A new look at the components of the petroleum system of the South Caspian Basin

Detailed knowledge of the petroleum systems of mature petroleum provinces i s often lacking. This has often resulted from the early discovery of most o f these provinces' reserves. The South Caspian basin represents one such pr ovince. The "modern" era of petroleum exploration in the South Caspian bega n more than 100 years ago. Even with this long history several aspects of t he petroleum system have remained largely speculation. Among the key questi ons are: What is the primary hydrocarbon source for the region? Where and when has hydrocarbon generation taken place? Can exploration be extended beyond known stratigraphic and areal limits? Recent integrated geologic and geochemical studies have attempted to answer these and other questions for the petroleum system of the South Caspian ba sin. These studies attempted to overcome prior problems associated with lim ited sample availability, severity of surface weathering, data quality and completeness. This study confirmed the Maykop Suite as the primary oil sour ce, but restricted its stratigraphic distribution. Organic carbon content w ithin the Maykop ranged upward to 14.28 wt.%, with hydrocarbon generation p otentials (S-2) approaching 75 mg HC/g rock. Source rock deposition was cyc lic and included both oil- and gas-prone episodes. Geochemically, the regio n's oils are highly variable as a result of their complex and variable migr ation and alteration histories rather than their derivation from multiple s ource rock units. The oils display similar molecular and isotopic compositi ons to the extracted Maykop samples confirming their genetic relationship. The range in carbon isotope values reflects secular changes within the Mayk op Suite source. Unlike some earlier studies, this study showed that all of the oils were generated within the conventional "oil-window" (R-o approxim ate to 0.9-1.0%). This places the source system at depths in excess of 5 km and emphasizes the role that vertical migration played in the development of the basin's hydrocarbon accumulations. There is evidence that migration was episodic, possibly related to punctuated generation or the opening of c onduits through the activity of mud diapirs. Model results also suggest tha t the rapid Plio-Pleistocene subsidence and sedimentation were the primary controls on hydrocarbon generation.