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

Abstract

The 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 lowpermeability 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, lowamplitude 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.

Characteristics and Controlling Factors of the Chang 8 Oil Reservoir in the Yanchi Area of the Ordos Basin: A Case Study of Well G20

“…Well G20 is located in the Yanchi area that crosses the Tianhuan depression and the western margin thrust belt, in which the faults are well developed. Compared with the center of the basin, the Chang 7 source rocks in this area are not well developed [31][32][33], but still have certain hydrocarbon generation capacity. Meanwhile, the reservoir characteristics show that the area has the conditions for oil enrichment.…”

Distribution of Mid‐deeply Buried Sand‐bodies and their Hydrocarbon Significance at Basin Margins: Case Study of the Paleogene in the Eastern Liuzan Area of the Nanpu Sag, Bohai Bay Basin, China

“…Multiple map overlays is an easy and effective method for studying the hydrocarbon distribution patterns of lithological reservoirs (Xu et al, 2008(Xu et al, , 2009Liu et al, 2020). As such, we comprehensively analyzed and summarized the lithological reservoir distribution pattern Fig.…”

Controls on Gas-Reservoir Formation in the Benxi Formation in the Ganquan–Fuxian Area of the Ordos Basin, China

“…Some tidal flat, channel, and tidal delta sandstone facies occurred between the barrier margins and between barriers [30,31]. The existence of double barriers [37][38][39] may be related to sea level rise and barrier migration during the early and middle depositional stages of the Benxi Formation [40] or to the occurrence of relatively strong hydrodynamic conditions between ancient karst highlands and grooves. With the sea level rise, the barrier in the Ben 2 member migrated approximately 5 km southwest compared to that in the Ben 1 member.…”