Linking source rock to expelled hydrocarbons using diamondoids: An integrated approach from the Northern Gulf of Mexico

Nanodiamonds (diamondoids) are the ideal geochemical tool for hydrocarbon-source correlation in high-maturity fields since they are thermally stable and outlast biomarkers (molecular fossils), which deteriorate with thermal maturity. Diamondoids form in the shallow subsurface as organic material reacts with clay minerals in source rock, thereby preserving information about the precursor organic compounds and parent minerals. The unique suites of diamondoids generated by these combinations allow direct linkage between expelled hydrocarbons and source rock facies. We compared diamondoid distributions obtained from oils produced from both Eagle Ford Fm. and Austin Chalk Fm. in South-Central Texas, to rock extracts taken from equivalent intervals in a cored pilot hole. The targeted well is in the gas window where biomarkers are of limited use. Diamondoid distributions from oils produced from laterals in the Eagle Ford Fm. and the Austin Chalk Fm. matched extracts from those formations, proving that local sourcing is recognizable in expelled hydrocarbons. A second phase of this project included a direct comparison of results from pilot hole extracts to diamondoids analyzed from 15 oils selected from producing Eagle Ford wells. We showed a close match between the diamondoid chemistry of produced oils regionally and source samples from the Eagle Ford Formation in the cored pilot well. However, samples from immediately adjacent formations (Buda and Upson) were discordant, containing higher concentrations and different distributions of diamondoids when compared to the Austin and Eagle Ford systems. These results spurred a third phase of investigation that included generating diamondoid data from extracts of known Mesozoic Northern Gulf of Mexico (NGoM) source rocks stratigraphically beneath the Austin and Eagle Ford system. These included the Aptian-Albian Pearsall group and several Jurassic formations (Oxfordian Smackover Fm., Kimmeridgian Haynesville Fm., and Tithonian Bossier Fm.). The outlier samples from phases 1 and 2 are matched with deeper source rocks when comparing their diamondoid geochemistry to those of extracts from cored Jurassic source samples. These Jurassic intervals and migration pathways were then vetted with seismic data and integrated attribute analysis. The results of this work show: 1.expelled hydrocarbons can be linked to parent source rock using diamondoid composition; 2. diamondoids can be used to discriminate co-sourcing in intervals where routine biomarker analyses would fail; 3. Samples of well-known Mesozoic source rocks in the northern Gulf of Mexico (Cenomanian-Turonian Eagle Ford Fm., Tithonian Bossier Fm., Kimmeridgian Haynesville Fm., and Oxfordian Smackover Fm.) can be independently recognized using diamondoids, and 4. seismic anisotropy and fluid escape attributes helped delineate both deep-seated, through-going faults and intervals of high fracture density, allowing for connectivity and migration from the deeper source rocks illuminated by diamondoid analyses.