Variations in Organic Facies and Sedimentary Environments of the La Luna Formation and Machiques Member in the Perija Mountain Range, Venezuela

Abstract

In a detailed geochemical study of core samples from four oil wells and outcrop samples from four sections, the vertical and areal variations of organic facies within the main source rocks in the western part of the Maracaibo Basin have been performed. In addition, oils and oil seeps of the basin have been geochemically correlated to each one of these facies. Based on geochemical and micropaleontological data, two organic facies related to variations in sedimentary environment are recognized in the La Luna Formation (Cenomanian-Santonian). The sulphur richer organic facies includes the Cenomanian intermiddle shales and the shales from the Cenomanian-Turonian boundary, both characterized by higher values of total organic carbon, bitumen, hydrogen index and benzotiophenes content. This facies probably is an earlier generation source rock than the rest of the La Luna Formation and its correlation with crude-oils indicates that most of the crudes display a distribution of hopane and sulphur aromatic compounds typical of these more anoxic facies. Within the Machiques Member of the Apon Formation (Aptian), two organic/lithological facies have been identified. These are transgressive basal shales and the overlying regressive mudstones with an upward decrease in hydrocarbon potential, anoxia and paleobathymetry. This member presents no benzotiophenes andmore » differences in the relationships between terpanes and aromatic hydrocarbons which allow us distinguish it from La Luna Formation extracts. This is a powerful technique to recognize the genesis of crude oils derived from a particular marine source rock interval in the Maracaibo Basin.« less