Source Rock Quality Variations within a Sequence Stratigraphic Framework of the Paleocene-Eocene Terrestrial Formation, Termit Basin, Niger

Termit Basin was filled with lacustrine deposits during its second rifting process in Paleocene-Eocene, this formation can be divided into 2 third-order sequences and 6 system tracts based on well-log and seismic data. The... [ view full abstract ]

Termit Basin was filled with lacustrine deposits during its second rifting process in Paleocene-Eocene, this formation can be divided into 2 third-order sequences and 6 system tracts based on well-log and seismic data. The occurrence and hydrocarbon potential of source rocks in this strata are strongly controlled by stratigraphic and geographic position.
Source quality can be accessed by total organic carbon (TOC), the amount of volatile hydrocabons (S1) and thermogenic hydrocarbons(S2) and HI index (mg hydrocarbon[HC]/g [TOC]). Source quality tends to be low in the lowatand systems tracts (LST) with the low values of TOC(0.49~2.32, average 1.12), S1+S2(0.38~16.94, average 2.69), HI(28~253, average 135), increasing to a maximum around the maximum flooding surface(MSF) and pro-delta area in the transgressive systems tracts (TST) with the high value of TOC(1.31~7.42, average 3.3), S1+S2(1.41~27.45, average13.25), HI(164~734, average 387), then decreasing to a moderate level in the highstand systems tracts (HST). Meanwhile, some molecular organic geochemical characteristics of source rock are also controlled by sequence development, such as Pr/Ph value was high (1.91~2.22) in the LST and the bottom of TST, while relatively low (1.38~1.51) around the MFS and the bottom of HST; Triaromatic dinosterane index(TDSI) tends to be low in delta area of LST and TST, increasing to a relatively high value in the shore-shallow lake, semi-deep or deep lake environment of TST and HST.
It can be concluded that 1) unlike the marine clastic sedimentary environment, the pro-delta deposit zone in the TST of lacustrine rift basin tends to be the most enrichment area of organic matter, followed by condensed section around the MFS; 2) The source quality was relatively low in the LST, even in the semi-deep or deep lake area of TST not shows high hydrocarbon potential, it may be the integrated effect of oxidation-prone environment and organic matter input limitation. 3) Therefore, for the lacustrine rift basin, high-quality source rock is not limited in the depocenter of basin, it can also developed in the pro-delta area of tectonic slope closed to the depocenter.