Geostatistical Analysis of Facies Distributions in a Buried-Valley Aquifer

David F. Dominic and Robert W. Ritzi Jr.

In this study we combine sedimentary facies models with geostatistical analyses to quantify facies distributions in a way that is useful for predicting reservoir heterogeneity. The Miami Valley aquifer system in southwestern Ohio is composed of contrasting aquifer (sand and gravel) and aquitard (diamicton, clay) hydrofacies. The sand and gravel was deposited by proglacial streams on valley-wide outwash plains, the diamicton was deposited as till, and the clay was deposited in a valley-wide proglacial lake. Variation of the volume of aquitard hydrofacies with depth indicates than upper and lower aquifers are separated by a discontinuous aquitard. However, less than half the volume of the aquitard interval is composed of aquitard hydrofacies, indicating that erosion was a dominant process and that the aquifers are largely interconnected. Thus, predicting heterogeneity depends to a large degree on understanding what controlled the erosion processes associated with proglacial streams. To understand these controls, we analyzed the distribution and spatial correlation of the hydrofacies.

Variograms indicate that in all three areas correlation is greatest parallel to the local trend of the bedrock valley. Ranges in the maximum principal direction (700 m) and in the minimum principal direction (300 m) are also similar and suggest the typical scale over which hydrofacies are correlated. These ranges are presumably related to the preserved extent of individual channel segments and undissected till or Iacustrine silt and clay between channel belts. The predominance of sand and gravel in the valley fill may reflect the alignment of ice margins with the pre-existing bedrock valley.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California