Fracture and Sandstone Diagenesis of the Lance –Upper Mesa Verde Formations, Pinedale Region, Wyoming

Malicse, Ariel¹, Calum Macaulay², Candyce Beck-Brake³ (1) Bellaire Technology Center, Houston, TX (2) Technology Applications & Research, Houston, TX (3) Shell Exploration and Production, Houston, TX

The Lance and Upper Mesa Verde Formations in the Pinedale region of Wyoming represent “tight gas” sandstones where gas production could be enhanced by natural fractures. Fractures in these formations are confined to the fluvial channel sandstones and are bound by calcareous and clay-rich paleosols. To fully understand fracture diagenesis, we investigated the minerals in fractures, host sandstones, and paleosols using data obtained from petrographic, fluid inclusion, and isotope analyses.

With the exception of barite, the host sandstones and fractures follow the same paragenetic history. Calcite dominates the fracture-fill, +/- barite and kaolinite. The calcites in cements and in fractures have a similar range of carbon and oxygen isotopic compositions. In contrast, the ä18O values of the calcareous paleosols are less negative and ä13C values are variable. Paleosol calcite has an isotopic composition that is distinctly different from pore-filling and fracture-filling calcite. Some evidence exists that paleosol calcite may have been dissolved and re-precipitated as cements and as fracture-fills during the burial history.

Fluid inclusion analyses indicate an overlap in the homogenization temperatures of calcite cements and fracture-filling calcite. Fluid inclusion analysis of barite further constrained the fracture timing. The results of fluid inclusion analyses were correlated with the burial history model for the study area. From this correlation and the crosscutting relationships of authigenic minerals we have reconstructed the timing of fracture development and re-opening.