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Locating Tight Shale Sweet Spots Using Diamondoid and Biomarker Analyses


Mapping the extent of oil cracking is one of the most important factors in determining sweetspots for producing liquids from tight shales. This is because oil cracking: (1) generates light oil and gas, which can solubilize oil and act as a carrier during production; (2) can generate pressure within the formation to drive production and, in some cases, possibly even fracture the rock; (3) may create porosity and permeability through the conversion of liquid to gas which can then escape; (4) can crack large oil molecules (such as asphaltenes) that can block pore throats to smaller molecules, allowing for mobility during production. Because of the importance of oil cracking, thermal maturity measurements on tight shale organic matter is crucial and maturity maps of most of the major tight shales in the United States have been made. The measurements generally include vitrinite reflectance and Rock-Eval. Although these analyses can be related indirectly to oil cracking, neither of these methods is a direct measurement. Furthermore, vitrinite reflectance and Tmax are made on the immobile organic matter which may or may not be of the same maturity as the fluids. We propose diamondoid and biomarker analyses on tight shale extracts as a means of supplementing Rock-Eval and/or vitrinite data. Diamondoid analysis, besides being a direct method of measuring oil cracking (Dahl et al. 1999), is made on the fluids within the rock. Comparing the maturity of the mobile and immobile phases one can identify migrated fluids, and where fluids have migrated into the rock, one might expect that they can also be more easily extracted from these zones. Furthermore, although the maturity of the immobile phase should change very little over a relatively small vertical section, e.g. 10s of meters, the maturity of mobile fluids can vary if migration of more mature fluids along bedding planes in certain zones within the section has occurred. Again, if fluids can migrate in, one might assume they will be more easily produced. An additional advantage of performing diamondoid and biomarker analyses on tight shale extracts is that it can be difficult to get reliable vitrinite numbers and Tmax values can be confusing. Oil cracking and maturity determinations by biomarkers and diamondoids can obviously be used in conjunction with vitrinite and Rock-Eval results to provide more accurate oil cracking and maturity determinations.