Mineralogy of the Green River Formation in the Piceance Creek Basin, Colorado

Abstract

Quantitative X-Ray Diffraction (XRD) analysis was performed on 82 samples spanning the Green River Formation in the Piceance Creek Basin of Colorado. Samples represented basin margin (outcrop sections in Douglas Pass), and basin center (Savage 24-1 and Shell 23X-2 core) depositional environments. Bulk mineralogy is integrated into a stratigraphic framework of lake stages, incorporating variations in organic richness. Twenty-eight Douglas Pass samples consist of argillaceous, siliceous, feldspathic and dolomitic mudstone and siltstone composed predominantly of clay minerals, quartz, carbonate minerals, feldspar, and analcime. Fifteen Shell 23X-2 well samples cover the lower third of the basin center section. Data for 35 additional Shell 23-X2 samples from the U. S. Geological Survey core database were added to the dataset. The lower 24 Shell 23-X2 samples consist of argillaceous and siliceous mudstone and siltstone composed of clay minerals, carbonate minerals, quartz, feldspar and dawsonite. The upper 26 samples overlap the lower John Savage well section. Thirty-nine Savage 24-1 well samples cover the upper 2/3 of the basin center section. With the upper part of the Shell 23-X2 well samples, they consist of feldspathic and dolomitic mudstone and siltstone composed of feldspar, carbonate minerals, quartz, nahcolite [NahCO3], dawsonite [NaAl(OH)2(CO3)], and clay minerals. Buddingtonite [(NH4)AlSi3O8•0.5H2O] comprises a significant fraction of the feldspar.

Overall, basin center samples are clay-poor (6 %), and organic matter-rich (12 %), whereas basin margin samples are clay-rich (22 %), with more diverse clay types, and less organic matter (7 %). Divalent carbonates are dominantly dolomite-ankerite, with calcite essentially absent in the middle basin center, but commonly present in the basin margin section. Buddingtonite, nahcolite, dawsonite, and halite (NaCl) are common in the basin center, but sparse to absent in the basin margin, whereas analcime (NaAlSi2O6•H2O) is abundant in the basin margin (max 37 %), and sparse in the basin center.

Changes in mineralogy are related to lake stages (as defined by Tanavsuu-Milkeviciene and Sarg 2012), including a sharp reduction in clay and increase in authigenic dawsonite and feldspar content during Stage 2 (transitional lake), and a fall in quartz and dawsonite near the end of Stage 3 (rapidly fluctuating lake) in the basin center. These changes are attributed to reactions controlled by variations in salinity, alkalinity, pH, and silica concentration. Halite and nahcolite reflect saturation in the deep lake center. Differences between basin margin and center likely reflect varying lake levels but persistent stratification of the lake throughout most of its history. Mineralogic variations in the Green River Formation have relevance to other organic-rich shale and mudstone formations in lacustrine and potentially even in marine environments where organic matter preservation is enhanced by water column stratification.