eastern portion of the Al-Jawf sub-basin were used for the source rock geochemistry and basin modeling study.Results from the geochemical and basin modeling indicate that the Arwa Member is a self contained source-reservoir whose shales are considered as gas-prone source rocks, and have generated large amounts of thermogenic gas through secondary cracking of oils at high thermal maturity levels. The geochemical results reveal that the Arwa shales currently contain Type III and IV kerogen in a gas-window maturity stage and are shale-gas resources.The basin models illustrate that the late Jurassic to early Miocene age was the peak-oil generation window of the Arwa shale source rock, with a transformation ratio (TR) of 10–85%. Most of the oil was expelled along micro-fractures caused by the pressure of oil generated within the Arwa shales, which was then trapped in the carbonate reservoir rocks within the Arwa Member itself. The oil retained in the Arwa carbonate rocks was partially and/or completely cracked into gas between the early Miocene and the present-day. This thermogenic gas was generated due to high thermal maturity caused by Tertiary volcanic rocks. Assuming a thickness of 360–1590 m and partial and/or complete cracking of retained oil to gas, the Arwa Shale Member could have a huge gas-generation potential.