Quartz Cementation in a Deep and Hot Sandstone Reservoir: the Devonian Jauf in Ghawar Field, Saudi Arabia

Abstract

The estuarine to shallow marine sandstones of the Devonian Jauf Formation form a deep and hot gas reservoir where clay coatings on detrital grains are essential for the preservation of porosity. In the absence of clay coats, sandstones have lost almost all porosity due to massive cementation with porefilling quartz. However, sandstones with extensively clay coated grains also commonly appear to contain high percentages of quartz cement, which is thought to have nucleated on detrital quartz grains at breaks in the clay coats and then grown out into the adjacent pore space. The origin of quartz cement in the clay-coated sandstones and the controls on clay coat distribution are the focus of ongoing research. The development of quartz cement in the Jauf reservoir was studied by measuring clay coat surface coverage of quartz grains in a suite of samples encompassing the range of quartz cement content and porosity values. It appears that sandstones with less than 90 % surface coverage are pervasively cemented with quartz, causing almost complete porosity loss in those samples. In those samples, large parts of the quartz grain surface were unprotected, allowing quartz to nucleate on many detrital quartz grains. Porosity is only preserved in sandstones with clay coat surface coverage above 90 %. These samples show a rough trend of decreasing quartz cementation with increasing clay coat coverage, although quartz cement abundance displays considerable variation for any one clay coat coverage value. This suggests that breaks in clay coats played a profound role in quartz cementation, although other factors could also be important. In practice, the frequency of breaks in clay coats may be evaluated by counting the number of quartz grains showing associated quartz cement in a thin section. This was tested on an abundantly quartzcemented sandstone with measured clay coat coverage of 99.8 %. This sample contains a high frequency of quartz grains with associated quartz cement, suggesting the frequent occurrence of breaks in clay coats. The frequent breaks would have facilitated the extensive quartz cement growth.