GEOCHEMICAL AND HYDROLOGIC CONTROLS ON CALCITE CEMENTATION WITHIN THE SIERRA LADRONES FORMATION AND ALONG THE LOMA BLANCA FAULT, RIO GRANDE RIFT, NM

Cementation is one of the most important diagenetic processes controlling reservoir and aquifer permeability. It commences almost immediately after deposition and continues over a wide range of burial depths and salinities.

Understanding the processes responsible for the growth of calcite cements and concretions in the Sierra Ladrones Formation (Pliocene-Pleistocene) is of particular importance in New Mexico, as the unit is an important aquifer for many urban and rural communities. Previous studies have shown that most concretions within the Sierra Ladrones Formation are elongated parallel to paleo-groundwater flow and paleocurrent directions. To date, no detailed studies have attempted to quantify the geochemical and hydrologic controls on calcite cements and concretions.

The present study assesses the geochemical and hydrologic mechanisms responsible for the formation of calcite concretions in sand and sandstone. We hypothesize that cementation in the formation and associated faults formed as a result of focused flow in regions of high permeability. We further hypothesize that precipitation within high permeability zones is facilitated by nucleation kinetics. Several questions will be addressed in this study. These questions are related to the roles of chemical diffusion and advection, nucleation kinetics, and effects of permeability heterogeneities in forming concretions. Additionally, mechanism responsible for cementation along fault zones will be investigated.

These questions will be addressed using a reactive transport model of carbonate diagenesis using a series of idealized and site-specific numerical experiments. In these experiments, geochemical and hydrologic parameters are varied over their known range of uncertainties. The model is constrained by field observations of calcite cementation within the Sierra Ladrones Formation, NM.