Geochemical comparison of oil-bearing fluid inclusions and produced oil from the Toro sandstone, Papua New Guinea

Abstract

A detailed study was carried out to compare the geochemistry of the crude oil in the Lower Cretaceous Toro sandstone reservoir in the Iagifu-7X well with oil trapped in fluid inclusions in this sandstone in the Iagifu-7X and P'nyang-2X wells in the Papuan Fold Belt. The drill stem test (DST) oils and the oils trapped in fluid inclusions show different source- and maturity-dependent geochemical parameters. Biomarkers suggest that the DST oils at Iagifu-7X were derived from clay-rich, mixed marine/terrestrial source rocks deposited under oxic conditions, probably Middle to Late Jurassic mudstones. The fluid inclusion oils were derived from a less terrestrially-influenced marine source rock deposited under less oxic conditions. The fluid inclusion oils contain 1,2,7-trimethylnaphthalene and oleanane biomarkers indicative of angiosperm input and a Cretaceous or younger source rock. These oils are more mature (calculated reflectance [ R c] = 1.02%) than the reservoired oil at Iagifu-7X ( R c = 0.84%) and condensate at P'nyang-2X ( R c = 0.84%). Source and thermal maturity parameters for all three fluid inclusion samples are similar, suggesting that they were derived from similar source rock facies. Furthermore, samples with predominantly blue vs. white fluorescing oil inclusions have no discernible geochemical maturity difference, suggesting that the use of fluorescence colours as a qualitative guide to the maturity of the trapped oil is not valid in this case. Fluid inclusion oils were trapped prior to charge of the reservoir with oil at the Iagifu-7X and gas condensate at the P'nyang-2X wells. The fluid inclusion oils therefore represent an early oil charge from a deeply buried, probably Cretaceous source rock, which started migrating into the Toro sandstone during the Miocene. At Iagifu this early oil was probably diluted by a larger volume of Jurassic oil generated at the end of the Miocene. The absence of fluid inclusions with compositions like the currently reservoired oil is interpreted to be due to oil charging an already oil-saturated zone, when diagenesis is likely to have been inhibited. At the P'nyang well, the transition from oil (as represented by the oil inclusions) to gas condensate was most likely caused by gas displacement.