Factors Controlling Hydrocarbon Accumulation in Jurassic Reservoirs in the Southwest Ordos Basin, NW China

Abstract

AbstractThe sedimentary, paleogeomorphological and reservoir characteristics of the Jurassic Yan'an Formation in the southwestern Ordos Basin, northwestern China, were studied by means of casting thin sections, scanning electron microscopy, inclusion analysis and identification of low‐amplitude structures. A model for reservoir formation is established, and the controlling effects of sedimentary facies, paleotopography, low‐amplitude structures and formation water on oil reservoirs are revealed. There are significant differences in the sedimentary characteristics, structural morphology and paleowater characteristics between the reservoirs above the Yan 10 Member and those in the Yan 9 to Yan 7 Members. The Yan 10 Member contains fluvial sediments, whereas the Yan 9 to Yan 7 members contain delta‐plain anastomosing‐river deposits. The distribution of high‐permeability reservoir is controlled by pre‐Jurassic paleogeomorphology and sedimentary facies. Some of these facies exhibit high porosity and high permeability in a low‐permeability background. The main hydrocarbon accumulation period was the late Early Cretaceous, filling was continuous, and the charging strength altered from weak to strong and then from strong to weak. The Yan 10 reservoir is mainly controlled by the paleogeomorphology: hydrocarbons migrated upward at a high speed through the unconformity surface, and accumulated in the favorable traps formed by paleogeomorphic structural units, such as gentle slopes or channel island. Furthermore, groundwater alternation in these areas was relatively stagnant, providing good reservoir preservation conditions. The reservoirs in the Yan 9 and higher members are controlled by the sedimentary facies, low‐amplitude structure and paleowater characteristics. Hydrocarbons migrated through the three‐dimensional delivery system, influenced by favorable sedimentary facies and high‐salinity groundwater, then accumulated in the favorable low‐amplitude structural traps that formed during the hydrocarbon production period.