Online Journal for E&P Geoscientists

Al-Ajmi, Hussain F.^*1
(1) water Resources Development Department, Ministry of Water and Electricity, Riyadh, Saudi Arabia.

In the frame of aquifer studies in Saudi Arabia, we measured porosity and permeability of sandstones of the Palaeozoic Wajid Sandstone in order to get a data base for a regional groundwater model. The Wajid sandstone is subdivided into five formations, which differ in their dominant depositional environment. From the base to the top these are: (i) Dibsiyah Formation representing a shallow-marine clastic shelf, (ii) Sanamah Formation containing glacial to proglacial sediments most of them deposited in tunnel valleys, (iii) Qusaibah Formation characterized by high-stand shales, (iv) Khusayyayn Formation showing large-scale tabular cross-bedded tidal sands, and the (v) Juwayl Formation again containing glacial to proglacial sediments mostly deposited in tunnel valleys and overlain by shallow-marine tidal beds.

Altogether 380 samples were taken in the outcrop area most of them along sedimentological logs to correlate poroperm properties with sedimentary facies and/or diagenetic features. From oriented samples cylindric plugs were drilled. With a minipermeameter all three directions were measured to analyse anisotropy. A column permeameter was used to measure intrinsic permeability under controlled conditions in a sealed cell. In general, both, porosity and permeability are high for all formations with values typically between 0.5 to 5 Darcy and 20-30%, respectively. Nevertheless, permeability varies about several orders of magnitude among samples or between vertical and horizontal permeability within one sample. This anisotropy can be well explained by varying sedimentary facies and cementation. To better predict poroperm properties in the subsurface, we measured gamma ray and analyzed selected samples with polarization and raster electron microscopy (REM). The data obtained enhance the quality of the hydraulic interpretations of this aquifer system.