Reservoir Characterization of the Chicot Aquifer in Acadia Parish, Louisiana

By

MILNER, L. RILEY

Louisiana Geological Survey, Louisiana State University, Baton Rouge, LA

Rapid urban growth and increased use of ground water for agriculture and industry has brought the issue of aquifer sustainability to the forefront in many areas of the country and throughout the world. A drought in 1998 resulted in farmers finding salt water flowing into their fresh water wells in some coastal areas of Louisiana, and large decreases in water levels in many of the state’s aquifers. As a result, the governor and the legislature formed a water commission and advisory group to address the water issues of the state. The Louisiana Geological Survey is conducting a reservoir characterization and modeling study of the Chicot Aquifer system. The study began in Acadia Parish, and will eventually cover the whole aquifer.

Oil and gas and water well electric logs and drillers’ logs were used to identify the different stratigraphic divisions and hydrochemical facies of the Chicot Aquifer in Acadia Parish (i.e. top of the Chicot, the clay layer separating Upper and Lower Chicot, the fresh water/salt water interface, and the base of the Chicot Aquifer). Data from each division and facies were used to generate structure maps, isopach maps, and cross sections of the Chicot Aquifer. The Chicot Aquifer is of early Pleistocene age and overlies the older late Pliocene age Evangeline Aquifer. The top of the Chicot Aquifer is fairly uniform with a depth below ground surface (BGS) at approximately 100 feet throughout Acadia Parish. The base of the Chicot Aquifer has been mapped at a maximum depth of 1,376 feet BGS in the southern part of the parish, and as shallow as 540 feet BGS in the northern part of the parish. The structure contour maps developed for the study suggest the presence of linear trends possibly associated with regional faulting and radial faulting connected with salt domes. The clay dividing the Upper and Lower Chicot units is not laterally continuous. This dividing clay is found between approximately 300 to 500 feet BGS, and averages only 17 feet thick, ranging in thickness from zero (no clay) to 87 feet dipping to the South. The fresh water/salt water interface (top of the transition zone from fresh to more saline water) was found to begin as shallow as 415 feet BGS to a maximum depth of 1,115 feet BGS.

The stratigraphic and hydrochemical facies data is being input to a ground water model that will be used to investigate the ground water flow dynamics in the aquifer. The modeling effort will focus on both local and regional ground water flow, linking the two scales through the use of Telescopic Mesh Refinement (TMR).