Abstract:
Upper Indus Basin is considered to be one of the prolific petroleum production
Basins found in Pakistan. The initial discovery for the hydrocarbons in the Potwar subbasin was made in 1915, while in the Kohat Sub-Basin the discovery of hydrocarbons
started with a discovery from Chanda-01 by Oil and Gas Development Company
Limited in 1999 (OGDCL). Although there has been no such discovery of hydrocarbons
in Hazara Basin. However, some of the low-quality coal is found in the carbonaceous
shale of Patala formation (Late- Paleocene to Early- Eocene age).
Late- Paleocene to Early- Eocene Patala Formation is considered as important
potential source rock present in the Upper Indus Basin. The potential to generate
hydrocarbons is dependent on the type of organic matter, quantity of preserved organic
matter, and its thermal maturity. Outcrop samples from Potwar sub-basin and Hazara
Basin and well cuttings of Chanda-1 (Kohat Sub-Basin), Sadkal Center Deep-1, and
Sadkal-1 (Potwar sub-basin) were analyzed for geochemical characteristics to
understand the organic matter source, environment of deposition, thermal maturity, and
probable type of hydrocarbon generation and the outcrop samples of Patala Formation
from Potwar sub-basin are correalated with the outcrop sample from Hazara basin to
establish well-defined correlation based on biomarkers and screening analysis.
The quantification of organic content of Patala Formation demonstrates
different trend of organic richness. The well cuttings of Patala Formation fall in the
rank of fair to very good source rock, and outcrop samples appeared poor to excellent.
The variation in organic richness of outcrop samples of Potwar sub-basin and Hazara
Basin reflect facies variation. The qualitative assessment of Patala Formation was
carried out by Rock Eval pyrolysis that indicates thermal maturity, and hydrocarbon
potential, with reference to generated hydrocarbons (S1) and remaining potential (S2),
Patala Formation ranked as fair to good source rock. Patala Formation mainly
comprises of terrestrial organic matter, Type-III and in few cases Type-II and Type-II/
III (mix). Type-II and III kerogen is of both oil and gas prone, while type-III is gas
prone. In case of outcrop samples from Hazara Basin, very low values of hydrogen
index and accordingly T-max resulted from weathering, oxidation of organic matter.
RE shows immature source rock and Type-VI kerogen type.vi
The outcrop samples show higher abundance of aromatic hydrocarbons, being
> 50%. There is odd to even predominance or bimodal distribution for outcrop samples
in saturated hydrocarbon fingerprinting, OEP values are > 1.5 and CPI values of outcrop
samples are greater than one, indicating comparatively low maturity of source rock and
terrestrial input. However, Patala Formation shows higher concentration of higher nalkanes in all the outcrop samples.
Isoprenoid/n-alkane ratio indicates the terrigenous organic matter input for all
the outcrop samples. Outcrop samples show higher terrestrial source contribution than
algal or bacterial as terpane biomarkers fingerprint is dominated by hopanes, tricyclic
terpanes and tetracyclic terpane are not appeared in any of the terpane chromatogram
(m/z 191). Higher relative abundance of C29 sterane indicates higher terrestrial influence
with very little marine input.
Sterane maturity parameter, S/S+R of outcrop samples reflect source rock has
not attained equilibrium for isomers and values of these parameters are comparable to
the accepted value for onset of the oil window. ββ/ αα+ββ values indicate the Patala
Formation from Hazara Basin and Potwar sub-basin did not attain peak phase of
maturity for oil generation.
Oleanane biomarker is known as source and age indicator, it has origin in land
plants. Angiosperms evolved from Late Cretaceous to early Tertiary time period and
present in rocks of late Cretaceous or younger. All the outcrop samples have
demonstrated Oleanane peak in ion chromatograms, indicating terrestrial source input
and showing an age of Late- Paleocene to Early- Eocene to Patala Formation.