Spatial Distribution of Mechanical Properties of the Late Ordovician Glaciogenic Sandstones, SW Saudi Arabia

摘要

The Late Ordovician Sanamah Formation of the Wajid Group, exposed in Wadi Al-Dawasir area, southwest Saudi Arabia, is an analogue of the late Ordovician tight gas sandstone reservoirs in Rub’ Al- Khali Basin. Sanamah Formation consists mainly of glacial origin sandstones and conglomerates deposited in Paleo-valleys. The Formation is divided into three lithological units from the bottom up, massive to cross-bedded conglomeratic sandstone and conglomerate (S1), massive to cross-bedded coarse-grained sandstone (S2), and siltstone to medium-grained sandstone (S3). The three units (S1, S2, and S3) are the proximal, medial, and distal outwash facies, respectively. This study aims to investigate the vertical and lateral variations of the mechanical properties (Schmidt hammer rebound (R), unconfined compressive strength (UCS), Young’s modulus (E), and Poisson's ratio (V)) of the three lithological units of Sanamah Formation, and define their mechanical layers. The strength of the studied sandstone was measured in the field using Schmidt hammer rebound (R), and the sedimentology and stratigraphy were described in detail. Unconfined compressive strength (UCS), Young’ modulus (E), and Poisson's ratio (v) were measured in the laboratory. Petrography study conducted to delineate the mineral composition and diagenetic features (e.g. type and amount of cement materials) of the studied formation. The entire sequence of Sanamah Formation subdivides vertically into three major mechanical layers (MS1, MS2, and MS3), which correspond to the proximal (S1), medial (S2), and distal (S3) outwash facies units, respectively. The increase in the strength values was found to be associated with a decrease in the grain size and increase in the cementation degree of the studied sandstone. Therefore, the vertical and lateral distribution of the mechanical layers of the studied formation is believed to be controlled by the lithological and diagenetic alterations. In the lateral extent, the mechanical layers are not consistent with lithological boundaries, whereas, the proximal facies (S1) in the southern part of Wadi Al-Dawasir quadrangle can be mechanically correlated with the medial facies (S2) in the northern part. This lateral crossing of the mechanical layer over lithological boundaries is probably related to the lateral change in the sandstone diagenesis. Understanding the vertical and lateral changes of outwash facies of late Ordovician Sanamah Formation along with its diagenetic alterations can help to predict the mechanical behavior of the equivalent sandstone reservoir in Rub’ Al-Khali Basin. Thus, the result of this study will help to enhance hydrocarbon exploration from the tight gas sandstone reservoirs in Rub’ Al-Khali Basin