The Jurassic Petroleum System of the Papuan Basin Fold Belt, Papua New Guinea
The Papuan Basin of Papua New Guinea (PNG) has proven to be a highly prolific hydrocarbon province, containing ~7 bboe of resources, split into 5.5 bboe of gas and 1.5 bboe of oil. The majority of the resources have been discovered in the PNG foldbelt and are interpreted to be sourced primarily from Jurassic source rocks. Modelling of hydrocarbon distributions and phases is very challenging due to a complex geologic history and a hostile surface terrane affecting the acquisition of extensive and good quality data. In addition, CGR’s of the discovered gas pools can vary greatly (20 – 70 bbls / mmscf) over short distances, having a significant impact on the economics. Analysis of a regional data set which includes PVT reports, fluid and source rock geochemistry, maturity, fluid inclusion, AFTA, pressure and temperature data integrated into a multi 1-D model indicates the following: (1) The geochemical fingerprint of the fluids and source rocks point to a deltaic or paralic setting in the Jurassic. The degree of marine influence varies but overall the source rock is classified as Organofacies D/E. Pristane-phytane ratios of the liquids and source rocks range from 2 to 5 and the sterane homolog distribution plots in the “terrestrial” and “estuarine or bay” area of the classification of Huang and Meinschein (1979). The majority of the Upper Jurassic source rocks have TOC’s < 2 wt.% and HI’s < 200 mg / gTOC and this quality is insufficient to explain the distribution and type of fluids. The main contributors are considered to be the carbonaceous shales and coals of the middle Jurassic Magobu Formation which have TOC’s of up to 70 wt.% and HI’s of 500 mg / gTOC. These coals have been penetrated regionally by several wells but remain undersampled (2) Vitrinite reflectance data measured across the Jurassic section in foldbelt wells ranges from 0.7 – 1.3 %Ro and samples from the Magobu Formation range from 0.9 – 1.2 %Ro. This is a very good match with the fluid VReq maturities (GeoMark Research Inc., derived from GC-MSMS data) ranging from 0.89 – 1.22. %. Equivalent maturities derived from gas isotopes (C3 vs C2) range from 1.2 – 1.5 % (3) PVT data show the oils to be light with API ranging from 43° - 49° and GOR from 500 – 3400 scf / bbl. The condensates range from 46° – 67° API with CGR from 19 – 73 bbls / mmscf. All fluids are saturated at reservoir pressure and a crossplot of GLR vs. Psat (Fig. 1) shows a good definition of the dew point and bubble point curves for the system. The lack of occurrence of single phase oil results in uncertainty of the depth and GLR of the oil / gas window transition. This is overcome partly by integrating the vitrinite reflectance data to allow calibration against the expelled fluid GLR. (4) The integration of the source rock data into a 1D model calibrated to temperature, apatite fission track and fluid inclusion data suggests hydrocarbon expulsion occurred in the Early Tertiary and entrapment during the Late Miocene – Pliocene. The dynamic process of charge, ongoing trap formation and reservoir exposure creates a continuous change in the pressure regime which can explain the overall saturated fluid state. Large variance in CGR over relatively small distances is a consequence of variable reservoir pressure. (5) The distribution of the hydrocarbon phase in the foldbelt has previously been attributed to variance in the source rock composition (Waples and Wulff, 1996). An alternative explanation, supported by multi 1D modelling, is linked to the structural setting of the respective discoveries. Oil discoveries are primarily associated with structures that have undergone thin skinned deformation whereas condensate / gas discoveries are linked to thick skinned deformation. In the foreland, the liquids content of the gases is controlled by the prevailing pressures as well as the presence of a regional hydrodynamic system which extends from the foldbelt to the Gulf of Papua.
AAPG Datapages/Search and Discovery Article #90349 © 2019 AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis: Changing of the Guard from Late Mature Experts to Peak Generating Staff, Houston, Texas, March 4-6, 2019