Ma Zhongzhen*+, Chen Heping*, Yang Xiaofa*, Zhou Yubing*, Tian Zuoji*, Wang Dandan*, Liu Yaming* and Zhao Yongbin*

* Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, China.

+ corresponding author:

Key words: Cretaceous, Napo Formation, M1 Sandstones, charge history, petroleum geochemistry, UCM hump, fluid inclusions, biodegradation, Oriente Basin, Ecuador, Sub-Andean basins.

Major oil discoveries have recently been made in Block T in the north of the Oriente Basin, Ecuador. The oil is reservoired in the M1 Sandstones of the Upper Cretaceous Napo Formation. To investigate the origin and charge history of the petroleum, a detailed geochemical study was carried out on 43 crude oil samples from 42 producing wells in Block T together with fluid inclusion analyses of three core samples from two wells.

According to the results of GC/GC-MS analyses of the oil samples, the oils contain n-alkanes with a peak carbon number at C15-C17 and a subordinate peak at C25-C30. The nC21-/nC22+ ratio ranges from 0.64 to 1.69, and the Carbon Preference Index from 0.95 to 1.23. The odd-over-even predominance is 1.02-1.27. A cross-plot of C22/C21 versus C24/C23 tricyclic terpanes indicates that the source rock is a marine marl mixed with a small amount of terrigenous material. C27 regular steranes are more dominant than C28 Ëœ C29 and the C29/C27 ratio ranges from 0.67 to 0.94 indicating a source rock dominated by marine algal material with minor terrigenous input. Rc calculated using the MPI index was 0.83% to 1.11%, indicating that the oils were generated during the early to peak oil generation stage. A cross-plot of C29ααα20S/(20S+20R) versus C29αββ/(ββ+αα), and ratios of C31L-hopane 22S/(22S + 22R) and C32L-hopane 22S/(22S + 22R), gave similar maturity results.

The presence in the same oil samples of a complete n-alkane series together with an unresolved UCM hump and 25-norhopanes indicates at least two stages of oil charging, with severe biodegradation of the early-stage oil and a later charge of fresh, unaltered oil.

The homogenization temperatures of 36 fluid inclusions in samples from Block T wells F20 and F67 range from 81 to 95oC. A reconstructed burial and geothermal history of well F20 indicates that the M1 Sandstones in this area reached a temperature of 81oC at 19-16 Ma, after which temperatures increased continuously to 95-100oC at the present day. The homogenization temperatures of the analysed fluid inclusions combined with the geothermal history indicate that oil charging into the M1Sandstones began in the early Miocene and continues at the present day.

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