Genesis and distribution of secondary porosity in the deep horizon of Gaoliu area, Nanpu Sag

Abstract

Strong dissolution generated a secondary porosity interval between 3 250–3 800 m in deep sandstones of the Shahejie Formation in Gaoliu area. The porosities included secondary intraparticle porosity, interparticle porosity, moldic porosity, and dissolved cement porosity and fractures. The underground water of the Gaoliu area was fed with fresh water in the northern Yanshan folded mountains. The fresh water moved along the deep and big faults such as Baigezhuang, Xinanzhuang etc. under the gravity force, and then moved along with unconformities. The secondary porosities of deep horizon sandstones were formed by the leaching of ground water along these faults and the unconformities during the shallow burial process, and were preserved by the abnormal high pressure during the deep burial process. Faults and unconformities serve as the leaching path of ground water, and along with the abnormal high pressure, control the secondary porosities distribution. Good reservoirs are possibly at places near the Gaobei fault, footwall of Shuohe fault, Tanghai area, and Shichang subsag near the footwall of the Baigezhuang faults. Key words: Nanpu Sag, deep horizon, secondary porosity, freshwater leaching, abnormal pressure