Surface Geochemical Exploration Using Bacterial and Plant Bioindicators, Northern Neuquen Basin, Argentina*

Organothroph bacterial accumulation and trace elements present in soils and sediments are good indicators for active oil reservoirs due to microseepage of light hydrocarbon gases and other geogases that reach the surface through pore spaces or fracture systems at both local and crustal scales. Trace elements like V, Cr, Ni, Mo among others, are also carried upwards in the shape of microbubbles by means of diffusion and hydrothermal activity. The surface and subsurface alteration of sediments and soils is therefore the expression of the ascending gas plumes. In northern Neuquen Basin, Cerro de Los Leones area, a geochemical exploration survey took place based on transects with sampling points of 400 meters distance from one another. Transects were perpendicular to the structures and faults present in the area to show the contrast between anomalies and background values. Geological formations were taken into account when sampling the soil surface, identifying rock types, development and origin. Satellite image processing with NDVI and Tasseled Cap algorithms were used to discriminate plant photosynthetic efficiency and soil moisture. Soil samples for microbial analysis were taken at 40 cm depth and placed in sterilized jars. Using a modified MPOG method, edaphic bacterial colonies were counted in UCF/g. Also samples were taken from the same depth for trace element analysis using X-ray fluorescence. Plant community analysis involving Shannon Diversity index (H) and Specific Richness index (S) was also carried out to show the local effect of soil alteration in vegetation. Square sample stations (6 meters side) were designed for each sample point of one transect. A geomorphological feature such as soil salinity was taken into account to correct the results of plant indexes and better show the effect of microseeps on the ecosystem. Every result was integrated to seismic information to construct the possible arrangement of the petroleum system. This research helped to identify active reservoirs through permeable faults and structures.