Source-Rock and Carbonization Study, Maracaibo Basin, Venezuela

Abstract

The Maracaibo Basin of western Venezuela is composed of unmetamorphosed Cretaceous and Tertiary sediments deposited on top of a complex pre-Cretaceous basement consisting of igneous rocks and strongly-to-weakly metamorphosed sediments. All oil accumulations in the Maracaibo Basin originate from oil source rocks intercalated in the Cretaceous to Tertiary sedimentary fill of the basin. Originally, the organic-rich limestones of the Cenomanian to Coniacian La Luna Formation were considered the most important, if not the only, oil source rocks in this region. They are indeed classical marine oil source rocks developed quite coherently and with considerable thickness throughout the basin. Later, the possible presence of additional oil source rocks (e.g., in the Tertiary rock sequence) was also taken into consideration. To check this possibility, numerous samples of Upper Cretaceous and Tertiary sediments from different parts of the Maracaibo Basin were collected by MARAVEN, Caracas, and geochemically analyzed by KSEPL, Rijswijk, between 1976 and 1978. The results of the geochemical analyses and carbonization data have been integrated into the geological framework of th Maracaibo Basin. The results of this geochemical-geological study indicate that source rocks for oil are indeed present in various Tertiary rock formations. Nevertheless, these Tertiary source rocks are characteristically different from those of the La Luna Formation. They are mainly composed of hydrogen-rich land-plant matter. The chemical composition of extracts of these land-plant derived source rocks is markedly different from that of La Luna source-rock extracts. With regard to the regional distribution of the Tertiary source rocks, it has been found that they are restricted to certain areas of the Maracaibo Basin, particularly its western and southern parts. For an appraisal of the possible contribution of Tertiary source rocks to the oil accumulations in the Maracaibo Basin, it has been necessary to carry out regional carbonization studies and oil/source-rock extract correlations. The degree of maturity was determined by measuring the optical properties of macerals (in particular vitrinite; Patteisky and Teichmuller, 1960) and, additionally, by maturity calculations. The results of this maturity evaluation show that mature Tertiary sediments occur only in two areas: (1) in the northeastern part of the Maracaibo Basin (the Bolivar Coast region), and (2) the Colon District and the North Andean Foredeep. In the first area, Tertiary source rocks for oil have not been encountered. In the second region, land-plant derived Tertiary source rocks are locally well-developed and could have generated oil. Correlation of oils with source-rock extracts was previously based mainly on the porphyrin content of crude oils and source-rock extracts and on vanadium/nickel ratios. More modern correlations are based on gas-chromatographic and mass-spectrometric methods. Most crude oils investigated correlate well with La Luna source-rock extracts. On the other hand, none of the analyzed oils correlated with extracts of land-plant derived Tertiary source rocks.