Petrophysical properties of heavy oil-bearing carbonate rocks and their implications on petroleum system evolution. Insights from the Majella Massif

In this work, we investigate the role of hydrocarbons in changing the petrophysical properties of rocks by merging laboratory measurements, outcrops characterization, and subsurface data focusing on a carbonatebearing reservoir (Bolognano Formation) of the Majella massif. This reservoir represents an interesting analogue for subsurface carbonate reservoirs and is made of high porosity (up to 32%) ramp calcarenites saturated by hydrocarbon in form of bitumen. We have characterized in the laboratory density, porosity, compressive strength, Young's modulus and Poisson's ratio of both clean and heavy oil-bearing carbonate rocks of the Bolognano Formation. At ambient pressure the Vp are in the range of 3.49 km/s to 5.06 km/s for clean samples with an inverse relation with porosity, whilst the Vp range for HHC-bearing rocks are from 3.56 km/s to 4.56 km/s without any relation with measured porosity. These data show that porosity and presence of HHCs within the samples both affects seismic waves velocity. In particular, Vp and the derived dynamic Young's modulus generally strongly decrease with increasing porosity for clean samples and, for a fixed porosity, tends to increase with the presence of heavy-oil. This is confirmed also at higher (up to 100 MPa) confining pressure that has not a significant effect on Vp and Vs. Cyclic loading-unloading experiments show that the average compressive strength is 25 MPa and 38 MPa and average static Young's modulus are 5.7 GPa and 9.9 GPa, for clean and HHC-bearing samples respectively. Additionally, the permanent strain is always larger for clean samples. Field work, consisting in measuring fracture orientation in 11 outcrops of the same lithology, shows that both heavy oil-bearing and clean outcrops are characterized by two main trends NW-SE and NNE-SSW. However, the spacing data show that heavy-oil-bearing outcrops are much less fractured respect to clean outcrops. This suggests, assuming that the deformation occurred under the same stress state, that heavy oil-bearing outcrops are stiffer respect to clean outcrops, as supported by the laboratory measurements. We thus infer that in the study area hydrocarbons migration predates the last stage of deformation, limiting the fracturing in bitumen-bearing portions of the Bolognano Formation. At regional scale, the presence of buried oil fields located few kilometres away from the heavy-oil-bearing outcrops and pertaining to the same petroleum system, suggests a good lateral continuity of the reservoir giving clues about the key role played by primary porosity in the secondary migration that acted in this area.