The Role of Micro-Quartz Cementation in Porosity Preservation in Deep Paleozoic Sandstone Reservoirs, Saudi Arabia

By

Salem H. Shammari¹, Khalid S. Shahab¹

(1) Saudi Aramco, Dhahran 31311, Saudi Arabia

 For the first time in Paleozoic samples from Saudi Arabia, grain-coating microcrystalline quartz cement has been observed during petrographic and SEM examination of Pre-Khuff (Permo-Carboniferous) sandstones. The microquartz is found in quartz arenites from the deeply buried (>13000 feet) pre-Khuff section deposited in shallow to marginal marine environment. The micro-quartz crystals are 2-5 micrometers in length and grow in optical continuity with the parent detrital grain. These crystals occur as a dense coating on detrital grain surfaces and are intergrown with short (~ 5 micrometers) illite filaments, which are also attached to the detrital grain. Where there is incomplete coating of microcrystalline quartz, large euhedral macro-quartz crystals grow as optically continuous overgrowths partially occluding the intergranular porosity.

The presence of microcrystalline quartz cement apparently reduces the growth of the normal euhedral macro-quartz cement. In the absence or paucity of authigenic clays (e.g. illite and chlorite) it appears that the microquartz coatings play an important role in the preservation of relatively high (12-15%) porosity in these deeply buried sandstones. The recognition of such preservation due to the presence of these coatings may allow explorationists to successfully predict sands with relatively good reservoir quality in deep Paleozoic structures.