Permo-Carboniferous paralic depositional systems in the Daniudi gas field and its near-source box-type gas accumulation-forming model

Abstract：In the Ordos basin, three depositional systems formed during the evolution of the lower Upper Paleozoic strata from marine to continental facies; they are in ascending order the barrier coast-tidal flat, delta and alluvial plain-braided river depositional systems, in which there occurs a terrigenous clastic coal-bearing sequence that has wide facies zones and a persistent lateral distribution. These depositional systems provided abundant source rocks and reservoir rocks and occur extensively in the same horizon or overlap each other over short distances; as a result, gas can be accumulated even though the conditions of large-scale migration passageways are lacking in the interior of the basin. The Daniudi gas field is located in the north of the Yishaan slope of the Ordos basin, and its basic framework is a large multi-layered lithologic trap developed in the Upper Carboniferous-Lower Permian. Sealed and covered by the argillite of the widespread Upper Permian Upper Shihezi Formation with unusually high pressures, this trap exhibits the features of near-source box-type accumulation. This is the main Upper Paleozoic accumulation-forming manner in the interior of the Ordos basin.