Elemental Chemostratigraphy of Upper Cenomanian to Santonian Source Rock Sediments from Eastern Venezuelan Basin: A Useful Tool for High Resolution Correlation

Crespo de Cabrera, Sandra¹, Ian Jarvis² (1) PDVSA Exploration & Production, División Oriente, Puerto La Cruz, Venezuela (2) Kingston University, Kingston Upon Thames, United Kingdom

Elemental chemostratigraphy, carbon stable isotope and biostratigraphic studies were undertaken in the Chimana Grande Island and the Querecual River sections, located in the Eastern Venezuelan Basin, where the Querecual Formation was sampled at approximately 3 m interval. The ~ 600 m sections composed of foraminiferal and organic-rich carbonates and laminated mud-rocks, range from Upper Albian, RotaliporaticinensisZone to Santonian DicarinellaasymetricaZone. Biostratigraphic resolution is poor due to a paucity of age-diag-nostic planktonic foraminifera limited by sedimentation in eutrophic conditions.

A high resolution correlation is presented through the integration of foraminiferal data, total organic carbon (TOC), δ ¹³C(org) and major and trace elements (Al, Ca, Si, Fe, Ti, Mn, Ba, Zr, V, Ni). The formation displays CaCO3 values > 70% from Turonian to Coniacian, falling below 30 % in the Santonian and Campanian. The highest Al, Si and Fe values, controlled by detrital input into the basin, are observed in the Upper Turonian and Santonian, whereas Mn, V and Ti exhibit similar trends but are also high in the Coniacian. High Si/Al and Ba/Al in the Upper Turonian, Coniacian and part of the Santonian primarily reflect biological pro­ductivity but could also be affected by diagenetic processes. Ni/V ratios remain constantly low from the Upper Cenomanian to Santonian and increase abruptly from the middle part of the Santonian, indicating sedimentation in more oxygenated conditions. High surface-water productivity (probably enhanced by upwelling), and resulting anoxic conditions induced the accumulation of this organic-rich facies which constitute the main hydrocarbon source rock for eastern Venezuela