Abstract: Diagenesis, Porosity Evolution and Hydrocarbon Evaluation of Permian Reef Complexes in the Middle and Lower Yangtze Area

Six Permian reef complexes have been studied in the middle and lower Yangtze areas, from which reef-building processes, dolomitization, cementation, dissolution and fracturing could be recognized by a detailed description of thin sections under microscope. Data analysis from varieties of test of O-C stable isotope, trace elements, inclusion temperature and cathodoluminescence of thin sections suggested that the Permian reef complexes had undergone marine, evaporative marine, mixture of meteoric water and seawater, meteoric water and burial diagenetic environments. The diagenetic sequence was as follows: reef-building processes - marine cementation - evaporative marine dolomitization - dissolution in the meteoric vadose zone and cementation in the meteoric phreatic zone - meteoric water-seawater mixing dolomitization - alternation of shallow burial (I) cementation , dolomitization, and dissolution (T[1]/P[2]) - deep burial cementation (including J[3]/J[2] paleokarstification) - deep burial dissolution - deep burial dolomitization - dissolution and cementation in shallow burial (II) and supergene leaching and cementation. Based on the above studies, porosity evolution of the Permian reef complexes has been reconstructed. Three porosity peaks existed: a porosity of 30-40%, caused by evaporative marine dolomitization, meteoric water-seawater mixing dolomitization and dissolution in the meteoric vadose zone, could been inferred during eodiagenetic stage; a porosity of 10-15% caused by supergene leaching during T[1]/P[2], made the second peak; a porosity of 10-15%, which was the most important effective porosity, caused by organic acid dissolution at deep burial stage, made the third peak. Furthermore, the hydrocarbon potential of the Permian reef complexes was discussed according to the timing of tectonic, porosity and hydrocarbon evolution, which shows that in the Indosinian-Yanshanian period and Himalaya period there might exist favorable hydrocarbon migration, accumulation and preservation.  

AAPG Search and Discovery Article #90940©1997 AAPG Foundation Grants-in-Aid