Key structural periods and Permian tight gas accumulation response in Hangjinqi area, Ordos Basin

Abstract

Critical tectonic period is a critical time point that characterizes the genetic connection and causal relation (accumulation response) between tectonic effect and oil/gas accumulation in large superposition basins, it is also an important window for analyzing accumulation evolution and accumulation regularity of oil/gas. Based on the systematic review of the tectonic evolution history of the Ordos Basin, the tectonic evolution and deformation sequence since the Mesozoic in Hangjinqi area in the northern part of Ordos Basin were studied through the constraints of apatite fission tracks and other thermal chronological indicators. Two key structural periods including J3-K1 and K2-E1 were proposed. It was believed by combining with the understanding of the Carboniferous-Permian hydrocarbon-generating and reservoir-forming histories that the J3-K1 was the period of design finalization in the Hangjinqi area which formed a currant NE-trending structural framework high in the northeast and low in the southwest during which, lithologic gas reservoirs including the Carboniferous Taiyuan and the Permian Shanxi formations, first member of the Permian Shihezi Formation to the north of the Sanyanjing-Wulan Jilin Temple-Bo'erjiang Haizi fault were formed and corresponding to the period of great amount of hydrocarbon generated and expulsed from source rocks as well as the densification of sandstones. On the other hand, the K2-E1 period was the major stage for fault reactivation and the formation of NE-trending tectonic trap and NW-trending fault that led to the adjustment of early lithologic gas reser-voirs in the south and the formation of structural or structural-lithologic gas reservoirs in the north. The difference of accumulation on each sides of the main fault is obvious. The southern part was formed earlier than the northern part, and was dominated by lithologic gas reservoirs yet the northern part was dominated by structural-lithological gas reservoirs. The southern gas-bearing strata are dominated by deep strata, while those in the north are both shallow and deep ones. In Paleogene, lifting in the north caused loss of gas. The key accumulation and transformation periods transformed with space and time, which transformed accumulation factors differentially and finally constrained the enrichment and depletion of natural gas.