Characterizations of Middle–Lower Triassic Reservoir Quality and Gas Accumulations in the Northeastern Margin of the Ordos Basin, China

Abstract

New Middle–Lower Triassic gas reservoirs were found in the eastern margin of the Ordos Basin, which provides new possibilities for gas exploration in the Ordos Basin. However, there are few relevant studies on these gas reservoirs. In this study, we use porosity–permeability measurements, optical light microscopy, scanning electron microscopy, mercury intrusion capillary pressure experiments, core-scale low-field nuclear magnetic resonance experiments, fluid inclusion analysis, burial history reconstruction, organic geochemistry data, and wireline log data to systematically investigate the reservoir quality and accumulation pattern of Middle–Lower Triassic gas reservoirs. The Middle–Lower Triassic sandstones are mostly lithic arkose, with a small amount of feldspathic litharenite. In comparison to the upper Paleozoic tight sandstone, the Middle–Lower Triassic sandstones exhibit a superior pore structure and increased porosity and permeability. A significant portion of the Middle–Lower Triassic sandstones can be referred to as “conventional sandstones” (porosities >12% and permeabilities >1 mD). The source rocks of the Middle–Lower Triassic gas reservoirs are most likely the upper Paleozoic coal and organic-rich mudstone, and the accumulation of natural gas occurred in the Early Jurassic to Late Cretaceous. The gas accumulation in the sandstones of the Middle–Lower Triassic reservoirs was mainly driven by buoyancy. The initiation and extension of vertical faults with small displacement in the northeast margin of the Ordos Basin enabled gas to move through the faults from the deep upper Paleozoic source rocks to the Middle–Lower Triassic reservoirs. Since gas accumulation relies on buoyancy, conventional sandstone layers with good reservoir properties are more likely to reach high gas saturation and are the primary targets for exploration and development.