The determination and mapping of the extent of oil cracking is essential to evaluating tight shale plays and delineating " sweetspots." This is due to the fact that oil cracking:
generates light oil and gas, which can solubilize oil and act as a carrier during production;
can generate pressure within the formation to drive production and, in some cases, possibly even fracture the rock;
may create porosity and permeability through the conversion of liquid to gas which can then escape; and
it can crack large oil molecules (such as asphaltenes) which can block pore throats to smaller molecules, allowing for mobility during production.
Determination and mapping of oil cracking is generally performed indirectly on the immovable organic matter via vitrinite reflectance and Rock-Eval Tmax. In this paper, we propose a direct method of determining the extent of oil cracking on the fluids contained within, above and below the target tight shale. This method is based on nano-diamond (diamondoid) concentrations in rock extracts. Analyzing biomarker concentrations and distributions, along with diamondoid concentrations, provides a continuum allowing for the determination of fluid thermal maturities and cracking extents ranging from the Early Oil Window into the Dry Gas Window. By combining these fluid analyses with maturity determinations on the immovable organic matter (such as vitrinite reflectance and Tmax), we not only derive a better understanding of oil cracking, but are also able to see oil migration above, below and within the tight shale target. Example of this would be
zones where the maturity of the migrated fluids are markedly different than the maturities of the immovable organic matter; and
zones where the maturities and extent of oil cracking of the fluids themselves vary dramatically over short vertical distances.