Cantrell, Dave L.¹, P.K. Swart²
(1) Saudi Arabian Oil Company, N/A, Saudi Arabia
(2) University of Miami, FL

ABSTRACT: Enhanced Porosity Development in the Arab-D as a Result of Alteration by Hydrothermal Fluids

The Arab-D in the Ghawar field is the largest oil field in the world. A significant proportion of the production in this field is derived from highly porous and permeable dolomitized intervals (Super-K Intevals). Based on the relatively positive oxygen isotopic composition of the dolomites, it is believed that the dolomites formed early and that hyper-saline brines initially only partially dolomitized these sediments; during later burial, these partially dolomitized intervals may have been referentially susceptible to fracturing. Immediately adjacent to the Super-K intervals there is a trend towards more negative O isotopic values. These intervals also frequently contain small concentrations of quartz. It is proposed that the more negative oxygen isotopic values and the quartz result from the influence of high temperature solutions moving through the fractures. These solutions dissolve the undolomitized portions of these intervals and leave a small remnant geochemical signal in the dolomite. Hence we propose that although the main reservoir rocks occur in early dolomitized rocks, the process which was responsible for the formation of porosity and permeability resulted from high temperature fluids. As these fluids mainly dissolve rather than precipitate existing rocks, they often leave no geochemical evidence of their passage. Such mechanisms may be more important than previously thought in controlling the formation of dolomitized reservoirs.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.