Theoretical Framework

Abstract

Hydrocarbon-induced changes are measurable and mappable and have formed the basis of many different methods of surface exploration over the years. Unfortunately, understanding the complex physical, chemical, and biological processes responsible for these phenomena remains incomplete, with the result that these methods are viewed with skepticism and remain underutilized. The aim of this chapter is to provide an overview of the major changes induced by hydrocarbons (HCs) affecting rocks and surface sediments and their implications for non-seismic exploration methods, particularly for marine territories. It is proposed, moreover, to know about the interpretive procedure used to obtain and evaluate the geomorphic anomalies related to the hardening of the surface rocks on the HCs deposits. The bacterial oxidation of the vertically migrating hydrocarbons can directly or indirectly generate changes in the pH and Eh of the surroundings, changing also the stability fields of the different mineral species present in that environment. These changes result in the precipitation or solution and remobilization of various species and mineral elements, such that the rock column above an accumulation of oil becomes significant and measurably different from that of the laterally equivalent rocks, forming a type of “chimney.” Diagenetic carbonates and carbonate cements are among the most common hydrocarbon-induced alterations. In the seas, carbonates can form like slabs and debris, large mounds and pillars, hard terrain or carbonate cements that fill the porosity, all of which give rise to a positive residual relief. It should be pointed out that the positive residual geomorphic anomalies could also have a lithological or structural nature not linked to hydrocarbon microseepage processes. The interpretative procedure choose to obtain and evaluate the geomorphic anomalies related to the hardening of the surface rocks on the HCs deposits (determined from an assay-test method with known gas-oil targets data) has been the UAC at 500 m.