GEOCHEMICAL CHARACTERISTICS AND CHARGE HISTORY OF OIL IN THE UPPER CRETACEOUS M1 SANDSTONES (NAPO FORMATION) IN BLOCK T, ORIENTE BASIN, ECUADOR

Abstract

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 nC_21-/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 ≈ 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. R_c 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 95°C. A reconstructed burial and geothermal history of well F20 indicates that the M1 Sandstones in this area reached a temperature of 81°C at 19–16 Ma, after which temperatures increased continuously to 95–100°C 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.