USE OF PROTEIN ASSAYS TO QUANTIFY MICROORGANISMS IN A SANDSTONE RESERVOIR

Quantification of bacteria in geologic samples by point count analysis of SEM images is difficult due to the heterogeneous distribution of the microorganisms and the presence of exopolysaccharide slime that often covers and obscures both the mineral and organic components of the rock. The Lowry Modified Method is an alternative technique that uses a protein assay as a proxy for the amount of bacteria present in a sample. Experiments were conducted to determine the applicability of the Lowry Modified Method for quantification of in situ microorganisms in core samples from the Mississippian Carter sandstone from the North Blowhorn Creek Oil Unit in Lamar County, Alabama. The tests were performed on mineralogically dissimilar samples in order to investigate the relationship between bacterial abundance and rock composition.

The analyzed cores had been preserved anaerobically since they were produced. Each ~5 g sample of core was broken into pebble sized pieces, placed in a funnel filter unit, and covered in sterilized NBCU oil. A bi-weekly alternating nutritional regime of 0.02 % sodium nitrate/100 mL of distilled water and 0.006 % sodium phosphate/100 mL of distilled water was applied to the samples to induce growth of the in situ bacteria. Feeding experiments were performed in an anaerobic glove bag. After a month of nourishment, 1 g of each sample was crushed in a mortar and pestle, added to 1 mL of distilled water and filtered through a 0.22 µ filter. The filtrate was then mixed with 1 mL of Lowry reagent. At this point the bacterial cell walls were breached by the Lowry reagent, and the internal proteins were liberated. Lowry Modified Folin Phenol reagent was then added to the solution, which caused the proteins to agglomerate into chromophores. The amount of protein present was determined by using a spectrophotometer to measure the amount of light absorbed by the chromophores in the solutions.

The protein assays showed increased bacterial growth in each core sample. The greatest growth occurred in samples with large amounts of clay. SEM analysis confirmed that mineral surfaces in all the samples were covered with varying amounts of exopolysaccharide slime, which is a by-product of bacterial growth.