Petrological and petrophysical causes for porosity reduction by bitumen plugging, Uwainat limestone (Middle Jurassic), Dukhan field, Qatar

Abstract

The Uwainat Member at Dukhan field was deposited on a ramp and it represents depositional environments ranging from subtidal, protected shallow-marine to outer ramp environments. The subtidal, protected shallow-marine environment is represented by the stromatoporoid wackstone and packstone facies. The inner ramp environment is represented by peloidal intraclastic packstone and grainstone facies while the outer ramp environment is characterized by the bioclastic wackstone facies. Diagenetic changes have affected virtually all of the depositional characteristics of the Uwainat Member in the study area. The types of diagenesis observed in cores and thin sections include micritization, drusy calcite cements, fine to medium blocky calcite cements, leaching, stylolitization, dolomite associated with stylolites, late leaching, coarse blocky calcite cements, fracturing, and saddle dolomite. These diagenetic categories are interpreted to have been formed in the following diagenetic environments- 1) marine phreatic, 2) meteoric phreatic, 3) shallow subsurface, and 4) deep subsurface diagenetic environment. Porosity at Dukhan field is both depositional and diagenetic in origin. Depositionally formed pores correspond to original grain texture, packing, sorting, and fabric whereas the pores formed by diagenetic alteration are related to processes of leaching and pressure dissolution. Intergranular pores are the most abundant pore type present in Uwainat rocks; therefore, they are considered to be the primary contributors to the total Uwainat reservoir volume. The Uwainat reservoir contains such a large amount of bitumen that it creates a pore-plugging problem in Dukhan field. It occludes pore-space and therefore has a significant negative effect on poroperm characteristics. Depending on the amount present, it may reduce porosity by amounts ranging from 17% to 88% of total original porosity. Bitumen plugged zones are discontinuously distributed throughout the Uwainat reservoir and only two zones of bitumen plugged-zones (5-25 feet thick) appear to be continuous for a distance of up to ten miles in a North-South direction. This distribution closely matches the distribution of grain-supported rocks that were extensively leached to produce good porosity (25-40%) and permeability. It also suggests that effective porosity was the main controlling factor in the distribution of bitumen in the Uwainat reservoir.