Abstract: A Middle East Basin Center Hydrocarbon Accumulation in Paleozoic Rocks, Eastern Jordan, Western Iraq and Surrounding Regions

AHLBRANDT, T.S., O.A. OKASHEH and M.D. LEWAN

The Risha gas field in eastern Jordan is a basin-centered, tight gas reservoir. Fractures and associated secondary porosity development in the Ordovician Risha Member of the Dubeidib (Dubaydib) Formation provide the reservoir for a regionally extensive natural gas accumulation in rocks with matrix porosities generally in the 6-8% range (rarely up to 15%) and very low permeabilities. The Risha Member is a clastic unit about 150 meters thick. The gas is enriched in methane and contains minor amounts of ethane and carbon dioxide. The methane has a stable-carbon isotope signature indicative of thermogenic gas (-41 to -40 o/oo vs PDB). The black shale in the basal 10 to 20 meters of the overlying Silurian Mudawwara Formation, the most likely source of this gas accumulation, has total organic carbon (TOC) values ranging from 1 to 4 weight % and is at the end of the oil-generation stage (~1.2% Ro) in the field area. The two likely sources of this gas are mature type II kerogen and cracked Silurian-sourced oils within the down-dip basal Silurian shales. These shales are also the source rock for low-sulfur, high-gravity oil and associated gas in the Sirhan area of southern Jordan, where they have TOC values as high as 11 weight %. To date, 31 wells have penetrated the Risha Member in eastern Jordan and all have encountered natural gas; however only 5 wells are currently producing at a combined rate of about 30 MMCF/D sufficient to provide about 10% of the electrical generating needs of Jordan. Original field development concentrated in the North Risha area; however, the regional nature of the accumulation was confirmed by a 60 km southern extension in the drilling of seven additional wells. Gas composition is constant across the entire region. The Risha/Dubeidib Formation is highly susceptible to formation damage due to its authigenic clays including kaolinite (dominant), montmorillonite and chlorite. Enhanced production is related to secondary porosity developed along fractures or faults. The dominant subsurface fractures based upon Frac View (FMS) interpretations in the South Risha area are NNW--SSE or NW-SE and in a few cases a NE-SW trend. Basin-center accumulations such as at Risha represent a significant resource in the Middle East.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria