OVERVIEW OF THE SARAH FORMATION, NW SAUDI ARABIA: AN EXAMPLE FOR UNCONVENTIONAL TIGHT GAS SANDSTONES

Tight Gas Formations in Saudi Arabia, specifically Sarah Formation, has great economic potentials. The Sarah Formation cuts deeply into the Zarqa, Qasim, and Saq Formations. The base of the Sarah Formation represents a glacially formed incised valley. These valleys extend east-west to southwest-northeast directions. These glacial valleys extend E-W to SW-NE direction, have well defined geomorphologic features, and were filled by the glacio-fluvial, glacial, and glacio-marine sandstones. They show a well-organized lithofacies pattern in a predictable pattern. Numbers of glacial paleovalleys were observed in the Qasim and Ha’il regions and they start from the Arabian Shield in the west and gradually widens towards the east. They extend 50-60 km as well defined valleys but further east, towards the basin, they form lobate shape deltas. Their geometries, thickness variations, lithofacies stacking pattern, and sedimentary structures all indicate that the glacial facies is more abundant in the base of the Sarah Formation and also in the west (proximal parts) of the valleys. However, along the axis of the Sarah paleovalleys, they show very rapid facies variations. The grain size, bedding thickness, and the sedimentary structures indicate more glacio-fluvial deposition. Basinward the grain size of the sandstone becomes significantly fine-grained and in some places the top of the Sarah Formation show wave ripples and marine bioturbation. The Sarah Formation is directly overlain by the offshore marine shales of the Qusaiba Member of the Qalibah Formation, deposited during the Early Silurian regional marine transgression. The base of the Qusaiba Member is known as the Qusaiba “Hot Shale” facies and it is the most important source rock for the Paleozoic Reservoirs of Saudi Arabia. The close relationship between the most prolific source rock facies of the Qusaiba Member and the Sarah Sandstones have significantly increased the exploration potential of the Sarah Formation. However, the rapid lateral and vertical facies variations within the Sarah glacial paleovalleys create certain problems during the development stages. Several detailed stratigraphic / sedimentologic sections were measured and representative specimens were collected to understand the sedimentology and internal facies arrangements of the valleys.