Evolution of Diagenetic Fluids in Red Beds Reservoirs of Eocene Dongying Depression: Evidence from Fluid Inclusions

Wang, Jian; Cao, Yingchang; Feely, Martin; Song, Guoqi

Fluid inclusions hosted in authigenic minerals record abundant information of diagenetic fluids. 24 samples of sandstones of Ek1-Es4x from 17 wells were investigated to study the petrography of fluid inclusions. 10 of these samples were prepared as doubly polished wafers and further analyzed by microthermometry. Abundant liquid-rich two-phase aqueous inclusions occur in authigenic quartz, carbonate and ferro-carbonate cement as primary inclusions and in successive cracks of quartz grains as secondary inclusions. Fluid inclusions sizes are small being usually less than 10μm with the majority ranging between 2μm and 8μm in length. The composition of fluid inclusions in quartz is mainly NaCl-H2O, while in carbonate and ferro-carbonate is mainly CaCl2-H2O. Homogenization temperature (TH) and salinity of secondary inclusions in quartz grains is consistent with the primary inclusions in authigenic quartz. TH and salinity of inclusions in quartz are different from inclusions in carbonate and ferro-carbonate, and the differences also exist between inclusions in carbonate and ferro-carbonate. Based on the analysis of microthermometry, PVT simulation and burial history, the trapping temperature and pressure and stages of diagenetic fluids were established. Four stages of diagenetic fluids occurred. The active time of the first stage was before 31.3Ma, which was recorded by inclusions in carbonate with the salinity being 17-23 eq.wt%NaCl and pH being more than 8.7. Fluids were from syndeposit water and diagenetic fluids of mudstones. The active time of the second stage was between 31.3 and 26.4Ma, which was recorded by inclusions in quartz with the salinity being 3-7 eq.wt%NaCl and pH being 5.6-6.1. Fluids were rich of organic acid. The active time of the third stage was between 26.4 and 21.4Ma, which was recorded by inclusions in ferro-carbonate with the salinity being 15-20 eq.wt%NaCl and pH being more than 9. Fluids were mainly influenced by dehydration of gypsum and decarboxylation of organic acid. The active time of the fourth stage was after 21.4 Ma, which was recorded by inclusions in quartz with the salinity being 3-9 eq.wt%NaCl and pH being 5.5-6.0. Fluids were influenced by organic acid and carbon acid. The evolution of diagenetic fluids was characterized by alternating alkaline and acid. Two apparent formation fluid pressure rise-fall cycles occurred with the pressure coefficient being generally less than 1.2, which indicates the open diagenetic system.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013