New Mexico Geological Society Annual Spring Meeting — Abstracts

The Fengcheng Formation consists primarily of dolomitized, tuffaceous mUdstones and/or sandstones as well as dolomitic and/or silicified tuffs. Depositional environment is interpreted to be a semi-closed bay. The formation acts not only as a profilic reservoir rock but also as a rich source rock. Formational primary porosities were lost primarily through compaction in early diagenesis. Present porosities are dominantly secondarily derived from dolomite or zeolite dissolution, siliceous syneresis, and recrystallization porosity. porosity may be vugular in size.

Petroleum thermomaturation of the sediments has been a major diagenetic factor. During the immature stage, minor carbon dioxide is released by oxidation, sulfate reduction, and organic fermentation; the pH value resulting reflects an alkaline environment. Main diagenetic effects are the devitrification and hydration of tephras, dolomitization, and zeolitization. At the mature stage major release of carbon dioxide converts pore water from alkaline to acid resulting in the crystallization and dehydration of silica as well as the dissolution of zeolite and dolomite which created good secondary porosity. The high quantities of released silica from hydration and corrosion of the fine tuffaceous sediments were subsequently precipitated as chalcedony and quartz with resulting shrinkage porosity and syneresis. Destruction of secondary porosity is inhibited by the short time interval as well as distance of migration.