The microbial input in carbonate-anhydrite facies of a sabkha palaeoenvironment from Guatemala: A molecular approach

Abstract

Nine representative rock samplesincluding four anhydrites, an oil-stained dolomite and a laminated mudstone (source rock) from a sabkha-type palaeoenvironment (La Felicidad 1 well, Guatemala) have been compared in order to tentatively characterize each facies by a geochemical signature. Each sample has been studied by using basic parameters (TOC, Rock-Eval pyrolysis yields) and analyses of alkanes and aromatics by computerized GC-MS. The following families of compounds have been reviewed: n-alkanes, branched and cyclic alkanes, n-alkylbenzenes, benzohopanes, monoaromatic steroids, D-ring aromatic 8,14-secohopanoids, alkylnaphthalenes, alkylphenanthrenes, alkyldibenzothiophenes and triaromatic steroids. Despite close geochemical relationships of all the sample, a more in-depth investigation taking into account molecular patterns and semi-quantitative data reveals significant differences among the samples. Anhydrites may be differentiated from carbonates which illustrates the efficiency of organic geochemistry in environmental palaeoreconstruction even in organiccarbon-lean sediments (TOC = 0.04–0.08% in anhydrites). In these anoxic, marginally mature sediments the strong predominance of triterpanes over steranes (TT to ST > 20) associated with other molecular features leads to the conclusion that the main contribution to the organic matter of all facies is by bacteria. Several prominent components including novel families (iso-C 23 and iso-C 27 alkanes, tetracyclic terpanes, D-ring aromatic 8–14-secohopanoids, 1-phenylpentadecane, undecylnaphathalene and their mono- and dimethylated counterparts) have been interpreted as derived from halophilic bacteria. These conclusions may be extended to all crude oils from the same area which may be considered as true bacterially-derived petroleums. The La Felicidad 1 well data are discussed with reference to other palaeosabkha-type sequences from the SW Aquitaine Basin (France) in which the bacterial input is less important. However, in these sabkha sequences 1-phenylpentadecane, undecylnaphthalene and related structures have also been identified.