A NEW METHOD TO PREDICT ORGANIC MATTER POROSITY IN UNCONVENTIONAL SHALE RESERVOIRS

A new method is proposed to predict organic matter porosity in unconventional shale (mudstone) reservoirs based on a pyrobitumen porosity model that is constrained by extensive scanning electron microscopic (SEM) observations from several major North American shale plays. These observations reveal that most of the “effective” porosity within organic matter is secondary in origin, occurring within void-filling organic matter, interpreted as solid bitumen (pyrobitumen), rather than in kerogen, as assumed in previous shale reservoir quality prediction models. The origin of the void-filling solid bitumen is interpreted as a thermal alteration product of residual oil retained within intergranular pores (and other preserved voids) within the source rock (shale hydrocarbon reservoir). Pores are interpreted to develop within the solid bitumen because of thermal cracking and gas generation at elevated levels of thermal maturity to form insoluble pyrobitumen. The development of pores within pyrobitumen may be analogous to the formation of pores within petroleum coke during the refining of heavy crude oils. This new organic porosity prediction technique requires: 1) an estimate of the preserved intergranualr porosity at the onset of oil generation based on compaction curves and burial history models, 2) average peak oil saturation (oil saturation index), and 3) the fraction of organic matter converted to porosity (porosity conversion ratio). The last two parameters are derived from SEM digital image measurements from analogous shale reservoirs. Further research is required to refine and test the proposed porosity prediction method.