The organic geochemistry of sedimentary organic matter from the Algerian Sahara

Abstract

This study Is composed of three parts: (I) organic geochemical and organic petrological analysis of source rock samples from the Ghadames-El Borma basin are. described; (! I) organic geochemical characterisation of oils from a number of occurrences In Algeria has been carried out to give families or groups of different oil types; (iii) oiksource rock correlations In the Ghadames-El Borma basin between the oils In the Triassic reservoirs and, the putative source rock - sections are suggested. Sixty rock samples ranging from the-Triassic to Ordovician In age from the Ghadames-El Borma basin, have been Investigated using organic geochemical techniques In order to assess source rock potential. Rock-Eval pyrolysis data indicate that Devonian, constitute excellent - source rocks, Siludan and Ordovician are considered potentially ; noderate to good source beds, whereas Triassic sediments have little or no oil potential. Reflected and ultraviolet light I microscopy show that amorphous organic matter and Graptolite dominate the Ordovician strata, while Silurian and Devonlan'contain predominantly liptinitic materials such as acritarchs, algal bodies (Tasmanites) and spores; bitumen Is moderately abundant. Biological marker Investigation using capillary gas chromatography and the combined gas chromatography/mass spectrometry in energy Impact . -(GC/MS/El) and metastable ! on reaction monitoring (GC/MS/MRM) modes together with principal component analysis (PCA) have allowed a discrimination of samples Into three groups: (I) Ordovician, (ii) Silurian and Devonian and (iii) Triassic. Oil-oil correlation procedure has been carried out In areas with a number of different oil occurrences from southern and northern Algeria, namely from the Ghadames-El Borma, Sbaa, Illizi, Triassic province and Southeast Constantine basins. A range of approaches can be used which depend on the nature of the oils. In the present study, correlation parameters Include data derived from biological marker distributions as well as gasoline range hydrocarbons. Distributions of biological marker compounds together with principal component analysis (PCA) of sterane and terpane distributions reveal a discrimination of, the well numbers Into three groups: (1) Southeast Constantine (DK1, GKN1), Ghadames-El Borma, (ELB9), Sbaa (ODZI, DECI-11)A, in Amenas (TG22). (il) Ghadames-El Borma (KA2, ROM 1), Ain Amenas (ZR1 15) (iii)Gassi Touil (GT6, GT47), Rhourd-Nouss (RN48, RNSE6). Oils from Illizi basin (MRK16, STAI-18, STAH40 and MRK12) are poor In biological marker content and as yet cannot ba'assigned to any of the above groups. Maturity assessment has been based on paraffin index values (heptane and Isoheptane Indices) given by Thompson (1983) as well as steroid Isomerization and aromatization ratios. In this situation, oil mixing can be clearly seen using both gasoline derived I parameters and biological marker ratios (e. g. sterane Isomerization) if there Is no loss during migration. In the final part of this study,o il-sourcer ock correlationsh ave been considered for oils (Keskessa, El Borma and Rom field) and source rocks from the Ghadames-El Borma basin. However, such correlations were, In some cases, difficult to establish because molecular distributions may have changed during migration. Nevertheless, correlations procedures attempted using molecular parameters and PCA results of steranes and terpanes suggest that these particular oils may have been derived from Devonian or Silurian source rocks, as suggested by similarities In sterane and terpane compositions.