Origin of H₂S and Hydrocarbons from the Permian-Triassic Khuff Formation Using Fluid Inclusion Technology

Abdelghayoum S. Ahmed^1*, Richard Worden², and Tom Harland^1
1Saudi Aramco
²Liverpool University, UK
*[email protected]

Fluid-inclusion analysis was conducted from onshore and offshore fields in Saudi Arabia, supported by detailed petrography and integration with existing geochemistry, reservoir and sedimentological data. The objectives were to determine the temperature of thermochemical sulphate reduction (TSR) and H₂S generation, petroleum phases and API, gas-to-oil ratio (GOR) of liquid petroleum in inclusions, as well as filling history. Results confirmed that H₂S in the Khuff Formation is due to TSR. The absolute quantity of produced H₂S is re-flected by the quantity of anhydrite that was converted to calcite, and was taken to suggest degree of TSR advancement. Petrographic and fluid-inclusion data suggested that TSR was controlled by temperatures, distribution of finely disseminated anhydrite and hydrocarbon liquid or gas phase in the Khuff reservoirs. Fluid-inclusion analysis revealed that the initial charge was oil, later displaced by condensate and dry gas. The analysis also showed that TSR occurred at temperature between 110° and 135° C. Oil found as inclusions is consis-tent with the emplacement of a maturity-controlled sequence beginning with 35° API oil through to a 55° API oil at temperatures of up to 140° C, with methane-dominant gas em-placement above 140° C to approximately 155° C. The absence of oil inclusions in other wells could be due to: (1) oil never having resided in the Khuff Formation in these wells because they were too deep for a palaeo-oil floor, or the lack of source rock in local areas for the reservoir; (2) an insufficient number of samples were examined to find rare oil in-clusions; (3) oil, initially trapped, leaked-out of inclusions into free pore space and was then diluted by gas; and/or (4) oil cracking to gas within fluid inclusions.

AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain