A novel exploration technique using the microbial fingerprint of shallow sediment to detect hydrocarbon microseepage and predict hydrocarbon charge — An Argentinian case study

The fundamental assumption of many successful geochemical and geomicrobial technologies developed in the last 80 years is that hydrocarbons leak from subsurface accumulations vertically to the surface. Driven by buoyancy, the process involves sufficiently large volumes directly measurable or indirectly inferable from their surface expressions. Even when the additional hydrocarbons are not measurable, their presence slightly changes the environment, where complex microbial communities live, and acts as an evolutionary constraint on their development. Since the ecology of this ecosystem is very complicated, we propose to use the full-microbiome analysis of the shallow sediments samples instead of targeting a selected number of known species, and the use of machine learning for uncovering the meaningful correlations in these data. We achieve this by sequencing the microbial biomass and generating its “DNA fingerprint”, and by analyzing the abundance and distribution of the microbes over the dataset. The proposed technology uses machine learning as an accurate tool for determining the detailed interactions among the various microorganisms and their environment in the presence or absence of hydrocarbons, thus overcoming data complexity. In a proof-of-technology study, we have taken more than 1000 samples in the Neuqu謠Basin in Argentina over three distinct areas, namely, an oil field, a gas field, and a dry location outside the basin, and created several successful predictive models. A subset of randomly selected samples was kept outside of the training set and blinded by the client operator, providing the means for objectively validating the prediction performance of this methodology. Uncovering the blinded dataset after estimating the prospectivity revealed that most of these samples were correctly predicted. This very encouraging result shows that analyzing the microbial ecosystem in the shallow sediment can be an additional de-risking method for assessing hydrocarbon prospects and improving the Probability Of Success(POS) of a drilling campaign.

Mineralogical and geochemical characterization of hydrocarbon microseepage-induced sediments in part of Assam-Arakan Fold Belt, Cachar area, NE India

Long term hydrocarbon microseepages create a reducing environment on the surface rocks and sediments, which induces an array of mineralogical alterations. Among these alterations, the reduction of ferric iron minerals to ferrous iron minerals and precipitation of clay and carbonates are significant. Several studies have been carried out to characterize these hydrocarbon induced rocks/sediments. Almost all these studies have been carried out for arid to semi-arid climatic regions. The present study attempts to characterize the geochemical properties of the hydrocarbon induced sediments in part of Assam-Arakan Fold Belt (AAFB), NE India characterized by heavy rainfall. Reflectance spectroscopy, X-ray diffraction (XRD), X-ray fluorescence (XRF) and inductively coupled plasma emission - mass spectrometry (ICP-MS) studies have been carried out on the sediments. The reflectance spectroscopy reveals that microseepage-induced sediments have higher clay content and lesser ferric iron mineral content. Geochemical indices also suggest that the hydrocarbon-affected sediments are relatively more altered than the unaffected ones. Studies of trace element patterns indicate that the hydrocarbon-induced sediments are enriched in average Be, V, Cu, Zn, Ga, Zr, and Mo and are depleted in Li, Cr, Co, Ni, Rb, Sr, Sc, and Y. The normalized rare earth element (REE) distribution patterns are the same for both the microseepage affected and unaffected sediments though the microseepage-induced sediments are slightly depleted in the REEs. The present study, thus, points out that the hydrocarbon microseepage-induced alterations are also evident in the high precipitation terrains though the alteration levels are less pronounced than that of the arid to semi-arid climatic regions due to abundant surface and groundwater which mobilize the minerals/elements from the microseepage system and tries to homogenize the compositions.