Applied petroleum source rock evaluation and high-resolution sequence stratigraphy for unconventional reservoirs in La Luna Formation (Cenomanian- Santonian) Northwest Maracaibo Basin, Venezuela.

Abstract

Since the early 2000s, the exploitation of unconventional reservoirs has become very important around the world because of their high potential source of energy and their high economic value. Venezuela possesses a world-class, hydrocarbon source rock from one of the most prolific places for oil accumulation in the world. This source rock, the La Luna Formation, (Cretaceous in age) is located in eastern Venezuela’s Maracaibo Basin. One of the theories that make the La Luna one of the best source rocks in the world is its deposition in an extensive transgressive and dysaerobic setting that created the optimal conditions for the productivity and preservation of the organic matter. Local variations in depositional and diagenetic conditions have manifestly affected the preservation and dilution of organic matter to some degree, generating small-scale variability in the depositional environments, and thus creating a higher-quality source rock within the depositional sequence that can be more prospective than others. To understand the variability of the depositional conditions, variations in organic matter source, thermal maturity, depositional environments, the use of organic, inorganic geochemical and geological parameters were crucial in this study. Four cores and one outcrop located along the Northwestern Maracaibo Basin provided the database for this study. The methodology used was based on the integration of a comprehensive multiscale characterization for source rocks that encompasses reservoir and source rock evaluation to unravel the stratigraphic origin of the petroleum system. Eight facies identified in one of the most preserved and complete cores (La Luna IX) of the Maracaibo Basin showed a good relationship with what was observed and analyzed in the outcrop. Based on lithofacies characterization and inorganic analysis using X-ray Diffraction (XRD) and X-Ray Fluorescence (XRF), more cores were evaluated along the basin displaying essential variations in the mineralogical and elemental content of different La Luna Formation lithofacies. The analyses allowed construction of a proposed depositional model for the La Luna Formation in the Maracaibo Basin. Based upon 153 analyses from all the subsurface and outcrop samples, the average total organic carbon (TOC) values ranged between 3.85 and 9.10 wt.%. Rock-Eval pyrolysis and biomarker data showed the La Luna Formation is dominated by Type IIS kerogen, indicating an oil and gas prone marine organic matter origin. Pristane, phytane (Pr/Ph) and other biomarker ratios suggest anoxic conditions during deposition of the La Luna Formation. Distributions of regular steranes, hopanes, monoaromatic steroid hydrocarbons (MAS) and tentative identification of gammacerane suggest a hypersaline and marine carbonate depositional environment. Thermal maturity parameters indicate that most of the cores (with the exception LLIE) are within the oil window. Liquid hydrocarbons in the study area occur in the northwest and southwest areas, and condensates and dry/wet gases occur in the northeast. The lithofacies association, the sequence-stratigraphic framework, relative hydrocarbon potential (RHP), and biomarker analysis identified the depositional environment as an epicontinental sea developed in a shallow marine, upper shelf euxinic environment represented by a series of third order sequences of Highstand and Transgressive System Tracts overlying the erosional top of the underlying Cogollo Group (Maraca Formation). According to lithofacies, well log interpretation, and organo-facies, the La Luna Formation deposition is paleo-topographically controlled in some areas by the karstic-like shallow-water platform of the Early Cretaceous, analogous to the Hunton- Woodford Formation in the USA midcontinent (Slatt et al., 2012). This combined source rock evaluation composed of geological and geochemical parameters indicated an excellent potential as an unconventional reservoir for oil and gas in the study area. Geochemical analysis confirmed the excellent quality of the organo-facies with higher productivity and preservation indicating that the most prospective intervals for mixed hydrocarbons in La Luna Formation are located in the Upper La Luna in the northwest Maracaibo Basin. Conversely, paleo-topographic controls and variation in lithofacies illustrate that the Lower La Luna as the most prospective area for hydrocarbons towards the southwest Maracaibo Basin. By the application of a higher density sampling and the use of the biomarkers, current results enhance the previous work done by Liborius (2015). These stark differences show the tremendous value that biomarkers provide in the exploration of prospective source rocks. Not only do they help to identify paleoenvironmental changes and redox conditions, but they also depict the best organo-facies and accurate maturity parameters of the rock.