Diagenetic and Petrographic Trends and Development of Secondary Porosity in Upper Cretaceous Feldspathic Sandstones: Central Sacramento Basin
Karl A. Mertz, Jr.
Upper Cretaceous Forbes sandstones from the Sutter Buttes and Rumsey Hills areas of the Sacramento basin are compositionally and texturally immature, and have undergone significant burial diagenesis and selective development of secondary porosity. Analyses reveal sequential diagenetic events that can be divided into four primary zones. Zone 1 includes early events: (a) development of clay rims on framework constituents; (b) extensive mechanical compaction, which continues into deeper burial; and (c) early diagenetic dissolution of unstable framework components and selective replacement by illitic clays and/or calcite. Zone 2 represents a period of widespread cementation: (a) precipitation of K-feldspar and quartz overgrowths; (b) generation of intergranular ferroan calcit (or, locally, siderite) and mixed-layer illite-smectite (approximately 40-60% illite); and (c) albitization of K-feldspar (plus albite overgrowths). In zone 3, extensive dissolution and local development of secondary porosity occurred. Zone 4 reveals porosity-occluding diagenetic products associated with zone 3 leaching, such as kaolinite, chlorite, pyrite, calcite, and zeolites (e.g., heulandite, analcite), and indicates continued albitization of K-feldspar. Development of secondary porosity is highly localized within given reservoir units, ranging from extensively leached zones to areas where leaching is minimal (or where late-stage carbonate cements occlude porosity). Porosity enhancement is linked to kerogen decarboxylation processes and continues during catagenesis. Diagenetic rela ionships suggest that most samples have not experienced temperatures in excess of 140°-160°C. Modal data coincide closely with other Upper Cretaceous sandstones sourced primarily from the Cordilleran magmatic-arc system to the east and northeast ("dissected arc" sandstones; plutonic-volcanic source). Paleogeographic reconstructions and modal analyses suggest probable source terranes in both the Sierran and Idaho batholith arc complexes.
AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.