Since the discovery of conventional oil in 1907, the Golfo San Jorge Basin (GSJB) has become into one of the most prolific hydrocarbon basins of Argentina, having an estimated areal extension of 180,000 km2. To this date, scarce information has been publicly released about unconventional exploration on its main hydrocarbon source rock, the lacustrine, Lower Cretaceous Pozo D-129 Formation (D129 Fm.).
The evaluation of the D129 Fm. for unconventional opportunities in El Huemul Field (EH) started in year 2013 and led to a recent shale oil discovery in well EHa-4301, which tested oil not only during completion but also began production under regular field operation conditions. Furthermore, shale-oil occurrence was also observed in well EH-4324
To understand organic richness distribution, kerogen type, hydrocarbon potential and thermal maturity windows of the D129 Fm. source rock in the area, 411 cutting samples obtained from 19 wells distributed along operated concessions were submitted to typical geochemical analysis that included TOC, programmed pyrolysis and organic petrography. Results show that current organic contents corresponding to the deep present structural position of the EH Field attain TOC values higher than 3% with the tightest cloud ranging between 0.8 and 2%. Type I lacustrine kerogen is predominant in the deepest facies of D129 Fm., while enrichment with Type III stands out in the transition to terrestrial-influenced facies. Thermal maturity spans a wide range from marginal-early stages of the oil window to late oil and transition to the gas-condensate stages.
Oils obtained from the D129 Fm. source rock in the EH Field show good states of preservation, contrary to conventional oils in the area, which typically depict evidences of biodegradation, rejuvenation and mixing processes that involve renewed migration pulses. In terms of physical characteristics, the produced shale oil exhibits API gravities around 44°, reflecting an outstanding quality in comparison with average conventional hydrocarbons of the basin.
In terms of potential, the D129 Fm. Upper Section constitutes a shale-oil target having an average thickness of 250 m. Next, the D129 Fm. Middle Section represents a hybrid system (interbedding of shale and tight) with potential for oil and gas production. Last, the D129 Fm. Lower Section seems to integrate two unconventional targets having different potential and is expected to be within the gas window. Adding up all three sections, the total oil window may even exceed 700 m of thickness for potential hydrocarbon production.
