ABSTRACT: Implications of Open System Diagenesis in Sandstones of the Pennsylvanian Tyler Formation, Central Montana

Peter T. Stanton

Texturally and mineralogically mature quartzarenites in the Tyler Formation contain abundant pore-filling cements for which precipitation cannot be attributed to sources within the "closed system" of the sandstones. Four distinct diagenetic stages occur, each marked by a dramatic shift in pore fluid chemistry that can only be explained by "open system" controls. Stage I, typically consisting of pyrite and siderite authigenesis, reflects bacterial sulfate reduction and fermentation in a reducing substrate. Stage II, consisting of anhydrite, chlorite, dolomite, and quartz precipitation, implies relatively alkaline, oxidizing pore water, and is probably related to erosion of the overlying Alaska Bench Formation. Stage III includes cementation by kaolinite and ferroan dolomit , locally associated with oil infiltration, and resulted from thermal maturation of organic material within adjacent shales. Stage IV is marked by dissolution of dolomite and anhydrite, and subsequent precipitation of calcite and pyrite. Locally, significant secondary porosity is developed. Stage IV is due to regional uplift and fresh water flushing from Tyler outcrops to the west.

The classic dilemma of open system diagenesis is that it precludes the use of mass balance analysis for prediction of porosity loss by cementation or gain by dissolution. When diagenesis can be related to specific burial history events, however, it becomes an exploration tool. In the case of the Tyler Formation, porosity is most greatly impacted by unconformities associated with Stages II and IV. These can be studied by reconstructing paleostructure and predicting groundwater migration paths within the basin.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990