Significance of organic matter in Eocene turbidite sediments (SE Pyrenees, Spain).

Although turbidite deposits are classically considered to be good reservoir rocks for oil and gas, there are no reports concerning their source rock potential in the literature. The sediments from the Vallfogona Formation in the South-Eastern Pyrenees present numerous organic matter-rich levels interbedded in sandstones and coarse turbidite deposits. Two types of organic matter deposits were differentiated on the basis of organic geochemistry and petrography: type A and type B. Type A was deposited in a carbonate marine environment under hypersaline conditions as indicated mainly by even/odd n-alkane predominance, pristane and phytane ratio (Pr/Ph) < 1, presence of gammacerane, and trisnorneohopane over trisnorhopane ratio (Ts/Tm) > 1. Type B was deposited in a more mud-rich marine environment evidenced by the predominance of odd n-alkane, Pr/Ph > or = 1, Ts/Tm < 1, the absence of gammacerane, similar concentrations of the C(27) and C(29) regular steranes, and the greater abundance of C(27) diasteranes. Turbidite facies can be regarded as an environment where organic matter sedimentation is heterogeneous in type and amount. This study suggests that turbidite deposits with interbedded organic matter-rich levels may act as a combined source-reservoir system.