Petrography and geochemistry of authigenic chlorite from Cretaceous and Oligocene sandstones of the Texas-Louisiana Gulf Coast
Authigenic chlorite is a common component of sandstones of the Gulf of Mexico and many other sedimentary basins of the world. This study integrates petrographic and geochemical data to determine the geological and geochemical controls on the formation and distribution of authigenic chlorite in the Woodbine, Tuscaloosa, Frio, and Vicksburg sandstones of the Texas and Louisiana Gulf Coast. Petrographic and geochemical analyses of chlorite from diverse thermal regimes, burial histories and geochemical environments constrain the timing and origins of this authigenic clay mineral. The sandstones studied are very fine- to medium-grained quartzarenites, sublitharenites, lithic arkoses and feldspathic litharenites which contain greater than 1% authigenic chlorite. 7-10 μm thick isopachous grain rims are the dominant chlorite morphology in all of these sandstones. Petrographic and oxygen isotope analyses suggest that chlorite grain rims form at low temperatures (40°C) early in the paragenetic sequence. Paragenetic sequences, leucoxene/chlorite textural relationships, and ⁸⁷Sr /⁸⁶Sr isotope ratios suggest the dissolution of volcanic rock fragments is the principal source of components for the precipitation of authigenic chlorite. Labile volcanic rock fragments are present in all of the chloride sandstones studied. Sediment grain size, sorting, and intergranular volume do not control the precipitation of chlorite. All the chlorites analyzed have similar iron-rich chamosite compositions, which suggests a limited set of geochemical conditions control precipitation. Chlorite compositions are not influenced by sandstone age, geographic locality, sedimentary depositional system, burial history, or dominant chlorite morphology. In all of the sandstones studied, chlorite precipitation is coeval with diagenetic calcite, quartz, and illite. The presence of chlorite grain rims throughout individual oil and gas fields suggests that chlorite precipitates during regionally extensive diagenetic events. Chlorite grain rims do not hinder mechanical compaction and are chemically stable once precipitated. These properties make chlorite grain rims useful for recognizing framework grain dissolution, secondary porosity development, and mechanical compaction. Petrographers can effectively quantify chlorite volumes with thin section image analysis techniques. Petrographic evidence suggests chlorite does not interfere with quartz overgrowth formation except on the micron scale of individual crystal faces. Quartz overgrowths which nucleate on chlorite-covered surfaces and engulf chlorite crystals were commonly noted. Clay mineral interference with quartz and carbonate cementation is not a volumetrically significant process in these sandstones.