Diagenesis and Porosity Evolution of the Yongningzheng Formation Grainstone (Lower Triassic) in Southwest Guizhou, People's Republic of China

D. S. Ye, S. H. Lui, Z. D. Zeng, Y. B. Chen

The Youngningzheng Formation (Lower Triassic) is a shallow platform deposit consisting of micrites interbedded with oolitic grainstones. The diagenetic history and porosity evolution of these grainstones are very complicated. Both near-surface and burial diagenesis are obvious, based on both petrographical and geochemical studies.

The submarine diagenesis is characterized by micritization and isopachous fibrous cementation. The subaerial diagenesis can be subdivided into freshwater vadose and phreatic diagenesis. The former is characterized by dissolution, vadose silt, and infrequent pendant cements. The main processes of the latter are frequent cementation by equant calcite cements, syntaxial overgrowths and neomorphism. The main processes of mixing diagenesis are silicification and early dolomitization. The characteristics of burial diagenesis are sparry calcite cementation, later dolomitization, stylolitization, and associated infrequent dissolution, pyritization, fracturing, and oil emplacement.

The most important processes that influence porosity evolution are cementation, dissolution, and compaction. The primary porosity was about 40%. The porosity after the submarine cementation and compaction was about 25%. The porosity was 35% after freshwater dissolution. After the freshwater phreatic cementation, the porosity decreased to about 10%. The porosity was about 5% after burial cementation. During the period of oil emplacement, the porosity (i.e., the porosity of accumulation stage) was only 4-5% after stylolitization and associated infrequent dissolution. The present porosity (outside oil-bearing area) is just 1-2% after continuous cementation. The authors consider that neither primary nor present porosity (outside oil-bearing area) can be used to evaluate the reservoir qual ty. In order to evaluate reservoir quality correctly, only the porosity of accumulation stage can be used.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.